Comment by niemandhier

I’d expect that it’s probably not the ion movement but the global electric field, maybe even mechanical effects in the axons.

If you look at the Goldstein-Katz equations you see that the conductivities play an as important role as the concentrations.

Most of the voltage change is driven by changes in conductivities and not concentration changes, the ion movements across the membrane should be negligible.

Off course you have the free ions outside the cells than diffuse in a field gradient.