FUNCTIONAL ANATOMY
Each branch of the ANS exhibits a different anatomic motif, which
is recapitulated on a cellular and molecular level. The underlying theme of the
sympathetic nervous system is an amplification response,
whereas that of the parasympathetic nervous system is a discrete
and narrowly targeted response. The enteric nervous system is arranged
nontopographically, as would be appropriate for the viscera, and relies on the mechanism
of chemical coding to differentiate between nerves serving different functions.
Sympathetic Nervous System
The sympathetic nervous system originates from the spinal cord
in the thoracolumbar region, from the first thoracic through the second or third
lumbar segment. The pregnanglionic sympathetic neurons have cell bodies within the
horns of the spinal gray matter (i.e., the intermediolateral columns). Nerve fibers
from these cell bodies extend to three types of ganglia, grouped as paired sympathetic
chains, various unpaired distal plexuses, or terminal or collateral ganglia near
the target organ.
The 22 paired ganglia lie along either side of the vertebral column.
Nerve trunks connect these ganglia to each other, and gray rami communicantes connect
the ganglia to the spinal nerves. The preganglionic fibers leave the cord in the
anterior nerve roots, join the spinal nerve trunks, and enter the ganglion at that
level through the white (myelinated) ramus. Leaving the ganglion, postsynaptic fibers
reenter the spinal nerve through the gray (unmyelinated) ramus and then go on to
innervate pilomotor and sudomotor (sweat gland) effectors and blood vessels of the
skeletal muscle and skin ( Fig. 16-5
).
Sympathetic innervation of the trunk and limbs is carried by the spinal nerves.
The sympathetic distribution to the head and neck, enabling and
mediating vasomotor, pupillodilator, secretory, and pilomotor functions, comes from
the three ganglia of the cervical sympathetic chain. Preganglionic fibers of these
cervical structures originate in the upper thoracic segments. In 80% of people,
the stellate ganglion is formed by the fusion of the inferior cervical ganglion with
the first thoracic ganglion on each side.
Figure 16-5
Autonomic nervous system. Post, postganglionic neuron;
pre, preganglionic neuron; RC, ramus communicans. (From Ganong W: The autonomic
nervous system. In Ganong W [ed]: Review of Medical
Physiology, 15th ed. Norwalk, CT, Appleton & Lange, 1991, p 210.)
The unpaired prevertebral ganglia reside in the abdomen and pelvis
anterior to the vertebral column and are primarily the celiac, superior mesenteric,
aorticorenal, and inferior mesenteric ganglia. The celiac ganglion is innervated
by T5 through T12 and innervates the liver, spleen, kidney, pancreas, small bowel,
and proximal colon. Many preganglionic fibers from T5 to T12 may pass through the
paired paravertebral ganglia to form the splanchnic nerves; most do not synapse until
in the celiac ganglion, whereas others innervate the adrenal medulla. The superior
mesenteric ganglion innervates the distal colon, and the inferior mesenteric ganglion
serves the rectum, bladder, and genitals ( Fig.
16-6
). Postganglionic fibers arising from the synaptic links of upper
thoracic sympathetic fibers in vertebral ganglia form terminal cardiac, esophageal,
and pulmonary plexuses. These fibers also innervate the viscera of the abdomen and
pelvis. Ganglia of the third type, the terminal or collateral ganglia, are small,
few in number, and near their target organs. The adrenal medulla and other chromaffin
tissue are homologous to sympathetic ganglia, and all are derived embryonically from
neural crest cells; unlike sympathetic ganglia, the adrenal medulla releases epinephrine
and norepinephrine.
Sympathetic preganglionic fibers are relatively short because
sympathetic ganglia are generally close to the central nervous system (CNS), but
they are distant from the effector organs; therefore, postganglionic fibers run a
long course before innervating effector organs (see Fig.
16-1
). The distribution is also diffuse and is capable of amplification.
Pregnanglionic sympathetic fibers may pass through multiple ganglia before synapsing,
and terminal fibers may contact large numbers of postganglionic neurons. Terminal
fibers of preganglionic axons may synapse with more than 20 ganglia, and one cell
may be supplied by several preganglionic fibers. Sympathetic nerves need not synapse
solely in the ganglion of their origin, but they can course up and down the paired
ganglia of the spinal cord. Sympathetic response is not confined to segments from
which the stimulus originates. This allows for a more dramatic response, with diffuse
discharge of the sympathetic system.
Autonomic reflexes remain after transection of the spinal cord.
The autonomic reflexes are normally inhibited by supraspinal feedback, but this
supraspinal inhibition is lost with the division of the spinal tracts. After transection,
trivial stimuli may elicit an exaggerated sympathetic discharge (see "Autonomic Dysfunction").
