A synapse

When a nerve impulse reaches the synapse at the end of a neurone, it cannot pass directly to the next one. Instead, it triggers the neurone to release a chemical neurotransmitter. The neurotransmitter drifts across the gap between the two neurones. On reaching the other side, it fits into a tailor-made receptor on the surface of the target neurone, like a key in a lock. This docking process converts the chemical signal back into an electrical nerve impulse.