{"id":216,"date":"2024-11-15T11:07:52","date_gmt":"2024-11-15T16:07:52","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/chapter\/the-normal-neuromuscular-junction\/"},"modified":"2024-11-15T11:07:52","modified_gmt":"2024-11-15T16:07:52","slug":"the-normal-neuromuscular-junction","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/chapter\/the-normal-neuromuscular-junction\/","title":{"raw":"The neuromuscular junction and nervous control of skeletal muscle","rendered":"The neuromuscular junction and nervous control of skeletal muscle"},"content":{"raw":"\n<div class=\"textbox textbox--learning-objectives\"><header class=\"textbox__header\">\n<p class=\"textbox__title\">Learning Objectives<\/p>\n\n<\/header>\n<div class=\"textbox__content\">\n\n<span style=\"text-align: initial;font-size: 1em\">By the end of this section, you will be able to:<\/span>\n<div id=\"1\" class=\"ui-has-child-title\" data-type=\"abstract\"><section>\n<ul id=\"list-00001\">\n \t<li>Identify the components of the neuromuscular junction<\/li>\n \t<li>Describe excitation-contraction coupling and formation of a power stroke<\/li>\n \t<li>Explain how muscles contract and relax<\/li>\n \t<li>Describe the mechanisms of ATP regeneration for use in muscle contraction<\/li>\n<\/ul>\n<\/section><\/div>\n<\/div>\n<\/div>\n&nbsp;\n\n<section id=\"fs-id1040329\" data-depth=\"1\">\n<div id=\"fig-ch10_02_03\" class=\"os-figure\">\n<div class=\"os-caption-container\">&nbsp;<span style=\"font-family: 'Cormorant Garamond', serif;font-size: 1.602em\">The Neuromuscular Junction<\/span><\/div>\n<\/div>\n<\/section><section id=\"fs-id1990056\" data-depth=\"1\">\n<p id=\"fs-id1854958\">A specialization of the skeletal muscle is the site where a motor neuron\u2019s terminal meets the muscle fiber\u2014called the <span id=\"term-00016\" data-type=\"term\">neuromuscular junction (NMJ)<\/span>. This is where the muscle fiber first responds to electrical signaling by the motor neuron. Every skeletal muscle fiber in every skeletal muscle is innervated by a motor neuron at the NMJ. Excitation signals from the neuron are the only way to functionally activate the fiber to contract.<\/p>\n<span style=\"color: #008000\">The sequence of events that result in the contraction of an individual muscle fiber begins with a signal\u2014the neurotransmitter, acetylcholine (ACh)\u2014from the motor neuron innervating that muscle fiber. The sarcolemma, the membrane surrounding the fibers, will depolarize as positively charged sodium ions (Na<sup>+<\/sup>) enter. This is where the Excitation-Contraction Coupling begins.&nbsp;<\/span>\n<div id=\"fs-id2017671\" class=\"anatomy interactive ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\n<div class=\"os-note-body\">\n<div class=\"textbox shaded\">Every skeletal muscle fiber is supplied by a motor neuron at the NMJ. Watch this&nbsp;<a href=\"http:\/\/openstax.org\/l\/skelmuscfiber\" target=\"_blank\" rel=\"noopener nofollow\">video<\/a>&nbsp;to learn more about what happens at the NMJ. (a) What is the definition of a motor unit? (b) What is the structural and functional difference between a large motor unit and a small motor unit? (c) Can you give an example of each? (d) Why is the neurotransmitter acetylcholine degraded after binding to its receptor?<\/div>\n<\/div>\n<\/section><\/div>\n<\/section><section id=\"fs-id1909043\" data-depth=\"1\">\n<div id=\"fig-ch10_02_04\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_02_04\"><span id=\"fs-id1236880\" data-type=\"media\" data-alt=\" Alt text to come.\"><img id=\"9\" class=\"aligncenter\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/ff663ee127683f7dd83c225b6d39d277da9256ed\" alt=\" Alt text to come.\" width=\"473\" height=\"824\" data-media-type=\"image\/jpg\"><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure 12.43<\/span><\/strong>&nbsp;<span id=\"8\" class=\"os-title\" data-type=\"title\">Motor End-Plate and Innervation&nbsp;<\/span><span class=\"os-caption\">At the NMJ, the axon terminal releases acetylcholine (ACh). The motor end-plate is the location of the ACh-receptors in the muscle fiber sarcolemma. When ACh molecules are released, they diffuse across a minute space called the synaptic cleft and bind to the receptors.<\/span><\/div>\n<\/div>\n<h3 data-type=\"title\">Excitation-Contraction Coupling<\/h3>\n<p id=\"fs-id2252260\">All living cells have membrane potentials, or electrical gradients across their membranes. The inside of the membrane is usually around -60 to -90 mV, relative to the outside. This is referred to as a cell\u2019s membrane potential. Neurons and muscle cells can use their membrane potentials to generate electrical signals. They do this by controlling the movement of charged particles, called ions, across their membranes to create electrical currents. This is achieved by opening and closing specialized proteins in the membrane called ion channels. Although the currents generated by ions moving through these channel proteins are very small, they form the basis of both neural signaling and muscle contraction.<\/p>\n<p id=\"fs-id1421868\">Both neurons and skeletal muscle cells are electrically excitable, meaning that they are able to generate action potentials. An action potential is a special type of electrical signal that can travel along a cell membrane as a wave. This allows a signal to be transmitted quickly and faithfully over long distances.<\/p>\n<p id=\"eip-362\">For a skeletal muscle fiber to contract, its membrane must first be \u201cexcited\u201d\u2014in other words, it must be stimulated to fire an action potential. The muscle fiber action potential, which sweeps along the sarcolemma as a wave, is \u201ccoupled\u201d to the actual contraction through the release of calcium ions (Ca<sup>++<\/sup>) from the SR. Once released, the Ca<sup>++<\/sup>&nbsp;interacts with the shielding proteins, forcing them to move aside so that the actin-binding sites are available for attachment by myosin heads. The myosin then pulls the actin filaments toward the center, shortening the muscle fiber.<\/p>\n&nbsp;\n<p id=\"fs-id2023666\">Signaling begins when a neuronal&nbsp;<span id=\"term-00018\" data-type=\"term\">action potential<\/span>&nbsp;travels along the axon of a motor neuron, and then along the individual branches to terminate at the NMJ. At the NMJ, the axon terminal releases a chemical messenger, or&nbsp;<span id=\"term-00019\" data-type=\"term\">neurotransmitter<\/span>, called&nbsp;<span id=\"term-00020\" data-type=\"term\">acetylcholine (ACh)<\/span>. The ACh molecules diffuse across a minute space called the&nbsp;<span id=\"term-00021\" data-type=\"term\">synaptic cleft<\/span>&nbsp;and bind to ACh receptors located within the&nbsp;<span id=\"term-00022\" data-type=\"term\">motor end-plate<\/span>&nbsp;of the sarcolemma on the other side of the synapse. Once ACh binds, a channel in the ACh receptor opens and positively charged ions can pass through into the muscle fiber, causing it to&nbsp;<span id=\"term-00023\" data-type=\"term\">depolarize<\/span>, meaning that the membrane potential of the muscle fiber becomes less negative (closer to zero.)<\/p>\n<p id=\"eip-536\">As the membrane depolarizes, another set of ion channels called&nbsp;<span id=\"term-00024\" data-type=\"term\">voltage-gated sodium channels<\/span>&nbsp;are triggered to open. Sodium ions enter the muscle fiber, and an action potential rapidly spreads (or \u201cfires\u201d) along the entire membrane to initiate excitation-contraction coupling.