The Autonomic Nervous System
contributes sympathetic ﬁbers that run in several cranial nerves
and in the upper three or four cervical spinal nerves.
Besides serving the skin and blood vessels of the head, ﬁ-
bers from the superior cervical ganglion stimulate the dilator
muscles of the irises of the eyes, inhibit the nasal and salivary
glands (the reason your mouth goes dry when you are scared),
and innervate the smooth (tarsal) muscle that liFs the up-
per eyelid. Te superior cervical ganglion also sends direct
branches to the heart.
Pathways to the Thorax
Sympathetic preganglionic ﬁbers in-
nervating the thoracic organs originate at ±
. ²rom there the
preganglionic ﬁbers run to synapse in the cervical trunk ganglia.
Postganglionic ﬁbers emerging from the
enter cervical nerves C
(²igure 14.6). Some
of these ﬁbers innervate the heart via the cardiac plexus, and
some innervate the thyroid gland, but most serve the skin. Addi-
tionally, some ±
preganglionic ﬁbers synapse in the nearest
trunk ganglion, and the postganglionic ﬁbers pass directly to the
organ served. ²ibers to the heart, aorta, lungs, and esophagus
take this direct route. Along the way, they run through the plex-
uses associated with those organs.
Pathways with Synapses in Collateral Ganglia
Most of the preganglionic ﬁbers from ±
down synapse in collat-
eral ganglia, and so most of these ﬁbers enter and leave the sym-
pathetic trunks without synapsing. Tey form several nerves
, including the thoracic splanchnic
nerves (greater, lesser, and least) and the
Te splanchnic nerves contribute to a number of interweaving
nerve plexuses known collectively as the
abdominal aortic plexus
which clings to the surface of the abdominal aorta. Tis complex
plexus contains several ganglia that together serve the abdomi-
nopelvic viscera. ²rom superior to inferior, the most important
of these ganglia (and related subplexuses) are the
, named for the arteries with
which they most closely associate (²igure 14.6). Postganglionic
ﬁbers issuing from these ganglia generally travel to their target
organs in the company of the arteries serving these organs.
Pathways to the Abdomen
Sympathetic preganglionic ﬁbers
innervate the abdomen. Tey travel in the thoracic
splanchnic nerves to synapse mainly at the celiac and superior
mesenteric ganglia. Postganglionic ﬁbers issuing from these
ganglia serve the stomach, intestines (except the distal half of
the large intestine), liver, spleen, and kidneys.
Pathways to the Pelvis
Preganglionic ﬁbers innervating the
pelvis originate from ±
and then descend in the sympa-
thetic trunk to the lumbar and sacral trunk ganglia. Some ﬁbers
synapse there and the postganglionic ﬁbers run in lumbar and
sacral splanchnic nerves to plexuses on the lower aorta and in
the pelvis. Other preganglionic ﬁbers pass directly to these au-
tonomic plexuses and synapse in collateral ganglia, such as the
inferior mesenteric ganglion.
Postganglionic ﬁbers proceed from these plexuses to the pel-
vic organs (the urinary bladder and reproductive organs) and
cord segments, as shown in ²igure 14.3. Te ganglia vary in
size, position, and number, but typically there are 23 in each
sympathetic trunk—3 cervical, 11 thoracic, 4 lumbar, 4 sacral,
and 1 coccygeal.
Once a preganglionic axon reaches a trunk ganglion, one of
three things can happen to the axon (see the three pathways in
²igure 14.5b). Te preganglionic and postganglionic neurons can:
Synapse at the same level.
In this case, the synapse is in the
same trunk ganglion.
Synapse at a higher or lower level.
axon ascends or descends the sympathetic trunk to another
Synapse in a distant collateral ganglion.
axon passes through the trunk ganglion and emerges from
the sympathetic trunk without synapsing. Tese pregangli-
onic ﬁbers help form several
located anterior to
the vertebral column. Unlike sympathetic trunk ganglia,
the collateral ganglia are neither paired nor segmentally ar-
ranged. Tey occur only in the abdomen and pelvis.
Regardless of where the synapse occurs, all sympathetic gan-
glia are close to the spinal cord, and their postganglionic ﬁbers
are typically much longer than their preganglionic ﬁbers. Recall
that the opposite condition exists in the parasympathetic divi-
sion, an important anatomical distinction.
Pathways with Synapses in Trunk Ganglia
When synapses are made in sympathetic trunk ganglia, the
postganglionic axons enter the ventral (or dorsal) ramus of the
adjoining spinal nerves by way of communicating branches
gray rami communicantes
(²igure 14.5). ²rom there
they travel via branches of the rami to their eﬀectors, including
sweat glands and arrector pili muscles of the skin. Anywhere
along their path, the postganglionic axons may transfer over to
nearby blood vessels and innervate the vascular smooth muscle
all the way to the ﬁnal branches of the blood vessels.
Notice that the names of the rami communicantes—
—reﬂect their appearance, revealing whether or not their
ﬁbers are myelinated. Preganglionic ﬁbers composing the white
rami are myelinated. Postganglionic axons forming the gray
rami are not. (Tis has no relationship to the white and gray
matter of the CNS.)
Te white rami, which carry preganglionic axons to the sym-
pathetic trunks, are found only in the ±
regions of sympathetic outﬂow. However, gray rami carrying
postganglionic ﬁbers headed for the periphery issue from every
trunk ganglion from the cervical to the sacral region, allowing
sympathetic output to reach all parts of the body.
rami communicantes are associated only with the
. Tey never carry parasympathetic ﬁbers.
Pathways to the Head
Sympathetic preganglionic ﬁbers serv-
ing the head emerge from spinal cord segments ±
the sympathetic trunk to synapse with postganglionic neurons
superior cervical ganglion
. Tis ganglion