Chapter 14
The Autonomic Nervous System
529
14
the soma (sweat glands and the hair-raising arrector pili muscles
of the skin) require autonomic innervation and are served only
by sympathetic fibers. In addition, sympathetic fibers innervate
smooth muscle in the walls of all arteries and veins, both deep
and superficial. But we will explain these matters later—let’s get
on with the anatomy of the sympathetic division.
All preganglionic fibers of the sympathetic division arise
from cell bodies of preganglionic neurons in spinal cord seg-
ments T
1
through L
2
(Figure 14.3). For this reason, the sympa-
thetic division is also referred to as the
thoracolumbar division
(tho-rah
0
ko-lum
9
bar).
±e numerous preganglionic sympathetic neurons in the
gray matter of the spinal cord form the
lateral horns
(see Fig-
ures 12.28b, p. 467, and 12.29, p. 468). ±e lateral horns are just
posterolateral to the ventral horns that house somatic motor
neurons. (Parasympathetic preganglionic neurons in the sa-
cral cord are far less abundant than the comparable sympathetic
neurons in the thoracolumbar regions, so there are no lateral
horns in the sacral region of the spinal cord. ±is is a major
anatomical difference between the two divisions.)
A²er leaving the cord via the ventral root, preganglionic
sympathetic fibers pass through a
white ramus communi-
cans
[plural:
rami communicantes
(kom-mu
0
nĭ-kan
9
tēz)] to
enter an adjoining
sympathetic trunk ganglion
forming part
of the
sympathetic trunk
(or
sympathetic chain
,
Figure 14.5
).
Looking like strands of glistening white beads, the sympathetic
trunks flank each side of the vertebral column. ±ey consist of
the sympathetic ganglia and fibers running from one ganglion
to another. ±e sympathetic trunk ganglia are also called
chain
ganglia
or
paravertebral
(“near the vertebrae”)
ganglia
.
Although the sympathetic
trunks
extend from neck to pel-
vis, sympathetic
fibers
arise only from the thoracic and lumbar
When the main trunks of the vagus nerves reach the esophagus,
their fibers intermingle, forming the
anterior
and
posterior va-
gal trunks
, each containing fibers from both vagus nerves. ±ese
vagal trunks then “ride” the esophagus down to the abdominal
cavity. ±ere they send fibers
through
the large
abdominal aortic
plexus
[formed by a number of smaller plexuses (e.g.,
celiac, supe-
rior mesenteric
, and
hypogastric
) that run along the aorta] before
giving off branches to the abdominal viscera. ±e vagus nerves
innervate the liver, gallbladder, stomach, small intestine, kidneys,
pancreas, and the proximal half of the large intestine.
Sacral Part
±e sacral part of the parasympathetic division serves the pelvic
organs and the distal half of the large intestine. ±e sacral part
arises from neurons located in the lateral gray matter of spinal
cord segments S
2
–S
4
. Axons of these neurons run in the ventral
roots of the spinal nerves to the ventral rami and then branch off
to form the
pelvic splanchnic nerves
(splank
9
nik;
splanchni
5
viscera), which pass through the
inferior hypogastric (pelvic)
plexus
in the pelvic floor (Figure 14.4). Some preganglionic fibers
synapse with ganglia in this plexus, but most synapse in intramu-
ral ganglia in the walls of the following organs: distal half of the
large intestine, urinary bladder, ureters, and reproductive organs.
Sympathetic (Thoracolumbar) Division
±e sympathetic division is anatomically more complex than
the parasympathetic division, partly because it innervates more
organs. It supplies not only the visceral organs in the internal
body cavities but also all visceral structures in the superficial
(somatic) part of the body. ±is sounds impossible, but there is
an explanation—some glands and smooth muscle structures in
Table 14.1
Anatomical and Physiological Differences Between the Parasympathetic and Sympathetic Divisions
CHARACTERISTIC
PARASYMPATHETIC
SYMPATHETIC
Origin
Craniosacral part: brain stem nuclei of
cranial nerves III, VII, IX, and X; spinal
cord segments S
2
–S
4
.
Thoracolumbar part: lateral horns of gray matter of spinal cord
segments T
1
–L
2
.
Location of ganglia
Ganglia (terminal ganglia) are within
the visceral organ (intramural) or close
to the organ served.
Ganglia are within a few centimeters of CNS: alongside vertebral
column (sympathetic trunk ganglia) and anterior to vertebral column
(collateral, or prevertebral, ganglia).
Relative length of pre- and
postganglionic fibers
Long preganglionic; short
postganglionic.
Short preganglionic; long postganglionic.
Rami communicantes
None.
Gray and white rami communicantes. White rami contain myelinated
preganglionic fibers. Gray contain nonmyelinated postganglionic
fibers.
Degree of branching of
preganglionic fibers
Minimal.
Extensive.
Functional role
Maintenance functions; conserves and
stores energy; “rest and digest.”
Prepares body for activity; “fight or flight.”
Neurotransmitters
All preganglionic and postganglionic
fibers release ACh (cholinergic fibers).
All preganglionic fibers release ACh. Most postganglionic fibers
release norepinephrine (adrenergic fibers). Postganglionic
fibers serving sweat glands release ACh. Neurotransmitter
activity is augmented by release of adrenal medullary hormones
(norepinephrine and epinephrine).
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