Electrophysiology and Membrane Potential

How the separation of ions across a membrane creates an electrical potential, and how voltage-dependent channels turn that potential into the action potentials of excitable cells.

Definition

The membrane potential is the voltage difference across a cell membrane arising from selective ion permeability; electrophysiology is the study of these potentials and the currents that produce them.

Scope

This topic covers the origin of the resting membrane potential, the equivalent-circuit view of the membrane as capacitor and conductances, and the generation and propagation of the action potential. It treats the Hodgkin–Huxley description of excitability and the measurement techniques of electrophysiology at a conceptual level, while single-channel mechanism and transport energetics are handled in adjacent topics.

Core questions

• Why does a cell at rest hold a steady voltage across its membrane?
• How is the membrane usefully modelled as a capacitor in parallel with ionic conductances?
• What sequence of conductance changes generates an action potential?
• How does an action potential propagate along an excitable cell?

Key theories

Hodgkin–Huxley excitability

Voltage- and time-dependent sodium and potassium conductances acting across the capacitive membrane reproduce the action potential quantitatively, with regenerative sodium entry depolarising the cell and delayed potassium efflux restoring rest.

Resting potential from mixed permeabilities

The resting voltage is a weighted average of the equilibrium potentials of the permeant ions, captured by Goldman's constant-field expression, so it sits near but not at the potassium equilibrium because the membrane is mainly, not exclusively, potassium-permeable.

Mechanisms

Because the membrane is a thin insulator that separates charge, ion gradients set up by pumps produce a steady resting voltage determined mainly by the dominant resting permeability. Treating the membrane as a capacitor in parallel with voltage-dependent conductances, a suprathreshold depolarisation opens sodium channels, whose inward current further depolarises the cell in a regenerative spike; sodium channels then inactivate and potassium channels open, repolarising the membrane. Local currents spread this depolarisation to neighbouring regions, propagating the action potential.

Clinical relevance

Membrane excitability underlies nerve, muscle, and cardiac function and is the target of anaesthetics, antiarrhythmics, and antiepileptics; the biophysics here is educational background for that physiology and pharmacology, not clinical guidance.

History

Building on Cole's voltage-clamp technique, Hodgkin and Huxley's 1952 quantitative model of the squid axon explained the action potential in terms of ionic conductances and remains the foundation of electrophysiology, later refined by single-channel and molecular studies.

Key figures

• Alan Hodgkin
• Andrew Huxley
• Bernard Katz
• Kenneth Cole

Seminal works

• hodgkin1952
• goldman1943

Frequently asked questions

Why is the resting potential negative inside?

At rest the membrane is most permeable to potassium, which leaves the cell down its gradient until the building negative interior voltage opposes further loss, leaving the inside negative relative to the outside.

What makes the action potential all-or-none?

Once depolarisation crosses threshold, sodium-channel opening is regenerative and drives a full spike regardless of how strong the original stimulus was, so the response has a fixed shape.