5. skeletal muscles

5/25/2018 5. Skeletal Muscles

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PHYSIOLOGY OF MUSCLE TISSUE

In vertebrates, three types of muscle cells can be identified on the basis of their structure andfunctions:

skeletal muscle- is the muscle directly attached to the bones of the skeleton and its role is tomaintain posture and to move the limbs

cardiac muscle is the muscle of the heart

smooth muscle - is the muscle that lines the blood vessels and modifies theirs diameter, andthe hollow organs in the body, in which it causes propulsion of the content.

THE SKELETAL MUSCLE

I. The structure of the skeletal muscleSkeletal muscles comprise about 40% of the mass of the average human body and they are

formed by thousands of elongated cells called muscle fibers running in parallel. Individual musclefibers are surrounded by a sheath of connective tissue, called endomysium. Groups of muscle fibersare bound together by connective tissue called perymisium to form bundles called fascicules. Thewhole muscle is surrounded by a coat of connective tissue, called epimysium. The connective tissuecontains collagen and elastic fibers that merge with the connective tissue of the tendons where it

serves to transmit the mechanical force generated by the muscle to the skeleton. Within the body ofthe skeletal muscle there are specialized sense organs, called muscle spindles that play a role in theregulation of muscle length.One muscle fiber is a multinucleate cell, in which the nuclei lie peripherally just under the cellmembrane. In large limb muscles, fibers may reach a length of 30 cm with a diameter of 100 m. Thecell membrane (the sarcolemma) surrounds the cytoplasm (the sarcoplasm). The sarcoplasm containsseveral hundreds to thousands of contractile elements, the myofibrils, each of which is 12 m indiameter. One myofibril is composed by as many as 10,000 repeating units called sarcomeres. Thesarcomere is the fundamental contractile unit within skeletal muscle. Because of the repeating units inmyofibrils the muscle fiber appear to have repeated cross-striations (easily seen under the lightmicroscope). For this reason the skeletal muscles are also called striated muscles. The sarcolemma isinvaginated at each sarcomere to form blind-ending transverse tubes (T tubules) that run into the

centre of the fiber. These T tubules have fundamental role in the activating the muscle contraction.Running longitudinally between the repeating T tubules are blind-ending membrane tubes or sacscalled the sarcoplasmic reticulum. The ends of the tubes of the sarcoplasmic reticulum, terminalcisternae, are situated closely to the membranes of the T tubules, forming triads. A triad is consistedof a T tubule and 2 terminal cisternae on either side. The triad is the site of excitationcontractioncoupling.

Ultrastructure of the striated muscleWhen a muscle fiber is observed by polarized light, it is seen as alternating dark and light zones,because they refract differently the light. A bands are regions that appear dark with the polarized lightmicroscope. I bands appear light, because they do not refract light. Each I band is divided by acharacteristic line known as Z line. The Z line crosses all neighboring myofibrils and attaches to theinternal side, keeping the pale and dark discs of neighboring myofibrils aligned. The unit between

successive Z lines is a sarcomere (functional unit of the striated muscle). Myosin-containing, thickfilaments lie in the center of an A band. Thin filaments contain the proteins actin, tropomyosin andtroponin; they insert on Z lines at the edges of the sarcomere and overlap thin filaments at the edgesof the A band. Each thick filament is surrounded by 6 thin filaments (hexagonal symmetry). When amuscle fiber is at rest, the fiber is elongated. The region that contains only thick filaments is situated inthe center of the A band, appears lighter and is called the H zone. Some protein molecules connectadjacent thick filaments. These proteins form a line in the center of the H zone called the M line. Atrest, the length of the sarcomere is about 2.5 m.


