Explain how an impulse is transmitted between nerve cells.

An impulse reaches the pre-synaptic neurone, as it does so, the impulse stimulates voltage-gated calcium channels to open, allowing an influx of calcium ions into the pre-synaptic bouton. This stimulates vesicles containing ACh to move towards the pre-synaptic membrane and fuse with it, releasing the ACh into the synaptic cleft via exocytosis. Once in the synaptic cleft, the ACh diffuses across and binds to sodium receptors on the post-synaptic membrane. This causes sodium channels on the membrane to open, allowing an influx of sodium into the neuron, eventually resulting in depolarisation and the creation of an impulse.