Brain Mass Concepts

Richard Norman rsnorman at mediaone.net
Wed Sep 12 22:27:38 EST 2001

On Wed, 12 Sep 2001 17:49:36 -0500, "Miles Robinson"
<m-robinson7 at northwestern.edu> wrote:

>    I have a basic question about brain mass studies. It has been supposed
>that the brain mass to body mass ratio can give realtively good quantitative
>measure of "intelligence". However, when looking at the brain as a control
>system for the body, it's hard to overlook that fact that a larger body mass
>does not necissarily require a larger brain to maintain equivalent
>functionality. For example, a single muscle doesn't require more neurons as
>it gets larger during the evolutionary process as far as I can see. I reason
>this way because an entire muscle moves together and thus the whole organ
>gets the same signal from the brain, so why is it necessary for multiple
>neurons to control something that only requires as single signal? Is it not
>more reasonable to use the critera of number of organ systems or sub-systems
>vs. brain mass to quantitatively evaluate intelligence? Or better yet the
>number of body systems to brain divisions?
>Your attention appreciated
>EE student at Northwestern U.
>(Not a neuroscience student)

I seriously question your supposition that brain mass/body mass ration
correlates with intelligence.  Do you have citations for that?
Certainly you are not suggestive a correlation over the range of
ratios exhibited by human variability!

Also, a "muscle" does NOT act as a single signal.  Vertebrate muscles
are composed of motor units, each controlled by a single motor neuron.
Large muscles tend to contain more motor units, that is more neurons,
to control them.  The division into motor units is important in
maintaing fine control of magnitude of contration.  There also may
some limit on the size of a motor unit -- the number of muscle fibers
that a single motor axon is capable of innervating.  This may be a
support infrastructure limitation -- how much transmitter is the axon
capable of producing and releasing?  Again, this requires large
muscles to contain more motor units.

According to your argument, a whale should have the same size brain
as, say, a shrew.  It has the same number of organ systems and organs
which do pretty much the same things.  So the same size brain should
be capable of managing a mammalian body, no matter the size.

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