Explain how a nerve impulse is transmitted across a synapse from one neurone to the next.

When an action potential reaches the presynaptic terminal, voltage‑gated Ca²⁺ channels open and Ca²⁺ enters the cell. The influx triggers vesicles to fuse with the membrane and release neurotransmitter into the synaptic cleft. The neurotransmitter diffuses across the cleft and binds to specific receptors on the postsynaptic membrane. Binding opens ion channels, causing ion flux that depolarises the postsynaptic membrane and generates a new action potential. The synapse is unidirectional because neurotransmitters are released only from the presynaptic side and receptors are only on the postsynaptic side.