<\/p>\n<p id=\"fs-id1698692\">Propagation of an action potential along the sarcolemma is the excitation portion of excitation-contraction coupling. Recall that this excitation actually triggers the release of calcium ions (Ca<sup>++<\/sup>) from its storage in the cell\u2019s sarcoplasmic reticulum (SR). For the action potential to reach the membrane of the SR, there are periodic invaginations in the sarcolemma, called <span id=\"term-00025\" data-type=\"term\">T-tubules<\/span>&nbsp;(\u201cT\u201d stands for \u201ctransverse\u201d). You will recall that the diameter of a muscle fiber can be up to 100&nbsp;<em data-effect=\"italics\">\u03bc<\/em>m, so these T-tubules ensure that the membrane can get close to the SR in the sarcoplasm. The arrangement of a T-tubule with the membranes of SR on either side is called a&nbsp;<span id=\"term-00026\" data-type=\"term\">triad<\/span>&nbsp;(<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-2-skeletal-muscle#fig-ch10_02_05\">Figure 10.7<\/a>). The triad surrounds the cylindrical structure called a&nbsp;<span id=\"term-00027\" data-type=\"term\">myofibril<\/span>, which contains actin and myosin.<\/p>\n\n<div id=\"fig-ch10_02_05\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_02_05\"><span id=\"fs-id1916516\" data-type=\"media\" data-alt=\" Alt text to come.\"><img id=\"11\" class=\"aligncenter\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/d9b10442a6f0569d0b57588ff01150c417d02884\" alt=\" Alt text to come.\" width=\"600\" height=\"382\" data-media-type=\"image\/jpg\"><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure 12.44<\/span><\/strong>&nbsp;<span id=\"10\" class=\"os-title\" data-type=\"title\">The T-tubule&nbsp;<\/span><span class=\"os-caption\">Narrow T-tubules permit the conduction of electrical impulses. The SR functions to regulate intracellular levels of calcium. Two terminal cisternae (where enlarged SR connects to the T-tubule) and one T-tubule comprise a triad\u2014a \u201cthreesome\u201d of membranes, with those of SR on two sides and the T-tubule sandwiched between them.<\/span><\/div>\n<\/div>\n<p id=\"fs-id1361245\">The T-tubules carry the action potential into the interior of the cell, which triggers the opening of calcium channels in the membrane of the adjacent SR, causing Ca<sup>++<\/sup>&nbsp;to diffuse out of the SR and into the sarcoplasm. It is the arrival of Ca<sup>++<\/sup> in the sarcoplasm that initiates contraction of the muscle fiber by its contractile units, or sarcomeres. This sets the stage for the power stroke of the sliding filament theory.<\/p>\n\n<div id=\"1\" class=\"ui-has-child-title\" data-type=\"abstract\"><\/div>\n<div id=\"fig-ch10_03_01\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_03_01\"><span id=\"fs-id2254387\" data-type=\"media\" data-alt=\"The top panel in this figure shows the interaction of a motor neuron with a muscle fiber and how the release of acetylcholine into the muscle cells leads to the release of calcium. The middle panel shows how calcium release activates troponin and leads to muscle contraction. The bottom panel shows an image of a muscle fiber being shortened and producing tension.\"><img id=\"3\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/d27415657173fa90321a95a51a9637b20d168b70\" alt=\"The top panel in this figure shows the interaction of a motor neuron with a muscle fiber and how the release of acetylcholine into the muscle cells leads to the release of calcium. The middle panel shows how calcium release activates troponin and leads to muscle contraction. The bottom panel shows an image of a muscle fiber being shortened and producing tension.\" width=\"400\" data-media-type=\"image\/jpg\"><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure 12.45<\/span>&nbsp;<\/strong><span id=\"2\" class=\"os-title\" data-type=\"title\"><strong>Contraction of a Muscle Fiber.<\/strong>&nbsp;<\/span><em><span class=\"os-caption\">A cross-bridge forms between actin (thin filament) and the myosin (thick filament) heads triggering contraction.&nbsp;<\/span><\/em><\/div>\n<\/div>\n<div id=\"fig-ch10_03_02\" class=\"os-figure\">\n<div class=\"os-caption-container\"><\/div>\n<\/div>\n<section id=\"fs-id2131675\" data-depth=\"1\">\n<h2 data-type=\"title\">The Sliding Filament Model of Contraction<\/h2>\n<p id=\"fs-id1478868\"><span style=\"color: #008000\">The sliding filament model of contraction can be used to demonstrate the process of muscle contraction (<a class=\"autogenerated-content\" style=\"color: #008000\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_03\">Figure 10.10<\/a>).<\/span> When signaled by a motor neuron, a skeletal muscle fiber contracts as <span style=\"color: #008000\">actin is pulled and then slides past the myosin<\/span>&nbsp;within the fiber\u2019s sarcomeres. The sliding can only occur when myosin-binding sites on the actin filaments are exposed by a series of steps as explained above.<\/p>\n<span class=\"os-caption\">When a sarcomere contracts, the Z lines, <span style=\"color: #008000\">or the junction of actin filaments marking the boundaries of each sarcomere, <\/span>move closer together. &nbsp;The I bands, <span style=\"color: #008000\">or areas of myosin only, <\/span>becomes smaller. The A band, <span style=\"color: #008000\">or areas of both actin and myosin,<\/span> stays the same width. At full contraction, the actin and myosin filaments overlap completely. <\/span>\n\n<span style=\"color: #0000ff\">LABEL RELAXED VS CONTRACTED MUSCLE &amp; ACTIN VS MYOSIN &amp; SARCOMERE IN IMAGE + Change fig #s if necessary<\/span>:\n<div id=\"fig-ch10_03_03\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_03_03\"><span id=\"fs-id2269514\" data-type=\"media\" data-alt=\"This diagram shows how muscle contracts. The top panel shows the stretched filaments and the bottom panel shows the compressed filaments.\"><img id=\"7\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/1463f0ef934552382ad130bf14cd23dd9e1e779a\" alt=\"This diagram shows how muscle contracts. The top panel shows the stretched filaments and the bottom panel shows the compressed filaments.\" width=\"450\" data-media-type=\"image\/jpg\"><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure&nbsp;<\/span><span class=\"os-number\">12.46 <\/span><span class=\"os-title\" data-type=\"title\">The Sliding Filament Model of Muscle Contraction. <\/span><\/strong><em><span style=\"color: #008000\"><span style=\"text-align: initial;font-size: 1em\">Z lines anchor the ends of actin filaments marking the boundaries of each sarcomere. The <\/span><span style=\"text-align: initial;font-size: 1em\">M line is the attachment site for thick filaments and is in the center of the A band. <\/span><span style=\"text-align: initial;font-size: 1em\">The H band is a region which contains only myosin while<\/span><span style=\"text-align: initial;font-size: 1em\"> I bands contain only actin. The<\/span><\/span><span style=\"text-align: initial;font-size: 1em\"><span style=\"color: #008000\"> A band is the site where filament movement starts and contains both actin and myosin and has a dense appearance. A myofibril is composed of many sarcomeres running along its length; thus, myofibrils and muscle cells contract as the sarcomeres contract.<\/span><\/span><\/em><\/div>\n<div><\/div>\n<div><span style=\"font-family: 'Cormorant Garamond', serif;font-size: 1.602em;font-weight: bold\">The Power Stroke, ATP and Muscle Contraction<\/span><\/div>\n<\/div>\n<\/section><section id=\"fs-id1582849\" data-depth=\"1\">\n<p id=\"fs-id1645190\"><span style=\"color: #008000\">Tropomyosin (a protein that winds around actin and covers actin's myosin-binding sites) binds to troponin to form a complex<\/span>. This troponin-tropomyosin complex prevents the myosin \u201cheads\u201d from binding to the active sites on the actin microfilaments. To initiate muscle contraction, tropomyosin has to expose the myosin-binding site on an actin filament. The first step in the process of contraction is for <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> to <span style=\"color: #008000\">bind to <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+ <\/sup>binding sites on troponin<\/span> so that tropomyosin can slide away from the binding sites on the actin strands. This allows the myosin heads to bind to these exposed binding sites and form cross-bridges. The actin filaments are then pulled by the myosin heads to slide past the myosin filaments toward the center of the sarcomere. But each head can only pull a very short distance before it has reached its limit and must be \u201cre-cocked\u201d <span style=\"color: #008000\">(or reset) <\/span>before it can pull again, a step that requires energy provided by ATP.