Explain how chemical transfer across a synapse occurs.

The nerve impulse will reach the end of the presynaptic neuron, where voltage-gated calcium channels are opened due to depolarisation. Calcium ions then diffuse into the presynaptic neuron, resulting in vesicles containing neurotransmitters to fuse with the presynaptic membrane. The neurotransmitter is then released into the synaptic cleft via exocytosis. It diffuses across the synaptic cleft and binds to receptors in the postsynaptic neuron. This generates an electrical signal, carrying out the message. Finally, the neurotransmitter in the synaptic cleft is either recycled or broken down.