Resting Potential

The Loaded Spring

Every neuron holds a static electrical charge across its membrane even when completely silent. This baseline, the resting potential, sits at about $-70\text{ mV}$ (inside negative relative to outside) and acts as a loaded spring ready to fire.

Two forces maintain this state. First, ion gradients create concentration differences. Second, selective permeability lets potassium ($K^+$) leak out far more easily than sodium ($Na^+$) leaks in, pulling the voltage close to the potassium equilibrium.

Common pitfall: Treating $-70\text{ mV}$ as a neutral state. It is not zero; it is a heavily defended, polarized baseline powered by continuous cellular work.

Keeping the Charge

Left alone, leaking ions would eventually destroy the gradient. The sodium-potassium pump prevents collapse by spending one ATP to export three $Na^+$ ions while importing two $K^+$ ions.

Because it moves three positive charges out for every two in, the pump is electrogenic, adding a small direct negativity on top of the diffusion potential.

$$V_{rest} \approx -70\text{ mV}$$

If ATP supply stops, the pump halts. The resting potential does not vanish instantly, but it decays steadily as passive leaks drain the ion gradients.