{"id":222,"date":"2024-11-15T11:07:55","date_gmt":"2024-11-15T16:07:55","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/chapter\/normal-muscular-structure-and-function-incl-histology\/"},"modified":"2024-11-15T11:07:55","modified_gmt":"2024-11-15T16:07:55","slug":"normal-muscular-structure-and-function-incl-histology","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/chapter\/normal-muscular-structure-and-function-incl-histology\/","title":{"raw":"Needed as we merged with other chapter re: Muscle Fiber Contraction and Relaxation (copied from GJ Betts, not cloned)","rendered":"Needed as we merged with other chapter re: Muscle Fiber Contraction and Relaxation (copied from GJ Betts, not cloned)"},"content":{"raw":"\n<div id=\"1\" class=\"ui-has-child-title\" data-type=\"abstract\"><header><header class=\"textbox__header\"><\/header><\/header><section>\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<p id=\"para-00001\">By the end of this section, you will be able to:<\/p>\n\n<ul id=\"list-00001\">\n \t<li><span style=\"color: #339966\">Identify<\/span>&nbsp;the components involved in muscle contraction<\/li>\n \t<li>Explain how muscles contract and relax<\/li>\n \t<li><span style=\"color: #339966\">Describe the mechanisms of ATP regeneration for use in muscle contraction<\/span><\/li>\n<\/ul>\n<\/div>\n<\/div>\n&nbsp;\n\n<\/section><section><span style=\"text-align: initial;font-size: 1em\">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 fiber. The <span style=\"color: #008000\">sarcolemma, the membrane surrounding the fibers, <\/span>will depolarize as positively charged sodium ions (Na<\/span><sup style=\"text-align: initial\">+<\/sup><span style=\"text-align: initial;font-size: 1em\">) enter. <span style=\"color: #008000\">This triggers an action potential that spreads to the rest of the membrane which will depolarize (including the T-tubules) and triggers the release of calcium ions (Ca<\/span><\/span><span style=\"color: #008000\"><sup style=\"text-align: initial\">2+<\/sup><\/span><span style=\"text-align: initial;font-size: 1em\"><span style=\"color: #008000\">) stored in the sarcoplasmic reticulum (SR). <\/span>The Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup><span style=\"text-align: initial;font-size: 1em\">&nbsp;then initiates contraction, which is sustained by ATP (<\/span><a class=\"autogenerated-content\" style=\"text-align: initial;font-size: 1em\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_01\">Figure 10.8<\/a><span style=\"text-align: initial;font-size: 1em\">). As long as Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup><span style=\"text-align: initial;font-size: 1em\"> ions remain in the sarcoplasm to bind to troponin, which keeps the <span style=\"color: #339966\">myosin<\/span>-binding sites <span style=\"color: #339966\">o<\/span><span style=\"color: #008000\">n actin exposed<\/span>, and as long as ATP is available to drive the cross-bridge cycling and the pulling of actin <span style=\"color: #008000\">(thin filament)<\/span> strands by myosin <span style=\"color: #008000\">(thick filament)<\/span>, the muscle fiber will continue to shorten to an anatomical limit.<\/span><\/section><\/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&nbsp;<\/span><span class=\"os-number\">10.8<\/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<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\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\">10.9<\/span>&nbsp;<\/strong><span id=\"4\" class=\"os-title\" data-type=\"title\"><strong>Relaxation of a Muscle Fiber.