Figure 22-2 The working of a chemical synapse, the motor nerve ending, including some of the apparatus for transmitter synthesis. The large, intracellular structures are mitochondria. Acetylcholine, synthesized from choline and acetate by acetylcoenzyme A, is transported into coated vesicles, which are moved to release sites. A presynaptic action potential, which triggers calcium influx through specialized proteins (Ca²⁺ channels), causes the vesicles to fuse with the membrane and discharge transmitter. Membrane from the vesicle is retracted from the nerve membrane and recycled. Each vesicle can undergo various degrees of release of contents—from incomplete to complete. The transmitter is inactivated by diffusion, catabolism, or reuptake. The inset provides a magnified view of a synaptic vesicle. Quanta of acetylcholine together with ATP are stored in the vesicle and covered by vesicle membrane proteins. Synaptophysin is a vesicle membrane component glycoprotein. Synaptotagmin is the vesicle's calcium sensor. Phosphorylation of another membrane protein, synapsin, facilitates vesicular trafficking to the release site. Synaptobrevin (VAMP) is a SNARE protein involved in attaching the vesicle to the release site (see Fig. 22-3 ). ACh, acetylcholine, acetyl CoA, acetyl coenzyme A; CAT, choline acetyltransferase.