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

When a nerve impulse reaches the axon terminal of the presynaptic neuron, voltage‑gated calcium channels open and calcium enters the terminal. The influx of Ca²⁺ triggers synaptic vesicles to fuse with the membrane and release neurotransmitter into the synaptic cleft.
The neurotransmitter molecules diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane. Binding opens ion channels or activates second‑messenger systems, causing a change in membrane potential.
If the depolarisation is sufficient to reach threshold, an action potential is generated in the postsynaptic neuron, continuing the signal.
The myelin sheath surrounding the axon provides electrical insulation, increasing conduction velocity and ensuring the impulse travels efficiently to the next neuron.