{"id":5210,"date":"2017-05-01T22:12:04","date_gmt":"2017-05-01T22:12:04","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=5210"},"modified":"2024-04-26T02:39:46","modified_gmt":"2024-04-26T02:39:46","slug":"neural-stimulation-of-muscle-contraction","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/neural-stimulation-of-muscle-contraction\/","title":{"raw":"Neural Stimulation of Muscle Contraction","rendered":"Neural Stimulation of Muscle Contraction"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Identify the role of the brain in muscle movement<\/li>\r\n<\/ul>\r\n<\/div>\r\n<h3 data-type=\"title\">Excitation\u2013Contraction Coupling<\/h3>\r\n<p id=\"fs-idm56103456\">Excitation\u2013contraction coupling is the link (transduction) between the action potential generated in the sarcolemma and the start of a muscle contraction. The trigger for calcium release from the sarcoplasmic reticulum into the sarcoplasm is a neural signal. Each skeletal muscle fiber is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle. The area of the sarcolemma on the muscle fiber that interacts with the neuron is called the\u00a0<span id=\"term1881\" data-type=\"term\">motor end plate<\/span>. The end of the neuron\u2019s axon is called the synaptic terminal, and it does not actually contact the motor end plate. A small space called the synaptic cleft separates the synaptic terminal from the motor end plate. Electrical signals travel along the neuron\u2019s axon, which branches through the muscle and connects to individual muscle fibers at a neuromuscular junction.<\/p>\r\n<p id=\"fs-idm72592000\">The ability of cells to communicate electrically requires that the cells expend energy to create an electrical gradient across their cell membranes. This charge gradient is carried by ions, which are differentially distributed across the membrane. Each ion exerts an electrical influence and a concentration influence. Just as milk will eventually mix with coffee without the need to stir, ions also distribute themselves evenly, if they are permitted to do so. In this case, they are not permitted to return to an evenly mixed state.<\/p>\r\n<p id=\"fs-idm105942816\">The sodium\u2013potassium ATPase uses cellular energy to move K<sup>+<\/sup>\u00a0ions inside the cell and Na<sup>+<\/sup>\u00a0ions outside. This alone accumulates a small electrical charge, but a big concentration gradient. There is lots of K<sup>+<\/sup>\u00a0in the cell and lots of Na<sup>+<\/sup>\u00a0outside the cell. Potassium is able to leave the cell through K<sup>+<\/sup>\u00a0channels that are open 90% of the time, and it does. However, Na<sup>+<\/sup>\u00a0channels are rarely open, so Na<sup>+<\/sup>\u00a0remains outside the cell. When K<sup>+<\/sup>\u00a0leaves the cell, obeying its concentration gradient, that effectively leaves a negative charge behind. So at rest, there is a large concentration gradient for Na<sup>+<\/sup>\u00a0to enter the cell, and there is an accumulation of negative charges left behind in the cell. This is the resting membrane potential. Potential in this context means a separation of electrical charge that is capable of doing work. It is measured in volts, just like a battery. However, the transmembrane potential is considerably smaller (0.07 V); therefore, the small value is expressed as millivolts (mV) or 70 mV. Because the inside of a cell is negative compared with the outside, a minus sign signifies the excess of negative charges inside the cell, \u221270 mV.<\/p>\r\n<p id=\"fs-idp61065856\">If an event changes the permeability of the membrane to Na<sup>+<\/sup>\u00a0ions, they will enter the cell. That will change the voltage. This is an electrical event, called an action potential, that can be used as a cellular signal. Communication occurs between nerves and muscles through neurotransmitters. Neuron action potentials cause the release of neurotransmitters from the synaptic terminal into the synaptic cleft, where they can then diffuse across the synaptic cleft and bind to a receptor molecule on the motor end plate. The motor end plate possesses junctional folds\u2014folds in the sarcolemma that create a large surface area for the neurotransmitter to bind to receptors. The receptors are actually sodium channels that open to allow the passage of Na<sup>+<\/sup>\u00a0into the cell when they receive a neurotransmitter signal.<\/p>\r\n<p id=\"fs-idm8239040\">Acetylcholine (ACh) is a neurotransmitter released by motor neurons that binds to receptors in the motor end plate. Neurotransmitter release occurs when an action potential travels down the motor neuron\u2019s axon, resulting in altered permeability of the synaptic terminal membrane and an influx of calcium. The Ca<sup>2+<\/sup>\u00a0ions allow synaptic vesicles to move to and bind with the presynaptic membrane (on the neuron), and release neurotransmitter from the vesicles into the synaptic cleft. Once released by the synaptic terminal, ACh diffuses across the synaptic cleft to the motor end plate, where it binds with ACh receptors. As a neurotransmitter binds, these ion channels open, and Na<sup>+<\/sup>\u00a0ions cross the membrane into the muscle cell. This reduces the voltage difference between the inside and outside of the cell, which is called depolarization. As ACh binds at the motor end plate, this depolarization is called an end-plate potential. The depolarization then spreads along the sarcolemma, creating an action potential as sodium channels adjacent to the initial depolarization site sense the change in voltage and open. The action potential moves across the entire cell, creating a wave of depolarization.