The Synapse

A synapse is the specialised junction across which an impulse in one neuron (the presynaptic neuron) influences a second neuron or an effector cell (the postsynaptic cell). Synapses are of two basic kinds, distinguished by how the signal actually crosses the junction.

At an electrical synapse, the presynaptic and postsynaptic membranes lie extremely close together and are connected by channel proteins forming gap junctions, which allow ionic current to flow directly from one cell's cytoplasm into the next. Because there is no chemical intermediary to release and act on receptors, transmission across an electrical synapse is almost instantaneous, and current can in principle flow in either direction across the junction. Electrical synapses are comparatively uncommon in the human nervous system, occurring at a limited number of sites where extremely rapid, synchronised firing of a group of neurons is required.

Chemical synapses are far more common in humans. Here the presynaptic and postsynaptic membranes are separated by a narrow, fluid-filled gap called the synaptic cleft, so there is no direct cytoplasmic continuity. Transmission occurs in a defined sequence: the arriving nerve impulse depolarizes the presynaptic axon terminal; this opens voltage-gated calcium (Ca2+) channels in the terminal membrane, and Ca2+ flows in from the synaptic cleft; the rise in intracellular Ca2+ causes synaptic vesicles (which store a chemical messenger called a neurotransmitter — for example, acetylcholine) to fuse with the presynaptic membrane and release their neurotransmitter into the cleft by exocytosis; the neurotransmitter molecules diffuse across the narrow cleft and bind to specific receptor proteins on the postsynaptic membrane; this binding opens ion channels in the postsynaptic membrane, producing a change in its potential — either an excitatory postsynaptic potential that brings the postsynaptic cell closer to firing its own impulse, or an inhibitory postsynaptic potential that makes it less likely to fire; and finally the neurotransmitter is removed from the cleft (broken down by an enzyme or taken back up into the presynaptic terminal) so that the synapse is reset and ready to transmit the next signal. …