{"id":1490,"date":"2019-08-06T14:14:00","date_gmt":"2019-08-06T18:14:00","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/chapter\/unit-9-nervous-system-2\/"},"modified":"2025-06-23T15:06:33","modified_gmt":"2025-06-23T19:06:33","slug":"unit-9-nervous-system-2","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/chapter\/unit-9-nervous-system-2\/","title":{"raw":"Unit 9: The Nervous System","rendered":"Unit 9: The Nervous System"},"content":{"raw":"<div class=\"unit-9.-nervous-system\">\r\n<div class=\"textbox shaded\">\r\n\r\n<strong>Unit Outline<\/strong>\r\n\r\n<a href=\"#9.1\"><strong>Part 1:<\/strong> The Anatomical and Functional Organization of the Nervous System<\/a>\r\n<ul>\r\n \t<li><a href=\"#9.1a\">Anatomical Divisions<\/a><\/li>\r\n \t<li><a href=\"#9.1b\">Functional Divisions<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#9.2\"><strong>Part 2:<\/strong> Nervous Tissue<\/a>\r\n<ul>\r\n \t<li><a href=\"#9.2a\">Neurons<\/a><\/li>\r\n \t<li><a href=\"#9.2b\">Glial cells<\/a><\/li>\r\n \t<li><a href=\"#9.2c\">Myelin<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#9.3\"><strong>Part 3:<\/strong> The Central Nervous System<\/a>\r\n<ul>\r\n \t<li><a href=\"#9.3a\">The Cerebrum<\/a><\/li>\r\n \t<li><a href=\"#9.3b\">The Diencephalon<\/a><\/li>\r\n \t<li><a href=\"#9.3c\">The Brainstem<\/a><\/li>\r\n \t<li><a href=\"#9.3d\">The Cerebellum<\/a><\/li>\r\n \t<li><a href=\"#9.3e\">The Spinal Cord<\/a><\/li>\r\n \t<li><a href=\"#9.3f\">The Meninges<\/a><\/li>\r\n \t<li><a href=\"#9.3g\">The Ventricular System and Cerebrospinal Fluid Circulation<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#9.4\"><strong>Part 4:<\/strong> The Peripheral Nervous System<\/a>\r\n<ul>\r\n \t<li><a href=\"#9.4a\">Ganglia<\/a><\/li>\r\n \t<li><a href=\"#9.4b\">Nerves<\/a><\/li>\r\n \t<li><a href=\"#9.4c\">The Somatic Nervous System<\/a><\/li>\r\n \t<li><a href=\"#9.4d\">The Autonomic Nervous System<\/a><\/li>\r\n<\/ul>\r\n<a href=\"#9.5\"><strong>Part 5:<\/strong> Neuronal Signalling<\/a>\r\n<ul style=\"margin-top: 1.42857em;margin-bottom: 1.42857em\">\r\n \t<li><a href=\"#9.5a\">Ion channels and the Resting Membrane Potential<\/a><\/li>\r\n \t<li><a href=\"#9.5b\">Generation of an Action Potential<\/a><\/li>\r\n \t<li><a href=\"#9.5c\">Propagation of Action Potentials<\/a><\/li>\r\n \t<li><a href=\"#9.5d\">Neurotransmission<\/a><\/li>\r\n<\/ul>\r\n<h2><a href=\"#P\">Practice Questions<\/a><\/h2>\r\n<\/div>\r\n<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\r\n<p class=\"textbox__title\"><strong>Learning Objectives<\/strong><\/p>\r\n\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nAt the end of this unit, you should be able to:\r\n<p class=\"hanging-indent\"><strong>I. <\/strong>Describe the organization of the nervous system and explain the functions of its principal components.<\/p>\r\n<p class=\"hanging-indent\"><strong>II. <\/strong>Describe the structure of the following: neuron, glia, ganglion, nerve, gray matter, tract, white matter, sensory neuron, motor neuron.<\/p>\r\n<p class=\"hanging-indent\"><strong>III. <\/strong>Name, locate and describe the functions of the main areas of the human brain.<\/p>\r\n<p class=\"hanging-indent\"><strong>IV.<\/strong> Describe the structure and explain the functions of the spinal cord.<\/p>\r\n<p class=\"hanging-indent\"><strong>V. <\/strong>Describe the components of a reflex arc and explain how a reflex arc works.<\/p>\r\n<p class=\"hanging-indent\"><strong>V<\/strong><strong>I.<\/strong> Describe the function of the autonomic nervous system (ANS) and compare the specific functions of the parasympathetic and sympathetic divisions of the ANS.<\/p>\r\n<p class=\"hanging-indent\"><strong>VII.<\/strong> Describe the resting membrane potential of a neuron and explain how it is maintained.<\/p>\r\n<p class=\"hanging-indent\"><strong>VIII<\/strong><strong>.<\/strong> Explain how a neuronal action potential is generated.<\/p>\r\n<p class=\"hanging-indent\"><strong>IX.<\/strong> Explain how neuronal action potentials travel down the axon.<\/p>\r\n<p class=\"hanging-indent\"><strong>X. <\/strong>Explain the process of neurotransmission, and name three different neurotransmitters.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\r\n<p class=\"textbox__title\"><strong>Learning Objectives and Guiding Questions<\/strong><\/p>\r\n\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\nAt the end of this unit, you should be able to complete all the following tasks, including answering the guiding questions associated with each task.\r\n<p class=\"hanging-indent\"><strong>I. <\/strong>Describe the organization of the nervous system and explain the functions of its principal components.<\/p>\r\n\r\n<ol>\r\n \t<li>Draw a flow chart demonstrating the relationships between, and stating the main function of each of the following components of the nervous system:\r\n<ul>\r\n \t<li>Central nervous system<\/li>\r\n \t<li>Peripheral nervous system<\/li>\r\n \t<li>Sensory neurons<\/li>\r\n \t<li>Motor neurons<\/li>\r\n \t<li>Somatic nervous system<\/li>\r\n \t<li>Autonomic nervous system<\/li>\r\n \t<li>Sympathetic nervous system<\/li>\r\n \t<li>Parasympathetic nervous system<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>Are the twelve cranial nerves considered part of the central nervous system, or the peripheral nervous system? Explain how you know.<\/li>\r\n \t<li>Are the dorsal root ganglia considered part of the central or peripheral nervous system? Explain how you know.<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>II. <\/strong>Describe the structure of the following: neuron, glia, ganglion, nerve, gray matter, tract, white matter, sensory neuron, motor neuron.<\/p>\r\n\r\n<ol>\r\n \t<li>Name the parts of a typical neuron and describe their functions.<\/li>\r\n \t<li>Compare and contrast the location, structure, and function of:\r\n<ul>\r\n \t<li>Neurons and glia<\/li>\r\n \t<li>Nerves and tracts<\/li>\r\n \t<li>White matter and nerves<\/li>\r\n \t<li>White matter and gray matter<\/li>\r\n \t<li>Nerves and ganglia<\/li>\r\n \t<li>Ganglia and gray matter<\/li>\r\n \t<li>Sensory and motor neurons<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>III. <\/strong>Name, locate and describe the functions of the main areas of the human brain.<\/p>\r\n\r\n<ol>\r\n \t<li>Describe the general anatomy of the brain, including the location of the lobes.<\/li>\r\n \t<li>Where in the brain would you find the cell bodies of neurons? Where would you find their axons? Describe how you can tell just by looking at a (cut) brain with the naked eye.<\/li>\r\n \t<li>Describe the location and function of each of the following areas of the human brain:\r\n<ul>\r\n \t<li>Cerebrum<\/li>\r\n \t<li>Diencephalon<\/li>\r\n \t<li>Thalamus<\/li>\r\n \t<li>Hypothalamus<\/li>\r\n \t<li>Brain stem<\/li>\r\n \t<li>Midbrain<\/li>\r\n \t<li>Pons<\/li>\r\n \t<li>Medulla oblongata<\/li>\r\n \t<li>Cerebellum<\/li>\r\n<\/ul>\r\n<\/li>\r\n \t<li>What are the names of the three meninges, and where are they located?<\/li>\r\n \t<li>What are the names of the four ventricles, and where are they located?<\/li>\r\n \t<li>Describe the path taken by cerebrospinal fluid through the brain.<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>IV.<\/strong> Describe the structure and explain the functions of the spinal cord.<\/p>\r\n\r\n<ol>\r\n \t<li>Where in the spinal cord would you find the cell bodies of neurons? Where would you find their axons? Describe how you can tell just by looking at a (cut) spinal cord with the naked eye.<\/li>\r\n \t<li>What are some of the functions of the spinal cord?<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>V. <\/strong>Describe the components of a reflex arc and explain how a reflex arc works.<\/p>\r\n\r\n<ol>\r\n \t<li>Describe the events that take place from the moment the knee is tapped to the moment when the leg extends during the patellar reflex, including the role of each of the structures involved.<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>V<\/strong><strong>I.<\/strong> Describe the function of the autonomic nervous system (ANS) and compare the specific functions of the parasympathetic and sympathetic divisions of the ANS.<\/p>\r\n\r\n<ol>\r\n \t<li>Compare the sympathetic and parasympathetic nervous system based on the:\r\n<ul>\r\n \t<li>Physiological situation to which they respond<\/li>\r\n \t<li>Location and neurotransmitter of the central (preganglionic) neuron<\/li>\r\n \t<li>Location and neurotransmitter of the ganglionic neuron<\/li>\r\n<\/ul>\r\n<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>VII.<\/strong> Describe the resting membrane potential of a neuron and explain how it is maintained.<\/p>\r\n\r\n<ol>\r\n \t<li>Describe the gating mechanism of ligand-gated, voltage-gated, mechanically-gated and leakage ion channels.<\/li>\r\n \t<li>What is the typical resting membrane potential of an animal cell, and what factors contribute to it?<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>VIII<\/strong><strong>.<\/strong> Explain how a neuronal action potential is generated.<\/p>\r\n\r\n<ol>\r\n \t<li>Draw a fully annotated figure plotting membrane potential vs. time as an action potential passes a specific location in an axon\u2019s membrane. Include in your annotations labels explaining the main mechanisms that underlie each shift in membrane potential.<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>IX.<\/strong> Explain how neuronal action potentials travel down the axon.<\/p>\r\n\r\n<ol>\r\n \t<li>Compare the mechanism by which nerve impulses are conducted in unmyelinated and myelinated axons.<\/li>\r\n<\/ol>\r\n<p class=\"hanging-indent\"><strong>X. <\/strong>Explain the process of neurotransmission, and name three different neurotransmitters.<\/p>\r\n\r\n<ol>\r\n \t<li>Create an annotated diagram (or series of diagrams) showing how neurons communicate with each other:<\/li>\r\n \t<li>Describe the mechanism by which an action potential travels from the cell body to the axon terminals of a neuron.<\/li>\r\n \t<li>Describe the mechanisms that return a neuron to its resting state (resting membrane potential) once an action potential has passed.<\/li>\r\n \t<li>Describe the intracellular events that occur in a neuron once an action potential reaches a synaptic end bulb.<\/li>\r\n \t<li>Describe how an excitatory neurotransmitter causes an action potential to be produced in a postsynaptic cell.<\/li>\r\n \t<li>Name at least three specific neurotransmitters: one from the cholinergic system, one amino acid that acts as a neurotransmitter, and one neuropeptide.<\/li>\r\n \t<li>What factor(s) determines whether a neurotransmitter has an excitatory or inhibitory effect on a cell exposed to that neurotransmitter?<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n&nbsp;\r\n<h2><strong><a id=\"9.1\"><\/a>Part 1: Anatomical and Functional Organization of the Nervous System<\/strong><\/h2>\r\n<p style=\"text-align: justify\">The picture you have in your mind of the nervous system probably includes the<strong> brain<\/strong>, the nervous tissue contained within the cranium, and the <strong>spinal cord<\/strong>, the extension of nervous tissue within the vertebral column. That suggests it is made of two organs\u2014and you may not even think of the spinal cord as an organ\u2014but the nervous system is a very complex structure. Within the brain, many different and separate regions are responsible for many different and separate functions. It is as if the nervous system is composed of many organs that all look similar and can only be differentiated using tools such as the microscope or electrophysiology. In comparison, it is easy to see that the stomach is different than the esophagus or the liver, so you can imagine the digestive system as a collection of specific organs.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.1a\"><\/a>Anatomical Divisions<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The nervous system can be divided into two major regions: the central and peripheral nervous systems. The <strong>central nervous system (CNS)<\/strong> is the brain and spinal cord, and the <strong>peripheral nervous system (PNS)<\/strong> is everything else (Figures 1 and 2). The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the central nervous system is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery\u2014meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and [pb_glossary id=\"2376\"]peripheral[\/pb_glossary] is not necessarily universal.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"600\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image1-2.png\" alt=\"image\" width=\"600\" height=\"536\" \/> <strong>Figure 1. Central and Peripheral Nervous System.<\/strong> The structures of the peripheral nervous system are referred to as ganglia and nerves, which can be seen as distinct structures. The equivalent structures in the central nervous system are not obvious from this overall perspective and are best examined in prepared tissue under the microscope.[\/caption]\r\n\r\nNervous tissue, present in both the central and peripheral nervous system, contains two basic types of cells: neurons and glial (or neuroglial) cells. A <strong>[pb_glossary id=\"2340\"]glial cell[\/pb_glossary]<\/strong> is one of a variety of cells that provide a framework of tissue that supports the neurons and their activities. The<strong> [pb_glossary id=\"2181\"]neuron[\/pb_glossary]<\/strong> is the more functionally important of the two, in terms of the communicative function of the nervous system. To describe the functional divisions of the nervous system, it is important to understand the structure of a neuron. Neurons are cells and therefore have a <strong>[pb_glossary id=\"2447\"]soma[\/pb_glossary]<\/strong>, or cell body, but they also have extensions of the cell; each extension is generally referred to as a <strong>[pb_glossary id=\"2448\"]process[\/pb_glossary]<\/strong>. There is one important process that every neuron has called an <strong>[pb_glossary id=\"2345\"]axon[\/pb_glossary]<\/strong>, which is the fiber that connects a neuron with its target. Another type of process that branches off from the soma is the [pb_glossary id=\"2342\"]dendrite[\/pb_glossary].\r\n\r\n[caption id=\"attachment_1753\" align=\"alignnone\" width=\"897\"]<img class=\"wp-image-1753 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/Unit-9-anatomical-organization-nervous-system-fixed-no-caption-897x1024.png\" alt=\"\" width=\"897\" height=\"1024\" \/> <strong>Figure 2. The Anatomical Organization of the Nervous System.<\/strong>[\/caption]\r\n<p style=\"text-align: justify\"><strong>Dendrites<\/strong> are responsible for receiving most of the input from other neurons. Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"695\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image3-3.png\" alt=\"image\" width=\"695\" height=\"502\" \/> <strong>Figure 3. Gray Matter and White Matter.<\/strong> A brain removed during an autopsy, with a partial section removed, shows white matter surrounded by gray matter. Gray matter makes up the outer cortex of the brain. (credit: modification of work by \u201cSuseno\u201d\/ Wikimedia Commons)[\/caption]\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"874\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image4-2.png\" alt=\"image\" width=\"874\" height=\"468\" \/> <strong>Figure 4. What Is a Nucleus?<\/strong> (a) The nucleus of an atom contains its protons and neutrons. (b) The nucleus of a cell is the organelle that contains DNA. (c) A nucleus in the central nervous system is a localized center of function with the cell bodies of several neurons, shown here circled in red. (credit c: \u201cWas a bee\u201d\/Wikimedia Commons)[\/caption]\r\n<p style=\"text-align: justify\">These two regions within nervous system structures are often referred to as <strong>[pb_glossary id=\"2449\"]gray matter[\/pb_glossary]<\/strong> (the regions with many cell bodies and dendrites) or <strong>[pb_glossary id=\"2450\"]white matter[\/pb_glossary]<\/strong> (the regions with many axons). The colors ascribed to these regions are what would be seen in \u201cfresh,\u201d or unstained, nervous tissue (Figure 3). Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because [pb_glossary id=\"2345\"]axons[\/pb_glossary] are insulated by a lipid-rich substance called <strong>[pb_glossary id=\"2343\"]myelin[\/pb_glossary]<\/strong>. Lipids can appear as white (\u201cfatty\u201d) material, much like the fat on a raw piece of chicken or beef. Actually, gray matter may have that color ascribed to it because next to the white matter, it is just darker\u2014hence, gray.<\/p>\r\n<p style=\"text-align: justify\">The distinction between gray matter and white matter is most often applied to central nervous tissue, which has large regions that can be seen with the unaided eye. When looking at peripheral structures, often a microscope is used and the tissue is stained with artificial colors. That is not to say that central nervous tissue cannot be stained and viewed under a microscope, but unstained tissue is most likely from the central nervous system \u2014for example, a frontal section of the brain or cross section of the spinal cord.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"541\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image5-2.png\" alt=\"image\" width=\"541\" height=\"434\" \/> <strong>Figure 5. Optic Nerve Versus Optic Tract.<\/strong> This drawing of the connections of the eye to the brain shows the optic nerve extending from the eye to the chiasm, where the structure continues as the optic tract. The same axons extend from the eye to the brain through these two bundles of fibers, but the chiasm represents the border between peripheral and central.[\/caption]\r\n<p style=\"text-align: justify\">Regardless of the appearance of stained or unstained tissue, the cell bodies of neurons or axons can be located in discrete anatomical structures that need to be named. Those names are specific to whether the structure is central or peripheral. A localized collection of neuron cell bodies in the central nervous system is referred to as a <strong>[pb_glossary id=\"2451\"]nucleus[\/pb_glossary]<\/strong>. In the peripheral nervous system, a cluster of neuron cell bodies is referred to as a <strong>[pb_glossary id=\"2452\"]ganglion[\/pb_glossary]<\/strong>. The term nucleus has a few different meanings within anatomy and physiology. It is the center of an atom, where protons and neutrons are found; it is the center of a cell, where the DNA is found; and it is a center of some function in the central nervous system (Figure 4). There is also a potentially confusing use of the word ganglion (plural = ganglia) that has a historical explanation. In the central nervous system, there is a group of [pb_glossary id=\"2451\"]nuclei[\/pb_glossary] that are connected together and were once called the basal ganglia before \u201cganglion\u201d became accepted as a description for a peripheral structure. Some sources refer to this group of nuclei as the \u201cbasal nuclei\u201d to avoid confusion.<\/p>\r\n\r\n<table style=\"border-collapse: collapse;width: 0%\" border=\"0\"><caption>Table 1: Structures of the Central and Peripheral Nervous System<\/caption>\r\n<tbody>\r\n<tr>\r\n<td style=\"width: 31.3686%\"><\/td>\r\n<th style=\"width: 8.8856%\" scope=\"col\"><strong>CNS<\/strong><\/th>\r\n<th style=\"width: 19.4091%\" scope=\"col\"><strong>PNS<\/strong><\/th>\r\n<\/tr>\r\n<tr>\r\n<th style=\"width: 31.3686%\" scope=\"row\">Group of neuron cell bodies (i.e., gray matter)<\/th>\r\n<td style=\"width: 8.8856%\">Nucleus<\/td>\r\n<td style=\"width: 19.4091%\">Ganglion<\/td>\r\n<\/tr>\r\n<tr>\r\n<th style=\"width: 31.3686%\" scope=\"row\">Bundle of axons (i.e., white matter)<\/th>\r\n<td style=\"width: 8.8856%\">Tract<\/td>\r\n<td style=\"width: 19.4091%\">Nerve<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<p style=\"text-align: justify\">Terminology applied to bundles of axons also differs depending on location. A bundle of axons, or fibers, found in the central nervous system is called a <strong>[pb_glossary id=\"2454\"]tract[\/pb_glossary]<\/strong> whereas the same thing in the peripheral nervous system would be called a <strong>[pb_glossary id=\"2453\"]nerve[\/pb_glossary]<\/strong>. There is an important point to make about these terms, which is that they can both be used to refer to the same bundle of [pb_glossary id=\"2345\"]axons[\/pb_glossary]. When those axons are in the peripheral nervous system, the term is nerve, but if they are central nervous system, the term is [pb_glossary id=\"2454\"]tract[\/pb_glossary]. The most obvious example of this is the axons that project from the retina into the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract. There is a specific place where the name changes, which is the optic chiasm, but they are still the same axons (Figure 5). A similar situation outside of science can be described for some roads. For example, you might know of a street named Canada Way in the city of Burnaby. If you travel south long enough on this road, eventually you will leave Burnaby and enter the city of New Westminster. In New Westminster, Canada Way changes its name to Eighth Street. That is the idea behind the naming of the retinal axons. In the peripheral nervous system, they are called the optic nerve, and in the central nervous system, they are the optic tract. Table 1 helps to clarify which of these terms apply to the central or peripheral nervous systems.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.1b\"><\/a>Functional Divisions<\/strong><\/h5>\r\n<p style=\"text-align: justify\">There are two ways to consider how the nervous system is divided functionally. First, the basic functions of the nervous system are sensation, integration, and response. Secondly, control of the body can be [pb_glossary id=\"2455\"]somatic[\/pb_glossary] or [pb_glossary id=\"2456\"]autonomic[\/pb_glossary]\u2014divisions that are largely defined by the structures that are involved in the response (Figure 6). There is also a region of the peripheral nervous system that is called the enteric nervous system that is responsible for a specific set of the functions within the realm of autonomic control related to gastrointestinal functions.<\/p>\r\n<p style=\"text-align: justify\"><em>Basic Functions: Sensation, Integration, and Response<\/em><\/p>\r\n<p style=\"text-align: justify\">The nervous system is involved in receiving information about the environment around us (sensation) and generating responses to that information (motor responses). The nervous system can be divided into regions that are responsible for <strong>sensation <\/strong>(sensory functions) and for the <strong>response<\/strong> (motor functions). But there is a third function that needs to be included. Sensory input needs to be integrated with other sensations, as well as with memories, emotional state, or learning (cognition). Some regions of the nervous system are termed <strong>integration<\/strong> or association areas. The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response.<\/p>\r\n<p style=\"text-align: justify\">The first major function of the nervous system is <strong>sensation<\/strong>\u2014receiving information about the environment to gain input about what is happening outside the body (or, sometimes, within the body). The sensory functions of the nervous system register the presence of a particular event in the external or internal environment, known as a <strong>stimulus<\/strong>. The senses we think of most are the \u201cbig five\u201d: taste, smell, touch, sight, and hearing. The stimuli for taste and smell are both chemical substances ([pb_glossary id=\"2066\"]molecules[\/pb_glossary], [pb_glossary id=\"2063\"]compounds[\/pb_glossary], [pb_glossary id=\"2093\"]ions[\/pb_glossary], etc.), touch is physical or mechanical stimuli that interact with the skin, sight is light stimuli, and hearing is the perception of sound, which is a physical stimulus similar to some aspects of touch. There are actually more senses than just those, but that list represents the major senses. Those five are all senses that receive stimuli from the outside world, and of which there is conscious perception. Additional sensory stimuli might be from the internal environment (inside the body), such as the stretch of an organ wall or the concentration of certain ions in the blood.<\/p>\r\n<p style=\"text-align: justify\">Stimuli that are received by sensory structures are communicated to the nervous system where that information is processed. This is called <strong>integration<\/strong>. Stimuli are compared with, or integrated with, other stimuli, memories of previous stimuli, or the state of a person at a particular time. This leads to the specific response that will be generated. Seeing a baseball pitched to a batter will not automatically cause the batter to swing. The trajectory of the ball and its speed will need to be considered. Maybe the count is three balls and one strike, and the batter wants to let this pitch go by in the hope of getting a walk to first base. Or maybe the batter\u2019s team is so far ahead, it would be fun to just swing away.<\/p>\r\n<p style=\"text-align: justify\">The nervous system produces a <strong>response<\/strong> on the basis of the stimuli perceived by sensory structures. An obvious response would be the movement of muscles, such as withdrawing a hand from a hot stove, but there are broader uses of the term. The nervous system can cause the contraction of all three types of muscle tissue. For example, skeletal muscle contracts to move the skeleton, cardiac muscle is influenced as heart rate increases during exercise, and smooth muscle contracts as the digestive system moves food along the digestive tract. Responses also include the neural control of glands in the body as well, such as the production and secretion of sweat by the [pb_glossary id=\"2457\"]eccrine[\/pb_glossary] and [pb_glossary id=\"2458\"]apocrine[\/pb_glossary] sweat glands found in the skin to lower body temperature.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"893\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image7-2.png\" alt=\"image\" width=\"893\" height=\"703\" \/> <strong>Figure 6. The Functional Organization of the Nervous System.<\/strong> The diagram represents the divisions of the nervous system involved in each of the basic functions: sensation (receiving and processing information from the external and internal environment), integration (comparing the sensory input with stored information and with other sensory inputs in order for the body to react appropriately) and response (most commonly, a motor command generated by the somatic nervous system or the autonomic nervous system).[\/caption]\r\n<p style=\"text-align: justify\">Responses can be divided into those that are [pb_glossary id=\"2280\"]voluntary[\/pb_glossary] or conscious (contraction of skeletal muscle) and those that are involuntary (contraction of smooth muscles, regulation of cardiac muscle, activation of glands). Voluntary responses are governed by the [pb_glossary id=\"2455\"]somatic nervous system[\/pb_glossary] and [pb_glossary id=\"2333\"]involuntary[\/pb_glossary] responses are governed by the [pb_glossary id=\"2456\"]autonomic nervous system[\/pb_glossary], which are discussed in the next section.<\/p>\r\n<p style=\"text-align: justify\"><em>Somatic, Autonomic and Enteric Nervous Systems<\/em><\/p>\r\n<p style=\"text-align: justify\">The nervous system can be divided into two parts mostly on the basis of a functional difference in responses. The <strong>[pb_glossary id=\"2455\"]somatic nervous system[\/pb_glossary] (SNS)<\/strong> is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle, but those contractions are not always voluntary in the sense that you have to want to perform them. Some somatic motor responses are reflexes, and often happen without a conscious decision to perform them. If your friend jumps out from behind a corner and yells \u201cBoo!\u201d you will be startled and you might scream or leap back. You didn\u2019t decide to do that, and you may not have wanted to give your friend a reason to laugh at your expense, but it is a reflex involving skeletal muscle contractions. Other motor responses become automatic (in other words, unconscious) as a person learns motor skills (referred to as \u201chabit learning\u201d or \u201cprocedural memory\u201d).<\/p>\r\n<p style=\"text-align: justify\">The <strong>[pb_glossary id=\"2456\"]autonomic nervous system[\/pb_glossary] (ANS)<\/strong> is responsible for involuntary control of the body, usually for the sake of [pb_glossary id=\"2264\"]homeostasis[\/pb_glossary] (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. But when you are nervous, you might start sweating also. That is not homeostatic, it is the physiological response to an emotional state.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image8-3.png\" alt=\"image\" width=\"975\" height=\"531\" \/> <strong>Figure 7. Somatic, Autonomic, and Enteric Structures of the Nervous System.<\/strong> Somatic structures include the spinal nerves, both motor and sensory fibers, as well as the sensory ganglia (posterior root ganglia and cranial nerve ganglia). Autonomic structures are found in the nerves also, but include the sympathetic and parasympathetic ganglia. The enteric nervous system includes the nervous tissue within the organs of the digestive tract.[\/caption]\r\n<p style=\"text-align: justify\">There is another division of the nervous system that describes functional responses. The <strong>[pb_glossary id=\"2459\"]enteric nervous system[\/pb_glossary] (ENS)<\/strong> is responsible for controlling the smooth muscle and glandular tissue in your digestive system. It is a large part of the peripheral nervous system, and is not dependent on the central nervous system. It is sometimes valid, however, to consider the enteric system to be a part of the autonomic system because the neural structures that make up the enteric system are a component of the autonomic output that regulates digestion (Figure 7). There are some differences between the two, but for our purposes here there will be a good bit of overlap.<\/p>\r\n&nbsp;\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"190\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image10-2.png\" alt=\"image\" width=\"190\" height=\"185\" \/> Watch <a href=\"https:\/\/youtu.be\/qPix_X-9t7E\">this Crash Course video<\/a> for an overview of the nervous system! Direct link: <a href=\"https:\/\/youtu.be\/qPix_X-9t7E\">https:\/\/youtu.be\/qPix_X-9t7E<\/a>[\/caption]\r\n<h2><strong><a id=\"9.2\"><\/a>Part 2: Nervous Tissue<\/strong><\/h2>\r\n<p style=\"text-align: justify\">Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons are the primary type of cell that most anyone associates with the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glial cells, or glia, are known to play a supporting role for nervous tissue. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.2a\"><\/a>Neurons<\/strong><\/h5>\r\n<p style=\"text-align: justify\">[pb_glossary id=\"2181\"]Neurons[\/pb_glossary] are the cells considered to be the basis of nervous tissue. They are responsible for the electrical signals that communicate information about sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. An important part of the function of neurons is in their structure, or shape. The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible.<\/p>\r\n<p style=\"text-align: justify\"><em>Parts of a Neuron<\/em><\/p>\r\n<p style=\"text-align: justify\">As you learned in the first section, the main part of a neuron is the cell body, which is also known as the [pb_glossary id=\"2447\"]soma[\/pb_glossary] (soma = \u201cbody\u201d). The cell body contains the [pb_glossary id=\"2221\"]nucleus[\/pb_glossary] and most of the major organelles. But what makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as [pb_glossary id=\"2448\"]processes[\/pb_glossary]. Neurons are usually described as having one, and only one, axon\u2014a fibre that emerges from the cell body and projects to target cells (Figure 8). That single [pb_glossary id=\"2345\"]axon[\/pb_glossary] can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse, which is communicated to one or more cells. The other processes of the neuron are [pb_glossary id=\"2342\"]dendrites[\/pb_glossary] (Figure 8), which receive information from other neurons at specialized areas of contact called [pb_glossary id=\"2344\"]synapses[\/pb_glossary]. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a [pb_glossary id=\"2460\"]polarity[\/pb_glossary]\u2014meaning that information flows in this one direction.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"640\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image11-2.png\" alt=\"image\" width=\"640\" height=\"427\" \/> <strong>Figure 8. Parts of a Neuron.<\/strong> The major parts of the neuron are labeled on a multipolar neuron from the central nervous system.[\/caption]\r\n<p style=\"text-align: justify\">Where the axon emerges from the cell body, there is a special region referred to as the [pb_glossary id=\"2461\"]axon hillock[\/pb_glossary]. This is a tapering of the cell body toward the axon fibre. Within the axon hillock, the cytoplasm changes to a solution of limited components called axoplasm. Because the axon hillock represents the beginning of the axon, it is also referred to as the initial segment.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"642\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image12-2.png\" alt=\"image\" width=\"642\" height=\"414\" \/> <strong>Figure 9. Neuron Classification by Shape.<\/strong> Unipolar cells have one process that includes both the axon and dendrite. Bipolar cells have two processes, the axon and a dendrite. Multipolar cells have more than two processes, the axon and two or more dendrites.[\/caption]\r\n<p style=\"text-align: justify\">Many axons are wrapped by an insulating substance called myelin, which is actually made from [pb_glossary id=\"2340\"]glial cells[\/pb_glossary]. [pb_glossary id=\"2343\"]Myelin[\/pb_glossary] acts as insulation much like the plastic or rubber that is used to insulate electrical wires. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an [pb_glossary id=\"2345\"]axon[\/pb_glossary]. Each gap is called a node of Ranvier and is important to the way that electrical signals travel down the axon. The length of the axon between each gap, which is wrapped in myelin, is referred to as an [pb_glossary id=\"2462\"]axon segment[\/pb_glossary]. At the end of the axon is the [pb_glossary id=\"2463\"]axon terminal[\/pb_glossary], where there are usually several branches extending toward the target cell, each of which ends in an enlargement called a [pb_glossary id=\"2464\"]synaptic end bulb[\/pb_glossary]. These bulbs are what make the connection with the target cell at the [pb_glossary id=\"2344\"]synapse[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\"><em>Types of Neurons<\/em><\/p>\r\n<p style=\"text-align: justify\">There are many neurons in the nervous system\u2014a number in the trillions. And there are many different types of neurons. They can be classified by many different criteria. The first way to classify them is by the number of [pb_glossary id=\"2448\"]processes[\/pb_glossary] attached to the cell body. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron\u2019s [pb_glossary id=\"2460\"]polarity[\/pb_glossary] (Figure 9).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"705\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image13-2.png\" alt=\"image\" width=\"705\" height=\"442\" \/> <strong>Figure 10. Other Neuron Classifications.<\/strong> Three examples of neurons that are classified on the basis of other criteria. (a) The pyramidal cell is a multipolar cell with a cell body that is shaped something like a pyramid. (b) The Purkinje cell in the cerebellum was named after the scientist who originally described it. (c) Olfactory neurons are named for the functional group with which they belong.[\/caption]\r\n<p style=\"text-align: justify\">Neurons can also be classified on the basis of where they are found, who found them, what they do, or even what chemicals they use to communicate with each other. Some neurons referred to in this section on the nervous system are named on the basis of those sorts of classifications (Figure 10). For example, a [pb_glossary id=\"2465\"]multipolar[\/pb_glossary] neuron that has a very important role to play in a part of the brain called the [pb_glossary id=\"2466\"]cerebellum[\/pb_glossary] is known as a Purkinje (commonly pronounced per-KIN-gee) cell. It is named after the anatomist who discovered it (Jan Evangilista Purkinje, 1787\u20131869).<\/p>\r\n<p style=\"text-align: justify\"><strong><a id=\"9.2b\"><\/a>Glial Cells<\/strong><\/p>\r\n<p style=\"text-align: justify\">Glial cells, or [pb_glossary id=\"2340\"]neuroglia[\/pb_glossary] or simply glia, are the other type of cell found in nervous tissue. They are considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. The name glia comes from the Greek word that means \u201cglue,\u201d and was coined by the German pathologist Rudolph Virchow, who wrote in 1856: \u201cThis connective substance, which is in the brain, the spinal cord, and the special sense nerves, is a kind of glue (neuroglia) in which the nervous elements are planted.\u201d Today, research into nervous tissue has shown that there are many deeper roles that these cells play. And research may find much more about them in the future.<\/p>\r\n\r\n<table style=\"border-collapse: collapse;width: 100%\" border=\"0\"><caption>Table 2: Glial Cell Types by Location and Basic Function<\/caption>\r\n<tbody>\r\n<tr>\r\n<th style=\"width: 23.87%\" scope=\"col\"><strong>CNS glia<\/strong><\/th>\r\n<th style=\"width: 19.209%\" scope=\"col\"><strong>PNS glia<\/strong><\/th>\r\n<th style=\"width: 56.9209%\" scope=\"col\"><strong>Basic function<\/strong><\/th>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 23.87%\">Astrocyte<\/td>\r\n<td style=\"width: 19.209%\">Satellite cell<\/td>\r\n<td style=\"width: 56.9209%\">Support<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 23.87%\">Oligodendrocyte<\/td>\r\n<td style=\"width: 19.209%\">Schwann cell<\/td>\r\n<td style=\"width: 56.9209%\">Insulation, myelination<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 23.87%\">Microglia<\/td>\r\n<td style=\"width: 19.209%\">-<\/td>\r\n<td style=\"width: 56.9209%\">Immune surveillance, phagocytosis<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"width: 23.87%\">Ependymal cell<\/td>\r\n<td style=\"width: 19.209%\">-<\/td>\r\n<td style=\"width: 56.9209%\">Creating cerebrospinal fluid<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<p style=\"text-align: justify\">There are six types of glial cells (Table 2). Four of them are found in the central nervous system (Figure 11) and two are found in the peripheral nervous system (Figure 12). For reference, Table 2 outlines some common characteristics and functions of the various glial cell types, but the specific names and roles of the glial cell types are not examinable material in this course.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"635\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image15-3.png\" alt=\"image\" width=\"635\" height=\"473\" \/> <strong>Figure 11. Glial Cells of the Central Nervous System.<\/strong> The central nervous system has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the central nervous system in several ways.[\/caption]\r\n<h5><strong style=\"text-align: justify\"><a id=\"9.2c\"><\/a>Myelin<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The insulation for axons in the nervous system is provided by glial cells: [pb_glossary id=\"2468\"]oligodendrocytes[\/pb_glossary] in the central nervous system, and [pb_glossary id=\"2469\"]Schwann cells[\/pb_glossary] in the peripheral nervous system. Whereas the manner in which either cell is associated with the [pb_glossary id=\"2462\"]axon segment[\/pb_glossary], or segments, that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. [pb_glossary id=\"2343\"]Myelin[\/pb_glossary] is a [pb_glossary id=\"2161\"]lipid[\/pb_glossary]-rich sheath that surrounds the [pb_glossary id=\"2345\"]axon[\/pb_glossary] and by doing so creates a [pb_glossary id=\"2470\"]myelin sheath[\/pb_glossary] that facilitates the transmission of electrical signals along the axon. The lipids are essentially the [pb_glossary id=\"2166\"]phospholipids[\/pb_glossary] of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"535\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image16-3.png\" alt=\"image\" width=\"535\" height=\"346\" \/> <strong>Figure 12. Glial Cells of the Peripheral Nervous System.<\/strong> The peripheral nervous system has satellite cells and Schwann cells.[\/caption]\r\n<h2 style=\"text-align: left\"><strong><a id=\"9.3\"><\/a>Part 3:<\/strong><strong>\u00a0The Central Nervous System<\/strong><\/h2>\r\n<p style=\"text-align: justify\">The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person\u2019s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3a\"><\/a>The Cerebrum<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the <strong>[pb_glossary id=\"2471\"]cerebrum[\/pb_glossary]<\/strong> with two distinct halves, a right and left <strong>[pb_glossary id=\"2472\"]cerebral hemisphere[\/pb_glossary]<\/strong> (Figure 13). Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The cerebrum comprises of a continuous, wrinkled and thin layer of [pb_glossary id=\"2449\"]gray matter[\/pb_glossary] that wraps around both hemispheres, the <strong>[pb_glossary id=\"2473\"]cerebral cortex[\/pb_glossary]<\/strong><strong>,<\/strong> and several deep [pb_glossary id=\"2451\"]nuclei[\/pb_glossary]. A <strong>[pb_glossary id=\"2474\"]gyrus[\/pb_glossary]<\/strong> (plural = gyri) is the ridge of one of those wrinkles, and a <strong>[pb_glossary id=\"2475\"]sulcus[\/pb_glossary]<\/strong> (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the [pb_glossary id=\"2473\"]cerebral cortex[\/pb_glossary] (Figure 14).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"929\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image17-2.png\" alt=\"image\" width=\"929\" height=\"429\" \/> <strong>Figure 13. The Cerebrum.<\/strong> The cerebrum is a large component of the central nervous system in humans, and the most obvious aspect of it is the folded surface called the cerebral cortex. Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for communication between the two hemispheres of the cerebral cortex.[\/caption]\r\n<p style=\"text-align: justify\">Different regions of the cerebral cortex can be associated with particular functions, a concept known as localization of function. In the early 1900s, a German neuroscientist named Korbinian Brodmann performed an extensive study of the microscopic anatomy (cytoarchitecture) of the cerebral cortex and divided the cortex into 52 separate regions on the basis of the histology of the cortex. His work resulted in a system of classification known as [pb_glossary id=\"2476\"]Brodmann\u2019s areas[\/pb_glossary], which is still used today to describe the anatomical distinctions within the cortex The results from Brodmann\u2019s work on the anatomy align very well with the functional differences within the cortex. For example, Areas 17 and 18 in the occipital lobe are responsible for primary visual perception. That visual information is complex, so it is processed in the temporal and parietal lobes as well.<\/p>\r\n<p style=\"text-align: justify\">Beneath the cerebral cortex are sets of nuclei known as [pb_glossary id=\"2477\"]<strong>bas<\/strong><strong>al nuclei<\/strong>[\/pb_glossary] that augment cortical processes (Figure 15). Some of the basal nuclei in the forebrain, for example, serve as the primary location for [pb_glossary id=\"2478\"]acetylcholine[\/pb_glossary] production, which modulates the overall activity of the cortex, possibly leading to greater attention to sensory stimuli. Alzheimer\u2019s disease is associated with a loss of neurons in the cholinergic basal forebrain nuclei. Some other basal nuclei control the initiation of movement. For example, while a student is sitting in a classroom listening to a lecture, the basal nuclei will keep an urge to jump up and scream from actually happening. (The basal nuclei are also referred to as the basal ganglia, although that is potentially confusing because the term ganglia is typically used for peripheral structures.)<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"591\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image18-2.png\" alt=\"image\" width=\"591\" height=\"489\" \/> <strong>Figure 14. Lobes of the Cerebral Cortex.<\/strong> The cerebral cortex is divided into four lobes. Extensive folding increases the surface area available for cerebral functions. (The names of the main sulci are provided but they are not required as examinable material in this course.)[\/caption]\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"547\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image19-2.png\" alt=\"image\" width=\"547\" height=\"422\" \/> <strong>Figure 15. Frontal Section of Cerebral Cortex and Basal Nuclei.<\/strong> The major components of the basal nuclei, shown in a frontal section of the brain, are the caudate (just lateral to the lateral ventricle), the putamen (inferior to the caudate and separated by the large white-matter structure called the internal capsule), and the globus pallidus (medial to the putamen). (The names of these nuclei are not required as examinable material in this course.)[\/caption]\r\n\r\n&nbsp;\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3b\"><\/a>The Diencephalon<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The word [pb_glossary id=\"2479\"]diencephalon[\/pb_glossary] translates to \u201cthrough brain.\u201d It is the connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the peripheral nervous system all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with <strong>[pb_glossary id=\"2480\"]olfaction[\/pb_glossary]<\/strong>, or the sense of smell, which connects directly with the [pb_glossary id=\"2471\"]cerebrum[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\">The diencephalon is deep beneath the cerebrum and constitutes the walls of the [pb_glossary id=\"2481\"]third ventricle[\/pb_glossary]. The diencephalon can be described as any region of the brain with \u201cthalamus\u201d in its name. The two major regions of the diencephalon are the [pb_glossary id=\"2482\"]thalamus[\/pb_glossary] itself and the hypothalamus (Figure 16). There are other structures, such as the <strong>[pb_glossary id=\"2483\"]epithalamus[\/pb_glossary]<\/strong>, which contains the pineal gland, and the <strong>[pb_glossary id=\"2484\"]subthalamus[\/pb_glossary]<\/strong>, which includes the subthalamic nucleus, one of the basal nuclei.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"598\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image20-2.png\" alt=\"image\" width=\"598\" height=\"471\" \/> <strong>Figure 16. The Diencephalon.<\/strong> The diencephalon is composed primarily of the thalamus and hypothalamus, which together define the walls of the third ventricle. The thalami are two elongated, ovoid structures on either side of the midline that make contact in the middle. The hypothalamus is inferior and anterior to the thalamus, culminating in a sharp angle to which the pituitary gland is attached.[\/caption]\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\"><em>Thalamus<\/em><\/p>\r\n<p style=\"text-align: justify\">The thalamus is a collection of nuclei that relay information between the [pb_glossary id=\"2473\"]cerebral cortex[\/pb_glossary] and the periphery, spinal cord, or brain stem. All sensory information, except for the sense of smell, passes through the thalamus before processing by the cortex. [pb_glossary id=\"2345\"]Axons[\/pb_glossary] from the peripheral sensory organs, or intermediate nuclei, [pb_glossary id=\"2344\"]synapse[\/pb_glossary] in the thalamus, and thalamic neurons project directly to the [pb_glossary id=\"2471\"]cerebrum[\/pb_glossary]. It is a requisite synapse in any sensory pathway, except for olfaction. The thalamus does not just pass the information on, it also processes that information. For example, the portion of the thalamus that receives visual information will influence what visual stimuli are important, or what receives attention. The [pb_glossary id=\"2471\"]cerebrum[\/pb_glossary] also sends information down to the [pb_glossary id=\"2482\"]thalamus[\/pb_glossary], which usually communicates motor commands.<\/p>\r\n<p style=\"text-align: justify\"><em>Hypothalamus<\/em><\/p>\r\n<p style=\"text-align: justify\">Inferior and slightly anterior to the thalamus is the [pb_glossary id=\"2440\"]hypothalamus[\/pb_glossary], the other major region of the [pb_glossary id=\"2479\"]diencephalon[\/pb_glossary]. The hypothalamus is a collection of nuclei that are largely involved in regulating [pb_glossary id=\"2264\"]homeostasis[\/pb_glossary]. The hypothalamus is the executive region in charge of the [pb_glossary id=\"2456\"]autonomic nervous system[\/pb_glossary] and the [pb_glossary id=\"2273\"]endocrine[\/pb_glossary] system through its regulation of the anterior [pb_glossary id=\"2485\"]pituitary gland[\/pb_glossary]. Other parts of the hypothalamus are involved in memory and emotion as part of the [pb_glossary id=\"2486\"]limbic system[\/pb_glossary].<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3c\"><\/a>The Brain Stem<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The [pb_glossary id=\"2487\"]midbrain[\/pb_glossary] and [pb_glossary id=\"2488\"]hindbrain[\/pb_glossary] (composed of the pons and the medulla oblongata, or medulla for short) are collectively referred to as the brain stem (Figure 17). The structure emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord. The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem. The majority of cranial nerves connect through the brain stem and provide the brain with the sensory input and\/or motor output associated with the head and neck, for example most of the special senses, eye movement, and swallowing. As discussed below, the brainstem controls many involuntary functions vital for survival like breathing rate, heart rate, blood pressure, and consciousness.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"618\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image21-2.png\" alt=\"image\" width=\"618\" height=\"470\" \/> <strong>Figure 17. The Brain Stem.<\/strong> The brain stem includes three regions: the midbrain, the pons, and the medulla.[\/caption]\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\"><em>Midbrain<\/em><\/p>\r\n<p style=\"text-align: justify\">The midbrain includes four bumps known as the colliculi (singular = colliculus), which means \u201clittle hill\u201d in Latin. The <strong>[pb_glossary id=\"2492\"]inferior colliculus[\/pb_glossary]<\/strong> is the inferior pair of these enlargements and is part of the auditory brain stem pathway that relays information to the [pb_glossary id=\"2471\"]cerebrum [\/pb_glossary]for conscious perception of sound. The <strong>[pb_glossary id=\"2493\"]superior colliculus[\/pb_glossary]<\/strong> is the superior pair of structures which integrates visual, auditory and somatosensory information to allow rapid head, eye and body movement towards external stimuli, like a loud noise.<\/p>\r\n<p style=\"text-align: justify\"><em>Pons<\/em><\/p>\r\n<p style=\"text-align: justify\">It is visible on the anterior surface of the brain stem as the thick bundle of [pb_glossary id=\"2450\"]white matter[\/pb_glossary]\u00a0attached to the [pb_glossary id=\"2466\"]cerebellum[\/pb_glossary]. The word [pb_glossary id=\"2489\"]pons\u00a0[\/pb_glossary]comes from the Latin word for bridge; it bridges the midbrain and the medulla and is the main connection between the cerebellum and the brain stem. In conjunction with the medulla it helps regulate vital functions, including respiratory rate (as will be <a href=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/chapter\/unit-6-the-respiratory-system\/\">discussed further in BIOL 1203\/9<\/a>). Through its connection to the cerebellum, the pons helps produce coordinated movement and good balance.<\/p>\r\n<p style=\"text-align: justify\"><em>M<\/em><em>edulla oblongata (or medulla)<\/em><\/p>\r\nThe [pb_glossary id=\"2449\"]gray matter[\/pb_glossary]\u00a0of the midbrain and pons continues into the [pb_glossary id=\"2490\"]medulla oblongata[\/pb_glossary] (also known as the medulla but should not be confused with the medulla in the kidney or adrenal glands; known as the renal medulla and adrenal medulla, respectively). This diffuse region of gray matter throughout the brain stem, known as the <strong>[pb_glossary id=\"2494\"]reticular formation[\/pb_glossary]<\/strong>, is related to sleep and wakefulness, general brain activity and attention. The medulla contains [pb_glossary id=\"2456\"]autonomic[\/pb_glossary]\u00a0nuclei with motor neurons that control the rate and force of heart contraction, the diameter of blood vessels, \u00a0the rate and depth of breathing, among other essential physiological processes, like swallowing.\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3d\"><\/a>The Cerebellum<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The cerebellum, as the name suggests, is the \u201clittle brain.\u201d It is covered in [pb_glossary id=\"2474\"]gyri[\/pb_glossary] and [pb_glossary id=\"2475\"]sulci[\/pb_glossary] like the cerebrum, and looks like a miniature version of that part of the brain (Figure 18). The cerebellum integrates motor commands from the cerebral cortex with sensory feedback from the periphery, allowing for the coordination and precise execution of motor activities, such as walking, cycling, writing or playing a musical instrument.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"639\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image22-2.png\" alt=\"image\" width=\"639\" height=\"791\" \/> <strong>Figure 18. The Cerebellum.<\/strong> The cerebellum is situated on the posterior surface of the brain stem. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of the inferior olive. Output goes to the midbrain, which sends a descending signal to the spinal cord.[\/caption]\r\n\r\n&nbsp;\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.33\"><\/a>Th<\/strong><strong>e Spinal Cord<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Whereas the brain develops out of expansions of the neural tube into primary and then secondary vesicles, the spinal cord maintains the tube structure and is only specialized into certain regions.<\/p>\r\n<p style=\"text-align: justify\">The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. The name of a spinal cord region corresponds to the level at which spinal nerves pass through the intervertebral [pb_glossary id=\"2500\"]foramina[\/pb_glossary]. Immediately adjacent to the brain stem is the [pb_glossary id=\"2501\"]cervical[\/pb_glossary] region, followed by the [pb_glossary id=\"2503\"]thoracic[\/pb_glossary], then the [pb_glossary id=\"2502\"]lumbar[\/pb_glossary], and finally the sacral region (Figures 24 and 25).<\/p>\r\n<p style=\"text-align: justify\"><em>Gray Horns<\/em><\/p>\r\n<p style=\"text-align: justify\">In cross-section, the [pb_glossary id=\"2449\"]gray matter[\/pb_glossary] of the spinal cord has the appearance of an ink-blot test, with the spread of the gray matter on one side replicated on the other\u2014a shape reminiscent of a bulbous capital \u201cH.\u201d As shown in Figure 19, the gray matter is subdivided into regions that are referred to as horns.<\/p>\r\n<p style=\"text-align: justify\">The <strong>[pb_glossary id=\"2504\"]posterior horn[\/pb_glossary]<\/strong> is responsible for sensory processing. The <strong>[pb_glossary id=\"2505\"]anterior horn[\/pb_glossary]<\/strong> sends out motor signals to the skeletal muscles. The <strong>[pb_glossary id=\"2506\"]lateral horn[\/pb_glossary]<\/strong>, which is only found in the thoracic, upper lumbar, and [pb_glossary id=\"2508\"]sacral[\/pb_glossary] regions, is the central component of the [pb_glossary id=\"2507\"]sympathetic division[\/pb_glossary] of the[pb_glossary id=\"2456\"] autonomic nervous system[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\">Some of the largest neurons of the spinal cord are the [pb_glossary id=\"2465\"]multipolar[\/pb_glossary] motor neurons in the anterior horn. The fibres that cause contraction of skeletal muscles are the axons of these neurons. The motor neuron that causes contraction of the big toe, for example, is located in the sacral spinal cord. The axon that has to reach all the way to the belly of that muscle may be a metre in length. The neuronal cell body that maintains that long fiber must be quite large, possibly several hundred micrometres in diameter, making it one of the largest cells in the body.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"603\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image23-2.png\" alt=\"image\" width=\"603\" height=\"691\" \/> <strong>Figure 19. Cross-section of Spinal Cord.<\/strong> The cross-section of a thoracic spinal cord segment shows the posterior, anterior, and lateral horns of gray matter, as well as the posterior, anterior, and lateral columns of white matter. LM \u00d7 40. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)[\/caption]\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\"><em>White Columns<\/em><\/p>\r\n<p style=\"text-align: justify\">Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns. <strong>[pb_glossary id=\"2509\"]Ascending tracts[\/pb_glossary]<\/strong> of nervous system fibres in these columns carry sensory information up to the brain, whereas <strong>[pb_glossary id=\"2510\"]descending tracts[\/pb_glossary]<\/strong> carry motor commands from the brain.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"189\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image24-2.png\" alt=\"image\" width=\"189\" height=\"193\" \/> Watch <a href=\"https:\/\/youtu.be\/q8NtmDrb_qo\">this Crash Course video<\/a> for an overview of the central nervous system! (Direct link: <a href=\"https:\/\/youtu.be\/q8NtmDrb_qo\">https:\/\/youtu.be\/q8NtmDrb_qo<\/a>)[\/caption]\r\n\r\n&nbsp;\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3f\"><\/a>The Meninges<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The outer surface of the central nervous system is covered by a series of membranes composed of connective tissue called the [pb_glossary id=\"2496\"]meninges[\/pb_glossary], which protect the brain. The [pb_glossary id=\"2497\"]dura mater[\/pb_glossary] is a thick fibrous layer and a strong protective sheath over the entire brain and spinal cord. It is anchored to the inner surface of the cranium and vertebral cavity. The [pb_glossary id=\"2498\"]arachnoid mater[\/pb_glossary] is a membrane of thin fibrous tissue that forms a loose sac around the central nervous system. Beneath the arachnoid is a thin, filamentous mesh called the [pb_glossary id=\"2511\"]arachnoid trabeculae[\/pb_glossary], which looks like a spider web, giving this layer its name. Directly adjacent to the surface of the central nervous system is the [pb_glossary id=\"2512\"]pia mater[\/pb_glossary], a thin fibrous membrane that follows the convolutions of [pb_glossary id=\"2474\"]gyri[\/pb_glossary] and [pb_glossary id=\"2475\"]sulci[\/pb_glossary] in the cerebral cortex and fits into other grooves and indentations (Figures 20).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"787\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image25-2.png\" alt=\"image\" width=\"787\" height=\"419\" \/> <strong>Figure 20. Meningeal Layers of Superior Sagittal Sinus.<\/strong> The layers of the meninges in the longitudinal fissure of the superior sagittal sinus are shown, with the dura mater adjacent to the inner surface of the cranium, the pia mater adjacent to the surface of the brain, and the arachnoid and subarachnoid space between them. An arachnoid villus is shown emerging into the dural sinus to allow CSF to filter back into the blood for drainage.[\/caption]\r\n\r\n&nbsp;\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3g\"><\/a>The Ventricular System<\/strong><strong> and Cerebrospinal Fluid Circulation<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Cerebrospinal fluid (CSF) circulates throughout and around the central nervous system. cerebrospinal fluid is produced in special structures to perfuse through the nervous tissue of the central nervous system and is continuous with the [pb_glossary id=\"2129\"]interstitial fluid[\/pb_glossary]. Specifically, cerebrospinal fluid circulates to remove metabolic wastes from the interstitial fluids of nervous tissues and return them to the blood stream. The <strong>[pb_glossary id=\"2513\"]ventricles[\/pb_glossary]<\/strong> are the open spaces within the brain where [pb_glossary id=\"2137\"]cerebrospinal fluid[\/pb_glossary] circulates. In some of these spaces, cerebrospinal fluid is produced by filtering of the blood that is performed by a specialized membrane known as a [pb_glossary id=\"2514\"]choroid plexus[\/pb_glossary]. The cerebrospinal fluid circulates through all of the ventricles to eventually emerge into the subarachnoid space where it will be reabsorbed into the blood.<\/p>\r\n<p style=\"text-align: justify\">There are four ventricles within the brain, all of which developed from the original hollow space within the neural tube, the central canal. The first two are named the [pb_glossary id=\"2515\"]lateral ventricles[\/pb_glossary] and are deep within the [pb_glossary id=\"2471\"]cerebrum[\/pb_glossary]. These ventricles are connected to the [pb_glossary id=\"2481\"]third ventricle[\/pb_glossary] by two openings called the interventricular foramina. The third ventricle is the space between the left and right sides of the [pb_glossary id=\"2479\"]diencephalon[\/pb_glossary], which opens into the [pb_glossary id=\"2491\"]cerebral aqueduct[\/pb_glossary] that passes through the [pb_glossary id=\"2487\"]midbrain[\/pb_glossary]. The aqueduct opens into the [pb_glossary id=\"2516\"]fourth ventricle[\/pb_glossary], which is the space between the [pb_glossary id=\"2466\"]cerebellum[\/pb_glossary] and the [pb_glossary id=\"2489\"]pons[\/pb_glossary] and upper [pb_glossary id=\"2490\"]medulla[\/pb_glossary] (Figure 21).<\/p>\r\n<p style=\"text-align: justify\">The ventricular system opens up to the [pb_glossary id=\"2517\"]subarachnoid space[\/pb_glossary] from the fourth ventricle. The single median aperture and the pair of lateral apertures connect to the subarachnoid space so that cerebrospinal fluid can flow through the ventricles and around the outside of the central nervous system. Cerebrospinal fluid is produced within the ventricles by a type of specialized membrane called a [pb_glossary id=\"2514\"]choroid plexus[\/pb_glossary]. [pb_glossary id=\"2518\"]Ependymal cells[\/pb_glossary] (a type of glial cell; see Figure 11) surround blood capillaries and filter the blood to make cerebrospinal fluid. The fluid is a clear solution with a limited amount of the constituents of blood. It is essentially water, small molecules, and [pb_glossary id=\"2134\"]electrolytes[\/pb_glossary]. Oxygen and carbon dioxide are dissolved into the cerebrospinal fluid, as they are in blood, and can diffuse between the fluid and the nervous tissue.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"835\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image26-2.png\" alt=\"image\" width=\"835\" height=\"534\" \/> <strong>Figure 21. Cerebrospinal Fluid Circulation.<\/strong> The choroid plexus in the four ventricles produce CSF, which is circulated through the ventricular system and then enters the subarachnoid space through the median and lateral apertures. The CSF is then reabsorbed into the blood at the arachnoid granulations, where the arachnoid membrane emerges into the dural sinuses.[\/caption]\r\n\r\n&nbsp;\r\n<p style=\"text-align: justify\"><em>Cerebrospinal Fluid Circulation<\/em><\/p>\r\n<p style=\"text-align: justify\">The [pb_glossary id=\"2514\"]choroid plexuses[\/pb_glossary] are found in all four [pb_glossary id=\"2513\"]ventricles[\/pb_glossary]. Observed in dissection, they appear as soft, fuzzy structures that may still be pink, depending on how well the circulatory system is cleared in preparation of the tissue. The CSF is produced from components extracted from the blood, so its flow out of the ventricles is tied to the pulse of cardiovascular circulation.<\/p>\r\n<p style=\"text-align: justify\">From the [pb_glossary id=\"2515\"]lateral ventricles[\/pb_glossary], the CSF flows into the [pb_glossary id=\"2481\"]third ventricle[\/pb_glossary], where more CSF is produced, and then through the [pb_glossary id=\"2491\"]cerebral aqueduct[\/pb_glossary] into the [pb_glossary id=\"2516\"]fourth ventricle[\/pb_glossary] where even more CSF is produced. A very small amount of CSF is filtered at any one of the plexuses, for a total of about 500 millilitres daily, but it is continuously made and pulses through the ventricular system, keeping the fluid moving. From the fourth ventricle, CSF can continue down the central canal of the spinal cord, but this is essentially a cul-de-sac, so more of the fluid leaves the ventricular system and moves into the [pb_glossary id=\"2517\"]subarachnoid space[\/pb_glossary] through the median and lateral apertures.<\/p>\r\n<p style=\"text-align: justify\">Within the subarachnoid space, the cerebrospinal fluid flows around all of the central nervous system, providing two important functions. As with elsewhere in its circulation, the cerebrospinal fluid picks up metabolic wastes from the nervous tissue and moves it out of the central nervous system. It also acts as a liquid cushion for the brain and spinal cord. By surrounding the entire system in the subarachnoid space, it provides a thin buffer around the organs within the strong, protective [pb_glossary id=\"2497\"]dura mater[\/pb_glossary]. The [pb_glossary id=\"2519\"]arachnoid granulations[\/pb_glossary] are outpocketings of the arachnoid membrane into the [pb_glossary id=\"2520\"]dural sinuses[\/pb_glossary] so that cerebrospinal fluid can be reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head and neck through the [pb_glossary id=\"2521\"]jugular veins[\/pb_glossary], along with the rest of the circulation for blood, to be re-oxygenated by the lungs and wastes to be filtered out by the kidneys (Table 3).<\/p>\r\n\r\n<table style=\"border-collapse: collapse;width: 0%\" border=\"0\"><caption>Table 3: Components of Cerebrospinal Fluid Circulation<\/caption>\r\n<tbody>\r\n<tr>\r\n<td style=\"width: 11.4609%\"><\/td>\r\n<th style=\"width: 12.4494%\" scope=\"col\"><strong>Lateral ventricles<\/strong><\/th>\r\n<th style=\"width: 16.6869%\" scope=\"col\"><strong>Third ventricle<\/strong><\/th>\r\n<th style=\"width: 12.3083%\" scope=\"col\"><strong>Cerebral aqueduct<\/strong><\/th>\r\n<th style=\"width: 24.1727%\" scope=\"col\"><strong>Fourth ventricle<\/strong><\/th>\r\n<th style=\"width: 8.63598%\" scope=\"col\"><strong>Central canal<\/strong><\/th>\r\n<th style=\"width: 14.2857%\" scope=\"col\"><strong>Subarachnoid space<\/strong><\/th>\r\n<\/tr>\r\n<tr>\r\n<th style=\"width: 11.4609%\" scope=\"row\"><strong>Location<\/strong><\/th>\r\n<td style=\"width: 12.4494%\">Cerebrum<\/td>\r\n<td style=\"width: 16.6869%\">Diencephalon<\/td>\r\n<td style=\"width: 12.3083%\">Midbrain<\/td>\r\n<td style=\"width: 24.1727%\">Between pons\/upper medulla oblongata and cerebellum<\/td>\r\n<td style=\"width: 8.63598%\">Spinal cord<\/td>\r\n<td style=\"width: 14.2857%\">External to entire central nervous system<\/td>\r\n<\/tr>\r\n<tr>\r\n<th style=\"width: 11.4609%\" scope=\"row\"><strong>Blood vessel structure<\/strong><\/th>\r\n<td style=\"width: 12.4494%\">Choroid plexus<\/td>\r\n<td style=\"width: 16.6869%\">Choroid plexus<\/td>\r\n<td style=\"width: 12.3083%\">None<\/td>\r\n<td style=\"width: 24.1727%\">Choroid plexus<\/td>\r\n<td style=\"width: 8.63598%\">None<\/td>\r\n<td style=\"width: 14.2857%\">Arachnoid granulations<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<h2 style=\"text-align: left\"><strong><a id=\"9.4\"><\/a>Part 4<\/strong><strong>:<\/strong><strong> The Peripheral Nervous System<\/strong><\/h2>\r\n<p style=\"text-align: justify\">The peripheral nervous system is not as contained as the central nervous system because it is defined as everything that is not the central nervous system. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the peripheral nervous system, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the [pb_glossary id=\"2459\"]enteric nervous system[\/pb_glossary] and are a special subset of the peripheral nervous system.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.4a\"><\/a>Ganglia<\/strong><\/h5>\r\n<p style=\"text-align: justify\">A [pb_glossary id=\"2452\"]ganglion[\/pb_glossary] is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a [pb_glossary id=\"2523\"]<strong>dorsal <\/strong><strong>root ganglion<\/strong>[\/pb_glossary]. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the central nervous system through the dorsal nerve root.<\/p>\r\n<p style=\"text-align: justify\">The other major category of ganglia, those of the [pb_glossary id=\"2456\"]autonomic nervous system[\/pb_glossary], will be examined later in this chapter.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"631\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image28-2.png\" alt=\"image\" width=\"631\" height=\"353\" \/> <strong>Figure 22. Dorsal Root Ganglion.<\/strong> The cell bodies of sensory neurons, which are unipolar neurons by shape, are seen in this photomicrograph. Also, the fibrous region is composed of the axons of these neurons that are passing through the ganglion to be part of the dorsal nerve root (tissue source: canine). LM \u00d7 40. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)[\/caption]\r\n\r\n&nbsp;\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.4b\"><\/a>Nerves<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Bundles of axons in the peripheral nervous system are referred to as [pb_glossary id=\"2453\"]nerves[\/pb_glossary]. These structures in the periphery are different than the central counterpart, called a [pb_glossary id=\"2454\"]tract[\/pb_glossary]. Nerves are composed of more than just nervous tissue. They have [pb_glossary id=\"2194\"]connective tissues[\/pb_glossary] invested in their structure, as well as blood vessels supplying the tissues with nourishment. Nerves are associated with the region of the central nervous system to which they are connected, either as cranial nerves (12 pairs) connected to the brain or spinal nerves (31 pairs) connected to the spinal cord.<\/p>\r\n<p style=\"text-align: justify\">The [pb_glossary id=\"2524\"]cranial nerves[\/pb_glossary] are primarily responsible for the sensory and motor functions of the head and neck, although one of these nerves, the [pb_glossary id=\"2525\"]vagus[\/pb_glossary], targets organs in the [pb_glossary id=\"2393\"]thoracic[\/pb_glossary]\u00a0and abdominal cavities as part of the parasympathetic nervous system. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the [pb_glossary id=\"2345\"]axons[\/pb_glossary] in these nerves originate out of sensory ganglia external to the cranium or motor [pb_glossary id=\"2451\"]nuclei[\/pb_glossary] within the brain stem.<\/p>\r\n<p style=\"text-align: justify\">All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibres, both [pb_glossary id=\"2455\"]somatic[\/pb_glossary] and [pb_glossary id=\"2456\"]autonomic[\/pb_glossary], emerge as the ventral nerve root. The [pb_glossary id=\"2523\"]dorsal root ganglion[\/pb_glossary] for each nerve is an enlargement of the spinal nerve.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.4c\"><\/a>The Somatic Ne<\/strong><strong>rvous System<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The [pb_glossary id=\"2455\"]somatic nervous system[\/pb_glossary] is traditionally considered a division within the peripheral nervous system. However, this misses an important point: somatic refers to a functional division, whereas peripheral refers to an anatomic division. The somatic nervous system is responsible for our conscious perception of the environment and for our [pb_glossary id=\"2280\"]voluntary[\/pb_glossary] responses to that perception by means of [pb_glossary id=\"2334\"]skeletal muscles[\/pb_glossary]. Peripheral sensory neurons receive input from environmental stimuli, but the neurons that produce motor responses originate in the central nervous system. The distinction between the structures of the peripheral and central nervous systems and the functions of the somatic and autonomic systems can most easily be demonstrated through a simple <strong>reflex<\/strong>, an automatic response that the nervous system produces in response to specific stimuli. The neurons and neural pathways responsible for a reflex action constitute the <strong>[pb_glossary id=\"2526\"]reflex arc[\/pb_glossary]<\/strong>. One of the simplest reflex acts is the <strong>stretch reflex<\/strong><strong>,<\/strong> by which the nervous system responds to the stretching of a muscle (the stimulus) with contraction of that same muscle (the response). This response protects the muscle from over-stretching, but more importantly, it has a crucial role in maintaining posture and balance. The <strong>patellar reflex<\/strong> (or knee-jerk reflex) is an example of stretch reflex and it occurs through the following steps (Figure 23):<\/p>\r\n\r\n<ul>\r\n \t<li>Tapping of the patellar tendon with a hammer causes the stretching of muscle fibres in the [pb_glossary id=\"2527\"]quadriceps[\/pb_glossary] muscle, which stimulates sensory neurons innervating those fibres.<\/li>\r\n \t<li>In the sensory neuron, a nerve impulse ([pb_glossary id=\"2341\"]action potential[\/pb_glossary]) is generated, which travels along the sensory nerve fibre from the muscle, through the dorsal root ganglion, to the spinal cord.<\/li>\r\n \t<li>The sensory neuron stimulates a motor neuron in the ventral horn of the spinal cord.<\/li>\r\n \t<li>That motor neuron sends a nerve impulse (action potential) along its [pb_glossary id=\"2345\"]axon[\/pb_glossary].<\/li>\r\n \t<li>This impulse reaches the quadriceps muscle, causing its contraction and the extension of the leg (a kick).<\/li>\r\n<\/ul>\r\n<p style=\"text-align: justify\">The sensory neuron can also activate an interneuron (e.g., Figure 23), which inhibits the motor neuron responsible for the contraction of the [pb_glossary id=\"2528\"]antagonistic[\/pb_glossary] muscle to [pb_glossary id=\"2527\"]quadriceps[\/pb_glossary] (i.e. [pb_glossary id=\"3028\"]hamstring[\/pb_glossary]).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"562\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image29-2.png\" alt=\"image\" width=\"562\" height=\"376\" \/> <strong>Figure 23. The Patellar Reflex.<\/strong> The stimulus (stretching of the quadriceps muscle caused by tapping on the tendon) triggers a nerve impulse in a sensory neuron, which synapses with and stimulated a motor neuron, leading to the contraction of the quadriceps. (credit: www.backyardbrains.com\/experiments\/Musclekneejerk, protected under Creative Commons License)[\/caption]\r\n<p style=\"text-align: justify\">Another example of a simple spinal reflex is the <strong>withdrawal reflex<\/strong><strong>,<\/strong> which occurs, for example, when you touch a hot stove and pull your hand away. This reflex occurs through a similar sequence of steps:<\/p>\r\n\r\n<ul>\r\n \t<li style=\"text-align: justify\">Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage.<\/li>\r\n \t<li style=\"text-align: justify\">In a sensory neuron, a nerve impulse ([pb_glossary id=\"2341\"]action potential[\/pb_glossary]) is generated, which travels along the sensory nerve fibre from the skin, through the [pb_glossary id=\"2523\"]dorsal root ganglion[\/pb_glossary], to the spinal cord.<\/li>\r\n \t<li style=\"text-align: justify\">The sensory neuron stimulates a motor neuron in the ventral horn motor of the spinal cord.<\/li>\r\n \t<li style=\"text-align: justify\">That motor neuron sends a nerve impulse (action potential) along its axon.<\/li>\r\n \t<li style=\"text-align: justify\">This impulse reaches the [pb_glossary id=\"2530\"]biceps brachii[\/pb_glossary], causing contraction of the muscle and flexion of the forearm at the elbow to withdraw the hand from the hot stove.<\/li>\r\n<\/ul>\r\n<p style=\"text-align: justify\">The basic withdrawal reflex includes sensory input (the painful stimulus), central processing (the [pb_glossary id=\"2344\"]synapse[\/pb_glossary] in the spinal cord), and motor output (activation of a ventral motor neuron that causes contraction of the [pb_glossary id=\"2530\"]biceps brachii[\/pb_glossary]). As seen for the patellar reflex, the withdrawal reflex can also include inhibition of the [pb_glossary id=\"2528\"]antagonistic[\/pb_glossary] muscle ([pb_glossary id=\"2531\"]triceps brachii[\/pb_glossary] in our example). Another possible motor output of the withdrawal reflex is cross extension: counterbalancing movement on the other side of the body by stimulation of the extensor muscles in the [pb_glossary id=\"2532\"]contralateral[\/pb_glossary] limb.<\/p>\r\n<p style=\"text-align: justify\">The somatic nervous system also controls voluntary movement and more complex motor functions. For example, reading of this text starts with visual sensory input to the retina, which then projects to the [pb_glossary id=\"2482\"]thalamus[\/pb_glossary], and on to the [pb_glossary id=\"2473\"]cerebral cortex[\/pb_glossary]. A sequence of regions of the cerebral cortex process the visual information, starting in the primary visual cortex of the occipital lobe, and resulting in the conscious perception of these letters. Subsequent cognitive processing results in understanding of the content. As you continue reading, regions of the cerebral cortex in the frontal lobe plan how to move the eyes to follow the lines of text. The output from the cortex causes activity in motor neurons in the brain stem that cause movement of the [pb_glossary id=\"2533\"]extraocular[\/pb_glossary] muscles through the third, fourth, and sixth cranial nerves. This example also includes sensory input (the retinal projection to the thalamus), central processing (the thalamus and subsequent cortical activity), and motor output (activation of neurons in the brain stem that lead to coordinated contraction of extraocular muscles).<\/p>\r\n<p style=\"text-align: justify\"><strong>The Autonomic Nervous System<\/strong><\/p>\r\n<p style=\"text-align: justify\">The [pb_glossary id=\"2456\"]autonomic nervous system[\/pb_glossary] is often associated with the \u201cfight-or-flight response,\u201d which refers to the preparation of the body to either run away from a threat or to stand and fight in the face of that threat. To suggest what this means, consider the (very unlikely) situation of seeing a lioness hunting out on the savannah. Though this is not a common threat that humans deal with in the modern world, it represents the type of environment in which the human species thrived and adapted. The spread of humans around the world to the present state of the modern age occurred much more quickly than any species would adapt to environmental pressures such as predators. However, the reactions modern humans have in the modern world are based on these prehistoric situations. If your boss is walking down the hallway on Friday afternoon looking for \u201cvolunteers\u201d to come in on the weekend, your response is the same as the prehistoric human seeing the lioness running across the savannah: fight or flight.<\/p>\r\n<p style=\"text-align: justify\">Most likely, your response to your boss\u2014not to mention the lioness\u2014would be flight. Run away! The autonomic system is responsible for the physiological response to make that possible, and hopefully successful. Adrenaline starts to flood your circulatory system. Your heart rate increases. Sweat glands become active. The bronchi of the lungs dilate to allow more air exchange. Pupils dilate to increase visual information. Blood pressure increases in general, and blood vessels dilate in skeletal muscles. Time to run. Similar physiological responses would occur in preparation for fighting off the threat.<\/p>\r\n<p style=\"text-align: justify\">This response should sound a bit familiar. The autonomic nervous system is tied into emotional responses as well, and the fight-or-flight response probably sounds like a panic attack. In the modern world, these sorts of reactions are associated with anxiety as much as with response to a threat. It is engrained in the nervous system to respond like this. In fact, the adaptations of the autonomic nervous system probably predate the human species and are likely to be common to all mammals, and perhaps shared by many animals. That lioness might herself be threatened in some other situation<\/p>\r\n<p style=\"text-align: justify\">However, the autonomic nervous system is not just about responding to threats. Besides the fight-or-flight response, there are the responses referred to as \u201crest and digest.\u201d If that lioness is successful in her hunting, then she is going to rest from the exertion. Her heart rate will slow. Breathing will return to normal. The digestive system has a big job to do. Much of the function of the autonomic system is based on the connections within an autonomic, or visceral, reflex.<\/p>\r\n<p style=\"text-align: justify\">As we have seen, the nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system. The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis.<\/p>\r\n<p style=\"text-align: justify\">The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. In addition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the <strong>[pb_glossary id=\"2507\"]sympathetic division[\/pb_glossary]<\/strong> and the <strong>[pb_glossary id=\"2534\"]parasympathetic division[\/pb_glossary]<\/strong>. The sympathetic system is associated with the <strong>fight-or-flight response<\/strong>, and parasympathetic activity is referred to by the epithet of <strong>rest and digest<\/strong>. At each target [pb_glossary id=\"2439\"]effector[\/pb_glossary], dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. One causes heart rate to increase, whereas the other causes heart rate to decrease.