Chapter 29
Visceral Motor Pathways
Organization of the Visceral Motor System
Targets of Visceral Motor Outflow
General Features of Peripheral Visceral Motor Outflow
Preganglionic Visceral Motor Neurons
Postganglionic Visceral Motor Neurons
Sympathetic Preganglionic Neurons
Internal Organization of Sympathetic Ganglia
Functional and Chemical Coding
Receptor Types in Sympathetic Targets
Preganglionic and Postganglionic Neurons
Parasympathetic Outflow Pathways
Functional and Chemical Coding
Receptor Types in Parasympathetic Targets
Regulation of Visceral Motor Outflow
The primary function of the visceral motor system is the regulation of cardiovascular, respiratory, digestive, urinary, integumentary, and reproductive organs. These organs are the main effectors of homeostasis, the maintenance of a stable internal environment against perturbing influences, both external and internal. In general, visceral motor neurons innervate smooth and cardiac muscles and glandular epithelium or structures made up of combinations of these tissues.
OVERVIEW
The visceral motor (autonomic) system ensures that tissues of the body receive appropriate nutrients, electrolytes, and oxygen and that functions such as osmolarity and temperature are properly regulated. The nervous system contributes significantly to the control and coordination of homeostatic mechanisms in response to continually changing requirements. Two overlapping control systems influence visceral effectors. One is humoral (endocrine). Hormonal responses tend to develop slowly, but the effects are prolonged. The other system is neural (autonomic). Visceral motor responses tend to be immediate, but their effects are short term.
The endocrine and autonomic systems are interdependent. They are both under the control of widely distributed central nervous system (CNS) structures, which generate commands after integrating inputs from a wide variety of sources. Thus visceral motor output is influenced by emotional status as well as by sensory signals reporting conditions inside and outside the body.
The visceral motor system has two major subdivisions, sympathetic and parasympathetic. In addition, neurons located in the wall of the alimentary canal form a somewhat autonomous component called the enteric nervous system. This is sometimes regarded as a third subdivision of the autonomic system. Within each of these components are populations of chemically coded, target-specific neurons.
ORGANIZATION OF THE VISCERAL MOTOR SYSTEM
Targets of Visceral Motor Outflow
The autonomic system provides neural control of smooth muscle, cardiac muscle, or glandular secretory cells or combinations of these tissues. For example, the gut wall is composed of smooth muscle and glandular epithelium. The sympathetic and parasympathetic divisions have overlapping and generally antagonistic influences on those viscera located in body cavities and on some structures of the head, such as the iris (Table 29-1). There are also visceral targets in the body wall and limbs. These are found in skeletal muscle (blood vessels) and in the skin (blood vessels, sweat glands, and arrector pili muscles). Visceral structures of the body wall and extremities are generally regulated by the sympathetic division alone. The sympathetic outflow thus has a global distribution in that it innervates visceral structures in all parts of the body, whereas the parasympathetic outflow is restricted to targets in the head and body cavities (Table 29-1).
Table 29-1 Comparison of Effects of Sympathetic and Parasympathetic Activity on Some Visceral Functions
+, positive effect; −, negative effect; 0, no effect; ±, variable effect.
General Features of Peripheral Visceral Motor Outflow
There are similarities and differences between the neural control of skeletal muscle and visceral effectors such as smooth muscle (Fig. 29-1). As described in Chapter 24, lower motor neurons (alpha motor neurons) function as the final common pathway linking the CNS to skeletal muscle fibers (Fig. 29-1A). Similarly, sympathetic and parasympathetic outflows serve as the final but often dual common neural pathway from the CNS to visceral effectors. However, unlike the somatic motor system, the peripheral visceral motor pathway consists of two neurons (Fig. 29-1B, C). The first, the preganglionic neuron, has its cell body in either the brainstem or the spinal cord. Its axon projects as a thinly myelinated preganglionic fiber to an autonomic ganglion. The second, the postganglionic neuron, has its cell body in the ganglion and sends an unmyelinated axon (postganglionic fiber) to visceral effector cells such as smooth muscle. In general, parasympathetic ganglia are close to the effector tissue and sympathetic ganglia are close to the CNS. Consequently, parasympathetic pathways typically have long preganglionic fibers and short postganglionic fibers, whereas sympathetic pathways more often have short preganglionic fibers and long postganglionic fibers.
Figure 29-1. Comparison of somatic motor outflow (A) with sympathetic (B) and parasympathetic (C) outflow.
Visceral motor neurons and their targets are not organized into discrete motor units like those of the somatic motor system. Recall that an alpha motor neuron makes synaptic contacts with a definite group of skeletal muscle fibers over which it has exclusive control. In contrast, the terminal branches of a postganglionic visceral motor axon typically have a series of swellings containing neurotransmitter vesicles along their length, giving them a beaded (varicose) appearance (Fig. 29-1B, C). The neurotransmitters released from these terminals may act on effector cells at a distance of up to 100 µm. Moreover, unlike skeletal muscle fibers, cardiac muscle fibers and the smooth muscle cells of some organs are electrically coupled by gap junctions. Because of this, neurochemical signaling to a few cells is sufficient to regulate a large group of cells that act as a unit. The main features of sympathetic and parasympathetic divisions are summarized in Table 29-2.
Table 29-2 Comparison of the Sympathetic and Parasympathetic Divisions of Autonomic Outflow
DEVELOPMENT
Preganglionic Visceral Motor Neurons
Cell bodies of these neurons are located in nuclei or cell columns embryologically derived from the visceral efferent cell column. This column arises from neuroblasts in the basal (motor) plate of the brainstem and spinal cord portions of the neural tube.
