Autonomic Nervous System

Richard M Wagers cortical-NOT at earthlink.net
Tue Oct 20 23:45:49 EST 1998


I didn't want to comment on the original post.  Though good questions, this
topic is _huge_ and is difficult to write briefly on.  But I feel someone
needs to add _something?_ to the post by "rand at mindless.com".  I mean no
disrespect, only that I may help to clarify it.  Maybe someone else will help
clarify _my_ response.

> {original question}

> >So why do the sympathetic neurones have a long preganglionic neurone and a
> short postganglionic neurone, >and why is the ganglion found near to (or in
> the) effector?? Also why do parasympathetic fibres only leave >from the
> base of the brain and the spine (forgot the proper names!)??

> rand at mindless.com wrote:
> I think the parasympathetic nerves only leave from the base (sacral?)
> area of the base and spine because they don't need the body to respond
> to anything, thus no receptors.

The question could be answered from a clinical perspective or an anatomical
one.  Very different really.  Anatomically, you need only study the
embryological development to find your answers.  Simple.  But clinical...
The question was asked in a simple way and a simple answer was requested.
Actually, the simplest answer I can give (to the difference in length of pre-
and post-ganglionic fibers)  is "specificity".  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.
There certainly are receptors for both SNS and PNS postganglionic neurons and
there certainly are functions of both systems.  Broadly, the functions counter
one another and therefore act to balance the system.  They differ in a few
ways (types of neurotransmitters/receptors utilized, rate of effect, etc.),
but are also similar in a few ways.
The similarities are often left out when the subject is learned, but are
clinically (at least) important.  One example: all of the ganglia of the ANS
were formed from the same embryologic tissue, they are forever homologously
related and are forever linked functionally.

Some people, who thouroughly understand a topic, have the ability to summarize
beautifully and concisely all the "important" aspects of the topic.  I
certainly haven't done that here, but maybe one of you will do it.

Richard





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