<\/p>\r\n<p style=\"text-align: justify\"><em>Sympathetic Division of the Autonomic Nervous System<\/em><\/p>\r\n<p style=\"text-align: justify\">To respond to a threat\u2014to fight or to run away\u2014the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together. Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system to a wide array of ganglia that project to the many effector organs simultaneously. The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change.<\/p>\r\n<p style=\"text-align: justify\">The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. It is referred to as the <strong>[pb_glossary id=\"2535\"]thoracolumbar system[\/pb_glossary]<\/strong> to reflect this anatomical basis. A <strong>[pb_glossary id=\"2536\"]central neuron[\/pb_glossary]<\/strong> in the lateral horn of any of these spinal regions projects to [pb_glossary id=\"2452\"]ganglia[\/pb_glossary] adjacent to the vertebral column through the ventral spinal roots. The majority of ganglia of the sympathetic system belong to a network of <strong>[pb_glossary id=\"2537\"]sympathetic chain ganglia[\/pb_glossary]<\/strong> that runs alongside the vertebral column. The ganglia appear as a series of clusters of neurons linked by axonal bridges. A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices (Figure 24, wherein the \u201ccircuits\u201d of the sympathetic system are intentionally simplified).<\/p>\r\n<p style=\"text-align: justify\">An axon from the central neuron that projects to a sympathetic ganglion is referred to as a <strong>[pb_glossary id=\"2538\"]preganglionic fibre[\/pb_glossary] <\/strong>or neuron, and represents the output from the central nervous system to the ganglion. Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibres are relatively short, and they are myelinated. A <strong>[pb_glossary id=\"2539\"]postganglionic fibre[\/pb_glossary]<\/strong>\u2014the axon from a ganglionic neuron that projects to the target effector\u2014represents the output of a ganglion that directly influences the organ. Compared with the preganglionic fibres, postganglionic sympathetic fibres are long because of the relatively greater distance from the ganglion to the target [pb_glossary id=\"2439\"]effector[\/pb_glossary]. These fibres are unmyelinated. (Note that the term \u201cpostganglionic neuron\u201d may be used to describe the projection from a ganglion to the target. The problem with that usage is that the cell body is in the ganglion, and only the fibre is postganglionic. Typically, the term neuron applies to the entire cell.)<\/p>\r\n<p style=\"text-align: justify\">One type of preganglionic sympathetic fibre does not terminate in a ganglion. These are the axons from central sympathetic neurons that project to the <strong>[pb_glossary id=\"2540\"]adrenal medulla[\/pb_glossary]<\/strong>, the interior portion of the [pb_glossary id=\"2541\"]adrenal gland[\/pb_glossary]. These axons are still referred to as preganglionic fibres, but the target is not a [pb_glossary id=\"2452\"]ganglion[\/pb_glossary]. The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"826\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image30-2.png\" alt=\"image\" width=\"826\" height=\"1220\" \/> <strong>Figure 24. The Sympathetic Division of the Autonomic Nervous System.<\/strong> Neurons from the lateral horn of the spinal cord (preganglionic nerve fibers - solid lines)) project to the chain ganglia on either side of the vertebral column or to collateral (prevertebral) ganglia that are anterior to the vertebral column in the abdominal cavity. Axons from these ganglionic neurons (postganglionic nerve fibers - dotted lines) then project to target effectors throughout the body. (The names of specific ganglia and nerves, as well as their target organs, are not examinable material in this course.)[\/caption]\r\n<p style=\"text-align: justify\">The projections of the [pb_glossary id=\"2507\"]sympathetic division[\/pb_glossary] of the [pb_glossary id=\"2456\"]autonomic nervous system[\/pb_glossary] diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease. Sweat gland secretion should also increase as part of an integrated response. All of those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons\u2014a single preganglionic sympathetic neuron may have 10\u201320 targets. An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets. At the level of the spinal cord at which the preganglionic sympathetic fibre exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion. Some branches will extend up or down to a different level of the chain ganglia. Other branches will pass through the chain ganglia and project through one of the [pb_glossary id=\"2542\"]splanchnic nerves[\/pb_glossary] to a collateral ganglion. Finally, some branches may project through the splanchnic nerves to the adrenal medulla. All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs.<\/p>\r\n<p style=\"text-align: justify\"><em>Parasympathetic Division of the Autonomic Nervous System<\/em><\/p>\r\nWhen not responding to an immediate threat, the [pb_glossary id=\"2534\"]parasympathetic system[\/pb_glossary] is generally more active than the sympathetic system.\u00a0 Many of the same effectors in the body are innervated by both divisions of the autonomic nervous system, but activation of each division tends to have opposing effects.\u00a0 Sympathetic system activation tends to increase activity in the respiratory, cardiovascular, and musculoskeletal systems while reducing activity in the digestive system.\u00a0 Parasympathetic system activation on the other hand tends to <em>decrease<\/em> activity in the respiratory, cardiovascular, and musculoskeletal systems while <em>increasing<\/em> activity in the digestive, urinary, and reproductive systems.\u00a0 Generally speaking, the activity of the many organs that receive input from both systems is dependent on whether neurons of the parasympathetic or sympathetic system are releasing more of their [pb_glossary id=\"2172\"]neurotransmitter[\/pb_glossary] onto each organ at a given time.\r\n<p style=\"text-align: justify\">The parasympathetic division of the autonomic nervous system is named because its central neurons are located on either side of the thoracolumbar region of the spinal cord (para- = \u201cbeside\u201d or \u201cnear\u201d). The parasympathetic system can also be referred to as the <strong>[pb_glossary id=\"2543\"]craniosacral system[\/pb_glossary]<\/strong> (or outflow) because the preganglionic neurons are located in [pb_glossary id=\"2451\"]nuclei[\/pb_glossary] of the brain stem and the lateral horn of the sacral spinal cord.<\/p>\r\n<p style=\"text-align: justify\">The connections, or \u201ccircuits,\u201d of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences (Figure 25). The preganglionic fibres from the cranial region travel in cranial nerves, whereas [pb_glossary id=\"2538\"]preganglionic fibres[\/pb_glossary] from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia, which are located near - or even within - the target organ. The [pb_glossary id=\"2539\"]postganglionic fibre[\/pb_glossary] projects from the terminal ganglia a short distance to the effector. These [pb_glossary id=\"2452\"]ganglia[\/pb_glossary] are often referred to as intramural ganglia when they are found within the walls target effector, or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibres are long and the postganglionic fibres are short because the ganglia are close to - and sometimes within - the target effectors.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"158\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image31-2.png\" alt=\"image\" width=\"158\" height=\"156\" \/> Watch <a href=\"https:\/\/youtu.be\/71pCilo8k4M\">this Crash Course video<\/a> for an overview of the autonomic nervous system! (Direct link: <a href=\"https:\/\/youtu.be\/71pCilo8k4M\">https:\/\/youtu.be\/71pCilo8k4M<\/a>)[\/caption]\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"826\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image32-2.png\" alt=\"image\" width=\"826\" height=\"1228\" \/> <strong>Figure 25. The Parasympathetic Division of the Autonomic Nervous System.<\/strong> Neurons from brain-stem nuclei, or from the lateral horn of the sacral spinal cord, project to terminal ganglia near or within the various organs of the body. Axons from these ganglionic neurons then project the short distance to those target effectors. (The names of specific ganglia and nerves, as well as their target organs, are not examinable material in this course.)[\/caption]\r\n\r\n<em>\u00a0<\/em>\r\n<p style=\"text-align: justify\"><em>Chemical Signaling i<\/em><em>n the Autonomic Nervous System<\/em><\/p>\r\n<p style=\"text-align: justify\">Where an autonomic neuron connects with a target, there is a [pb_glossary id=\"2344\"]synapse[\/pb_glossary]. The electrical signal of the [pb_glossary id=\"2341\"]action potential[\/pb_glossary] causes the release of a signaling molecule, which will bind to [pb_glossary id=\"2270\"]receptor[\/pb_glossary] proteins on the target cell. Synapses of the autonomic system are classified as either <strong>[pb_glossary id=\"2544\"]cholinergic[\/pb_glossary]<\/strong>, meaning that <strong>[pb_glossary id=\"2478\"]acetylcholine[\/pb_glossary] (<\/strong><strong>ACh<\/strong><strong>)<\/strong> is released, or <strong>[pb_glossary id=\"2545\"]adrenergic[\/pb_glossary]<\/strong>, meaning that <strong>[pb_glossary id=\"2546\"]norepinephrine[\/pb_glossary]<\/strong> is released. The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds.<\/p>\r\n<p style=\"text-align: justify\">The term adrenergic should remind you of the word adrenaline, which is associated with the fight-or-flight response described at the beginning of the chapter. Adrenaline and epinephrine are two names for the same molecule. The adrenal gland (in Latin, ad- = \u201con top of\u201d; renal = \u201ckidney\u201d) secretes adrenaline. The ending \u201c-ine\u201d refers to the chemical being derived, or extracted, from the [pb_glossary id=\"2541\"]adrenal gland[\/pb_glossary]. A similar construction from Greek instead of Latin results in the word [pb_glossary id=\"2547\"]epinephrine[\/pb_glossary] (epi- = \u201cabove\u201d; nephr- = \u201ckidney\u201d). In scientific usage, epinephrine is preferred in the United States, whereas adrenaline is preferred in Great Britain, because \u201cadrenalin\u201d was once a registered, proprietary drug name in the United States. Though the drug is no longer sold, the convention of referring to this molecule by the two different names persists. Similarly, norepinephrine and noradrenaline are two names for the same molecule.<\/p>\r\n<p style=\"text-align: justify\">All [pb_glossary id=\"2538\"]preganglionic fibres[\/pb_glossary], both [pb_glossary id=\"2507\"]sympathetic[\/pb_glossary] and [pb_glossary id=\"2534\"]parasympathetic[\/pb_glossary], release ACh. The postganglionic parasympathetic fibres also release ACh. Postganglionic sympathetic fibers release norepinephrine, except for fibers that project to sweat glands and to blood vessels associated with skeletal muscles, which release ACh.<\/p>\r\n<p style=\"text-align: justify\">Signaling molecules can belong to two broad groups. [pb_glossary id=\"2172\"]Neurotransmitters[\/pb_glossary] are released at synapses, whereas hormones are released into the bloodstream. These are simplistic definitions, but they can help to clarify this point. [pb_glossary id=\"2478\"]Acetylcholine[\/pb_glossary] can be considered a neurotransmitter because it is released by [pb_glossary id=\"2345\"]axons[\/pb_glossary] at synapses. The adrenergic system, however, presents a challenge. Postganglionic sympathetic fibres release [pb_glossary id=\"2546\"]norepinephrine[\/pb_glossary], which can be considered a neurotransmitter. But the adrenal medulla releases epinephrine and norepinephrine into circulation, so they should be considered [pb_glossary id=\"2171\"]hormones[\/pb_glossary].<\/p>\r\n\r\n<h2 style=\"text-align: left\"><strong><a id=\"9.5\"><\/a>Part <\/strong><strong>5<\/strong><strong>: Neuronal <\/strong><strong>Signalling<\/strong><\/h2>\r\n<p style=\"text-align: justify\">Having looked at the components of nervous tissue, and the basic anatomy of the nervous system, next comes an understanding of how nervous tissue is capable of communicating within the nervous system. Before getting to the nuts and bolts of how this works, an illustration of how the components come together will be helpful (summarized in Figure 26).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image33-2.png\" alt=\"image\" width=\"975\" height=\"681\" \/> <strong>Figure 26. Testing the Water.<\/strong> (1) The sensory neuron has endings in the skin that sense a stimulus such as water temperature. The strength of the signal that starts here is dependent on the strength of the stimulus. (2) The graded potential from the sensory endings, if strong enough, will initiate an action potential at the initial segment of the axon (which is immediately adjacent to the sensory endings in the skin). (3) The axon of the peripheral sensory neuron enters the spinal cord and contacts another neuron in the gray matter. The contact is a synapse where another graded potential is caused by the release of a chemical signal from the axon terminals. (4) An action potential is initiated at the initial segment of this neuron and travels up the sensory pathway to a region of the brain called the thalamus. Another synapse passes the information along to the next neuron. (5) The sensory pathway ends when the signal reaches the cerebral cortex. (6) After integration with neurons in other parts of the cerebral cortex, a motor command is sent from the precentral gyrus of the frontal cortex. (7) The upper motor neuron sends an action potential down to the spinal cord. The target of the upper motor neuron is the dendrites of the lower motor neuron in the gray matter of the spinal cord. (8) The axon of the lower motor neuron emerges from the spinal cord in a nerve and connects to a muscle through a neuromuscular junction to cause contraction of the target muscle.[\/caption]\r\n<p style=\"text-align: justify\">Imagine you are about to take a shower. You have turned on the faucet to start the water as you prepare to get in the shower. After a few minutes, you expect the water to be a temperature that will be comfortable to enter. So you put your hand out into the spray of water. What happens next depends on how your nervous system interacts with the stimulus of the water temperature and what you do in response to that stimulus.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"577\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image34-2.png\" alt=\"image\" width=\"577\" height=\"316\" \/> <strong>Figure 27. The Sensory Input.<\/strong> Receptors in the skin sense the temperature of the water.[\/caption]\r\n<p style=\"text-align: justify\">Found in the skin of your fingers or toes is a type of sensory receptor that is sensitive to temperature, called a <strong>[pb_glossary id=\"2548\"]thermoreceptor[\/pb_glossary]<\/strong>. When you place your hand under the shower (Figure 27), the cell membrane of the thermoreceptors changes its electrical state (voltage). The amount of change is dependent on the strength of the stimulus (how hot the water is). This is called a <strong>[pb_glossary id=\"2549\"]graded potential[\/pb_glossary]<\/strong>. If the stimulus is strong, the voltage of the cell membrane will change enough to generate an electrical signal that will travel down the [pb_glossary id=\"2345\"]axon[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\">The voltage at which such a signal is generated is called the <strong>threshold<\/strong>, and the resulting electrical signal is called an <strong>[pb_glossary id=\"2341\"]action potential[\/pb_glossary]<\/strong>. In this example, the action potential travels\u2014a process known as <strong>propagation<\/strong>\u2014along the axon from the axon hillock to the [pb_glossary id=\"2463\"]axon terminals[\/pb_glossary] and into the [pb_glossary id=\"2464\"]synaptic end bulbs[\/pb_glossary]. When this signal reaches the end bulbs, it causes the release of a signaling molecule called a <strong>[pb_glossary id=\"2172\"]neurotransmitter[\/pb_glossary]<\/strong>.<\/p>\r\n<p style=\"text-align: justify\">The [pb_glossary id=\"2172\"]neurotransmitter[\/pb_glossary] diffuses across the short distance of the [pb_glossary id=\"2344\"]synapse[\/pb_glossary] and binds to a [pb_glossary id=\"2270\"]receptor[\/pb_glossary] protein of the target neuron. When the molecular signal binds to the receptor, the cell membrane of the target neuron changes its electrical state and a new graded potential begins. If that graded potential is strong enough to reach threshold, the second neuron generates an action potential at its axon hillock. The target of this neuron is another neuron in the <strong>[pb_glossary id=\"2482\"]thalamus[\/pb_glossary]<\/strong> of the brain, the part of the central nervous system that acts as a relay for sensory information. At another synapse, neurotransmitter is released and binds to its receptor. The thalamus then sends the sensory information to the <strong>cerebral cortex<\/strong>, the outermost layer of gray matter in the brain, where conscious perception of that water temperature begins. Within the [pb_glossary id=\"2550\"]cerebral cortex,[\/pb_glossary] information is processed among many neurons, integrating the stimulus of the water temperature with other sensory stimuli, with your emotional state (you just aren\u2019t ready to wake up; the bed is calling to you), memories (perhaps of the lab notes you have to study before a quiz). Finally, a plan is developed about what to do, whether that is to turn the temperature up, turn the whole shower off and go back to bed, or step into the shower. To do any of these things, the cerebral cortex has to send a command out to your body to move muscles (Figure 28).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"580\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image35-3.png\" alt=\"image\" width=\"580\" height=\"463\" \/> <strong>Figure 28. The Motor Response.<\/strong> On the basis of the sensory input and the integration in the central nervous system, a motor response is formulated and executed.[\/caption]\r\n<p style=\"text-align: justify\">A region of the cortex is specialized for sending signals down to the spinal cord for movement. The <strong>upper motor neuron<\/strong> is in this region, called the primary <strong>motor cortex<\/strong>, which has an axon that extends all the way down the spinal cord. At the level of the spinal cord at which this axon makes a synapse, a graded potential occurs in the cell membrane of a <strong>lower motor neuron<\/strong>. This second motor neuron is responsible for causing muscle fibres to contract. In the manner described in the chapter on muscle tissue, an action potential travels along the motor neuron axon into the periphery. The axon terminates on muscle fibers at the [pb_glossary id=\"2551\"]neuromuscular junction[\/pb_glossary]. Acetylcholine is released at this specialized synapse, which causes the muscle action potential to begin, following a large potential known as an end plate potential. When the lower motor neuron excites the muscle fiber, it contracts. All of this occurs in a fraction of a second, but this story is the basis of how the nervous system functions.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong>Ion Channels and the Resting Membrane<\/strong><strong> Potential<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The functions of the nervous system\u2014sensation, integration, and response\u2014depend on the functions of the neurons underlying these pathways. To understand how neurons are able to communicate, it is necessary to describe the role of an <strong>excitable membrane<\/strong> in generating these signals. The basis of this communication is the [pb_glossary id=\"2341\"]action potential[\/pb_glossary], which demonstrates how changes in the membrane can constitute a signal. (The way these signals work in more variable circumstances involves graded potentials.)<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image36-2.png\" alt=\"image\" width=\"975\" height=\"417\" \/> <strong>Figure 29. Cell Membrane and Transmembrane Proteins.<\/strong> The cell membrane is composed of a phospholipid bilayer and has many transmembrane proteins, including different types of channel proteins that serve as ion channels.[\/caption]\r\n<p style=\"text-align: justify\">Most cells in the body make use of charged particles, [pb_glossary id=\"2093\"]ions[\/pb_glossary], to build up a charge across the cell membrane. Cells make use of the cell membrane to regulate ion movement between the extracellular fluid and cytosol. As you learned in the chapter on cells, the cell membrane is primarily responsible for regulating what can cross the membrane and what stays on only one side. The cell membrane is a [pb_glossary id=\"2166\"]phospholipid[\/pb_glossary] bilayer, so only substances that can pass directly through the hydrophobic core can diffuse through unaided. Charged particles, which are [pb_glossary id=\"2178\"]hydrophilic[\/pb_glossary] by definition, cannot pass through the cell membrane without assistance (Figure 29). Transmembrane proteins, specifically [pb_glossary id=\"2250\"]channel proteins[\/pb_glossary], make this possible. Several passive ion channels, as well as [pb_glossary id=\"2254\"]active transport[\/pb_glossary] pumps, are necessary to generate a transmembrane potential and an action potential. Ion channels are pores that allow specific charged particles to cross the membrane in response to an existing [pb_glossary id=\"2255\"]concentration gradient[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\">Of special interest is the [pb_glossary id=\"2259\"]carrier protein[\/pb_glossary] referred to as the sodium\/potassium pump that moves sodium ions (Na<sup>+<\/sup>) out of a cell and potassium ions (K<sup>+<\/sup>) into a cell, thus regulating ion concentration on both sides of the cell membrane. The sodium\/potassium pump requires energy in the form of [pb_glossary id=\"2074\"]adenosine triphosphate[\/pb_glossary] (ATP), so it is also referred to as an ATPase. As was explained in the cell chapter, the concentration of Na<sup>+<\/sup> is higher outside the cell than inside, and the concentration of K<sup>+<\/sup> is higher inside the cell than outside. That means that this pump is moving the ions against the concentration gradients for sodium and potassium, which is why it requires energy. In fact, the pump basically maintains those concentration gradients.<\/p>\r\n<p style=\"text-align: justify\">Ion channels do not always freely allow ions to diffuse across the membrane. Some are opened by certain events, meaning the channels are <strong>gated<\/strong>.<\/p>\r\n<p style=\"text-align: justify\">A <strong>[pb_glossary id=\"2552\"]ligand-gated channel[\/pb_glossary]<\/strong> opens because a signaling molecule, a ligand, binds to the extracellular region of the channel. This type of channel is also known as an ionotropic receptor because when the ligand, known as a neurotransmitter in the nervous system, binds to the protein, ions cross the membrane changing its charge (Figure 30).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"985\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image37-2.png\" alt=\"image\" width=\"985\" height=\"509\" \/> <strong>Figure 30. Ligand-Gated Channels.<\/strong> When the ligand, in this case the neurotransmitter acetylcholine, binds to a specific location on the extracellular surface of the channel protein, the pore opens to allow select ions through. The ions, in this case, are cations of sodium, calcium, and potassium.[\/caption]\r\n<p style=\"text-align: justify\">A <strong>[pb_glossary id=\"2553\"]mechanically gated channel[\/pb_glossary]<\/strong> opens because of a physical distortion of the cell membrane. Many channels associated with the sense of touch ([pb_glossary id=\"2554\"]somatosensation[\/pb_glossary]) are mechanically gated. For example, as pressure is applied to the skin, these channels open and allow ions to enter the cell. Similar to this type of channel would be the channel that opens on the basis of temperature changes, as in testing the water in the shower (Figure 31).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"991\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image38-3.png\" alt=\"image\" width=\"991\" height=\"480\" \/> <strong>Figure 31. Mechanically Gated Channels.<\/strong> When a mechanical change occurs in the surrounding tissue, such as pressure or touch, the channel is physically opened. Thermoreceptors work on a similar principle. When the local tissue temperature changes, the protein reacts by physically opening the channel.[\/caption]\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"975\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image39-2.png\" alt=\"image\" width=\"975\" height=\"475\" \/> <strong>Figure 32. Voltage-Gated Channels.<\/strong> Voltage-gated channels open when the transmembrane voltage changes around them. Amino acids in the structure of the protein are sensitive to charge and cause the pore to open to the selected ion.[\/caption]\r\n\r\nA <strong>[pb_glossary id=\"2555\"]voltage-gated channel[\/pb_glossary]<\/strong> is a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Normally, the inner portion of the membrane is at a negative voltage. When that voltage becomes less negative, the channel begins to allow [pb_glossary id=\"2093\"]ions[\/pb_glossary] to cross the membrane (Figure 32).\r\n<p style=\"text-align: justify\">A <strong>[pb_glossary id=\"2556\"]leakage channel[\/pb_glossary]<\/strong> is randomly gated, meaning that it opens and closes at random, hence the reference to leaking. There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states. Leakage channels contribute to the resting transmembrane voltage of the excitable membrane (Figure 33).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"985\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image40-2.png\" alt=\"image\" width=\"985\" height=\"461\" \/> <strong>Figure 33. Leakage Channels.<\/strong> In certain situations, ions need to move across the membrane randomly. The particular electrical properties of certain cells are modified by the presence of this type of channel.[\/caption]\r\n<p style=\"text-align: justify\">The electrical state of the cell membrane can have several variations. These are all variations in the <strong>[pb_glossary id=\"2557\"]membrane potential[\/pb_glossary]<\/strong>. A potential is a distribution of charge across the cell membrane, measured in millivolts (mV). The standard is to compare the inside of the cell relative to the outside, so the membrane potential is a value representing the charge on the intracellular side of the membrane based on the outside being zero, relatively speaking (Figure 34).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"985\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image41-2.png\" alt=\"image\" width=\"985\" height=\"463\" \/> <strong>Figure 34. Measuring Charge across a Membrane with a Voltmeter.<\/strong> A recording electrode is inserted into the cell and a reference electrode is outside the cell. By comparing the charge measured by these two electrodes, the transmembrane voltage is determined. It is conventional to express that value for the cytosol relative to the outside.[\/caption]\r\n<p style=\"text-align: justify\">The concentration of ions in [pb_glossary id=\"2127\"]extracellular[\/pb_glossary] and [pb_glossary id=\"2126\"]intracellular fluids[\/pb_glossary] is largely balanced, with a net neutral charge. However, a slight difference in charge occurs right at the membrane surface, both internally and externally. It is the difference in this very limited region that has all the power in neurons (and muscle cells) to generate electrical signals, including action potentials.<\/p>\r\n<p style=\"text-align: justify\">Before these electrical signals can be described, the resting state of the membrane must be explained. When the cell is at rest, and the ion channels are closed (except for leakage channels which randomly open), ions are distributed across the membrane in a very predictable way. The concentration of Na<sup>+<\/sup> outside the cell is 10 times greater than the concentration inside. Also, the concentration of K<sup>+<\/sup> inside the cell is greater than outside. The [pb_glossary id=\"2130\"]cytosol[\/pb_glossary] contains a high concentration of [pb_glossary id=\"2095\"]anions[\/pb_glossary], in the form of [pb_glossary id=\"2150\"]phosphate[\/pb_glossary] ions and negatively charged proteins. Large anions are a component of the inner cell membrane, including specialized [pb_glossary id=\"2166\"]phospholipids[\/pb_glossary] and proteins associated with the inner leaflet of the membrane (leaflet is a term used for one side of the lipid bilayer membrane). The negative charge is localized in the large anions.<\/p>\r\nWith the ions distributed across the membrane at these concentrations, the difference in charge is measured at -70 mV, the value described as the <strong>[pb_glossary id=\"2558\"]resting membrane potential[\/pb_glossary]<\/strong>. The exact value measured for the resting membrane potential varies between cells, but -70 mV is the most commonly recorded value. This voltage would actually be much lower except for the contributions of some important proteins in the membrane. Leakage channels K<sup>+<\/sup> channels allow K<sup>+<\/sup> to slowly move out of the cells. To a much lesser extent, leakage Na<sup>+<\/sup> channels allow Na<sup>+<\/sup> to slowly move into the cell. The constant activity of the Na<sup>+<\/sup>\/K<sup>+<\/sup> pump maintains the ion gradients. This may appear to be a waste of energy, but each has a role in maintaining the membrane potential.\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.5b\"><\/a>Generation of an<\/strong><strong> Action Potential<\/strong><\/h5>\r\n<p style=\"text-align: justify\">Resting membrane potential describes the steady state of the cell, which is a dynamic process that is balanced by ion leakage and ion pumping. Without any outside influence, it will not change. To get an electrical signal started, the membrane potential has to change.<\/p>\r\n<p style=\"text-align: justify\">This starts with a channel opening for Na<sup>+<\/sup> in the membrane. Because the concentration of Na<sup>+<\/sup> is higher outside the cell than inside the cell by a factor of 10, ions will rush into the cell that are driven largely by the concentration gradient. Because sodium is a positively charged ion, it will change the relative voltage immediately inside the cell relative to immediately outside. The resting potential is the state of the membrane at a voltage of -70 mV, so the sodium cation entering the cell will cause it to become less negative. This is known as <strong>[pb_glossary id=\"2559\"]depolarization[\/pb_glossary]<\/strong>, meaning the membrane potential moves toward zero.<\/p>\r\n<p style=\"text-align: justify\">The concentration gradient for Na<sup>+<\/sup> is so strong that it will continue to enter the cell even after the membrane potential has become zero, so that the voltage immediately around the pore begins to become positive. The electrical gradient also plays a role, as negative proteins below the membrane attract the sodium ion. The membrane potential will reach +30 mV by the time sodium has entered the cell.<\/p>\r\n<p style=\"text-align: justify\">As the membrane potential reaches +30 mV, other [pb_glossary id=\"2555\"]voltage-gated channels[\/pb_glossary] are opening in the membrane. These channels are specific for the potassium ion. A concentration gradient acts on K<sup>+<\/sup>, as well. As K<sup>+<\/sup> starts to leave the cell, taking a positive charge with it, the membrane potential begins to move back toward its resting voltage. This is called <strong>[pb_glossary id=\"2560\"]repolarization[\/pb_glossary]<\/strong>, meaning that the membrane voltage moves back toward the -70 mV value of the [pb_glossary id=\"2558\"]resting membrane potential[\/pb_glossary].<\/p>\r\n<p style=\"text-align: justify\">Repolarization returns the membrane potential to the -70 mV value that indicates the resting potential, but it actually overshoots that value. Potassium ions reach equilibrium when the membrane voltage is below -70 mV, so a period of <strong>[pb_glossary id=\"2561\"]hyperpolarization[\/pb_glossary]<\/strong> occurs while the K<sup>+<\/sup> channels are open. Those K<sup>+<\/sup> channels are slightly delayed in closing, accounting for this short overshoot.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"682\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image42-2.png\" alt=\"image\" width=\"682\" height=\"427\" \/> <strong>Figure 35. Graph of Action Potential.<\/strong> Plotting voltage measured across the cell membrane against time, the action potential begins with depolarization, followed by repolarization, which goes past the resting potential into hyperpolarization, and finally the membrane returns to rest.[\/caption]\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"237\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image43-2.png\" alt=\"image\" width=\"237\" height=\"231\" \/> Watch <a href=\"https:\/\/youtu.be\/OZG8M_ldA1M\">this Crash Course video<\/a> to learn more about the action potential! Direct link: <a href=\"https:\/\/youtu.be\/OZG8M_ldA1M\">https:\/\/youtu.be\/OZG8M_ldA1M<\/a>[\/caption]\r\n<p style=\"text-align: justify\">What has been described here is the action potential, which is presented as a graph of voltage over time (Figure 35). It is the electrical signal that nervous tissue generates for communication. The change in the membrane voltage from -70 mV at rest to +30 mV at the end of depolarization is a 100-mV change. That can also be written as a 0.1-V change. To put that value in perspective, think about a battery. An AA battery that you might find in a television remote has a voltage of 1.5 V, or a 9-V battery (the rectangular battery with two posts on one end) is, obviously, 9 V. The change seen in the action potential is one or two orders of magnitude less than the charge in these batteries. In fact, the [pb_glossary id=\"2557\"]membrane potential[\/pb_glossary] can be described as a battery. A charge is stored across the membrane that can be released under the correct conditions. A battery in your remote has stored a charge that is \u201creleased\u201d when you push a button.<\/p>\r\n<p style=\"text-align: justify\">The question is, now, what initiates the [pb_glossary id=\"2341\"]action potential[\/pb_glossary]? The description above conveniently glosses over that point. But it is vital to understanding what is happening. The membrane potential will stay at the resting voltage until something changes. The description above just says that a Na<sup>+<\/sup> channel opens. Now, to say \u201ca channel opens\u201d does not mean that one individual transmembrane protein changes. Instead, it means that one kind of channel opens. There are a few different types of channels that allow Na<sup>+<\/sup> to cross the membrane. A [pb_glossary id=\"2552\"]ligand-gated[\/pb_glossary] Na<sup>+<\/sup> channel will open when a [pb_glossary id=\"2172\"]neurotransmitter[\/pb_glossary] binds to it and a [pb_glossary id=\"2553\"]mechanically gated[\/pb_glossary] Na<sup>+<\/sup> channel will open when a physical stimulus affects a sensory receptor (like pressure applied to the skin compresses a touch receptor). Whether it is a [pb_glossary id=\"2172\"]neurotransmitter[\/pb_glossary] binding to its receptor protein or a sensory stimulus activating a sensory receptor cell, some stimulus gets the process started. Sodium starts to enter the cell and the membrane becomes less negative.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"631\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image44.png\" alt=\"image\" width=\"631\" height=\"377\" \/> <strong>Figure 36. Stages of an Action Potential.<\/strong> Plotting voltage measured across the cell membrane against time, the events of t6he action potential can be related to specific changes in the membrane voltage. (1) At rest, the membrane voltage is -70 mV. (2) The membrane begins to depolarize when an external stimulus is applied. (3) The membrane voltage begins a rapid rise toward +30 mV. (4) The membrane voltage starts to return to a negative value. (5) Repolarization continues past the resting membrane voltage, resulting in hyperpolarization. (6) The membrane voltage returns to the resting value shortly after hyperpolarization.[\/caption]\r\n<p style=\"text-align: justify\">A third type of channel that is an important part of [pb_glossary id=\"2559\"]depolarization[\/pb_glossary] in the action potential is the voltage-gated Na<sup>+<\/sup> channel. The channels that start depolarizing the membrane because of a stimulus help the cell to depolarize from -70 mV to -55 mV. Once the membrane reaches that voltage, the [pb_glossary id=\"2555\"]voltage-gated[\/pb_glossary] Na<sup>+<\/sup> channels open. This is what is known as the threshold. Any depolarization that does not change the membrane potential to -55 mV or higher will not reach threshold and thus will not result in an action potential. Also, any stimulus that depolarizes the membrane to -55 mV or beyond will cause a large number of channels to open and an action potential will be initiated.<\/p>\r\n<p style=\"text-align: justify\">Because of the threshold, the action potential can be likened to a digital event\u2014it either happens or it does not. If the threshold is not reached, then no action potential occurs. If depolarization reaches -55 mV, then the action potential continues and runs all the way to +30 mV, at which K<sup>+<\/sup> causes [pb_glossary id=\"2560\"]repolarization[\/pb_glossary], including the [pb_glossary id=\"2561\"]hyperpolarizing[\/pb_glossary] overshoot. Also, those changes are the same for every action potential, which means that once the threshold is reached, the exact same thing happens. A stronger stimulus, which might [pb_glossary id=\"2559\"]depolarize[\/pb_glossary] the membrane well past threshold, will not make a \u201cbigger\u201d action potential. Action potentials are \u201call or none.\u201d Either the membrane reaches the threshold and everything occurs as described above, or the membrane does not reach the threshold and nothing else happens. All action potentials peak at the same voltage (+30 mV), so one action potential is not bigger than another. Stronger stimuli will initiate multiple action potentials more quickly, but the individual signals are not bigger. Thus, for example, you will not feel a greater sensation of pain, or have a stronger muscle contraction, because of the size of the action potential because they are not different sizes.<\/p>\r\n<p style=\"text-align: justify\">As we have seen, the depolarization and repolarization of an action potential are dependent on two types of channels (the voltage-gated Na<sup>+<\/sup> channel and the voltage-gated K<sup>+<\/sup> channel). The voltage-gated Na<sup>+<\/sup> channel actually has two gates. One is the activation gate, which opens when the membrane potential crosses -55 mV. The other gate is the inactivation gate, which closes after a specific period of time\u2014on the order of a fraction of a millisecond. When a cell is at rest, the activation gate is closed and the inactivation gate is open. However, when the threshold is reached, the activation gate opens, allowing Na+ to rush into the cell. Timed with the peak of [pb_glossary id=\"2559\"]depolarization[\/pb_glossary], the inactivation gate closes. During repolarization, no more sodium can enter the cell. When the membrane potential passes -55 mV again, the activation gate closes. After that, the inactivation gate re-opens, making the channel ready to start the whole process over again.<\/p>\r\n<p style=\"text-align: justify\">The voltage-gated K<sup>+<\/sup> channel has only one gate, which is sensitive to a membrane voltage of -50 mV. However, it does not open as quickly as the voltage-gated Na<sup>+<\/sup> channel does. It might take a fraction of a millisecond for the channel to open once that voltage has been reached. The timing of this coincides exactly with when the Na<sup>+<\/sup> flow peaks, so voltage-gated K<sup>+<\/sup> channels open just as the voltage-gated Na<sup>+<\/sup> channels are being inactivated. As the membrane potential repolarizes and the voltage passes -50 mV again, the channel closes\u2014again, with a little delay. Potassium continues to leave the cell for a short while and the membrane potential becomes more negative, resulting in the hyperpolarizing overshoot. Then the channel closes again and the membrane can return to the resting potential because of the ongoing activity of the non-gated channels and the Na<sup>+<\/sup>\/K<sup>+<\/sup> pump. All of this takes place within approximately 2 milliseconds (Figure 36). While an action potential is in progress, another one cannot be initiated. That effect is referred to as the <strong>[pb_glossary id=\"2562\"]refractory period[\/pb_glossary]<\/strong>.