A synapse is a junction between two neurons, or between a neuron and another type of cell, that allows electrical or chemical signals to pass from one to the other. The vast majority of synapses in the nervous system are chemical synapses, in which communication between neurons takes place via chemicals called neurotransmitters. Electrical synapses are much less common, and are found mostly in the central nervous system.
The word “synapse” comes from the Greek συνάπτειν (synaptein), meaning “to join together or fasten.” This reflects the fact that at a chemical synapse, the membranes of the two cells are joined together very closely, with only a small gap (the synaptic cleft) separating them. Electrical synapses are even more intimate, with direct contact between the cells’ membrane proteins.
The function of a synapse is to transmit signals from one neuron to another (or from a neuron to another type of cell). This transmission can be either excitatory or inhibitory: an excitatory signal will tend to make the post-synaptic neuron fire an action potential (and thus send its own signals onwards), while an inhibitory signal will tend to reduce or stop neuronal firing. In general, most chemical neurotransmitters areexcitatory at low concentrations but become inhibitory at high concentrations; however there are some notable exceptions such as GABA (gamma-aminobutyric acid), which is always inhibitory.
Synapses can be classified according to their morphology (the way they look under a microscope), their physiology (how they function) or their location in the body. Morphologically, they can be either axodendritic, dendrodendritic or axosomatic; physiologically they can be either electrical or chemical; and anatomically they can be located either pre-synaptically or post-synaptically. Most commonly, however, they are simply classified according to whether they are excitatory or inhibitory.
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