Human Physiology, 14th edition (2016)

The Autonomic Nervous System 261

Organs Type of Receptors Reflex effects

Lungs Stretch receptors Further inhalation inhibited; increase in cardiac rate and vasodilation stimulated

Type J receptors Stimulated by pulmonary congestion—produces feelings of breathlessness and

causes a reflex fall in cardiac rate and blood pressure

Aorta Chemoreceptors Stimulated by rise in CO 2 and fall in O 2 —produces increased rate of breathing, rise in

heart rate, and vasoconstriction

Baroreceptors Stimulated by increased blood pressure—produces a reflex decrease in heart rate

Heart Atrial stretch receptors Antidiuretic hormone secretion inhibited, thus increasing the volume of urine excreted

Stretch receptors in ventricles Produces a reflex decrease in heart rate and vasodilation

Gastrointestinal tract Stretch receptors Feelings of satiety, discomfort, and pain

Table 9.8 | Sensory Receptors Stimulate Afferent Fibers in the Vagus, Which Transmit to
the Medulla Oblongata and Cause Autonomic Reflexes

CLINICAL APPLICATION

The somatic motor system is clearly under voluntary con-

trol, whereas the autonomic nervous system operates

without conscious involvement. However, scientists have

verified that, using biofeedback techniques, a person can

be trained to have limited but significant control over some

Clinical Investigation CLUES

Sofia monitored her pulse rate in an attempt to calm her

anxiety.

• How does the autonomic system control the pulse
  rate?


• What technique is she employing, and can it work?

The limbic system is a group of fiber tracts and nuclei that
form a ring around the brain stem (chapter 8, section 8.2). It
includes the cingulate gyrus of the cerebral cortex, the hypothal-
amus, the fornix (a fiber tract), the hippocampus, and the amyg-
daloid nucleus (see fig. 8.15). The limbic system is involved in
basic emotional drives, such as anger, fear, sex, and hunger. The
involvement of the limbic system with the control of autonomic
function is responsible for the visceral responses that are char-
acteristic of these emotional states. Blushing, pallor, fainting,
breaking out in a cold sweat, a racing heartbeat, and “butterflies
in the stomach” are only some of the many visceral reactions
that accompany emotions as a result of autonomic activation.
The autonomic correlates of motion sickness—nausea,
sweating, and cardiovascular changes—are eliminated by cut-
ting the motor tracts of the cerebellum. This demonstrates that
impulses from the cerebellum to the medulla oblongata influ-
ence activity of the autonomic nervous system. Experimental
and clinical observations have also demonstrated that the frontal
and temporal lobes of the cerebral cortex influence lower brain
areas as part of their involvement in emotion and personality.
Studies indicate that aging is associated with increased lev-
els of sympathetic nervous system activity. This represents an
increased level of tonic sympathetic tone in healthy adults, not
an increased response to stress. It has been suggested that the
higher tonic levels of sympathetic nerve activity may promote
increased catabolism, generating heat and helping to combat
the greater amounts of adipose tissue in the elderly. However,
chronically elevated sympathetic tone may increase the risk of
hypertension and cardiovascular diseases.

| CHECKPOINT

6. Define adrenergic and cholinergic and use these
   terms to describe the neurotransmitters of different
   autonomic nerve fibers.


7. List the effects of sympathoadrenal stimulation on
   different effector organs. In each case, indicate whether
   the effect is due to alpha- or beta-receptor stimulation.


8. Describe the effects of the drug atropine and
   explain these effects in terms of the actions of the
   parasympathetic system.


9. Explain how the effects of the sympathetic and
   parasympathetic systems can be antagonistic,
   cooperative, or complementary. Include specific
   examples of these different types of effects in your
   explanation.

autonomic responses. This training involves the use of

devices that monitor body changes; for example, devices

can monitor brain waves (electroencephalograph or EEG),

muscle tension (electromyography, or EMG), skin conductiv-

ity (affected by sweating and monitored by GSR—galvanic

skin response), and others. Autonomic functions may be

trained to help control blood pressure, chronic pain, head-

ache, urinary incontinence, anxiety, and other conditions.