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 [28]. 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.