<\/p>\n<p id=\"fs-id2030918\">For actin filaments to continue to slide past myosin filaments during muscle contraction, myosin heads <span style=\"color: #008000\">must:<\/span><\/p>\n\n<ol>\n \t<li><span style=\"color: #008000\">Pull the actin at the binding sites<\/span><\/li>\n \t<li><span style=\"text-align: initial;font-size: 1em;color: #008000\">Detach<\/span><\/li>\n \t<li><span style=\"text-align: initial;font-size: 1em;color: #008000\">Re-cock (reset)<\/span><\/li>\n \t<li><span style=\"text-align: initial;font-size: 1em\"><span style=\"color: #008000\">Repeat (attach to more binding sites)<\/span><\/span><\/li>\n<\/ol>\nThis repeated movement is known as the cross-bridge cycle. This motion of the myosin heads is similar to the oars when an individual rows a boat: The paddle of the oars (the myosin heads) pull, are lifted from the water (detach), repositioned (re-cocked) and then immersed again to pull (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_04\">Figure 10.11<\/a>).\n<div id=\"fig-ch10_03_04\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_03_04\"><span id=\"fs-id2142612\" data-type=\"media\" data-alt=\"This multipart figure shows the mechanism of skeletal muscle contraction. In the top panel, the ADP and inorganic phosphate molecules are bound to the myosin motor head. In the middle panel, the ADP and phosphate come off the myosin motor and the direction of the power stroke is shown. In the bottom panel, a molecule of ATP is shown to bind the myosin motor head and the motor is reset.\"><img id=\"9\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/86ea61f9cc667f8fdba8e214b3cefcb16b60676e\" alt=\"This multipart figure shows the mechanism of skeletal muscle contraction. In the top panel, the ADP and inorganic phosphate molecules are bound to the myosin motor head. In the middle panel, the ADP and phosphate come off the myosin motor and the direction of the power stroke is shown. In the bottom panel, a molecule of ATP is shown to bind the myosin motor head and the motor is reset.\" width=\"450\" data-media-type=\"image\/jpg\"><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure&nbsp;<\/span><span class=\"os-number\">12.47<\/span>&nbsp;<\/strong><span id=\"8\" class=\"os-title\" data-type=\"title\"><strong>Skeletal Muscle Contraction.<\/strong>&nbsp;<\/span><em><span class=\"os-caption\">(a) The active site on actin is exposed as <span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup> binds to troponin. (b) The myosin (thick filament) head is attracted to actin (thin filament), and myosin binds actin at its actin-binding site, forming the cross-bridge. (c) During the power stroke, the phosphate generated in the previous contraction cycle is released. This results in the myosin head pivoting toward the center of the sarcomere, after which the attached ADP and phosphate group are released. (d) A new molecule of ATP attaches to the myosin head, causing the cross-bridge to detach. (e) The myosin head hydrolyzes ATP to ADP and phosphate, which returns the myosin to the cocked position.<\/span><\/em><\/div>\n<\/div>\n<p id=\"fs-id1471902\">Cross-bridge formation occurs when the myosin head attaches to the actin while adenosine diphosphate (ADP) and inorganic phosphate (P<sub>i<\/sub>) are still bound to myosin (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_04\">Figure 10.11a,b<\/a>). P<sub>i<\/sub> is then released, causing myosin to form a stronger attachment to the actin, after which the myosin head moves toward the M-line, pulling the actin along with it. <span style=\"color: #008000\">The filaments move approximately 10 nm toward the M-line. This movement is called the <span id=\"term-00001\" data-type=\"term\">power stroke<\/span><\/span>&nbsp;(<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_04\">Figure 10.11c<\/a>). In the absence of ATP, the myosin head will not detach from actin.<\/p>\n<p id=\"fs-id1236880\">One part of the myosin head attaches to the binding site on the actin, but has another binding site for ATP <span style=\"color: #008000\">that allows the myosin head to detach from the actin<\/span> (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_04\">Figure 10.11d<\/a>). After this occurs, ATP is converted to ADP and P<sub>i<\/sub>&nbsp;by the intrinsic&nbsp;<span id=\"term-00002\" data-type=\"term\">ATPase<\/span>&nbsp;activity of myosin. The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_04\">Figure 10.11e<\/a>). The myosin head is now in position for further movement.<\/p>\n<p id=\"fs-id2004997\">When the myosin head is cocked, myosin is in a high-energy configuration. This energy is expended as the myosin head moves through the power stroke. At the end of the power stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released; however, the formed cross-bridge is still in place, and actin and myosin are bound together. As long as ATP is available, it readily attaches to myosin, the cross-bridge cycle can recur, and muscle contraction can continue.<\/p>\n<p id=\"fs-id2059667\">Note that each thick filament of roughly 300 myosin molecules has multiple myosin heads, and many cross-bridges form and break continuously during muscle contraction. Multiply this by all of the sarcomeres in one myofibril, all the myofibrils in one muscle fiber, and all of the muscle fibers in one skeletal muscle, and you can understand why so much ATP is needed to keep skeletal muscles working. In fact, it is the loss of ATP that results in the rigor mortis observed soon after someone dies. With no further ATP production possible, there is no ATP available for myosin heads to detach from the <span style=\"color: #339966\">myosin<\/span>-binding sites <span style=\"color: #339966\">on actin<\/span>, so the cross-bridges stay in place, causing the rigidity in the skeletal muscles.<\/p>\n\n<h2>Relaxation of a Skeletal Muscle<\/h2>\nThings happen very quickly in the world of excitable membranes (just think about how quickly you can snap your fingers as soon as you decide to do it). Immediately following depolarization of the membrane, it repolarizes, re-establishing the negative membrane potential. Meanwhile, the ACh in the synaptic cleft is degraded by the enzyme acetylcholinesterase (AChE) so that the ACh cannot rebind to a receptor and reopen its channel, which would cause unwanted extended muscle excitation and contraction.\n\n<section id=\"fs-id2141622\" data-depth=\"1\">\n<p id=\"fs-id2023667\">Muscle contraction usually stops when signaling from the motor neuron ends. <span style=\"color: #008000\">This <\/span>repolarizes the sarcolemma and T-tubules, and closes the voltage-gated <span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup> channels in the SR. <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> ions are then pumped back into the SR, which causes the tropomyosin to reshield the binding sites on the actin strands. A muscle also can stop contracting when it runs out of ATP and becomes fatigued (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_02\">Figure 10.9<\/a>).<\/p>\nThus, this&nbsp; combination of repolarized membranes (in both the motor neuron and sarcolemma) and the lack of Ca+2 available for troponin causes the skeletal muscle to relax and prepare for the next contraction.\n<div id=\"fig-ch10_03_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_03_02\"><span id=\"fs-id805157\" data-type=\"media\" data-alt=\"The top panel in this figure shows the interaction of a motor neuron with a muscle fiber and how calcium is being absorbed into the muscle fiber. This results in the relaxation of the thin and thick filaments as shown in the bottom panel.