Some Current Concepts of Synaptic Organization

The accumulation of literature dealing with the structure and function of synapses presents the synaptologist with a formidable problem. The diverse interests now en- compassed by synaptology, and the many facets of neurobiology mirrored in these interests, make the task of reviewing synaptic organization a major one. Selection must be made and, if the reader is not to be misled, biases must be exposed. My frame of reference is the presynaptic terminal, that is, the enlarged termination of the axon (Figs. 1 and 2). This includes the specialized presynaptic membrane run- ning alongside the cleft region and associated with the dense projections and presyn- tic vesicular grid (Figs. 1 and 8). Within the cytoplasm of the terminal are the synaptic and coated vesicles, mitochondria, the micromamentous presynaptic network and possibly microtubules. My approach to the presynaptic terminal will rely principally on mor- phological concepts, although biochemical features of the composition of the ju- tional region are essential for a basic understanding of synaptic organization and ref- erence to these will also be made.