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.5c\"><\/a>Propagation of Action Potential<\/strong><strong>s<\/strong><\/h5>\r\n[caption id=\"\" align=\"alignnone\" width=\"891\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image45.png\" alt=\"image\" width=\"891\" height=\"626\" \/> <strong>Figure 37. Propagation of an Action Potential Along an Unmyelinated Axon.<\/strong>[\/caption]\r\n<p style=\"text-align: justify\">The action potential is initiated at the beginning of the axon, at what is called the initial segment. There is a high density of voltage-gated Na<sup>+<\/sup> channels so that rapid depolarization can take place here. Going down the length of the axon, the action potential is propagated because more voltage-gated Na<sup>+<\/sup> channels are opened as the depolarization spreads. This spreading occurs because Na<sup>+<\/sup> enters through the channel and moves along the inside of the cell membrane. As the Na+ moves, or flows, a short distance along the cell membrane, its positive charge depolarizes a little more of the cell membrane. As that depolarization spreads, new voltage-gated Na<sup>+<\/sup> channels open and more ions rush into the cell, spreading the depolarization a little farther (Figure 37).<\/p>\r\n<p style=\"text-align: justify\">Because voltage-gated Na<sup>+<\/sup> channels are inactivated at the peak of the depolarization, they cannot be opened again for a brief time\u2014the absolute refractory period. Because of this, depolarization spreading back toward previously opened channels has no effect. The action potential must propagate toward the axon terminals; as a result, the polarity of the neuron is maintained, as mentioned above.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"621\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image46.png\" alt=\"image\" width=\"621\" height=\"229\" \/> <strong>Figure 38. Propagation of an Action Potential Along a Myelinated Axon.<\/strong> Nodes of Ranvier are gaps in myelin coverage along axons. Nodes contain voltage-gated K<sup>+<\/sup> and Na<sup>+<\/sup> channels. Action potentials travel down the axon by jumping from one node to the next. This diagram shows the nodes of Ranvier and the internodal (myelinated) segments with approximately the same length. This is not accurate: in real axons, the segments with myelin are about one thousand times longer than the nodes![\/caption]\r\n<p style=\"text-align: justify\">Propagation, as described above, applies to unmyelinated axons. When [pb_glossary id=\"2343\"]myelination[\/pb_glossary] is present, the action potential propagates differently (Figure 38). Sodium ions that enter the cell at the initial segment start to spread along the length of the [pb_glossary id=\"2462\"]axon segment[\/pb_glossary], but there are no voltage-gated Na<sup>+<\/sup> channels until the first [pb_glossary id=\"2563\"]node of Ranvier[\/pb_glossary]. Because there is not constant opening of these channels along the axon segment, the depolarization spreads at an optimal speed. The distance between nodes (1-3 mm) is the optimal distance to keep the membrane still depolarized above threshold at the next node. As Na<sup>+<\/sup> spreads along the inside of the membrane of the axon segment, the charge starts to dissipate. If the node were any farther down the axon, that depolarization would have fallen off too much for voltage-gated Na<sup>+<\/sup> channels to be activated at the next [pb_glossary id=\"2563\"]node of Ranvier[\/pb_glossary]. If the nodes were any closer together, the speed of propagation would be slower.<\/p>\r\n<p style=\"text-align: justify\">Propagation along an unmyelinated axon is referred to as <strong>[pb_glossary id=\"2565\"]continuous conduction[\/pb_glossary]<\/strong>; along the length of a myelinated axon, it is [pb_glossary id=\"2564\"]<strong>saltatory<\/strong><strong> conduction<\/strong>[\/pb_glossary]. Continuous conduction is slow because there are always voltage-gated Na<sup>+<\/sup> channels opening, and more and more Na<sup>+<\/sup> is rushing into the cell. Saltatory conduction is faster because the action potential basically jumps from one node to the next (saltare = \u201cto leap\u201d), and the new influx of Na<sup>+<\/sup> renews the depolarized membrane. Along with the myelination of the axon, the diameter of the axon can influence the speed of conduction. Much as water runs faster in a wide river than in a narrow creek, Na<sup>+<\/sup>-based depolarization spreads faster down a wide axon than down a narrow one. This concept is known as resistance and is generally true for electrical wires or plumbing, just as it is true for axons, although the specific conditions are different at the scales of electrons or ions versus water in a river.<\/p>\r\n\r\n<h5 style=\"text-align: justify\"><strong><a id=\"9.5d\"><\/a>Neuro<\/strong><strong>transmission<\/strong><\/h5>\r\n<p style=\"text-align: justify\">The electrical changes taking place within a neuron, as described in the previous section, are similar to a light switch being turned on. A stimulus starts the depolarization, but the action potential runs on its own once a threshold has been reached. The question is now, \u201cWhat flips the light switch on?\u201d Temporary changes to the cell membrane voltage can result from neurons receiving information from the environment, or from the action of one neuron on another. These special types of potentials influence a neuron and determine whether an action potential will occur or not. Many of these transient signals originate at the <strong>[pb_glossary id=\"2344\"]synapse[\/pb_glossary]<\/strong>, the connection between electrically active cells.<\/p>\r\n<p style=\"text-align: justify\">There are two types of synapses: chemical synapses and electrical synapses. In a chemical synapse, a chemical signal\u2014namely, a [pb_glossary id=\"2172\"]neurotransmitter[\/pb_glossary]\u2014is released from one cell and it affects the other cell. In an electrical synapse, there is a direct connection between the two cells so that ions can pass directly from one cell to the next. If one cell is depolarized in an electrical synapse, the joined cell also depolarizes because the ions pass between the cells. Chemical synapses involve the transmission of chemical information from one cell to the next. This section will concentrate on the chemical type of synapse.<\/p>\r\n<p style=\"text-align: justify\">An example of a chemical synapse is the neuromuscular junction described in the chapter on muscle tissue. In the nervous system, there are many more synapses that are essentially the same as the neuromuscular junction. All synapses have common characteristics, which can be summarized in this list:<\/p>\r\n\r\n<ul>\r\n \t<li>presynaptic element<\/li>\r\n \t<li>neurotransmitter (packaged in vesicles)<\/li>\r\n \t<li>synaptic cleft<\/li>\r\n \t<li>receptor proteins<\/li>\r\n \t<li>postsynaptic element<\/li>\r\n \t<li>neurotransmitter elimination or re-uptake<\/li>\r\n<\/ul>\r\n<p style=\"text-align: justify\">Synaptic transmission (or neurotransmission) takes place through the following steps (Figure 39):<\/p>\r\n\r\n<ul>\r\n \t<li style=\"text-align: justify\">An action potential reaches the [pb_glossary id=\"2463\"]axon terminal[\/pb_glossary].<\/li>\r\n \t<li style=\"text-align: justify\">The change in voltage causes [pb_glossary id=\"2555\"]voltage-gated[\/pb_glossary] Ca<sup>2+<\/sup> channels in the membrane of the synaptic end bulb to open.<\/li>\r\n \t<li style=\"text-align: justify\">The concentration of Ca<sup>2+<\/sup> increases inside the end bulb, and Ca<sup>2+<\/sup> ions associate with proteins in the outer surface of neurotransmitter vesicles facilitating the merging of the [pb_glossary id=\"2225\"]vesicle[\/pb_glossary] with the presynaptic membrane. The neurotransmitter is then released through [pb_glossary id=\"2271\"]exocytosis[\/pb_glossary] into the small gap between the cells, known as the <strong>[pb_glossary id=\"2566\"]synaptic cleft[\/pb_glossary]<\/strong>.<\/li>\r\n \t<li style=\"text-align: justify\">Once in the synaptic cleft, the neurotransmitter diffuses the short distance to the postsynaptic membrane and can interact with neurotransmitter receptors. Receptors are specific for the neurotransmitter, and the two fit together like a key and lock. One neurotransmitter binds to its receptor and will not bind to receptors for other neurotransmitters, making the binding a specific chemical event.<\/li>\r\n \t<li style=\"text-align: justify\">The interaction of the neurotransmitter with the receptor can result in [pb_glossary id=\"2559\"]depolarization[\/pb_glossary] or [pb_glossary id=\"2561\"]hyperpolarization[\/pb_glossary] of the postsynaptic cell membrane, leading to excitation of the postsynaptic cell (and possibly the generation of a new action potential) or inhibition, respectively.<\/li>\r\n \t<li style=\"text-align: justify\">The neurotransmitter is removed from the synaptic cleft by [pb_glossary id=\"2256\"]diffusion[\/pb_glossary], due to the action of [pb_glossary id=\"2173\"]enzymes[\/pb_glossary] that break it down chemically or by transporters in the presynaptic cell membrane.<\/li>\r\n<\/ul>\r\n<em>\u00a0<\/em>\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"907\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image47.png\" alt=\"image\" width=\"907\" height=\"1139\" \/> <strong>Figure 39. Synaptic Transmission.<\/strong> The pre-synaptic neuron signals a postsynaptic neuron by releasing neurotransmitter across the synaptic cleft.[\/caption]\r\n\r\n<em>\u00a0<\/em>\r\n<p style=\"text-align: justify\"><em>Neurotransmitter Systems<\/em><\/p>\r\n<p style=\"text-align: justify\">There are several systems of neurotransmitters that are found at various synapses in the nervous system (Figure 40).\u00a0 In this course, you are not required to know all the neurotransmitters, but only to be able to provide one example of a neurotransmitter from each of the systems below.<\/p>\r\n\r\n<ul>\r\n \t<li style=\"text-align: justify\">[pb_glossary id=\"2179\"]<strong>Amino <\/strong><strong>acids<\/strong>[\/pb_glossary]: This includes glutamate (Glu), GABA (gamma-aminobutyric acid, a derivative of glutamate), and glycine (Gly).<\/li>\r\n \t<li style=\"text-align: justify\"><strong>B<\/strong><strong>iogenic amines<\/strong>: This is a group of neurotransmitters that are enzymatically made from amino acids. For example, the neurotransmitter serotonin is made from tryptophan. Other biogenic amines are made from tyrosine, and include dopamine, [pb_glossary id=\"2546\"]norepinephrine[\/pb_glossary], and [pb_glossary id=\"2547\"]epinephrine[\/pb_glossary]. The chemical epinephrine (epi- = \u201con\u201d; \u201c-nephrine\u201d = kidney) is also known as adrenaline (renal = \u201ckidney\u201d), and [pb_glossary id=\"2546\"]norepinephrine[\/pb_glossary] is sometimes referred to as noradrenaline. The [pb_glossary id=\"2541\"]adrenal gland[\/pb_glossary] produces epinephrine and norepinephrine to be released into the blood stream as hormones.<\/li>\r\n \t<li style=\"text-align: justify\"><strong>C<\/strong><strong>holinergic system<\/strong>: It is the system based on [pb_glossary id=\"2478\"]acetylcholine[\/pb_glossary]. This includes the [pb_glossary id=\"2551\"]neuromuscular junction[\/pb_glossary] as an example of a cholinergic synapse, but [pb_glossary id=\"2544\"]cholinergic[\/pb_glossary] synapses are found in other parts of the nervous system. They are in the [pb_glossary id=\"2456\"]autonomic nervous system[\/pb_glossary], as well as distributed throughout the brain.<\/li>\r\n \t<li style=\"text-align: justify\"><strong>N<\/strong><strong>europeptide<\/strong><strong>s:<\/strong> These are neurotransmitter molecules made up of chains of amino acids connected by [pb_glossary id=\"2180\"]peptide bonds[\/pb_glossary]. This is what a protein is, but the term protein implies a certain length to the molecule. Some neuropeptides are quite short, such as met-enkephalin, which is five amino acids long. Others are long, such as beta-endorphin, which is 31 amino acids long. Neuropeptides are often released at synapses in combination with another neurotransmitter, and they often act as hormones in other systems of the body, such as vasoactive intestinal peptide (VIP) or substance P.<\/li>\r\n<\/ul>\r\n<p style=\"text-align: justify\">The effect of a neurotransmitter on the postsynaptic element is entirely dependent on the receptor protein. First, if there is no receptor protein in the membrane of the postsynaptic element, then the neurotransmitter has no effect. The depolarizing or hyperpolarizing effect is also dependent on the receptor. For example, when acetylcholine binds to a type of receptor called nicotinic receptor, the postsynaptic cell is depolarized. This is because the receptor is a [pb_glossary id=\"2094\"]cation[\/pb_glossary] channel and positively charged Na<sup>+<\/sup> will rush into the cell. However, when acetylcholine binds to another type of receptor called muscarinic receptor, of which there are several variants, it might cause depolarization or hyperpolarization of the target cell.<\/p>\r\n<p style=\"text-align: justify\">On the other hand, the amino acid neurotransmitters, glutamate, glycine, and GABA, are almost exclusively associated with just one effect. Glutamate is considered an excitatory amino acid, but only because Glu receptors in the adult cause depolarization of the postsynaptic cell. Glycine and GABA are considered inhibitory amino acids, again because their receptors cause hyperpolarization.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"alignnone\" width=\"783\"]<img src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image48.png\" alt=\"image\" width=\"783\" height=\"1113\" \/> <strong>Figure 40. Examples of Neurotransmitters.<\/strong> Shown are some examples of major transmitters, their chemical structures and some of their functions.[\/caption]\r\n\r\n<div class=\"textbox textbox--exercises\"><header class=\"textbox__header\">\r\n<p class=\"textbox__title\"><a id=\"P\"><\/a>Practice questions<\/p>\r\n\r\n<\/header>\r\n<div class=\"textbox__content\">\r\n\r\n<strong>Part 1:<\/strong> The Anatomical and Functional Organization of the Nervous System\r\n\r\n[h5p id=\"138\"]\r\n\r\n[h5p id=\"139\"]\r\n\r\n<strong>Part 2:<\/strong> Nervous Tissue\r\n\r\n[h5p id=\"140\"]\r\n\r\n[h5p id=\"141\"]\r\n\r\n[h5p id=\"142\"]\r\n\r\n<strong>Part 3:<\/strong> The Central Nervous System\r\n\r\n[h5p id=\"143\"]\r\n\r\n[h5p id=\"144\"]\r\n\r\n<strong>Part 4:<\/strong> The Peripheral Nervous System\r\n\r\n[h5p id=\"147\"]\r\n\r\n[h5p id=\"150\"]\r\n\r\n<strong>Part 5:<\/strong> Neuronal Signalling\r\n\r\n[h5p id=\"152\"]\r\n\r\n[h5p id=\"149\"]\r\n\r\n[h5p id=\"153\"]\r\n\r\n[h5p id=\"154\"]\r\n\r\n<\/div>\r\n<\/div>\r\n&nbsp;\r\n\r\n<\/div>","rendered":"<div class=\"unit-9.-nervous-system\">\n<div class=\"textbox shaded\">\n<p><strong>Unit Outline<\/strong><\/p>\n<p><a href=\"#9.1\"><strong>Part 1:<\/strong> The Anatomical and Functional Organization of the Nervous System<\/a><\/p>\n<ul>\n<li><a href=\"#9.1a\">Anatomical Divisions<\/a><\/li>\n<li><a href=\"#9.1b\">Functional Divisions<\/a><\/li>\n<\/ul>\n<p><a href=\"#9.2\"><strong>Part 2:<\/strong> Nervous Tissue<\/a><\/p>\n<ul>\n<li><a href=\"#9.2a\">Neurons<\/a><\/li>\n<li><a href=\"#9.2b\">Glial cells<\/a><\/li>\n<li><a href=\"#9.2c\">Myelin<\/a><\/li>\n<\/ul>\n<p><a href=\"#9.3\"><strong>Part 3:<\/strong> The Central Nervous System<\/a><\/p>\n<ul>\n<li><a href=\"#9.3a\">The Cerebrum<\/a><\/li>\n<li><a href=\"#9.3b\">The Diencephalon<\/a><\/li>\n<li><a href=\"#9.3c\">The Brainstem<\/a><\/li>\n<li><a href=\"#9.3d\">The Cerebellum<\/a><\/li>\n<li><a href=\"#9.3e\">The Spinal Cord<\/a><\/li>\n<li><a href=\"#9.3f\">The Meninges<\/a><\/li>\n<li><a href=\"#9.3g\">The Ventricular System and Cerebrospinal Fluid Circulation<\/a><\/li>\n<\/ul>\n<p><a href=\"#9.4\"><strong>Part 4:<\/strong> The Peripheral Nervous System<\/a><\/p>\n<ul>\n<li><a href=\"#9.4a\">Ganglia<\/a><\/li>\n<li><a href=\"#9.4b\">Nerves<\/a><\/li>\n<li><a href=\"#9.4c\">The Somatic Nervous System<\/a><\/li>\n<li><a href=\"#9.4d\">The Autonomic Nervous System<\/a><\/li>\n<\/ul>\n<p><a href=\"#9.5\"><strong>Part 5:<\/strong> Neuronal Signalling<\/a><\/p>\n<ul style=\"margin-top: 1.42857em;margin-bottom: 1.42857em\">\n<li><a href=\"#9.5a\">Ion channels and the Resting Membrane Potential<\/a><\/li>\n<li><a href=\"#9.5b\">Generation of an Action Potential<\/a><\/li>\n<li><a href=\"#9.5c\">Propagation of Action Potentials<\/a><\/li>\n<li><a href=\"#9.5d\">Neurotransmission<\/a><\/li>\n<\/ul>\n<h2><a href=\"#P\">Practice Questions<\/a><\/h2>\n<\/div>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\"><strong>Learning Objectives<\/strong><\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p>At the end of this unit, you should be able to:<\/p>\n<p class=\"hanging-indent\"><strong>I. <\/strong>Describe the organization of the nervous system and explain the functions of its principal components.<\/p>\n<p class=\"hanging-indent\"><strong>II. <\/strong>Describe the structure of the following: neuron, glia, ganglion, nerve, gray matter, tract, white matter, sensory neuron, motor neuron.<\/p>\n<p class=\"hanging-indent\"><strong>III. <\/strong>Name, locate and describe the functions of the main areas of the human brain.<\/p>\n<p class=\"hanging-indent\"><strong>IV.<\/strong> Describe the structure and explain the functions of the spinal cord.<\/p>\n<p class=\"hanging-indent\"><strong>V. <\/strong>Describe the components of a reflex arc and explain how a reflex arc works.<\/p>\n<p class=\"hanging-indent\"><strong>V<\/strong><strong>I.<\/strong> Describe the function of the autonomic nervous system (ANS) and compare the specific functions of the parasympathetic and sympathetic divisions of the ANS.<\/p>\n<p class=\"hanging-indent\"><strong>VII.<\/strong> Describe the resting membrane potential of a neuron and explain how it is maintained.<\/p>\n<p class=\"hanging-indent\"><strong>VIII<\/strong><strong>.<\/strong> Explain how a neuronal action potential is generated.<\/p>\n<p class=\"hanging-indent\"><strong>IX.<\/strong> Explain how neuronal action potentials travel down the axon.<\/p>\n<p class=\"hanging-indent\"><strong>X. <\/strong>Explain the process of neurotransmission, and name three different neurotransmitters.<\/p>\n<\/div>\n<\/div>\n<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\"><strong>Learning Objectives and Guiding Questions<\/strong><\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p>At the end of this unit, you should be able to complete all the following tasks, including answering the guiding questions associated with each task.<\/p>\n<p class=\"hanging-indent\"><strong>I. <\/strong>Describe the organization of the nervous system and explain the functions of its principal components.<\/p>\n<ol>\n<li>Draw a flow chart demonstrating the relationships between, and stating the main function of each of the following components of the nervous system:\n<ul>\n<li>Central nervous system<\/li>\n<li>Peripheral nervous system<\/li>\n<li>Sensory neurons<\/li>\n<li>Motor neurons<\/li>\n<li>Somatic nervous system<\/li>\n<li>Autonomic nervous system<\/li>\n<li>Sympathetic nervous system<\/li>\n<li>Parasympathetic nervous system<\/li>\n<\/ul>\n<\/li>\n<li>Are the twelve cranial nerves considered part of the central nervous system, or the peripheral nervous system? Explain how you know.<\/li>\n<li>Are the dorsal root ganglia considered part of the central or peripheral nervous system? Explain how you know.<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>II. <\/strong>Describe the structure of the following: neuron, glia, ganglion, nerve, gray matter, tract, white matter, sensory neuron, motor neuron.<\/p>\n<ol>\n<li>Name the parts of a typical neuron and describe their functions.<\/li>\n<li>Compare and contrast the location, structure, and function of:\n<ul>\n<li>Neurons and glia<\/li>\n<li>Nerves and tracts<\/li>\n<li>White matter and nerves<\/li>\n<li>White matter and gray matter<\/li>\n<li>Nerves and ganglia<\/li>\n<li>Ganglia and gray matter<\/li>\n<li>Sensory and motor neurons<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>III. <\/strong>Name, locate and describe the functions of the main areas of the human brain.<\/p>\n<ol>\n<li>Describe the general anatomy of the brain, including the location of the lobes.<\/li>\n<li>Where in the brain would you find the cell bodies of neurons? Where would you find their axons? Describe how you can tell just by looking at a (cut) brain with the naked eye.<\/li>\n<li>Describe the location and function of each of the following areas of the human brain:\n<ul>\n<li>Cerebrum<\/li>\n<li>Diencephalon<\/li>\n<li>Thalamus<\/li>\n<li>Hypothalamus<\/li>\n<li>Brain stem<\/li>\n<li>Midbrain<\/li>\n<li>Pons<\/li>\n<li>Medulla oblongata<\/li>\n<li>Cerebellum<\/li>\n<\/ul>\n<\/li>\n<li>What are the names of the three meninges, and where are they located?<\/li>\n<li>What are the names of the four ventricles, and where are they located?<\/li>\n<li>Describe the path taken by cerebrospinal fluid through the brain.<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>IV.<\/strong> Describe the structure and explain the functions of the spinal cord.<\/p>\n<ol>\n<li>Where in the spinal cord would you find the cell bodies of neurons? Where would you find their axons? Describe how you can tell just by looking at a (cut) spinal cord with the naked eye.<\/li>\n<li>What are some of the functions of the spinal cord?<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>V. <\/strong>Describe the components of a reflex arc and explain how a reflex arc works.<\/p>\n<ol>\n<li>Describe the events that take place from the moment the knee is tapped to the moment when the leg extends during the patellar reflex, including the role of each of the structures involved.<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>V<\/strong><strong>I.<\/strong> Describe the function of the autonomic nervous system (ANS) and compare the specific functions of the parasympathetic and sympathetic divisions of the ANS.<\/p>\n<ol>\n<li>Compare the sympathetic and parasympathetic nervous system based on the:\n<ul>\n<li>Physiological situation to which they respond<\/li>\n<li>Location and neurotransmitter of the central (preganglionic) neuron<\/li>\n<li>Location and neurotransmitter of the ganglionic neuron<\/li>\n<\/ul>\n<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>VII.<\/strong> Describe the resting membrane potential of a neuron and explain how it is maintained.<\/p>\n<ol>\n<li>Describe the gating mechanism of ligand-gated, voltage-gated, mechanically-gated and leakage ion channels.<\/li>\n<li>What is the typical resting membrane potential of an animal cell, and what factors contribute to it?<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>VIII<\/strong><strong>.<\/strong> Explain how a neuronal action potential is generated.<\/p>\n<ol>\n<li>Draw a fully annotated figure plotting membrane potential vs. time as an action potential passes a specific location in an axon\u2019s membrane. Include in your annotations labels explaining the main mechanisms that underlie each shift in membrane potential.<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>IX.<\/strong> Explain how neuronal action potentials travel down the axon.<\/p>\n<ol>\n<li>Compare the mechanism by which nerve impulses are conducted in unmyelinated and myelinated axons.<\/li>\n<\/ol>\n<p class=\"hanging-indent\"><strong>X. <\/strong>Explain the process of neurotransmission, and name three different neurotransmitters.<\/p>\n<ol>\n<li>Create an annotated diagram (or series of diagrams) showing how neurons communicate with each other:<\/li>\n<li>Describe the mechanism by which an action potential travels from the cell body to the axon terminals of a neuron.<\/li>\n<li>Describe the mechanisms that return a neuron to its resting state (resting membrane potential) once an action potential has passed.<\/li>\n<li>Describe the intracellular events that occur in a neuron once an action potential reaches a synaptic end bulb.<\/li>\n<li>Describe how an excitatory neurotransmitter causes an action potential to be produced in a postsynaptic cell.<\/li>\n<li>Name at least three specific neurotransmitters: one from the cholinergic system, one amino acid that acts as a neurotransmitter, and one neuropeptide.<\/li>\n<li>What factor(s) determines whether a neurotransmitter has an excitatory or inhibitory effect on a cell exposed to that neurotransmitter?<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<h2><strong><a id=\"9.1\"><\/a>Part 1: Anatomical and Functional Organization of the Nervous System<\/strong><\/h2>\n<p style=\"text-align: justify\">The picture you have in your mind of the nervous system probably includes the<strong> brain<\/strong>, the nervous tissue contained within the cranium, and the <strong>spinal cord<\/strong>, the extension of nervous tissue within the vertebral column. That suggests it is made of two organs\u2014and you may not even think of the spinal cord as an organ\u2014but the nervous system is a very complex structure. Within the brain, many different and separate regions are responsible for many different and separate functions. It is as if the nervous system is composed of many organs that all look similar and can only be differentiated using tools such as the microscope or electrophysiology. In comparison, it is easy to see that the stomach is different than the esophagus or the liver, so you can imagine the digestive system as a collection of specific organs.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.1a\"><\/a>Anatomical Divisions<\/strong><\/h5>\n<p style=\"text-align: justify\">The nervous system can be divided into two major regions: the central and peripheral nervous systems. The <strong>central nervous system (CNS)<\/strong> is the brain and spinal cord, and the <strong>peripheral nervous system (PNS)<\/strong> is everything else (Figures 1 and 2). The brain is contained within the cranial cavity of the skull, and the spinal cord is contained within the vertebral cavity of the vertebral column. It is a bit of an oversimplification to say that the central nervous system is what is inside these two cavities and the peripheral nervous system is outside of them, but that is one way to start to think about it. In actuality, there are some elements of the peripheral nervous system that are within the cranial or vertebral cavities. The peripheral nervous system is so named because it is on the periphery\u2014meaning beyond the brain and spinal cord. Depending on different aspects of the nervous system, the dividing line between central and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2376\">peripheral<\/a> is not necessarily universal.<\/p>\n<figure style=\"width: 600px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image1-2.png\" alt=\"image\" width=\"600\" height=\"536\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 1. Central and Peripheral Nervous System.<\/strong> The structures of the peripheral nervous system are referred to as ganglia and nerves, which can be seen as distinct structures. The equivalent structures in the central nervous system are not obvious from this overall perspective and are best examined in prepared tissue under the microscope.<\/figcaption><\/figure>\n<p>Nervous tissue, present in both the central and peripheral nervous system, contains two basic types of cells: neurons and glial (or neuroglial) cells. A <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2340\">glial cell<\/a><\/strong> is one of a variety of cells that provide a framework of tissue that supports the neurons and their activities. The<strong> <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2181\">neuron<\/a><\/strong> is the more functionally important of the two, in terms of the communicative function of the nervous system. To describe the functional divisions of the nervous system, it is important to understand the structure of a neuron. Neurons are cells and therefore have a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2447\">soma<\/a><\/strong>, or cell body, but they also have extensions of the cell; each extension is generally referred to as a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2448\">process<\/a><\/strong>. There is one important process that every neuron has called an <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">axon<\/a><\/strong>, which is the fiber that connects a neuron with its target. Another type of process that branches off from the soma is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2342\">dendrite<\/a>.<\/p>\n<figure id=\"attachment_1753\" aria-describedby=\"caption-attachment-1753\" style=\"width: 897px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-1753 size-large\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/Unit-9-anatomical-organization-nervous-system-fixed-no-caption-897x1024.png\" alt=\"\" width=\"897\" height=\"1024\" srcset=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/Unit-9-anatomical-organization-nervous-system-fixed-no-caption-897x1024.png 897w, https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/Unit-9-anatomical-organization-nervous-system-fixed-no-caption-263x300.png 263w, https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/Unit-9-anatomical-organization-nervous-system-fixed-no-caption-768x876.png 768w, https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/Unit-9-anatomical-organization-nervous-system-fixed-no-caption-65x74.png 65w, https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/Unit-9-anatomical-organization-nervous-system-fixed-no-caption-225x257.png 225w, https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/Unit-9-anatomical-organization-nervous-system-fixed-no-caption-350x399.png 350w, https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/Unit-9-anatomical-organization-nervous-system-fixed-no-caption.png 1021w\" sizes=\"auto, (max-width: 897px) 100vw, 897px\" \/><figcaption id=\"caption-attachment-1753\" class=\"wp-caption-text\"><strong>Figure 2. The Anatomical Organization of the Nervous System.<\/strong><\/figcaption><\/figure>\n<p style=\"text-align: justify\"><strong>Dendrites<\/strong> are responsible for receiving most of the input from other neurons. Looking at nervous tissue, there are regions that predominantly contain cell bodies and regions that are largely composed of just axons.<\/p>\n<figure style=\"width: 695px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image3-3.png\" alt=\"image\" width=\"695\" height=\"502\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 3. Gray Matter and White Matter.<\/strong> A brain removed during an autopsy, with a partial section removed, shows white matter surrounded by gray matter. Gray matter makes up the outer cortex of the brain. (credit: modification of work by \u201cSuseno\u201d\/ Wikimedia Commons)<\/figcaption><\/figure>\n<figure style=\"width: 874px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image4-2.png\" alt=\"image\" width=\"874\" height=\"468\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 4. What Is a Nucleus?<\/strong> (a) The nucleus of an atom contains its protons and neutrons. (b) The nucleus of a cell is the organelle that contains DNA. (c) A nucleus in the central nervous system is a localized center of function with the cell bodies of several neurons, shown here circled in red. (credit c: \u201cWas a bee\u201d\/Wikimedia Commons)<\/figcaption><\/figure>\n<p style=\"text-align: justify\">These two regions within nervous system structures are often referred to as <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2449\">gray matter<\/a><\/strong> (the regions with many cell bodies and dendrites) or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2450\">white matter<\/a><\/strong> (the regions with many axons). The colors ascribed to these regions are what would be seen in \u201cfresh,\u201d or unstained, nervous tissue (Figure 3). Gray matter is not necessarily gray. It can be pinkish because of blood content, or even slightly tan, depending on how long the tissue has been preserved. But white matter is white because <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">axons<\/a> are insulated by a lipid-rich substance called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2343\">myelin<\/a><\/strong>. Lipids can appear as white (\u201cfatty\u201d) material, much like the fat on a raw piece of chicken or beef. Actually, gray matter may have that color ascribed to it because next to the white matter, it is just darker\u2014hence, gray.<\/p>\n<p style=\"text-align: justify\">The distinction between gray matter and white matter is most often applied to central nervous tissue, which has large regions that can be seen with the unaided eye. When looking at peripheral structures, often a microscope is used and the tissue is stained with artificial colors. That is not to say that central nervous tissue cannot be stained and viewed under a microscope, but unstained tissue is most likely from the central nervous system \u2014for example, a frontal section of the brain or cross section of the spinal cord.<\/p>\n<figure style=\"width: 541px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image5-2.png\" alt=\"image\" width=\"541\" height=\"434\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 5. Optic Nerve Versus Optic Tract.<\/strong> This drawing of the connections of the eye to the brain shows the optic nerve extending from the eye to the chiasm, where the structure continues as the optic tract. The same axons extend from the eye to the brain through these two bundles of fibers, but the chiasm represents the border between peripheral and central.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Regardless of the appearance of stained or unstained tissue, the cell bodies of neurons or axons can be located in discrete anatomical structures that need to be named. Those names are specific to whether the structure is central or peripheral. A localized collection of neuron cell bodies in the central nervous system is referred to as a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2451\">nucleus<\/a><\/strong>. In the peripheral nervous system, a cluster of neuron cell bodies is referred to as a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2452\">ganglion<\/a><\/strong>. The term nucleus has a few different meanings within anatomy and physiology. It is the center of an atom, where protons and neutrons are found; it is the center of a cell, where the DNA is found; and it is a center of some function in the central nervous system (Figure 4). There is also a potentially confusing use of the word ganglion (plural = ganglia) that has a historical explanation. In the central nervous system, there is a group of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2451\">nuclei<\/a> that are connected together and were once called the basal ganglia before \u201cganglion\u201d became accepted as a description for a peripheral structure. Some sources refer to this group of nuclei as the \u201cbasal nuclei\u201d to avoid confusion.<\/p>\n<table style=\"border-collapse: collapse;width: 0%\">\n<caption>Table 1: Structures of the Central and Peripheral Nervous System<\/caption>\n<tbody>\n<tr>\n<td style=\"width: 31.3686%\"><\/td>\n<th style=\"width: 8.8856%\" scope=\"col\"><strong>CNS<\/strong><\/th>\n<th style=\"width: 19.4091%\" scope=\"col\"><strong>PNS<\/strong><\/th>\n<\/tr>\n<tr>\n<th style=\"width: 31.3686%\" scope=\"row\">Group of neuron cell bodies (i.e., gray matter)<\/th>\n<td style=\"width: 8.8856%\">Nucleus<\/td>\n<td style=\"width: 19.4091%\">Ganglion<\/td>\n<\/tr>\n<tr>\n<th style=\"width: 31.3686%\" scope=\"row\">Bundle of axons (i.e., white matter)<\/th>\n<td style=\"width: 8.8856%\">Tract<\/td>\n<td style=\"width: 19.4091%\">Nerve<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p style=\"text-align: justify\">Terminology applied to bundles of axons also differs depending on location. A bundle of axons, or fibers, found in the central nervous system is called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2454\">tract<\/a><\/strong> whereas the same thing in the peripheral nervous system would be called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2453\">nerve<\/a><\/strong>. There is an important point to make about these terms, which is that they can both be used to refer to the same bundle of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">axons<\/a>. When those axons are in the peripheral nervous system, the term is nerve, but if they are central nervous system, the term is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2454\">tract<\/a>. The most obvious example of this is the axons that project from the retina into the brain. Those axons are called the optic nerve as they leave the eye, but when they are inside the cranium, they are referred to as the optic tract. There is a specific place where the name changes, which is the optic chiasm, but they are still the same axons (Figure 5). A similar situation outside of science can be described for some roads. For example, you might know of a street named Canada Way in the city of Burnaby. If you travel south long enough on this road, eventually you will leave Burnaby and enter the city of New Westminster. In New Westminster, Canada Way changes its name to Eighth Street. That is the idea behind the naming of the retinal axons. In the peripheral nervous system, they are called the optic nerve, and in the central nervous system, they are the optic tract. Table 1 helps to clarify which of these terms apply to the central or peripheral nervous systems.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.1b\"><\/a>Functional Divisions<\/strong><\/h5>\n<p style=\"text-align: justify\">There are two ways to consider how the nervous system is divided functionally. First, the basic functions of the nervous system are sensation, integration, and response. Secondly, control of the body can be <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2455\">somatic<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\">autonomic<\/a>\u2014divisions that are largely defined by the structures that are involved in the response (Figure 6). There is also a region of the peripheral nervous system that is called the enteric nervous system that is responsible for a specific set of the functions within the realm of autonomic control related to gastrointestinal functions.<\/p>\n<p style=\"text-align: justify\"><em>Basic Functions: Sensation, Integration, and Response<\/em><\/p>\n<p style=\"text-align: justify\">The nervous system is involved in receiving information about the environment around us (sensation) and generating responses to that information (motor responses). The nervous system can be divided into regions that are responsible for <strong>sensation <\/strong>(sensory functions) and for the <strong>response<\/strong> (motor functions). But there is a third function that needs to be included. Sensory input needs to be integrated with other sensations, as well as with memories, emotional state, or learning (cognition). Some regions of the nervous system are termed <strong>integration<\/strong> or association areas. The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response.<\/p>\n<p style=\"text-align: justify\">The first major function of the nervous system is <strong>sensation<\/strong>\u2014receiving information about the environment to gain input about what is happening outside the body (or, sometimes, within the body). The sensory functions of the nervous system register the presence of a particular event in the external or internal environment, known as a <strong>stimulus<\/strong>. The senses we think of most are the \u201cbig five\u201d: taste, smell, touch, sight, and hearing. The stimuli for taste and smell are both chemical substances (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2066\">molecules<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2063\">compounds<\/a>, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2093\">ions<\/a>, etc.), touch is physical or mechanical stimuli that interact with the skin, sight is light stimuli, and hearing is the perception of sound, which is a physical stimulus similar to some aspects of touch. There are actually more senses than just those, but that list represents the major senses. Those five are all senses that receive stimuli from the outside world, and of which there is conscious perception. Additional sensory stimuli might be from the internal environment (inside the body), such as the stretch of an organ wall or the concentration of certain ions in the blood.<\/p>\n<p style=\"text-align: justify\">Stimuli that are received by sensory structures are communicated to the nervous system where that information is processed. This is called <strong>integration<\/strong>. Stimuli are compared with, or integrated with, other stimuli, memories of previous stimuli, or the state of a person at a particular time. This leads to the specific response that will be generated. Seeing a baseball pitched to a batter will not automatically cause the batter to swing. The trajectory of the ball and its speed will need to be considered. Maybe the count is three balls and one strike, and the batter wants to let this pitch go by in the hope of getting a walk to first base. Or maybe the batter\u2019s team is so far ahead, it would be fun to just swing away.