Skeletal muscle structure & function

SKELETAL MUSCLE STRUCTURE & FUNCTION

SKELETAL MUSCLE STRUCTURE &MECHANISM OF CONTRACTION Dr. Dinesh T,Junior resident.

20-11-20111Jipmer Physiologist

Discussion topicsIntroductionMyocyte- structure, developmentMuscle proteinsSarcomereSarcotubular systemExcitation- contraction couplingMolecular basis of muscle contractionSummaryMuscle relaxationConclusion


IntroductionHuman body contains over 400 skeletal muscles40-50% of total body weight

Functions of skeletal muscleBody movement (Locomotion)Maintenance of postureRespirationDiaphragm and intercostal contractionsCommunication (Verbal and Facial)Constriction of organs and vesselsPeristalsis of intestinal tractVasoconstriction of b.v. and other structures (pupils) Production of body heat (Thermogenesis)


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Skeletal Muscle CharacteristicsSlide 6.3Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Most are attached by tendons to bonesCells are multinucleateStriated have visible bandingVoluntary subject to conscious controlCells are surrounded and bundled by connective tissue = great force, but tires easily20-11-20114Jipmer Physiologist

TermsSarcolemma = Cell membraneSarcoplasm = CytoplasmSarcoplasmic Reticulum = Endoplasmic ReticulumSarcosomes = Mitochontria


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Structure of skeletal muscle:connective tissue coveringEpimysiumSurrounds entire musclePerimysiumSurrounds bundles of muscle fibersEndomysiumSurrounds individual muscle fibers20-11-20116Jipmer Physiologist

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Skeletal muscle structureComposed of muscle cells (fibers), connective tissue, blood vessels, nervesFibers are long, cylindrical, and multinucleatedTend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length


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Develop from myoblasts; numbers remain constantStriated appearanceNuclei are peripherally located


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Embryologic origin:


Muscle fiber anatomySarcolemma - cell membraneSurrounds the sarcoplasm (cytoplasm of fiber)Contains many of the same organelles seen in other cellsAn abundance of the oxygen-binding protein myoglobinPunctuated by openings called the transverse tubules (T-tubules)Narrow tubes that extend into the sarcoplasm at right angles to the surfaceFilled with extracellular fluidMyofibrils -cylindrical structures within muscle fiberAre bundles of protein filaments (=myofilaments)Two types of myofilamentsActin filaments (thin filaments)Myosin filaments (thick filaments)At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemmaWhen myofibril shortens, muscle shortens (contracts)


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Muscle proteinsContractile proteins Actin- thin myofilimentMyosin- thick filamentRegulatory proteinsTropomyosinTroponinAttachment proteins Titin, nebulin, alpha actinin, dystrophin


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Structure of Actin and Myosin


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Actin (Thin) MyofilamentsThin Filament: composed of 3 major proteinsF (fibrous) actinTropomyosinTroponinTwo strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere.Composed of G actin monomers each of which has a myosin-binding site Actin site can bind myosin during muscle contraction. Tropomyosin: an elongated protein winds along the groove of the F actin double helix. Troponin is composed of three subunits: Tn-A : binds to actinTn-T :binds to tropomyosin,Tn-C :binds to calcium ions.


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Myosin (Thick) MyofilamentMany elongated myosin molecules shaped like golf clubs. Single filament contains roughly 300 myosin moleculesMolecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally. Myosin headsCan bind to active sites on the actin molecules to form cross-bridges. (Actin binding site)Attached to the rod portion by a hinge region that can bend and straighten during contraction. Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction


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Sarcomeres: Z Disk to Z DiskSarcomere - repeating functional units of a myofibrilAbout 10,000 sarcomeres per myofibril, end to endEach is about 2 m longDifferences in size, density, and distribution of thick and thin filaments gives the muscle fiber a banded or striated appearance.A bands: a dark band; full length of thick (myosin) filamentM line - protein to which myosins attachH zone - thick but NO thin filamentsI bands: a light band; from Z disks to ends of thick filamentsThin but NO thick filamentsExtends from A band of one sarcomere to A band of the next sarcomereZ disk: filamentous network of protein. Serves as attachment for actin myofilamentsTitin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment


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Sarcoplasmic Reticulum (SR)SR is an elaborate, smooth endoplasmic reticulum runs longitudinally and surrounds each myofibrilForm chambers called terminal cisternae on either side of the T-tubulesA single T-tubule and the 2 terminal cisternae form a triadSR stores Ca++ when muscle not contractingWhen stimulated, calcium released into sarcoplasm SR membrane has Ca++ pumps that function to pump Ca++ out of the sarcoplasm back into the SR after contraction


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Sarcoplasmic Reticulum (SR)Figure 9.5


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Muscular ContractionThe sliding filament modelMuscle shortening occurs due to the movement of the actin filament over the myosin filamentFormation of cross-bridges between actin and myosin filamentsReduction in the distance between Z-lines of the sarcomere


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Sliding Filament TheoryRest uncharged ATP cross-bridge complexExcitation-coupling charged ATP cross-bridge complex, turned onContraction actomyosin ATP > ADP & Pi + energyRecharging reload cross-bridge with ATPRelaxation cross-bridges turned off20-11-201129Jipmer Physiologist

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Sliding Filament Model of ContractionThin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degreeIn the relaxed state, thin and thick filaments overlap only slightlyUpon stimulation, myosin heads bind to actin and sliding begins20-11-201130Jipmer Physiologist

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Cross-Bridge Formation in Muscle Contraction


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Myosin ATPase CycleFrom: Stryer 1988. Biochemistry. Freeman eds. Based on: Lymn and Taylor. 1971. Biochemistry 10: 4617

PiADP20-11-201132Jipmer Physiologist


EM Shows Different TM positions along the Actin FilamentRelaxed Ca-Activated Rigor Ca-ActivatedX-bridge From: Craig & Lehman.. 2001. J. Mol. Biol 311: 1027.20-11-201134Jipmer Physiologist


Excitation-Contraction CouplingMechanism where an action potential causes muscle fiber contractionInvolvesSarcolemmaTransverse or T tubulesTerminal cisternaeSarcoplasmic reticulumCa2+Troponin


Sources of ATP for Muscle Contraction


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Energy SourcesATP provides immediate energy for muscle contractions from 3 sourcesCreatine phosphateDuring resting conditions stores energy to synthesize ATPAnaerobic respirationOccurs in absence of oxygen and results in breakdown of glucose to yield ATP and lactic acidAerobic respirationRequires oxygen and breaks down glucose to produce ATP, carbon dioxide and waterMore efficient than anaerobic


Energy for Muscle ContractionSlide 6.24Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Direct phosphorylationMuscle cells contain creatine phosphate (CP)CP is a high-energy moleculeAfter ATP is depleted, ADP is leftCP transfers energy to ADP, to regenerate ATPCP supplies are exhausted in about 20 seconds

Figure 6.10a20-11-201139Jipmer Physiologist

Energy for Muscle ContractionSlide 6.26aCopyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Anaerobic glycolysisReaction that breaks down glucose without oxygenGlucose is broken down to pyruvic acid to produce some ATPPyruvic acid is converted to lactic acidFigure 6.10b


Energy for Muscle ContractionSlide 6.25Copyright 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Aerobic RespirationSeries of metabolic pathways that occur in the mitochondriaGlucose is broken down to carbon dioxide and water, releasing energyThis is a slower reaction that requires continuous oxygenFigure 6.10c


SUMMARY OF MUSCLE CONTRACTION -VIDEO 20-11-201142Jipmer Physiologist

THANK U20-11-201143Jipmer Physiologist

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Skeletal muscle structure & function

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