Describe what happens when a nerve impulse arrives at a synapse.

A synapse is a junction between a neurone and the next cell. When an action potential arrives at the presynaptic membrane, voltage-gated calcium ion channels open. Calcium ions (Ca2+) diffuse into the synaptic knob. The increase in calcium ions causes vesicles containing neurotransmitter to move towards and fuse with the presynaptic membrane. Vesicles release the neurotransmitter into the gap between two neurones known as the synaptic cleft. Neurotransmitter molecules bind to receptors on the postsynaptic membrane, causing sodium ions channels to open. The influx of sodium ions into the postsynaptic membrane causes depolarisation and if threshold level is reached, an action potential will be generated.