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| The
Autonomic Nervous System (ANS) maintains primary
neural control of the heart |
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| Your
autonomic nervous system (ANS) is the part of your nervous
system that functions to sustain your life by controlling
your heart, lungs, digestive system, blood pressure, immune
system, certain of your reflexes, fluid balance, pupil diameter,
sweating, and sexual function.
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The
ANS influences every cell in the body through its two
branches: the sympathetic nervous system (sympathetics)
and the parasympathetic nervous system (parasympathetics).
In general, the sympathetics are responsible for mediating
energy expenditure, while the parasympathetics are responsible
for energy conservation and restoration. For example,
the sympathetics mediate the "fight or flight"
response and the body's response to stress, pain, and
cold. Thus, the sympathetics cause higher heart rates
and respiratory rates, shunting blood from the extremities
to core organs and muscles (e.g., running or shivering),
etc.
The
parasympathetics mediate resting states after meals and
at night, digestion and nutrient storage, and recovery
states by helping to coordinate immune responses and healing.
Thus, the parasympathetics cause slower heart rates and
respiratory rates, sleep, increased gastrointestinal track
motility, increased peripheral vascular flow, blood flow
to all cells, liver and kidneys, and venous return to
the heart. The sympathetic and parasympathetic branches
of the ANS work together to maintain homeostasis.
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When
the ANS affects a change in the body (e.g., heart rate
or respiratory rate), it works only to cause the change.
The ANS then returns to its baseline state. So, periodic
excursions in one or the other branch from baseline are
normal and expected as long as the ANS returns to baseline
in a timely manner. Persistently elevated levels of tone
in one or the other branch are not healthy.
The
general action of each of the branches of the ANS is to
oppose the other. As one branch begins to work the other
branch begins to return it to baseline. Consequently,
persistently elevated tone in one branch can result in
a persistently depressed tone in the other. This only
serves to compound an unhealthy situation. So, balance
between the branches is as important as overall tone in
each of the branches.
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It
has been learned that the parasympathetic nervous system
can change faster than the sympathetic nervous system. Thus,
as the sympathetics start to mediate a stress response the
parasympathetics immediately begin to counter it. If the
parasympathetics were not faster than the sympathetics,
then any stress response could send the heart into tachycardia
and onto ventricular fibrillation before the parasympathetics
could act to prevent it. The parasympathetics, through the
Vagus, are the main controlling influence on respiratory
activity. Increases in respiratory analysis are caused by
increases in parasympathetic tone. Parasympathetic input
to the heart is through fibers that synapse deep in the
myocardium. Sympathetic influence on the heart is through
surface synapses. Due to this arrangement the parasympathetics
are more sensitive to heart damage (i.e., infarct, ischemia,
or cardiomyopathies). Since the parasympathetics are faster
to respond, it is usually the branch that is first to indicate
changes in health status anywhere in the body. |
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