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The contractile proteinsIn fact, skeletal muscle fibers contain 2 contractile proteins, myosin and actin, and 2 regulatoryproteins, tropomyosin and troponin.Myosin:it is a fibrilar protein. Each myosin molecule consists of two heavy (2000 amino acids) alpha-helical protein chains and is 150 nm long. About 250 molecules make up a thick filament in asarcomere. One individual molecule has a long thin tail region and two thicker head regions that areextended by an arm outwards to form cross-bridges with the neighboring thin filaments. The protruding

arm and head are called the cross-bridges. Each cross-bridge is flexible, allowing the head to beextended far outward from the body of the myosin filament and to be brought close to the body. Theglobular head not only contain sites for attaching to actin but also function as an ATP-ase enzyme.This feature of the myosin molecule is essential for contraction.

Actin: the actin molecule is a globular protein (G-actin). The actin of the thin filament in a sarcomereis a polymerized form called F-actin. The thin filament is composed of two F-actin filaments woundtogether like two strands of beads. One polymerized actine contains about 300 molecules of globularactine. Each bead of actin possesses a site for attaching of the globular heads of myosin.Tropomyosin (molecular weight 70 kDa) is an elongated protein polymer that is wrapped around theactin filament, lying in the ditch made by the 2 filaments of actin, and partly obscures its binding sites.In such a position, myosin heads cannot bind to actin and cannot create a power stroke.Troponin is a complex of three proteins associated with tropomyosin. Troponin I is inhibitory, troponin

T binds to tropomyosin and troponin C binds reversibly to Ca2+.

When Ca2+

binds to subunit C of thetroponin, it changes the protein conformation, thus pulling tropomyosin away from the myosin-bindingsites. In such a position, myosin heads can bind and carry out their power stroke.

! Myoglobin is not a contractile protein; it is an iron-containing protein similar to hemoglobin in redblood cells. Myoglobin combines with oxygen and stores it until needed. According to the myoglobincontent, the muscle fibers divide in:

- white fibers/fast fibers: low myoglobin content, larger fibers, anaerobic metabolism, fewmitochondria; they are fast contracting fibers and get easily fatigued.

- red fibers/slow fibers: much myoglobin, many mitochondria, smaller fibers, they contract moreslowly, but do not get so easily fatigued.

Every muscle in the body contains both white and red fibers. The muscles that react very rapidly are

composed mainly of fast fibers, while those contracting slowly, but for a prolonged period of time arecomposed mainly of slow fibers.

Muscle= Organized arrays of muscle fibers

Myofiber/Musc le fiber= A single multinucleate muscle cell containing all the usual cell organellesplus many myofibrils

Sarcomere= The unit of contractile activity composed mainly of actin and myosin and extending fromZ line to Z line in a myofibril

Thin filament = Composed of a linear array Thick filament= Composed of hundreds of longof hundreds of globular, actin monomers contractile myosin molecules arranged in a staggered ina double helical arrangement; contains also side by side complexregulatory proteins


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II. The mechanism of skeletal muscle contraction

A. Excitation-contraction couplingCurrent understanding of the molecular events underlying muscle contraction is explained by

the sliding filament theory of muscle contraction. The model is applicable to smooth, skeletal, cardiac,and other contractile activity, including mechanochemical events such as single cell locomotion.

Skeletal muscle, like the nerve cell, is an excitable tissue. Each skeletal muscle fiber isinnervated by a motor neuron. As it enters a muscle, the axon of the motor neuron branches and eachbranch supplies a single muscle fiber. An action potential in the motor neuron, that reaches theneuromuscular junction, triggers an action potential that propagates along the whole length of themuscle fiber. The action potential that reaches the T tubes causes Ca++voltage-gated channels in thesarcoplasmic reticulum to open. As a result, Ca++stored in the sarcoplasmic reticulum is released intothe sarcoplasma and triggers the contraction of the muscle fiber, allowing the contractile protein tointeract.

B. Molecular mechanism of contraction - sliding filament theory of contractionMyosin in thick filaments and actin in thin filaments work together to shorten the sarcomere

during muscle contraction. The model of this rhythmic process is called the Sliding Filament

Mechanism.The main feature of muscle contraction is the interaction of actin, myosin and ATP. This

fundamental process of contraction is regulated by the tropomyosin-troponin-Ca2+system. The actinmolecule without the presence of troponin-tropomyosin complex binds strongly with myosin molecules.