\"><img id=\"5\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/aa1cd0ddb04d7fe1e9990764a2609235d96ac0c5\" alt=\"The top panel in this figure shows the interaction of a motor neuron with a muscle fiber and how calcium is being absorbed into the muscle fiber. This results in the relaxation of the thin and thick filaments as shown in the bottom panel.\" width=\"380\" data-media-type=\"image\/jpg\"><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure&nbsp;<\/span><span class=\"os-number\">12.48<\/span><\/strong><span id=\"4\" class=\"os-title\" data-type=\"title\"><strong>.<\/strong> Muscle relaxes as <\/span><em><span class=\"os-caption\"><span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+ <\/sup>ions are pumped back into the SR.<\/span><\/em><\/div>\n<\/div>\n<\/section><section id=\"fs-id2094906\" data-depth=\"1\">\n<h2 data-type=\"title\"><\/h2>\n<h2 data-type=\"title\">Muscle Strength<\/h2>\n<p id=\"fs-id1725236\">The number of skeletal muscle fibers in a given muscle is genetically determined and does not change. Muscle strength is directly related to the amount of myofibrils and sarcomeres within each fiber. Factors, such as hormones and stress (and artificial anabolic steroids), acting on the muscle can increase the production of sarcomeres and myofibrils within the muscle fibers. <span style=\"color: #008000\">This change is called hypertrophy, <\/span>which results in the increased mass and bulk in a skeletal muscle. Likewise, decreased use of a skeletal muscle - whether due to inactivity or lack of nervous stimulation - results in atrophy, where the number of sarcomeres and myofibrils disappear (but not the number of muscle fibers).<\/p>\n\n<\/section><\/section><\/section><section id=\"fs-id1040329\" data-depth=\"1\">\n<figure data-id=\"fig-ch10_02_03\"><\/figure>\n<div class=\"textbox textbox--examples\"><header class=\"textbox__header\">\n<p class=\"textbox__title\">Disorders of the Muscular System<\/p>\n\n<\/header>\n<div class=\"textbox__content\">\n<p id=\"fs-id1932393\">Duchenne muscular dystrophy (DMD), <span style=\"color: #008000\">one of several diseases collectively referred to as \u201cmuscular dystrophy,\" is a progressive weakening of the skeletal muscles<\/span>. DMD is caused by a lack of the protein dystrophin, which helps <span style=\"color: #008000\">actin<\/span>&nbsp;bind to the sarcolemma. Without sufficient dystrophin, muscle contractions cause the sarcolemma to tear, causing an influx of <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup>, leading to cellular damage and muscle fiber degradation. Over time, muscle mass is lost, and greater functional impairments develop.<\/p>\n<p id=\"fs-id1417377\">DMD is an inherited disorder caused by a <span style=\"color: #008000\">mutation in the gene that codes for dystrophin in an<\/span>&nbsp;abnormal X chromosome. It primarily affects males, and it is usually diagnosed in early childhood. DMD usually first appears as difficulty with balance and motion, and then progresses to an inability to walk. It continues progressing upward in the body from the lower extremities to the upper body, where it affects the muscles responsible for breathing and circulation. It ultimately causes death due to respiratory failure, and those afflicted do not usually live past their 20s.<\/p>\n\n<\/div>\n<\/div>\n<section id=\"fs-id2094906\" data-depth=\"1\">\n<div id=\"fs-id1409378\" class=\"anatomy disorders ui-has-child-title\" data-type=\"note\"><section>\n<div id=\"fs-id2095890\" class=\"anatomy interactive ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<h1>Section Review<\/h1>\n<span style=\"color: #339966\">Muscle contraction starts with ACh release which prompts the sarcolemma to depolarize (Na<sup>+<\/sup> enters), creating an action potential. <span style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\"><span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup><\/span> is then released from the SR which binds to troponin so that tropomyosin slides away to keep myosin-binding sites exposed. This allows myosin heads to bind to actin for contraction. <\/span><span style=\"color: #339966\">In the sliding filament model of muscle contraction, actin is pulled and slides past myosin. In a power stroke, phosphate and ADP (hydrolyzed from the ATP used to release the myosin head from the previous contraction cycle) is released, causing the myosin head to have a stronger attachment to actin. The myosin head then moves actin towards the M line (sarcomere center). Myosin detaches from actin when ATP attaches to the myosin head and resets the head for more contraction cycles. <\/span><span style=\"color: #339966\">Repolarization occurs when ACh has stopped being released and <span style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\"><span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup><\/span> returns to the SR. In the absence of <span style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\"><span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup><\/span>, tropomyosin reshields the myosin-binding sites on actin which prevents myosin heads from binding.<\/span>\n<h1 class=\"os-title\" style=\"text-align: justify\" data-type=\"title\"><span class=\"os-title-label\">Interactive Link Questions<\/span><\/h1>\n<p class=\"os-title\" data-type=\"title\"><span style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\">The release of <span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup> initiates muscle contractions. Watch this <\/span><a style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\" href=\"http:\/\/openstax.org\/l\/calciumrole\" target=\"_blank\" rel=\"noopener nofollow\">video<\/a><span style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\"> to learn more about the role of <span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup>.<\/span><\/p>\n<p data-type=\"title\">[h5p id=\"331\"]<\/p>\n\n<h1>Review Questions<\/h1>\n[h5p id=\"343\"]\n\n<\/div>\n<\/section><\/div>\n<\/section><\/section><section id=\"fs-id1909043\" data-depth=\"1\">\n<h1>Adaption<\/h1>\n<p style=\"text-align: start\">This chapter was adapted by Valerie Swanston and Eva M Su&nbsp; from the following texts:<\/p>\n<p style=\"text-align: start\"><a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-2-skeletal-muscle\" target=\"_blank\" rel=\"noopener\">Skeletal Muscle<\/a>&nbsp;<strong>in&nbsp;<\/strong><a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/\">Anatomy and Physiology<\/a>&nbsp;by&nbsp;OSCRiceUniversity&nbsp;is licensed under a&nbsp;<a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">Creative Commons Attribution 4.0 International License<\/a><\/p>\n<span style=\"color: #99cc00\"><a style=\"color: #99cc00\" href=\"https:\/\/www.coursehero.com\/study-guides\/cuny-csi-ap-1\/muscular-levels-of-organization\/\">Muscular Levels of Organization <\/a>from by&nbsp; <a href=\"https:\/\/www.nursinghero.com\/study-guides\/cuny-csi-ap-1\/muscular-levels-of-organization\">Nursing Hero <\/a>licensed under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">Creative Commons Attribution 4.0 International License<\/a><\/span>\n\n&nbsp;\n\n<\/section>\n","rendered":"<div class=\"textbox textbox--learning-objectives\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\">Learning Objectives<\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p><span style=\"text-align: initial;font-size: 1em\">By the end of this section, you will be able to:<\/span><\/p>\n<div id=\"1\" class=\"ui-has-child-title\" data-type=\"abstract\">\n<section>\n<ul id=\"list-00001\">\n<li>Identify the components of the neuromuscular junction<\/li>\n<li>Describe excitation-contraction coupling and formation of a power stroke<\/li>\n<li>Explain how muscles contract and relax<\/li>\n<li>Describe the mechanisms of ATP regeneration for use in muscle contraction<\/li>\n<\/ul>\n<\/section>\n<\/div>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<section id=\"fs-id1040329\" data-depth=\"1\">\n<div id=\"fig-ch10_02_03\" class=\"os-figure\">\n<div class=\"os-caption-container\">&nbsp;<span style=\"font-family: 'Cormorant Garamond', serif;font-size: 1.602em\">The Neuromuscular Junction<\/span><\/div>\n<\/div>\n<\/section>\n<section id=\"fs-id1990056\" data-depth=\"1\">\n<p id=\"fs-id1854958\">A specialization of the skeletal muscle is the site where a motor neuron\u2019s terminal meets the muscle fiber\u2014called the <span id=\"term-00016\" data-type=\"term\">neuromuscular junction (NMJ)<\/span>. This is where the muscle fiber first responds to electrical signaling by the motor neuron. Every skeletal muscle fiber in every skeletal muscle is innervated by a motor neuron at the NMJ. Excitation signals from the neuron are the only way to functionally activate the fiber to contract.