<\/p>\r\n<p id=\"fs-idm177862864\">ACh is broken down by the enzyme\u00a0<span id=\"term1882\" data-type=\"term\">acetylcholinesterase<\/span>\u00a0(AChE) into acetyl and choline. AChE resides in the synaptic cleft, breaking down ACh so that it does not remain bound to ACh receptors, which would cause unwanted extended muscle contraction<span style=\"font-size: 1rem; text-align: initial;\"> (Figure 1).<\/span><\/p>\r\n\r\n\r\n[caption id=\"attachment_2813\" align=\"aligncenter\" width=\"700\"]<img class=\"wp-image-2813\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/08003754\/Figure_38_04_06f.png\" alt=\"There are four steps in the start of a muscle contraction. Step 1: Acetylcholine released from synaptic vesicles in the axon terminal binds to receptors on the muscle cell plasma membrane. Step 2: An action potential is initiated that travels down the T tubule. Step 3: Calcium ions are released from the sarcoplasmic reticulum in response to the change in voltage. Step 4: Calcium ions bind to troponin, exposing active sites on actin. Cross-bridge formation occurs and muscles contract. Three additional steps are part of the end of a muscle contraction. Step 5: Acetylcholine is removed from the synaptic cleft by acetylcholinesterase. Step 6: Calcium ions are transported back into the sarcoplasmic reticulum. Step 7: Tropomyosin covers active sites on actin preventing cross-bridge formation, so the muscle contraction ends.\" width=\"700\" height=\"765\" \/> Figure 1.\u00a0This diagram shows excitation-contraction coupling in a skeletal muscle contraction. The sarcoplasmic reticulum is a specialized endoplasmic reticulum found in muscle cells.[\/caption]\r\n\r\nAfter depolarization, the membrane returns to its resting state. This is called repolarization, during which voltage-gated sodium channels close. Potassium channels continue at 90% conductance. Because the plasma membrane sodium\u2013potassium ATPase always transports ions, the resting state (negatively charged inside relative to the outside) is restored. The period immediately following the transmission of an impulse in a nerve or muscle, in which a neuron or muscle cell regains its ability to transmit another impulse, is called the refractory period. During the refractory period, the membrane cannot generate another action potential. The refractory period allows the voltage-sensitive ion channels to return to their resting configurations. The sodium potassium ATPase continually moves Na<sup>+<\/sup>\u00a0back out of the cell and K<sup>+<\/sup>\u00a0back into the cell, and the K<sup>+<\/sup>\u00a0leaks out leaving negative charge behind. Very quickly, the membrane repolarizes, so that it can again be depolarized.\r\n<div class=\"textbox\"><a href=\"http:\/\/www.ucl.ac.uk\/~sjjgsca\/muscleSlidingFilament.html\" target=\"_blank\" rel=\"noopener\">Visit this page for another demonstration of this theory.<\/a><\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>\u00a0Practice Question<\/h3>\r\nThe deadly nerve gas Sarin irreversibly inhibits acetycholinesterase. What effect would Sarin have on muscle contraction?\r\n\r\n[reveal-answer q=\"528437\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"528437\"]In the presence of Sarin, acetycholine is not removed from the synapse, resulting in continuous stimulation of the muscle plasma membrane. At first, muscle activity is intense and uncontrolled, but the ion gradients dissipate, so electrical signals in the T-tubules are no longer possible. The result is paralysis, leading to death by asphyxiation.[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/1ab9b040-f70b-45f2-b64a-7853c3ba999f\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Identify the role of the brain in muscle movement<\/li>\n<\/ul>\n<\/div>\n<h3 data-type=\"title\">Excitation\u2013Contraction Coupling<\/h3>\n<p id=\"fs-idm56103456\">Excitation\u2013contraction coupling is the link (transduction) between the action potential generated in the sarcolemma and the start of a muscle contraction. The trigger for calcium release from the sarcoplasmic reticulum into the sarcoplasm is a neural signal. Each skeletal muscle fiber is controlled by a motor neuron, which conducts signals from the brain or spinal cord to the muscle. The area of the sarcolemma on the muscle fiber that interacts with the neuron is called the\u00a0<span id=\"term1881\" data-type=\"term\">motor end plate<\/span>. The end of the neuron\u2019s axon is called the synaptic terminal, and it does not actually contact the motor end plate. A small space called the synaptic cleft separates the synaptic terminal from the motor end plate. Electrical signals travel along the neuron\u2019s axon, which branches through the muscle and connects to individual muscle fibers at a neuromuscular junction.