On Tue, 3 Feb 2004 18:47:32 +0000 (UTC), kola9809{at}aol.com (Kola9809)
wrote:


>My premise is that for a process to happen systematically, specific information
>is needed. In our case, extracellular information (chemical signals) need to be
>provided to the pineal cells for activating genes involved in the synthesis of
>melatonin. This is why no other type of cell in the body does activate those
>genes.  
>I will break down my question by trying to explain "Why?" the
information for
>activation of melatonin genes is necessary, as well as
"Where?" and "How?" is
>it generated.
>
>Why is information for activation of melatonin genes necessary? Pineal cells
>are uniquely capable of activating those genes and synthesizing melatonin. The
>reason is that only those cells receive the necessary information, i.e.
>chemical signals from the suprachiasmatic neurons. Having exactly the same set
>of genes, the rest of the body cells do not activate those genes because they
>do not receive that information. I believe we all agree on this.
>
>Where does this information for activating melatonin genes in the pineal cells
>come from?
>The suprachiasmatic signal itself is the chemical output of processing in the
>above-mentioned circuit of the electrical signal into which retinal neurons
>have converted an external stimulus, the day-night (circadian) cycle.
>This conversion as well, is not a random process, but very specific and results
>from a computational process taking place in those neurons. It turns an
>otherwise neutral stimulus intu a cue, an instruction for activating a signal
>cascade that leads to specific gene activation for an adaptive phenotypic
>result.
>The processing takes place in the above mentioned neural circuit and it is the
>neural circuit that generates the information. Correct me if I'm wrong.
> 
>How is this information generated in neural circuits?
>Experimental evidence shows that the chemical output, or information for
>activating genes in neurons is generated computationally, by processing
>internal and external stimuli according to a general model that essentially
>comprises:
>-reception of internal/external signals by interoceptors/exteroceptors,
>-conversion of those stimuli into specific patterns of electrical signals, 
>-reconfiguration of synaptic morphology of the neural circuit and resulting
>change in the computational properties of the circuit, which
>- changes  the electrical/chemical output of the circuit, leading to
>- specific activation/inactivation of target genes in neurons.
>
>I hope I have clarified the meaning of my sentence on the information necessary
>for activation of genes. 
> 

There are many ways that the informal notion of "information" can be
expressed.  First, there is a specific nervous pathway from the visual
system through the brain to the pineal gland.  That consists of
specific cells growing axons in certain specific directions but not in
others and forming synaptic connections with certain specific cells
but not with others.  In addition, there are ways in which nerve cells
form many contacts with many cells but somehow experience trims down
most of the connections to leave only certain ones remaining. There is
"information" involved in all these choices.  We do know many things
about trophic factors and cell recognition proteins that assist in
making specific neuronal connections, but do not know the whole story
in most cases.  The pathway leading from the eye to the pineal is no
more involved or complicated than any other "specified" or
"determined" neural pathway -- the visual system, the auditory system,
the limbic system, etc.  These all involve specific connections
between specific neuron types.

Once the pathway is made, there is another form of "information", the
presence of absence of the light signal. But that is trivial.  Light
and dark change the activity of cells in the visual pathway which
changes the activity of cells connecting to the pineal which changes
the level of response in the pineal gland cells.  This form of
information is the traditional notion of what nervous systems do --
process and transmit information in the form of electrical signals.

Finally, there is "information" is the cell signaling machinery
whereby synaptic activation of pineal gland cells activates specific
cell processes leading to the production and release of melatonin
orthe activation an inactivation of the genes involved in melatonin
synthesis. This is no different from the fact that the cell signaling
machinery in a pancreatic beta cell causes insulin secretion or the
cell signaling machinery in a muscle cell causes contraction.

So there are three kinds (at least) of information involved --
specificity of nerve pathways, neuronal signals, and cell responses to
stimulation.  Each of these is known in many details. 

Where does all this "information" come from?  These pathways seem to
be genetically coded in the same way that the knee-jerk reflex is
genetically coded. There is a tremendous amount of research on just
how genetic information can produce specific nerve circuits.  Many
invertebrates have individually identifiable and genetically
determined neurons that have very specific functions in producing and
controlling behavior.  The roundworm, Caenorhabditis elegans, is the
ultimate example since every individual worm in the species, except
for mutations, has exactly the same number of cells in their body, The
cells form in exactly the same way from the fertilized egg, are
located in exactly the same location, and do exactly the same thing.
The genome is completely known and their is a major research effort in
working out how all this really works.

Where does the genetic "information" come from?  That is what
evolution is all about.
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