Functional communication between neurons occurs at specialized junctions called synapses. The most common types of synapses in the brain use chemicals, (more specifically, neurotransmitters,) to communicate between neurons. These are called chemical synapses.
A presynaptic element, an
axon, and a postsynaptic element, for example a dendritic spine, are in
close apposition at the synapse but not in direct contact. The pre- and
postsynaptic membranes are separated by a gap, the synaptic cleft.
Chemical transmitters bridge this gap by diffusing from release sites on
the presynaptic side to receptors on the postsynaptic side.
A variety of
ultrastructural specializations occur at the synapse enabling unambiguous
identification of the pre- and postsynaptic partners. Within the
presynaptic axonal bouton, clouds of synaptic vesicles are prominent;
mitochondria may be present, as well as tubules of endoplasmic reticulum.
A characteristic feature of the synapse is the accumulation of opaque
material on the cytoplasmic face of the postsynaptic membrane. This
material is refered to as the postsynaptic density. The density represents
the aggregation of neurotransmitter receptors and signaling proteins
essential for chemical synaptic transmission.
Since the late 1950s, the
ultrastructural features of individual synapses have been studied
extensively using snap-shots obtained via electron microscopy. Gray
classified two types of synapses within the brain based on the
ultrastructural characteristics of the presynaptic (vesicle-bearing) and
postsynaptic partners (length of apposed membrane, membrane thickenings
and synaptic cleft):
These two categories were
further distinguished by their locations: Type 1 synapses were found on
dendritic spines and dendrite shafts, whereas Type 2 synapses occurred
primarily on dendrite shafts and neuronal cell bodies. Virtually
synonymous with Gray's nomenclature are the terms:
described by Colonnier.
Colonnier extended the observations of Gray using aldehyde-fixed brain. In
aldehyde-fixed tissue, asymmetric synapses include axons that contain
predominantly round or spherical vesicles and form synapses that are
distinguished by a thickened, postsynaptic density. In contrast, symmetric
synapses involve axons that contain clusters of vesicles that are
predominantly flattened or elongate in their appearance. The pre-and
postsynaptic membranes are more parallel than the surrounding nonsynaptic
membrane, and the synapse does not contain a prominent postsynaptic
density. Click here (164K) to view Colonier's description of asymmetric
and symmetric synapses.
The sterotypical and most
abundant synapse in the central nervous system is the asymmetric synapse
occurring between an axon and a dendritic spine. Other synaptic
relationships exist and involve different parts of the neuron. For
instance, axo-axonic, somato-axonic, somato-dendritic, dendro-axonic, and
dendro-dendritic synapses can occur and provide alternate mechanisms for
functional communication between neurons.
Structural and functional classifications of axons, dendrites and their synapses are still emerging. The use of electrophysiology, laser scanning, and serial electron microscopy, together with 3D computer-aided reconstruction, facilitate the study of neurons and the intricacies of their synapses within the brain.