Post-synaptic potentials spread through muscle via the transverse tubular system, an extension of the sarcolemma

An excitatory action potential is produced when a threshold is reached and sodium channels are activated bringing about a massive influx of sodium ions
The density of sodium channels is highest at the initial segment of the axon (the “axon hillock” of motor neurones or the first node of Ranvier for myelinated sensory neurones).
The membrane potential reaches positive values, reaching a threshold triggering an action potential
Repolarisation later occurs due to a decrease in sodium conductance and slow increase in potassium conductance allowing potassium to leave the cell

The tubular system is a continuation of the sarcoplasmic reticulum and is a calcium ion reservoir
Action potential stimulates calcium release from this tubular system by opening DHP receptors
Calcium binds to troponin C associated with tropomyosin on the actin filament.

The displacement of tropomyosin frees the myosin binding sites in the actin grooves
The myosin can combine with actin to form “cross-bridges”
Myosin magnesium-dependent ATPase generates energy which effects the movement of the myosin heads and muscle contraction
Calcium accumulates back in the SR via the action of ATPase