mats_trash at hotmail.com
Wed Feb 6 14:20:11 EST 2002
"Rich Cooper" <richcooper1 at mindspring.com> wrote in message news:<a3pfjd$b8k$1 at slb5.atl.mindspring.net>...
> Richard Norman <rnorman at umich.edu> wrote > > Do you simply mean sampling at
> a higher frequency to get data of
> > > higher resolution?
> Yes, on the basis that differing conditions may have some effect on
> the output impulse shape, and the higher frequencies could possibly
> be used to detect the variation in impulse shape.
> [snip] That leaves the electrical signature of one
> > > > impulse versus another as the most likely way to distinguish
> > > > messages, assuming that they exist at all. Although the NTs
> > > > may be stored up for hours before an impulse is generated,
> > > > conformation might change as charge distributions change.
> > > > Also, the DNA/RNA bases might carry different charges
> > > > that are more dynamic, reflecting message content. Or
> > > > maybe not; its just a thought worth looking into.
> > >
> > > What do you mean by conformation? Most of the major NTs are nothing
> > > more than single amino acids and variations thereon. There's not much
> > > to play with to use different 'conformations'. Are you suggesting
> > > that say glutamate in different conformations binds to different
> > > receptors (or to the same receptors with differing affinity)?
> I am not a biochemist; my background is EE and CS with a
> medical engineering minor. So don't take my statements about
> neural chemistry as anything authoritative. I'm simply responding to
> the Science News article that seems to indicate there is some kind
> of message being transferred in an impulse other than a simple
> on/off message. Given that speculation, I'm trying to explore how
> such a message might be encoded and transferred among neurons.
Well that is partly true, neurons are not simple logic gates, and
several substances modulate the way in which they react to inputs from
other neurons. You could imagine it has higher levels of organisation
that regulate the respose of the neurons to the 'basic' on/off
mechanisms that glutamate and GABA mediate. (Though of course the
excitatory and inhibitory roles of glutamate and GABA are not as clear
cut as that) Transmitters such as dopamine and serotonin modulate ion
channel activity amongst other things while chemicals such as NO and
arachidonic acid may play more global roles. However a tremendous
amount is still unkown about these mechanisms.
One problem I think many people have when approaching neuroscience is
that they forget the brain is made up of cells that are not at all
that different from those in the liver, skin or any other part of the
body. The images of electrical impulses as sweeping flashes of light
betray the fact that in terms of ions involved action potentials are
relatively minor events and further the cell is still a hub of protein
synthesis and gene regulation etc etc. I think many people grasp the
electrical properties because of their dualistic connotations (light,
etherealness(?)) as being the most important function of neruons. In
truth, though the action potential is the eventual manifestation there
is much going on 'behind the scenes' such as meatbolisom, protein
synthesis and gene regulation, just as in every other cell. What is
remarkable is that these functions sustain our minds.
Furthermore the engineering approach to the mind often forgets the
lessons of evolution. The action potential itself is mainly mediated
by sodium and potassium ions and their respective channels. However
there was no direction to forming this mechanism, it just evolved.
Sodium ions don't 'know about' potassium ions or vice versa, they just
happen to work together fortiutously to allow behaviour. Originally
there may have been just the sodium channel, but the biochemical
changes they induced could equally have became relevant to behaviour.
Further to the last point, what if channels had evolved not for
charged ions but for say, water and glucose (both relatively simple to
channel) and the flux of these across the lipid membrane induced
muscles to contract. Would the idea of action potentials giving rise
to mind be so 'sexy' then?
The main reason why we consider action potentials to be the substrate
of mind is that action potentials can induce muscle contraction, i.e.
they can 'cause' behaviour. I think it is more than possible that
action potenitals may bear no relation to consciousness, which could
have evolved through a totally different biochemical system which
eventually impinges on the action potential system to produce
behaviour. Of course this is speculation but no more so than the
blindly held belief that action potentials give rise to mind.
> The time-of-flight description applies to any consistent combination
> of pulses impinging on one neuron. Suppose one consistent combination
> of pulses is consistently recognized and a message is generated based
> on that consistent combination. That mechanism could explain how
> specific messages are output by one neuron, and input by others,
> to form complex learned patterns. Its this correlated detection
> of a complex situation that I'm most interested in.
> > > I don't get the bit of DNA/RNA... are you suggesting that they are
> > > transmitters or that the charge on the bases (??) affects the protein
> > > synthesised? and that membrane potetnial affects these charges and
> > > subsequently alters protein product?
> I'm suggesting that the molecule transmitted across the synapse
> contains a message encoded in the molecule itself. Then I'm speculating
> about how the encoding could be carried. From your comments, you
> seem to have some ideas - how would you think messages might be
> encoded between neurons?
First of all we must agree that cells are mechanistic. Subsequently
asking whether messages are contained within transmitters or within
the receiveing cell is meaningless. Certainly, similar molecules that
have been subject to slight modification (e.g. arachidonic acid and
the cannabinoid anandamide are very similar (anadamide being
arachidonic acid with an ethanolamine group I think)) can induce
different postsynpatic events. But if neurons had not evolved these
different receptor mechanisms there would be no different 'messages'
even though the enzyme than produces anandamide from arachidonic acid
may still be there. What happens in the post synaptic cell on binding
a transmitter depends of course on the transmitter but also on the way
the post-synpatic cell is organised to react to it. Another example -
glutamate - this is an Amino acid that is a fundamental constituent of
proteins but also induces action potentials in post-synaptic neurones
(glitamate binds to a receptor protein, which itself contains
glutamate as a constituent). Is it carrying different messages in
these different roles? ATP, the energy 'currency' of all cells is also
a neurotransmitter. In each case it has very different effects. The
ATP molecule doesn't 'decide' to have these different effects at
different times its just that proteins have evolved to react to ATP in
> Those are exactly the kinds of considerations I'm referring to. The real
> AP observed at the output of the neuron depends on a lot of past history,
> and all these differences in one AP versus another could contain the
> message encodings I'm describing.
I have to ask: to whom or what is this message directed? Fine, if
subtle different in action potential amplitude or other parameters
induce different events in the postsynaptic cell then you can say a
'message' has been passed. But these are just evolved mechanisms
whereby mutations have allowed cells to react to stimuli in a greater
variety of ways and produce outputs of greater variety.
Thinks (biologically anyway) are not the way they are for any
particular reason, action potentials are one way that life has evolved
on this planet to get around and survive. If it were any other way we
would be asking the same questions.
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