<\/p>\n<p style=\"text-align: justify\">The nervous system produces a <strong>response<\/strong> on the basis of the stimuli perceived by sensory structures. An obvious response would be the movement of muscles, such as withdrawing a hand from a hot stove, but there are broader uses of the term. The nervous system can cause the contraction of all three types of muscle tissue. For example, skeletal muscle contracts to move the skeleton, cardiac muscle is influenced as heart rate increases during exercise, and smooth muscle contracts as the digestive system moves food along the digestive tract. Responses also include the neural control of glands in the body as well, such as the production and secretion of sweat by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2457\">eccrine<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2458\">apocrine<\/a> sweat glands found in the skin to lower body temperature.<\/p>\n<figure style=\"width: 893px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image7-2.png\" alt=\"image\" width=\"893\" height=\"703\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 6. The Functional Organization of the Nervous System.<\/strong> The diagram represents the divisions of the nervous system involved in each of the basic functions: sensation (receiving and processing information from the external and internal environment), integration (comparing the sensory input with stored information and with other sensory inputs in order for the body to react appropriately) and response (most commonly, a motor command generated by the somatic nervous system or the autonomic nervous system).<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Responses can be divided into those that are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2280\">voluntary<\/a> or conscious (contraction of skeletal muscle) and those that are involuntary (contraction of smooth muscles, regulation of cardiac muscle, activation of glands). Voluntary responses are governed by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2455\">somatic nervous system<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2333\">involuntary<\/a> responses are governed by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\">autonomic nervous system<\/a>, which are discussed in the next section.<\/p>\n<p style=\"text-align: justify\"><em>Somatic, Autonomic and Enteric Nervous Systems<\/em><\/p>\n<p style=\"text-align: justify\">The nervous system can be divided into two parts mostly on the basis of a functional difference in responses. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2455\">somatic nervous system<\/a> (SNS)<\/strong> is responsible for conscious perception and voluntary motor responses. Voluntary motor response means the contraction of skeletal muscle, but those contractions are not always voluntary in the sense that you have to want to perform them. Some somatic motor responses are reflexes, and often happen without a conscious decision to perform them. If your friend jumps out from behind a corner and yells \u201cBoo!\u201d you will be startled and you might scream or leap back. You didn\u2019t decide to do that, and you may not have wanted to give your friend a reason to laugh at your expense, but it is a reflex involving skeletal muscle contractions. Other motor responses become automatic (in other words, unconscious) as a person learns motor skills (referred to as \u201chabit learning\u201d or \u201cprocedural memory\u201d).<\/p>\n<p style=\"text-align: justify\">The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\">autonomic nervous system<\/a> (ANS)<\/strong> is responsible for involuntary control of the body, usually for the sake of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2264\">homeostasis<\/a> (regulation of the internal environment). Sensory input for autonomic functions can be from sensory structures tuned to external or internal environmental stimuli. The motor output extends to smooth and cardiac muscle as well as glandular tissue. The role of the autonomic system is to regulate the organ systems of the body, which usually means to control homeostasis. Sweat glands, for example, are controlled by the autonomic system. When you are hot, sweating helps cool your body down. That is a homeostatic mechanism. But when you are nervous, you might start sweating also. That is not homeostatic, it is the physiological response to an emotional state.<\/p>\n<figure style=\"width: 975px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image8-3.png\" alt=\"image\" width=\"975\" height=\"531\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 7. Somatic, Autonomic, and Enteric Structures of the Nervous System.<\/strong> Somatic structures include the spinal nerves, both motor and sensory fibers, as well as the sensory ganglia (posterior root ganglia and cranial nerve ganglia). Autonomic structures are found in the nerves also, but include the sympathetic and parasympathetic ganglia. The enteric nervous system includes the nervous tissue within the organs of the digestive tract.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">There is another division of the nervous system that describes functional responses. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2459\">enteric nervous system<\/a> (ENS)<\/strong> is responsible for controlling the smooth muscle and glandular tissue in your digestive system. It is a large part of the peripheral nervous system, and is not dependent on the central nervous system. It is sometimes valid, however, to consider the enteric system to be a part of the autonomic system because the neural structures that make up the enteric system are a component of the autonomic output that regulates digestion (Figure 7). There are some differences between the two, but for our purposes here there will be a good bit of overlap.<\/p>\n<p>&nbsp;<\/p>\n<figure style=\"width: 190px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image10-2.png\" alt=\"image\" width=\"190\" height=\"185\" \/><figcaption class=\"wp-caption-text\">Watch <a href=\"https:\/\/youtu.be\/qPix_X-9t7E\">this Crash Course video<\/a> for an overview of the nervous system! Direct link: <a href=\"https:\/\/youtu.be\/qPix_X-9t7E\">https:\/\/youtu.be\/qPix_X-9t7E<\/a><\/figcaption><\/figure>\n<h2><strong><a id=\"9.2\"><\/a>Part 2: Nervous Tissue<\/strong><\/h2>\n<p style=\"text-align: justify\">Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons are the primary type of cell that most anyone associates with the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glial cells, or glia, are known to play a supporting role for nervous tissue. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.2a\"><\/a>Neurons<\/strong><\/h5>\n<p style=\"text-align: justify\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2181\">Neurons<\/a> are the cells considered to be the basis of nervous tissue. They are responsible for the electrical signals that communicate information about sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. An important part of the function of neurons is in their structure, or shape. The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible.<\/p>\n<p style=\"text-align: justify\"><em>Parts of a Neuron<\/em><\/p>\n<p style=\"text-align: justify\">As you learned in the first section, the main part of a neuron is the cell body, which is also known as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2447\">soma<\/a> (soma = \u201cbody\u201d). The cell body contains the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2221\">nucleus<\/a> and most of the major organelles. But what makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2448\">processes<\/a>. Neurons are usually described as having one, and only one, axon\u2014a fibre that emerges from the cell body and projects to target cells (Figure 8). That single <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">axon<\/a> can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse, which is communicated to one or more cells. The other processes of the neuron are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2342\">dendrites<\/a> (Figure 8), which receive information from other neurons at specialized areas of contact called <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2344\">synapses<\/a>. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2460\">polarity<\/a>\u2014meaning that information flows in this one direction.<\/p>\n<figure style=\"width: 640px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image11-2.png\" alt=\"image\" width=\"640\" height=\"427\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 8. Parts of a Neuron.<\/strong> The major parts of the neuron are labeled on a multipolar neuron from the central nervous system.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Where the axon emerges from the cell body, there is a special region referred to as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2461\">axon hillock<\/a>. This is a tapering of the cell body toward the axon fibre. Within the axon hillock, the cytoplasm changes to a solution of limited components called axoplasm. Because the axon hillock represents the beginning of the axon, it is also referred to as the initial segment.<\/p>\n<figure style=\"width: 642px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image12-2.png\" alt=\"image\" width=\"642\" height=\"414\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 9. Neuron Classification by Shape.<\/strong> Unipolar cells have one process that includes both the axon and dendrite. Bipolar cells have two processes, the axon and a dendrite. Multipolar cells have more than two processes, the axon and two or more dendrites.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Many axons are wrapped by an insulating substance called myelin, which is actually made from <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2340\">glial cells<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2343\">Myelin<\/a> acts as insulation much like the plastic or rubber that is used to insulate electrical wires. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">axon<\/a>. Each gap is called a node of Ranvier and is important to the way that electrical signals travel down the axon. The length of the axon between each gap, which is wrapped in myelin, is referred to as an <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2462\">axon segment<\/a>. At the end of the axon is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2463\">axon terminal<\/a>, where there are usually several branches extending toward the target cell, each of which ends in an enlargement called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2464\">synaptic end bulb<\/a>. These bulbs are what make the connection with the target cell at the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2344\">synapse<\/a>.<\/p>\n<p style=\"text-align: justify\"><em>Types of Neurons<\/em><\/p>\n<p style=\"text-align: justify\">There are many neurons in the nervous system\u2014a number in the trillions. And there are many different types of neurons. They can be classified by many different criteria. The first way to classify them is by the number of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2448\">processes<\/a> attached to the cell body. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron\u2019s <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2460\">polarity<\/a> (Figure 9).<\/p>\n<figure style=\"width: 705px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image13-2.png\" alt=\"image\" width=\"705\" height=\"442\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 10. Other Neuron Classifications.<\/strong> Three examples of neurons that are classified on the basis of other criteria. (a) The pyramidal cell is a multipolar cell with a cell body that is shaped something like a pyramid. (b) The Purkinje cell in the cerebellum was named after the scientist who originally described it. (c) Olfactory neurons are named for the functional group with which they belong.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Neurons can also be classified on the basis of where they are found, who found them, what they do, or even what chemicals they use to communicate with each other. Some neurons referred to in this section on the nervous system are named on the basis of those sorts of classifications (Figure 10). For example, a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2465\">multipolar<\/a> neuron that has a very important role to play in a part of the brain called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2466\">cerebellum<\/a> is known as a Purkinje (commonly pronounced per-KIN-gee) cell. It is named after the anatomist who discovered it (Jan Evangilista Purkinje, 1787\u20131869).<\/p>\n<p style=\"text-align: justify\"><strong><a id=\"9.2b\"><\/a>Glial Cells<\/strong><\/p>\n<p style=\"text-align: justify\">Glial cells, or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2340\">neuroglia<\/a> or simply glia, are the other type of cell found in nervous tissue. They are considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. The name glia comes from the Greek word that means \u201cglue,\u201d and was coined by the German pathologist Rudolph Virchow, who wrote in 1856: \u201cThis connective substance, which is in the brain, the spinal cord, and the special sense nerves, is a kind of glue (neuroglia) in which the nervous elements are planted.\u201d Today, research into nervous tissue has shown that there are many deeper roles that these cells play. And research may find much more about them in the future.<\/p>\n<table style=\"border-collapse: collapse;width: 100%\">\n<caption>Table 2: Glial Cell Types by Location and Basic Function<\/caption>\n<tbody>\n<tr>\n<th style=\"width: 23.87%\" scope=\"col\"><strong>CNS glia<\/strong><\/th>\n<th style=\"width: 19.209%\" scope=\"col\"><strong>PNS glia<\/strong><\/th>\n<th style=\"width: 56.9209%\" scope=\"col\"><strong>Basic function<\/strong><\/th>\n<\/tr>\n<tr>\n<td style=\"width: 23.87%\">Astrocyte<\/td>\n<td style=\"width: 19.209%\">Satellite cell<\/td>\n<td style=\"width: 56.9209%\">Support<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 23.87%\">Oligodendrocyte<\/td>\n<td style=\"width: 19.209%\">Schwann cell<\/td>\n<td style=\"width: 56.9209%\">Insulation, myelination<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 23.87%\">Microglia<\/td>\n<td style=\"width: 19.209%\">&#8211;<\/td>\n<td style=\"width: 56.9209%\">Immune surveillance, phagocytosis<\/td>\n<\/tr>\n<tr>\n<td style=\"width: 23.87%\">Ependymal cell<\/td>\n<td style=\"width: 19.209%\">&#8211;<\/td>\n<td style=\"width: 56.9209%\">Creating cerebrospinal fluid<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p style=\"text-align: justify\">There are six types of glial cells (Table 2). Four of them are found in the central nervous system (Figure 11) and two are found in the peripheral nervous system (Figure 12). For reference, Table 2 outlines some common characteristics and functions of the various glial cell types, but the specific names and roles of the glial cell types are not examinable material in this course.<\/p>\n<figure style=\"width: 635px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image15-3.png\" alt=\"image\" width=\"635\" height=\"473\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 11. Glial Cells of the Central Nervous System.<\/strong> The central nervous system has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the central nervous system in several ways.<\/figcaption><\/figure>\n<h5><strong style=\"text-align: justify\"><a id=\"9.2c\"><\/a>Myelin<\/strong><\/h5>\n<p style=\"text-align: justify\">The insulation for axons in the nervous system is provided by glial cells: <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2468\">oligodendrocytes<\/a> in the central nervous system, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2469\">Schwann cells<\/a> in the peripheral nervous system. Whereas the manner in which either cell is associated with the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2462\">axon segment<\/a>, or segments, that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2343\">Myelin<\/a> is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2161\">lipid<\/a>-rich sheath that surrounds the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">axon<\/a> and by doing so creates a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2470\">myelin sheath<\/a> that facilitates the transmission of electrical signals along the axon. The lipids are essentially the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2166\">phospholipids<\/a> of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.<\/p>\n<figure style=\"width: 535px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image16-3.png\" alt=\"image\" width=\"535\" height=\"346\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 12. Glial Cells of the Peripheral Nervous System.<\/strong> The peripheral nervous system has satellite cells and Schwann cells.<\/figcaption><\/figure>\n<h2 style=\"text-align: left\"><strong><a id=\"9.3\"><\/a>Part 3:<\/strong><strong>\u00a0The Central Nervous System<\/strong><\/h2>\n<p style=\"text-align: justify\">The brain and the spinal cord are the central nervous system, and they represent the main organs of the nervous system. The spinal cord is a single structure, whereas the adult brain is described in terms of four major regions: the cerebrum, the diencephalon, the brain stem, and the cerebellum. A person\u2019s conscious experiences are based on neural activity in the brain. The regulation of homeostasis is governed by a specialized region in the brain. The coordination of reflexes depends on the integration of sensory and motor pathways in the spinal cord.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3a\"><\/a>The Cerebrum<\/strong><\/h5>\n<p style=\"text-align: justify\">The iconic gray mantle of the human brain, which appears to make up most of the mass of the brain, is the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2471\">cerebrum<\/a><\/strong> with two distinct halves, a right and left <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2472\">cerebral hemisphere<\/a><\/strong> (Figure 13). Many of the higher neurological functions, such as memory, emotion, and consciousness, are the result of cerebral function. The cerebrum comprises of a continuous, wrinkled and thin layer of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2449\">gray matter<\/a> that wraps around both hemispheres, the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2473\">cerebral cortex<\/a><\/strong><strong>,<\/strong> and several deep <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2451\">nuclei<\/a>. A <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2474\">gyrus<\/a><\/strong> (plural = gyri) is the ridge of one of those wrinkles, and a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2475\">sulcus<\/a><\/strong> (plural = sulci) is the groove between two gyri. The pattern of these folds of tissue indicates specific regions of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2473\">cerebral cortex<\/a> (Figure 14).<\/p>\n<figure style=\"width: 929px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image17-2.png\" alt=\"image\" width=\"929\" height=\"429\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 13. The Cerebrum.<\/strong> The cerebrum is a large component of the central nervous system in humans, and the most obvious aspect of it is the folded surface called the cerebral cortex. Deep within the cerebrum, the white matter of the corpus callosum provides the major pathway for communication between the two hemispheres of the cerebral cortex.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Different regions of the cerebral cortex can be associated with particular functions, a concept known as localization of function. In the early 1900s, a German neuroscientist named Korbinian Brodmann performed an extensive study of the microscopic anatomy (cytoarchitecture) of the cerebral cortex and divided the cortex into 52 separate regions on the basis of the histology of the cortex. His work resulted in a system of classification known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2476\">Brodmann\u2019s areas<\/a>, which is still used today to describe the anatomical distinctions within the cortex The results from Brodmann\u2019s work on the anatomy align very well with the functional differences within the cortex. For example, Areas 17 and 18 in the occipital lobe are responsible for primary visual perception. That visual information is complex, so it is processed in the temporal and parietal lobes as well.<\/p>\n<p style=\"text-align: justify\">Beneath the cerebral cortex are sets of nuclei known as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2477\"><strong>bas<\/strong><strong>al nuclei<\/strong><\/a> that augment cortical processes (Figure 15). Some of the basal nuclei in the forebrain, for example, serve as the primary location for <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2478\">acetylcholine<\/a> production, which modulates the overall activity of the cortex, possibly leading to greater attention to sensory stimuli. Alzheimer\u2019s disease is associated with a loss of neurons in the cholinergic basal forebrain nuclei. Some other basal nuclei control the initiation of movement. For example, while a student is sitting in a classroom listening to a lecture, the basal nuclei will keep an urge to jump up and scream from actually happening. (The basal nuclei are also referred to as the basal ganglia, although that is potentially confusing because the term ganglia is typically used for peripheral structures.)<\/p>\n<figure style=\"width: 591px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image18-2.png\" alt=\"image\" width=\"591\" height=\"489\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 14. Lobes of the Cerebral Cortex.<\/strong> The cerebral cortex is divided into four lobes. Extensive folding increases the surface area available for cerebral functions. (The names of the main sulci are provided but they are not required as examinable material in this course.)<\/figcaption><\/figure>\n<figure style=\"width: 547px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image19-2.png\" alt=\"image\" width=\"547\" height=\"422\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 15. Frontal Section of Cerebral Cortex and Basal Nuclei.<\/strong> The major components of the basal nuclei, shown in a frontal section of the brain, are the caudate (just lateral to the lateral ventricle), the putamen (inferior to the caudate and separated by the large white-matter structure called the internal capsule), and the globus pallidus (medial to the putamen). (The names of these nuclei are not required as examinable material in this course.)<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3b\"><\/a>The Diencephalon<\/strong><\/h5>\n<p style=\"text-align: justify\">The word <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2479\">diencephalon<\/a> translates to \u201cthrough brain.\u201d It is the connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the peripheral nervous system all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2480\">olfaction<\/a><\/strong>, or the sense of smell, which connects directly with the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2471\">cerebrum<\/a>.<\/p>\n<p style=\"text-align: justify\">The diencephalon is deep beneath the cerebrum and constitutes the walls of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2481\">third ventricle<\/a>. The diencephalon can be described as any region of the brain with \u201cthalamus\u201d in its name. The two major regions of the diencephalon are the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2482\">thalamus<\/a> itself and the hypothalamus (Figure 16). There are other structures, such as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2483\">epithalamus<\/a><\/strong>, which contains the pineal gland, and the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2484\">subthalamus<\/a><\/strong>, which includes the subthalamic nucleus, one of the basal nuclei.<\/p>\n<figure style=\"width: 598px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image20-2.png\" alt=\"image\" width=\"598\" height=\"471\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 16. The Diencephalon.<\/strong> The diencephalon is composed primarily of the thalamus and hypothalamus, which together define the walls of the third ventricle. The thalami are two elongated, ovoid structures on either side of the midline that make contact in the middle. The hypothalamus is inferior and anterior to the thalamus, culminating in a sharp angle to which the pituitary gland is attached.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><em>Thalamus<\/em><\/p>\n<p style=\"text-align: justify\">The thalamus is a collection of nuclei that relay information between the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2473\">cerebral cortex<\/a> and the periphery, spinal cord, or brain stem. All sensory information, except for the sense of smell, passes through the thalamus before processing by the cortex. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">Axons<\/a> from the peripheral sensory organs, or intermediate nuclei, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2344\">synapse<\/a> in the thalamus, and thalamic neurons project directly to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2471\">cerebrum<\/a>. It is a requisite synapse in any sensory pathway, except for olfaction. The thalamus does not just pass the information on, it also processes that information. For example, the portion of the thalamus that receives visual information will influence what visual stimuli are important, or what receives attention. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2471\">cerebrum<\/a> also sends information down to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2482\">thalamus<\/a>, which usually communicates motor commands.<\/p>\n<p style=\"text-align: justify\"><em>Hypothalamus<\/em><\/p>\n<p style=\"text-align: justify\">Inferior and slightly anterior to the thalamus is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2440\">hypothalamus<\/a>, the other major region of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2479\">diencephalon<\/a>. The hypothalamus is a collection of nuclei that are largely involved in regulating <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2264\">homeostasis<\/a>. The hypothalamus is the executive region in charge of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\">autonomic nervous system<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2273\">endocrine<\/a> system through its regulation of the anterior <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2485\">pituitary gland<\/a>. Other parts of the hypothalamus are involved in memory and emotion as part of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2486\">limbic system<\/a>.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3c\"><\/a>The Brain Stem<\/strong><\/h5>\n<p style=\"text-align: justify\">The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2487\">midbrain<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2488\">hindbrain<\/a> (composed of the pons and the medulla oblongata, or medulla for short) are collectively referred to as the brain stem (Figure 17). The structure emerges from the ventral surface of the forebrain as a tapering cone that connects the brain to the spinal cord. The major ascending and descending pathways between the spinal cord and brain, specifically the cerebrum, pass through the brain stem. The majority of cranial nerves connect through the brain stem and provide the brain with the sensory input and\/or motor output associated with the head and neck, for example most of the special senses, eye movement, and swallowing. As discussed below, the brainstem controls many involuntary functions vital for survival like breathing rate, heart rate, blood pressure, and consciousness.<\/p>\n<figure style=\"width: 618px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image21-2.png\" alt=\"image\" width=\"618\" height=\"470\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 17. The Brain Stem.<\/strong> The brain stem includes three regions: the midbrain, the pons, and the medulla.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><em>Midbrain<\/em><\/p>\n<p style=\"text-align: justify\">The midbrain includes four bumps known as the colliculi (singular = colliculus), which means \u201clittle hill\u201d in Latin. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2492\">inferior colliculus<\/a><\/strong> is the inferior pair of these enlargements and is part of the auditory brain stem pathway that relays information to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2471\">cerebrum <\/a>for conscious perception of sound. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2493\">superior colliculus<\/a><\/strong> is the superior pair of structures which integrates visual, auditory and somatosensory information to allow rapid head, eye and body movement towards external stimuli, like a loud noise.<\/p>\n<p style=\"text-align: justify\"><em>Pons<\/em><\/p>\n<p style=\"text-align: justify\">It is visible on the anterior surface of the brain stem as the thick bundle of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2450\">white matter<\/a>\u00a0attached to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2466\">cerebellum<\/a>. The word <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2489\">pons\u00a0<\/a>comes from the Latin word for bridge; it bridges the midbrain and the medulla and is the main connection between the cerebellum and the brain stem. In conjunction with the medulla it helps regulate vital functions, including respiratory rate (as will be <a href=\"https:\/\/pressbooks.bccampus.ca\/dcbiol120312094thed\/chapter\/unit-6-the-respiratory-system\/\">discussed further in BIOL 1203\/9<\/a>). Through its connection to the cerebellum, the pons helps produce coordinated movement and good balance.<\/p>\n<p style=\"text-align: justify\"><em>M<\/em><em>edulla oblongata (or medulla)<\/em><\/p>\n<p>The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2449\">gray matter<\/a>\u00a0of the midbrain and pons continues into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2490\">medulla oblongata<\/a> (also known as the medulla but should not be confused with the medulla in the kidney or adrenal glands; known as the renal medulla and adrenal medulla, respectively). This diffuse region of gray matter throughout the brain stem, known as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2494\">reticular formation<\/a><\/strong>, is related to sleep and wakefulness, general brain activity and attention. The medulla contains <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\">autonomic<\/a>\u00a0nuclei with motor neurons that control the rate and force of heart contraction, the diameter of blood vessels, \u00a0the rate and depth of breathing, among other essential physiological processes, like swallowing.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3d\"><\/a>The Cerebellum<\/strong><\/h5>\n<p style=\"text-align: justify\">The cerebellum, as the name suggests, is the \u201clittle brain.\u201d It is covered in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2474\">gyri<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2475\">sulci<\/a> like the cerebrum, and looks like a miniature version of that part of the brain (Figure 18). The cerebellum integrates motor commands from the cerebral cortex with sensory feedback from the periphery, allowing for the coordination and precise execution of motor activities, such as walking, cycling, writing or playing a musical instrument.<\/p>\n<figure style=\"width: 639px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image22-2.png\" alt=\"image\" width=\"639\" height=\"791\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 18. The Cerebellum.<\/strong> The cerebellum is situated on the posterior surface of the brain stem. Descending input from the cerebellum enters through the large white matter structure of the pons. Ascending input from the periphery and spinal cord enters through the fibers of the inferior olive. Output goes to the midbrain, which sends a descending signal to the spinal cord.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.33\"><\/a>Th<\/strong><strong>e Spinal Cord<\/strong><\/h5>\n<p style=\"text-align: justify\">Whereas the brain develops out of expansions of the neural tube into primary and then secondary vesicles, the spinal cord maintains the tube structure and is only specialized into certain regions.<\/p>\n<p style=\"text-align: justify\">The length of the spinal cord is divided into regions that correspond to the regions of the vertebral column. The name of a spinal cord region corresponds to the level at which spinal nerves pass through the intervertebral <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2500\">foramina<\/a>. Immediately adjacent to the brain stem is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2501\">cervical<\/a> region, followed by the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2503\">thoracic<\/a>, then the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2502\">lumbar<\/a>, and finally the sacral region (Figures 24 and 25).<\/p>\n<p style=\"text-align: justify\"><em>Gray Horns<\/em><\/p>\n<p style=\"text-align: justify\">In cross-section, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2449\">gray matter<\/a> of the spinal cord has the appearance of an ink-blot test, with the spread of the gray matter on one side replicated on the other\u2014a shape reminiscent of a bulbous capital \u201cH.\u201d As shown in Figure 19, the gray matter is subdivided into regions that are referred to as horns.<\/p>\n<p style=\"text-align: justify\">The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2504\">posterior horn<\/a><\/strong> is responsible for sensory processing. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2505\">anterior horn<\/a><\/strong> sends out motor signals to the skeletal muscles. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2506\">lateral horn<\/a><\/strong>, which is only found in the thoracic, upper lumbar, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2508\">sacral<\/a> regions, is the central component of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2507\">sympathetic division<\/a> of the<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\"> autonomic nervous system<\/a>.<\/p>\n<p style=\"text-align: justify\">Some of the largest neurons of the spinal cord are the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2465\">multipolar<\/a> motor neurons in the anterior horn. The fibres that cause contraction of skeletal muscles are the axons of these neurons. The motor neuron that causes contraction of the big toe, for example, is located in the sacral spinal cord. The axon that has to reach all the way to the belly of that muscle may be a metre in length. The neuronal cell body that maintains that long fiber must be quite large, possibly several hundred micrometres in diameter, making it one of the largest cells in the body.<\/p>\n<figure style=\"width: 603px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image23-2.png\" alt=\"image\" width=\"603\" height=\"691\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 19. Cross-section of Spinal Cord.<\/strong> The cross-section of a thoracic spinal cord segment shows the posterior, anterior, and lateral horns of gray matter, as well as the posterior, anterior, and lateral columns of white matter. LM \u00d7 40. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><em>White Columns<\/em><\/p>\n<p style=\"text-align: justify\">Just as the gray matter is separated into horns, the white matter of the spinal cord is separated into columns. <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2509\">Ascending tracts<\/a><\/strong> of nervous system fibres in these columns carry sensory information up to the brain, whereas <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2510\">descending tracts<\/a><\/strong> carry motor commands from the brain.<\/p>\n<figure style=\"width: 189px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image24-2.png\" alt=\"image\" width=\"189\" height=\"193\" \/><figcaption class=\"wp-caption-text\">Watch <a href=\"https:\/\/youtu.be\/q8NtmDrb_qo\">this Crash Course video<\/a> for an overview of the central nervous system! (Direct link: <a href=\"https:\/\/youtu.be\/q8NtmDrb_qo\">https:\/\/youtu.be\/q8NtmDrb_qo<\/a>)<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3f\"><\/a>The Meninges<\/strong><\/h5>\n<p style=\"text-align: justify\">The outer surface of the central nervous system is covered by a series of membranes composed of connective tissue called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2496\">meninges<\/a>, which protect the brain. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2497\">dura mater<\/a> is a thick fibrous layer and a strong protective sheath over the entire brain and spinal cord. It is anchored to the inner surface of the cranium and vertebral cavity. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2498\">arachnoid mater<\/a> is a membrane of thin fibrous tissue that forms a loose sac around the central nervous system. Beneath the arachnoid is a thin, filamentous mesh called the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2511\">arachnoid trabeculae<\/a>, which looks like a spider web, giving this layer its name. Directly adjacent to the surface of the central nervous system is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2512\">pia mater<\/a>, a thin fibrous membrane that follows the convolutions of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2474\">gyri<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2475\">sulci<\/a> in the cerebral cortex and fits into other grooves and indentations (Figures 20).<\/p>\n<figure style=\"width: 787px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image25-2.png\" alt=\"image\" width=\"787\" height=\"419\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 20. Meningeal Layers of Superior Sagittal Sinus.<\/strong> The layers of the meninges in the longitudinal fissure of the superior sagittal sinus are shown, with the dura mater adjacent to the inner surface of the cranium, the pia mater adjacent to the surface of the brain, and the arachnoid and subarachnoid space between them. An arachnoid villus is shown emerging into the dural sinus to allow CSF to filter back into the blood for drainage.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.3g\"><\/a>The Ventricular System<\/strong><strong> and Cerebrospinal Fluid Circulation<\/strong><\/h5>\n<p style=\"text-align: justify\">Cerebrospinal fluid (CSF) circulates throughout and around the central nervous system. cerebrospinal fluid is produced in special structures to perfuse through the nervous tissue of the central nervous system and is continuous with the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2129\">interstitial fluid<\/a>. Specifically, cerebrospinal fluid circulates to remove metabolic wastes from the interstitial fluids of nervous tissues and return them to the blood stream. The <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2513\">ventricles<\/a><\/strong> are the open spaces within the brain where <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2137\">cerebrospinal fluid<\/a> circulates. In some of these spaces, cerebrospinal fluid is produced by filtering of the blood that is performed by a specialized membrane known as a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2514\">choroid plexus<\/a>. The cerebrospinal fluid circulates through all of the ventricles to eventually emerge into the subarachnoid space where it will be reabsorbed into the blood.<\/p>\n<p style=\"text-align: justify\">There are four ventricles within the brain, all of which developed from the original hollow space within the neural tube, the central canal. The first two are named the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2515\">lateral ventricles<\/a> and are deep within the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2471\">cerebrum<\/a>. These ventricles are connected to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2481\">third ventricle<\/a> by two openings called the interventricular foramina. The third ventricle is the space between the left and right sides of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2479\">diencephalon<\/a>, which opens into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2491\">cerebral aqueduct<\/a> that passes through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2487\">midbrain<\/a>. The aqueduct opens into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2516\">fourth ventricle<\/a>, which is the space between the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2466\">cerebellum<\/a> and the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2489\">pons<\/a> and upper <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2490\">medulla<\/a> (Figure 21).