At rest, the myosin-binding site on the actin molecule is covered by the tropomysin-troponin complex.When Ca concentration inside the cell increases (it is released from sracoplasmic reticulum), thisinflux of calcium ions bind to troponin complex and thus it changes its conformation. This, at its tunr,changes the position of tropomyosin the myosin-binding sites of actin become exposed. Thus actinand the myosin head groups can interact. When a head attaches to an active site on actin, thisattachment causes profound changes in the intramolecular forces between the arm and the head ofthe cross bridge. The new alignment of forces causes the head to tilt toward the arm and drag theactin filament along with it. Then, immediately after tilting, the head automatically breaks away from

the active site, returns to its perpendicular position, combines with a new actin site and tilts again.Thus, the heads of the cross-bridges bend back and forth, pulling the ends of the actin filamentstoward the center of the thick filament of myosin. As the muscle contracts, the thick and thin filamentsslide past each other, moving the Z lines of the sarcomere closer together. The thick myosin filaments,seen as the A band, stay at a fixed length but the I band shortens as the actin slides into the myosin.The cycle continues to operate as long as the binding sites for cross-bridge formation are exposed.Therefore, control of contraction is linked to the regulation of troponin and tropomyosin. Thecrossbridges are uniformly distributed along the thick filaments with the exception of a short bare zonein the middle. The tension is the algebraic sum of the tension produced at each individual site. At orabove rest length the tension is directly proportional to the number of crossbridges in the overlapregion between thick and thin filaments. Below rest length, when the thin filaments meet in the centerof the A band or they start to interact with the oppositely directed crossbridge sites past the bare

zone (in the middle of the sarcomere), tension drops off.For muscle contraction, the large amount of energy that is required comes from ATP hydrolysis.Before contraction begins, ATP bound to the heads of myosin and is cleaved by it into ADP, aninorganic phosphate group and energy. In this state the heads are oriented perpendicularly toward theactin, but there is no interaction yet. When the binding sites on actin molecules become active, themyosin and actin interact as described above. The energy that allows the slide of the two filaments isgiven by the energy already stored by the conformational change in the head when ATP has beencleaved. Once the head is tilted, ADP and inorganic phosphate are released and a new ATP moleculeis bound. The binding of the new ATP leads to disclosure of myosin heads from actin. The cleavage ofthis new ATP molecule will cause the head to take a perpendicular position on the body, and thus fix a


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new active site of actin. With a new ATP a new cycle may begin and the cycling may continue till theregulatory mechanism stops the interaction of actin and myosin.

C. RelaxationNormally, cessation of contractile activity and a state of relaxation follow electrical quiescence

at the myoneural junction. The sarcoplasmic membrane returns to its resting electrical potential (about60 mV more positive outside), as does the entire T tubule system and the SR membrane.

Subsequently, sarcoplasmic calcium is pumped back into the SR cisternae by an extremely active CaATPase (calcium pump), which comprises one of the main proteins of the SR membrane. Relaxationis a result of the decrease of Ca concentration in sarcoplasm, following the influx of Ca back into thesarcoplasmic reticulum. Because relaxation is an energy-dependant process, it is an active process.

III. Energetics of musc le contraction

Muscle action requires energy for excitation-contraction coupling, contraction and relaxation. Acontinuous supply of ATP is needed for the muscle to function. The muscle contraction involves thetransformation of chemical energy from ATP into mechanical energy. Energy from ATP also powersthe Ca pump in the sarcoplasmic reticulum membrane to return the calcium ions into the sarcoplasmicreticulum, thus allowing relaxation.

a. ATP: the energy for contraction is derived from the hydrolysis of ATP. The amount of ATP in musclefiber is low, of about 4 mmoles/kg wet tissue. The available ATP can maintain the contraction for a fewseconds (about 8 twitches). Cleaving one phosphate group from ATP liberates 7.3 kcal/mole:

