Autonomic Nervous System

Richard Hall rhall at
Wed Oct 21 08:41:52 EST 1998

At 11:45 PM -0500 10/20/98, Richard M Wagers wrote:

>[cut]  The autonomic nervous system
>(ANS) is that portion of the nervous system which automatically regulates the
>vital functions of the body.  The sympathetic nervous system (SNS) is very
>fast and specific, contrasted with the slower, less specific parasympatheic
>n.s. (PNS).  PNS is sometimes (probably A-level) refered to as "craniosacral"
>because it arises from the brain stem nuclei (CN 5,7,9,10) and the sacral
>cord.  The primary nerons of the SNS are located in the thoracolumbar cord,
>specifically in the IML.

I believe the sympathetic division of the ANS is considered to be LESS
specific than the parasympathetic division.

1.  parasympathetic post ganglionics converge on specific target tissues
while sympathetic fibers tyically diverge and innervate larger regions.
True the vagus meanders, but it's pattern of innervation is very focused.
Sympathetic stimulation effects seem to linger, while parasympathetic tone
requires rapid adjustments.  However awkwardly stated: the general effect
of sympathetic activity has a longer time course and broader scope than
parasympathetic activity.

2.  The spinal orgins (within the lateral horn of the cord) of sympathetic
fibers reflects the somatic organization of the body and is consistent with
the effects of sympathetic activity on peripheral, superficial blood flow
and eccrine functions as well as general effects on visceral functions.
The craniosacral PNS has no periperal, superficial innervation pattern and
instead has a primarily visceral innervation pattern.

3.  Sympathetic activities tend to involve energy expenditures,
mobilization of energy reserves, augmenting blood flow, enhancing pulmonary
function, and suppression of parasympathetic activity.  These are broad
spectrum, low frequency..petal to the metal types of functions.
Parasympathetic activity seems capable of more selective actions with more
precise (high frequency) modifications.

- Consider the roles of sympathetic and parasympathetic activity on the
pupillary reflex.  Absence of parasympathetic tone results in a slow
dilation of the pupil.  Illumination of the retina results in abrupt
pupillary constriction.  Guess which is controlled primarily by
parasympathetic neurons.

-Consider heart rate.  Vagal tone has immediate effect while elevated
sympathetic activity has long lasting effects primarily on cardiac muscle
contractility.  Indeed, studies of heart rate variability assume that low
frequency variations reflect sympathetic activity while high frequency
variations reflect parasympathetic activity.

4.  As to the speed of response, are you referring the the speed of
conduction in pre and post ganglionics?  Or are your referring to the time
of response? Or some other parameter?  Oh!  What do you mean by " (probably


Richard Hall
Comparative Animal Physiologist
Division of Sciences and Mathematics
University of the Virgin Islands
St. Thomas, USVI  00802

rhall at

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