Membrane potential

A membrane potential is an electric potential difference across a cell membrane. We use the conventionTaking the external electric potential as zero makes $V_m<0$ mean that the inside is at lower electric potential. “Negative inside” describes a difference, not an absolute electric potential, and does not imply that the entire cytosol has a large net negative electric charge.

Consider a cell membrane permeable only to potassium ions, with higher potassium concentration inside. Diffusion initially favors outward transport. If balancing negative ions cannot follow, that loss of positive electric charge produces a small inner excess of negative electric charge and an outer positive excess. The resulting electric field favors inward potassium transport. At the Nernst equilibrium potential, chemical and electrical contributions to electrochemical potential balance; ions still cross both ways, with zero net transport.^[ref]

In living biological cells, several ions can permeate and pumps maintain their concentration differences through active transport. Pumps transporting net electric charge also contribute electric current. Thus $V_m=E_{\mathrm{K}}$ is a special limit, not a universal rule; membrane potential need not always be negative.^[ref] The existing reversal potential example shows how opposing electric currents can balance without each ion reaching thermodynamic equilibrium. This makes selective permeability a way to convert chemical differences into electrical signals: changing the available ion channels changes which electrochemical gradients influence the voltage.