<\/p>\n<p style=\"text-align: justify\">The ventricular system opens up to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2517\">subarachnoid space<\/a> from the fourth ventricle. The single median aperture and the pair of lateral apertures connect to the subarachnoid space so that cerebrospinal fluid can flow through the ventricles and around the outside of the central nervous system. Cerebrospinal fluid is produced within the ventricles by a type of specialized membrane called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2514\">choroid plexus<\/a>. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2518\">Ependymal cells<\/a> (a type of glial cell; see Figure 11) surround blood capillaries and filter the blood to make cerebrospinal fluid. The fluid is a clear solution with a limited amount of the constituents of blood. It is essentially water, small molecules, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2134\">electrolytes<\/a>. Oxygen and carbon dioxide are dissolved into the cerebrospinal fluid, as they are in blood, and can diffuse between the fluid and the nervous tissue.<\/p>\n<figure style=\"width: 835px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image26-2.png\" alt=\"image\" width=\"835\" height=\"534\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 21. Cerebrospinal Fluid Circulation.<\/strong> The choroid plexus in the four ventricles produce CSF, which is circulated through the ventricular system and then enters the subarachnoid space through the median and lateral apertures. The CSF is then reabsorbed into the blood at the arachnoid granulations, where the arachnoid membrane emerges into the dural sinuses.<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<p style=\"text-align: justify\"><em>Cerebrospinal Fluid Circulation<\/em><\/p>\n<p style=\"text-align: justify\">The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2514\">choroid plexuses<\/a> are found in all four <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2513\">ventricles<\/a>. Observed in dissection, they appear as soft, fuzzy structures that may still be pink, depending on how well the circulatory system is cleared in preparation of the tissue. The CSF is produced from components extracted from the blood, so its flow out of the ventricles is tied to the pulse of cardiovascular circulation.<\/p>\n<p style=\"text-align: justify\">From the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2515\">lateral ventricles<\/a>, the CSF flows into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2481\">third ventricle<\/a>, where more CSF is produced, and then through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2491\">cerebral aqueduct<\/a> into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2516\">fourth ventricle<\/a> where even more CSF is produced. A very small amount of CSF is filtered at any one of the plexuses, for a total of about 500 millilitres daily, but it is continuously made and pulses through the ventricular system, keeping the fluid moving. From the fourth ventricle, CSF can continue down the central canal of the spinal cord, but this is essentially a cul-de-sac, so more of the fluid leaves the ventricular system and moves into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2517\">subarachnoid space<\/a> through the median and lateral apertures.<\/p>\n<p style=\"text-align: justify\">Within the subarachnoid space, the cerebrospinal fluid flows around all of the central nervous system, providing two important functions. As with elsewhere in its circulation, the cerebrospinal fluid picks up metabolic wastes from the nervous tissue and moves it out of the central nervous system. It also acts as a liquid cushion for the brain and spinal cord. By surrounding the entire system in the subarachnoid space, it provides a thin buffer around the organs within the strong, protective <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2497\">dura mater<\/a>. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2519\">arachnoid granulations<\/a> are outpocketings of the arachnoid membrane into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2520\">dural sinuses<\/a> so that cerebrospinal fluid can be reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head and neck through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2521\">jugular veins<\/a>, along with the rest of the circulation for blood, to be re-oxygenated by the lungs and wastes to be filtered out by the kidneys (Table 3).<\/p>\n<table style=\"border-collapse: collapse;width: 0%\">\n<caption>Table 3: Components of Cerebrospinal Fluid Circulation<\/caption>\n<tbody>\n<tr>\n<td style=\"width: 11.4609%\"><\/td>\n<th style=\"width: 12.4494%\" scope=\"col\"><strong>Lateral ventricles<\/strong><\/th>\n<th style=\"width: 16.6869%\" scope=\"col\"><strong>Third ventricle<\/strong><\/th>\n<th style=\"width: 12.3083%\" scope=\"col\"><strong>Cerebral aqueduct<\/strong><\/th>\n<th style=\"width: 24.1727%\" scope=\"col\"><strong>Fourth ventricle<\/strong><\/th>\n<th style=\"width: 8.63598%\" scope=\"col\"><strong>Central canal<\/strong><\/th>\n<th style=\"width: 14.2857%\" scope=\"col\"><strong>Subarachnoid space<\/strong><\/th>\n<\/tr>\n<tr>\n<th style=\"width: 11.4609%\" scope=\"row\"><strong>Location<\/strong><\/th>\n<td style=\"width: 12.4494%\">Cerebrum<\/td>\n<td style=\"width: 16.6869%\">Diencephalon<\/td>\n<td style=\"width: 12.3083%\">Midbrain<\/td>\n<td style=\"width: 24.1727%\">Between pons\/upper medulla oblongata and cerebellum<\/td>\n<td style=\"width: 8.63598%\">Spinal cord<\/td>\n<td style=\"width: 14.2857%\">External to entire central nervous system<\/td>\n<\/tr>\n<tr>\n<th style=\"width: 11.4609%\" scope=\"row\"><strong>Blood vessel structure<\/strong><\/th>\n<td style=\"width: 12.4494%\">Choroid plexus<\/td>\n<td style=\"width: 16.6869%\">Choroid plexus<\/td>\n<td style=\"width: 12.3083%\">None<\/td>\n<td style=\"width: 24.1727%\">Choroid plexus<\/td>\n<td style=\"width: 8.63598%\">None<\/td>\n<td style=\"width: 14.2857%\">Arachnoid granulations<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h2 style=\"text-align: left\"><strong><a id=\"9.4\"><\/a>Part 4<\/strong><strong>:<\/strong><strong> The Peripheral Nervous System<\/strong><\/h2>\n<p style=\"text-align: justify\">The peripheral nervous system is not as contained as the central nervous system because it is defined as everything that is not the central nervous system. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the peripheral nervous system, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2459\">enteric nervous system<\/a> and are a special subset of the peripheral nervous system.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.4a\"><\/a>Ganglia<\/strong><\/h5>\n<p style=\"text-align: justify\">A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2452\">ganglion<\/a> is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2523\"><strong>dorsal <\/strong><strong>root ganglion<\/strong><\/a>. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the central nervous system through the dorsal nerve root.<\/p>\n<p style=\"text-align: justify\">The other major category of ganglia, those of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\">autonomic nervous system<\/a>, will be examined later in this chapter.<\/p>\n<figure style=\"width: 631px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image28-2.png\" alt=\"image\" width=\"631\" height=\"353\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 22. Dorsal Root Ganglion.<\/strong> The cell bodies of sensory neurons, which are unipolar neurons by shape, are seen in this photomicrograph. Also, the fibrous region is composed of the axons of these neurons that are passing through the ganglion to be part of the dorsal nerve root (tissue source: canine). LM \u00d7 40. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/figcaption><\/figure>\n<p>&nbsp;<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.4b\"><\/a>Nerves<\/strong><\/h5>\n<p style=\"text-align: justify\">Bundles of axons in the peripheral nervous system are referred to as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2453\">nerves<\/a>. These structures in the periphery are different than the central counterpart, called a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2454\">tract<\/a>. Nerves are composed of more than just nervous tissue. They have <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2194\">connective tissues<\/a> invested in their structure, as well as blood vessels supplying the tissues with nourishment. Nerves are associated with the region of the central nervous system to which they are connected, either as cranial nerves (12 pairs) connected to the brain or spinal nerves (31 pairs) connected to the spinal cord.<\/p>\n<p style=\"text-align: justify\">The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2524\">cranial nerves<\/a> are primarily responsible for the sensory and motor functions of the head and neck, although one of these nerves, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2525\">vagus<\/a>, targets organs in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2393\">thoracic<\/a>\u00a0and abdominal cavities as part of the parasympathetic nervous system. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">axons<\/a> in these nerves originate out of sensory ganglia external to the cranium or motor <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2451\">nuclei<\/a> within the brain stem.<\/p>\n<p style=\"text-align: justify\">All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibres, both <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2455\">somatic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\">autonomic<\/a>, emerge as the ventral nerve root. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2523\">dorsal root ganglion<\/a> for each nerve is an enlargement of the spinal nerve.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.4c\"><\/a>The Somatic Ne<\/strong><strong>rvous System<\/strong><\/h5>\n<p style=\"text-align: justify\">The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2455\">somatic nervous system<\/a> is traditionally considered a division within the peripheral nervous system. However, this misses an important point: somatic refers to a functional division, whereas peripheral refers to an anatomic division. The somatic nervous system is responsible for our conscious perception of the environment and for our <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2280\">voluntary<\/a> responses to that perception by means of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2334\">skeletal muscles<\/a>. Peripheral sensory neurons receive input from environmental stimuli, but the neurons that produce motor responses originate in the central nervous system. The distinction between the structures of the peripheral and central nervous systems and the functions of the somatic and autonomic systems can most easily be demonstrated through a simple <strong>reflex<\/strong>, an automatic response that the nervous system produces in response to specific stimuli. The neurons and neural pathways responsible for a reflex action constitute the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2526\">reflex arc<\/a><\/strong>. One of the simplest reflex acts is the <strong>stretch reflex<\/strong><strong>,<\/strong> by which the nervous system responds to the stretching of a muscle (the stimulus) with contraction of that same muscle (the response). This response protects the muscle from over-stretching, but more importantly, it has a crucial role in maintaining posture and balance. The <strong>patellar reflex<\/strong> (or knee-jerk reflex) is an example of stretch reflex and it occurs through the following steps (Figure 23):<\/p>\n<ul>\n<li>Tapping of the patellar tendon with a hammer causes the stretching of muscle fibres in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2527\">quadriceps<\/a> muscle, which stimulates sensory neurons innervating those fibres.<\/li>\n<li>In the sensory neuron, a nerve impulse (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2341\">action potential<\/a>) is generated, which travels along the sensory nerve fibre from the muscle, through the dorsal root ganglion, to the spinal cord.<\/li>\n<li>The sensory neuron stimulates a motor neuron in the ventral horn of the spinal cord.<\/li>\n<li>That motor neuron sends a nerve impulse (action potential) along its <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">axon<\/a>.<\/li>\n<li>This impulse reaches the quadriceps muscle, causing its contraction and the extension of the leg (a kick).<\/li>\n<\/ul>\n<p style=\"text-align: justify\">The sensory neuron can also activate an interneuron (e.g., Figure 23), which inhibits the motor neuron responsible for the contraction of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2528\">antagonistic<\/a> muscle to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2527\">quadriceps<\/a> (i.e. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_3028\">hamstring<\/a>).<\/p>\n<figure style=\"width: 562px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image29-2.png\" alt=\"image\" width=\"562\" height=\"376\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 23. The Patellar Reflex.<\/strong> The stimulus (stretching of the quadriceps muscle caused by tapping on the tendon) triggers a nerve impulse in a sensory neuron, which synapses with and stimulated a motor neuron, leading to the contraction of the quadriceps. (credit: www.backyardbrains.com\/experiments\/Musclekneejerk, protected under Creative Commons License)<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Another example of a simple spinal reflex is the <strong>withdrawal reflex<\/strong><strong>,<\/strong> which occurs, for example, when you touch a hot stove and pull your hand away. This reflex occurs through a similar sequence of steps:<\/p>\n<ul>\n<li style=\"text-align: justify\">Sensory receptors in the skin sense extreme temperature and the early signs of tissue damage.<\/li>\n<li style=\"text-align: justify\">In a sensory neuron, a nerve impulse (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2341\">action potential<\/a>) is generated, which travels along the sensory nerve fibre from the skin, through the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2523\">dorsal root ganglion<\/a>, to the spinal cord.<\/li>\n<li style=\"text-align: justify\">The sensory neuron stimulates a motor neuron in the ventral horn motor of the spinal cord.<\/li>\n<li style=\"text-align: justify\">That motor neuron sends a nerve impulse (action potential) along its axon.<\/li>\n<li style=\"text-align: justify\">This impulse reaches the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2530\">biceps brachii<\/a>, causing contraction of the muscle and flexion of the forearm at the elbow to withdraw the hand from the hot stove.<\/li>\n<\/ul>\n<p style=\"text-align: justify\">The basic withdrawal reflex includes sensory input (the painful stimulus), central processing (the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2344\">synapse<\/a> in the spinal cord), and motor output (activation of a ventral motor neuron that causes contraction of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2530\">biceps brachii<\/a>). As seen for the patellar reflex, the withdrawal reflex can also include inhibition of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2528\">antagonistic<\/a> muscle (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2531\">triceps brachii<\/a> in our example). Another possible motor output of the withdrawal reflex is cross extension: counterbalancing movement on the other side of the body by stimulation of the extensor muscles in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2532\">contralateral<\/a> limb.<\/p>\n<p style=\"text-align: justify\">The somatic nervous system also controls voluntary movement and more complex motor functions. For example, reading of this text starts with visual sensory input to the retina, which then projects to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2482\">thalamus<\/a>, and on to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2473\">cerebral cortex<\/a>. A sequence of regions of the cerebral cortex process the visual information, starting in the primary visual cortex of the occipital lobe, and resulting in the conscious perception of these letters. Subsequent cognitive processing results in understanding of the content. As you continue reading, regions of the cerebral cortex in the frontal lobe plan how to move the eyes to follow the lines of text. The output from the cortex causes activity in motor neurons in the brain stem that cause movement of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2533\">extraocular<\/a> muscles through the third, fourth, and sixth cranial nerves. This example also includes sensory input (the retinal projection to the thalamus), central processing (the thalamus and subsequent cortical activity), and motor output (activation of neurons in the brain stem that lead to coordinated contraction of extraocular muscles).<\/p>\n<p style=\"text-align: justify\"><strong>The Autonomic Nervous System<\/strong><\/p>\n<p style=\"text-align: justify\">The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\">autonomic nervous system<\/a> is often associated with the \u201cfight-or-flight response,\u201d which refers to the preparation of the body to either run away from a threat or to stand and fight in the face of that threat. To suggest what this means, consider the (very unlikely) situation of seeing a lioness hunting out on the savannah. Though this is not a common threat that humans deal with in the modern world, it represents the type of environment in which the human species thrived and adapted. The spread of humans around the world to the present state of the modern age occurred much more quickly than any species would adapt to environmental pressures such as predators. However, the reactions modern humans have in the modern world are based on these prehistoric situations. If your boss is walking down the hallway on Friday afternoon looking for \u201cvolunteers\u201d to come in on the weekend, your response is the same as the prehistoric human seeing the lioness running across the savannah: fight or flight.<\/p>\n<p style=\"text-align: justify\">Most likely, your response to your boss\u2014not to mention the lioness\u2014would be flight. Run away! The autonomic system is responsible for the physiological response to make that possible, and hopefully successful. Adrenaline starts to flood your circulatory system. Your heart rate increases. Sweat glands become active. The bronchi of the lungs dilate to allow more air exchange. Pupils dilate to increase visual information. Blood pressure increases in general, and blood vessels dilate in skeletal muscles. Time to run. Similar physiological responses would occur in preparation for fighting off the threat.<\/p>\n<p style=\"text-align: justify\">This response should sound a bit familiar. The autonomic nervous system is tied into emotional responses as well, and the fight-or-flight response probably sounds like a panic attack. In the modern world, these sorts of reactions are associated with anxiety as much as with response to a threat. It is engrained in the nervous system to respond like this. In fact, the adaptations of the autonomic nervous system probably predate the human species and are likely to be common to all mammals, and perhaps shared by many animals. That lioness might herself be threatened in some other situation<\/p>\n<p style=\"text-align: justify\">However, the autonomic nervous system is not just about responding to threats. Besides the fight-or-flight response, there are the responses referred to as \u201crest and digest.\u201d If that lioness is successful in her hunting, then she is going to rest from the exertion. Her heart rate will slow. Breathing will return to normal. The digestive system has a big job to do. Much of the function of the autonomic system is based on the connections within an autonomic, or visceral, reflex.<\/p>\n<p style=\"text-align: justify\">As we have seen, the nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system. The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis.<\/p>\n<p style=\"text-align: justify\">The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. In addition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2507\">sympathetic division<\/a><\/strong> and the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2534\">parasympathetic division<\/a><\/strong>. The sympathetic system is associated with the <strong>fight-or-flight response<\/strong>, and parasympathetic activity is referred to by the epithet of <strong>rest and digest<\/strong>. At each target <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2439\">effector<\/a>, dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. One causes heart rate to increase, whereas the other causes heart rate to decrease.<\/p>\n<p style=\"text-align: justify\"><em>Sympathetic Division of the Autonomic Nervous System<\/em><\/p>\n<p style=\"text-align: justify\">To respond to a threat\u2014to fight or to run away\u2014the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together. Additionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system to a wide array of ganglia that project to the many effector organs simultaneously. The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change.<\/p>\n<p style=\"text-align: justify\">The sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. It is referred to as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2535\">thoracolumbar system<\/a><\/strong> to reflect this anatomical basis. A <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2536\">central neuron<\/a><\/strong> in the lateral horn of any of these spinal regions projects to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2452\">ganglia<\/a> adjacent to the vertebral column through the ventral spinal roots. The majority of ganglia of the sympathetic system belong to a network of <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2537\">sympathetic chain ganglia<\/a><\/strong> that runs alongside the vertebral column. The ganglia appear as a series of clusters of neurons linked by axonal bridges. A diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices (Figure 24, wherein the \u201ccircuits\u201d of the sympathetic system are intentionally simplified).<\/p>\n<p style=\"text-align: justify\">An axon from the central neuron that projects to a sympathetic ganglion is referred to as a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2538\">preganglionic fibre<\/a> <\/strong>or neuron, and represents the output from the central nervous system to the ganglion. Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibres are relatively short, and they are myelinated. A <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2539\">postganglionic fibre<\/a><\/strong>\u2014the axon from a ganglionic neuron that projects to the target effector\u2014represents the output of a ganglion that directly influences the organ. Compared with the preganglionic fibres, postganglionic sympathetic fibres are long because of the relatively greater distance from the ganglion to the target <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2439\">effector<\/a>. These fibres are unmyelinated. (Note that the term \u201cpostganglionic neuron\u201d may be used to describe the projection from a ganglion to the target. The problem with that usage is that the cell body is in the ganglion, and only the fibre is postganglionic. Typically, the term neuron applies to the entire cell.)<\/p>\n<p style=\"text-align: justify\">One type of preganglionic sympathetic fibre does not terminate in a ganglion. These are the axons from central sympathetic neurons that project to the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2540\">adrenal medulla<\/a><\/strong>, the interior portion of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2541\">adrenal gland<\/a>. These axons are still referred to as preganglionic fibres, but the target is not a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2452\">ganglion<\/a>. The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures.<\/p>\n<figure style=\"width: 826px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image30-2.png\" alt=\"image\" width=\"826\" height=\"1220\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 24. The Sympathetic Division of the Autonomic Nervous System.<\/strong> Neurons from the lateral horn of the spinal cord (preganglionic nerve fibers &#8211; solid lines)) project to the chain ganglia on either side of the vertebral column or to collateral (prevertebral) ganglia that are anterior to the vertebral column in the abdominal cavity. Axons from these ganglionic neurons (postganglionic nerve fibers &#8211; dotted lines) then project to target effectors throughout the body. (The names of specific ganglia and nerves, as well as their target organs, are not examinable material in this course.)<\/figcaption><\/figure>\n<p style=\"text-align: justify\">The projections of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2507\">sympathetic division<\/a> of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\">autonomic nervous system<\/a> diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease. Sweat gland secretion should also increase as part of an integrated response. All of those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons\u2014a single preganglionic sympathetic neuron may have 10\u201320 targets. An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets. At the level of the spinal cord at which the preganglionic sympathetic fibre exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion. Some branches will extend up or down to a different level of the chain ganglia. Other branches will pass through the chain ganglia and project through one of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2542\">splanchnic nerves<\/a> to a collateral ganglion. Finally, some branches may project through the splanchnic nerves to the adrenal medulla. All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs.<\/p>\n<p style=\"text-align: justify\"><em>Parasympathetic Division of the Autonomic Nervous System<\/em><\/p>\n<p>When not responding to an immediate threat, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2534\">parasympathetic system<\/a> is generally more active than the sympathetic system.\u00a0 Many of the same effectors in the body are innervated by both divisions of the autonomic nervous system, but activation of each division tends to have opposing effects.\u00a0 Sympathetic system activation tends to increase activity in the respiratory, cardiovascular, and musculoskeletal systems while reducing activity in the digestive system.\u00a0 Parasympathetic system activation on the other hand tends to <em>decrease<\/em> activity in the respiratory, cardiovascular, and musculoskeletal systems while <em>increasing<\/em> activity in the digestive, urinary, and reproductive systems.\u00a0 Generally speaking, the activity of the many organs that receive input from both systems is dependent on whether neurons of the parasympathetic or sympathetic system are releasing more of their <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2172\">neurotransmitter<\/a> onto each organ at a given time.<\/p>\n<p style=\"text-align: justify\">The parasympathetic division of the autonomic nervous system is named because its central neurons are located on either side of the thoracolumbar region of the spinal cord (para- = \u201cbeside\u201d or \u201cnear\u201d). The parasympathetic system can also be referred to as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2543\">craniosacral system<\/a><\/strong> (or outflow) because the preganglionic neurons are located in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2451\">nuclei<\/a> of the brain stem and the lateral horn of the sacral spinal cord.<\/p>\n<p style=\"text-align: justify\">The connections, or \u201ccircuits,\u201d of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences (Figure 25). The preganglionic fibres from the cranial region travel in cranial nerves, whereas <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2538\">preganglionic fibres<\/a> from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia, which are located near &#8211; or even within &#8211; the target organ. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2539\">postganglionic fibre<\/a> projects from the terminal ganglia a short distance to the effector. These <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2452\">ganglia<\/a> are often referred to as intramural ganglia when they are found within the walls target effector, or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibres are long and the postganglionic fibres are short because the ganglia are close to &#8211; and sometimes within &#8211; the target effectors.<\/p>\n<figure style=\"width: 158px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image31-2.png\" alt=\"image\" width=\"158\" height=\"156\" \/><figcaption class=\"wp-caption-text\">Watch <a href=\"https:\/\/youtu.be\/71pCilo8k4M\">this Crash Course video<\/a> for an overview of the autonomic nervous system! (Direct link: <a href=\"https:\/\/youtu.be\/71pCilo8k4M\">https:\/\/youtu.be\/71pCilo8k4M<\/a>)<\/figcaption><\/figure>\n<figure style=\"width: 826px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image32-2.png\" alt=\"image\" width=\"826\" height=\"1228\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 25. The Parasympathetic Division of the Autonomic Nervous System.<\/strong> Neurons from brain-stem nuclei, or from the lateral horn of the sacral spinal cord, project to terminal ganglia near or within the various organs of the body. Axons from these ganglionic neurons then project the short distance to those target effectors. (The names of specific ganglia and nerves, as well as their target organs, are not examinable material in this course.)<\/figcaption><\/figure>\n<p><em>\u00a0<\/em><\/p>\n<p style=\"text-align: justify\"><em>Chemical Signaling i<\/em><em>n the Autonomic Nervous System<\/em><\/p>\n<p style=\"text-align: justify\">Where an autonomic neuron connects with a target, there is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2344\">synapse<\/a>. The electrical signal of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2341\">action potential<\/a> causes the release of a signaling molecule, which will bind to <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2270\">receptor<\/a> proteins on the target cell. Synapses of the autonomic system are classified as either <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2544\">cholinergic<\/a><\/strong>, meaning that <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2478\">acetylcholine<\/a> (<\/strong><strong>ACh<\/strong><strong>)<\/strong> is released, or <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2545\">adrenergic<\/a><\/strong>, meaning that <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2546\">norepinephrine<\/a><\/strong> is released. The terms cholinergic and adrenergic refer not only to the signaling molecule that is released but also to the class of receptors that each binds.<\/p>\n<p style=\"text-align: justify\">The term adrenergic should remind you of the word adrenaline, which is associated with the fight-or-flight response described at the beginning of the chapter. Adrenaline and epinephrine are two names for the same molecule. The adrenal gland (in Latin, ad- = \u201con top of\u201d; renal = \u201ckidney\u201d) secretes adrenaline. The ending \u201c-ine\u201d refers to the chemical being derived, or extracted, from the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2541\">adrenal gland<\/a>. A similar construction from Greek instead of Latin results in the word <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2547\">epinephrine<\/a> (epi- = \u201cabove\u201d; nephr- = \u201ckidney\u201d). In scientific usage, epinephrine is preferred in the United States, whereas adrenaline is preferred in Great Britain, because \u201cadrenalin\u201d was once a registered, proprietary drug name in the United States. Though the drug is no longer sold, the convention of referring to this molecule by the two different names persists. Similarly, norepinephrine and noradrenaline are two names for the same molecule.<\/p>\n<p style=\"text-align: justify\">All <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2538\">preganglionic fibres<\/a>, both <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2507\">sympathetic<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2534\">parasympathetic<\/a>, release ACh. The postganglionic parasympathetic fibres also release ACh. Postganglionic sympathetic fibers release norepinephrine, except for fibers that project to sweat glands and to blood vessels associated with skeletal muscles, which release ACh.<\/p>\n<p style=\"text-align: justify\">Signaling molecules can belong to two broad groups. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2172\">Neurotransmitters<\/a> are released at synapses, whereas hormones are released into the bloodstream. These are simplistic definitions, but they can help to clarify this point. <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2478\">Acetylcholine<\/a> can be considered a neurotransmitter because it is released by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">axons<\/a> at synapses. The adrenergic system, however, presents a challenge. Postganglionic sympathetic fibres release <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2546\">norepinephrine<\/a>, which can be considered a neurotransmitter. But the adrenal medulla releases epinephrine and norepinephrine into circulation, so they should be considered <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2171\">hormones<\/a>.<\/p>\n<h2 style=\"text-align: left\"><strong><a id=\"9.5\"><\/a>Part <\/strong><strong>5<\/strong><strong>: Neuronal <\/strong><strong>Signalling<\/strong><\/h2>\n<p style=\"text-align: justify\">Having looked at the components of nervous tissue, and the basic anatomy of the nervous system, next comes an understanding of how nervous tissue is capable of communicating within the nervous system. Before getting to the nuts and bolts of how this works, an illustration of how the components come together will be helpful (summarized in Figure 26).<\/p>\n<figure style=\"width: 975px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image33-2.png\" alt=\"image\" width=\"975\" height=\"681\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 26. Testing the Water.<\/strong> (1) The sensory neuron has endings in the skin that sense a stimulus such as water temperature. The strength of the signal that starts here is dependent on the strength of the stimulus. (2) The graded potential from the sensory endings, if strong enough, will initiate an action potential at the initial segment of the axon (which is immediately adjacent to the sensory endings in the skin). (3) The axon of the peripheral sensory neuron enters the spinal cord and contacts another neuron in the gray matter. The contact is a synapse where another graded potential is caused by the release of a chemical signal from the axon terminals. (4) An action potential is initiated at the initial segment of this neuron and travels up the sensory pathway to a region of the brain called the thalamus. Another synapse passes the information along to the next neuron. (5) The sensory pathway ends when the signal reaches the cerebral cortex. (6) After integration with neurons in other parts of the cerebral cortex, a motor command is sent from the precentral gyrus of the frontal cortex. (7) The upper motor neuron sends an action potential down to the spinal cord. The target of the upper motor neuron is the dendrites of the lower motor neuron in the gray matter of the spinal cord. (8) The axon of the lower motor neuron emerges from the spinal cord in a nerve and connects to a muscle through a neuromuscular junction to cause contraction of the target muscle.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Imagine you are about to take a shower. You have turned on the faucet to start the water as you prepare to get in the shower. After a few minutes, you expect the water to be a temperature that will be comfortable to enter. So you put your hand out into the spray of water. What happens next depends on how your nervous system interacts with the stimulus of the water temperature and what you do in response to that stimulus.<\/p>\n<figure style=\"width: 577px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image34-2.png\" alt=\"image\" width=\"577\" height=\"316\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 27. The Sensory Input.<\/strong> Receptors in the skin sense the temperature of the water.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Found in the skin of your fingers or toes is a type of sensory receptor that is sensitive to temperature, called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2548\">thermoreceptor<\/a><\/strong>. When you place your hand under the shower (Figure 27), the cell membrane of the thermoreceptors changes its electrical state (voltage). The amount of change is dependent on the strength of the stimulus (how hot the water is). This is called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2549\">graded potential<\/a><\/strong>. If the stimulus is strong, the voltage of the cell membrane will change enough to generate an electrical signal that will travel down the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2345\">axon<\/a>.<\/p>\n<p style=\"text-align: justify\">The voltage at which such a signal is generated is called the <strong>threshold<\/strong>, and the resulting electrical signal is called an <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2341\">action potential<\/a><\/strong>. In this example, the action potential travels\u2014a process known as <strong>propagation<\/strong>\u2014along the axon from the axon hillock to the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2463\">axon terminals<\/a> and into the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2464\">synaptic end bulbs<\/a>. When this signal reaches the end bulbs, it causes the release of a signaling molecule called a <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2172\">neurotransmitter<\/a><\/strong>.<\/p>\n<p style=\"text-align: justify\">The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2172\">neurotransmitter<\/a> diffuses across the short distance of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2344\">synapse<\/a> and binds to a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2270\">receptor<\/a> protein of the target neuron. When the molecular signal binds to the receptor, the cell membrane of the target neuron changes its electrical state and a new graded potential begins. If that graded potential is strong enough to reach threshold, the second neuron generates an action potential at its axon hillock. The target of this neuron is another neuron in the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2482\">thalamus<\/a><\/strong> of the brain, the part of the central nervous system that acts as a relay for sensory information. At another synapse, neurotransmitter is released and binds to its receptor. The thalamus then sends the sensory information to the <strong>cerebral cortex<\/strong>, the outermost layer of gray matter in the brain, where conscious perception of that water temperature begins. Within the cerebral cortex, information is processed among many neurons, integrating the stimulus of the water temperature with other sensory stimuli, with your emotional state (you just aren\u2019t ready to wake up; the bed is calling to you), memories (perhaps of the lab notes you have to study before a quiz). Finally, a plan is developed about what to do, whether that is to turn the temperature up, turn the whole shower off and go back to bed, or step into the shower. To do any of these things, the cerebral cortex has to send a command out to your body to move muscles (Figure 28).<\/p>\n<figure style=\"width: 580px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image35-3.png\" alt=\"image\" width=\"580\" height=\"463\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 28. The Motor Response.<\/strong> On the basis of the sensory input and the integration in the central nervous system, a motor response is formulated and executed.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">A region of the cortex is specialized for sending signals down to the spinal cord for movement. The <strong>upper motor neuron<\/strong> is in this region, called the primary <strong>motor cortex<\/strong>, which has an axon that extends all the way down the spinal cord. At the level of the spinal cord at which this axon makes a synapse, a graded potential occurs in the cell membrane of a <strong>lower motor neuron<\/strong>. This second motor neuron is responsible for causing muscle fibres to contract. In the manner described in the chapter on muscle tissue, an action potential travels along the motor neuron axon into the periphery. The axon terminates on muscle fibers at the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2551\">neuromuscular junction<\/a>. Acetylcholine is released at this specialized synapse, which causes the muscle action potential to begin, following a large potential known as an end plate potential. When the lower motor neuron excites the muscle fiber, it contracts. All of this occurs in a fraction of a second, but this story is the basis of how the nervous system functions.