For transmission across the synapse to be initiated, an action potential must arrive at the pre-synaptic knob and open voltage- gated ion channels. Once the channels are open, calcium ions are able to diffuse into the synaptic knob. This rapid influx in calcium causes the vesicles containing acetylcholine to fuse with the presynaptic membrane. Acetylcholine can then be released by exocytosis into the cleft and diffuse across until it reaches receptor sites on the postsynaptic membrane. These receptors are on the sodium ion channel, so when binding occurs, the channels open and there is an influx of sodium ions into the postsynaptic neurone. These sodium ions can then generate a new action potential and the process can occur again in the next synapse.