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You could also send an email to the <a href="https://meta.wikimedia.org/wiki/Special:MyLanguage/Stewards" class="extiw" title="m:Special:MyLanguage/Stewards">stewards</a> <a href="https://meta.wikimedia.org/wiki/Special:MyLanguage/VRT" class="extiw" title="m:Special:MyLanguage/VRT">VRT</a> queue at <kbd>stewards@wikimedia.org</kbd> including all above details. </p> </div> <p>Other useful links: <a href="https://meta.wikimedia.org/wiki/Global_blocks" class="extiw" title="m:Global blocks">Global blocks</a> · <a href="/wiki/Help:I_have_been_blocked" title="Help:I have been blocked">Help:I have been blocked</a> </p> </div></li></ul><hr /> <div id="viewsourcetext">You can view and copy the source of this page:</div><textarea readonly="" accesskey="," id="wpTextbox1" cols="80" rows="25" style="" class="mw-editfont-monospace" lang="en" dir="ltr" name="wpTextbox1">===Skeletal muscle=== [[File:1022 Muscle Fibers (small).jpg|right|thumb|Organization of skeletal muscle]] Excluding reflexes, all [[skeletal muscle]] contractions occur as a result of signals originating in the brain. The brain sends electrochemical signals through the [[nervous system]] to the [[motor neuron]] that [[nerve|innervates]] several muscle fibers.<ref>{{Cite book |last=Tassinary |title=Handbook of Psychophysiology |last2=Cacioppo |publisher=Cambridge University Press |year=2000 |isbn=978-0-521-62634-7 |editor-last=Cacioppo |editor-first=John T. |edition=Second |location=Cambridge |chapter=The Skeletomotor system: surface electromyography |editor-last2=Tassinary |editor-first2=Luois G. |editor-last3=Berntson |editor-first3=Gary G.}}</ref> In the case of some [[reflexes]], the signal to contract can originate in the [[spinal cord]] through a feedback loop with the grey matter. Other actions such as locomotion, breathing, and chewing have a reflex aspect to them: the contractions can be initiated either consciously or unconsciously. ====Neuromuscular junction==== [[File:1009 Motor End Plate and Innervation.jpg|right|thumb|Structure of neuromuscular junction.]] A [[neuromuscular junction]] is a [[chemical synapse]] formed by the contact between a [[motor neuron]] and a [[muscle fiber]].<ref name="Levitan 2015">{{Cite book |last=Levitan |first=Irwin |title=The Neuron: Cell and Molecular Biology |last2=Kaczmarek |first2=Leonard |date=19 August 2015 |publisher=Oxford University Press |isbn=978-0199773893 |edition=4th |location=New York, NY |pages=153–328 |chapter=Intercellular communication}}</ref> It is the site in which a motor neuron transmits a signal to a muscle fiber to initiate muscle contraction. The sequence of events that results in the depolarization of the muscle fiber at the neuromuscular junction begins when an action potential is initiated in the cell body of a motor neuron, which is then propagated by [[saltatory conduction]] along its axon toward the neuromuscular junction. Once it reaches the [[Chemical synapse|terminal bouton]], the action potential causes a [[Calcium in biology|{{chem|Ca|2+}} ion]] influx into the terminal by way of the [[voltage-gated calcium channel]]s. The [[Calcium signaling|{{chem|Ca|2+}} influx]] causes [[synaptic vesicles]] containing the neurotransmitter [[acetylcholine]] to fuse with the plasma membrane, releasing acetylcholine into the [[synaptic cleft]] between the motor neuron terminal and the neuromuscular junction of the skeletal muscle fiber. Acetylcholine diffuses across the synapse and binds to and activates [[nicotinic acetylcholine receptors]] on the neuromuscular junction. Activation of the nicotinic receptor opens its intrinsic [[sodium]]/[[potassium]] channel, causing sodium to rush in and potassium to trickle out. As a result, the [[sarcolemma]] reverses polarity and its voltage quickly jumps from the resting membrane potential of -90mV to as high as +75mV as sodium enters. The membrane potential then becomes hyperpolarized when potassium exits and is then adjusted back to the resting membrane potential. This rapid fluctuation is called the end-plate potential.