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The Autonomic Nervous System
When we are awake, we are constantly aware of sensory input from our external environment, and we consciously plan how to react to it. When we are asleep, the nervous system has a variety of mechanisms to dissociate cortical function from sensory input and somatic motor output. Among these mechanisms are closing the eyes, blocking the transmission of sensory impulses to the cortex as they pass through the thalamus, and effecting a nearly complete paralysis of skeletal muscles during rapid eye movement (REM) sleep to keep us from physically acting out our dreams.
The conscious and discontinuous nature of cortical brain function stands in sharp contrast to that of those parts of the nervous system responsible for control of our internal environment. These “autonomic” processes never stop attending to the wide range of metabolic, cardiopulmonary, and other visceral requirements of our body. Autonomic control continues whether we are awake and attentive, preoccupied with other activities, or asleep. While we are awake, we are unaware of most visceral sensory input, and we avoid any conscious effort to act on it unless it induces distress. In most cases, we have no awareness of motor commands to the viscera, and most individuals can exert voluntary control over visceral motor output in only minor ways. Consciousness and memory are frequently considered the most important functions of the human nervous system, but it is the visceral control system—including the autonomic nervous system (ANS) —that makes life and higher cortical function possible.
We have a greater understanding of the physiology of the ANS than of many other parts of the nervous system, largely because it is reasonably easy to isolate peripheral neurons and to study them. As a result of its accessibility, the ANS has served as a key model system for the elucidation of many principles of neuronal and synaptic function.
Organization of the Visceral Control System
The ANS has sympathetic, parasympathetic, and enteric divisions
Output from the central nervous system (CNS) travels along two anatomically and functionally distinct pathways: the somatic motor neurons, which innervate striated skeletal muscle; and the autonomic motor neurons, which innervate smooth muscle, cardiac muscle, secretory epithelia, and glands. All viscera are richly supplied by efferent axons from the ANS that constantly adjust organ function.
The autonomic nervous system (from the Greek for “self-governing,” functioning independently of the will) was first defined by Langley in 1898 as including the local nervous system of the gut and the efferent neurons innervating glands and involuntary muscle. Thus, this definition of the ANS includes only efferent neurons and enteric neurons. Since that time, it has become clear that the efferent ANS cannot easily be dissociated from visceral afferents as well as from those parts of the CNS that control the output to the ANS and those that receive interoceptive input. 
N14-1 This larger visceral control system monitors afferents from the viscera and the rest of the body, compares this input with current and anticipated needs, and controls output to the body's organ systems.
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Types of Afferent Input
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Exteroceptive
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Proprioceptive
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Interoceptive
The ANS has three divisions: sympathetic, parasympathetic, and enteric. The sympathetic and parasympathetic divisions of the ANS are the two major efferent pathways controlling targets other than skeletal muscle ( Fig. 14-1 ). Each innervates target tissue by a two-synapse pathway. The cell bodies of the first neurons lie within the CNS. These preganglionic neurons are found in columns of cells in the brainstem and spinal cord and send axons out of the CNS to make synapses with postganglionic neurons in peripheral ganglia interposed between the CNS and their target cells. Axons from these postganglionic neurons then project to their targets. The sympathetic and parasympathetic divisions can act independently of each other. However, in general, they work synergistically to control visceral activity and often act in opposite ways, like an accelerator and brake to regulate visceral function. An increase in output of the sympathetic division occurs under conditions such as stress, anxiety, physical activity, fear, or excitement, whereas parasympathetic output increases during sedentary activity, eating, or other “vegetative” behavior.
The enteric division of the ANS is a collection of afferent neurons, interneurons, and motor neurons that form networks of neurons called plexuses (from the Latin “to braid”) that surround the gastrointestinal (GI) tract. It can function as a separate and independent nervous system, but it is normally controlled by the CNS through sympathetic and parasympathetic fibers.
