On Tue, 3 Feb 2004 18:47:32 +0000 (UTC), [email hidden] (Kola9809)
Quoted message said: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.
