resting membrane potential

The resting membrane potential is the steady electrical charge a heart cell carries when it is quiet and waiting between beats. The inside of the cell is negative relative to the outside — a kind of charged-up readiness, like a battery sitting fully primed before it is used.

In a typical working ventricular cell this baseline sits around minus 90 millivolts. It exists because the cell membrane keeps ions unevenly distributed: potassium is concentrated inside, sodium and calcium outside, and the membrane at rest is most permeable to potassium. The constant background of potassium leaking outward, maintained by ion pumps, holds the inside at its negative resting value. This stored charge is the potential energy that a depolarization later releases.

Not every heart cell has a true resting potential. Pacemaker cells in the sinoatrial and atrioventricular nodes lack a stable baseline — their voltage drifts upward continuously, which is the basis of their automaticity. The resting value also depends on potassium balance, so abnormal blood potassium levels shift it and can dangerously alter excitability: high potassium makes the resting potential less negative and the cell harder to fire reliably, which is why severe hyperkalemia is a cardiac emergency.