How does an action potential travel between two neurones?

When an action potential reaches the end of one neurone, it cannot simply move to the next one as there is a gap (the synapse or synaptic cleft) between the two neurones. Instead, when the action potential reaches the end of the first neurone, also called the presynaptic knob, the change in voltage causes voltage-gated calcium channels in the presynaptic membrane to open. Due to a difference in ion concentration, calcium ions (Ca²⁺ ions) then flow into the presynaptic knob. This causes vesicles containing a neurotransmitter (e.g. acetylcholine) to migrate towards the membrane and fuse to release their contents into the synaptic cleft via a process known as exocytosis. The neurotransmitter then binds to complementary receptors on the postsynaptic membrane of the next neurone and causes sodium ion (Na⁺) channels to open and allow sodium ions to rush into the postsynaptic neurone. If enough Na+ enters, it will cause a depolarisation of the postsynaptic neurone, and so start an action potential in the next neurone.