Resting Membrane Potential

Core Idea

Steady-state voltage (~−70 mV) maintained by Na+/K+ ATPase gradients and differential permeability. At rest, K+ conductance dominates.

Explainer

To understand the resting membrane potential, start with what the Na+/K+ ATPase has already accomplished: it has used ATP to pump Na+ out and K+ in against their respective concentration gradients, creating a cell interior that is high in K+ (~140 mM) and low in Na+ (~15 mM) relative to the extracellular fluid. This pump runs continuously and is the engine that makes everything else possible. Without it, the gradients — and the resting potential — would collapse.

With those gradients in place, consider what happens at the membrane. At rest, the membrane contains many open K+ leak channels and very few open Na+ channels. K+ ions, driven by their concentration gradient (high inside → low outside), flow outward through the leak channels. As positive charges leave, the inside of the membrane becomes increasingly negative. This growing negativity exerts an electrical pull back on K+ — and at some point the outward chemical drive and the inward electrical pull balance exactly. That balance point is the K+ equilibrium potential, approximately −90 mV.

The resting membrane potential sits at about −70 mV rather than −90 mV because the membrane is not perfectly impermeable to Na+. A small but nonzero Na+ conductance allows a trickle of Na+ to flow inward (driven by both its concentration gradient and the negative interior), nudging the potential slightly positive of E_K. The Goldman equation formalizes this: the resting potential is a conductance-weighted average of all ion equilibrium potentials, dominated by K+ but slightly offset by Na+ and Cl−.

A common misconception is that the Na+/K+ ATPase is directly pumping the membrane to −70 mV the way a battery charges a capacitor. In reality, the pump's direct electrogenic contribution (3 Na+ out, 2 K+ in per cycle — net −1 charge per cycle) accounts for only a few millivolts. What the pump actually does is maintain the concentration gradients that the leak channels then convert into voltage. If you block the pump with ouabain, the resting potential does not collapse immediately — the gradients can sustain the potential for some time before dissipating.

Understanding the resting membrane potential is essential for what comes next: action potentials. The −70 mV resting state is like a compressed spring. When voltage-gated Na+ channels open (e.g., upon sufficient depolarization), Na+ rushes inward down both its concentration and electrical gradients, and the membrane rapidly depolarizes toward +60 mV. The resting potential you have just studied is the baseline from which that explosive reversal begins.