Outline the process which occurs when an action potential arrives at a synapse

A synapse is a gap between two neurons and it can be thought of in 3 parts:
The presynaptic neuron - The neuron at the start of the synapse.
The synaptic cleft - The gap between the two neurons
The postsynaptic neuron - The neuron at the end of the synapse, on the other side of the synaptic cleft
Action Potentials can't pass through the synaptic cleft and so instead they transmit nervous impulses via chemicals called neurotransmitters. Acetylcholine and Noradrenaline are examples of neurotransmitters. This process happens in a few stages:
1.When the action potential reaches the post-synaptic membrane it causes a positive voltage in this area. This causes voltage-gated calcium channels to open so calcium ions move into the cell
2.Within the cell there are vesicles containing neurotransmitters. When the calcium ions move into the cell this causes the vesicles to move towards the cell membrane and fuse with it, releasing the neurotransmitters into the synaptic cleft
3.The neurotransmitters travel by diffusion across the synaptic cleft and bind to complementary receptors on the post-synaptic membrane
4.This stimulates sodium ion channels to open on the post-synaptic membrane and sodium ions move into the cell. This initiates depolarisation in the post-synaptic neuron which will lead to an action potential if the threshold potential is met.
5.The neurotransmitter is then broken down by an enzyme; for example, acetylcholine is broken down by the enzyme acetylcholinesterase. This stops the synapse from permanently being activated.