ATP= ADP + Pi + 7,3 kcal

b. Creatine phosphate in the muscle is a phosphorylated form of creatine to store energy. Theimmediate synthesis of ATP is achieved from creatinphosphate present in muscle:

Creatine phosphate + ADP = creatine + ATP

When ATP concentrations in the muscle fibers begin to fall due to muscle contraction, an enzymecalled creatine phosphokinase catalyzes the transfer of the phosphoryl group from the CP to ADP,regenerating ATP. The amount of creatine phosphate present in the muscle is about ten times that of

ATP and allows the muscle contraction for about 100 twitches. Even so, the energy available formuscle contraction maintains the muscle activity for less than a minute. Then the muscle must switchto glucose metabolism.

c. Glucose- phosphocreatine is generated by glucose. Two different pathways are involved in themetabolism of glucose: one anaerobic and one aerobic. The anaerobic process occurs in thecytoplasm and is only moderately efficient, because it generates only 2 ATP/glucose molecule.

Anaerobic metabolism can support about 600 twitches. When oxygen is available, the aerobiccycle takes place in the mitochondria and is results in the greatest release of energy, because it

generates 38 ATP/glucose molecule. As the name implies, though, it requires oxygen. Aerobiccellular respirationproduces ATP slowly, but can produce large amounts of ATP over time if thereis a sufficient blood supply and myoglobin (red pigment that stores oxygen) stores. Aerobicmetabolism can support about 20.000 twitches. During heavy exercise, insufficient oxygen foroxidative metabolism can be delivered to the muscle, so ATP will be generated by anaerobicpathway. This is less efficient in generating ATP than the aerobic pathway. In the beginning of alight physical effort, the energy is delivered via an anaerobic pathway for approximately 20-45seconds, until an appropriate blood supply reaches the muscle. This adequate amount of blood inthe muscle fiber brings enough oxygen for energy being obtained via an aerobic pathway. After 2minutes this aerobic pathways becomes the main source of energy. In prolonged exercise, the ATP


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is derived either by oxidative metabolism of glucose and fats or by glycogen breakdown to lactate(anaerobic activity). During anaerobic activity hydrogen ions, lactate and phosphate ionsaccumulate inside the muscle fiber, causing a weaker and weaker activity known as muscularfatigue. Muscle fibers that use aerobic respiration take longer to fatigue.

IV. The properties of musclesA. Exci tab il ity-represents the ability of a muscle to respond to stimuli. (All kinds of muscle cells

respond to nervous stimuli, but smooth and cardiac muscles respond also to certain chemical stimuli).

B. Contractility represents the property of a muscle to produce a force. There are 2 types of musclecontraction: isometric and isotonic. Isometric contractions are contractions in which the muscle isdeveloping force but not shortening and no visible movement is seen. In this case, the forcedeveloped by the muscle equals the force (load) it is contracting against. An example of this would beholding a book in the same position or some of masticatory muscles contractions. Isotoniccontractions are contractions in which the muscle fibers change length to lift a load, but exert aconstant tension. Examples of this type of contraction areMost complex movements are a combination of isotonic and isometric contractions, because alwaysther are changes of both tension and length of a muscle. Such contractions are called auxotonic.

C. Muscle forceIt is the force generated by a muscle contraction. The muscle force MF is calculated according to theformula:MF = A x L x 10,

Where A= the cross sectional area (in cm2) and L= the length of the muscle during shortening (in m)Muscle force varies between 3.6-10kg/ cm2The amount of force exerted by a muscle depends on:

1. the initial length of the muscle(initial length of sarcomers): the generated force is atmaximum when the initial length of the sarcomer is of 2,2 nm. At this length the interdigitation betweenthe thin and thick filament is optimal and the maximum number of cross bridges can be established.When the sarcomere has a length of more than 2.2 nm or les than 2.2 nm, the force generated by themuscle is lower.

