Describe the transmission of a nerve impulse across a synapse.

An action potential arrives at a presynaptic neurone, which causes calcium ion channels to open. Calcium ions enter the presynaptic knob, which causes synaptic vesicles containing neurotransmitter to detach from the cytoskeleton. These move towards the presynaptic membrane. The vesicles fuse with the membrane and release the neurotransmitter which diffuses across the synaptic cleft. The neurotransmitter binds to complementary receptors on the postsynaptic membrane, which opens sodium ion channels. This leads to depolarisation and if this reaches the threshold, an action potential will fire.