The resting membrane potential of a real cell is not set by any single ion but is the permeability-weighted combination of the individual equilibrium (Nernst) potentials of every ion that can cross the membrane. Each ion possesses its own equilibrium potential determined by its transmembrane concentration gradient and charge — potassium and chloride driving the potential negative, sodium and calcium driving it positive — and relative permeability, expressed as each ion's fractional share of total membrane crossings summing to unity, acts as a weighting coefficient, so the resulting potential lies nearest the equilibrium potential of the dominant permeant ion. The concept belongs to cell physiology and membrane biophysics, extends the single-ion Nernst treatment to multi-ion membranes (the Goldman-type weighted-sum formulation), and explains how changing permeability alone — for example by opening sodium channels — shifts the membrane potential from strongly negative to positive without any change in ion concentrations.
The resting membrane potential of a real cell is not set by any single ion but is the permeability-weighted combination of the individual equilibrium (Nernst) potentials of every ion that can cross t…