> You are right to be sceptical. In teaching about the action potential
> I always emphasize that the tiny amounts of Na and K that cross the
> membrane during a single action potential are so small as to cause
> virtually no change in intracellular or extracellular concentrations.
> However the situation changes when you consider cells making thousands
> of action potentials (twenty/second for a minute) and when you
> consider thousands of cells all active in a small volume. The
> extracellular space is also quite confined.
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.