<\/p>\n<p><span style=\"color: #008000\">The sequence of events that result in the contraction of an individual muscle fiber begins with a signal\u2014the neurotransmitter, acetylcholine (ACh)\u2014from the motor neuron innervating that muscle fiber. The sarcolemma, the membrane surrounding the fibers, will depolarize as positively charged sodium ions (Na<sup>+<\/sup>) enter. This is where the Excitation-Contraction Coupling begins.&nbsp;<\/span><\/p>\n<div id=\"fs-id2017671\" class=\"anatomy interactive ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<div class=\"textbox shaded\">Every skeletal muscle fiber is supplied by a motor neuron at the NMJ. Watch this&nbsp;<a href=\"http:\/\/openstax.org\/l\/skelmuscfiber\" target=\"_blank\" rel=\"noopener nofollow\">video<\/a>&nbsp;to learn more about what happens at the NMJ. (a) What is the definition of a motor unit? (b) What is the structural and functional difference between a large motor unit and a small motor unit? (c) Can you give an example of each? (d) Why is the neurotransmitter acetylcholine degraded after binding to its receptor?<\/div>\n<\/div>\n<\/section>\n<\/div>\n<\/section>\n<section id=\"fs-id1909043\" data-depth=\"1\">\n<div id=\"fig-ch10_02_04\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_02_04\"><span id=\"fs-id1236880\" data-type=\"media\" data-alt=\"Alt text to come.\"><img loading=\"lazy\" decoding=\"async\" id=\"9\" class=\"aligncenter\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/ff663ee127683f7dd83c225b6d39d277da9256ed\" alt=\"Alt text to come.\" width=\"473\" height=\"824\" data-media-type=\"image\/jpg\" \/><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure 12.43<\/span><\/strong>&nbsp;<span id=\"8\" class=\"os-title\" data-type=\"title\">Motor End-Plate and Innervation&nbsp;<\/span><span class=\"os-caption\">At the NMJ, the axon terminal releases acetylcholine (ACh). The motor end-plate is the location of the ACh-receptors in the muscle fiber sarcolemma. When ACh molecules are released, they diffuse across a minute space called the synaptic cleft and bind to the receptors.<\/span><\/div>\n<\/div>\n<h3 data-type=\"title\">Excitation-Contraction Coupling<\/h3>\n<p id=\"fs-id2252260\">All living cells have membrane potentials, or electrical gradients across their membranes. The inside of the membrane is usually around -60 to -90 mV, relative to the outside. This is referred to as a cell\u2019s membrane potential. Neurons and muscle cells can use their membrane potentials to generate electrical signals. They do this by controlling the movement of charged particles, called ions, across their membranes to create electrical currents. This is achieved by opening and closing specialized proteins in the membrane called ion channels. Although the currents generated by ions moving through these channel proteins are very small, they form the basis of both neural signaling and muscle contraction.<\/p>\n<p id=\"fs-id1421868\">Both neurons and skeletal muscle cells are electrically excitable, meaning that they are able to generate action potentials. An action potential is a special type of electrical signal that can travel along a cell membrane as a wave. This allows a signal to be transmitted quickly and faithfully over long distances.<\/p>\n<p id=\"eip-362\">For a skeletal muscle fiber to contract, its membrane must first be \u201cexcited\u201d\u2014in other words, it must be stimulated to fire an action potential. The muscle fiber action potential, which sweeps along the sarcolemma as a wave, is \u201ccoupled\u201d to the actual contraction through the release of calcium ions (Ca<sup>++<\/sup>) from the SR. Once released, the Ca<sup>++<\/sup>&nbsp;interacts with the shielding proteins, forcing them to move aside so that the actin-binding sites are available for attachment by myosin heads. The myosin then pulls the actin filaments toward the center, shortening the muscle fiber.<\/p>\n<p>&nbsp;<\/p>\n<p id=\"fs-id2023666\">Signaling begins when a neuronal&nbsp;<span id=\"term-00018\" data-type=\"term\">action potential<\/span>&nbsp;travels along the axon of a motor neuron, and then along the individual branches to terminate at the NMJ. At the NMJ, the axon terminal releases a chemical messenger, or&nbsp;<span id=\"term-00019\" data-type=\"term\">neurotransmitter<\/span>, called&nbsp;<span id=\"term-00020\" data-type=\"term\">acetylcholine (ACh)<\/span>. The ACh molecules diffuse across a minute space called the&nbsp;<span id=\"term-00021\" data-type=\"term\">synaptic cleft<\/span>&nbsp;and bind to ACh receptors located within the&nbsp;<span id=\"term-00022\" data-type=\"term\">motor end-plate<\/span>&nbsp;of the sarcolemma on the other side of the synapse. Once ACh binds, a channel in the ACh receptor opens and positively charged ions can pass through into the muscle fiber, causing it to&nbsp;<span id=\"term-00023\" data-type=\"term\">depolarize<\/span>, meaning that the membrane potential of the muscle fiber becomes less negative (closer to zero.)<\/p>\n<p id=\"eip-536\">As the membrane depolarizes, another set of ion channels called&nbsp;<span id=\"term-00024\" data-type=\"term\">voltage-gated sodium channels<\/span>&nbsp;are triggered to open. Sodium ions enter the muscle fiber, and an action potential rapidly spreads (or \u201cfires\u201d) along the entire membrane to initiate excitation-contraction coupling.<\/p>\n<p id=\"fs-id1698692\">Propagation of an action potential along the sarcolemma is the excitation portion of excitation-contraction coupling. Recall that this excitation actually triggers the release of calcium ions (Ca<sup>++<\/sup>) from its storage in the cell\u2019s sarcoplasmic reticulum (SR). For the action potential to reach the membrane of the SR, there are periodic invaginations in the sarcolemma, called <span id=\"term-00025\" data-type=\"term\">T-tubules<\/span>&nbsp;(\u201cT\u201d stands for \u201ctransverse\u201d). You will recall that the diameter of a muscle fiber can be up to 100&nbsp;<em data-effect=\"italics\">\u03bc<\/em>m, so these T-tubules ensure that the membrane can get close to the SR in the sarcoplasm. The arrangement of a T-tubule with the membranes of SR on either side is called a&nbsp;<span id=\"term-00026\" data-type=\"term\">triad<\/span>&nbsp;(<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-2-skeletal-muscle#fig-ch10_02_05\">Figure 10.7<\/a>). The triad surrounds the cylindrical structure called a&nbsp;<span id=\"term-00027\" data-type=\"term\">myofibril<\/span>, which contains actin and myosin.<\/p>\n<div id=\"fig-ch10_02_05\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_02_05\"><span id=\"fs-id1916516\" data-type=\"media\" data-alt=\"Alt text to come.\"><img loading=\"lazy\" decoding=\"async\" id=\"11\" class=\"aligncenter\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/d9b10442a6f0569d0b57588ff01150c417d02884\" alt=\"Alt text to come.