THE
AUTONOMIC NERVOUS SYSTEM
by O.W. Henson, Jr., Ph.D.
University of North Carolina, Chapel Hill
Some Anatomical Generalizations
l. The autonomic nervous system (ANS) is an involuntary motor (efferent) system supplying smooth muscle, cardiac muscle and glands.
2. The ANS is accompanied by sensory (afferent) nerve fibers. Thus, depriving an organ or region of its autonomic innervation (by nerve section) also deprives that organ or region of its afferent innervation.
3. Autonomic nerves are typically composed of a two-neuron chain. One neuron has its cell body in the central nervous system while the other is outside the CNS. Since nerve cell bodies outside the CNS are located in a ganglion, the fibers associated with the first type are called “preganglionic” and those associated with the second type are “postganglionic”. Recall that motor nerves that are not autonomic arise from cell bodies in the central nervous system (CNS) and are distributed directly to effectors (skeletal muscle).
4 Although “involuntary”, the autonomic nervous system is regulated by higher centers. The best known of these centers is the hypothalamus which has descending projections to cell bodies of the preganglionic neurons. Other areas of the central nervous system affect the activities of the hypothalamus.
5. The autonomic nervous system consists of two divisions:
a) the sympathetic (or thoracolumbar) division in which the preganglionic cells are located in the thoracic and first two lumbar segments of the spinal cord.
b) the parasympathetic (or craniosacral) division in which the preganglionic neurons are located in the brain stem and in sacral (S2 - S4) segments of the spinal cord.
6. The two divisions of the autonomic nervous system are not infrequently said to be antagonists in the sense of their having opposite effects (e.g., stimulation of appropriate sympathetic fibers accelerate the heart rate and stimulation of appropriate parasympathetic nerves slows the heart rate). This generalization may be overdrawn. It is probably more accurate to presume that the two usually supply different effectors.
The sympathetic division
1. The cell bodies giving rise to preganglionic fibers are located in the intermediolateral column (lateral horn) of the gray matter in spinal cord segments Tl through L2.
2. Preganglionic fibers leave the spinal cord with the ventral roots of spinal nerves arising from cord segments Tl-L2.
3. Right and left sympathetic trunks extend from the base of the skull to the region of the coccyx; at their distal ends, the right and left trunks are fused.
4. At intervals along the length of each sympathetic trunk are collections of nerve cell bodies. Each such collection is termed a sympathetic trunk ganglion.
5. The preganglionic sympathetic nerve fibers are typically myelinated. They enter the spinal nerves formed by the merging of the dorsal and ventral roots of Tl-L2 near the intervertebral foramina. They then pass through the foramina and then exit from the anterior rami of the spinal nerves to the sympathetic trunk via a white ramus communicans (Pl.= rami communicantes). It should be obvious that white rami communicantes only occur in spinal nerves T1 to L2.
6. Once in the trunk the preganglionic nerve fibers may do one of several things:
a) Synapse with one or more cell bodies in that ganglion.
b) Pass up or down the sympathetic trunk to synapse with cells in some other ganglion of the trunk.
c) Pass through the sympathetic trunk (without synapsing)to reach the cell bodies in other ganglia called collateral or prevertebral ganglia. The fibers directed toward these ganglia, viscera, or aorta constitute “splanchnic nerves”.
7. The postganglionic fibers which arise from sympathetic trunk ganglia take one of two courses:
a) They may rejoin the anterior ramus of a spinal nerve (by way of a gray ramus communicans) for distribution to the periphery via anterior and posterior rami of the spinal nerves. In the periphery they innervate smooth muscle (notably in the walls of blood vessels, erector pili muscles, and sweat glands). It should be remembered that gray rami communicantes occur throughout the length of the sympathetic chain and are not restricted to the T1-L2 level.
b) They may pass directly to nearby viscera, such as the heart, lungs, esophagus, etc., often passing along the walls of arteries to reach their destinations.
8. Other important considerations:
a) sympathetic preganglionic fiber may synapse with many ganglion cells. Hence the effect of stimulation may be widespread.
b) ganglion cells are usually located at some distance from the effectors. Accordingly, postganglionic sympathetic fibers are usually long fibers.
c) The terminations of most, but not all, sympathetic postganglionic fibers release a substance (norepinephrine) resembling epinephrine (adrenalin). Such postganglionic fibers are commonly called adrenergic fibers.
d) The effects elicited by the action of the sympathetic division of the ANS are typically effects useful in “fight or flight”. These include dilation of the pupil, increase in heart rate, elevation of blood pressure, diversion of blood from the alimentary tract to skeletal muscles, etc.
9. Four general effects of lesions involving the interruption of sympathetic fibers are:
a) Dilation of blood vessels with increased blood flow. Thus, the skin in the affected region tends to become warmer and more red in color than the surrounding skin. (i.e. the surgical interruption of sympathetic fibers may be used to improve blood flow).
b) Lack of sweating in the involved area. “Sweat tests” are used clinically to evaluate the integrity of sympathetic innervation in cutaneous areas).
c) A constricted pupil on the side of a lesion involving fibers distributed to the head. The sympathetic nerves normally dilate the pupil.
d) Paralysis of arrector pili muscles such that “goose bumps” do not appear when the skin is chilled. Testing for the reflex appearance of goose bumps may be of diagnostic value.
The parasympathetic division
1. Has preganglionic cell bodies in the brainstem and in sacral segments 2, 3 and 4 of the spinal cord.
The fibers of cells in the brainstem are in the oculomotor (III), facial (VII), glossopharyngeal (IX), and vagus (X) nerves. They innervate smooth muscles of the eye (III), lacrimal and salivary glands (VII and IX), and smooth muscles of the thoracic and abdominal viscera (X).
The fibers arising from the sacral portion of the spinal cord run in the ventral roots of spinal nerves S2, 3 and 4. They enter the anterior primary rami of these nerves and travel in pelvic splanchnic nerves to reach the pelvic plexus and descending colon. These fibers are not associated with the sympathetic trunk.
2. The ganglion cells of the parasympathetic system are located in or on the wall of the organs supplied or in specific ganglia located near the organs supplied. Hence the postganglionic fibers are short.
3 Except for the vagus nerves, the area of distribution of parasympathetic nerves is somewhat limited. The number of synaptic connections is smaller than in the sympathetic division. Accordingly, the effects of the parasympathetic division tend to be local rather than widespread.
4. Most postganglionic parasympathetic fibers release acetylcholine at their terminations. These fibers are, hence, often called cholinergic fibers. They may also release a variety of peptides that influence smooth muscle activity.
5. Speaking generally, the parasympathetic division typically promotes visceral activity and repose. As examples, it provokes constriction of the pupil, slowing of the heart rate, salivation, flow of gastric juice, muscular activity of the alimentary tract, etc.