An equilibrium (Nernst) potential is the transmembrane voltage at which the electrical force on a permeant ion exactly offsets the chemical force of its concentration gradient, and it is a stable attractor: any imposed perturbation of membrane voltage is self-correcting so long as the concentration gradient and the selective permeability are maintained, because net ion flux resumes in whichever direction restores the balance. Its existence requires two independent and jointly necessary conditions — a concentration gradient supplying the driving tendency, and membrane permeability supplying the pathway — with either alone yielding no potential. The magnitude is given by Vm = 61.5 · log([ion]out/[ion]in) for a monovalent ion, the constant being scaled by the ion's valence, and the concept belongs to cell and circulatory physiology as the quantitative foundation on which resting potential and excitability are built.
An equilibrium (Nernst) potential is the transmembrane voltage at which the electrical force on a permeant ion exactly offsets the chemical force of its concentration gradient, and it is a stable att…