Describe what happens when an action potential reaches an axon terminal.

When an action potential reaches an axon terminal, the depolarisation from causes voltage-gated calcium ion channels in the plasma membrane of the synaptic knob to open. As there is a much higher concentration of calcium ions on the outside of the cell, Ca²⁺ ions travel down their concentration gradient into the cell. The influx of calcium ions triggers the formation of synaptic vesicles containing the neurotransmitter (at a cholinergic synapse, the neurotransmitter will be acetylcholine). The cytoskeleton of the cell is then able to move these vesicles towards the pre-synaptic membrane, once the vesicles arrive here, they fuse with the membraane and the neurotransmitter is released into the synaptic cleft by exocytosis. The neurotransmitter diffuses across the cleft until it reaches the post-synaptic membrane. Here, it binds to specific receptors which triggers the firing of an action potential in the next nerve cell. The neurotransmitter in the cleft is then broken down by enzymes (eg acetylcholinesterase) and reabsorbed into the synapse.