Excitation-contraction coupling: the link between the surface and the interior of a muscle cell

The control of the contractile state of a muscle cell, a process called excitation-contraction coupling, involves a sequence of steps and is passed along through a series of cellular structures. The excitation of the fibre, initiated at the motor end-plate, spreads rapidly over the surface of the fibre and into the fibre along the T-system networks as an action potential. This excitation is coupled to the sarcoplasmic reticulum (SR) by a mechanism not yet identified with certainty. The SR then responds by releasing stored calcium from its interior compartment into the myoplasm. The diffusion and binding of this calcium to regulatory proteins initiates the mechanical events of contraction, which subsequently are turned off when calcium is removed from these regulatory binding sites. Some of this calcium may temporarily bind to proteins in the myoplasm, but eventually it gets pumped back into the internal compartment of the SR by a calcium-ATPase in the SR membrane. The system then is poised for a repeat performance.