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 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 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 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 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