> >There's also evidence frequently cited in the optical imaging circles
that
> >optical signals arise naturally due to light scattering associated with
> >water/ion movement and cell swelling, but it provides a minor overall
> >contribution to the signal. It seems to me, though, that this result
> >suggests that there are some substantial changes taking place with cell
> >firing even independent of the hemodynamic changes.
> >
>> Water movement and cell volume changes are probably rather smaller,
> though. The number of ions (or moles of ions) moving inward in
> generally quite comparable to the number moving outward so the osmotic
> pressure is not likely to change much.
>> This assumes that it is mostly Na and K doing the charge transfer. A
> cell can depolarize because of Na entry and then repolarize at least
> partly because of Cl entry, which can produce a significant osmotic
> effect. Alternatively, it takes twice as many K leaving to equal the
> depolarization produce by Ca entry.
>> Still, it is quite true that there are many other cellular changes
> occurring with nerve activity than simple changes in membrane
> potential.
I'll have to dig up the old paper to determine exactly what processes they
were attributing this light scattering to. I've gotten my information
second-hand from a Grinvald review.
