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Neurotransmitters

Richard Norman rsnorman at mw.mediaone.net
Thu Feb 18 23:08:06 EST 1999


Bill Gates wrote in message <36CC846B.A870B48F at microsoft.com>...
>
>  In my neurobiology class, we were discussing the abundance of
>neurotransmitters; and my prof asked us for guesses of the cause.
She
>put me on the spot, so I suggested that perhaps certain regions of
the
>brain primarily respond to certain transmitters, and other regions
are
>more responsive to another subset.  Thus, it would be possible for
the
>brain to regulate activity in various regions by simply controlling
the
>concentration of various transmitters.
>    My prof didn't buy this explanation (and I don't entirely blame
her,
>since I pretty much made it up from my meager knowledge of
>neurotransmitters), and instead suggested that the abundance is to
>facilitate "fine tuning" of the transmitter action.  She further
>suggested that part of the abundance is due to evolutionary
"garbage".
>I don't entirely buy her explanation.  Would anybody be able to shed
>some light on this issue?
>

Having just taught a course on receptors and cell signalling and now
am into teaching one on neurobiology, I would have to agree with
your wise professor.  The small amine transmitters are not all that
numerous, but the profusion of proteins certainly is a problem! And
the profusion of different receptors for the same transmitter is also
a problem!

One important fact is that neurons really didn't "invent" anything new
to accomplish their special functions.  A tremendous variety of cells
(and organisms from many kingdoms) use membrane potentials and
second messenger systems for intracellular signalling and control.
And the profusion of receptors and second messenger systems is
also a tremendous puzzle.

My guess is that evolution hit upon a few really useful tricks -- G
proteins,
cyclic nucleotides, kinases, protein phosphorylation, Ca binding,
etc. and then (probably by gene duplication followed by mutation)
multiplied the number of possibilities -- all slightly different.  The
idea is
that if you can produce one potassium channel, you can produce a
dozen,
all slightly different.  If you can produce one receptor, you can
produce a
whole series, again all slightly different.

Now you have all these cells using a whole diversity of transmitters,
receptors, and intermediate signalling systems all doing slightly
different things.  Evolution then has the task of whittling down the
ineffective ones and retaining the useful ones, which happens in
different ways in different cells with different circumstances.  It is
a
molecular reflection of the way evolution works with organisms --
produce a tremendous diversity and then let selection shape what
you want to keep.

The problem with your suggestion is that transmitters are not
restricted
to specific brain areas and that cells in any one brain area use a
wide
variety of transmitters (at least different types of cells, a
population of
similar morphological cells will, of course, be similar chemically).

But, then, I also imagine your prof was more interested in seeing
if her class had any imagination and could invent at least reasonable
ideas based on what you had learned, rather than in the "correct"
answer!







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