Chapter 14
The Autonomic Nervous System
539
14
blocker. Why might his doctor have done this? What does a
beta-blocker do?
For answers, see Appendix H.
Developmental Aspects
of the ANS
Describe several effects of aging on the autonomic nervous
system.
ANS preganglionic neurons derive from the embryonic
neu-
ral tube
, as do somatic motor neurons. ANS structures in the
PNS—postganglionic neurons, the adrenal medulla, and all au-
tonomic ganglia—derive from the
neural crest
(along with all
sensory neurons) (see Figure 12.33,
2
).
Neural crest cells reach their ultimate destinations by migrat-
ing along growing axons. Forming ganglia receive axons from
preganglionic neurons in the spinal cord or brain and send their
axons to synapse with their effector cells in the body periphery.
Tis process depends on the presence of
nerve growth factor
,
and is guided by a number of signaling chemicals similar to
those acting in the CNS.
During youth, impairments of ANS function are usually due
to injuries to the spinal cord or autonomic nerves. In old age,
the efficiency of the ANS declines. At least part of the problem is
due to structural changes in some preganglionic axon terminals,
which become congested with neurofilaments.
Many elderly people complain of constipation (a result of
reduced gastrointestinal tract motility), and of dry eyes and fre-
quent eye infections (both a result of a diminished ability to
form tears). Additionally, when they stand up they may have
fainting episodes due to
orthostatic hypotension
(
ortho
5
straight;
stat
5
standing), low blood pressure following changes
in position. Orthostatic hypotension occurs because aging pres-
sure receptors become less responsive to changes in blood pres-
sure, and aging cardiovascular centers fail to maintain healthy
blood pressure.
Tese problems are distressing, but not usually life threaten-
ing, and most can be managed by lifestyle changes or artificial
aids. For example, changing position slowly gives the sympa-
thetic nervous system time to adjust blood pressure, and eye
drops (artificial tears) can alleviate the dry-eye problem.
Check Your Understanding
10.
Which embryonic structure gives rise to both the autonomic
ganglia and the adrenal medulla?
For answers, see Appendix H.
In this chapter, we have described the structure and function
of the ANS, one arm of the motor division of the peripheral
nervous system. Because virtually every organ system still to be
considered depends on autonomic controls, you will be hearing
more about the ANS in chapters that follow. Now that we have
explored most of the nervous system, this is a good time to ex-
amine how it interacts with the rest of the body as summarized
in
System Connections
, p. 540.
8.
Which part of the brain is the main integration center of
the ANS? Which part exerts the most direct influence over
autonomic functions?
For answers, see Appendix H.
Homeostatic Imbalances
of the ANS
Explain the relationship of some types of hypertension,
Raynaud’s disease, and autonomic dysreflexia to disorders
of autonomic function.
Te ANS is involved in nearly every important process that goes
on in the body, so it is not surprising that abnormalities of auto-
nomic function can have far-reaching effects. Most autonomic
disorders reflect exaggerated or deficient controls of smooth
muscle activity. Te most devastating involve blood vessels and
include conditions such as hypertension, Raynaud’s disease, and
autonomic dysreflexia.
Hypertension
, or high blood pressure, may result from an
overactive sympathetic vasoconstrictor response promoted
by continuous high levels of stress. Hypertension is always se-
rious because it forces the heart to work harder, which may
precipitate heart disease, and increases the wear and tear on
artery walls. Hypertension is sometimes treated with adrener-
gic receptor–blocking drugs that counteract the effects of the
sympathetic nervous system on the cardiovascular system. We
will discuss hypertension in more detail in Chapter 19.
Raynaud’s disease
is characterized by intermittent attacks
causing the skin of the fingers and toes to become pale, then
cyanotic (bluish) and painful. Commonly provoked by expo-
sure to cold or emotional stress, it is an exaggerated vasocon-
striction response. Te severity of Raynaud’s disease ranges
from merely uncomfortable to severe blood vessel constric-
tion that causes ischemia and gangrene (tissue death).
Vasodilators (for example, adrenergic blockers) usually
suffice. ±o treat very severe cases, preganglionic sympathetic
fibers serving the affected regions are severed (a procedure
called
sympathectomy
). Te involved vessels then dilate, re-
establishing adequate blood flow to the region.
Autonomic dysreflexia
is a life-threatening condition involving
uncontrolled activation of autonomic neurons. It occurs in a
majority of individuals with quadriplegia and in others with
spinal cord injuries above the ±
6
level, usually in the first year
a²er injury. Te usual trigger is a painful stimulus to the skin
or an overfilled visceral organ, such as the urinary bladder.
Arterial blood pressure skyrockets to life-threatening levels,
which may rupture a blood vessel in the brain, precipitating
stroke. Symptoms include headache, flushed face, sweating
above the level of the injury, and cold, clammy skin below. Te
precise mechanism of autonomic dysreflexia is not yet clear.
Check Your Understanding
9.
Jackson works long, stress-filled shifts as an air traffic
controller at a busy airport. His doctor has prescribed a beta-
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