<\/p>\n<p id=\"fs-idm72592000\">The ability of cells to communicate electrically requires that the cells expend energy to create an electrical gradient across their cell membranes. This charge gradient is carried by ions, which are differentially distributed across the membrane. Each ion exerts an electrical influence and a concentration influence. Just as milk will eventually mix with coffee without the need to stir, ions also distribute themselves evenly, if they are permitted to do so. In this case, they are not permitted to return to an evenly mixed state.<\/p>\n<p id=\"fs-idm105942816\">The sodium\u2013potassium ATPase uses cellular energy to move K<sup>+<\/sup>\u00a0ions inside the cell and Na<sup>+<\/sup>\u00a0ions outside. This alone accumulates a small electrical charge, but a big concentration gradient. There is lots of K<sup>+<\/sup>\u00a0in the cell and lots of Na<sup>+<\/sup>\u00a0outside the cell. Potassium is able to leave the cell through K<sup>+<\/sup>\u00a0channels that are open 90% of the time, and it does. However, Na<sup>+<\/sup>\u00a0channels are rarely open, so Na<sup>+<\/sup>\u00a0remains outside the cell. When K<sup>+<\/sup>\u00a0leaves the cell, obeying its concentration gradient, that effectively leaves a negative charge behind. So at rest, there is a large concentration gradient for Na<sup>+<\/sup>\u00a0to enter the cell, and there is an accumulation of negative charges left behind in the cell. This is the resting membrane potential. Potential in this context means a separation of electrical charge that is capable of doing work. It is measured in volts, just like a battery. However, the transmembrane potential is considerably smaller (0.07 V); therefore, the small value is expressed as millivolts (mV) or 70 mV. Because the inside of a cell is negative compared with the outside, a minus sign signifies the excess of negative charges inside the cell, \u221270 mV.<\/p>\n<p id=\"fs-idp61065856\">If an event changes the permeability of the membrane to Na<sup>+<\/sup>\u00a0ions, they will enter the cell. That will change the voltage. This is an electrical event, called an action potential, that can be used as a cellular signal. Communication occurs between nerves and muscles through neurotransmitters. Neuron action potentials cause the release of neurotransmitters from the synaptic terminal into the synaptic cleft, where they can then diffuse across the synaptic cleft and bind to a receptor molecule on the motor end plate. The motor end plate possesses junctional folds\u2014folds in the sarcolemma that create a large surface area for the neurotransmitter to bind to receptors. The receptors are actually sodium channels that open to allow the passage of Na<sup>+<\/sup>\u00a0into the cell when they receive a neurotransmitter signal.<\/p>\n<p id=\"fs-idm8239040\">Acetylcholine (ACh) is a neurotransmitter released by motor neurons that binds to receptors in the motor end plate. Neurotransmitter release occurs when an action potential travels down the motor neuron\u2019s axon, resulting in altered permeability of the synaptic terminal membrane and an influx of calcium. The Ca<sup>2+<\/sup>\u00a0ions allow synaptic vesicles to move to and bind with the presynaptic membrane (on the neuron), and release neurotransmitter from the vesicles into the synaptic cleft. Once released by the synaptic terminal, ACh diffuses across the synaptic cleft to the motor end plate, where it binds with ACh receptors. As a neurotransmitter binds, these ion channels open, and Na<sup>+<\/sup>\u00a0ions cross the membrane into the muscle cell. This reduces the voltage difference between the inside and outside of the cell, which is called depolarization. As ACh binds at the motor end plate, this depolarization is called an end-plate potential. The depolarization then spreads along the sarcolemma, creating an action potential as sodium channels adjacent to the initial depolarization site sense the change in voltage and open. The action potential moves across the entire cell, creating a wave of depolarization.<\/p>\n<p id=\"fs-idm177862864\">ACh is broken down by the enzyme\u00a0<span id=\"term1882\" data-type=\"term\">acetylcholinesterase<\/span>\u00a0(AChE) into acetyl and choline. AChE resides in the synaptic cleft, breaking down ACh so that it does not remain bound to ACh receptors, which would cause unwanted extended muscle contraction<span style=\"font-size: 1rem; text-align: initial;\"> (Figure 1).