A presynaptic complement of membrane-bound synaptic vesicles exists. These are spherical vesicles in excitatory nerve endings, shown in Figure 8.6. In inhibitory neurons the synaptic vesicle are often flattened as shown in Figure 8.7.
Regions of functional contacts between neurons (synapses) have distinct morphological characteristics. Although a great deal of variation exists in the size and shape of boutons of individual neurons, synapses can be identified by the presence of the following: The nerve ending often has aggregations of dense material in the cytoplasm immediately adjacent to the membrane on the pre- and postsynaptic side of the junction (these are known as presynaptic density or postsynaptic density, respectively.) This dense material on the presynaptic side is thought to be the site of vesicle attachment. The dense material on the postsynaptic side is a site where receptor proteins and channels are prevalent.
Many mitochondria are present, especially in the nerve terminal; and
There is a distinct synaptic cleft or intercellular space of approximately 20-40 nm.
An endoplasmic reticulum is present that regulates Ca2+ level.
Endosomal membrane that functions in the recycling of synaptic vesicles.