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muscle tissue.docx

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University of Toronto Scarborough
Biological Sciences
Kenneth Welch

Muscle Tissue Skeletal muscle tissue and the Muscular System Three types of muscle • Skeletal – attached to bone • Cardiac – found in the heart • Smooth – lines hollow organs Skeletal muscle functions • Produce skeletal movement • Maintain posture and body position • Support soft tissues • Guard entrances and exits • Maintain body temperature Anatomy of Skeletal Muscle Organization of connective tissues • Epimysium surrounds muscle • Perimysium sheathes bundles of muscle fibers • Epimysium and perimysium contain blood vessels and nerves • Endomysium covers individual muscle fibers • Tendons or aponeuroses attach muscle to bone or muscle Skeletal muscle fibers • Sarcolemma (cell membrane) • Sarcoplasm (muscle cell cytoplasm) • Sarcoplasmic reticulum (modified ER) • T-tubules and myofibrils aid in contraction • Sarcomeres – regular arrangement of myofibrils Myofibrils • Thick and thin filaments • Organized regularly Muscle Fiber Thin filaments • F-actin • Nebulin • Tropomyosin • Covers active sites on G-actin • Troponin • Binds to G-actin and holds tropomyosin in place Thick filaments • Bundles of myosin fibers around titan core • Myosin molecules have elongate tail, globular head • Heads form cross-bridges during contraction • Interactions between G-actin and myosin prevented by tropomyosin during rest Sliding filament theory • Explains the relationship between thick and thin filaments as contraction proceeds • Cyclic process beginning with calcium release from SR • Calcium binds to troponin • Trponin moves, moving tropomyosin and exposing actin active site • Myosin head forms cross bridge and bends toward H zone • ATP allows release of cross bridge The Contraction of Skeletal Muscle Tension • Created when muscles contract • Series of steps that begin with excitation at the neuromuscular junction • Calcium release • Thick/thin filament interaction • Muscle fiber contraction • Tension Control of skeletal muscle activity occurs at the neuromuscular junction • Action potential arrives at synaptic terminal • ACh released into synaptic cleft • ACh binds to receptors on post-synaptic neuron • Action potential in sarcolemma Excitation/contraction coupling • Action potential along T-tubule causes release of calcium from cisternae of SR • Initiates contraction cycle • Attachment • Pivot • Detachment • Return Relaxation • Acetylcholinesterase breaks down ACh • Limits the duration of contraction Tension Production Tension production by muscle fibers • All or none principle • Amount of tension depends on number of cross bridges formed • Skeletal muscle contracts most forcefully over a narrow ranges of resting lengths • Twitch • Cycle of contraction, relaxation produced by a single stimulus • Treppe • Repeated stimulation after relaxation phase has been completed Summation • Repeated stimulation before relaxation phase has been completed • Wave summation = one twitch is added to another • Incomplete tetanus = muscle never relaxes completely • Complete tetanus = relaxation phase is eleminated Tension production by skeletal muscles • Internal tension generated inside contracting muscle fibers • External tension generated in extracellular fibers • Motor units • All the muscle fibers innervated by one neuron • Precise control of movement determined by number and size of motor unit • Muscle tone • Stabilizes bones and joints Tension production by s
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