<\/span><\/p>\n<div id=\"attachment_2813\" style=\"width: 710px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2813\" class=\"wp-image-2813\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/08003754\/Figure_38_04_06f.png\" alt=\"There are four steps in the start of a muscle contraction. Step 1: Acetylcholine released from synaptic vesicles in the axon terminal binds to receptors on the muscle cell plasma membrane. Step 2: An action potential is initiated that travels down the T tubule. Step 3: Calcium ions are released from the sarcoplasmic reticulum in response to the change in voltage. Step 4: Calcium ions bind to troponin, exposing active sites on actin. Cross-bridge formation occurs and muscles contract. Three additional steps are part of the end of a muscle contraction. Step 5: Acetylcholine is removed from the synaptic cleft by acetylcholinesterase. Step 6: Calcium ions are transported back into the sarcoplasmic reticulum. Step 7: Tropomyosin covers active sites on actin preventing cross-bridge formation, so the muscle contraction ends.\" width=\"700\" height=\"765\" \/><\/p>\n<p id=\"caption-attachment-2813\" class=\"wp-caption-text\">Figure 1.\u00a0This diagram shows excitation-contraction coupling in a skeletal muscle contraction. The sarcoplasmic reticulum is a specialized endoplasmic reticulum found in muscle cells.<\/p>\n<\/div>\n<p>After depolarization, the membrane returns to its resting state. This is called repolarization, during which voltage-gated sodium channels close. Potassium channels continue at 90% conductance. Because the plasma membrane sodium\u2013potassium ATPase always transports ions, the resting state (negatively charged inside relative to the outside) is restored. The period immediately following the transmission of an impulse in a nerve or muscle, in which a neuron or muscle cell regains its ability to transmit another impulse, is called the refractory period. During the refractory period, the membrane cannot generate another action potential. The refractory period allows the voltage-sensitive ion channels to return to their resting configurations. The sodium potassium ATPase continually moves Na<sup>+<\/sup>\u00a0back out of the cell and K<sup>+<\/sup>\u00a0back into the cell, and the K<sup>+<\/sup>\u00a0leaks out leaving negative charge behind. Very quickly, the membrane repolarizes, so that it can again be depolarized.<\/p>\n<div class=\"textbox\"><a href=\"http:\/\/www.ucl.ac.uk\/~sjjgsca\/muscleSlidingFilament.html\" target=\"_blank\" rel=\"noopener\">Visit this page for another demonstration of this theory.<\/a><\/div>\n<div class=\"textbox exercises\">\n<h3>\u00a0Practice Question<\/h3>\n<p>The deadly nerve gas Sarin irreversibly inhibits acetycholinesterase. What effect would Sarin have on muscle contraction?<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q528437\">Show Answer<\/span><\/p>\n<div id=\"q528437\" class=\"hidden-answer\" style=\"display: none\">In the presence of Sarin, acetycholine is not removed from the synapse, resulting in continuous stimulation of the muscle plasma membrane. At first, muscle activity is intense and uncontrolled, but the ion gradients dissipate, so electrical signals in the T-tubules are no longer possible. The result is paralysis, leading to death by asphyxiation.<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_1ab9b040-f70b-45f2-b64a-7853c3ba999f\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/1ab9b040-f70b-45f2-b64a-7853c3ba999f?iframe_resize_id=assessment_practice_id_1ab9b040-f70b-45f2-b64a-7853c3ba999f\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-5210\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction<\/li><li>Human Physiology. <strong>Provided by<\/strong>: Wikibooks. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"https:\/\/en.wikibooks.org\/wiki\/Human_Physiology\">https:\/\/en.wikibooks.org\/wiki\/Human_Physiology<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by-sa\/4.0\/\">CC BY-SA: Attribution-ShareAlike<\/a><\/em><\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":18,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction\"},{\"type\":\"cc\",\"description\":\"Human Physiology\",\"author\":\"\",\"organization\":\"Wikibooks\",\"url\":\"https:\/\/en.wikibooks.org\/wiki\/Human_Physiology\",\"project\":\"\",\"license\":\"cc-by-sa\",\"license_terms\":\"\"}]","CANDELA_OUTCOMES_GUID":"bb11ca62-2d5e-4533-88ae-3c855f46f8c1, 33f24a21-27e1-4b50-a589-5ba69632afa6","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-5210","chapter","type-chapter","status-publish","hentry"],"part":3796,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/5210","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":14,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/5210\/revisions"}],"predecessor-version":[{"id":8700,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/5210\/revisions\/8700"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/3796"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/5210\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=5210"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=5210"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=5210"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=5210"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}