Excitation–contraction (EC) coupling is the coordination of two processes that are needed for the generation of force; the transmission of the nerve stimulus to the triad followed by the release of calcium from the cisternae of the sarcoplasmic reticulum and the interaction between actin and myosin that forms cross-bridges. Many (but not all) of the molecular events during E–C coupling have been defined. Briefly, the action potential that arrives to the muscle fiber membrane is conducted to the interior of the muscle cell via the transverse tubular (T tubule) system. The nervous impulse arrives in the triad where the T tubule is in close proximity with the terminal cisternae of the sarcoplasmic reticulum that stores calcium. A voltage sensor subunit of the dihydropiridine receptors on the T tubule opens and allows an inward current of calcium. This calcium current triggers the opening of the ryanodine receptors in the terminal cisternae of the sarcoplasmic reticulum and releases large amounts of calcium into the sarcoplasm. The calcium released into the sarcoplasm then binds to the regulatory protein troponin C on the actin thin myofilament. This initiates a series of molecular events that displace the tropomyosin blocking the active site of the actin filament.
A detailed description and discussion of the events that follow is beyond the scope of this brief review. Suffice it to say that the exposure of the active site on actin allows the binding of the head of the myosin molecule with actin. ATP and the ATPase located in the myosin head, facilitate the detachment of myosin from actin (a cross-bridge formed in a previous contraction) and the formation of a new cross-bridge. The detailed structure of the myosin head was described for the first time in 1993 and contributed significantly to our understanding of the mechanics and physiology of this process. As mentioned above, the end-result of this sequence of events is the sliding of the actin and myosin filaments and the generation of force. Recent evidence suggests that this sequence of events within the cell is modulated by several genes including MTMR14, MG29, and KLF15.