On Mon, 29 Nov 2004 18:22:45 -0500, "MZ"
<zarellam at removetwcny.rr.comspam> wrote:
>> 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.
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