Describe how a nerve impulse travels from neurone A to neurone B.

As an action potential arrives at A (the presynaptic terminal), pre-formed vesicles containing a chemical neurotransmitter fuse with the presynaptic membrane, releasing the chemical neurotransmitter into the synaptic cleft. The neurotransmitter molecules then diffuse across the synaptic cleft and bind to receptors on neuron B, at the postsynaptic membrane. If the concentration of neurotransmitters reaching B is sufficiently high (above the threshold), an electrical impulse will be generated in neuron B.