\" width=\"600\" height=\"382\" data-media-type=\"image\/jpg\" \/><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure 12.44<\/span><\/strong>&nbsp;<span id=\"10\" class=\"os-title\" data-type=\"title\">The T-tubule&nbsp;<\/span><span class=\"os-caption\">Narrow T-tubules permit the conduction of electrical impulses. The SR functions to regulate intracellular levels of calcium. Two terminal cisternae (where enlarged SR connects to the T-tubule) and one T-tubule comprise a triad\u2014a \u201cthreesome\u201d of membranes, with those of SR on two sides and the T-tubule sandwiched between them.<\/span><\/div>\n<\/div>\n<p id=\"fs-id1361245\">The T-tubules carry the action potential into the interior of the cell, which triggers the opening of calcium channels in the membrane of the adjacent SR, causing Ca<sup>++<\/sup>&nbsp;to diffuse out of the SR and into the sarcoplasm. It is the arrival of Ca<sup>++<\/sup> in the sarcoplasm that initiates contraction of the muscle fiber by its contractile units, or sarcomeres. This sets the stage for the power stroke of the sliding filament theory.<\/p>\n<div id=\"1\" class=\"ui-has-child-title\" data-type=\"abstract\"><\/div>\n<div id=\"fig-ch10_03_01\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_03_01\"><span id=\"fs-id2254387\" data-type=\"media\" data-alt=\"The top panel in this figure shows the interaction of a motor neuron with a muscle fiber and how the release of acetylcholine into the muscle cells leads to the release of calcium. The middle panel shows how calcium release activates troponin and leads to muscle contraction. The bottom panel shows an image of a muscle fiber being shortened and producing tension.\"><img decoding=\"async\" id=\"3\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/d27415657173fa90321a95a51a9637b20d168b70\" alt=\"The top panel in this figure shows the interaction of a motor neuron with a muscle fiber and how the release of acetylcholine into the muscle cells leads to the release of calcium. The middle panel shows how calcium release activates troponin and leads to muscle contraction. The bottom panel shows an image of a muscle fiber being shortened and producing tension.\" width=\"400\" data-media-type=\"image\/jpg\" \/><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure 12.45<\/span>&nbsp;<\/strong><span id=\"2\" class=\"os-title\" data-type=\"title\"><strong>Contraction of a Muscle Fiber.<\/strong>&nbsp;<\/span><em><span class=\"os-caption\">A cross-bridge forms between actin (thin filament) and the myosin (thick filament) heads triggering contraction.&nbsp;<\/span><\/em><\/div>\n<\/div>\n<div id=\"fig-ch10_03_02\" class=\"os-figure\">\n<div class=\"os-caption-container\"><\/div>\n<\/div>\n<section id=\"fs-id2131675\" data-depth=\"1\">\n<h2 data-type=\"title\">The Sliding Filament Model of Contraction<\/h2>\n<p id=\"fs-id1478868\"><span style=\"color: #008000\">The sliding filament model of contraction can be used to demonstrate the process of muscle contraction (<a class=\"autogenerated-content\" style=\"color: #008000\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_03\">Figure 10.10<\/a>).<\/span> When signaled by a motor neuron, a skeletal muscle fiber contracts as <span style=\"color: #008000\">actin is pulled and then slides past the myosin<\/span>&nbsp;within the fiber\u2019s sarcomeres. The sliding can only occur when myosin-binding sites on the actin filaments are exposed by a series of steps as explained above.<\/p>\n<p><span class=\"os-caption\">When a sarcomere contracts, the Z lines, <span style=\"color: #008000\">or the junction of actin filaments marking the boundaries of each sarcomere, <\/span>move closer together. &nbsp;The I bands, <span style=\"color: #008000\">or areas of myosin only, <\/span>becomes smaller. The A band, <span style=\"color: #008000\">or areas of both actin and myosin,<\/span> stays the same width. At full contraction, the actin and myosin filaments overlap completely. <\/span><\/p>\n<p><span style=\"color: #0000ff\">LABEL RELAXED VS CONTRACTED MUSCLE &amp; ACTIN VS MYOSIN &amp; SARCOMERE IN IMAGE + Change fig #s if necessary<\/span>:<\/p>\n<div id=\"fig-ch10_03_03\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_03_03\"><span id=\"fs-id2269514\" data-type=\"media\" data-alt=\"This diagram shows how muscle contracts. The top panel shows the stretched filaments and the bottom panel shows the compressed filaments.\"><img decoding=\"async\" id=\"7\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/1463f0ef934552382ad130bf14cd23dd9e1e779a\" alt=\"This diagram shows how muscle contracts. The top panel shows the stretched filaments and the bottom panel shows the compressed filaments.\" width=\"450\" data-media-type=\"image\/jpg\" \/><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure&nbsp;<\/span><span class=\"os-number\">12.46 <\/span><span class=\"os-title\" data-type=\"title\">The Sliding Filament Model of Muscle Contraction. <\/span><\/strong><em><span style=\"color: #008000\"><span style=\"text-align: initial;font-size: 1em\">Z lines anchor the ends of actin filaments marking the boundaries of each sarcomere. The <\/span><span style=\"text-align: initial;font-size: 1em\">M line is the attachment site for thick filaments and is in the center of the A band. <\/span><span style=\"text-align: initial;font-size: 1em\">The H band is a region which contains only myosin while<\/span><span style=\"text-align: initial;font-size: 1em\"> I bands contain only actin. The<\/span><\/span><span style=\"text-align: initial;font-size: 1em\"><span style=\"color: #008000\"> A band is the site where filament movement starts and contains both actin and myosin and has a dense appearance. A myofibril is composed of many sarcomeres running along its length; thus, myofibrils and muscle cells contract as the sarcomeres contract.<\/span><\/span><\/em><\/div>\n<div><\/div>\n<div><span style=\"font-family: 'Cormorant Garamond', serif;font-size: 1.602em;font-weight: bold\">The Power Stroke, ATP and Muscle Contraction<\/span><\/div>\n<\/div>\n<\/section>\n<section id=\"fs-id1582849\" data-depth=\"1\">\n<p id=\"fs-id1645190\"><span style=\"color: #008000\">Tropomyosin (a protein that winds around actin and covers actin&#8217;s myosin-binding sites) binds to troponin to form a complex<\/span>. This troponin-tropomyosin complex prevents the myosin \u201cheads\u201d from binding to the active sites on the actin microfilaments. To initiate muscle contraction, tropomyosin has to expose the myosin-binding site on an actin filament. The first step in the process of contraction is for <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> to <span style=\"color: #008000\">bind to <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+ <\/sup>binding sites on troponin<\/span> so that tropomyosin can slide away from the binding sites on the actin strands. This allows the myosin heads to bind to these exposed binding sites and form cross-bridges. The actin filaments are then pulled by the myosin heads to slide past the myosin filaments toward the center of the sarcomere. But each head can only pull a very short distance before it has reached its limit and must be \u201cre-cocked\u201d <span style=\"color: #008000\">(or reset) <\/span>before it can pull again, a step that requires energy provided by ATP.<\/p>\n<p id=\"fs-id2030918\">For actin filaments to continue to slide past myosin filaments during muscle contraction, myosin heads <span style=\"color: #008000\">must:<\/span><\/p>\n<ol>\n<li><span style=\"color: #008000\">Pull the actin at the binding sites<\/span><\/li>\n<li><span style=\"text-align: initial;font-size: 1em;color: #008000\">Detach<\/span><\/li>\n<li><span style=\"text-align: initial;font-size: 1em;color: #008000\">Re-cock (reset)<\/span><\/li>\n<li><span style=\"text-align: initial;font-size: 1em\"><span style=\"color: #008000\">Repeat (attach to more binding sites)<\/span><\/span><\/li>\n<\/ol>\n<p>This repeated movement is known as the cross-bridge cycle. This motion of the myosin heads is similar to the oars when an individual rows a boat: The paddle of the oars (the myosin heads) pull, are lifted from the water (detach), repositioned (re-cocked) and then immersed again to pull (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_04\">Figure 10.11<\/a>).