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14
The Autonomic Nervous System
Overview
(pp. 524–527)
Comparison of the Somatic and Autonomic
Nervous Systems (pp. 525–526)
ANS Divisions (pp. 526–527)
ANS Anatomy
(pp. 527–533)
T
he human body is exquisitely sensitive to changes in its internal
environment,
and engages in a lifelong struggle to balance competing demands
for resources under ever-changing conditions. Although all body systems contrib-
ute, the stability of our internal environment depends largely on the
autonomic nervous
system (ANS)
, the system of motor neurons that innervates smooth and cardiac muscle
and glands
(Figure 14.1)
.
At every moment, signals stream from visceral organs into the CNS, and autonomic nerves
make adjustments as necessary to ensure optimal support for body activities. In response to
changing conditions, the ANS shunts blood to “needy” areas, speeds or slows heart rate,
adjusts blood pressure and body temperature, and increases or decreases stomach secretions.
Most of this fine-tuning occurs without our awareness or attention. Can you tell when
your arteries are constricting or your pupils are dilating? Probably not—but if you’ve
ever been stuck in a checkout line, and your full bladder was contracting as if it had a
mind of its own, you’ve been very aware of visceral activity. Te ANS controls all these
functions, both those we’re aware of and those we’re not. Indeed, as the term
autonomic
(
auto
5
self;
nom
5
govern) implies, this motor subdivision of the peripheral nervous
system has a certain amount of functional independence. Te ANS is also called the
involuntary nervous system
, which reflects its subconscious control, or the
general
visceral motor system
, which indicates the location of most of its effectors.
Overview
Define autonomic nervous system and explain its relationship to the peripheral
nervous system.
Compare the somatic and autonomic nervous systems relative to effectors, efferent
pathways, and neurotransmitters released.
Compare and contrast the functions of the parasympathetic and sympathetic divisions.
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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.
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keep you on track.
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.
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