I didn't mean to suggest that there is no energy released when ions
flow down their electrochemical gradients. Certainly the energy
goes into the production of the electrical current with the associated
change in charge distribution (electrical potential) across the membrane.
But that is far different from saying that the energy can be picked up
by a single protein molecule to be channeled into something like the
production of photons. The electrical energy is widely distributed
along the cell -- current loops flow for hundreds of micrometers and
even millimeters. The space constant is a measure of this spread.
The coupling you mention indicates that the flow of ions can be
coupled into the flow of water. This presumably occurs in the narrow
ion selective portion of the channel where there is certainly interaction
between the ion and the channel protein. But again, there is little
opportunity for the bulk of the energy to be captured by the protein.
There are, to be sure, other places where the situation is different.
For example, in the active transport of sugars and amino acids across
the brush border membrane of intestinal or kidney tubule epithelia,
the energy from the movement of sodium into the cell is coupled into
the movement of the co-transport ligand (the sugar or amino acid)
into the cell. But that is NOT a "pore" or "channel". Instead, the
active transport works by binding the sodium on one side of the
membrane under one condition of concentration and electrical
potential and releasing on the other, under a different condition of
concentration and electrical potential. The difference in energy
between the binding step and the unbinding step is captured by the
protein.
But the experimental evidence for ions traversing a water-filled channel
essentially at the rate indicated by diffusion (plus electrophoresis)
is rather compelling. And in this model it is hard to imagine how
the channel can absorb enough of the energy to be useful as in
independent computational element.
And I don't buy your argument about the human brain being different
from the peripheral system or, for that matter, from a bacterium.
Well, a bacterium is a little extreme. Say, from a eukaryotic protist.
There is a fairly limited bag of molecular tricks that cells have available
to do everything they must do. And the molecular machinery was
quite well worked out early in evolution. Eukaryotes added a bit to
the tool kit and complex multicellular eukaryotes added another
bunch of things mostly related to how to regulate the genome during
development. But the basic cellular processes of transport, motility,
energy metabolism, and signaling are very well preserved. I think
it would be very unlikely to find radically different processes occurring
in the mammalian CNS than in any other animal. And, since I happen
to be a comparative animal physiologist (as is the "other Richard"),
I am quite sure that if there really were something to the coupled photon
transfer hypothesis, then some weird invertebrate would have seized
upon that mechanism and specialized it the nth degree to produce
a noticeable and interesting capability. There are many weird things
in this world, mostly still unknown. And the idea is interesting and
intriguing, otherwise I wouldn't waste my time arguing about it.
But I wouldn't bet it has any legs.
"Iain McClatchie" <iain at 10xinc.com> wrote in message
news:39A432F6.83EA7571 at 10xinc.com...
> Richard> All the studies [...] indicate that ions move in a
> Richard> dissipative flow by electrochemical diffusion through
> Richard> essentially water filled channels, although with some local
> Richard> interaction with ion selectivity sites in the channel.
> Richard> There is no opportunity for the energy released to be
> Richard> coupled to any other process without significantly
> Richard> interfering with measured flow rates.
>> If, as you claim, there is no energy coupled into the ion channel,
> then there can be no computation.
>> "Simulations of ion channels - watching ions and water move"
> Mark S.P. Sansom a mark at biop.ox.ac.uk, Indira H. Shrivastava a,
> Kishani M. Ranatunga b and Graham R. Smith a
> Trends in Biochemical Sciences 2000, 25:368-374
>> quote:
> The motion of ions within pores seems also, in general, to be
> restricted relative to their motion in bulk solution. [....]
> (e.g. an approximately threefold reduction for K+ ions versus an
> ~12-fold reduction for water for a hexameric Alm helix-bundle
> channel).
>> Ions, water, and channel are all coupled:
>> In the narrow selectivity filter of the bacterial K+ channel
> (i.e. the region of the channel that discriminates between
> different species of ions) a column of water molecules and
> K+ ions moves in a concerted fashion.
>> Now I'm not arguing that the ion channels in this bacterium are
> trading photons, but it does appear that there is ample coupling
> between ions and channel protein, giving opportunity to couple energy
> into the protein. Given that the channel is greatly slowing the
> diffusion of ions relative to bulk water, it would appear that a
> fair bit of the ions' potential energy is being dissipated into the
> channel structure somehow. In the case of the bacterium and the
> human peripheral nervous system, I would expect this energy to be
> dissipated as heat with no computation. But in the human CNS, with
> its difficult-to-model complicated ion channels, the energy may be
> exchanged between ion channels before dissipation, which would lead
> to computation.
>> I think Dr. Skaggs is onto a bigger problem with this screwy idea:
> I don't have any easily testable predictions yet. Any ideas?
>> -Iain McClatchie 650-364-0520 voice
>http://www.10xinc.com 650-364-0530 FAX
>iain at 10xinc.com 650-906-8832 cell
