When an action potential arrives at a neuromuscular junction it stimulates calcium ion channels to open. Calcium ions diffuse from the synapse into the synaptic knob. This causes vesicles to fuse with the presynaptic membrane and release acetylcholine. Acetylcholine binds to receptors on the post-synaptic membrane opening sodium channels. The influx of sodium ions causes depolarisation of the sarcolemma.