[Neuroscience] Re: Are the dorsal roots parts of the somatic nervous system?

r norman via neur-sci%40net.bio.net (by r_s_norman from comcast.net)
Sat Jul 18 10:32:21 EST 2009

On Sat, 18 Jul 2009 08:05:58 -0700 (PDT), GreenXenon
<glucegen1x from gmail.com> wrote:

>On Jul 18, 5:13 am, r norman <r_s_nor... from comcast.net> wrote:
>> On Fri, 17 Jul 2009 20:57:44 -0700 (PDT), GreenXenon
>> <glucege... from gmail.com> wrote:
>> >Hi:
>> >Are the dorsal roots parts of the somatic nervous system?
>> The distinction between somatic and autonomic is primarily a
>> functional one.  Do you say that the spinal cord is part of the
>> somatic nervous system?
>The spinal cord is part of the central nervous system. The somatic
>nervous system is a division of the peripheral nervous system. So
>*no*, the spinal cord is *not* part of the somatic nervous system.
>However, the dorsal roots are part of the peripheral nervous system,
>so I was wondering if they are also part of the somatic division of
>the peripheral nervous system.
>> Both somatic and autonomic use the spinal
>> cord and both somatic and autonomic use the dorsal roots.   So the
>> dorsal roots are sensory, both somatic and autonomic.  However the
>> sensory side of the autonomic system at the spinal level  is often
>> ignored in elementary treatments, leaving the impression that the
>> dorsal roots are  somatic.
>Are there any purely-somatic afferent nerve roots in the human body?

I always found that defining "somatic" to be strictly peripheral a
rather questionable and arbitrary notion.  Does a single sensory
neuron entering the dorsal root suddenly become "non-somatic" the
instant it plunges into the spinal cord?  Does a motor neuron axon
suddenly acquire the "somatic" label only as it enters the ventral
root?   Two microns this way a cell is somatic, two microns the other
way the same cell is not.

However, to answer your question, the facial and trigeminal cranial
nerves are generally considered to be somatic sensory without
autonomic sensory function and the special sensory cranial nerves:
vision, olfaction, audition and balance, taste might be considered
sort of somatic.  Of course these are not spinal roots. 

There are sensory cells in the gut which are considered autonomic and
enter the spinal cord through the dorsal roots but I really don't know
exactly how they are distributed across each level of the spinal

It is also difficult to make absolute generalizations about anything
in biology or physiology.  There are lots of very small nerve fibers
all over the place, there are efferent fibers in supposedly purely
sensory nerves and there are reports about dorsal root efferents and
ventral root afferents.

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