<\/strong> <\/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 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><strong><span class=\"os-title-label\">Figure&nbsp;<\/span><span class=\"os-number\">10.10<\/span>&nbsp;<span class=\"os-title\" data-type=\"title\">The Sliding Filament Model of Muscle Contraction. <\/span><\/strong><\/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\">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\">10.11<\/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>\n<section id=\"fs-id2141622\" data-depth=\"1\">\n<p id=\"fs-id1639118\">Relaxing skeletal muscles begin with the motor neuron, <span style=\"color: #008000\">which stops releasing ACh into the synapse at the<\/span> <span style=\"color: #008000\">neuromuscular junction (NMJ)<\/span>. The muscle fiber will repolarize, which closes the gates in the SR where <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> was being released. ATP-driven pumps will move <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> out of the sarcoplasm back into the SR. This results in the \u201creshielding\u201d of the myosin-binding sites on the thin filaments. Without the ability to form cross-bridges between the thin and thick filaments, the muscle fiber loses its tension and relaxes.<\/p>\n\n<\/section><section id=\"fs-id2094906\" data-depth=\"1\">\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 results in atrophy, where the number of sarcomeres and myofibrils disappear (but not the number of muscle fibers). It is common for a limb in a cast to show atrophied muscles when the cast is removed. <span style=\"color: #008000\">Certain diseases, such as polio, can also<\/span>&nbsp;show atrophied muscles.<\/p>\n\n<\/section><\/section><section id=\"fs-id2164808\" data-depth=\"1\">\n<h2 data-type=\"title\">Sources of ATP<\/h2>\n<p id=\"fs-id2072704\">In addition to its direct role in the cross-bridge cycle, ATP also provides the energy for the active-transport <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> pumps in the SR. Muscle contraction does not occur without sufficient amounts of ATP. The amount of ATP stored in muscle is very low, only sufficient to power a few seconds worth of contractions. As it is broken down, ATP must therefore be regenerated and replaced quickly to allow for sustained contraction. There are three mechanisms by which ATP can be regenerated: creatine phosphate metabolism, anaerobic glycolysis, and fermentation and aerobic respiration.<\/p>\n\n<h3><span style=\"color: #008000\">Creatine Phosphate Metabolism<\/span><\/h3>\n<p id=\"fs-id1898661\"><span id=\"term-00003\" data-type=\"term\">Creatine phosphate<\/span>&nbsp;is a molecule that can store energy in its phosphate bonds. In a resting muscle, excess ATP transfers its energy to creatine, producing ADP and creatine phosphate. This acts as an energy reserve that can be used to quickly create more ATP. When the muscle starts to contract and needs energy, creatine phosphate transfers its phosphate back to ADP to form ATP and creatine. This reaction is catalyzed by the enzyme creatine kinase and occurs very quickly; thus, creatine phosphate-derived ATP powers the first few seconds of muscle contraction. However, creatine phosphate can only provide approximately 15 seconds worth of energy, at which point another energy source has to be used (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_05\">Figure 10.12a<\/a>).<\/p>\n\n<h3><span style=\"color: #008000\">Anaerobic Glycolysis<\/span><\/h3>\n<p id=\"fs-id2142755\">As the ATP produced by creatine phosphate is depleted, muscles turn to glycolysis as an ATP source.&nbsp;<span id=\"term-00004\" data-type=\"term\">Glycolysis<\/span> is an anaerobic process that breaks down glucose (sugar) to produce ATP; however, glycolysis cannot generate ATP as quickly as creatine phosphate. Thus, the switch to glycolysis results in a slower rate of ATP availability to the muscle. The sugar used in glycolysis can be provided by blood glucose or by metabolizing glycogen that is stored in the muscle. The breakdown of one glucose molecule produces two ATP and two molecules of <span id=\"term-00005\" data-type=\"term\">pyruvic acid<\/span>, which can be used in aerobic respiration or converted to lactic acid <span style=\"color: #008000\">when oxygen levels are low <\/span>(<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_05\">Figure 10.12b<\/a>).