Communication between neurons usually occurs from the axon terminal of the transmitting neuron (presynaptic side) to the receptive region of the receiving neuron (postsynaptic side). This specialized interneuronal complex is a synapse, or synaptic junction. Some synapses are located between an axon and a dendrite (axodendritic synapses, which tend to be excitatory), whereas others are located between an axon and a nerve cell body (axosomatic synapses, which tend to be inhibitory). Still other synapses are located between an axon terminal and another axon; these axoaxonic synapses modulate transmitter release by the postsynaptic axon. Synaptic transmission permits information from many presynaptic neurons to converge on a single postsynaptic neuron. Some large cell bodies receive several thousand synapses.
Impulse transmission at most synaptic sites involves the release of a chemical transmitter substance at other sites, current passes directly from cell to cell through specialized junctions called electrical synapses, or gap junctions. Electrical synapses are most common in invertebrate nervous systems, although they are found in a small number of sites in the mammalian CNS. Chemical synapses have several distinctive characteristics: synaptic vesicles on the presynaptic side, a synaptic cleft, and a dense thickening of the cell membrane on both the receiving cell and the presynaptic side. Synaptic vesicles contain neurotransmitters, and each vesicle contains a small packet, or quanta, of transmitter. When the synaptic terminal is depolarized (by an action potential in its parent axon), there is an influx of calcium. This calcium influx leads to phosphorylation of a class of proteins called synapsins. After phosphorylation of synapsins, synaptic vesicles dock at the presynaptic membrane facing the synaptic cleft, fuse with it, and release their transmitter.
