Figure 1. Excitation-contraction coupling in cardiac and skeletal muscle.

A) Figure showing the principals of Ca²⁺ cycling in excitation-contraction (E-C) coupling of the cardiomyocyte. Upon depolarization of the sarcolemma, L-type Ca²⁺ channels open to allow Ca²⁺ to enter the cell. This Ca²⁺ will in turn activate the juxtaposed RyR and more Ca²⁺ will be released. The bulk of cytoplasmic [Ca²⁺] increase is due to RyR-mediated release of Ca²⁺ from the SR. Cytoplasmic Ca²⁺ will trigger myofilament activity so that contraction can occur. To relax, cytoplasmic Ca²⁺ is pumped back into the SR via the SR Ca²⁺ ATPase (SERCA2a) and extruded out of the cell through the sarcolemmal Na+-Ca²⁺ exchanger (NCX).

B) Depicts Ca²⁺ handling in skeletal muscle E-C coupling. L-type Ca²⁺ channels at the sarcolemma and RyR1 in the SR membrane are in close vicinity with each other. Depolarization of the sarcolemma causes the L-type Ca²⁺ channel to interact with and activate the RyR independent of entry of extracellular Ca²⁺. Virtually all of the increase in cytoplasmic [Ca²⁺] that is needed for myofilament activation and force generation comes from SR Ca²⁺ release. The Ca²⁺ is cleared from the cytoplasm via the SERCA. The absence of transsarcolemmal Ca²⁺ fluxes underscores the importance of RyR1 Ca²⁺ release function in skeletal muscle ECC.