The Resting Membrane Potential

Why a neuron sits at about −70 millivolts: unequal ion concentrations, a membrane leaky mainly to potassium, the sodium-potassium pump, and the equations — Nernst and Goldman — that describe them.

Put a fine electrode inside a resting neuron and another outside, and you will measure a voltage of roughly −60 to −70 millivolts: the inside is negative relative to the outside. This is the resting membrane potential. It arises from three ingredients.

1. Unequal ion concentrations

Inside the neuron, potassium (K⁺) is concentrated — around 140 millimolar, roughly 30 times more than outside. Outside, sodium (Na⁺) and chloride (Cl⁻) are concentrated. The cell also contains large negatively charged proteins that can't leave.

2. A membrane leaky mostly to potassium

At rest, the membrane contains many open potassium "leak" channels and few open sodium channels. Potassium therefore drifts out, down its concentration gradient. But each K⁺ ion that leaves takes a positive charge with it, making the inside more negative — which pulls potassium back. The two forces balance at the potassium equilibrium potential, about −90 mV, given by the Nernst equation. The membrane's small leak of sodium inward pulls the real resting potential a little above that, to about −70 mV. The Goldman–Hodgkin–Katz equation combines the contributions of all the ions weighted by how permeable the membrane is to each.

3. The pump that keeps it going

Left alone, the leaks would slowly run the gradients down. The sodium-potassium pump (Na⁺/K⁺-ATPase), discovered by Jens Christian Skou (Nobel Prize 1997), uses energy from ATP to push 3 sodium ions out for every 2 potassium ions it brings in. This pumping is a major reason the brain is so energy-hungry — a large share of its energy budget goes to restoring ion gradients after signalling.

Why it matters

The resting potential is stored energy. Opening channels lets ions rush down their gradients and changes the voltage — the basis of every synaptic potential and action potential. Disturb it — for example, high potassium in the blood depolarises cells — and nerves and heart muscle can stop working properly.