How does an action potential travel from one neurone to another?

The action potential arriving at the presynaptic membrane causes the voltage gated calcium ion channels on the presynaptic membrane to open. This causes an influx of calcium ions into the presynaptic neurone which triggers synaptic vesicles, containing neurotransmitter, to travel to the presynaptic membrane where they then fuse with the membrane to release their contents into the synaptic cleft. The neurotransmitter diffuses across the synaptic cleft to where it is received and binds to specific receptors on the postsynaptic membrane, causing the sodium ion channels on the postsynaptic neurone to open. Once the sodium ion channels are open sodium ions flow into the postsynaptic neurone, this causes depolarisation which triggers a new action potential.