<\/p>\n\n<h3><span style=\"color: #008000\">Lactic Acid Fermentation and Aerobic Respiration<\/span><\/h3>\nIf oxygen is available, pyruvic acid is used in aerobic respiration. However, if oxygen is not available, pyruvic acid is converted to&nbsp;<span id=\"term-00006\" data-type=\"term\">lactic acid<\/span>, which may contribute to muscle fatigue. This conversion allows the recycling of the enzyme NAD<sup>+<\/sup>&nbsp;from NADH, which is needed for glycolysis to continue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. Glycolysis itself cannot be sustained for very long (approximately 1 minute of muscle activity), but it is useful in facilitating short bursts of high-intensity output. This is because glycolysis does not utilize glucose very efficiently, producing a net gain of two ATPs per molecule of glucose, and the end product of lactic acid, which may contribute to muscle fatigue as it accumulates.\n<p id=\"fs-id1909043\"><span id=\"term-00007\" data-type=\"term\">Aerobic respiration<\/span>&nbsp;is the breakdown of glucose or other nutrients in the presence of oxygen (O<sub>2<\/sub>) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria. The inputs for aerobic respiration include glucose circulating in the bloodstream, pyruvic acid, and fatty acids. Aerobic respiration is much more efficient than anaerobic glycolysis, producing approximately 36 ATPs per molecule of glucose versus four from glycolysis. However, aerobic respiration cannot be sustained without a steady supply of O<sub>2<\/sub>&nbsp;to the skeletal muscle and is much slower (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_05\">Figure 10.12c<\/a>). To compensate, muscles store small amount of excess oxygen in proteins call myoglobin, allowing for more efficient muscle contractions and less fatigue. Aerobic training also increases the efficiency of the circulatory system so that O<sub>2<\/sub>&nbsp;can be supplied to the muscles for longer periods of time.<\/p>\n\n<div id=\"fig-ch10_03_05\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_03_05\"><span id=\"fs-id1700003\" data-type=\"media\" data-alt=\"This figure shows the metabolic processes in muscle. The top panel shows the reactions in resting muscle. The middle panel shows glycolysis and aerobic respiration and the bottom panel shows cellular respiration in mitochondria.\"><img id=\"11\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/b54437d9c1a12b07842921d529ee26b6b8a788a5\" alt=\"This figure shows the metabolic processes in muscle. The top panel shows the reactions in resting muscle. The middle panel shows glycolysis and aerobic respiration and the bottom panel shows cellular respiration in mitochondria.\" width=\"430\" 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\">10.12<\/span><\/strong><span id=\"10\" class=\"os-title\" data-type=\"title\"><strong>Muscle Metabolism.<\/strong><\/span><em><span class=\"os-caption\">(a) Creatine conversion (b) Glycolysis of one glucose produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or converted to lactic acid. (c) Aerobic respiration is the breakdown of glucose in the presence of oxygen (O<sub>2<\/sub>) to produce carbon dioxide, water, and ATP.&nbsp;<\/span><\/em><\/div>\n<\/div>\n<p id=\"fs-id2302886\">Muscle fatigue occurs when a muscle can no longer contract in response to signals from the nervous system. The exact causes of muscle fatigue are not fully known, but certain factors <span style=\"color: #008000\">have been noted:<\/span><\/p>\n\n<ol>\n \t<li><span style=\"color: #008000\">ATP is needed for normal muscle contraction, and as ATP reserves are reduced, muscle function may decline. Although, this may be more of a factor in brief, intense muscle output rather than sustained, lower intensity efforts.<\/span><\/li>\n \t<li><span style=\"color: #008000\">Lactic acid buildup may lower intracellular pH, affecting enzyme and protein activity.<\/span><\/li>\n \t<li><span style=\"color: #008000\">Imbalances in Na<sup>+<\/sup>&nbsp;and K<sup>+<\/sup> levels as a result of membrane depolarization may disrupt <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> flow out of the SR.<\/span><\/li>\n \t<li><span style=\"color: #008000\">Long periods of sustained exercise may damage the SR and the sarcolemma, resulting in impaired <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> regulation.