Chapter 14
The Autonomic Nervous System
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14
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 ±
1
–±
6
. ²rom there the
preganglionic fibers run to synapse in the cervical trunk ganglia.
Postganglionic fibers emerging from the
middle
and
inferior
cervical ganglia
enter cervical nerves C
4
–C
8
(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 ±
1
–±
6
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 ±
5
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
called
splanchnic nerves
, including the thoracic splanchnic
nerves (greater, lesser, and least) and the
lumbar
and
sacral
splanchnic nerves
.
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
celiac
,
superior
mesenteric
, and
inferior mesenteric
, 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
from ±
5
to L
2
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 ±
10
to L
2
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:
1
Synapse at the same level.
In this case, the synapse is in the
same trunk ganglion.
2
Synapse at a higher or lower level.
Te preganglionic
axon ascends or descends the sympathetic trunk to another
trunk ganglion.
3
Synapse in a distant collateral ganglion.
Te preganglionic
axon passes through the trunk ganglion and emerges from
the sympathetic trunk without synapsing. Tese pregangli-
onic fibers help form several
splanchnic nerves
and synapse
in
collateral
, or
prevertebral
,
ganglia
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
called
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—
white
or
gray
—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 ±
1
–L
2
cord segments,
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.
Note that
rami communicantes are associated only with the
sympathetic division
. Tey never carry parasympathetic fibers.
Pathways to the Head
Sympathetic preganglionic fibers serv-
ing the head emerge from spinal cord segments ±
1
–±
4
and ascend
the sympathetic trunk to synapse with postganglionic neurons
in the
superior cervical ganglion
(Figure 14.6)
. Tis ganglion
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