synapse

A synapse is the junction between one neuron and the next cell in its path. The two membranes are close but not touching: there is a narrow gap, the synaptic cleft, and the electrical impulse cannot jump it. So the signal changes form. When the impulse reaches the end of the axon, it makes the terminal release neurotransmitters, chemicals stored in small vesicles, into the gap. Those molecules diffuse across and bind receptors on the far membrane, and that binding starts a new impulse in the next neuron — or, at a neuromuscular junction, makes a muscle fiber contract. Because the signal is re-created rather than continued, a synapse can pass it on, hold it back or sum several inputs, which is where the nervous system gets its ability to compute rather than merely conduct. A synapse is also the reason a drug that mimics or blocks one neurotransmitter changes behavior: it acts at the junction, not on the impulse itself.