Despite important difference between skeletal and cardiac muscle, the general scheme for excitation-contraction (E-C) is similar. Electrical excitation of the surface membrane leads to an action potential which propagates as a wave of depolarization along the surface and along the transverse (T) tubules. The depolarization of the T-tubule overlying the terminal cisternae of the SR induces the release of Ca2+ from SR. The Ca2+ released from SR then binds to TN-C which activates contraction. Cellular Ca2+ movement in the heart is somewhat complex because of the presence of Ca2+ channels and transport system in the sarcolemma.
There is a recurring theme that skeletal muscle contraction depends almost exclusively on Ca2+ released from SR with insignificant Ca2+ entry across the sarcolemma during a normal twitch. Cardiac muscle contraction, on the other hand, depends on both Ca2+ entry across the sarcolemma and Ca2+ release from the SR. There are notable differences in the ultrastructure: Skeletal muscle has an extensive and well organized SR network, abutting the narrow T-tubules. In contrast, the SR of cardiac muscle is rather sparse and less organized, and surrounded with T-tubules of much larger diameter. In addtion cardiac myocytes are only 0.02 nm thick, whereas the diameter of the skeletal muscle fibers goes up to 0.2 nm. Thus, a substance from the extracellular space reaches the center of the heart cell much faster than the center of the skeletal muscle cell. In general the structure of the cardiac cell is consistent with a larger role of the transsarcolemmal Ca2+ fluxes.
Major events in cardiac E-C coupling (courtesy of Dr.Pieter de Tombe): Excitation
Ca2+ induced Ca2+ release Activation of contractile proteins
Ca2+ reuptake into the SR and Ca2+ extrusion leading to relaxation
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