Ion Channels and Membrane Potential

The electrical signalling of neurons begins with the membrane potential — the voltage difference across the cell membrane — and with the ion channels that control how ions cross it. By selectively allowing ions such as sodium, potassium, and calcium to flow, and by opening and closing in response to voltage or ligands, ion channels set the resting potential and generate the action potential. This topic surveys the ionic basis of neuronal excitability.

Definition

The membrane potential is the voltage across a cell's membrane arising from unequal ion distributions and selective permeability; ion channels are membrane proteins that conduct specific ions and, by opening and closing, generate the resting potential and the action potential.

Scope

The topic covers the resting membrane potential and its ionic basis, the structure and selectivity of ion channels, voltage- and ligand-gated channels, and the ionic mechanism of the action potential as described by Hodgkin and Huxley. It is a reference survey of biophysical mechanisms and does not provide clinical guidance.

Core questions

• What establishes the resting membrane potential of a neuron?
• How do ion channels select for and conduct specific ions?
• How do voltage-gated channels generate the action potential?
• How do ligand- and voltage-gated channels differ in what opens them?

Key concepts

• Resting membrane potential
• Ion gradients and selective permeability
• Voltage-gated sodium and potassium channels
• Ion channel selectivity filter
• Ligand-gated channels
• Action potential threshold and propagation

Key theories

Ionic theory of the action potential

Hodgkin and Huxley provided a quantitative description showing that the action potential results from voltage-dependent changes in the membrane's permeability to sodium and potassium, formalising neuronal excitability in mathematical terms.

Mechanisms

At rest, ion pumps maintain concentration gradients and the membrane is selectively permeable, chiefly to potassium, producing a negative resting potential. A depolarisation that reaches threshold opens voltage-gated sodium channels, whose rapid inward current drives the rising phase of the action potential; subsequent sodium-channel inactivation and the opening of voltage-gated potassium channels repolarise the membrane — the dynamics Hodgkin and Huxley quantified. The selectivity of channels for particular ions is determined by their pore structure; Doyle and colleagues revealed how a selectivity filter allows a potassium channel to conduct potassium while excluding sodium. Ligand-gated channels, by contrast, open in response to neurotransmitter binding rather than voltage.

Clinical relevance

Abnormalities of ion channels underlie a class of disorders affecting nerve and muscle, and many drugs and toxins act by modifying channel function, so the biophysics in this topic provides background for understanding excitability disorders and neuropharmacology. The entry is educational and is not a basis for diagnosis or treatment.

Evidence & guidelines

The topic is grounded in biophysics and structural biology rather than clinical guidelines, drawing on the Hodgkin-Huxley analysis of excitability, structural studies of channel selectivity, and standard references on ion channels.

History

The ionic basis of the action potential was established by Hodgkin and Huxley's mid-twentieth-century experiments on the squid giant axon, which yielded a quantitative model of excitability. Later voltage-clamp and patch-clamp methods characterised individual channel currents, and atomic-resolution structures, beginning with the potassium channel, explained how channels achieve their ion selectivity, integrating physiology with molecular structure.

Key figures

• Alan Hodgkin
• Andrew Huxley
• Roderick MacKinnon
• Bertil Hille

Seminal works

• hodgkin-huxley-1952
• doyle-1998

Frequently asked questions

What is the resting membrane potential?

It is the steady voltage difference across a neuron's membrane when it is not signalling, arising from ion concentration gradients and the membrane's selective permeability, chiefly to potassium, and is typically negative inside relative to outside.

How do voltage-gated channels make the action potential?

When the membrane depolarises to threshold, voltage-gated sodium channels open and let sodium rush in to produce the spike, then they inactivate while potassium channels open to restore the negative resting potential.