In twitch muscle, an action potential propagating along the surface can lead to mechanical contraction of the entire cross section of the fibre. The processes involve a depolarization of the membranes of the transverse tubular system which, in turn, causes a release of calcium from its intracellular storage location, the sarcoplasmic reticulum. It seems that a change in potential across the first structure can trigger the release from the second, adjacent structure. If the time and voltage dependent ionic currents are blocked, small movements of charge can be detected when a fibre is depolarized from a normal resting potential to a potential at which contraction is activated. These charge movements, which do not behave as currents passing through ionic channels, may be part of a trigger mechanism.