Illustration of a junction, or synapse, between two nerve cells (neurons). As the electrical signal (arrow) reaches the presynaptic end (upper centre) of a neuron it triggers the release of neurotransmitters (orange spheres) across the gap, or synaptic cleft, between the two cells. The neurotransmitters bind to receptors on the postsynaptic membrane (lower centre), changing the membrane's excitability and triggering an electrical impulse.