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Two Major Subdivisions - Basic Human Physiology

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(1) The various hollow organs of the body whose walls have smooth muscle
tissue in them. Examples are the blood vessels and the gut.
(2) The glands.
b. The visceral organs are innervated by the ANS. This results in a "visceral
motor system." For most of us, the control of the visceral organs is automatic, that is,
without conscious control. However, recent research demonstrates that conscious
control of some of the visceral organs is possible after proper training.
12-11. TWO MAJOR SUBDIVISIONS
The ANS is organized into two major subdivisions--the sympathetic and the
parasympathetic nervous systems.
a. The neurons of the sympathetic nervous system originate in the thoracic and
lumbar regions of the spinal cord. Thus, it is also known as the thoraco-lumbar outflow.
b. Some of the neurons of the parasympathetic nervous system originate in
nuclei of the brainstem. Others originate in the sacral region of the spinal cord. Thus,
the parasympathetic nervous system is also known as the cranio-sacral outflow.
c. In the ANS, there are always two neurons (one after the other) connecting
the CNS with the visceral organ. The cell bodies of the second neurons form a
collection outside the CNS, called a ganglion. Processes of these postganglionic
neurons extend to the visceral organs. Those processes going to peripheral visceral
organs are included with the peripheral nerves.
12-12. EQUILIBRIUM
Under ordinary circumstances, the sympathetic and parasympathetic nervous
system have opposite effects upon any given visceral organ. That is, one system will
stimulate the organ to action, and the other system will inhibit it. The interplay of these
two systems helps visceral organs to function within a stable equilibrium. This tendency
to produce an equilibrium is called homeostasis.
12-13. RESPONSE TO STRESS
Under conditions of stress, the sympathetic nervous system produces a
"fight-or-flight" response. In other words, it mobilizes all of the energy producing
structures of the body. Simultaneously, it inhibits those structures that do not contribute
to the mobilization of energy. For example, the sympathetic nervous system makes the
heart beat faster. Later, as equilibrium is restored, the parasympathetic nervous system
slows the heart down.
MD0007
12-11



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