<\/span><\/li>\n<\/ol>\n<p id=\"fs-id1841712\">Intense muscle activity results in an&nbsp;<span id=\"term-00008\" data-type=\"term\">oxygen debt<\/span>, which is the amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction. Oxygen is required to restore ATP and creatine phosphate levels, convert lactic acid to pyruvic acid, and, in the liver, to convert lactic acid into glucose or glycogen. Other systems used during exercise also require oxygen, and all of these combined processes result in the increased breathing rate that occurs after exercise. Until the oxygen debt has been met, oxygen intake is elevated, even after exercise has stopped.<\/p>\n\n<\/section>&nbsp;\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\n<span style=\"color: #339966\">ATP is an important component that allows our muscles to properly contract and relax. ATP can be regenerated via the following pathways:<\/span>\n<ol>\n \t<li><span style=\"color: #339966\">Creatine phosphate metabolism<\/span><\/li>\n \t<li><span style=\"color: #339966;text-align: initial;font-size: 1em\">Anaerobic glycolysis<\/span><\/li>\n \t<li><span style=\"color: #339966\">Lactic Acid Fermentation &amp; Aerobic respiration<\/span><\/li>\n<\/ol>\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<h1>Adaptation<\/h1>\n<p style=\"text-align: start\">This chapter was adapted by Eva M. Su from the following texts:<\/p>\n<p style=\"text-align: start\"><a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation\" target=\"_blank\" rel=\"noopener\">Muscle fiber contraction and relaxation<\/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\n<\/div>\n<\/section><\/div>\n<\/section>\n","rendered":"<div id=\"1\" class=\"ui-has-child-title\" data-type=\"abstract\">\n<header><\/header>\n<header class=\"textbox__header\"><\/header>\n<section>\n<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 id=\"para-00001\">By the end of this section, you will be able to:<\/p>\n<ul id=\"list-00001\">\n<li><span style=\"color: #339966\">Identify<\/span>&nbsp;the components involved in muscle contraction<\/li>\n<li>Explain how muscles contract and relax<\/li>\n<li><span style=\"color: #339966\">Describe the mechanisms of ATP regeneration for use in muscle contraction<\/span><\/li>\n<\/ul>\n<\/div>\n<\/div>\n<p>&nbsp;<\/p>\n<\/section>\n<section><span style=\"text-align: initial;font-size: 1em\">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 fiber. The <span style=\"color: #008000\">sarcolemma, the membrane surrounding the fibers, <\/span>will depolarize as positively charged sodium ions (Na<\/span><sup style=\"text-align: initial\">+<\/sup><span style=\"text-align: initial;font-size: 1em\">) enter. <span style=\"color: #008000\">This triggers an action potential that spreads to the rest of the membrane which will depolarize (including the T-tubules) and triggers the release of calcium ions (Ca<\/span><\/span><span style=\"color: #008000\"><sup style=\"text-align: initial\">2+<\/sup><\/span><span style=\"text-align: initial;font-size: 1em\"><span style=\"color: #008000\">) stored in the sarcoplasmic reticulum (SR). <\/span>The Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup><span style=\"text-align: initial;font-size: 1em\">&nbsp;then initiates contraction, which is sustained by ATP (<\/span><a class=\"autogenerated-content\" style=\"text-align: initial;font-size: 1em\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_01\">Figure 10.8<\/a><span style=\"text-align: initial;font-size: 1em\">). As long as Ca<sup>2<\/sup><\/span><sup style=\"text-align: initial\">+<\/sup><span style=\"text-align: initial;font-size: 1em\"> ions remain in the sarcoplasm to bind to troponin, which keeps the <span style=\"color: #339966\">myosin<\/span>-binding sites <span style=\"color: #339966\">o<\/span><span style=\"color: #008000\">n actin exposed<\/span>, and as long as ATP is available to drive the cross-bridge cycling and the pulling of actin <span style=\"color: #008000\">(thin filament)<\/span> strands by myosin <span style=\"color: #008000\">(thick filament)<\/span>, the muscle fiber will continue to shorten to an anatomical limit.