<ref name="Saladin, Kenneth S. 2015">Saladin, Kenneth S., Stephen J. Sullivan, and Christina A. Gan. Anatomy & Physiology: The Unity of Form and Function. 7th ed. New York: McGraw-Hill Education, 2015. Print.</ref> The voltage-gated ion channels of the sarcolemma next to the end plate open in response to the end plate potential. They are sodium and potassium specific and only allow one through. This wave of ion movements creates the action potential that spreads from the motor end plate in all directions.<ref name="Saladin, Kenneth S. 2015" /> If action potentials stop arriving, then acetylcholine ceases to be released from the terminal bouton. The remaining acetylcholine in the synaptic cleft is either degraded by active [[acetylcholine esterase]] or reabsorbed by the synaptic knob and none is left to replace the degraded acetylcholine. ====Excitation–contraction coupling==== Excitation–contraction coupling (ECC) is the process by which a [[Action potential#Muscular action potentials|muscular action potential]] in the muscle fiber causes [[myofibril]]s to contract. In skeletal muscles, excitation–contraction coupling relies on a direct coupling between two key proteins, the [[sarcoplasmic reticulum]] (SR) calcium release channel identified as the [[ryanodine receptor 1]] (RYR1) and the [[Voltage-dependent calcium channel|voltage-gated L-type calcium channel]] identified as [[dihydropyridine receptor]]s, (DHPRs). DHPRs are located on the sarcolemma (which includes the surface sarcolemma and the [[transverse tubules]]), while the RyRs reside across the SR membrane. The close apposition of a transverse tubule and two SR regions containing RyRs is described as a triad and is predominantly where excitation–contraction coupling takes place. [[File:Interactions_within_Excitation-contraction_Coupling.jpg|thumb|Picture showing the different interactions within the ECC pathway]] Excitation–contraction coupling (ECC) occurs when depolarization of skeletal muscles (usually through neural innervation) results in a muscle action potential. This action potential spreads across the muscle's surface and into the muscle fiber's network of [[T-tubule]]s, depolarizing the inner portion of the muscle fiber. This activates dihydropyridine receptors in the [[terminal cisternae]], which are in close proximity to ryanodine receptors in the adjacent [[Endoplasmic reticulum#Sarcoplasmic reticulum|sarcoplasmic reticulum]]. The activated dihydropyridine receptors physically interact with ryanodine receptors to activate them via foot processes (involving conformational changes that allosterically activates the ryanodine receptors). As ryanodine receptors open, Ca<sup>2+</sup> is released from the sarcoplasmic reticulum into the local junctional space and diffuses into the bulk cytoplasm to cause a [[calcium spark]].<ref name="Lanner a003996">{{Cite journal |last=Lanner |first=Johanna T. |last2=Georgiou |first2=Dimitra K. |last3=Joshi |first3=Aditya D. |last4=Hamilton |first4=Susan L. |date=November 2010 |title=Ryanodine receptors: structure, expression, molecular details, and function in calcium release |url=https://pubmed.ncbi.nlm.nih.gov/20961976 |journal=Cold Spring Harbor Perspectives in Biology |volume=2 |issue=11 |pages=a003996 |doi=10.1101/cshperspect.a003996 |issn=1943-0264 |pmc=2964179 |pmid=20961976}}</ref> The action potential creates a near synchronous activation of thousands of [[calcium sparks]] and causes a cell-wide increase in calcium giving rise to the upstroke of the [[calcium transient]]. The Ca<sup>2+</sup> released into the cytosol binds to [[Troponin C]] by the [[actin filaments]]. This bond allows the actin filaments to perform [[cross-bridge cycling]], producing force and, in some situations, motion. When the desired motion is accomplished, relaxation can be achieved quickly through numerous pathways. Relaxation is quickly achieved through a Ca<sup>2+</sup> buffer with various cytoplasmic proteins binding to Ca<sup>2+</sup> with very high affinity.