Sympathetic preganglionic neurons originate from spinal segments T1 to L3 and synapse with postganglionic neurons in paravertebral or prevertebral ganglia
Preganglionic Neurons
The cell bodies of preganglionic sympathetic motor neurons are located in the thoracic and upper lumbar spinal cord between levels T1 and L3. At these spinal levels, autonomic neurons lie in the intermediolateral cell column, or lateral horn, between the dorsal and ventral horns ( Fig. 14-2 ). Axons from preganglionic sympathetic neurons exit the spinal cord through the ventral roots along with axons from somatic motor neurons. After entering the spinal nerves, sympathetic efferents diverge from somatic motor axons to enter the white rami communicantes. These rami, or branches, are white because most preganglionic sympathetic axons are myelinated.
Paravertebral Ganglia
Axons from preganglionic neurons enter the nearest sympathetic paravertebral ganglion through a white ramus. These ganglia lie adjacent to the vertebral column. Although preganglionic sympathetic fibers emerge only from levels T1 to L3, the chain of sympathetic ganglia extends all the way from the upper part of the neck to the coccyx, where the left and right sympathetic chains merge in the midline and form the coccygeal ganglion. In general, one ganglion is positioned at the level of each spinal root, but adjacent ganglia are fused in some cases. The most rostral ganglion, the superior cervical ganglion, arises from fusion of C1 to C4 and supplies the head and neck. The next two ganglia are the middle cervical ganglion, which arises from fusion of C5 and C6, and the inferior cervical ganglion (C7 and C8), which is usually fused with the first thoracic ganglion to form the stellate ganglion. Together, the middle cervical and stellate ganglia, along with the upper thoracic ganglia, innervate the heart, lungs, and bronchi. The remaining paravertebral ganglia supply organs and portions of the body wall in a segmental fashion.
After entering a paravertebral ganglion, a preganglionic sympathetic axon has one or more of three fates. It may (1) synapse within that segmental paravertebral ganglion, (2) travel up or down the sympathetic chain to synapse within a neighboring paravertebral ganglion, or (3) enter the greater or lesser splanchnic nerve to synapse within one of the ganglia of the pre vertebral plexus.
Prevertebral Ganglia
The prevertebral plexus lies in front of the aorta and along its major arterial branches and includes the prevertebral ganglia and interconnected fibers ( Fig. 14-3 ). The major prevertebral ganglia are named according to the arteries that they are adjacent to and include the celiac, superior mesenteric, aorticorenal, and inferior mesenteric ganglia. Portions of the prevertebral plexus extend down the major arteries and contain other named and unnamed ganglia and plexuses of nerve fibers, which altogether make up a dense and extensive network of sympathetic neuron cell bodies and nerve fibers.
Each pre ganglionic sympathetic fiber synapses on many post ganglionic sympathetic neurons that are located within one or several nearby paravertebral or prevertebral ganglia. It has been estimated that each preganglionic sympathetic neuron branches and synapses on as many as 200 postganglionic neurons, which enables the sympathetic output to have more widespread effects. However, any impulse arriving at its target end organ has only crossed a single synapse between the preganglionic and postganglionic sympathetic neurons.
Postganglionic Neurons
The cell bodies of postganglionic sympathetic neurons that are located within para vertebral ganglia send out their axons through the nearest gray rami communicantes, which rejoin the spinal nerves (see Fig. 14-2 ). These rami are gray because most postganglionic axons are unmyelinated. Because preganglionic sympathetic neurons are located only in the thoracic and upper lumbar spinal segments (T1 to L3), white rami are found only at these levels ( Fig. 14-4 , left panel). However, because each sympathetic ganglion sends out postganglionic axons, gray rami are present at all spinal levels from C2 or C3 to the coccyx. Postganglionic sympathetic axons from paravertebral and prevertebral ganglia travel to their target organs within other nerves or by traveling along blood vessels. Because the paravertebral and prevertebral sympathetic ganglia lie near the spinal cord and thus relatively far from their target organs, the postganglionic axons of the sympathetic division tend to be long. On their way to reach their targets, some postganglionic sympathetic axons travel through para sympathetic terminal ganglia or cranial nerve ganglia without synapsing. N14-2
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Tracing of Nerve Tracts Using Pseudorabies Virus
The CNS neuroanatomy of autonomic control has been difficult to define experimentally. However, a technique developed by Arthur Loewy and his colleagues that traces nerve tracts with the pseudorabies virus has helped to define more clearly the central pathways for autonomic control. For example, if axons of preganglionic sympathetic neurons are exposed to pseudorabies virus, the virus is transported back into the cell bodies, where it replicates. After a delay of several days, neurons that make synapses with these preganglionic neurons (i.e., “premotor” neurons) become infected and the virus is transported to their cell bodies. After longer periods of incubation, neurons farther upstream are also infected. Histological staining can then be used at different time points to visualize neurons that contain the virus at each level upstream.
