For the nerve terminal to respond rapidly and repeatedly, synaptic vesicles need to be replenished very quickly after they discharge. Thus, most synaptic vesicles are generated not from the Golgi membrane in the nerve cell body but by local recycling from the presynaptic plasma membrane in the nerve terminals (Figure 13–68). Similarly, newly made membrane components of the synaptic vesicles are initially delivered to the plasma membrane by the constitutive secretory pathway and then retrieved by endocytosis. But instead of fusing with endosomes, most of the endocytic vesicles immediately fill with neurotransmitter to become synaptic vesicles.
The membrane components of a synaptic vesicle include transporters specialized for the uptake of neurotransmitter from the cytosol, where the small-molecule neurotransmitters that mediate fast synaptic signaling are synthesized. Once filled with neurotransmitter, the synaptic vesicles can be used again (see Figure 13–68). Because synaptic vesicles are abundant and relatively uniform in size, they can be purified in large numbers and, consequently, are the best-characterized organelle of the cell, in that all of their membrane components have been identified by quantitative proteomic analyses (Figure 13–69). Extending this analysis to a complete presynaptic terminal, allows us to model the crowded environment in which these reactions occur.
secretory vesicle Membrane components are Quickly removed from the plasma Membrane
When a secretory vesicle fuses with the plasma membrane, its contents are discharged from the cell by exocytosis, and its membrane becomes part of the plasma membrane. Although this should greatly increase the surface area of the plasma membrane, it does so only transiently, because membrane components are removed from the surface by endocytosis almost as fast as they are added by exocytosis, a process reminiscent of the endocytic–exocytic cycle discussed earlier. After their removal from the plasma membrane, the proteins of the secretory
Figure 13–68 The formation of synaptic vesicles in a nerve cell. These tiny uniform vesicles are found only in nerve cells and in some endocrine cells, where they store and secrete small-molecule neurotransmitters. The import of neurotransmitter directly into the small endocytic vesicles that form from the plasma membrane is mediated by membrane transporters that function as antiports and are driven by an h+ gradient maintained by v-aTpase h+ pumps in the vesicle membrane (discussed in chapter 11).