<ref>{{Cite journal |last=Calderón |first=Juan C. |last2=Bolaños |first2=Pura |last3=Caputo |first3=Carlo |date=March 2014 |title=The excitation-contraction coupling mechanism in skeletal muscle |url=https://pubmed.ncbi.nlm.nih.gov/28509964 |journal=Biophysical Reviews |volume=6 |issue=1 |pages=133–160 |doi=10.1007/s12551-013-0135-x |issn=1867-2450 |pmc=5425715 |pmid=28509964}}</ref> These cytoplasmic proteins allow for quick relaxation in fast twitch muscles. Although slower, the [[SERCA|sarco/endoplasmic reticulum calcium-ATPase]] (SERCA) actively pumps Ca<sup>2+</sup> back into the sarcoplasmic reticulum, resulting in a permanent relaxation until the next action potential arrives.<ref name="Lanner a003996"/> Mitochondria also participate in Ca<sup>2+</sup> reuptake, ultimately delivering their gathered Ca<sup>2+</sup> to SERCA for storage in the sarcoplasmic reticulum. A few of the relaxation mechanisms (NCX, Ca2+ pumps and Ca2+ leak channels) move Ca2+ completely out of the cells as well.<ref>{{Cite journal |last=Webb |first=R. Clinton |date=December 2003 |title=SMOOTH MUSCLE CONTRACTION AND RELAXATION |url=https://www.physiology.org/doi/10.1152/advan.00025.2003 |journal=Advances in Physiology Education |language=en |volume=27 |issue=4 |pages=201–206 |doi=10.1152/advan.00025.2003 |issn=1043-4046}}</ref> As Ca<sup>2+</sup> concentration declines to resting levels, Ca2+ releases from Troponin C, disallowing cross bridge-cycling, causing the force to decline and relaxation to occur. Once relaxation has fully occurred, the muscle is able to contract again, thus fully resetting the cycle. ====Sliding filament theory==== [[File:Sarcomere.svg|right|thumb|Sliding filament theory: A sarcomere in relaxed (above) and contracted (below) positions]] The [[sliding filament theory]] describes a process used by [[muscle]]s to contract. It is a cycle of repetitive events that cause a thin filament to slide over a thick filament and generate tension in the muscle.<ref name="Saladin 2012">{{Cite book |last=Saladin |first=Kenneth |title=Anatomy and Physiology: The Unity of Form and Function |publisher=McGraw Hill |year=2012 |isbn=978-0-07-337825-1 |location=New York}}</ref> It was independently developed by [[Andrew F. Huxley|Andrew Huxley]] and [[Rolf Niedergerke]] and by [[Hugh Huxley]] and [[Jean Hanson]] in 1954.<ref name="Huxley AF, Niedergerke R 1954 971–973">{{Cite journal |vauthors=Huxley AF, Niedergerke R |year=1954 |title=Structural Changes in Muscle During Contraction: Interference Microscopy of Living Muscle Fibres |journal=Nature |volume=173 |issue=4412 |pages=971–973 |bibcode=1954Natur.173..971H |doi=10.1038/173971a0 |pmid=13165697 |s2cid=4275495}}</ref><ref name="Huxley H, Hanson J 1954 973–976">{{Cite journal |vauthors=Huxley H, Hanson J |year=1954 |title=Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation |journal=Nature |volume=173 |issue=4412 |pages=973–976 |bibcode=1954Natur.173..973H |doi=10.1038/173973a0 |pmid=13165698 |s2cid=4180166}}</ref> Physiologically, this contraction is not uniform across the sarcomere; the central position of the thick filaments becomes unstable and can shift during contraction but this is countered by the actions of the elastic myofilament of [[titin]]. This fine myofilament maintains uniform tension across the sarcomere by pulling the thick filament into a central position.