Resting Membrane Potential and Ion Distribution

The resting membrane potential is the steady voltage difference across the plasma membrane of an unstimulated cell, with the interior negative relative to the outside, typically around -60 to -90 mV in neurons. It is the product of unequal ion distribution across the membrane and the membrane's selective permeability to those ions.

Definition

The resting membrane potential is the transmembrane voltage maintained by an unstimulated excitable cell, set by the equilibrium between ionic concentration gradients and the membrane's relative permeabilities, and held near the potassium equilibrium potential.

Scope

This topic explains why a resting cell holds a stable negative voltage and how the asymmetric distribution of potassium, sodium, and chloride gives rise to it. It covers the contributions of each major ion and the dominant role of potassium at rest, and it distinguishes the resting state from the active voltage changes treated elsewhere.

Core questions

• What ion concentrations characterise the inside and outside of a resting cell?
• Which ion dominates the resting potential and why?
• How do small sodium and chloride permeabilities shift the resting potential away from the potassium equilibrium value?

Key concepts

• Transmembrane voltage
• Intracellular vs extracellular ion concentrations
• Potassium dominance at rest
• Sodium and chloride contributions
• Relative permeability
• Steady state versus equilibrium

Key theories

Selective-permeability account of the resting potential

The resting voltage is determined by which ions can cross the membrane and how easily; because potassium permeability greatly exceeds sodium permeability at rest, the potential lies close to the potassium equilibrium potential but is offset positive by sodium leak.

Mechanisms

A resting neuron keeps potassium concentrated inside and sodium and chloride concentrated outside. The membrane at rest is highly permeable to potassium through open leak channels and only weakly permeable to sodium. Potassium diffuses outward down its concentration gradient, carrying positive charge out and leaving the interior negative; the growing negativity opposes further efflux, so the voltage approaches the potassium equilibrium potential. A small persistent sodium influx, and in many cells chloride movement, holds the actual resting potential slightly positive of the potassium equilibrium value. Hodgkin and Katz (1949) demonstrated this multi-ion dependence directly by varying external sodium and showing predictable shifts in the membrane voltage.

Clinical relevance

Because the resting potential sets how readily a cell can be excited, shifts in extracellular potassium or other ions change membrane excitability in nerve, muscle, and heart. This entry describes that mechanistic dependence as background physiology and is not guidance for managing any condition.

Evidence & guidelines

The quantitative account derives from squid-axon recordings and is standard content in physiology and biophysics textbooks; it is mechanistic reference material rather than clinical guideline content.

History

Julius Bernstein's early membrane theory proposed that the resting potential reflected selective permeability to potassium. Hodgkin and Katz (1949) extended this to a multi-ion view, showing that sodium permeability also matters, and Goldman's 1943 constant-field analysis gave the relationship its quantitative form.

Key figures

• Alan Hodgkin
• Bernard Katz
• David E. Goldman
• Julius Bernstein

Seminal works

• hodgkin-katz-1949
• goldman-1943

Frequently asked questions

Why is the inside of a resting neuron negative?

Because the resting membrane is mostly permeable to potassium, potassium leaves the cell down its concentration gradient and carries positive charge out, leaving net negative charge inside until the electrical pull balances further efflux.

Is the resting potential an equilibrium?

Not exactly. It is a steady state held near the potassium equilibrium potential; continuous small ion leaks are offset by active pumping, so the gradients and voltage stay constant without true equilibrium.