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Module 3: The Peripheral and Autonomic Nervous Systems

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The Autonomic Nervous System

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Control of Visceral Activity
The autonomic nervous system (ANS) is that portion of the nervous system concerned with commands for smooth muscle tissue, cardiac muscle tissue, and glands.

The term visceral organs may be used to include:
• The various hollow organs of the body whose walls have smooth muscle tissue in them. For example, the blood vessels and the gut.
• The glands.

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.

ANS
The ANS is organized into two major subdivisions--the sympathetic and the parasympathetic nervous systems.

Sympathetic
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.

Parasympathetic
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.

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.

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.

Summary screen
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.