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Unformatted text preview: Lecture 23: Muscle Physiology Reading: chapter 8, section: skeletal muscle, pgs 257-264 (pgs 253-259, if using 6 th edition) (pgs 257-263, if using 5 th edition) Types of Muscle (3 types) skeletal muscle striated, voluntary cardiac muscle striated, involuntary smooth muscle unstriated, involuntary Skeletal Muscle Organization Muscle- population of elongated muscle fibers held together by connective tissue and connected at either end by tendons ) . Muscle Fiber- single, multinucleated cell composed of a population of elongated myofibrils . Myofibril- elongated, cylindrically-shaped contractile elements composed of a population of sarcomeres connected end-to-end. Sarcomere- the smallest unit of a muscle cell containing all of the elements necessary for contraction. Composed of interdigitating and partially-overlapping thick and thin filaments . Structural components - Z line - region where thin filaments from adjacent sarcomeres join - A band- region with thick filaments - I band- region without thick filaments - H z o n e- region without thin filaments - M line- region in the center of the thick filaments Thick Filament- special assemblies of hundreds of myosin protein molecules organized into elongated fibers. Myosin- cytoskeletal protein composed of two interwoven subunits, each with a long tail and a globular head region. actin binding site- specialized region of the myosin head capable of binding to actin . myosin ATPase- specialized region of the myosin head capable of ATP hydrolysis. Thin Filament- specialized assemblies of three proteins, actin, tropomyosin and troponin , arranged to form an elongated double helical strand. Actin- Globular cytoskeletal protein linked to form two long chains arranged in a double helical strand. Tropomyosin- pairs of threadlike filamentous proteins that lie alongside the groove formed by the actin helix. Troponin- protein complex composed of three subunits, one that binds to actin, one that binds to tropomyosin, and one that binds Ca ++ . Multiple copies of this complex are bound to the strands of actin and tropomyosin. Cross-Bridge Activity myosin binds to actin via cross-bridges. Cross-bridge activity is necessary for actin and myosin molecules to slide across each other allowing the muscle to contract. 4 steps : 1) Binding Myosin cross bridge binds to actin molecules 2) Power Stroke Cross bridge bends, pulling thin myofilament (actin) inward 3) Detachment Cross bridge detaches at end of power stroke and returns to original conformation 4) Binding Cross bridge binds to more distal actin molecule; cycle repeats Consequences of cross-bridge activity 1) Sarcomere shortens 2) H zone becomes shorter 3) I band becomes shorter 4) A bands maintains the same width 5) Individual actin and myosin fibers maintain a constant length Role of Ca++ in turning on cross bridges 1) When muscle fiber relaxed, cross-bridge binding site is covered by the troponin- tropomyosin complex 2) When muscle fiber is excited, Ca++ binds to troponin, pulling the troponin- tropomyosin complex aside to expose cross-bridge binding sites Lecture 24: Muscle Physiology...
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- Spring '08