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part 7
 

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    part 7 part 7 Presentation Transcript

    • Chapter 11: MUSCLE
      SECTION A
      SKELETAL MUSCLE
      Structure
      Molecular Mechanisms of
      Contraction
      Sliding-Filament Mechanism
      Roles of Troponin, Tropomyosin, and
      Calcium in Contraction
      Excitation-Contraction Coupling
      Membrane Excitation: The
      Neuromuscular Junction
      Roxanne Trina C. Ferrer, MD
      EllaineShiela Marie G. Corpuz, MD
      BS Biology 4
    • Excitation-Contraction Coupling
      refers to the sequence of events by which an action potential in the plasma membrane of a muscle fiber leads to cross-bridge activity by the certain mechanisms.
    • Excitation-Contraction Coupling
      The Skeletal Muscle
      has a plasma membrane that is excitable and capable of generating and propagating action potentials.
      lasts 1 to 2 ms and is completed before any signs of mechanical activity begin.
      once begun, the mechanical activity following an action potential may last 100 ms or more.
    • Excitation-Contraction Coupling
    • Excitation-Contraction Coupling
      The electrical activity in the plasma membrane does not directly act upon the contractile proteins
      but instead produces a state of increased cytosolic calcium concentrationwhich continues to activate the contractile apparatus long after the electrical activity in the membrane has ceased.
    • Excitation-Contraction Coupling
      In a resting muscle fiber
      the cystolic calcium concentration surrounding the thick and thin filaments is very low (about 10-7 mol/L).
      very few of the calcium binding sites on the troponin are occupied
      thus crossbridge activity is blocked by tropomyosin.
    • Excitation-Contraction Coupling
      Following an action potential…
      there is a rapid increase in cytosolic calcium concentration
      calcium binds to troponin
      removes the blocking effect of tropomyosin
      allows cross-bridge cycling.
    • Excitation-Contraction Coupling
      SarcoplasmicReticulum
      the source of the increased cytosoliccalcium within the muscle fiber.
      forms a series of sleevelike structures around each myofibril (one segment surrounding the A band and another the I band).
    • Excitation-Contraction Coupling
    • Excitation-Contraction Coupling
      Lateral Sacs
      two enlarged regions at the end of each segment that are connected to each other by a series of smaller tubular elements.
      store the calcium that is released following membrane excitation.
    • Excitation-Contraction Coupling
    • Excitation-Contraction Coupling
      Transverse Tubule (T tubule)
      a separate tubular structure with a membrane that is able to propagate action potentials over the surface of the muscle fiber and into its interior
      activates voltage-gated proteins in the T-tubule membrane that are physically or chemically linked to calcium-release channels in the membrane of the lateral sacs.
    • Excitation-Contraction Coupling
    • Excitation-Contraction Coupling
      Depolarization of the T tubule by an action potential…
      leads to the opening of the calcium channels in the lateral sacs
      allows calcium to diffuse from the calcium-rich lumen of the lateral sacs into the cytosol
      turn on all the cross bridges in the fiber.
    • Excitation-Contraction Coupling
      A contraction continues until calcium is removed from troponin.
      achieved by lowering the calcium concentration in the cytosol back to its pre-release level.
      primary active-transport proteins,Ca-ATPases,pump calcium ions from the cytosol back into the lumen of the reticulum
      requires a much longertime
    • Excitation-Contraction Coupling
      Hydrolysis of ATP by the Ca-ATPase in the sarcoplasmic reticulum…
      provides the energy for the active transport of calcium ions into the lateral sacs of the reticulum
      lowers cytosolic calcium to pre-release levels
      ends the contraction
      allows the muscle fiber to relax
    • END!<3