<ref name="pmid3680378">{{Cite journal |vauthors=Horowits R, Podolsky RJ |date=November 1987 |title=The positional stability of thick filaments in activated skeletal muscle depends on sarcomere length: evidence for the role of titin filaments. |url=http://jcb.rupress.org/cgi/reprint/105/5/2217 |journal=J. Cell Biol. |volume=105 |issue=5 |pages=2217–23 |doi=10.1083/jcb.105.5.2217 |pmc=2114850 |pmid=3680378}}</ref> =====Cross-bridge cycle===== [[File:1008 Skeletal Muscle Contraction.jpg|right|thumb|Cross-bridge cycle]] Cross-bridge cycling is a sequence of molecular events that underlies the sliding filament theory. A [[Sliding filament theory#Cross-bridge mechanism|cross-bridge]] is a myosin projection, consisting of two myosin heads, that extends from the thick filaments.<ref name="Widmaier et al 2008" /> Each myosin head has two binding sites: one for [[adenosine triphosphate]] (ATP) and another for actin. The binding of ATP to a myosin head detaches myosin from [[actin]], thereby allowing myosin to bind to another actin molecule. Once attached, the ATP is hydrolyzed by myosin, which uses the released energy to move into the "cocked position" whereby it binds weakly to a part of the actin binding site. The remainder of the actin binding site is blocked by [[tropomyosin]].<ref name="Enoka 2013">{{Cite book |last=Enoka |first=Roger M. |title=Principles of Neural Science |last2=Pearson |first2=Keir G. |date=2013 |publisher=McGraw-Hill Medical |isbn=978-0-071-39011-8 |editor-last=Kandel |editor-first=Eric R. |edition=5th |location=New York, NY |pages=768–789 |chapter=The motor unit and muscle action |editor-last2=Schwartz |editor-first2=James H. |editor-last3=Jessell |editor-first3=Thomas M. |editor-last4=Siegelbaum |editor-first4=Steven A. |editor-last5=Hudspeth |editor-first5=A. J.}}</ref> With the ATP hydrolyzed, the cocked myosin head now contains [[adenosine diphosphate]] (ADP) + [[Inorganic phosphate|P<sub>i</sub>]]. Two {{chem|Ca|2+}} ions bind to [[troponin C]] on the actin filaments. The troponin-{{chem|Ca|2+}} complex causes tropomyosin to slide over and unblock the remainder of the actin binding site. Unblocking the rest of the actin binding sites allows the two myosin heads to close and myosin to bind strongly to actin.<ref name="Enoka 2013" /> The myosin head then releases the inorganic phosphate and initiates a ''power stroke,'' which generates a force of 2 pN. The power stroke moves the actin filament inwards, thereby shortening the [[sarcomere]]. Myosin then releases ADP but still remains tightly bound to actin. At the end of the power stroke, ADP is released from the myosin head, leaving myosin attached to actin in a rigor state until another ATP binds to myosin. A lack of ATP would result in the rigor state characteristic of [[rigor mortis]]. Once another ATP binds to myosin, the myosin head will again detach from actin and another cross-bridge cycle occurs. Cross-bridge cycling is able to continue as long as there are sufficient amounts of ATP and {{chem|Ca|2+}} in the cytoplasm.<ref name="Enoka 2013" /> Termination of cross-bridge cycling can occur when {{chem|Ca|2+}} is [[Active transport|actively pumped]] back into the sarcoplasmic reticulum. When {{chem|Ca|2+}} is no longer present on the thin filament, the tropomyosin changes conformation back to its previous state so as to block the binding sites again. The myosin ceases binding to the thin filament, and the muscle relaxes. The {{chem|Ca|2+}} ions leave the troponin molecule to maintain the {{chem|Ca|2+}} ion concentration in the sarcoplasm. The active pumping of {{chem|Ca|2+}} ions into the sarcoplasmic reticulum creates a deficiency in the fluid around the myofibrils. This causes the removal of {{chem|Ca|2+}} ions from the troponin. Thus, the tropomyosin-troponin complex again covers the binding sites on the actin filaments and contraction ceases. ====Gradation of skeletal muscle contractions==== {{multiple image | direction = vertical | width = 219 | footer = Three types of skeletal muscle contractions | image1 = 1012_Muscle_Twitch_Myogram.jpg | alt1 = Twitch | caption1 = Twitch | image2 = 1013_Summation_Tetanus.jpg | alt2 = Summation and tetanus | caption2 = Summation and tetanus }} The strength of skeletal muscle contractions can be broadly separated into [[Muscle twitch|twitch]], summation, and [[tetanic contractions|tetanus]]. A twitch is a single contraction and relaxation cycle produced by an action potential within the muscle fiber itself.