On Thu, 23 Feb 2006 08:04:46 -0800, "Michael Grammer"
<mkgrammer at gmail.com> wrote:
<snip discussion about using slope of voltage -- dV/dt, in place of
> Regarding the
>Hodgkin and Katz reference, they probably used the slope of the ap as an
>estimate of the na current rather than the amplitude due to the
>contamination of the ap peak by k current(?).
My recollection (again, I am on vacation and don't have Hodgkin and
Katz with me) is that they used dV/dt specifically at V = 0 as a
measure of Na current since at V=0 there is no electrical force
causing ion movement or current. Hence the fact that the observed
slope of the rising edge of the action potential at V=0 varied
directly with Na concentration proved that the current must be caused
by Na entering the cell because of the concentration gradient, hence
that Na permeability increased drastically. Textbooks usually show
the figure showing that the amplitude of the action potential varied
with extracellular Na concentration but the paper based its conclusion
more forcefully on the slope at V=0.