Review of Medical Physiology - William F. Ganong 2002

Functions of the Nervous System
Autonomic Nervous System
Chemical Transmission of Excitation in Synapses of the Autonomic Nervous System

Excitation at synaptic contacts between preganglionic Neurons and neurons of autonomic ganglia, as well as between postganglionic fibers and autonomic effectors, is mediated by chemical Transmitters. The primary Neurotransmitters in this process are acetylcholine and norepinephrine. Dopamine is released in the interneurons of sympathetic ganglia, whereas GnRH is released in certain preganglionic neurons (Table 13-1). Slow excitatory responses are mediated by GnRH (see below). In addition, cotransmitters are released within the Autonomic Nervous system: for example, VIP alongside acetylcholine, and ATP and neuropeptide Y alongside norepinephrine. The chemistry of all these transmitters and the receptors they act upon is detailed in Chapter 4. The cotransmitter VIP induces bronchial dilation, suggesting the existence of a distinct VIP-producing non-adrenergic, non-cholinergic (NANC) nerve system innervating the smooth Muscle of the bronchioles (see Chapter 34).

Class="center">Table 13-1. Fast and slow responses of postganglionic sympathetic ganglion neurons

Chemical Division of the Autonomic Nervous System

Based on the type of chemical transmitter, the autonomic nervous system can be divided into cholinergic and noradrenergic components (Table 13-2). Cholinergic neurons include the following: all preganglionic neurons; anatomically parasympathetic postganglionic neurons; anatomically sympathetic postganglionic neurons innervating Sweat Glands; and anatomically sympathetic neurons terminating on Blood Vessels within skeletal Muscles, which induce vasodilation upon stimulation (sympathetic vasodilator nerves; see Chapter 31). The remaining postganglionic sympathetic neurons are noradrenergic or, apparently in the case of SIF Cells, adrenergic. The adrenal medulla is essentially a sympathetic ganglion in which the postganglionic neurons have lost their axons. These cells release norepinephrine, epinephrine, and—in some cases—dopamine directly into the bloodstream. Consequently, the axons of cholinergic preganglionic neurons terminating on adrenal medullary cells act as the secretory nerves of this gland.

Transmission of Excitation in Sympathetic Ganglia

At least in experimental animals, the responses elicited in postganglionic neurons upon stimulation of preganglionic fibers encompass not only a rapid depolarization (fast EPSP) that generates an Action Potential, but also a prolonged inhibitory postsynaptic potential (slow IPSP), a prolonged excitatory postsynaptic potential (slow EPSP), and a late slow EPSP (see Chapter 4). The late slow EPSP is markedly prolonged, lasting minutes rather than milliseconds. These slow responses apparently modulate and regulate neurotransmission within sympathetic ganglia. The initial depolarization is driven by the action of acetylcholine on nicotinic receptors. The slow IPSP appears to depend on dopamine released by ganglionic interneurons. Excitation within these interneurons is triggered by the activation of M₂ muscarinic receptors. The dopamine-releasing interneurons are small, intensely fluorescent (SIF) cells located within the ganglia. The Generation of the slow IPSP is not mediated by cyclic AMP, as might otherwise be assumed, given that it involves the D₂ receptor (see Chapter 4). The slow EPSP is elicited by acetylcholine acting on muscarinic receptors on the postganglionic neuronal membrane, whereas the late slow EPSP is mediated by GnRH or a structurally related peptide.

Table 13-2. Responses of effector Organs to autonomic nerve impulses and circulating catecholamines¹

¹ Modified from Hardman JG et al (editors): Goodman and Gilman’s The Pharmacological Basis of Therapeutics, 9th ed. McGraw-Hill, 1996

² If the receptor subtype is not precisely defined, the data are insufficient to characterize the response.

³ Varies depending on the phase of the Menstrual cycle, circulating estrogen and progesterone levels, pregnancy, and other factors.

⁴ On the palms and certain other areas ("adrenergic sweating").