Nervous system, autonomic definition - medical term
Part of the nervous system
that was once thought to be functionally independent of the brain.
The autonomic nervous system regulates key functions of the body
including the activity of the heart muscle (see below), the smooth
muscles (e.g., the muscles of the intestinal tract), and the glands.
The autonomic nervous system has two divisions: (1) the
sympathetic
nervous system that accelerates the heart rate,
constricts blood vessels, and raises blood pressure; and (2) the
parasympathetic
nervous system slows the heart rate, increases
intestinal and gland activity, and relaxes sphincter muscles.
The autonomic system, together with the SA (sinoatrial) and AV
(atrioventricular) nodes, is a major element in the
cardiac
conduction system
, the system that controls the heart rate. This
stunningly designed system generates electrical impulses and conducts
them throughout the muscle of the heart, stimulating the heart to
contract and pump blood.
The
SA node
is the heart's natural pacemaker. The SA
node consists of a cluster of cells that are situated in the upper
part of the wall of the right atrium (the right upper chamber of the
heart). The electrical impulses are generated there. The SA node is
also called the sinus node.
The electrical signal generated by the SA node moves from
cell to cell down through the heart until it reaches the
AV node
, a cluster of cells
situated in the center of the heart between the atria and ventricles.
The AV node serves as a gate that slows the electrical current
before the signal is permitted to pass down through to the
ventricles. This delay ensures that the atria have a chance to fully
contract before the ventricles are stimulated. After passing the AV
node, the electrical current travels to the ventricles along special
fibers embedded in the walls of the lower part of the heart.
The
autonomic nervous system
controls the firing of the SA
node
to trigger the start of the cardiac cycle. The autonomic nervous
system can transmit a message quickly to the SA node so it in turn
can increase the heart rate to twice normal within only 3 to 5
seconds. This quick response is important during exercise when the
heart has to increase its beating speed to keep up with the body's
increased demand for oxygen.
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