Describe the events that take place to generate an action potential at a nerve synapse

A synapse is a chemical junction between a neurone and another cell. This next cell is often another neurone or a muscle cell. In this case we will describe a synapse between a two neurone cells. When an action potential reaches the presynaptic knob, at the end of an axon, it depolarises the presynaptic membrane. This causes the voltage-gated calcium ion (Ca²⁺) channels to open. This allows Ca²⁺ to diffuse into the presynaptic knob down the concentration gradient. The Ca²⁺ activates vesicles containing neurotransmitter in the presynaptic knob and causes them to move towards the presynaptic membrane. Once the vesicles reach and fuse with this membrane they release their neurotransmitter molecules into the gap between the pre and postsynaptic knobs called the synaptic cleft. The neurotransmitter molecules then diffuse across this gap and bind to receptors on the postsynaptic membrane. This causes sodium ion channels to open and an influx of sodium ions into the postsynaptic knob. This then causes depolarisation of the postsynaptic membrane and if the threshold voltage is reached an action potential is generated and continues down the dendron of the next neurone.