<\/span><\/section>\n<\/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&nbsp;<\/span><span class=\"os-number\">10.8<\/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<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<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\">10.9<\/span>&nbsp;<\/strong><span id=\"4\" class=\"os-title\" data-type=\"title\"><strong>Relaxation of a Muscle Fiber.<\/strong> <\/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 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><strong><span class=\"os-title-label\">Figure&nbsp;<\/span><span class=\"os-number\">10.10<\/span>&nbsp;<span class=\"os-title\" data-type=\"title\">The Sliding Filament Model of Muscle Contraction. <\/span><\/strong><\/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\">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\">10.11<\/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<section id=\"fs-id2141622\" data-depth=\"1\">\n<p id=\"fs-id1639118\">Relaxing skeletal muscles begin with the motor neuron, <span style=\"color: #008000\">which stops releasing ACh into the synapse at the<\/span> <span style=\"color: #008000\">neuromuscular junction (NMJ)<\/span>. The muscle fiber will repolarize, which closes the gates in the SR where <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> was being released. ATP-driven pumps will move <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> out of the sarcoplasm back into the SR. This results in the \u201creshielding\u201d of the myosin-binding sites on the thin filaments. Without the ability to form cross-bridges between the thin and thick filaments, the muscle fiber loses its tension and relaxes.<\/p>\n<\/section>\n<section id=\"fs-id2094906\" data-depth=\"1\">\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 results in atrophy, where the number of sarcomeres and myofibrils disappear (but not the number of muscle fibers). It is common for a limb in a cast to show atrophied muscles when the cast is removed. <span style=\"color: #008000\">Certain diseases, such as polio, can also<\/span>&nbsp;show atrophied muscles.<\/p>\n<\/section>\n<\/section>\n<section id=\"fs-id2164808\" data-depth=\"1\">\n<h2 data-type=\"title\">Sources of ATP<\/h2>\n<p id=\"fs-id2072704\">In addition to its direct role in the cross-bridge cycle, ATP also provides the energy for the active-transport <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> pumps in the SR. Muscle contraction does not occur without sufficient amounts of ATP. The amount of ATP stored in muscle is very low, only sufficient to power a few seconds worth of contractions. As it is broken down, ATP must therefore be regenerated and replaced quickly to allow for sustained contraction. There are three mechanisms by which ATP can be regenerated: creatine phosphate metabolism, anaerobic glycolysis, and fermentation and aerobic respiration.<\/p>\n<h3><span style=\"color: #008000\">Creatine Phosphate Metabolism<\/span><\/h3>\n<p id=\"fs-id1898661\"><span id=\"term-00003\" data-type=\"term\">Creatine phosphate<\/span>&nbsp;is a molecule that can store energy in its phosphate bonds. In a resting muscle, excess ATP transfers its energy to creatine, producing ADP and creatine phosphate. This acts as an energy reserve that can be used to quickly create more ATP. When the muscle starts to contract and needs energy, creatine phosphate transfers its phosphate back to ADP to form ATP and creatine. This reaction is catalyzed by the enzyme creatine kinase and occurs very quickly; thus, creatine phosphate-derived ATP powers the first few seconds of muscle contraction. However, creatine phosphate can only provide approximately 15 seconds worth of energy, at which point another energy source has to be used (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_05\">Figure 10.12a<\/a>).<\/p>\n<h3><span style=\"color: #008000\">Anaerobic Glycolysis<\/span><\/h3>\n<p id=\"fs-id2142755\">As the ATP produced by creatine phosphate is depleted, muscles turn to glycolysis as an ATP source.