Chapter 14
The Autonomic Nervous System
541
14
Chapter Summary
For more chapter study tools, go to the Study Area of
MasteringA&P at
www.masteringaandp.com
.
There you will find:
Interactive Physiology
A&PFlix
Practice Anatomy Lab
PhysioEx
Videos, Practice Quizzes and Tests, MP3 Tutor Sessions,
Case Studies, and much more!
1.
Te autonomic nervous system (ANS) is the motor division of the
PNS that controls visceral activities, with the goal of maintaining
internal homeostasis.
Overview
(pp. 524–527)
Comparison of the Somatic and Autonomic Nervous
Systems
(pp. 525–526)
1.
Te somatic (voluntary) nervous system provides motor fibers to
skeletal muscles. Te autonomic (involuntary or visceral motor)
nervous system provides motor fibers to smooth and cardiac
muscles and glands.
2.
In the somatic division, a single motor neuron forms the efferent
pathway from the CNS to the effectors. Te efferent pathway
of the autonomic division consists of a two-neuron chain: the
preganglionic neuron in the CNS and the postganglionic neuron
in a ganglion.
3.
Acetylcholine, the neurotransmitter of somatic motor neurons, is
stimulatory to skeletal muscle fibers. Neurotransmitters released
by autonomic motor neurons (acetylcholine and norepinephrine)
may excite or inhibit target cells.
Nervous System II; Topic: Synaptic Transmission, pp. 8–11.
ANS Divisions
(pp. 526–527)
4.
Te ANS consists of two divisions, parasympathetic and
sympathetic, which normally exert antagonistic effects on many
of the same target organs.
5.
Te parasympathetic division (the rest-digest system) conserves
body energy and maintains body activities at basal levels.
6.
Parasympathetic effects include constricted pupils, glandular
secretion, increased digestive tract motility, and smooth muscle
activity leading to elimination of feces and urine.
7.
Te sympathetic division prepares the body for activity (the fight-
or-flight system).
8.
Sympathetic responses include dilated pupils, increased heart
rate, increased blood pressure, dilated bronchioles of the
lungs, increased blood glucose levels, and sweating. During
exercise, sympathetic vasoconstriction shunts blood from the
skin and digestive viscera to the heart, brain, and skeletal muscles.
ANS Anatomy
(pp. 527–533)
Parasympathetic (Craniosacral) Division
(pp. 527–529)
1.
Parasympathetic preganglionic neurons arise from the brain stem
and from the sacral (S
2
–S
4
) region of the spinal cord.
2.
Preganglionic fibers synapse with postganglionic neurons in
terminal ganglia located in (intramural ganglia) or close to their
effector organs. Preganglionic fibers are long; postganglionic
fibers are short.
3.
Cranial fibers arise in the brain stem nuclei of cranial nerves
III, VII, IX, and X and synapse in ganglia of the head, thorax,
and abdomen. Te vagus nerves serve virtually all organs of the
thoracic and abdominal cavities.
4.
Sacral fibers (S
2
–S
4
) issue from the lateral region of the cord and
form pelvic splanchnic nerves that innervate the pelvic viscera. Te
preganglionic axons do not travel within rami communicantes.
Sympathetic (Thoracolumbar) Division
(pp. 529–533)
5.
Preganglionic sympathetic neurons arise from the lateral horns of
the spinal cord from the level of ±
1
through L
2
.
6.
Preganglionic axons leave the cord via white rami
communicantes and enter the sympathetic trunk ganglia in the
sympathetic trunk. An axon may synapse in a trunk ganglion
at the same or at a different level, or it may issue from the
sympathetic trunk without synapsing. Preganglionic fibers are
short; postganglionic fibers are long.
7.
When the synapse occurs in a trunk ganglion, the postganglionic
fiber may enter the spinal nerve ramus via the gray ramus
communicans to travel to the body periphery. Postganglionic
fibers issuing from the cervical ganglia also serve visceral organs
and blood vessels of the head, neck, and thorax.
8.
When synapses do not occur in the trunk ganglia, the
preganglionic fibers form splanchnic nerves (thoracic, lumbar,
and sacral). Most splanchnic nerve fibers synapse in collateral
ganglia, and the postganglionic fibers serve the abdominal
viscera. Exceptions are that (1) some splanchnic nerve fibers
synapse with cells of the adrenal medulla, and (2) some
lumbar and sacral splanchnic nerve fibers
do
synapse in trunk
ganglia.
Visceral Reflexes
(p. 533)
9.
Visceral reflex arcs have the same components as somatic reflexes:
receptor, sensory neuron, integration center, motor neurons,
effector.
10.
Cell bodies of visceral sensory neurons are located in dorsal root
ganglia, sensory ganglia of cranial nerves, or autonomic ganglia.
Visceral afferents are located in spinal nerves and in virtually all
autonomic nerves.
ANS Physiology
(pp. 533–539)
Neurotransmitters and Receptors
(pp. 533–535)
1.
Autonomic motor neurons release two major neurotransmitters,
acetylcholine (ACh) and norepinephrine (NE). Based on the
neurotransmitter they release, fibers are classified as cholinergic
(ACh) or adrenergic (NE).
2.
ACh is released by all preganglionic fibers and all
parasympathetic postganglionic fibers. NE is released by all
sympathetic postganglionic fibers except those serving the sweat
glands of the skin.
3.
Neurotransmitter effects depend on the receptors to which
the neurotransmitter binds. Cholinergic (ACh) receptors are
classified as nicotinic or muscarinic. Adrenergic (NE) receptors
are classified as
a
1
or
a
2
, or β
1
, β
2
, or β
3
.
Nervous System II; Topic: Synaptic Transmission, pp. 8–11, 14.
previous page 575 Human Anatomy and Physiology (9th ed ) 2012 read online next page 577 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off