the liver and the brain

r norman rsn_ at
Tue Aug 31 19:50:43 EST 2004

On 31 Aug 2004 16:03:12 -0700, rscanlon at (ray scanlon)

>David Longley writes:
>> Don't you think sleight-of-hand and metaphysics deserves derision?
>Well, then let's drop all the sleight-of-hand and metaphysical
>prejudices and talk about the brain.
>Since the neural net (interneurons) appeared in Cnidaria, what has
>changed? For one thing, the DNA has evolved to a point where it is
>able to construct a whole series of motor program generators, groups
>of neurons that when triggered produce a motor act. These generators
>can be modified by experience but they are not learned. We are born
>with them.
>The location in the nervous system of some of these motor program
>generators can be more or less specified.
>Orofaciopharyngeal movements: facial expression, vocalization,
>licking, chewing, and swallowing in the dorsolateral hindbrain.
>Reaching, grasping, and manipulating in the spinal cord (cervical
>There are more, of course, but we take these for starters.
>(The specialist talks about controllers, imitators, motoneuron pools,
>The important thing is that these are not, repeat not, learned—they
>are constructed by the DNA. Under vocalization are the phonemes—also
>provided by the DNA.
>Some synapses in the nervous system can be altered by experience. This
>allows us to string the phonemes together into language.
>This lays the groundwork for a scientific explanation of brain action.

Central pattern generators are widespread and are well studied in a
variety of invertebrates.  In these animals, individual identifiable
and named neurons are definitely genetically programmed to have very
specific "wiring diagrams" and behavioral roles.

It is quite likely that very similar types of circuits do exist in the
human CNS, although not with individually identifiable and nameable
neurons.  And, as in the type of examples you gave, they are probably
important contributors (though certainly not complete determinants) of
such low-level behaviors as breathing, chewing, swallowing,
locomotion, etc.  The existence of reflexes also proves that certainly
there do exist specialized genetically programmed wiring patterns
between specific neurons in the mammalian and human nervous system.

At the same time, it is also certain that synaptic plasticity
triggered by usage and experience -- learning, in short -- plays an
enormous role in any "real" type of mammalian behavior.  

So it really is somewhat a trivial statement to say that the
combination of genetically programmed cells plus nerve connections
that can be modified by plasticity together lay the groundwork for a
scientific explanation of brain action.  You might as well say,
simply, that cellular activity lays the groundwork for a scientific
explanation of brain action.

There are enormous technical problems in trying to determine to what
extent, if any, that any specific behavioral act in humans is
controlled by or even initially produced by the genetically determined
systems you describe.  And to imply that genetically determined
factors play a major role in significant aspects of human behavior is
to make a statement so fraught with political implications and
possible misuse (with numerous historic examples of such misuse) that
it does invite the personal invective you complain about.

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