&nbsp;<span id=\"term-00004\" data-type=\"term\">Glycolysis<\/span> is an anaerobic process that breaks down glucose (sugar) to produce ATP; however, glycolysis cannot generate ATP as quickly as creatine phosphate. Thus, the switch to glycolysis results in a slower rate of ATP availability to the muscle. The sugar used in glycolysis can be provided by blood glucose or by metabolizing glycogen that is stored in the muscle. The breakdown of one glucose molecule produces two ATP and two molecules of <span id=\"term-00005\" data-type=\"term\">pyruvic acid<\/span>, which can be used in aerobic respiration or converted to lactic acid <span style=\"color: #008000\">when oxygen levels are low <\/span>(<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_05\">Figure 10.12b<\/a>).<\/p>\n<h3><span style=\"color: #008000\">Lactic Acid Fermentation and Aerobic Respiration<\/span><\/h3>\n<p>If oxygen is available, pyruvic acid is used in aerobic respiration. However, if oxygen is not available, pyruvic acid is converted to&nbsp;<span id=\"term-00006\" data-type=\"term\">lactic acid<\/span>, which may contribute to muscle fatigue. This conversion allows the recycling of the enzyme NAD<sup>+<\/sup>&nbsp;from NADH, which is needed for glycolysis to continue. This occurs during strenuous exercise when high amounts of energy are needed but oxygen cannot be sufficiently delivered to muscle. Glycolysis itself cannot be sustained for very long (approximately 1 minute of muscle activity), but it is useful in facilitating short bursts of high-intensity output. This is because glycolysis does not utilize glucose very efficiently, producing a net gain of two ATPs per molecule of glucose, and the end product of lactic acid, which may contribute to muscle fatigue as it accumulates.<\/p>\n<p id=\"fs-id1909043\"><span id=\"term-00007\" data-type=\"term\">Aerobic respiration<\/span>&nbsp;is the breakdown of glucose or other nutrients in the presence of oxygen (O<sub>2<\/sub>) to produce carbon dioxide, water, and ATP. Approximately 95 percent of the ATP required for resting or moderately active muscles is provided by aerobic respiration, which takes place in mitochondria. The inputs for aerobic respiration include glucose circulating in the bloodstream, pyruvic acid, and fatty acids. Aerobic respiration is much more efficient than anaerobic glycolysis, producing approximately 36 ATPs per molecule of glucose versus four from glycolysis. However, aerobic respiration cannot be sustained without a steady supply of O<sub>2<\/sub>&nbsp;to the skeletal muscle and is much slower (<a class=\"autogenerated-content\" href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation#fig-ch10_03_05\">Figure 10.12c<\/a>). To compensate, muscles store small amount of excess oxygen in proteins call myoglobin, allowing for more efficient muscle contractions and less fatigue. Aerobic training also increases the efficiency of the circulatory system so that O<sub>2<\/sub>&nbsp;can be supplied to the muscles for longer periods of time.<\/p>\n<div id=\"fig-ch10_03_05\" class=\"os-figure\">\n<figure data-id=\"fig-ch10_03_05\"><span id=\"fs-id1700003\" data-type=\"media\" data-alt=\"This figure shows the metabolic processes in muscle. The top panel shows the reactions in resting muscle. The middle panel shows glycolysis and aerobic respiration and the bottom panel shows cellular respiration in mitochondria.\"><img decoding=\"async\" id=\"11\" src=\"https:\/\/openstax.org\/apps\/archive\/20221109.213337\/resources\/b54437d9c1a12b07842921d529ee26b6b8a788a5\" alt=\"This figure shows the metabolic processes in muscle. The top panel shows the reactions in resting muscle. The middle panel shows glycolysis and aerobic respiration and the bottom panel shows cellular respiration in mitochondria.\" width=\"430\" 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\">10.12<\/span><\/strong><span id=\"10\" class=\"os-title\" data-type=\"title\"><strong>Muscle Metabolism.<\/strong><\/span><em><span class=\"os-caption\">(a) Creatine conversion (b) Glycolysis of one glucose produces two ATP and two molecules of pyruvic acid, which can be used in aerobic respiration or converted to lactic acid. (c) Aerobic respiration is the breakdown of glucose in the presence of oxygen (O<sub>2<\/sub>) to produce carbon dioxide, water, and ATP.