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part 7


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part 7

  1. 1. Chapter 11: MUSCLE<br />SECTION A<br />SKELETAL MUSCLE<br />Structure<br />Molecular Mechanisms of<br />Contraction<br />Sliding-Filament Mechanism<br />Roles of Troponin, Tropomyosin, and<br />Calcium in Contraction<br />Excitation-Contraction Coupling<br />Membrane Excitation: The<br />Neuromuscular Junction<br />Roxanne Trina C. Ferrer, MD<br />EllaineShiela Marie G. Corpuz, MD<br />BS Biology 4<br />
  2. 2. Excitation-Contraction Coupling<br />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.<br />
  3. 3. Excitation-Contraction Coupling<br />The Skeletal Muscle<br />has a plasma membrane that is excitable and capable of generating and propagating action potentials.<br />lasts 1 to 2 ms and is completed before any signs of mechanical activity begin.<br />once begun, the mechanical activity following an action potential may last 100 ms or more.<br />
  4. 4. Excitation-Contraction Coupling<br />
  5. 5. Excitation-Contraction Coupling<br />The electrical activity in the plasma membrane does not directly act upon the contractile proteins <br />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.<br />
  6. 6. Excitation-Contraction Coupling<br />In a resting muscle fiber<br />the cystolic calcium concentration surrounding the thick and thin filaments is very low (about 10-7 mol/L).<br />very few of the calcium binding sites on the troponin are occupied<br />thus crossbridge activity is blocked by tropomyosin.<br />
  7. 7. Excitation-Contraction Coupling<br />Following an action potential…<br />there is a rapid increase in cytosolic calcium concentration<br />calcium binds to troponin<br />removes the blocking effect of tropomyosin<br />allows cross-bridge cycling.<br />
  8. 8. Excitation-Contraction Coupling<br />SarcoplasmicReticulum<br />the source of the increased cytosoliccalcium within the muscle fiber.<br />forms a series of sleevelike structures around each myofibril (one segment surrounding the A band and another the I band).<br />
  9. 9. Excitation-Contraction Coupling<br />
  10. 10. Excitation-Contraction Coupling<br />Lateral Sacs<br />two enlarged regions at the end of each segment that are connected to each other by a series of smaller tubular elements.<br />store the calcium that is released following membrane excitation.<br />
  11. 11. Excitation-Contraction Coupling<br />
  12. 12. Excitation-Contraction Coupling<br />Transverse Tubule (T tubule)<br />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<br />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.<br />
  13. 13. Excitation-Contraction Coupling<br />
  14. 14. Excitation-Contraction Coupling<br />Depolarization of the T tubule by an action potential…<br />leads to the opening of the calcium channels in the lateral sacs<br />allows calcium to diffuse from the calcium-rich lumen of the lateral sacs into the cytosol<br />turn on all the cross bridges in the fiber.<br />
  15. 15.
  16. 16. Excitation-Contraction Coupling<br />A contraction continues until calcium is removed from troponin.<br />achieved by lowering the calcium concentration in the cytosol back to its pre-release level.<br />primary active-transport proteins,Ca-ATPases,pump calcium ions from the cytosol back into the lumen of the reticulum<br />requires a much longertime<br />
  17. 17.
  18. 18. Excitation-Contraction Coupling<br />Hydrolysis of ATP by the Ca-ATPase in the sarcoplasmic reticulum… <br />provides the energy for the active transport of calcium ions into the lateral sacs of the reticulum<br />lowers cytosolic calcium to pre-release levels<br /> ends the contraction<br />allows the muscle fiber to relax<br />
  19. 19. END!<3<br />