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CHAPTER
14
The Autonomic Nervous System
Objectives
Overview
1. Define autonomic nervous system and explain its relationship to the peripheral nervous
system.
2. Compare the somatic and autonomic nervous systems relative to effectors, efferent
pathways, and neurotransmitters released.
3. Compare and contrast the functions of the parasympathetic and sympathetic divisions.
ANS Anatomy
4. For the parasympathetic and sympathetic divisions, describe the site of CNS origin,
locations of ganglia, and general fiber pathways.
ANS Physiology
5. Define cholinergic and adrenergic fibers, and list the different types of their receptors.
6. Describe the clinical importance of drugs that mimic or inhibit adrenergic or cholinergic
effects.
7. State the effects of the parasympathetic and sympathetic divisions on the following
organs: heart, blood vessels, gastrointestinal tract, lungs, adrenal medulla, and external
genitalia.
8. Describe autonomic nervous system controls.
Homeostatic Imbalances of the ANS
9. Explain the relationship of some types of hypertension, Raynaud’s disease, and
autonomic dysreflexia to disorders of autonomic function.
Developmental Aspects of the ANS
10. Describe several effects of aging on the autonomic nervous system.
Lecture Outline
I. Overview (pp. 524–527, Figs. 14.1–14.2)
A. Comparison of the Somatic and Autonomic Nervous Systems (pp. 525–526; Figs.
14.1–14.2)
1. The somatic nervous system stimulates skeletal muscles, while the ANS innervates
cardiac and smooth muscle and glands.
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2. In the somatic nervous system, the cell bodies of the neurons are in the spinal cord and
their axons extend to the skeletal muscles they innervate.
3. The ANS consists of a two-neuron chain in which the cell body of the first neuron, the
preganglionic neuron, resides in the spinal cord, and synapses with a second neuron,
the postganglionic neuron, reside within an autonomic ganglion outside the CNS.
4. The neurotransmitter released by the somatic motor neurons is acetylcholine, which
always has an excitatory effect; the neurotransmitters released by the ANS are epinephrine and acetylcholine, and both may have either an excitatory or an inhibitory
effect.
5. There is overlap between the somatic and autonomic nervous systems, and most body
responses to changing internal and external stimuli involve both skeletal muscle
activity and visceral organ responses.
B. ANS Divisions (pp. 526–527; Fig. 14.3)
1. There are two ANS divisions, both usually serving the same visceral organs, but
having opposing effects.
a. The parasympathetic division keeps body energy use as low as possible while
directing digestion and elimination activities.
b. The sympathetic division prepares the body to respond to an emergency or threatening situation (or vigorous exercise) by promoting essential cardiovascular and liver
(and other) effects, while inhibiting nonessential effects, such as GI motility.
II. ANS Anatomy (pp. 527–533; Figs. 14.3–14.7; Table 14.1)
A. Sympathetic and parasympathetic divisions differ in the site of origin, relative lengths of
fibers, and location of ganglia (p. 527, Fig. 14.3).
B. Parasympathetic (Craniosacral) Division (pp. 527–529; Figs. 14.3–14.4; Table 14.1)
1. The preganglionic axons extend from the CNS nearly all the way to the structures
to be innervated, where they synapse with postganglionic neurons in the terminal
ganglia.
2. The cranial outflow consists of preganglionic fibers that run in the oculomotor, facial,
glossopharyngeal, and vagus cranial nerves.
3. The rest of the large intestine and the pelvic organs are served by the sacral outflow,
which arises from neurons located in the lateral gray matter of spinal cord segments
S2–S4.
C. Sympathetic (Thoracolumbar) Division (pp. 529–533; Figs. 14.3, 14.5–14.6; Table 14.1)
1. The sympathetic division supplies the visceral organs in the internal body cavities but
also all visceral structures in the somatic part of the body.
2. When synapses are made in chain ganglia, the postganglionic axons enter the ventral
(or dorsal) ramus of the adjoining spinal nerves by way of communicating branches
called gray rami communicantes.
