Figure 1. A. Nernst equation. At body temperature, the potassium equilibrium potential (which represents the great majority of resting membrane potential) is a constant times the logarithm of the ratio of concentrations of potassium outside and inside the cell. B. Goldman-Hodgkins-Katz equation. This is similar to the Nernst equation except that it adds a factor for permeability. If an ion is impermeable, its contribution to the membrane potential (V) is nil. The higher the relative permeability, the closer the membrane will be to the equilibrium potential for that ion.