Membrane Permeability and Ionic Equilibrium Potentials

Selective membrane permeability is what turns ion concentration gradients into a voltage. Ions cross the membrane only through channels that favour particular species, and for each ion there is an equilibrium potential, given by the Nernst equation, at which its diffusive and electrical forces exactly balance.

Definition

Selective permeability is the property by which ion channels allow particular ions to cross the membrane; the equilibrium (Nernst) potential for an ion is the membrane voltage at which the electrical force on that ion exactly opposes its concentration gradient, so there is no net flux.

Scope

This topic covers how ion-selective channels make the membrane permeable to some ions and not others, and how the Nernst equation defines the equilibrium potential for each permeant ion. It explains the equilibrium potential as the reference point toward which each ion's permeability drives the membrane voltage. The combined steady-state voltage from several ions is treated in the Goldman-Hodgkin-Katz topic.

Core questions

• How do channels make the membrane selectively permeable to one ion over another?
• What does the Nernst equation calculate, and what does the equilibrium potential mean?
• Why do potassium and sodium have such different equilibrium potentials?

Key concepts

• Ion-selective channels
• Selectivity filter
• Nernst equation
• Equilibrium (reversal) potential
• Concentration gradient versus electrical gradient
• Permeability versus conductance

Key theories

Equilibrium-potential principle

For a single permeant ion the membrane reaches a voltage, the Nernst potential, at which the electrical force across the membrane exactly balances the concentration gradient, producing no net movement; this defines the target voltage each ion's permeability pulls toward.

Mechanisms

Ion channels span the membrane and pass ions through a selectivity filter that discriminates between species by size and coordination chemistry; the potassium channel structure resolved by Doyle and colleagues (1998) showed how backbone carbonyls mimic the hydration shell of potassium to select it over sodium. When the membrane is permeable to a single ion, that ion diffuses down its concentration gradient until the charge it moves builds an opposing electrical force; the voltage at which the two forces balance is the equilibrium potential, calculated by the Nernst equation from the ratio of external to internal concentrations. Different ions therefore have different equilibrium potentials, and the membrane voltage is pulled toward the equilibrium potential of whichever ion is currently most permeant. Hodgkin and Katz (1949) showed experimentally that changing external ion concentrations shifts the membrane voltage as this framework predicts.

Clinical relevance

Equilibrium potentials and channel selectivity underlie why changes in extracellular ion concentration alter excitability and why channel-targeting drugs and toxins affect nerve and muscle. This entry presents those relationships as mechanistic background and offers no diagnostic or treatment guidance.

Evidence & guidelines

The Nernst relationship is a thermodynamic result confirmed in countless electrophysiology experiments, and channel selectivity is established by structural and functional studies; this is standard biophysics reference material, not guideline content.

History

Walther Nernst formulated the equation linking concentration ratio to electrochemical potential at the end of the nineteenth century. Its application to excitable membranes matured with the squid-axon work of Hodgkin and Katz (1949), and the molecular basis of the selectivity that the framework assumes was finally visualised in the potassium channel structure of Doyle and colleagues (1998).

Key figures

• Walther Nernst
• Alan Hodgkin
• Bernard Katz
• Roderick MacKinnon
• Bertil Hille

Seminal works

• hodgkin-katz-1949
• doyle-1998

Frequently asked questions

What is an equilibrium potential?

It is the membrane voltage at which the electrical force on a particular ion exactly cancels its concentration gradient, so the ion has no net tendency to move; the Nernst equation calculates it from the ion's concentrations on either side.

How does a channel select for one ion over another?

A narrow selectivity filter coordinates the preferred ion in a way that substitutes for its water shell while excluding ions of the wrong size or charge geometry, as shown for potassium channels by structural studies.