Neurons communicate with each other and with other cells of the body, such as muscle and secretory cells.
In Chapter 4 the generation of the action potential and its rapid conduction down the axon, to the presynaptic terminal, was discussed. Using these processes, the neuron can rapidly notify its presynaptic terminals, often located far from its cell body, to initiate the transfer of information to other cells.
Such communication occurs between cells rapidly, and often focally, at specialized junctions called synapses (Greek, “junction" or “to bind tightly”). Synaptic transmission between cells can be either electrical or chemical. At electrical synapses, ionic current flows directly between presynaptic and postsynaptic cells as the mediator for signal transmission. Although electrical synapses in the mammalian nervous system appear to be more widespread than originally thought, synaptic transmission is more frequently mediated by a chemical messenger. Released from the presynaptic terminals on arrival of the action potential, this chemical messenger rapidly diffuses to the postsynaptic cell membrane, where it binds with receptors. This binding initiates a postsynaptic change in function, often generating a postsynaptic potential.The best-understood chemical synapse is that between a motor neuron and a skeletal muscle cell (fiber): the neuromuscular synapse, also known as the neuromuscular junction (Figure 5-1). Given the emphasis in Section II of this text on posture and locomotion, this synapse is the focus of this chapter. Synaptic communication at the neuromuscular junction is fundamentally similar to that between neurons, although there is greater variety in the specifics of neuron-to- neuron synaptic transmission, as also discussed.