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Autonomic Nervous System

Richard M Wagers cortical at earthlink.net
Wed Oct 21 11:33:27 EST 1998

Richard Hall wrote:  [cut]
>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.

You're exactly right.  My response here, in a state of critical sleep deprivation,
was worded incorrectly.

> 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.

Yes, the extent of divergence (of preganglionic fibers) and difference in the
release of NT's create that.  The diffuse adrenergic effects of the SNS and the
specific cholinergic effects of PNS postganglionic fibers.

> - 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.

Edinger-Westphall is the rostral most representation of the IML.  Anatomists have
said it is distinct from sympathetic nuclei, but clinically we must recoginize
that the ANS exists as a homologus column of neurons, functionally linked, and
integrated with the rest of the neuraxis.  It is derived from the same embryologic
tissue.  If we consider only activation of  EW we can say it has a
pupilloconstrictor effect.  Not a big deal.  Care to guess the impact of
neocortical integration on pupillary reaction??

> 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?

Sorry to be ambiguous.  Clearly an arguement can be made that the conduction rate
of the PNS is faster than SNS.  The preganglionic fibers of both systems are B
type and the postganlionic are C type fibers.  B fibers have less of an after
depolarization sequence, therefore they can have a much faster spike train.
Because the PNS has much longer preganglionic (B fibers) it's response is more
rapid, but doesnt' last as long.  The effects of SNS activation are slower and
more diffuse/generalized (that is opposite my original incorrect post...I don't
know what I was thinking), but last longer as a consequence of catecholamine
release into the bloodstream.

> -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.
OK.  But since both SNS and PNS are tonically active, what drives the flux to meet
metabolic demand?  It is a specific function of the ANS to control the diameter
of  vessels, heart rate, respiration, etc., etc.  all to assure appropriate
delivery of fuel and substrate.  The hypothalamus is the main excitatory input
into ANS, firing especially through the mesencephalic reticular formation, which
fires to the IML bilaterally.  Why then, since these pathways exist bilaterally,
do we normally only see summation of the contralateral side?

> Oh!  What do you mean by " (probably
> A-level)"?

Nothing insulting I assure you.  There's to much of that around here already.

Thank you for your comments and corrections

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