538
UNIT 3
Regulation and Integration of the Body
14
Hypothalamic Controls
As we noted, the hypothalamus is the main integration center of
the autonomic nervous system. In general, anterior hypothalamic
regions direct parasympathetic functions, and posterior areas di-
rect sympathetic functions. Hypothalamic centers exert their ef-
fects both directly and via relays through the
reticular formation
,
which in turn influences the preganglionic motor neurons in the
brain stem and spinal cord (Figure 14.8). Te hypothalamus, act-
ing through the ANS, coordinates heart activity, blood pressure,
body temperature, water balance, and endocrine activity.
Te hypothalamus also mediates our reactions to fear via
its associations with the amygdala and the periaqueductal gray
matter. Emotional responses of the limbic system of the cere-
brum to danger and stress signal the hypothalamus to activate
the sympathetic system to fight-or-flight status. In this way, the
hypothalamus serves as the keystone of the emotional and vis-
ceral brain. Trough its centers, emotions influence ANS func-
tion and behavior.
Cortical Controls
Originally, scientists believed the ANS was not subject to vol-
untary controls. However, we have all had occasions when just
remembering a frightening event made our heart race (sympa-
thetic response) or the thought of a favorite food, pecan pie for
example, made our mouth water (parasympathetic response).
Tese inputs converge on the hypothalamus through its con-
nections to the limbic lobe.
Additionally, studies have shown that voluntary cortical con-
trol of visceral activities is possible—a capability untapped by
most people.
Influence of Biofeedback on Autonomic Function
Biofeed-
back
is a way of becoming aware of physiological conditions
(such as heart rate and blood pressure), with the goal of being
able to influence them consciously. During
biofeedback train-
ing
, subjects are connected to monitoring devices that detect
and amplify changes in these physiological processes. Te feed-
back comes in the form of flashing lights or audible tones.
Subjects try to alter or control some “involuntary” function
by concentrating on calming, pleasant thoughts. Te monitor
allows them to identify changes in the desired direction, so they
can recognize the feelings associated with these changes and
learn to produce the changes at will.
Biofeedback techniques have been successful in helping indi-
viduals plagued by migraine headaches. Some cardiac patients also
use them to manage stress and reduce their risk of heart attack.
However, biofeedback training is time-consuming and o±en frus-
trating, and the training equipment is expensive and difficult to use.
Check Your Understanding
6.
Name the division of the ANS that does each of the following:
increases digestive activity; increases blood pressure; dilates
bronchioles; decreases heart rate; stimulates the adrenal
medulla to release its hormones; causes ejaculation.
7.
Would you find nicotinic receptors on skeletal muscle?
Smooth muscle? Eccrine sweat glands? The adrenal medulla?
CNS neurons?
Control of Autonomic Function
Although the ANS is not usually considered to be under vol-
untary control, its activity is regulated by CNS controls in the
spinal cord, brain stem, hypothalamus, and cerebral cortex
(Figure 14.8)
. In general, the hypothalamus is the integrative
center at the top of the ANS control hierarchy. From there,
orders flow to lower and lower CNS centers for execution.
Although the cerebral cortex may modify the workings of the
ANS, it does so at the subconscious level and by acting through
limbic system structures on hypothalamic centers.
Brain Stem and Spinal Cord Controls
Te hypothalamus is the “boss,” but the brain stem reticular for-
mation appears to exert the most
direct
influence over autonomic
functions (see Figure 12.14 on p. 446). For example, certain mo-
tor centers in the ventrolateral medulla (
cardiac
and
vasomotor cen-
ters
) reflexively regulate heart rate and blood vessel diameter. Other
medullary regions oversee gastrointestinal activities. Most sensory
impulses involved in these autonomic reflexes reach the brain stem
via vagus nerve afferents. Midbrain centers (
oculomotor nuclei
) con-
trol the muscles concerned with pupil diameter and lens focus.
Defecation and micturition (urination) reflexes that empty
the rectum and urinary bladder are integrated at the spinal cord
level but are subject to conscious inhibition. We will describe all
of these autonomic reflexes in later chapters in relation to the
organ systems they serve.
Cerebral cortex
(frontal lobe)
Limbic system
(emotional input)
Communication at
subconscious level
Hypothalamus
The “boss”: Overall
integration of ANS
Spinal cord
Reflexes for urination,
defecation, erection,
and ejaculation
Brain stem
(reticular formation, etc.)
Regulates pupil size, heart,
blood pressure, airflow,
salivation, etc.
Figure 14.8
Levels of ANS control.
The hypothalamus stands at the
top of the control hierarchy as the integrator of ANS activity, but it is in-
fluenced by subconscious cerebral inputs via limbic system connections.
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