<\/p>\n<h5 style=\"text-align: justify\"><strong>Ion Channels and the Resting Membrane<\/strong><strong> Potential<\/strong><\/h5>\n<p style=\"text-align: justify\">The functions of the nervous system\u2014sensation, integration, and response\u2014depend on the functions of the neurons underlying these pathways. To understand how neurons are able to communicate, it is necessary to describe the role of an <strong>excitable membrane<\/strong> in generating these signals. The basis of this communication is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2341\">action potential<\/a>, which demonstrates how changes in the membrane can constitute a signal. (The way these signals work in more variable circumstances involves graded potentials.)<\/p>\n<figure style=\"width: 975px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image36-2.png\" alt=\"image\" width=\"975\" height=\"417\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 29. Cell Membrane and Transmembrane Proteins.<\/strong> The cell membrane is composed of a phospholipid bilayer and has many transmembrane proteins, including different types of channel proteins that serve as ion channels.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Most cells in the body make use of charged particles, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2093\">ions<\/a>, to build up a charge across the cell membrane. Cells make use of the cell membrane to regulate ion movement between the extracellular fluid and cytosol. As you learned in the chapter on cells, the cell membrane is primarily responsible for regulating what can cross the membrane and what stays on only one side. The cell membrane is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2166\">phospholipid<\/a> bilayer, so only substances that can pass directly through the hydrophobic core can diffuse through unaided. Charged particles, which are <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2178\">hydrophilic<\/a> by definition, cannot pass through the cell membrane without assistance (Figure 29). Transmembrane proteins, specifically <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2250\">channel proteins<\/a>, make this possible. Several passive ion channels, as well as <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2254\">active transport<\/a> pumps, are necessary to generate a transmembrane potential and an action potential. Ion channels are pores that allow specific charged particles to cross the membrane in response to an existing <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2255\">concentration gradient<\/a>.<\/p>\n<p style=\"text-align: justify\">Of special interest is the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2259\">carrier protein<\/a> referred to as the sodium\/potassium pump that moves sodium ions (Na<sup>+<\/sup>) out of a cell and potassium ions (K<sup>+<\/sup>) into a cell, thus regulating ion concentration on both sides of the cell membrane. The sodium\/potassium pump requires energy in the form of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2074\">adenosine triphosphate<\/a> (ATP), so it is also referred to as an ATPase. As was explained in the cell chapter, the concentration of Na<sup>+<\/sup> is higher outside the cell than inside, and the concentration of K<sup>+<\/sup> is higher inside the cell than outside. That means that this pump is moving the ions against the concentration gradients for sodium and potassium, which is why it requires energy. In fact, the pump basically maintains those concentration gradients.<\/p>\n<p style=\"text-align: justify\">Ion channels do not always freely allow ions to diffuse across the membrane. Some are opened by certain events, meaning the channels are <strong>gated<\/strong>.<\/p>\n<p style=\"text-align: justify\">A <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2552\">ligand-gated channel<\/a><\/strong> opens because a signaling molecule, a ligand, binds to the extracellular region of the channel. This type of channel is also known as an ionotropic receptor because when the ligand, known as a neurotransmitter in the nervous system, binds to the protein, ions cross the membrane changing its charge (Figure 30).<\/p>\n<figure style=\"width: 985px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image37-2.png\" alt=\"image\" width=\"985\" height=\"509\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 30. Ligand-Gated Channels.<\/strong> When the ligand, in this case the neurotransmitter acetylcholine, binds to a specific location on the extracellular surface of the channel protein, the pore opens to allow select ions through. The ions, in this case, are cations of sodium, calcium, and potassium.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">A <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2553\">mechanically gated channel<\/a><\/strong> opens because of a physical distortion of the cell membrane. Many channels associated with the sense of touch (<a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2554\">somatosensation<\/a>) are mechanically gated. For example, as pressure is applied to the skin, these channels open and allow ions to enter the cell. Similar to this type of channel would be the channel that opens on the basis of temperature changes, as in testing the water in the shower (Figure 31).<\/p>\n<figure style=\"width: 991px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image38-3.png\" alt=\"image\" width=\"991\" height=\"480\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 31. Mechanically Gated Channels.<\/strong> When a mechanical change occurs in the surrounding tissue, such as pressure or touch, the channel is physically opened. Thermoreceptors work on a similar principle. When the local tissue temperature changes, the protein reacts by physically opening the channel.<\/figcaption><\/figure>\n<figure style=\"width: 975px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image39-2.png\" alt=\"image\" width=\"975\" height=\"475\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 32. Voltage-Gated Channels.<\/strong> Voltage-gated channels open when the transmembrane voltage changes around them. Amino acids in the structure of the protein are sensitive to charge and cause the pore to open to the selected ion.<\/figcaption><\/figure>\n<p>A <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2555\">voltage-gated channel<\/a><\/strong> is a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Normally, the inner portion of the membrane is at a negative voltage. When that voltage becomes less negative, the channel begins to allow <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2093\">ions<\/a> to cross the membrane (Figure 32).<\/p>\n<p style=\"text-align: justify\">A <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2556\">leakage channel<\/a><\/strong> is randomly gated, meaning that it opens and closes at random, hence the reference to leaking. There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states. Leakage channels contribute to the resting transmembrane voltage of the excitable membrane (Figure 33).<\/p>\n<figure style=\"width: 985px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image40-2.png\" alt=\"image\" width=\"985\" height=\"461\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 33. Leakage Channels.<\/strong> In certain situations, ions need to move across the membrane randomly. The particular electrical properties of certain cells are modified by the presence of this type of channel.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">The electrical state of the cell membrane can have several variations. These are all variations in the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2557\">membrane potential<\/a><\/strong>. A potential is a distribution of charge across the cell membrane, measured in millivolts (mV). The standard is to compare the inside of the cell relative to the outside, so the membrane potential is a value representing the charge on the intracellular side of the membrane based on the outside being zero, relatively speaking (Figure 34).<\/p>\n<figure style=\"width: 985px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image41-2.png\" alt=\"image\" width=\"985\" height=\"463\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 34. Measuring Charge across a Membrane with a Voltmeter.<\/strong> A recording electrode is inserted into the cell and a reference electrode is outside the cell. By comparing the charge measured by these two electrodes, the transmembrane voltage is determined. It is conventional to express that value for the cytosol relative to the outside.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">The concentration of ions in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2127\">extracellular<\/a> and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2126\">intracellular fluids<\/a> is largely balanced, with a net neutral charge. However, a slight difference in charge occurs right at the membrane surface, both internally and externally. It is the difference in this very limited region that has all the power in neurons (and muscle cells) to generate electrical signals, including action potentials.<\/p>\n<p style=\"text-align: justify\">Before these electrical signals can be described, the resting state of the membrane must be explained. When the cell is at rest, and the ion channels are closed (except for leakage channels which randomly open), ions are distributed across the membrane in a very predictable way. The concentration of Na<sup>+<\/sup> outside the cell is 10 times greater than the concentration inside. Also, the concentration of K<sup>+<\/sup> inside the cell is greater than outside. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2130\">cytosol<\/a> contains a high concentration of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2095\">anions<\/a>, in the form of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2150\">phosphate<\/a> ions and negatively charged proteins. Large anions are a component of the inner cell membrane, including specialized <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2166\">phospholipids<\/a> and proteins associated with the inner leaflet of the membrane (leaflet is a term used for one side of the lipid bilayer membrane). The negative charge is localized in the large anions.<\/p>\n<p>With the ions distributed across the membrane at these concentrations, the difference in charge is measured at -70 mV, the value described as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2558\">resting membrane potential<\/a><\/strong>. The exact value measured for the resting membrane potential varies between cells, but -70 mV is the most commonly recorded value. This voltage would actually be much lower except for the contributions of some important proteins in the membrane. Leakage channels K<sup>+<\/sup> channels allow K<sup>+<\/sup> to slowly move out of the cells. To a much lesser extent, leakage Na<sup>+<\/sup> channels allow Na<sup>+<\/sup> to slowly move into the cell. The constant activity of the Na<sup>+<\/sup>\/K<sup>+<\/sup> pump maintains the ion gradients. This may appear to be a waste of energy, but each has a role in maintaining the membrane potential.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.5b\"><\/a>Generation of an<\/strong><strong> Action Potential<\/strong><\/h5>\n<p style=\"text-align: justify\">Resting membrane potential describes the steady state of the cell, which is a dynamic process that is balanced by ion leakage and ion pumping. Without any outside influence, it will not change. To get an electrical signal started, the membrane potential has to change.<\/p>\n<p style=\"text-align: justify\">This starts with a channel opening for Na<sup>+<\/sup> in the membrane. Because the concentration of Na<sup>+<\/sup> is higher outside the cell than inside the cell by a factor of 10, ions will rush into the cell that are driven largely by the concentration gradient. Because sodium is a positively charged ion, it will change the relative voltage immediately inside the cell relative to immediately outside. The resting potential is the state of the membrane at a voltage of -70 mV, so the sodium cation entering the cell will cause it to become less negative. This is known as <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2559\">depolarization<\/a><\/strong>, meaning the membrane potential moves toward zero.<\/p>\n<p style=\"text-align: justify\">The concentration gradient for Na<sup>+<\/sup> is so strong that it will continue to enter the cell even after the membrane potential has become zero, so that the voltage immediately around the pore begins to become positive. The electrical gradient also plays a role, as negative proteins below the membrane attract the sodium ion. The membrane potential will reach +30 mV by the time sodium has entered the cell.<\/p>\n<p style=\"text-align: justify\">As the membrane potential reaches +30 mV, other <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2555\">voltage-gated channels<\/a> are opening in the membrane. These channels are specific for the potassium ion. A concentration gradient acts on K<sup>+<\/sup>, as well. As K<sup>+<\/sup> starts to leave the cell, taking a positive charge with it, the membrane potential begins to move back toward its resting voltage. This is called <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2560\">repolarization<\/a><\/strong>, meaning that the membrane voltage moves back toward the -70 mV value of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2558\">resting membrane potential<\/a>.<\/p>\n<p style=\"text-align: justify\">Repolarization returns the membrane potential to the -70 mV value that indicates the resting potential, but it actually overshoots that value. Potassium ions reach equilibrium when the membrane voltage is below -70 mV, so a period of <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2561\">hyperpolarization<\/a><\/strong> occurs while the K<sup>+<\/sup> channels are open. Those K<sup>+<\/sup> channels are slightly delayed in closing, accounting for this short overshoot.<\/p>\n<figure style=\"width: 682px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image42-2.png\" alt=\"image\" width=\"682\" height=\"427\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 35. Graph of Action Potential.<\/strong> Plotting voltage measured across the cell membrane against time, the action potential begins with depolarization, followed by repolarization, which goes past the resting potential into hyperpolarization, and finally the membrane returns to rest.<\/figcaption><\/figure>\n<figure style=\"width: 237px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image43-2.png\" alt=\"image\" width=\"237\" height=\"231\" \/><figcaption class=\"wp-caption-text\">Watch <a href=\"https:\/\/youtu.be\/OZG8M_ldA1M\">this Crash Course video<\/a> to learn more about the action potential! Direct link: <a href=\"https:\/\/youtu.be\/OZG8M_ldA1M\">https:\/\/youtu.be\/OZG8M_ldA1M<\/a><\/figcaption><\/figure>\n<p style=\"text-align: justify\">What has been described here is the action potential, which is presented as a graph of voltage over time (Figure 35). It is the electrical signal that nervous tissue generates for communication. The change in the membrane voltage from -70 mV at rest to +30 mV at the end of depolarization is a 100-mV change. That can also be written as a 0.1-V change. To put that value in perspective, think about a battery. An AA battery that you might find in a television remote has a voltage of 1.5 V, or a 9-V battery (the rectangular battery with two posts on one end) is, obviously, 9 V. The change seen in the action potential is one or two orders of magnitude less than the charge in these batteries. In fact, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2557\">membrane potential<\/a> can be described as a battery. A charge is stored across the membrane that can be released under the correct conditions. A battery in your remote has stored a charge that is \u201creleased\u201d when you push a button.<\/p>\n<p style=\"text-align: justify\">The question is, now, what initiates the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2341\">action potential<\/a>? The description above conveniently glosses over that point. But it is vital to understanding what is happening. The membrane potential will stay at the resting voltage until something changes. The description above just says that a Na<sup>+<\/sup> channel opens. Now, to say \u201ca channel opens\u201d does not mean that one individual transmembrane protein changes. Instead, it means that one kind of channel opens. There are a few different types of channels that allow Na<sup>+<\/sup> to cross the membrane. A <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2552\">ligand-gated<\/a> Na<sup>+<\/sup> channel will open when a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2172\">neurotransmitter<\/a> binds to it and a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2553\">mechanically gated<\/a> Na<sup>+<\/sup> channel will open when a physical stimulus affects a sensory receptor (like pressure applied to the skin compresses a touch receptor). Whether it is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2172\">neurotransmitter<\/a> binding to its receptor protein or a sensory stimulus activating a sensory receptor cell, some stimulus gets the process started. Sodium starts to enter the cell and the membrane becomes less negative.<\/p>\n<figure style=\"width: 631px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image44.png\" alt=\"image\" width=\"631\" height=\"377\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 36. Stages of an Action Potential.<\/strong> Plotting voltage measured across the cell membrane against time, the events of t6he action potential can be related to specific changes in the membrane voltage. (1) At rest, the membrane voltage is -70 mV. (2) The membrane begins to depolarize when an external stimulus is applied. (3) The membrane voltage begins a rapid rise toward +30 mV. (4) The membrane voltage starts to return to a negative value. (5) Repolarization continues past the resting membrane voltage, resulting in hyperpolarization. (6) The membrane voltage returns to the resting value shortly after hyperpolarization.<\/figcaption><\/figure>\n<p style=\"text-align: justify\">A third type of channel that is an important part of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2559\">depolarization<\/a> in the action potential is the voltage-gated Na<sup>+<\/sup> channel. The channels that start depolarizing the membrane because of a stimulus help the cell to depolarize from -70 mV to -55 mV. Once the membrane reaches that voltage, the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2555\">voltage-gated<\/a> Na<sup>+<\/sup> channels open. This is what is known as the threshold. Any depolarization that does not change the membrane potential to -55 mV or higher will not reach threshold and thus will not result in an action potential. Also, any stimulus that depolarizes the membrane to -55 mV or beyond will cause a large number of channels to open and an action potential will be initiated.<\/p>\n<p style=\"text-align: justify\">Because of the threshold, the action potential can be likened to a digital event\u2014it either happens or it does not. If the threshold is not reached, then no action potential occurs. If depolarization reaches -55 mV, then the action potential continues and runs all the way to +30 mV, at which K<sup>+<\/sup> causes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2560\">repolarization<\/a>, including the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2561\">hyperpolarizing<\/a> overshoot. Also, those changes are the same for every action potential, which means that once the threshold is reached, the exact same thing happens. A stronger stimulus, which might <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2559\">depolarize<\/a> the membrane well past threshold, will not make a \u201cbigger\u201d action potential. Action potentials are \u201call or none.\u201d Either the membrane reaches the threshold and everything occurs as described above, or the membrane does not reach the threshold and nothing else happens. All action potentials peak at the same voltage (+30 mV), so one action potential is not bigger than another. Stronger stimuli will initiate multiple action potentials more quickly, but the individual signals are not bigger. Thus, for example, you will not feel a greater sensation of pain, or have a stronger muscle contraction, because of the size of the action potential because they are not different sizes.<\/p>\n<p style=\"text-align: justify\">As we have seen, the depolarization and repolarization of an action potential are dependent on two types of channels (the voltage-gated Na<sup>+<\/sup> channel and the voltage-gated K<sup>+<\/sup> channel). The voltage-gated Na<sup>+<\/sup> channel actually has two gates. One is the activation gate, which opens when the membrane potential crosses -55 mV. The other gate is the inactivation gate, which closes after a specific period of time\u2014on the order of a fraction of a millisecond. When a cell is at rest, the activation gate is closed and the inactivation gate is open. However, when the threshold is reached, the activation gate opens, allowing Na+ to rush into the cell. Timed with the peak of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2559\">depolarization<\/a>, the inactivation gate closes. During repolarization, no more sodium can enter the cell. When the membrane potential passes -55 mV again, the activation gate closes. After that, the inactivation gate re-opens, making the channel ready to start the whole process over again.<\/p>\n<p style=\"text-align: justify\">The voltage-gated K<sup>+<\/sup> channel has only one gate, which is sensitive to a membrane voltage of -50 mV. However, it does not open as quickly as the voltage-gated Na<sup>+<\/sup> channel does. It might take a fraction of a millisecond for the channel to open once that voltage has been reached. The timing of this coincides exactly with when the Na<sup>+<\/sup> flow peaks, so voltage-gated K<sup>+<\/sup> channels open just as the voltage-gated Na<sup>+<\/sup> channels are being inactivated. As the membrane potential repolarizes and the voltage passes -50 mV again, the channel closes\u2014again, with a little delay. Potassium continues to leave the cell for a short while and the membrane potential becomes more negative, resulting in the hyperpolarizing overshoot. Then the channel closes again and the membrane can return to the resting potential because of the ongoing activity of the non-gated channels and the Na<sup>+<\/sup>\/K<sup>+<\/sup> pump. All of this takes place within approximately 2 milliseconds (Figure 36). While an action potential is in progress, another one cannot be initiated. That effect is referred to as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2562\">refractory period<\/a><\/strong>.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.5c\"><\/a>Propagation of Action Potential<\/strong><strong>s<\/strong><\/h5>\n<figure style=\"width: 891px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image45.png\" alt=\"image\" width=\"891\" height=\"626\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 37. Propagation of an Action Potential Along an Unmyelinated Axon.<\/strong><\/figcaption><\/figure>\n<p style=\"text-align: justify\">The action potential is initiated at the beginning of the axon, at what is called the initial segment. There is a high density of voltage-gated Na<sup>+<\/sup> channels so that rapid depolarization can take place here. Going down the length of the axon, the action potential is propagated because more voltage-gated Na<sup>+<\/sup> channels are opened as the depolarization spreads. This spreading occurs because Na<sup>+<\/sup> enters through the channel and moves along the inside of the cell membrane. As the Na+ moves, or flows, a short distance along the cell membrane, its positive charge depolarizes a little more of the cell membrane. As that depolarization spreads, new voltage-gated Na<sup>+<\/sup> channels open and more ions rush into the cell, spreading the depolarization a little farther (Figure 37).<\/p>\n<p style=\"text-align: justify\">Because voltage-gated Na<sup>+<\/sup> channels are inactivated at the peak of the depolarization, they cannot be opened again for a brief time\u2014the absolute refractory period. Because of this, depolarization spreading back toward previously opened channels has no effect. The action potential must propagate toward the axon terminals; as a result, the polarity of the neuron is maintained, as mentioned above.<\/p>\n<figure style=\"width: 621px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image46.png\" alt=\"image\" width=\"621\" height=\"229\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 38. Propagation of an Action Potential Along a Myelinated Axon.<\/strong> Nodes of Ranvier are gaps in myelin coverage along axons. Nodes contain voltage-gated K<sup>+<\/sup> and Na<sup>+<\/sup> channels. Action potentials travel down the axon by jumping from one node to the next. This diagram shows the nodes of Ranvier and the internodal (myelinated) segments with approximately the same length. This is not accurate: in real axons, the segments with myelin are about one thousand times longer than the nodes!<\/figcaption><\/figure>\n<p style=\"text-align: justify\">Propagation, as described above, applies to unmyelinated axons. When <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2343\">myelination<\/a> is present, the action potential propagates differently (Figure 38). Sodium ions that enter the cell at the initial segment start to spread along the length of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2462\">axon segment<\/a>, but there are no voltage-gated Na<sup>+<\/sup> channels until the first <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2563\">node of Ranvier<\/a>. Because there is not constant opening of these channels along the axon segment, the depolarization spreads at an optimal speed. The distance between nodes (1-3 mm) is the optimal distance to keep the membrane still depolarized above threshold at the next node. As Na<sup>+<\/sup> spreads along the inside of the membrane of the axon segment, the charge starts to dissipate. If the node were any farther down the axon, that depolarization would have fallen off too much for voltage-gated Na<sup>+<\/sup> channels to be activated at the next <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2563\">node of Ranvier<\/a>. If the nodes were any closer together, the speed of propagation would be slower.<\/p>\n<p style=\"text-align: justify\">Propagation along an unmyelinated axon is referred to as <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2565\">continuous conduction<\/a><\/strong>; along the length of a myelinated axon, it is <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2564\"><strong>saltatory<\/strong><strong> conduction<\/strong><\/a>. Continuous conduction is slow because there are always voltage-gated Na<sup>+<\/sup> channels opening, and more and more Na<sup>+<\/sup> is rushing into the cell. Saltatory conduction is faster because the action potential basically jumps from one node to the next (saltare = \u201cto leap\u201d), and the new influx of Na<sup>+<\/sup> renews the depolarized membrane. Along with the myelination of the axon, the diameter of the axon can influence the speed of conduction. Much as water runs faster in a wide river than in a narrow creek, Na<sup>+<\/sup>-based depolarization spreads faster down a wide axon than down a narrow one. This concept is known as resistance and is generally true for electrical wires or plumbing, just as it is true for axons, although the specific conditions are different at the scales of electrons or ions versus water in a river.<\/p>\n<h5 style=\"text-align: justify\"><strong><a id=\"9.5d\"><\/a>Neuro<\/strong><strong>transmission<\/strong><\/h5>\n<p style=\"text-align: justify\">The electrical changes taking place within a neuron, as described in the previous section, are similar to a light switch being turned on. A stimulus starts the depolarization, but the action potential runs on its own once a threshold has been reached. The question is now, \u201cWhat flips the light switch on?\u201d Temporary changes to the cell membrane voltage can result from neurons receiving information from the environment, or from the action of one neuron on another. These special types of potentials influence a neuron and determine whether an action potential will occur or not. Many of these transient signals originate at the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2344\">synapse<\/a><\/strong>, the connection between electrically active cells.<\/p>\n<p style=\"text-align: justify\">There are two types of synapses: chemical synapses and electrical synapses. In a chemical synapse, a chemical signal\u2014namely, a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2172\">neurotransmitter<\/a>\u2014is released from one cell and it affects the other cell. In an electrical synapse, there is a direct connection between the two cells so that ions can pass directly from one cell to the next. If one cell is depolarized in an electrical synapse, the joined cell also depolarizes because the ions pass between the cells. Chemical synapses involve the transmission of chemical information from one cell to the next. This section will concentrate on the chemical type of synapse.<\/p>\n<p style=\"text-align: justify\">An example of a chemical synapse is the neuromuscular junction described in the chapter on muscle tissue. In the nervous system, there are many more synapses that are essentially the same as the neuromuscular junction. All synapses have common characteristics, which can be summarized in this list:<\/p>\n<ul>\n<li>presynaptic element<\/li>\n<li>neurotransmitter (packaged in vesicles)<\/li>\n<li>synaptic cleft<\/li>\n<li>receptor proteins<\/li>\n<li>postsynaptic element<\/li>\n<li>neurotransmitter elimination or re-uptake<\/li>\n<\/ul>\n<p style=\"text-align: justify\">Synaptic transmission (or neurotransmission) takes place through the following steps (Figure 39):<\/p>\n<ul>\n<li style=\"text-align: justify\">An action potential reaches the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2463\">axon terminal<\/a>.<\/li>\n<li style=\"text-align: justify\">The change in voltage causes <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2555\">voltage-gated<\/a> Ca<sup>2+<\/sup> channels in the membrane of the synaptic end bulb to open.<\/li>\n<li style=\"text-align: justify\">The concentration of Ca<sup>2+<\/sup> increases inside the end bulb, and Ca<sup>2+<\/sup> ions associate with proteins in the outer surface of neurotransmitter vesicles facilitating the merging of the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2225\">vesicle<\/a> with the presynaptic membrane. The neurotransmitter is then released through <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2271\">exocytosis<\/a> into the small gap between the cells, known as the <strong><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2566\">synaptic cleft<\/a><\/strong>.<\/li>\n<li style=\"text-align: justify\">Once in the synaptic cleft, the neurotransmitter diffuses the short distance to the postsynaptic membrane and can interact with neurotransmitter receptors. Receptors are specific for the neurotransmitter, and the two fit together like a key and lock. One neurotransmitter binds to its receptor and will not bind to receptors for other neurotransmitters, making the binding a specific chemical event.<\/li>\n<li style=\"text-align: justify\">The interaction of the neurotransmitter with the receptor can result in <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2559\">depolarization<\/a> or <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2561\">hyperpolarization<\/a> of the postsynaptic cell membrane, leading to excitation of the postsynaptic cell (and possibly the generation of a new action potential) or inhibition, respectively.<\/li>\n<li style=\"text-align: justify\">The neurotransmitter is removed from the synaptic cleft by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2256\">diffusion<\/a>, due to the action of <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2173\">enzymes<\/a> that break it down chemically or by transporters in the presynaptic cell membrane.<\/li>\n<\/ul>\n<p><em>\u00a0<\/em><\/p>\n<figure style=\"width: 907px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image47.png\" alt=\"image\" width=\"907\" height=\"1139\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 39. Synaptic Transmission.<\/strong> The pre-synaptic neuron signals a postsynaptic neuron by releasing neurotransmitter across the synaptic cleft.<\/figcaption><\/figure>\n<p><em>\u00a0<\/em><\/p>\n<p style=\"text-align: justify\"><em>Neurotransmitter Systems<\/em><\/p>\n<p style=\"text-align: justify\">There are several systems of neurotransmitters that are found at various synapses in the nervous system (Figure 40).\u00a0 In this course, you are not required to know all the neurotransmitters, but only to be able to provide one example of a neurotransmitter from each of the systems below.<\/p>\n<ul>\n<li style=\"text-align: justify\"><a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2179\"><strong>Amino <\/strong><strong>acids<\/strong><\/a>: This includes glutamate (Glu), GABA (gamma-aminobutyric acid, a derivative of glutamate), and glycine (Gly).<\/li>\n<li style=\"text-align: justify\"><strong>B<\/strong><strong>iogenic amines<\/strong>: This is a group of neurotransmitters that are enzymatically made from amino acids. For example, the neurotransmitter serotonin is made from tryptophan. Other biogenic amines are made from tyrosine, and include dopamine, <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2546\">norepinephrine<\/a>, and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2547\">epinephrine<\/a>. The chemical epinephrine (epi- = \u201con\u201d; \u201c-nephrine\u201d = kidney) is also known as adrenaline (renal = \u201ckidney\u201d), and <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2546\">norepinephrine<\/a> is sometimes referred to as noradrenaline. The <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2541\">adrenal gland<\/a> produces epinephrine and norepinephrine to be released into the blood stream as hormones.<\/li>\n<li style=\"text-align: justify\"><strong>C<\/strong><strong>holinergic system<\/strong>: It is the system based on <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2478\">acetylcholine<\/a>. This includes the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2551\">neuromuscular junction<\/a> as an example of a cholinergic synapse, but <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2544\">cholinergic<\/a> synapses are found in other parts of the nervous system. They are in the <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2456\">autonomic nervous system<\/a>, as well as distributed throughout the brain.<\/li>\n<li style=\"text-align: justify\"><strong>N<\/strong><strong>europeptide<\/strong><strong>s:<\/strong> These are neurotransmitter molecules made up of chains of amino acids connected by <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2180\">peptide bonds<\/a>. This is what a protein is, but the term protein implies a certain length to the molecule. Some neuropeptides are quite short, such as met-enkephalin, which is five amino acids long. Others are long, such as beta-endorphin, which is 31 amino acids long. Neuropeptides are often released at synapses in combination with another neurotransmitter, and they often act as hormones in other systems of the body, such as vasoactive intestinal peptide (VIP) or substance P.<\/li>\n<\/ul>\n<p style=\"text-align: justify\">The effect of a neurotransmitter on the postsynaptic element is entirely dependent on the receptor protein. First, if there is no receptor protein in the membrane of the postsynaptic element, then the neurotransmitter has no effect. The depolarizing or hyperpolarizing effect is also dependent on the receptor. For example, when acetylcholine binds to a type of receptor called nicotinic receptor, the postsynaptic cell is depolarized. This is because the receptor is a <a class=\"glossary-term\" aria-haspopup=\"dialog\" aria-describedby=\"definition\" href=\"#term_1490_2094\">cation<\/a> channel and positively charged Na<sup>+<\/sup> will rush into the cell. However, when acetylcholine binds to another type of receptor called muscarinic receptor, of which there are several variants, it might cause depolarization or hyperpolarization of the target cell.<\/p>\n<p style=\"text-align: justify\">On the other hand, the amino acid neurotransmitters, glutamate, glycine, and GABA, are almost exclusively associated with just one effect. Glutamate is considered an excitatory amino acid, but only because Glu receptors in the adult cause depolarization of the postsynaptic cell. Glycine and GABA are considered inhibitory amino acids, again because their receptors cause hyperpolarization.<\/p>\n<figure style=\"width: 783px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-content\/uploads\/sites\/750\/2019\/08\/image48.png\" alt=\"image\" width=\"783\" height=\"1113\" \/><figcaption class=\"wp-caption-text\"><strong>Figure 40. Examples of Neurotransmitters.<\/strong> Shown are some examples of major transmitters, their chemical structures and some of their functions.<\/figcaption><\/figure>\n<div class=\"textbox textbox--exercises\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\"><a id=\"P\"><\/a>Practice questions<\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p><strong>Part 1:<\/strong> The Anatomical and Functional Organization of the Nervous System<\/p>\n<div id=\"h5p-138\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-138\" class=\"h5p-iframe\" data-content-id=\"138\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-1\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-139\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-139\" class=\"h5p-iframe\" data-content-id=\"139\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-2\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 2:<\/strong> Nervous Tissue<\/p>\n<div id=\"h5p-140\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-140\" class=\"h5p-iframe\" data-content-id=\"140\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-3\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-141\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-141\" class=\"h5p-iframe\" data-content-id=\"141\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-4\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-142\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-142\" class=\"h5p-iframe\" data-content-id=\"142\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-5\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 3:<\/strong> The Central Nervous System<\/p>\n<div id=\"h5p-143\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-143\" class=\"h5p-iframe\" data-content-id=\"143\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-6\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-144\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-144\" class=\"h5p-iframe\" data-content-id=\"144\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-7\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 4:<\/strong> The Peripheral Nervous System<\/p>\n<div id=\"h5p-147\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-147\" class=\"h5p-iframe\" data-content-id=\"147\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-10\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-150\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-150\" class=\"h5p-iframe\" data-content-id=\"150\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-12\"><\/iframe><\/div>\n<\/div>\n<p><strong>Part 5:<\/strong> Neuronal Signalling<\/p>\n<div id=\"h5p-152\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-152\" class=\"h5p-iframe\" data-content-id=\"152\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-13\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-149\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-149\" class=\"h5p-iframe\" data-content-id=\"149\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-14\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-153\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-153\" class=\"h5p-iframe\" data-content-id=\"153\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-15\"><\/iframe><\/div>\n<\/div>\n<div id=\"h5p-154\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-154\" class=\"h5p-iframe\" data-content-id=\"154\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"9-16\"><\/iframe><\/div>\n<\/div>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<div class=\"glossary\"><span class=\"screen-reader-text\" id=\"definition\">definition<\/span><template id=\"term_1490_2376\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2376\"><div tabindex=\"-1\"><p>Describes a position towards the outer edge (periphery) of a structure or organ system.