MUSCLE TISSUE

MUSCLE TISSUEBatari T. Umar1Muscle TissueResponsible for Movement of the body and its partsChanges in the shape and size of the internal organContains of connective tissueComposed of muscle cellsVery high metabolic activity Requires high oxygen and nutrientMust be attached to fibrous connective tissue2General StructureMuccle cellCell membrane sarcolemmaCytoplasma sarcoplasmaEndoplasmic reticulum sarcoplasmic reticulumMitocondria sarcosomMicrofilament myofibril3Sarcolemma - 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)Three types of myofilamentsActin filaments (thin filaments)Myosin filaments (thick filaments)Titin filaments (Elastic)At each end of the fiber, myofibrils are anchored to the inner surface of the sarcolemmaWhen myofibril shortens, muscle shortens (contracts)SR is an elaborate, smooth endoplasmic reticulum Runs longitudinally and surrounds each myofibrilForm chambers called terminal cisternae on either side of the T-tubules

A single T-tubule and the 2 terminal cisternae form a triad

SR 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

Connective Tissue of a MuscleEpimysium. Dense regular c.t. surrounding entire muscleSeparates muscle from surrounding tissues and organsConnected to the deep fasciaPerimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicleContains b.v and nervesEndomysium. Loose connective tissue that surrounds individual muscle fibersAlso contains b.v., nerves, and satellite cells (embryonic stem cells function in repair of muscle tissueCollagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis

Types of Muscle TissueSkeletal muscle

Cardiac muscle

Smooth muscle

Skeletal Muscle= Voluntary muscle= Striated muscleComposed of muscle cells (fibers), connective tissue, blood vessels, nervesFibers are long, cylindrical, and multinucleated1 mm - 4 cm in lengthDevelop from myoblasts; numbers remain constantStriated appearanceNuclei are peripherally located

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13The structureThe E.M shows that each myofibril is made up of repeating dark & light bandsIn the middle of the dark band is the M-lineIn the middle of the light band is the Z-lineThe repeating unit from one Z-line to the next is called the sarcomereA very high resolution E.M reveals that each myofibril is made up of parallel filaments.

Thick & thin filaments are linked at intervals called cross bridges, which actually stick out from the thick filaments The structure

17SarcomereSarcomere - 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; produced by thick (myosin) filamentM line - protein to which myosins attachH zone is that portion of the A band where the thick and thin filaments do not overlap.Sarcomere........I 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 sarcomere

Z disk: filamentous network of protein. Serves as attachment for actin myofilaments

Titin filaments: elastic chains of amino acids; keep thick and thin filaments in proper alignment

20MyofilamentsThick filaments1.6m long, 15 nm wideOccupy the A band (the central portion of sarcomere)The thin filaments run between and parallel to the thick filamentsMajor protein : Myosin

Thin filaments1.0 m long, 8 nm wideMajor protein :ActinTropomyosinTroponin

22Structure of Actin and Myosin

ActinLong filamentous polymers2 strands of globular monomers (G-actin)Double helical formation5.6 nm in diameter

TropomyosinLong, thin molecule40 nm length2 polypaptide chainsLocated in the groove between 2 actin strands

TroponinComplex of 3 subunitTn T attaches to tropomyosinTn C binds calcium ionsTn I inhibits the actin-myosin interactionAttached at spesific sites along each tropomyosin molecule

Myosin2 identical heavy chains & 2 pairs of light chainsHeavy chain twisted together as myosin tailsSmall globular projection at one end of each heavy chain form the headHave ATP binding sitesBind actin

Titin filament

Smooth MuscleElongated, fusiform, nonstriated cellsEnclosed by a thin basal lamina & a fine network reticular fibersThe length are varies from 20 m in small blood vessels to 500 m the pregnant uterus1 nucleus located centrallyFibers smaller than those in skeletal muscleNo sarcomeres thus NO striationsT tubules (-)6 major locations: inside the eye walls of vessels respiratory tubes digestive tubes urinary organs reproductive organs

MyofilamentsThick, thin and intermediate filamentsBundles of thick & thin filaments crisscross obliquely through the cellsThick filaments contains of myosin similar to myosin in sceletal muscleThin filamentsActin, tropomyosin and calmodulinCalmodulin Ca binding protein Intermediate filamentsDesmin major protein in all smooth musclesVimentin additional component of vascular smooth muscle

Cardiac muscleFound only in heart, form thick myocardiumCardiac muscle cells are single cells (not called fibers)Cells branchCells join at intercalated discsIDs are composites of desmosomes and gap junctionsAllow excitation in one fiber to spread quickly to adjoining fibers

1-2 nuclei in centerHere fiber = long row of joined cardiac muscle cellsInherent rhythmicity: each cell! (muscle cells beat separately without any stimulation)

Structure & function of contractile protein = skeletal muscleT tubules : larger & more numerousMitochondria occupy 40% of cytoplasmic volume (>> than skeletal muscle)

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