There are two major types of synapses (Figure 1): (1) chemical and (2) electrical.
Fig1. Physiological anatomy of a chemical synapse (A) and an electrical synapse (B).
Most of the synapses used for signal transmission in the central nervous system of the human being are chemical synapses. In these synapses, the first neuron secretes at its nerve ending synapse a chemical substance called a neurotransmitter (often called a transmitter substance), and this transmitter in turn acts on receptor proteins in the membrane of the next neuron to excite the neuron, inhibit it, or modify its sensitivity in some other way. More than 40 important neurotransmitters have been discovered thus far. Some of the best known are acetylcholine, norepinephrine, epinephrine, histamine, gamma-aminobutyric acid (GABA), glycine, serotonin, and glutamate.
In electrical synapses, the cytoplasms of adjacent cells are directly connected by clusters of ion channels called gap junctions that allow free movement of ions from the interior of one cell to the interior of the next cell. Such junctions were discussed in Chapter 4, and it is by way of gap junctions and other similar junctions that action potentials are transmitted from one smooth muscle fiber to the next in visceral smooth muscle and from one cardiac muscle cell to the next in cardiac muscle.
Although most synapses in the brain are chemical, electrical and chemical synapses may coexist and interact in the central nervous system. The bidirectional transmission of electrical synapses permits them to help coordinate the activities of large groups of interconnected neurons. For example, electrical synapses are useful in detecting the coincidence of simultaneous subthreshold depolarizations within a group of interconnected neurons; this enables increased neuronal sensitivity and pro motes synchronous firing of a group of interconnected neurons.
“One-Way” Conduction at Chemical Synapses. Chemical synapses have one exceedingly important characteristic that makes them highly desirable for transmitting nervous system signals. This characteristic is that they always transmit the signals in one direction—that is, from the neuron that secretes the neurotransmitter, called the presynaptic neuron, to the neuron on which the transmitter acts, called the postsynaptic neuron. This phenomenon is the principle of one-way conduction at chemical syn apses, and it is quite different from conduction through electrical synapses, which often transmit signals in either direction.
A one-way conduction mechanism allows signals to be directed toward specific goals. Indeed, it is this specific transmission of signals to discrete and highly focused areas both within the nervous system and at the terminals of the peripheral nerves that allows the nervous system to perform its myriad functions of sensation, motor control, memory, and many other functions.