Postganglionic Visceral Motor Neurons
Cell bodies of these multipolar neurons are located in autonomic ganglia, which may be either well-defined, encapsulated structures, such as the superior cervical ganglion, or clusters of somata found in nerve plexuses or in the walls and capsules of visceral organs. Like most primary sensory neurons, autonomic ganglion cells are derived from neural crest cells that migrate to appropriate locations during development.
One result of this cell migration is the advent, in the adult, of the myenteric (Auerbach) and the submucosal (Meissner) plexuses and the normal muscular and secretory functions of the intestinal wall.
Congenital megacolon, or Hirschsprung disease, results from a failure of these enteric neuronal precursor cells to migrate into the wall of the developing lower gut. As a result, the affected segment of the gut (the portion lacking enteric ganglion cells, the aganglionic segment), usually the colon, is paralyzed in a constricted state, with consequent distention of the proximal and normally innervated portion of the intestine (the portion containing enteric ganglion cells, the ganglionic segment) (Fig. 29-2). This disease is most commonly seen in the very young (newborn to 6 years) but may be seen in adults. Although the presentation of the disease is strikingly characteristic on a radiograph or with magnetic resonance imaging (Fig. 29-2), definitive diagnosis relies on a biopsy and histologic confirmation of a lack of enteric ganglion neurons in the affected segment. The treatment of choice is to resect the aganglionic segment and join the remaining normal portions of the gut.
Development of the autonomic nervous system requires an elaborate sequence of intercellular signaling that involves two major families of neurotrophic factors. One is the glial cell line–derived neurotrophic factor (GDNF) family, which consists of several distinct signaling molecules and their receptors. Mutations of one of these receptors, designated RET, is the underlying cause of some cases of congenital megacolon. The neurotrophins are the other large family of neurotrophic factors. As with the GDNF family, each neurotrophin regulates development and function of specific populations of peripheral nervous system and CNS neurons via binding to specific receptors. The existence of these neuronal growth factors was first demonstrated when the neurotrophin nerve growth factor (NGF) was discovered as a target-derived messenger molecule that is absolutely essential for survival and development of sympathetic postganglionic neurons (Fig. 29-3) as well as those primary sensory neurons that are involved in pain (Fig. 29-3). The pathologic changes in animals deprived of NGF or its high-affinity receptor are similar to those seen in patients with congenital insensitivity to pain with anhidrosis (hereditary sensory and autonomic neuropathy type IV), an autosomal recessive disease. Indeed, mutations that impair the function of the NGF receptor trkA have been identified in patients with this disease.
SYMPATHETIC DIVISION
Sympathetic Preganglionic Neurons
Although the sympathetic outflow influences visceral targets throughout the body, sympathetic preganglionic neurons are found only in spinal cord segments T1 through L2 (sometimes in C8 and L3). These cell bodies are located in Rexed lamina VII, primarily in the intermediolateral nucleus (cell column) of the lateral horn. The axons of these preganglionic cells exit the spinal cord in the ventral root and enter the sympathetic trunk via the white communicating ramus. A preganglionic fiber takes one of three courses after entering the sympathetic chain: (1) it may synapse at that level with postganglionic neurons whose unmyelinated axons join that spinal nerve via a gray communicating ramus; (2) it may ascend or descend in the sympathetic chain to synapse on postganglionic neurons whose axons either join spinal nerves or project to targets in the thoracic cavity or head; or (3) it may pass through the chain ganglion as a preganglionic fiber to form part of a splanchnic nerve (Fig. 29-4).
The sympathetic outflow for the entire body originates from thoracic and upper lumbar spinal cord segments. Although the segmental pattern of innervation is not straightforward, there is a general viscerotopic organization (Figs. 29-5 and 29-6). Neurons of the superior, middle, and inferior cervical ganglia receive input, via the sympathetic trunk, from preganglionic neurons of the upper thoracic spinal segments. Lower lumbar and sacral ganglia are supplied by neurons of the lower thoracic and upper lumbar spinal segments. The ganglia between these regions are supplied by their corresponding spinal levels. Thus visceral targets in the head, neck, and upper extremity as well as the viscera of the thoracic cavity are served by preganglionic sympathetic neurons in the upper thoracic segments. The main abdominal viscera and other targets in the trunk are served by the central and lower thoracic spinal cord segments, whereas the pelvic viscera, the lower trunk, and the lower extremity are served by the lower thoracic and upper lumbar spinal cord segments.
Figure 29-5. Sympathetic pathways to visceral targets in the body wall, limbs, and scalp and neck. Postganglionic sympathetic fibers that distribute to targets of the head are shown in Figure 29-6.
Preganglionic sympathetic neurons also target the adrenal gland (Fig. 29-6). Chromaffin cells of the adrenal medulla are related to sympathetic ganglion neurons in both derivation (neural crest) and function. These cells secrete catecholamines (mostly epinephrine) into the bloodstream in response to signals from preganglionic neurons. Thus the sympathetic system, through this endocrine pathway, regulates functions of cells that are not directly contacted by nerve terminals.
Sympathetic Ganglia
Cell bodies of sympathetic postganglionic neurons are generally grouped into discrete ganglia that are located at some distance from the target tissue. Most of them make up the sympathetic chain (paravertebral) ganglia and the prevertebral ganglia associated with the abdominal aorta or its large branches (the celiac, aorticorenal, superior mesenteric, and inferior mesenteric ganglia; see Fig. 29-6). In addition, small clusters of cell bodies are also scattered among nerve fibers in communicating rami, the sympathetic trunk between chain ganglia, and peripheral plexuses.
The sympathetic chain
