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

A nerve impulse is an electrical current that is transmitted down the axon of a neurone and to the synaptic knob. The impulse depolarises the membrane and causes a change in voltage across the membrane. This causes voltage-gated calcium (Ca) channels to open. Because Ca ions are more concentrated outside the cell than inside it, Ca enters the cell through the voltage gated channels. Ca then causes vesicles, containing neurotransmitter, to fuse with the cell membrane, causing release of neurotransmitter into the synaptic cleft - the space between the synaptic knob of one cell, and the dendrite of another.