3. The preganglionic fibers from T5 down synapse in collateral ganglia; thus these fibers
enter and leave the sympathetic chains without synapsing.
4. Some fibers of the thoracic splanchnic nerves terminate by synapsing with the
hormone-producing medullary cells of the adrenal cortex.
D. The visceral sensory neurons are the first link in autonomic reflexes, sending information
concerning chemical changes, stretch, and irritation of the viscera (p. 533; Fig. 14.7).
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III. ANS Physiology (pp. 533–539; Fig. 14.8; Tables 14.2–14.4)
A. Neurotransmitters and Receptors (pp. 533–535; Table 14.2)
1. Cholinergic receptors, such as nicotinic and muscarinic receptors, bind acetylcholine.
2. Adrenergic receptors alpha and beta bind to epinephrine.
B. Knowing the locations of the cholinergic and adrenergic receptor subtypes allows
specific drugs to be prescribed to obtain desired inhibitory or stimulatory effects on target
organs (p. 535; Table 14.3).
C. Interactions of the Autonomic Divisions (pp. 535–537; Table 14.4)
1. Most visceral organs receive dual innervation by both ANS divisions, allowing for a
dynamic antagonism to exist between the divisions and precise control of visceral
activity.
a. The sympathetic division increases heart rate, dilates airways, and inhibits digestion
and elimination while the body is under stress.
b. After the stress has passed, the parasympathetic division returns heart rate and
airway diameter to normal and allows digestion and elimination to resume.
2. Sympathetic tone throughout the vascular system allows the firing rate of sympathetic
neurons to control the diameter of blood vessels, regulating systemic blood pressure.
3. Parasympathetic tone is usually dominant in the heart, digestive system, and urinary
tracts, maintaining normal homeostatic levels of function unless overridden by the
sympathetic system during stress.
4. Sympathetic and parasympathetic divisions show a cooperative effect in the genitalia
during sexual excitement and release.
5. The sympathetic system has a unique role in control of the adrenal medulla, sweat
glands, arrector pili muscles of the skin, the kidneys, and most blood vessels.
6. The parasympathetic division exerts short-lived, localized control over its effectors,
whereas effects of the sympathetic division are persistent and widespread.
D. Control of Autonomic Functioning (pp. 538–539, Fig. 14.8)
1. The brain stem appears to exert the most direct influence over autonomic functions.
2. The hypothalamus is the main integration center for the autonomic nervous system.
3. Cortical or voluntary control of the autonomic nervous system may be possible.
4. Biofeedback training may enable a person to alter some involuntary functions.
IV. Homeostatic Imbalances of the ANS (p. 539)
A. Most autonomic disorders reflect exaggerated or deficient control of smooth muscle
(p. 539).
1. Hypertension, or high blood pressure, may result from an overactive sympathetic
vasoconstrictor response due to continuous high levels of stress.
2. Raynaud’s disease is characterized by intermittent attacks causing the skin of the
fingers and the toes to become pale, then cyanotic and painful.
3. Autonomic dysreflexia is a life-threatening condition involving uncontrolled activation of both somatic and autonomic motor neurons.
V. Developmental Aspects of the ANS (p. 539)
A. Embryonic and fetal development of the autonomic nervous system (p. 539).
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The Autonomic Nervous System
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1. ANS preganglionic neurons and somatic motor neurons derive from the embryonic
neural tube.
2. ANS structures found in the PNS (postganglionic neurons, adrenal medulla, and all
autonomic ganglia) derive from the neural crest.
3. Nerve growth factor is a protein secreted by target cells of the postganglionic axons
that directs the growth of axons toward their targets.
B. In youth, ANS dysfunction is usually due to injury to the spinal cord or autonomic nerves
(p. 539).
C. In old age the efficiency of the ANS begins to decline, partly due to structural changes of
some preganglionic axon terminals (p. 539).
1. Typical changes include constipation, dry eyes and frequent eye infections, and
orthostatic hypotension.
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