&nbsp;<\/span><\/em><\/div>\n<\/div>\n<p id=\"fs-id2302886\">Muscle fatigue occurs when a muscle can no longer contract in response to signals from the nervous system. The exact causes of muscle fatigue are not fully known, but certain factors <span style=\"color: #008000\">have been noted:<\/span><\/p>\n<ol>\n<li><span style=\"color: #008000\">ATP is needed for normal muscle contraction, and as ATP reserves are reduced, muscle function may decline. Although, this may be more of a factor in brief, intense muscle output rather than sustained, lower intensity efforts.<\/span><\/li>\n<li><span style=\"color: #008000\">Lactic acid buildup may lower intracellular pH, affecting enzyme and protein activity.<\/span><\/li>\n<li><span style=\"color: #008000\">Imbalances in Na<sup>+<\/sup>&nbsp;and K<sup>+<\/sup> levels as a result of membrane depolarization may disrupt <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> flow out of the SR.<\/span><\/li>\n<li><span style=\"color: #008000\">Long periods of sustained exercise may damage the SR and the sarcolemma, resulting in impaired <span style=\"text-align: initial;font-size: 1em\">Ca<\/span><sup style=\"text-align: initial\">2+<\/sup> regulation.<\/span><\/li>\n<\/ol>\n<p id=\"fs-id1841712\">Intense muscle activity results in an&nbsp;<span id=\"term-00008\" data-type=\"term\">oxygen debt<\/span>, which is the amount of oxygen needed to compensate for ATP produced without oxygen during muscle contraction. Oxygen is required to restore ATP and creatine phosphate levels, convert lactic acid to pyruvic acid, and, in the liver, to convert lactic acid into glucose or glycogen. Other systems used during exercise also require oxygen, and all of these combined processes result in the increased breathing rate that occurs after exercise. Until the oxygen debt has been met, oxygen intake is elevated, even after exercise has stopped.<\/p>\n<\/section>\n<p>&nbsp;<\/p>\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<p><span style=\"color: #339966\">ATP is an important component that allows our muscles to properly contract and relax. ATP can be regenerated via the following pathways:<\/span><\/p>\n<ol>\n<li><span style=\"color: #339966\">Creatine phosphate metabolism<\/span><\/li>\n<li><span style=\"color: #339966;text-align: initial;font-size: 1em\">Anaerobic glycolysis<\/span><\/li>\n<li><span style=\"color: #339966\">Lactic Acid Fermentation &amp; Aerobic respiration<\/span><\/li>\n<\/ol>\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<h1>Adaptation<\/h1>\n<p style=\"text-align: start\">This chapter was adapted by Eva M. Su from the following texts:<\/p>\n<p style=\"text-align: start\"><a href=\"https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/10-3-muscle-fiber-contraction-and-relaxation\" target=\"_blank\" rel=\"noopener\">Muscle fiber contraction and relaxation<\/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<\/div>\n<\/section>\n<\/div>\n<\/section>\n","protected":false},"author":1076,"menu_order":12,"template":"","meta":{"pb_show_title":"","pb_short_title":"","pb_subtitle":"","pb_authors":["j-gordon-betts-hpeufzyxws-ev84rcc31c","james-a-wise-qr37sjotpp-mnwkakfgvm","kelly-a-young-ebcqggfbjy-iimnzivb9c","eddie-johnson-spzb6fnj8w-vmoae6prlq","brandon-poe-hbyrw0vl1x","dean-h-kruse-lso7ax0e1z-ptydqmydyu","oksana-korol-uhzmfvuyzq-liok5zbnok","jody-e-johnson-rz4w6efp48-znjvie8axv","mark-womble-oqasblbzan-mgykbeyoow","peter-desaix-ocr1dcrbuy-tpfurfwhas"],"pb_section_license":""},"chapter-type":[],"contributor":[339],"license":[],"class_list":["post-222","chapter","type-chapter","status-publish","hentry","contributor-peter-desaix-ocr1dcrbuy-tpfurfwhas"],"part":195,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/pressbooks\/v2\/chapters\/222","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\/222\/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\/222\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/wp\/v2\/media?parent=222"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/pressbooks\/v2\/chapter-type?post=222"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/wp\/v2\/contributor?post=222"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/zoesandbox\/wp-json\/wp\/v2\/license?post=222"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}