In mammals, most synapses are chemical rather than electrical. Substances called neurotransmitters and neuromodulators - either a gas or, more commonly, a liquid, rather than an uninterrupted electrical signal - convey information from one neuron to another.
Examples of synapses are shown in the transmission electron micrographs to the right. Notice that the synapse is morphologically polarized; there are two elements (pre- and postsynaptic) on either side of a narrow part of extracellular space called the synaptic cleft. The presynaptic part has an accumulation of synaptic vesicles containing neurotransmitter.
i. type I (asymmetric pre- and postsynaptic cell membranes) vs. type II (symmetric pre- and postsynaptic cell membranes)
ii. S (spherical vesicles) vs. F (flattened vesicles) vs. P (pleomorphic vesicles) vs. G (granular or dense core vesicles)
A second way to classify a synapse is based on the
parts of the neurons that comprise the synapse. For example, an axodendritic synapse
is one where a region of the axon is presynaptic and a dendrite is postsynaptic.
There are all kinds of possible (and actual) synapses using the classification, but the
three most common are : axodendritic > axosomatic
> axoaxonic. Sometimes the dendrite ( and occasionally the soma)
will have a spinous process with which the presynaptic element makes a synapse.
In the electron micrograph, examples of axosomatic synapses are outlined in
blue, while axodendritic ones are encircled in red.
There are at least two other common ways to
categorize synapses in a mammal; they are based on:
i. the electrical change in the postsynaptic cell
(excitatory or inhibitory), and
ii. the neurotransmitter used (for example, cholinergic -
acetylcholine being a specific type,
catecholaminergic - dopamine being a specific example in this group)
