1103 Chapter 4. The Tissue Level of Organization
By the end of this section, you will be able to:
- Describe the general characteristics of a neuron, and relate those characteristics to its function
- Describe the structure and function of nervous tissue
Nervous tissue is characterized as being excitable and capable of sending and receiving electrochemical signals that provide the body with information. Two main classes of cells make up nervous tissue: the neuron and neuroglia (Figure 1). Neurons propagate information via electrochemical impulses, called action potentials, which are biochemically linked to the release of chemical signals. Neuroglia play an essential role in supporting neurons and modulating their information propagation.
Neurons display distinctive morphology, well suited to their role as conducting cells, with three main parts. The cell body includes most of the cytoplasm, the organelles, and the nucleus. Dendrites branch off the cell body and appear as thin extensions. A long “tail,” the axon, extends from the neuron body and can be wrapped in an insulating layer known as myelin, which is formed by accessory cells. The synapse is the gap between nerve cells, or between a nerve cell and its target, for example, a muscle or a gland, across which the impulse is transmitted by chemical compounds known as neurotransmitters. Neurons categorized as multipolar neurons have several dendrites and a single prominent axon. Bipolar neurons possess a single dendrite and axon with the cell body, while unipolar neurons have only a single process extending out from the cell body, which divides into a functional dendrite and into a functional axon. When a neuron is sufficiently stimulated, it generates an action potential that propagates down the axon towards the synapse. If enough neurotransmitters are released at the synapse to stimulate the next neuron or target, a response is generated.
The second class of neural cells comprises the neuroglia or glial cells, which have been characterized as having a simple support role. The word “glia” comes from the Greek word for glue. Recent research is shedding light on the more complex role of neuroglia in the function of the brain and nervous system. Astrocyte cells, named for their distinctive star shape, are abundant in the central nervous system. The astrocytes have many functions, including regulation of ion concentration in the intercellular space, uptake and/or breakdown of some neurotransmitters, and formation of the blood-brain barrier, the membrane that separates the circulatory system from the brain. Microglia protect the nervous system against infection but are not nervous tissue because they are related to macrophages. Oligodendrocyte cells produce myelin in the central nervous system (brain and spinal cord) while the Schwann cell produces myelin in the peripheral nervous system (Figure 2).
As appropriate to its role as a conductor of information, nervous tissue is found throughout the body. Nervous tissue is divided into different subsystems or divisions based primarily on its specific function. It is organized into two broad systems: the central nervous system (CNS) and the peripheral nervous system. The CNS includes the brain and the spinal cord and is responsible for integrating, and generating a response to, information from the peripheral nervous system. The peripheral nervous system is the part of the nervous system outside the CNS and is responsible for relaying information from other body tissues to the CNS, and for relaying commands from the CNS out to other body tissues.
The peripheral nervous system includes a sensory or afferent division, and a motor or efferent division. The sensory division carries information to the CNS and includes receptors found in the skin, skeletal muscles and joints as well as their associated nerve fibres in visceral organs and skeletal muscles. The motor division conducts information away from the CNS to skeletal muscles (somatic or voluntary nervous system) or to the heart, glands and muscles involved with the movement of materials throughout the body (autonomic or involuntary nervous system). The autonomic system includes the sympathetic nervous system that is activated during stress (“flight or fight”) and the parasympathetic nervous system that maintains body functions during rest (“rest and digest”).
The presence of the nervous system throughout the body and its organization allow it to receive, integrate and provide information to the entire body. This ensures that appropriate responses can occur among all body systems within an intact organism, both under normal conditions as well as during times of stress.