Like nerves, skeletal muscle is an excitable tissue.

As ACh binds on to the receptor sites, end-plate potentials are created over the surface of the muscle fibres. Once a threshold is reached, an action potential is propagated through the muscle fibre.

Ca²⁺ channels then open in the sarcoplasmic reticulum, which is a membranous sac that envelops the myofibrils. This triggers contraction of the fibre through a process called 'excitation-contraction coupling'. The Ca²⁺ initiates contraction by binding to Troponin C.

Muscle contraction occurs through cross-linkages between actin and myosin. The thick and thin filaments slide past each other producing shortening (Fig 1).

Select play to watch the animation.

The Ca²⁺ is then pumped back into the sarcoplasmic reticulum where it is stored until another next action potential arrives.