<ref name="Feher 2012">{{Cite book |last=Feher |first=Joseph |title=Quantitative Human Physiology: An Introduction |date=2012 |publisher=Academic Press |isbn=978-0-123-82163-8 |edition=1st |series=Academic Press Series in Biomedical Engineering |location=New York, NY |pages=239–248 |chapter=Chapter 3.4: Skeletal muscle mechanics}}</ref> The time between a stimulus to the motor nerve and the subsequent contraction of the innervated muscle is called the '''latent period''', which usually takes about 10 ms and is caused by the time taken for nerve action potential to propagate, the time for chemical transmission at the neuromuscular junction, then the subsequent steps in excitation-contraction coupling.<ref>{{Cite book |last=Khurana |first=Indu |title=Textbook Of Medical Physiology |date=2006 |publisher=Elsevier |edition=1st |pages=101–2 |chapter=Characteristics of muscle excitability and contractility}}</ref> If another muscle action potential were to be produced before the complete relaxation of a muscle twitch, then the next twitch will simply sum onto the previous twitch, thereby producing a ''summation''.<ref name="Smith">{{Cite journal |last=Smith |first=Ian C. |last2=Adam |first2=Helen |last3=Herzog |first3=Walter |date=April 2020 |title=A brief contraction has complex effects on summation of twitch pairs in human adductor pollicis |url=https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/EP088401 |journal=Experimental Physiology |volume=105 |issue=4 |pages=676–689 |doi=10.1113/ep088401 |pmid=32052487 |s2cid=211100581 |access-date=5 April 2022|doi-access=free }}</ref> Summation can be achieved in two ways:<ref>{{Cite journal |last=Shwedyk |first=E. |last2=Balasubramanian |first2=R. |last3=Scott |first3=R. N. |year=1977 |title=A nonstationary model for the Electromyogram |journal=IEEE Transactions on Biomedical Engineering |volume=24 |issue=5 |pages=417–424 |doi=10.1109/TBME.1977.326175 |pmid=892834 |s2cid=1770255}}</ref> ''frequency summation'' and ''multiple fiber summation''. In '''frequency summation''', the force exerted by the skeletal muscle is controlled by varying the frequency at which [[action potential]]s are sent to muscle fibers. Action potentials do not arrive at muscles synchronously, and, during a contraction, some fraction of the fibers in the muscle will be firing at any given time. In a typical circumstance, when humans are exerting their muscles as hard as they are consciously able, roughly one-third of the fibers in each of those muscles will fire at once{{Citation needed|date=August 2020}}, though this ratio can be affected by various physiological and psychological factors (including [[Golgi tendon organ]]s and [[Renshaw cell]]s). This 'low' level of contraction is a protective mechanism to prevent [[avulsion fracture|avulsion]] of the tendon—the force generated by a 95% contraction of all fibers is sufficient to damage the body. In '''multiple fiber summation''', if the central nervous system sends a weak signal to contract a muscle, the smaller [[motor unit]]s, being more excitable than the larger ones, are stimulated first. As the [[Summation (neurophysiology)|strength of the signal]] increases, more motor units are excited in addition to larger ones, with the largest motor units having as much as 50 times the contractile strength as the smaller ones. As more and larger motor units are activated, the force of muscle contraction becomes progressively stronger. A concept known as the size principle, allows for a gradation of muscle force during weak contraction to occur in small steps, which then become progressively larger when greater amounts of force are required. Finally, if the frequency of muscle action potentials