<\/p>\n<div id=\"fig-ch10_03_04\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_03_04\"><span id=\"fs-id2142612\" data-type=\"media\" data-alt=\"This multipart figure shows the mechanism of skeletal muscle contraction. In the top panel, the ADP and inorganic phosphate molecules are bound to the myosin motor head. In the middle panel, the ADP and phosphate come off the myosin motor and the direction of the power stroke is shown. In the bottom panel, a molecule of ATP is shown to bind the myosin motor head and the motor is reset.\"><img decoding=\"async\" id=\"9\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/86ea61f9cc667f8fdba8e214b3cefcb16b60676e\" alt=\"This multipart figure shows the mechanism of skeletal muscle contraction. In the top panel, the ADP and inorganic phosphate molecules are bound to the myosin motor head. In the middle panel, the ADP and phosphate come off the myosin motor and the direction of the power stroke is shown. In the bottom panel, a molecule of ATP is shown to bind the myosin motor head and the motor is reset.\" width=\"450\" data-media-type=\"image\/jpg\" \/><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure&nbsp;<\/span><span class=\"os-number\">12.47<\/span>&nbsp;<\/strong><span id=\"8\" class=\"os-title\" data-type=\"title\"><strong>Skeletal Muscle Contraction.<\/strong>&nbsp;<\/span><em><span class=\"os-caption\">(a) The active site on actin is exposed as <span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup> binds to troponin. (b) The myosin (thick filament) head is attracted to actin (thin filament), and myosin binds actin at its actin-binding site, forming the cross-bridge. (c) During the power stroke, the phosphate generated in the previous contraction cycle is released. This results in the myosin head pivoting toward the center of the sarcomere, after which the attached ADP and phosphate group are released. (d) A new molecule of ATP attaches to the myosin head, causing the cross-bridge to detach. (e) The myosin head hydrolyzes ATP to ADP and phosphate, which returns the myosin to the cocked position.<\/span><\/em><\/div>\n<\/div>\n<p id=\"fs-id1471902\">Cross-bridge formation occurs when the myosin head attaches to the actin while adenosine diphosphate (ADP) and inorganic phosphate (P<sub>i<\/sub>) are still bound to myosin (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_04\">Figure 10.11a,b<\/a>). P<sub>i<\/sub> is then released, causing myosin to form a stronger attachment to the actin, after which the myosin head moves toward the M-line, pulling the actin along with it. <span style=\"color: #008000\">The filaments move approximately 10 nm toward the M-line. This movement is called the <span id=\"term-00001\" data-type=\"term\">power stroke<\/span><\/span>&nbsp;(<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_04\">Figure 10.11c<\/a>). In the absence of ATP, the myosin head will not detach from actin.<\/p>\n<p id=\"fs-id1236880\">One part of the myosin head attaches to the binding site on the actin, but has another binding site for ATP <span style=\"color: #008000\">that allows the myosin head to detach from the actin<\/span> (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_04\">Figure 10.11d<\/a>). After this occurs, ATP is converted to ADP and P<sub>i<\/sub>&nbsp;by the intrinsic&nbsp;<span id=\"term-00002\" data-type=\"term\">ATPase<\/span>&nbsp;activity of myosin. The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_04\">Figure 10.11e<\/a>). The myosin head is now in position for further movement.<\/p>\n<p id=\"fs-id2004997\">When the myosin head is cocked, myosin is in a high-energy configuration. This energy is expended as the myosin head moves through the power stroke. At the end of the power stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released; however, the formed cross-bridge is still in place, and actin and myosin are bound together. As long as ATP is available, it readily attaches to myosin, the cross-bridge cycle can recur, and muscle contraction can continue.<\/p>\n<p id=\"fs-id2059667\">Note that each thick filament of roughly 300 myosin molecules has multiple myosin heads, and many cross-bridges form and break continuously during muscle contraction. Multiply this by all of the sarcomeres in one myofibril, all the myofibrils in one muscle fiber, and all of the muscle fibers in one skeletal muscle, and you can understand why so much ATP is needed to keep skeletal muscles working. In fact, it is the loss of ATP that results in the rigor mortis observed soon after someone dies. With no further ATP production possible, there is no ATP available for myosin heads to detach from the <span style=\"color: #339966\">myosin<\/span>-binding sites <span style=\"color: #339966\">on actin<\/span>, so the cross-bridges stay in place, causing the rigidity in the skeletal muscles.<\/p>\n<h2>Relaxation of a Skeletal Muscle<\/h2>\n<p>Things happen very quickly in the world of excitable membranes (just think about how quickly you can snap your fingers as soon as you decide to do it). Immediately following depolarization of the membrane, it repolarizes, re-establishing the negative membrane potential. Meanwhile, the ACh in the synaptic cleft is degraded by the enzyme acetylcholinesterase (AChE) so that the ACh cannot rebind to a receptor and reopen its channel, which would cause unwanted extended muscle excitation and contraction.<\/p>\n<section id=\"fs-id2141622\" data-depth=\"1\">\n<p id=\"fs-id2023667\">Muscle contraction usually stops when signaling from the motor neuron ends. <span style=\"color: #008000\">This <\/span>repolarizes the sarcolemma and T-tubules, and closes the voltage-gated <span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup> channels in the SR. <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> ions are then pumped back into the SR, which causes the tropomyosin to reshield the binding sites on the actin strands. A muscle also can stop contracting when it runs out of ATP and becomes fatigued (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_02\">Figure 10.9<\/a>).<\/p>\n<p>Thus, this&nbsp; combination of repolarized membranes (in both the motor neuron and sarcolemma) and the lack of Ca+2 available for troponin causes the skeletal muscle to relax and prepare for the next contraction.<\/p>\n<div id=\"fig-ch10_03_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_03_02\"><span id=\"fs-id805157\" data-type=\"media\" data-alt=\"The top panel in this figure shows the interaction of a motor neuron with a muscle fiber and how calcium is being absorbed into the muscle fiber. This results in the relaxation of the thin and thick filaments as shown in the bottom panel.\"><img decoding=\"async\" id=\"5\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/aa1cd0ddb04d7fe1e9990764a2609235d96ac0c5\" alt=\"The top panel in this figure shows the interaction of a motor neuron with a muscle fiber and how calcium is being absorbed into the muscle fiber. This results in the relaxation of the thin and thick filaments as shown in the bottom panel.\" width=\"380\" data-media-type=\"image\/jpg\" \/><\/span><\/figure>\n<div class=\"os-caption-container\"><strong><span class=\"os-title-label\">Figure&nbsp;<\/span><span class=\"os-number\">12.48<\/span><\/strong><span id=\"4\" class=\"os-title\" data-type=\"title\"><strong>.<\/strong> Muscle relaxes as <\/span><em><span class=\"os-caption\"><span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+ <\/sup>ions are pumped back into the SR.