and two SR regions containing RyRs is described as a triad and is predominantly where excitation-contraction coupling takes place.

Excitation-contraction coupling (ECC) occurs when depolarization of skeletal muscles (usually through neural innervation) results in a muscle action potential. This action potential spreads across the muscle's surface and into the muscle fiber's network of T-tubules, depolarizing the inner portion of the muscle fiber. This activates dihydropyridine receptors in the terminal cisternae,