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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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Neurotransmission in the Visceral Motor System

The neurotransmitter functions of the visceral motor system are of enormous importance in clinical practice, and drugs that act on the autonomic system are among the most important in the clinical armamentarium. Moreover, autonomic transmitters have played a major role in the history of efforts to understand synaptic function. Consequently, neurotransmission in the visceral motor system deserves special comment (see also Chapter 6).

Acetylcholine is the primary neurotransmitter of both sympathetic and parasympathetic preganglionic neurons. Nicotinic receptors on autonomic ganglion cells are ligand-gated ion channels that mediate a so-called fast EPSP (much like nicotinic receptors at the neuromuscular junction). In contrast, muscarinic acetylcholine receptors on ganglion cells are members of the 7-transmembrane G protein-linked receptor family, and they mediate slower synaptic responses (see Chapters 7 and 8). The primary action of muscarinic receptors in autonomic ganglion cells is to close K+ channels, making the neurons more excitable and generating a prolonged EPSP. As a result of these two acetylcholine receptor types, ganglionic synapses mediate both rapid excitation and a slower modulation of autonomic ganglion cell activity.

The postganglionic effects of autonomic ganglion cells on their smooth muscle, cardiac muscle, or glandular targets are mediated by two primary neurotransmitters: norepinephrine (NE) and acetylcholine (ACh). For the most part, sympathetic ganglion cells release norepinephrine onto their targets (a notable exception is the cholinergic sympathetic innervation of sweat glands), whereas parasympathetic ganglion cells typically release acetylcholine. As expected from the foregoing account, these two neurotransmitters usually have opposing effects on their target tissue—contraction versus relaxation of smooth muscle, for example.

As described in Chapters 6 to 8, the specific effects of either ACh or NE are determined by the type of receptor expressed in the target tissue, and the downstream signaling pathways to which these receptors are linked. Peripheral sympathetic targets generally have two subclasses of noradrenergic receptors in their cell membranes, referred to as α and β receptors. Like muscarinic ACh receptors, both α and β receptors and their subtypes belong to the 7-transmembrane G-protein-coupled class of cell surface receptors. The different distribution of these receptors in sympathetic targets allows for a variety of postsynaptic effects mediated by norepinephrine released from postganglionic sympathetic nerve endings (Table 21.2).

Table 21.2. Summary of Adrenergic Receptor Types and Some of Their Effects in Sympathetic Targets.

Table 21.2

Summary of Adrenergic Receptor Types and Some of Their Effects in Sympathetic Targets.

The effects of acetylcholine released by parasympathetic ganglion cells onto smooth muscles, cardiac muscle, and glandular cells also vary according to the subtypes of muscarinic cholinergic receptors found in the peripheral target (Table 21.3). The two major subtypes are known as M1 and M2 receptors, M1 receptors being found primarily in the gut and M2 receptors in the cardiovascular system (another subclass of muscarinic receptors, M3, occurs in both smooth muscle and glandular tissues). Muscarinic receptors are coupled to a variety of intracellular signal transduction mechanisms that modify K+ and Ca2+ channel conductances. They can also activate nitric oxide synthase, which promotes the local release of NO in some parasympathetic target tissues (see, for example, the section on autonomic control of sexual function).

Table 21.3. Summary of Cholinergic Receptor Types and Some of Their Effects in Parasympathetic Targets.

Table 21.3

Summary of Cholinergic Receptor Types and Some of Their Effects in Parasympathetic Targets.

In contrast to the relatively restricted responses generated by norepinephrine and acetylcholine released by sympathetic and parasympathetic ganglion cells, respectively, neurons of the enteric nervous system achieve an enormous diversity of target effects by virtue of many different neurotransmitters, most of which are neuropeptides associated with specific cell groups in either the myenteric or submucous plexuses mentioned earlier. The details of these agents and their actions are beyond the scope of this introductory account.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK10854

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