2. The frequency of stimulationA twitchis a single stimuluscontractionrelaxation sequence in a muscle fiber. Twitches vary in

duration between 7.5 msec in fast fibers to 100-150 msec in slow fibers.A single twitch can be divided into a latent period,a contraction phase,and a relaxation phase. Thelatent phase begins at stimulation and typically lasts about 10-15% of the contraction duration. Overthis period, the action potential is conducted all along the sarcolemma and the sarcoplasmic reticulumreleases calcium ions. The muscle fiber does not produce tension during the latent period, becausethe contraction did not beginn. In the contraction phase,tension rises to a peak. Contraction occursbecause calcium ions are binding to troponin, active sites on thin filaments are being exposed, andcross-bridge interactions are occurring. The contraction phase ends roughl. The relaxation phase thencontinues. During this period, calcium levels are falling, as calcium is being pumped back into thesarcoplasmic reticulum; active sites are being covered by tropomyosin, and the number of active

cross-bridges is declining.Wave summation and incomplete or unfused tetanus If a second stimulus arrives to the

muscle fiber before the relaxation phase has ended, a second, more powerful contraction occurs. Theaddition of one twitch to another in this way constitutes the summation of twitches.

Complete or fused tetanus is obtained by increasing the stimulation rate until the relaxationphase is eliminated. During complete tetanus, action potentials arrive so rapidly that the sarcoplasmicreticulum does not have time to reclaim the calcium ions. The high Ca2+ concentration in thecytoplasm maintains the contraction state, making it continuous. Within the body, all normal muscularcontractions involve complete tetanus


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Tension production increases as the rate of stimulation increases; during tetanus the tensiondeveloped is 4 times higher as compared to a single switch.

A single stimulation produces a single twitch. All normal activities involve sustained musclecontraction, as the motor nerves always conduct trains of impulses and not only one impulse. In otherwords, normally, the skeletal muscles receive repeated stimuli, thus the contraction obtained is alwaysa prolonged one.

After a long duration tetanic contraction sometimes contracture can occur. Contracture defines the

condition in which a delayed relaxation occurs and is due to exhaustion of energy sources in themuscles, so that relaxation cannot occur (remember relaxation is an active procees, requiring energy).

3.The number of active muscle fibers- the tension depends not only on the tension produced byeach individual muscle fiber, but also on the number of fibers contracting at a given time. A typicalskeletal muscle contains thousands of muscle fibers. Although some motor neurons control a fewmuscle fibers, most control hundreds of them. All the muscle fibers controlled by a single motorneuron constitute a motor unit. The size of a motor unit is an indication of how fine the control ofmovement can be. In the muscles of the eye, where precise control is extremely important, a motorneuron may control 46 muscle fibers. For the muscles in the leg a single motor neuron may control10002000 muscle fibers. For a whole muscle, the numbers of contracting fibers is determined by thenumber of motor units activated and the number of fibers per unit. When a weak stimulus reaches themuscle, only the smallest motor units become active. Then, as the neuron firing increases, more motor

units are activated. The steady increase in muscular tension produced by increasing the number ofactive motor units is called recruitment. Such recruitment is the main factor in increasing the strengthof contraction smoothly. Peak tension production occurs when all motor units in the muscle contract ina state of complete tetanus. Such powerful contractions do not last long, however, because theindividual muscle fibers soon use up their available energy reserves. During a sustained tetaniccontraction, motor units are activated on a rotating basis, so some of them are resting and recoveringwhile others are actively contracting. This is called asynchronous motor unit summation and lets eachmotor unit recover before it is stimulated again . As a result, when the muscles contract for sustainedperiods, they produce slightly less than maximal tension.

4. The cross sectional area of the muscle the force that is developed by a muscle dependsnot on its length, but on its cross sectional area. The cross sectional area depends, on its turn, on thenumber of myofibrils in that muscle, that act in parallel.

