When a neuron is not actively signaling, its membrane still maintains a steady voltage difference, typically around –70 mV, with the interior negative relative to the exterior. This resting potential is the baseline from which all electrical signaling departs.

It arises from the unequal distribution of ions—high potassium inside, high sodium outside—and from the membrane’s greater permeability to potassium at rest. The sodium-potassium pump continuously expends energy to maintain the gradients against leak. Because the resting potential is close to the potassium equilibrium potential, small changes in potassium conductance strongly influence it.

The resting potential determines the driving force on ions and the distance to threshold. Factors that alter it—changes in extracellular potassium, pump activity, or background conductances—can increase or decrease excitability and are implicated in conditions ranging from hyperkalemic periodic paralysis to spreading depression. Measuring resting potential is a basic step in any intracellular study of neuronal function.

Even at “rest,” the membrane is a dynamic equilibrium maintained by continuous ion flux and active transport.