Parasympathetic preganglionic neurons originate from the brainstem and sacral spinal cord and synapse with postganglionic neurons in ganglia located near target organs
The cell bodies of preganglionic parasympathetic neurons are located in the medulla, pons, and midbrain and in the S2 through S4 levels of the spinal cord (see Fig. 14-4 , right panel). Thus, unlike the sympathetic—or thoracolumbar —division, whose preganglionic cell bodies are in the thoracic and lumbar spinal cord, the parasympathetic—or craniosacral —division's preganglionic cell bodies are cranial and sacral. The preganglionic parasympathetic fibers originating in the brain distribute with four cranial nerves: the oculomotor nerve (CN III), the facial nerve (CN VII), the glossopharyngeal nerve (CN IX), and the vagus nerve (CN X). The preganglionic parasympathetic fibers originating in S2 through S4 distribute with the pelvic splanchnic nerves.
Postganglionic parasympathetic neurons are located in terminal ganglia that are more peripherally located and more widely distributed than are the sympathetic ganglia. Terminal ganglia often lie within the walls of their target organs. Thus, in contrast to the sympathetic division, postganglionic fibers of the parasympathetic division are short. In some cases, individual postganglionic parasympathetic neurons are found in isolation or in scattered cell groups rather than in encapsulated ganglia.
Cranial Nerves III, VII, and IX
The preganglionic parasympathetic neurons that are distributed with CN III, CN VII, and CN IX originate in three groups of nuclei.
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The Edinger-Westphal nucleus is a subnucleus of the oculomotor complex in the midbrain ( Fig. 14-5 ). Parasympathetic neurons in this nucleus travel in the oculomotor nerve (CN III) and synapse onto postganglionic neurons in the ciliary ganglion (see Fig. 14-4 , right panel). The postganglionic fibers project to two smooth muscles of the eye: the constrictor muscle of the pupil and the ciliary muscle, which controls the shape of the lens (see Fig. 15-6 ).
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The superior salivatory nucleus is in the rostral medulla (see Fig. 14-5 ) and contains parasympathetic neurons that project, through a branch of the facial nerve (CN VII), to the pterygopalatine ganglion (see Fig. 14-4 , right panel). The postganglionic fibers supply the lacrimal glands, which produce tears. Another branch of the facial nerve carries preganglionic fibers to the submandibular ganglion. The postganglionic fibers supply two salivary glands, the submandibular and sublingual glands.
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The inferior salivatory nucleus and the rostral part of the nucleus ambiguus in the rostral medulla (see Fig. 14-5 ) contain parasympathetic neurons that project through the glossopharyngeal nerve (CN IX) to the otic ganglion (see Fig. 14-4 , right panel). The postganglionic fibers supply a third salivary gland, the parotid gland.
Cranial Nerve X
Most parasympathetic output occurs through the vagus nerve (CN X). Cell bodies of vagal preganglionic parasympathetic neurons are found in the medulla within the nucleus ambiguus and the dorsal motor nucleus of the vagus (see Fig. 14-5 ). This nerve supplies parasympathetic innervation to all the viscera of the thorax and abdomen, including the GI tract between the pharynx and distal end of the colon (see Fig. 14-4 , right panel). Among other effects, electrical stimulation of the nucleus ambiguus results in contraction of striated muscle in the pharynx, larynx, and upper esophagus due to activation of somatic motor neurons (not autonomic), as well as slowing of the heart due to activation of vagal preganglionic parasympathetic neurons. Stimulation of the dorsal motor nucleus of the vagus induces many effects in the viscera, including initiation of secretion of gastric acid, insulin, and glucagon. Preganglionic parasympathetic fibers of the vagus nerve join the esophageal, pulmonary, and cardiac plexuses and travel to terminal ganglia that are located within their target organs.
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