As noted earlier, synapses are the junctions where neurons release a chemical neurotransmitter that acts on a postsynaptic target cell, which can be another neuron or a muscle or gland cell (see Figure 7-31). In this section, we focus on several key issues related to impulse transmission:
■ How neurotransmitters are packaged in membrane-bounded synaptic vesicles in the axon terminus
■ How arrival of an action potential at axon termini in presynaptic cells triggers secretion of neurotransmitters
■ How binding of neurotransmitters by receptors on post-synaptic cells leads to changes in their membrane potential
■ How neurotransmitters are removed from the synaptic cleft after stimulating postsynaptic cells
Neurotransmitter receptors fall into two broad classes: ligand-gated ion channels, which open immediately upon neurotransmitter binding, and G protein-coupled receptors. Neurotransmitter binding to a G protein-coupled receptor induces the opening or closing of a separate ion channel protein over a period of seconds to minutes. These "slow" neurotransmitter receptors are discussed in Chapter 13 along with G protein-coupled receptors that bind different types of ligands and modulate the activity of cytosolic proteins other than ion channels. Here we examine the structure and operation of the nicotinic acetylcholine receptor found at many nerve-muscle synapses. The first ligand-gated ion channel to be purified, cloned, and characterized at the molecular level, this receptor provides a paradigm for other neurotransmitter-gated ion channels.
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