<\/span><\/em><\/div>\n<\/div>\n<\/section>\n<section id=\"fs-id2094906\" data-depth=\"1\">\n<h2 data-type=\"title\"><\/h2>\n<h2 data-type=\"title\">Muscle Strength<\/h2>\n<p id=\"fs-id1725236\">The number of skeletal muscle fibers in a given muscle is genetically determined and does not change. Muscle strength is directly related to the amount of myofibrils and sarcomeres within each fiber. Factors, such as hormones and stress (and artificial anabolic steroids), acting on the muscle can increase the production of sarcomeres and myofibrils within the muscle fibers. <span style=\"color: #008000\">This change is called hypertrophy, <\/span>which results in the increased mass and bulk in a skeletal muscle. Likewise, decreased use of a skeletal muscle &#8211; whether due to inactivity or lack of nervous stimulation &#8211; results in atrophy, where the number of sarcomeres and myofibrils disappear (but not the number of muscle fibers).<\/p>\n<\/section>\n<\/section>\n<\/section>\n<section id=\"fs-id1040329\" data-depth=\"1\">\n<figure data-id=\"fig-ch10_02_03\"><\/figure>\n<div class=\"textbox textbox--examples\">\n<header class=\"textbox__header\">\n<p class=\"textbox__title\">Disorders of the Muscular System<\/p>\n<\/header>\n<div class=\"textbox__content\">\n<p id=\"fs-id1932393\">Duchenne muscular dystrophy (DMD), <span style=\"color: #008000\">one of several diseases collectively referred to as \u201cmuscular dystrophy,&#8221; is a progressive weakening of the skeletal muscles<\/span>. DMD is caused by a lack of the protein dystrophin, which helps <span style=\"color: #008000\">actin<\/span>&nbsp;bind to the sarcolemma. Without sufficient dystrophin, muscle contractions cause the sarcolemma to tear, causing an influx of <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup>, leading to cellular damage and muscle fiber degradation. Over time, muscle mass is lost, and greater functional impairments develop.<\/p>\n<p id=\"fs-id1417377\">DMD is an inherited disorder caused by a <span style=\"color: #008000\">mutation in the gene that codes for dystrophin in an<\/span>&nbsp;abnormal X chromosome. It primarily affects males, and it is usually diagnosed in early childhood. DMD usually first appears as difficulty with balance and motion, and then progresses to an inability to walk. It continues progressing upward in the body from the lower extremities to the upper body, where it affects the muscles responsible for breathing and circulation. It ultimately causes death due to respiratory failure, and those afflicted do not usually live past their 20s.<\/p>\n<\/div>\n<\/div>\n<section id=\"fs-id2094906\" data-depth=\"1\">\n<div id=\"fs-id1409378\" class=\"anatomy disorders ui-has-child-title\" data-type=\"note\">\n<section>\n<div id=\"fs-id2095890\" class=\"anatomy interactive ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<h1>Section Review<\/h1>\n<p><span style=\"color: #339966\">Muscle contraction starts with ACh release which prompts the sarcolemma to depolarize (Na<sup>+<\/sup> enters), creating an action potential. <span style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\"><span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup><\/span> is then released from the SR which binds to troponin so that tropomyosin slides away to keep myosin-binding sites exposed. This allows myosin heads to bind to actin for contraction. <\/span><span style=\"color: #339966\">In the sliding filament model of muscle contraction, actin is pulled and slides past myosin. In a power stroke, phosphate and ADP (hydrolyzed from the ATP used to release the myosin head from the previous contraction cycle) is released, causing the myosin head to have a stronger attachment to actin. The myosin head then moves actin towards the M line (sarcomere center). Myosin detaches from actin when ATP attaches to the myosin head and resets the head for more contraction cycles. <\/span><span style=\"color: #339966\">Repolarization occurs when ACh has stopped being released and <span style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\"><span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup><\/span> returns to the SR. In the absence of <span style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\"><span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup><\/span>, tropomyosin reshields the myosin-binding sites on actin which prevents myosin heads from binding.<\/span><\/p>\n<h1 class=\"os-title\" style=\"text-align: justify\" data-type=\"title\"><span class=\"os-title-label\">Interactive Link Questions<\/span><\/h1>\n<p class=\"os-title\" data-type=\"title\"><span style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\">The release of <span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup> initiates muscle contractions. Watch this <\/span><a style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\" href=\"http:\/\/openstax.org\/l\/calciumrole\" target=\"_blank\" rel=\"noopener nofollow\">video<\/a><span style=\"font-weight: normal;text-align: initial;font-family: Lora, serif;font-size: 1em\"> to learn more about the role of <span style=\"text-align: initial;font-size: 1em\">Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup>.<\/span><\/p>\n<p data-type=\"title\">\n<h1>Review Questions<\/h1>\n<\/div>\n<\/section>\n<\/div>\n<\/section>\n<\/section>\n<section id=\"fs-id1909043\" data-depth=\"1\">\n<h1>Adaption<\/h1>\n<p style=\"text-align: start\">This chapter was adapted by Valerie Swanston and Eva M Su&nbsp; from the following texts:<\/p>\n<p style=\"text-align: start\"><a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-2-skeletal-muscle\" target=\"_blank\" rel=\"noopener\">Skeletal Muscle<\/a>&nbsp;<strong>in&nbsp;<\/strong><a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/\">Anatomy and Physiology<\/a>&nbsp;by&nbsp;OSCRiceUniversity&nbsp;is licensed under a&nbsp;<a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">Creative Commons Attribution 4.0 International License<\/a><\/p>\n<p><span style=\"color: #99cc00\"><a style=\"color: #99cc00\" href=\"https:\/\/www.coursehero.com\/study-guides\/cuny-csi-ap-1\/muscular-levels-of-organization\/\">Muscular Levels of Organization <\/a>from by&nbsp; <a href=\"https:\/\/www.nursinghero.com\/study-guides\/cuny-csi-ap-1\/muscular-levels-of-organization\">Nursing Hero <\/a>licensed under a <a href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">Creative Commons Attribution 4.0 International License<\/a><\/span><\/p>\n<p>&nbsp;<\/p>\n<\/section>\n","protected":false},"author":1076,"menu_order":10,"template":"","meta":{"pb_show_title":"","pb_short_title":"","pb_subtitle":"","pb_authors":["j-gordon-betts-wmw0km2yc4-zwjtwys5dl","james-a-wise-twwfannsqv","kelly-a-young-2saui1agja-swe36vbrsh","eddie-johnson-ljviofnjlc-pdgaajcn0a","brandon-poe-wtgix7mvyg-tm2nlfxzlf","dean-h-kruse-fjrrkqc7wi-qwjdqlbqf5","oksana-korol-jv27eblhm3-72wdrwl9z4","jody-e-johnson-mjihsoknzs-2zewflajuk","mark-womble-jm1pj5aaoy","peter-desaix-nez4h62x7j-xmejobczo0"],"pb_section_license":""},"chapter-type":[],"contributor":[338],"license":[],"class_list":["post-216","chapter","type-chapter","status-publish","hentry","contributor-peter-desaix-nez4h62x7j-xmejobczo0"],"part":195,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/pressbooks\/v2\/chapters\/216","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/wp\/v2\/users\/1076"}],"version-history":[{"count":0,"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/pressbooks\/v2\/chapters\/216\/revisions"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/pressbooks\/v2\/parts\/195"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/pressbooks\/v2\/chapters\/216\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/wp\/v2\/media?parent=216"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/pressbooks\/v2\/chapter-type?post=216"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/wp\/v2\/contributor?post=216"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/wp\/v2\/license?post=216"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}