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2340\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2340\"><div tabindex=\"-1\"><p>Supportive neural cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2181\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2181\"><div tabindex=\"-1\"><p>Excitable neural cell that transfer nerve impulses.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2447\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2447\"><div tabindex=\"-1\"><p>In neurons, that portion of the cell that contains the nucleus; the cell body, as opposed to the cell processes (axons and dendrites).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2448\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2448\"><div tabindex=\"-1\"><p>In cells, an extension of a cell body; in the case of neurons, this includes the axon and dendrites.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2345\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2345\"><div tabindex=\"-1\"><p>Single process of the neuron that carries an electrical signal (action potential) away from the cell body toward a target cell.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2342\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2342\"><div tabindex=\"-1\"><p>One of many branchlike processes that extends from the neuron cell body and functions as a contact for incoming signals (synapses) from other neurons or sensory cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2449\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2449\"><div tabindex=\"-1\"><p>Regions of the nervous system containing cell bodies of neurons with few or no myelinated axons; actually may be more pink or tan in color, but called gray in contrast to white matter.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2450\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2450\"><div tabindex=\"-1\"><p>Regions of the nervous system containing mostly myelinated axons, making the tissue appear white because of the high lipid content of myelin.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2343\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2343\"><div tabindex=\"-1\"><p>Lipid-rich insulating substance surrounding the axons of many neurons, allowing for faster transmission of electrical signals.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2451\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2451\"><div tabindex=\"-1\"><p>(In nervous system) a localized collection of neuron cell bodies that are functionally related; a \u201ccenter\u201d of neural function (plural= nuclei).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2452\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2452\"><div tabindex=\"-1\"><p>Localized collection of neuron cell bodies in the peripheral nervous system.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2454\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2454\"><div tabindex=\"-1\"><p>Cord-like bundle of axons located in the peripheral nervous system that transmits sensory input and response output to and from the central nervous system.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2453\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2453\"><div tabindex=\"-1\"><p>Cord-like bundle of axons located in the peripheral nervous system that transmits sensory input and response output to and from the central nervous system.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2455\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2455\"><div tabindex=\"-1\"><p>Functional division of the nervous system that is concerned with conscious perception, voluntary movement, and skeletal muscle reflexes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2456\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2456\"><div tabindex=\"-1\"><p>Functional division of the nervous system that is responsible for homeostatic reflexes that coordinate control of cardiac and smooth muscle, as well as glandular tissue.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2066\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2066\"><div tabindex=\"-1\"><p>Two or more atoms covalently bonded together.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2063\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2063\"><div tabindex=\"-1\"><p>A substance composed of two or more different elements joined by chemical bonds.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2093\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2093\"><div tabindex=\"-1\"><p>Atom with an overall positive or negative charge. Many function as electrolytes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2457\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2457\"><div tabindex=\"-1\"><p>Type of sweat gland that is common throughout the skin surface; it produces a hypotonic sweat for thermoregulation.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2458\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2458\"><div tabindex=\"-1\"><p>Type of sweat gland that is associated with hair follicles in the armpits and genital regions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2280\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2280\"><div tabindex=\"-1\"><p>(In physiology) under conscious control of the brain.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2333\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2333\"><div tabindex=\"-1\"><p>(In physiology) though under nervous control (usually from the brain), control is not conscious.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2264\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2264\"><div tabindex=\"-1\"><p>Steady state of body systems that living organisms maintain.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2459\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2459\"><div tabindex=\"-1\"><p>Neural tissue associated with the digestive system that is responsible for nervous control through autonomic connections.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2221\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2221\"><div tabindex=\"-1\"><p>Cell\u2019s central organelle; contains the cell\u2019s DNA.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2344\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2344\"><div tabindex=\"-1\"><p>Narrow junction across which a chemical signal passes from neuron to the next, initiating a new electrical signal in the target cell.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2460\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2460\"><div tabindex=\"-1\"><p>Information flow in one direction.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2461\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2461\"><div tabindex=\"-1\"><p>Tapering of the neuron cell body that gives rise to the axon.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2462\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2462\"><div tabindex=\"-1\"><p>Single stretch of the axon insulated by myelin and bounded by nodes of Ranvier at either end (except for the first, which is after the initial segment, and the last, which is followed by the axon terminal).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2463\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2463\"><div tabindex=\"-1\"><p>End of the axon, where there are usually several branches extending toward the target cell.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2464\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2464\"><div tabindex=\"-1\"><p>Swelling at the end of an axon where neurotransmitter molecules are released onto a target cell across a synapse.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2465\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2465\"><div tabindex=\"-1\"><p>Shape of a neuron that has multiple processes\u2014the axon and two or more dendrites.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2466\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2466\"><div tabindex=\"-1\"><p>Region of the adult brain connected primarily to the pons that developed from the metencephalon (along with the pons) and is largely responsible for comparing information from the cerebrum with sensory feedback from the periphery through the spinal cord.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2468\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2468\"><div tabindex=\"-1\"><p>Glial cell type in the CNS that provides the myelin insulation for axons in tracts.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2469\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2469\"><div tabindex=\"-1\"><p>Glial cell type in the PNS that provides the myelin insulation for axons in nerves.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2161\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2161\"><div tabindex=\"-1\"><p>Class of nonpolar organic compounds built from hydrocarbons and distinguished by the fact that they are not soluble in water.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2470\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2470\"><div tabindex=\"-1\"><p>Lipid-rich layer of insulation that surrounds an axon, formed by oligodendrocytes in the CNS and Schwann cells in the PNS; facilitates the transmission of electrical signals.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2166\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2166\"><div tabindex=\"-1\"><p>An amphipathic lipid molecule containing a phosphate head (polar) and two fatty acid tails (non-polar). The major molecule comprising plasma membranes.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2471\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2471\"><div tabindex=\"-1\"><p>Region of the adult brain that develops from the telencephalon and is responsible for higher neurological functions such as memory, emotion, and consciousness.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2472\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2472\"><div tabindex=\"-1\"><p>One half of the bilaterally symmetrical cerebrum.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2473\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2473\"><div tabindex=\"-1\"><p>Outer gray matter covering the forebrain, marked by wrinkles and folds known as gyri and sulci.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2474\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2474\"><div tabindex=\"-1\"><p>Ridge formed by convolutions on the surface of the cerebrum or cerebellum.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2475\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2475\"><div tabindex=\"-1\"><p>Groove formed by convolutions in the surface of the cerebral cortex.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2476\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2476\"><div tabindex=\"-1\"><p>Mapping of regions of the cerebral cortex based on microscopic anatomy that relates specific areas to functional differences, as described by Brodmann in the early 1900s.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2477\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2477\"><div tabindex=\"-1\"><p>Nuclei of the cerebrum (with a few components in the upper brain stem and diencephalon) that are responsible for assessing cortical movement commands and comparing them with the general state of the individual through broad modulatory activity of dopamine neurons; largely related to motor functions, as evidenced through the symptoms of Parkinson\u2019s and Huntington\u2019s diseases.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2478\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2478\"><div tabindex=\"-1\"><p>An important neurotransmitter.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2479\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2479\"><div tabindex=\"-1\"><p>Region of the adult brain that retains its name from embryonic development and includes the thalamus and hypothalamus.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2480\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2480\"><div tabindex=\"-1\"><p>Referring to the sense of smell.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2481\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2481\"><div tabindex=\"-1\"><p>Portion of the ventricular system that is in the region of the diencephalon.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2482\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2482\"><div tabindex=\"-1\"><p>Major region of the diencephalon that is responsible for relaying information between the cerebrum and the hindbrain, spinal cord, and periphery.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2483\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2483\"><div tabindex=\"-1\"><p>Region of the diecephalon containing the pineal gland.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2484\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2484\"><div tabindex=\"-1\"><p>Nucleus within the basal nuclei that is part of the indirect pathway.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2440\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2440\"><div tabindex=\"-1\"><p>Region of the brain inferior to the thalamus that functions in neural and endocrine signaling, temperature regulation and control of the autonomic nervous system.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2273\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2273\"><div tabindex=\"-1\"><p>Tissue or organ that secretes hormones into the blood and lymph without ducts such that they may be transported to organs distant from the site of secretion.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2485\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2485\"><div tabindex=\"-1\"><p>Bean-sized organ suspended from the hypothalamus that produces, stores, and secretes hormones in response to hypothalamic stimulation (also called hypophysis).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2486\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2486\"><div tabindex=\"-1\"><p>Structures at the edge (limit) of the boundary between the forebrain and hindbrain that are most associated with emotional behavior and memory formation.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2487\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2487\"><div tabindex=\"-1\"><p>Middle region of the adult brain that develops from the mesencephalon.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2488\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2488\"><div tabindex=\"-1\"><p>Posterior region of the adult brain that develops from the rhombencephalon and includes the pons, medulla oblongata, and cerebellum.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2492\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2492\"><div tabindex=\"-1\"><p>Half of the midbrain tectum that is part of the brain stem auditory pathway.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2493\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2493\"><div tabindex=\"-1\"><p>Half of the midbrain tectum that is responsible for aligning visual, auditory, and somatosensory spatial perceptions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2489\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2489\"><div tabindex=\"-1\"><p>Portion of the brainstem connecting the medulla oblongata with the midbrain. Serves as a connection to cerebellum, as well as functions including sleep cycles and the origin of some cranial nerves.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2490\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2490\"><div tabindex=\"-1\"><p>Lowest (most inferior) part of the brain, controlling many autonomic functions including heart rate, breathing, and digestion.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2494\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2494\"><div tabindex=\"-1\"><p>Diffuse region of gray matter throughout the brain stem that regulates sleep, wakefulness, and states of consciousness.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2500\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2500\"><div tabindex=\"-1\"><p>General anatomical term for a hole or opening (usually in bone. Plural = foramina<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2501\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2501\"><div tabindex=\"-1\"><p>Neck<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2503\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2503\"><div tabindex=\"-1\"><p>Mid-back, where ribs attach to vertebrae.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2502\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2502\"><div tabindex=\"-1\"><p>Lower back, below the ribs.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2504\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2504\"><div tabindex=\"-1\"><p>Gray matter region of the spinal cord in which sensory input arrives, sometimes referred to as the dorsal horn.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2505\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2505\"><div tabindex=\"-1\"><p>Gray matter of the spinal cord containing multipolar motor neurons, sometimes referred to as the ventral horn.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2506\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2506\"><div tabindex=\"-1\"><p>Region of the spinal cord gray matter in the thoracic, upper lumbar, and sacral regions that is the central component of the sympathetic division of the autonomic nervous system.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2508\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2508\"><div tabindex=\"-1\"><p>Region of the sacrum, bone forming the back part of the pelvic cavity.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2507\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2507\"><div tabindex=\"-1\"><p>Branch of the autonomic nervous system associated with emergency systems (\"fight of flight\").<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2509\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2509\"><div tabindex=\"-1\"><p>Central nervous system fibers carrying sensory information from the spinal cord or periphery to the brain.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2510\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2510\"><div tabindex=\"-1\"><p>Central nervous system fibers carrying motor commands from the brain to the spinal cord or periphery.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2496\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2496\"><div tabindex=\"-1\"><p>Protective outer coverings of the CNS composed of connective tissue.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2497\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2497\"><div tabindex=\"-1\"><p>Tough, fibrous, outer layer of the meninges that is attached to the inner surface of the cranium and vertebral column and surrounds the entire CNS.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2498\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2498\"><div tabindex=\"-1\"><p>Middle layer of the meninges named for the spider-web\u2013like trabeculae that extend between it and the pia mater.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2511\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2511\"><div tabindex=\"-1\"><p>Filaments between the arachnoid and pia mater within the subarachnoid space.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2512\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2512\"><div tabindex=\"-1\"><p>Thin, innermost membrane of the meninges that directly covers the surface of the CNS.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2129\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2129\"><div tabindex=\"-1\"><p>Extracellular fluid in the small spaces between cells not contained within blood vessels.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2513\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2513\"><div tabindex=\"-1\"><p>Remnants of the hollow center of the neural tube that are spaces for cerebrospinal fluid to circulate through the brain.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2137\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2137\"><div tabindex=\"-1\"><p>Circulatory medium within the CNS that is produced by ependymal cells in the choroid plexus filtering the blood.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2514\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2514\"><div tabindex=\"-1\"><p>Specialized structures containing ependymal cells lining blood capillaries that filter blood to produce CSF in the four ventricles of the brain.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2515\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2515\"><div tabindex=\"-1\"><p>Portions of the ventricular system that are in the region of the cerebrum.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2491\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2491\"><div tabindex=\"-1\"><p>connection of the ventricular system between the third and fourth ventricles located in the midbrain.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2516\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2516\"><div tabindex=\"-1\"><p>The portion of the ventricular system that is in the region of the brain stem and opens into the subarachnoid space through the median and lateral apertures.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2517\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2517\"><div tabindex=\"-1\"><p>Space between the arachnoid mater and pia mater that contains CSF and the fibrous connections of the arachnoid trabeculae.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2518\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2518\"><div tabindex=\"-1\"><p>Glial cell type that filters blood at the choroid plexus.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2134\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2134\"><div tabindex=\"-1\"><p>A solution containing ions; sometimes referring to ions themselves.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2519\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2519\"><div tabindex=\"-1\"><p>Outpocket of the arachnoid membrane into the dural sinuses that allows for reabsorption of CSF into the blood.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2520\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2520\"><div tabindex=\"-1\"><p>Any of the venous structures surrounding the brain, enclosed within the dura mater, which drain blood from the CNS to the common venous return of the jugular veins.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2521\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2521\"><div tabindex=\"-1\"><p>One of a pair of major veins located in the neck region that  flows parallel to the common carotid artery that is more or less its counterpart; primarily drains blood from the brain, receives the superficial facial vein, and empties into the subclavian vein.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2523\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2523\"><div tabindex=\"-1\"><p>Sensory ganglion attached to the posterior nerve root of a spinal nerve.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2194\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2194\"><div tabindex=\"-1\"><p>Type of tissue that serves to hold in place, connect, and integrate the body\u2019s organs and systems.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2524\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2524\"><div tabindex=\"-1\"><p>One of twelve nerves connected to the brain that are responsible for sensory or motor functions of the head and neck.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2525\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2525\"><div tabindex=\"-1\"><p>Tenth cranial nerve; responsible for the autonomic control of organs in the thoracic and upper abdominal cavities.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2393\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2393\"><div tabindex=\"-1\"><p>Division of the anterior (ventral) cavity that houses the heart, lungs, esophagus, and trachea.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2334\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2334\"><div tabindex=\"-1\"><p>Usually attached to bone, under voluntary control, each cell is a fiber that is multinucleated and striated.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2526\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2526\"><div tabindex=\"-1\"><p>Circuit of a reflex that involves a sensory input and motor output, or an afferent branch and an efferent branch, and an integrating center to connect the two branches.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2527\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2527\"><div tabindex=\"-1\"><p>Four muscles, that extend and stabilize the knee.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2341\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2341\"><div tabindex=\"-1\"><p>Change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibres.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2528\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2528\"><div tabindex=\"-1\"><p>Muscle that opposes the action of an agonist.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_3028\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_3028\"><div tabindex=\"-1\"><p>Three long muscles on the back of the upper leg.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2530\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2530\"><div tabindex=\"-1\"><p>Two-headed muscle that crosses the shoulder and elbow joints to flex the forearm while assisting in supinating it and flexing the arm at the shoulder.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2531\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2531\"><div tabindex=\"-1\"><p>Three-headed muscle that extends the forearm.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2532\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2532\"><div tabindex=\"-1\"><p>Opposite side of the body.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2533\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2533\"><div tabindex=\"-1\"><p>One of six muscles originating out of the bones of the orbit and inserting into the surface of the eye which are responsible for moving the eye.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2534\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2534\"><div tabindex=\"-1\"><p>Division of the autonomic nervous system responsible for restful and digestive functions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2439\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2439\"><div tabindex=\"-1\"><p>Organ that can cause a change in a value.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2535\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2535\"><div tabindex=\"-1\"><p>Alternate name for the sympathetic division of the autonomic nervous system that is based on the anatomical location of central neurons in the lateral horn of the thoracic and upper lumbar spinal cord.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2536\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2536\"><div tabindex=\"-1\"><p>Specifically referring to the cell body of a neuron in the autonomic system that is located in the central nervous system, specifically the lateral horn of the spinal cord or a brain stem nucleus.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2537\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2537\"><div tabindex=\"-1\"><p>Series of ganglia adjacent to the vertebral column that receive input from central sympathetic neurons.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2538\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2538\"><div tabindex=\"-1\"><p>Axon from a central neuron in the autonomic nervous system that projects to and synapses with a ganglionic neuron; sometimes referred to as a preganglionic neuron.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2539\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2539\"><div tabindex=\"-1\"><p>Axon from a ganglionic neuron in the autonomic nervous system that projects to and synapses with the target effector; sometimes referred to as a postganglionic neuron.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2540\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2540\"><div tabindex=\"-1\"><p>Inner layer of the adrenal glands that plays an important role in the stress response by producing epinephrine and norepinephrine.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2541\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2541\"><div tabindex=\"-1\"><p>Endocrine glands located at the top of each kidney that are important for the regulation of the stress response, blood pressure and blood volume, water homeostasis, and electrolyte levels.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2542\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2542\"><div tabindex=\"-1\"><p>Paired nerves that carry both autonomic and sensory fibres to the internal organs.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2172\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2172\"><div tabindex=\"-1\"><p>Chemical signal that is released from the synaptic end bulb of a neuron to cause a change in the target cell.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2543\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2543\"><div tabindex=\"-1\"><p>Alternate name for the parasympathetic division of the autonomic nervous system that is based on the anatomical location of central neurons in brain-stem nuclei and the lateral horn of the sacral spinal cord; also referred to as craniosacral outflow.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2270\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2270\"><div tabindex=\"-1\"><p>Protein molecule that contains a binding site for another specific molecule (called a ligand).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2544\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2544\"><div tabindex=\"-1\"><p>Synapse at which acetylcholine is released and binds to the nicotinic or muscarinic receptor.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2545\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2545\"><div tabindex=\"-1\"><p>Synapse where norepinephrine is released, which binds to \u03b1- or \u03b2-adrenergic receptors.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2546\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2546\"><div tabindex=\"-1\"><p>Signaling molecule released as a neurotransmitter by most postganglionic sympathetic fibres as part of the sympathetic response, or as a hormone into the bloodstream from the adrenal medulla.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2547\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2547\"><div tabindex=\"-1\"><p>Signaling molecule released from the adrenal medulla into the bloodstream as part of the sympathetic response.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2171\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2171\"><div tabindex=\"-1\"><p>Secretion of an endocrine organ that travels via the bloodstream or lymphatics to induce a response in target cells or tissues in another part of the body.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2548\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2548\"><div tabindex=\"-1\"><p>Sensory receptor specialized for temperature stimuli.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2549\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2549\"><div tabindex=\"-1\"><p>Change in the membrane potential that varies in size, depending on the size of the stimulus that elicits it.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2550\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2550\"><div tabindex=\"-1\"><\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2551\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2551\"><div tabindex=\"-1\"><p>Synapse between the axon terminal of a motor neuron and the section of the membrane of a muscle fiber with receptors for the acetylcholine released by the terminal.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2178\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2178\"><div tabindex=\"-1\"><p>\"Water loving\"; a molecule or portion thereof that is polar and therefore water soluble.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2250\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2250\"><div tabindex=\"-1\"><p>Membrane-spanning protein that has an inner pore which allows the passage of one or more substances (a form of facilitated diffusion).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2254\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2254\"><div tabindex=\"-1\"><p>Form of transport across the cell membrane that requires input of cellular energy.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2255\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2255\"><div tabindex=\"-1\"><p>Difference in the concentration of a substance between two regions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2259\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2259\"><div tabindex=\"-1\"><p>Membrane-spanning protein that binds to substances it needs to transport, changes shape and moves the substance into or out of the cell (a form of facilitated diffusion, or active transport pumps when energy is required).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2074\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2074\"><div tabindex=\"-1\"><p>Nucleotide containing ribose and an adenine base that is essential in energy transfer.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2552\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2552\"><div tabindex=\"-1\"><p>A channel protein (facilitated diffusion) that is activated (opens) when a molecule (such as a neurotransmitter) binds to it.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2553\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2553\"><div tabindex=\"-1\"><p>Ion channel protein (facilitated diffusion) that opens when a physical event directly affects the structure of the protein.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2554\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2554\"><div tabindex=\"-1\"><p>Sense of touch.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2555\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2555\"><div tabindex=\"-1\"><p>Ion channel that opens because of a change in the charge distributed across the membrane where it is located.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2556\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2556\"><div tabindex=\"-1\"><p>Ion channel (facilitated diffusion) that opens randomly and is not gated to a specific event, also known as a non-gated channel.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2557\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2557\"><div tabindex=\"-1\"><p>Distribution of charge across the cell membrane, based on the charges of ions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2127\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2127\"><div tabindex=\"-1\"><p>Fluid outside cells (plasma or interstitial fluid).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2126\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2126\"><div tabindex=\"-1\"><p>Fluid inside cells.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2130\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2130\"><div tabindex=\"-1\"><p>Clear, semi-fluid medium of the cytoplasm, made up mostly of water.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2095\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2095\"><div tabindex=\"-1\"><p>Atom with a negative charge.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2150\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2150\"><div tabindex=\"-1\"><p>Chemical functional group, PO4-, a component of phospholipids and nucleic acids (including ATP).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2558\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2558\"><div tabindex=\"-1\"><p>The difference in voltage measured across a cell membrane under steady-state conditions, typically -70 mV.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2559\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2559\"><div tabindex=\"-1\"><p>Change in a cell membrane potential from rest toward zero.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2560\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2560\"><div tabindex=\"-1\"><p>Return of the membrane potential to its normally negative voltage at the end of the action potential.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2561\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2561\"><div tabindex=\"-1\"><p>Change in cell membrane potential below resting potential (&lt;-70mV).<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2562\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2562\"><div tabindex=\"-1\"><p>Time after the initiation of an action potential when another action potential cannot be generated.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2563\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2563\"><div tabindex=\"-1\"><p>Gap between two myelinated regions of an axon, allowing for strengthening of the electrical signal as it propagates down the axon.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2565\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2565\"><div tabindex=\"-1\"><p>Slow propagation of an action potential along an unmyelinated axon owing to voltage-gated Na+ channels located along the entire length of the cell membrane.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2564\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2564\"><div tabindex=\"-1\"><p>Quick propagation of the action potential along a myelinated axon owing to voltage-gated Na+ channels being present only at the nodes of Ranvier.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2225\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2225\"><div tabindex=\"-1\"><p>Membrane-bound structure that contains materials within or outside of the cell.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2271\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2271\"><div tabindex=\"-1\"><p>Export of a substance out of a cell by formation of a membrane-bound vesicle.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2566\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2566\"><div tabindex=\"-1\"><p>Small gap between cells in a chemical synapse where neurotransmitter diffuses from the presynaptic element to the postsynaptic element.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2256\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2256\"><div tabindex=\"-1\"><p>Movement of a substance from an area of higher concentration to one of lower concentration.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2173\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2173\"><div tabindex=\"-1\"><p>Molecule (usually a protein) that catalyzes chemical reactions.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2179\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2179\"><div tabindex=\"-1\"><p>Building block of proteins; characterized by an amino and carboxyl functional groups and a variable side-chain.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2180\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2180\"><div tabindex=\"-1\"><p>A type of covalent bond occurring between amino acids.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><template id=\"term_1490_2094\"><div class=\"glossary__definition\" role=\"dialog\" data-id=\"term_1490_2094\"><div tabindex=\"-1\"><p>Ion with a positive charge.<\/p>\n<\/div><button><span aria-hidden=\"true\">&times;<\/span><span class=\"screen-reader-text\">Close definition<\/span><\/button><\/div><\/template><\/div>","protected":false},"author":10,"menu_order":1,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1490","chapter","type-chapter","status-publish","hentry"],"part":472,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/pressbooks\/v2\/chapters\/1490","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/wp\/v2\/users\/10"}],"version-history":[{"count":25,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/pressbooks\/v2\/chapters\/1490\/revisions"}],"predecessor-version":[{"id":3268,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/pressbooks\/v2\/chapters\/1490\/revisions\/3268"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/pressbooks\/v2\/parts\/472"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/pressbooks\/v2\/chapters\/1490\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/wp\/v2\/media?parent=1490"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/pressbooks\/v2\/chapter-type?post=1490"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/wp\/v2\/contributor?post=1490"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/dcbiol110311092nded\/wp-json\/wp\/v2\/license?post=1490"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}