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Perhaps a dumb question.. but can I have yout input ?

kevin k-mck*enna at nwu.edu
Mon Jun 9 02:58:27 EST 1997

In article <01bc7346$ec6b6f20$bcd092cf at default>, "RANDY MORTELL"
<R.C.M. at worldnet.att.net> wrote:

>         Hi,
> I was having a discussion with a couple of friends and we disagreed on the
> following:
> 1). The brain originates / controls sexual urges.
> 2). The brain originates / controls all body functions (voluntary and
> involantary functions)  ie.. twitches etc....
> I really appreciate any input from everyone out there, particulary those
> with a medical backround.

1). Sexual urges originate in and are controlled by the central nervous
system. No question about it. However, these CNS commands are under the
influence of gonadal hormones. This depends a great deal on the species. In
many animals, females will not mate unless in estrous (heat) and males will
not mate if they have low testosterone levels (due to castration or
seasonal factors in some species). In the human, hormones do play a role,
but it is not as important. Another thing to bear in mind is that the
release of hormones from he gonads is also controlled and coordinated by
the CNS. There is an elaborate and interesting interaction between the CNS
and the endocrine organs. But ultimately, the conclusion is that sexual
urges are controlled by the CNS. If you are interested in this topic, there
is an excellent undergraduate textbook: Behavioral Endocrinology, edited by
J.B. Becker, S.M. Breedlove and D. Crews, MIT Press, 1993. 

2). This is a little trickier. Someone else mentioned that there are
reflexes that originate in the spinal cord. But I assume when you say
"brain" you mean the central nervous system, which includes the spinal
cord. The vast majority of voluntary and involuntary functions are indeed
controlled by the central nervous system. However, there are some clear
exceptions. If you cut all the nerves to the heart (such as occurs with a
heart transplant) the heart will continue to beat and pump blood. It will
change its force of contraction and output in response to changes in the
amount of blood coming back to the heart. So, here is a good example of a
bodily function which occurs in the absence of CNS control. The nervous
control of the heart is important for coordination with other activities,
but you can live without it. And of course, there are many many bodily
functions such as liver metabolism, immune function, insulin secretion,
etc. where the nervous system plays a *relatively* small role.

Another important exception is the gastrointestinal system. The GI system
does receive an innervation from the CNS via the autonomic nervous system.
But you can cut these nerves and the system will still work very well. The
reason for that is that there is a nervous system within the GI system,
called the enteric nervous system. There are probably as many nerve cells
in the enteric nervous system as there are in the spinal cord. The enteric
nervous system is responsible for the secretion of digestive fluids,
movement of material and absorption. All the CNS control of the GI system
is effected through the enteric nervous system.  So, I would have to say
that the answer to question 2 is no, all bodily functions are not
controlled by the CNS.

Hope this helped settle your bet.

Kevin E. McKenna, Ph.D.
Associate Professor of Physiology and Urology
Northwestern University Medical School
k-mckenna at nwu.edu

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