Overview
An action potential does not stay where it starts: it spreads along the membrane and, in muscle, is turned into a contraction. This topic covers the propagation of excitation, the structure of skeletal muscle with its T-tubules, the mechanism of contraction and the electromechanical coupling that releases Ca2+.
Propagation of the excited process
Excitation spreads by local currents: the depolarized region of the membrane is a current source for the neighbouring resting region, which is depolarized to threshold and fires in turn. The refractory period of the region just activated keeps the impulse travelling one way only. In unmyelinated fibres conduction is continuous and slow. In myelinated fibres the myelin insulates the axon, and the action potential jumps from one node of Ranvier to the next (saltatory conduction), which is faster and uses less energy. Velocity increases with fibre diameter and myelination, up to about 120 m/s. In skeletal muscle the action potential spreads over the sarcolemma and into the fibre along the T-tubules.