rnorman at umich.edu
Mon Feb 4 13:24:31 EST 2002
"mat" <mats_trash at hotmail.com> wrote in message
news:43525ce3.0202040159.498fa894 at posting.google.com...
> > For many purposes, instrumentation can treat one impulse as
> > indistinguishable from another. They're both versions of the
> > theoretically perfect Dirac impulse. But to understand the variations
> > in what makes one impulse different from another, you have to look
> > at small differences among them. You mentioned pulse width as one,
> > but there are infinitely many frequencies in a real, imperfect, impulse.
> > For example, the incoming pulses from one set of dendrites may
> > cause a slightly different response than another set of dendrites. The
> > timing of incoming pulses can sum, or interfere, in different ways. So
> > its not just a question of whether the output impulse is there, but
> > what characteristics that impulse has, which might convey information
> > about how the impulse was generated in the first place.
> Do you simply mean sampling at a higher frequency to get data of
> higher resolution?
> > Secondly, there is no reason to assume that the NTs crossing
> > the synaptic gap all have the same conformation. Yet identifying
> > possibly varying conformations is difficult, perhaps impossible
> > at this point in technology, since the conformation can change
> > as you measure it. 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 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?
> > So I'm not suggesting very high frequencies for some mystical
> > reason, but as a way of distinguishing among very subtle differences
> > in the impulses as a way to shed further light on whether the
> > impulses themselves convey messages other than the simple
> > event of an impulse occurrence.
> Different trasnmitters (and theis respective receptors) induce
> differing post-synaptic currents/potentials (and much work has already
> been done on it) and these summed in different ways to alter the
> probablility of firing an action potential. Further it would be very
> intersting to elucidate how different synpatic events alter things
> such as gene transcription and protein synthesis (but again, a lot of
> work has been done on this already notably by Kandel)
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. 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
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.
It is possible that different protein neurotransmitters may exist in
"conformations" that would influence their function. But it is hard to
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. 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
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