The autonomic nervous system (ANS) is the major involuntary, automatic portion of the nervous system and contrasts in several ways with the somatic (voluntary) nervous system. The anatomy, neurotransmitter chemistry, receptor characteristics, and functional integration of the ANS are discussed in this chapter. Major autonomic drug groups are discussed in Chapters 7 through 10. Drugs in many other groups have significant autonomic adverse effects.
ANATOMIC ASPECTS OF THE ANS
The motor (efferent) portion of the ANS is the major neural pathway for information transmission from the central nervous system (CNS) to the involuntary effector tissues (smooth muscle, cardiac muscle, and exocrine glands; Figure 6–1). Its two major subdivisions are the parasympathetic ANS (PANS) and the sympathetic ANS (SANS). The enteric nervous system (ENS) is a semiautonomous part of the ANS located in the gastrointestinal (GI) tract, with specific functions for the control of this organ system. The neuron cell bodies of the ENS are located in the myenteric plexus (plexus of Auerbach) and the submucous plexus (plexus of Meissner); these neurons send motor axons to GI muscle and secretory cells; they also provide sensory input to the parasympathetic and sympathetic nervous systems and receive motor output from them.
Schematic diagram comparing some features of the parasympathetic and sympathetic divisions of the autonomic nervous system with the somatic motor system. Parasympathetic ganglia are not shown as discrete structures because most of them are diffusely distributed in the walls of the organs innervated. Only 3 of the more than 20 sympathetic ganglia are shown. α and β, alpha and beta adrenoceptors; ACh, acetylcholine; D, dopamine; D1, dopamine1 receptors; Epi, epinephrine; M, muscarinic; N, nicotinic; NE, norepinephrine. (Reproduced with permission from Katzung BG, Vanderah TW: Basic & Clinical Pharmacology, 15th ed. New York, NY: McGraw Hill; 2021.)
There are many sensory (afferent) fibers in autonomic nerves. These are of considerable importance for the physiologic control of the involuntary organs but are directly influenced by only a few drugs. In contrast, many drugs have important effects on the motor functions of these organs, mediated by ANS synaptic receptors. The sensory fibers of the vagus nerve also seem to play an important but poorly understood role in modulating several CNS functions as evidenced by the development of chronic intermittent stimulation of the vagus as a therapy for refractory seizure disorders.
High-Yield Terms to Learn
|Adrenergic, noradrenergic ||A nerve ending that releases norepinephrine as the primary transmitter; also, a synapse in which norepinephrine is the primary transmitter |
|Adrenoceptor, adrenergic receptor ||A receptor that binds, and is activated by, one of the catecholamine transmitters or hormones (norepinephrine, epinephrine, dopamine) and related drugs |
|Baroreceptor reflex ||The homeostatic mechanism that maintains a constant mean arterial blood pressure; ...|