increases such that the muscle contraction reaches its peak force and plateaus at this level, then the contraction is a ''tetanus''. ====Length-tension relationship==== {{Further|Hill's muscle model}} [[File:Lengthtension.jpg|thumb|left|Muscle length versus isometric force]] Length-tension relationship relates the strength of an isometric contraction to the length of the muscle at which the contraction occurs. Muscles operate with greatest active tension when close to an ideal length (often their resting length). When stretched or shortened beyond this (whether due to the action of the muscle itself or by an outside force), the maximum active tension generated decreases.<ref name="1stLT">{{Cite journal |vauthors=Gordon AM, Huxley AF, Julian FJ |year=1966 |title=The variation in isometric tension with sarcomere length in vertebrate muscle fibres |journal=J. Physiol. |volume=184 |issue=1 |pages=170–92 |doi=10.1113/jphysiol.1966.sp007909 |pmc=1357553 |pmid=5921536}}</ref> This decrease is minimal for small deviations, but the tension drops off rapidly as the length deviates further from the ideal. Due to the presence of elastic proteins within a muscle cell (such as [[titin]]) and extracellular matrix, as the muscle is stretched beyond a given length, there is an entirely passive tension, which opposes lengthening. Combined, there is a strong resistance to lengthening an active muscle far beyond the peak of active tension. ====Force-velocity relationships==== [[Image:Muscle Force Velocity relationship.png|thumb|right|300px|alt=Force–velocity relationship of muscle contraction|Force–velocity relationship: right of the vertical axis concentric contractions (the muscle is shortening), left of the axis eccentric contractions (the muscle is lengthened under load); power developed by the muscle in red. Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). The optimal shortening velocity for power generation is approximately one-third of maximum shortening velocity.]] Force–velocity relationship relates the speed at which a muscle changes its length (usually regulated by external forces, such as load or other muscles) to the amount of force that it generates. Force declines in a hyperbolic fashion relative to the isometric force as the shortening velocity increases, eventually reaching zero at some maximum velocity. The reverse holds true for when the muscle is stretched – force increases above isometric maximum, until finally reaching an absolute maximum. This intrinsic property of active muscle tissue plays a role in the active damping of joints that are actuated by simultaneously active opposing muscles. In such cases, the force-velocity profile enhances the force produced by the lengthening muscle at the expense of the shortening muscle. This favoring of whichever muscle returns the joint to equilibrium effectively increases the damping of the joint. Moreover, the strength of the damping increases with muscle force. The motor system can thus actively control joint damping via the simultaneous contraction (co-contraction) of opposing muscle groups.<ref>{{Cite journal |last=Heitmann |first=Stewart |last2=Ferns |first2=Norm |last3=Breakpsear |first3=Michael |year=2011 |title=Muscle co-contraction modulates damping and joint stability in a three-link biomechanical limb |journal=Frontiers in Neurorobotics |volume=5 |pages=5 |doi=10.3389/fnbot.2011.00005 |issn=1662-5218 |pmc=3257849 |pmid=22275897 |doi-access=free}}</ref> </textarea><div class="templatesUsed"></div><p id="mw-returnto">Return to <a href="/wiki/Muscle_contraction" title="Muscle contraction">Muscle contraction</a>.</p> <noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?useformat=desktop&type=1x1&usesul3=0" alt="" width="1" height="1" style="border: none; position: absolute;"></noscript> <div class="printfooter" data-nosnippet="">Retrieved from "<a dir="ltr" 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