The sequence of events during synaptic transmission at the neuromuscular junction is summarized in Figure 8-16. The depolarization produced by an action potential in the synaptic terminal opens voltage-dependent calcium channels in the terminal membrane. Calcium ions enter the terminal down their concentration and electrical gradients, inducing synaptic vesicles filled with ACh to fuse with the plasma membrane facing the muscle cell. The ACh is thereby dumped into the synaptic cleft, and some of it diffuses to the muscle membrane and combines with specific receptors on ACh-activated channels in the muscle membrane. When ACh is bound, the channel opens and allows sodium and potassium ions to cross the membrane. This depolarizes the muscle membrane and triggers an all-or-none action potential in the muscle cell. The action of ACh is terminated by the enzyme acetylcholinesterase, which splits ACh into acetate and choline.

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