From our picture, it is clear that the reversal potential is reached when the potential on the inner segment is lowered or lifted in such a way that the orange sloping potential in Fig. 1.8 turns horizontal. To reach it (provided that the potential on the other side is kept fixed), the concentrations on the invasion site must be changed. The invasions resulting in the exact (but opposite) change of potentials from the two sides leads to different concentrations. The invasion acts as changing the potential at the corresponding surface side of the membrane. Given that the difference between the two potentials on the two surfaces defines the driving forces, the concentration on the other side is also affected. That is, the reversal potential exists, but its value depends on the context, see the discussion about invasions.
[24], page 7 provides a numeric estimation that ”a spherical cell of radius with a resting potential of stores about coulomb of charge just below the membrane and an equal but opposite amount of charge outside”. As we estimated above, a charge in the same order of magnitude (or above it) is involved in forming and action potential, so the assumption that the membrane is a kind of electric circuit with a fixed potential and the moving ion current does not change membrane’s potential, is far from reality. That amount of charge represents ions. From this we can estimate that the ions are at a distance from each other.
As the caption of Fig. 11.22 in [107] formulated: ”A small flow of ions carries sufficient charge to cause a large change in the membrane potential.”