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.
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?
[snip]
> Like Mat, I don't understand a lot of what you are really asking.
> The action potential is not at all like a Dirac impulse. It is a physical
> signal with a specific amplitude and time course, both of which are very
> important.
The Dirac impulse is simply an infinitely narrow, infinitely tall pulse
that contains a fixed integral area. All real impulses are necessarily
not Dirac impulses. The real ones have a shape that approximates
the ideal impulse, but the real ones could contain messages in many
ways. For example, the frequency domain representation of a real
pulse could contain phase, amplitude and sequence information that
don't show up at low frequencies, but which are observable at
high frequencies with wide passbands.
> Although we say the AP is "all-or-none", the fact is that
> different action potentials differ in details of amplitude and duration
and
> after potential and such depending on the history of firing, the metabolic
> state of the cell, the chemical environment (ion concentrations), possible
> activation (phosphorylation) of membrane channels, etc. And the details
> of the amplitude and time course of the AP in the presynaptic terminal
> can be important in determining just how much transmitter is released.
> Some synaptic modulators work by altering the presynaptic AP in just
> this way.
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.
The original question here was about what frequencies were adequate
for an encephalogram database. I'm trying to add to that question the
possibility that higher frequencies might lead to better understanding
of the content of the encephalogram. Specifically, the types of messages
(if they exist at all) could depend on the types of stimuli that would be
part of any encephalogram database. If the frequency bands recorded in
the database are much higher than the normal clinical signals, they can
be used to explore the kinds of stimuli (visual, auditory, linguistic, ...)
presented to the subject, versus any subsequent diagnosis, and possibly
could add to the quality of the database.
> In any event, the fact that you can write the AP waveform as a Fourier
> transform does not mean that the "frequencies" in the transform spectrum
> are at all meaningful for neurobiology. It is the way that the
time-domain
> signal influences the calcium-induced transmitter release that is
important.
I disagree. The higher frequencies would allow more subtle calculations
and correlations to be studied. Possibly they will have meaning beyond
the calcium-induced transmitter release in ways that can't be predicted.
But if there is an effort to build an encephalogram database that will be
in any way a "standard" for analysis, higher quality analysis can be done
on broader bandwidths. Whether the message concept is proven or
disproven depends on having high enough data quality to address it. And
there are other hypotheses that might be profitably studied if the data
quality is high enough, not just the message hypothesis.
> It is possible that different protein neurotransmitters may exist in
> different
> "conformations" that would influence their function. But it is hard to
> imagine
> any simple way that presynaptic electrical activity could influence that
> conformation. It would be even harder to imagine an experimental way of
> testing such a hypothesis.
With computer studies, statistical correlations can show useful hints
about where more depth is needed, even when the mechanism of the
process isn't understood at the time the correlation is found. The
correlation doesn't have to be simple, it just has to be statistically
significant.
>Many weird things do happen in biology. And
> we can dream up many even weirder things. But experimental scientists
> tend to completely discount any imaginings that cannot be tested
> experimentally.
Its easier to postulate mechanisms to be tested experimentally if you
have data to study. Any quality encephalogram database should provide
more opportunity for study, leading to those imaginings!
-Rich