Fig. 14. The chemical synapse.
When one nerve must pass an impulse on to the next cell, it does so by one of two general processes. Where speed is the critical factor, a direct electrical connection may exist between the two cells Where modulation is important, a chemical event intercedes between the arrival of the impulse at the terminal of one axon and the continuation of the signal in the next cell.
The juncture is referred to as a synapse. The cells are separated by a gap, the synaptic cleft. Upon appropriate stimulation the presynaptic cell releases neurotransmitter (commonly acetylcholine). The neurotransmitter then diffuses across the cleft to receptors on the postsynaptic membrane. Here the receptors respond by initiating depolarization of the membrane and the impulse is propagated in the postsynaptic cell.