C. Extensibility: is the ability to undergo passive stretching under the action of an external forceapplied on the muscle

D. Elasticity: is the ability of a muscle to return to its original shape after it has been stretched.

F. Trophycity: muscles work better the more they are used. When a muscle is stimulated either by anerve impulse or an electrical impulse, it maintains its function. Muscle atrophy is a decrease inmuscle mass caused by age, lack of use (broken bone) or denervation (loss of innervation). By thecontrary, hypertrophy is an increase in mass of a muscle that can be induced by exercise. Isometriccontraction rather than isotonic contraction produce hypertrophy. Hypertrophy results primarily fromthe growth of each muscle cell, by increasing the number of myofibrils rather than an increase in the

number of cells. It can take as long as two months for hypertrophy to begin.

G.Tonicity: themuscle tone is the slight degree of contraction produced even in resting muscles thatrenders the muscle firm and solid. The muscle tone is due to asynchronous contraction of variousmotor unit in a muscle. Thus, while some motor units are at rest, others are active, even when theentire muscle is not contracting. The active motor units changes constantly, so that no group becomesfatigued. The role of muscle tone is to stabilize the position of bones and joints (helps prevent sudden,uncontrolled changes in the position of bones and joints) and to smooth the movements of the body.


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V. Heat produc tionMuscle activity requires that chemical energy is converted to work. The percentage of the input

energy to the muscle that can be converted into work is about 20-25 %. The remaining is transformedinto heat. Skeletal muscle fibers are the main source of heat in our body. Muscles produce heat whenat rest; the amount of heat increases largely when they become active. This heat is composed by twocategories of heat, of approximately same amount: the initial heat is the heat produced duringcontraction and relaxation of muscle fibers and the tardy heat, produced for approximately 30 minutes

after finishing the muscle activity. This tardy heat is due to the energy reaction taking place in musclefibers after the work is done, for getting back the ATP, PC and glycogen stores of the muscle throughaerobic reactions. Heat released by muscles contributes to maintaining the normal body temperature.Heat release during strenuous exercise can rise the body temperature above normal.

VI. Electric activity of the muscleDuring depolarization and repolarization of the sarcolemma little electric currents are

generated. Electromyography is a procedure of recording electrical changes in muscle tissues.Electromyography is used in medicine mainly to differentiate between muscle and nerve diseases.

VII. Musc le fatigueThe impairment of muscle function due to prolonged or repeated contractions of skeletal muscles is

called fatigue. Impairment refers to both decreased force production and slower contractions.Contractions that involve stretch of the muscle also cause muscle weakness and damage, which takesmany days to recover from.Fatigue may be caused by factors within the muscle cells (peripheral fatigue) and diminishedactivation from the central nervous system (central fatigue). The relative importance of peripheralversuscentral fatigue depends on the type of physical activity.For peripheral fatiguehypoxia and accumulation of catabolic products are involved. During moderateexercise, in slow muscle fibers decreasing the deposits of glycogen is the cause of muscle fatigue. Infast muscle fibers, that use energy derived from anaerobic glycolysis, accumulation of lactic acid is themain cause of the muscle fatigue.Central fatigue is caused by 2 factors. First decreased muscle function is given by accumulation ofcatabolic products in the neurons within the nervous centers that control the voluntary activity.

Secondly, the cause of muscle fatigue is the exhaustion of neurotransmitters in the neuromuscularjunction. Central fatigue may be more prominent in elderly subjects.Training can make a difference in how long and fast both of the metabolic pathways work. Trainedathletes have a greater ability to quickly deliver oxygen to the working muscles which increases theability to use aerobic metabolism at higher exercise intensity. Trained athletes also develop a greaterefficiency in both energy deliver, and skill. Finally training may improve the way the body creates anduses the anaerobic systems so you can access ATP more readily

VII. Roles of the skeletal muscles1. Maintaining the body posture2. Movement3. Some striated muscles are sphincters (of the lips, of the lids, anal).

4. In maintaining the body temperature5. Diaphragm is a skeletal muscle responsible for pulmonary ventilation6. the upper 1/3 of the esophagus contains skeletal muscle that helps swallowing7. Aesthetic

5. skeletal muscles

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