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

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 concentration which 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

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).

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 longer time

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