> >There's also evidence frequently cited in the optical imaging circles
> >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
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.