When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob. Describe how.

When the action potential arrives at the presynaptic end of the neurone or nerve ending, the influx of ions along the axon body results in the depolarisation of the nerve cell and the presynaptic membrane. This depolarisation results in the protein or ion channels to open at the presynaptic membrane, allowing calcium ions to enter via facilitated diffusion from an area of high concentration to an area of low concentration within the presynaptic membrane. The influx of calcium ions then triggers the mechanism whereby presynaptic vesicles containing the neurotransmitter fuse with the presynaptic membrane, releasing the neurotransmitters into the synaptic cleft.