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The Autonomic Nervous System
Comparison of the Somatic and Autonomic
Nervous Systems (pp. 525–526)
ANS Divisions (pp. 526–527)
he human body is exquisitely sensitive to changes in its internal
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
, the system of motor neurons that innervates smooth and cardiac muscle
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
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
visceral motor system
, which indicates the location of most of its eﬀectors.
Deﬁne autonomic nervous system and explain its relationship to the peripheral
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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For the parasympathetic and sympathetic divisions, describe
the site of CNS origin, locations of ganglia, and general
Anatomically, the sympathetic and parasympathetic divisions
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
keep you on track.
Check Your Understanding
Name the three types of effectors of the autonomic nervous
Which relays instructions from the CNS to muscles more
quickly, the somatic nervous system or the ANS? Explain why.
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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