FIGURE 6–1 Schematic diagram comparing some anatomic and neurotransmitter features of autonomic and somatic motor nerves. Only the primary transmitter substances are shown. Parasympathetic ganglia are not shown because most are in or near the wall of the organ innervated. Cholinergic nerves are shown in blue, noradrenergic in red. Note that some sympathetic postganglionic fibers release acetylcholine rather than norepinephrine. Sympathetic nerves to the renal vasculature and kidney may release dopamine as well as norepinephrine during stress. The adrenal medulla, a modified sympathetic ganglion, receives sympathetic preganglionic fibers and releases epinephrine and nor-epinephrine into the blood. Not shown are the sacral preganglionic fibers that innervate the rectum, bladder, and genitalia. These fibers are probably sympathetic preganglionic nerves with cholinergic postganglionic fibers (see Box: Sympathetic Sacral Outflow). ACh, acetylcholine; D, dopamine; Epi, epinephrine; M, muscarinic receptors; N, nicotinic receptors; NE, norepinephrine.
CHAPTER 6 Introduction to Autonomic Pharmacology the thoracic, lumbar, and (according to new information) sacral spinal nerves. The parasympathetic preganglionic fibers leave the CNS through the cranial nerves (especially the third, seventh, ninth, and tenth).
Most thoracic and lumbar sympathetic preganglionic fibers are short and terminate in ganglia located in the paravertebral chains that lie on either side of the spinal column. Most of the remaining sympathetic preganglionic fibers are somewhat longer and terminate in prevertebral ganglia, which lie in front of the vertebrae, usually on the ventral surface of the aorta. From the ganglia, post-ganglionic sympathetic fibers run to the tissues innervated. Some preganglionic parasympathetic fibers terminate in parasympathetic ganglia located outside the organs innervated: the ciliary, pterygopalatine, submandibular, and otic ganglia. However, the majority of parasympathetic preganglionic fibers terminate on ganglion cells distributed diffusely or in networks in the walls of the innervated organs. Several pelvic ganglia are innervated by sacral preganglionic nerves that are ontogenetically similar to sympathetic preganglionic fibers (see Box: Sympathetic Sacral Outflow). Note that the terms “sympathetic” and “parasympathetic” are anatomic designations and do not depend on the type of transmitter chemical released from the nerve endings nor on the kind of effect—excitatory or inhibitory—evoked by nerve activity.
In addition to these clearly defined peripheral motor portions of the ANS, large numbers of afferent fibers run from the periphery to integrating centers, including the enteric plexuses in the gut, the autonomic ganglia, and the CNS. Many of the sensory pathways that end in the CNS terminate in the hypothalamus and medulla and evoke reflex motor activity that is carried to the effector cells by the efferent fibers described previously. There is increasing evidence that some of these sensory fibers also have peripheral motor functions.
The enteric nervous system (ENS) is a large and highly organized collection of neurons located in the walls of the gastrointestinal (GI) system (Figure 6–2). With over 150 million neurons, it
EC EC EC EC EC EPAN IPAN IN ACh, CGRP EN NP 5HT NP postganglionic fibers preganglionic fibers ACh INEN IPAN ACh, CGRP ACh NE ACh NPNP NE ACh EN IN IPAN 5HT 5HT 5HT5HT ACh SCAC ACSC SC