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Chapter 11:
MUSCLESECTION ASKELETAL MUSCLEStructureMolecular Mechanisms ofContractionSliding-Filament MechanismRoles of Troponin, Tropomyosin, andCalcium in ContractionExcitation-Contraction CouplingMembrane Excitation: TheNeuromuscular Junction
Roxanne Trina C. Ferrer, MDEllaine Shiela Marie G. Corpuz, MDBS Biology 4
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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.
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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.
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Excitation-Contraction Coupling
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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 concentration which continues to activate the contractile apparatus long after the electrical activity in the membrane has ceased.
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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.
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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.
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Excitation-Contraction Coupling
Sarcoplasmic Reticulum the source of the increased cytosolic
calcium within the muscle fiber.
forms a series of sleevelike structures around each myofibril (one segment surrounding the A band and another the I band).
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Excitation-Contraction Coupling
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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.
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Excitation-Contraction Coupling
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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.
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Excitation-Contraction Coupling
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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.
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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 longer time
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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
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END!<3