Chapter 14
The Autonomic Nervous System
527
14
Location of their ganglia.
Most parasympathetic ganglia are
located in the visceral effector organs. Sympathetic ganglia
lie close to the spinal cord.
Figure 14.3
illustrates these and other key differences, which
are summarized in
Table 14.1
.
We begin our detailed exploration of the ANS with the ana-
tomically simpler parasympathetic division.
Parasympathetic (Craniosacral) Division
Te parasympathetic division is also called the
craniosacral
division
because its preganglionic fibers spring from opposite
ends of the CNS—the brain stem and the sacral region of the
spinal cord
(Figure 14.4)
. Te preganglionic axons extend
from the CNS nearly all the way to the structures they inner-
vate. Tere the axons synapse with postganglionic neurons lo-
cated in
terminal ganglia
that lie close to or within the target
organs. Very short postganglionic axons issue from the terminal
ganglia and synapse with effector cells in their immediate area.
Dilates the bronchioles in the lungs, increasing ventilation
(and thus increasing oxygen delivery to body cells)
Causes the liver to release more glucose into the blood to ac-
commodate the increased energy needs of body cells
At the same time, the sympathetic division temporarily
damps nonessential activities, such as gastrointestinal tract mo-
tility. If you are running from a mugger, digesting lunch can
wait! It is far more important to give your muscles everything
they need to get you out of danger. In such active situations,
the sympathetic division generates a head of steam that enables
the body to cope with situations that threaten homeostasis. It
provides the optimal conditions for an appropriate response to
some threat, whether that response is to run, see distant objects
better, or think more clearly.
We have just looked at two extreme situations in which one
or the other branch of the ANS dominates. Tink of the para-
sympathetic division as the
D
division [digestion, defecation,
and diuresis (urination)], and the sympathetic division as the
E
division (exercise, excitement, emergency, embarrassment).
±able 14.4 (p. 536) presents a more detailed summary of how
each division affects various organs.
Remember, however, that the two ANS divisions rarely work
in an all-or-none fashion as described above. A dynamic antag-
onism exists between the divisions, and both make continuous
fine adjustments to maintain homeostasis.
Check Your Understanding
1.
Name the three types of effectors of the autonomic nervous
system.
2.
Which relays instructions from the CNS to muscles more
quickly, the somatic nervous system or the ANS? Explain why.
3.
Which branch of the ANS would predominate if you were
lying on the beach enjoying the sun and the sound of the
waves? Which branch would predominate if you were on a
surfboard and a shark appeared within a few feet of you?
For answers, see Appendix H.
ANS Anatomy
For the parasympathetic and sympathetic divisions, describe
the site of CNS origin, locations of ganglia, and general
fiber pathways.
Anatomically, the sympathetic and parasympathetic divisions
differ in
Sites of origin.
Parasympathetic fibers are craniosacral—
they originate in the brain (cranium) and sacral spinal cord.
Sympathetic fibers are thoracolumbar—they originate in the
thoracic and lumbar regions of the spinal cord.
Relative lengths of their fibers.
Te parasympathetic divi-
sion has long preganglionic and short postganglionic fibers.
Te sympathetic division has the opposite condition—the
preganglionic fibers are short and the postganglionic fibers
are long.
Salivary
glands
Eye
Skin*
Heart
Lungs
Liver
and gall-
bladder
Genitals
Pancreas
Eye
Lungs
Bladder
Liver and
gall-
bladder
Pancreas
Stomach
Cervical
Sympathetic
ganglia
Cranial
Lumbar
Thoracic
Genitals
Heart
Salivary
glands
Stomach
Bladder
Adrenal
gland
Parasympathetic
Sympathetic
Sacral
Brain stem
L
1
T
1
Figure 14.3
The subdivisions of the ANS.
The parasympathetic
and sympathetic divisions differ anatomically in the (1) sites where
their nerves originate, (2) relative lengths of their preganglionic and
postganglionic fibers, and (3) locations of their ganglia (indicated
here by synapse sites).
*Although sympathetic innervation to the skin is mapped to the cervical
region here, all nerves to the periphery carry postganglionic sympathetic
fibers.
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