Transmission at a Chemical Synapse

The sequence of events during neuromuscular synaptic transmission is summarized in Figure 8-1. When an action potential arrives at the end of the motor neuron nerve fiber, it invades a specialized structure called the synaptic terminal. Depolarization of the synaptic terminal induces release of a chemical messenger, which is stored inside the terminal. At the vertebrate neuromuscular junction, this chemical messenger is acetylcholine; the chemical structure of acetylcholine (abbreviated ACh) is shown in Figure 8-2. The ACh diffuses across the space separating the presynaptic motor neuron terminal from the postsynaptic muscle cell and alters the ionic permeability of the muscle cell. This change in ionic permeability then depolarizes the muscle cell membrane. The remainder of this chapter will be concerned with a detailed description of this basic sequence of events.

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