The Neuron at Rest
Every neuron holds a static electrical charge across its membrane even when it is transmitting nothing at all. This baseline, the resting potential, sits at about millivolts (the inside of the cell negative relative to the outside) and is not a passive accident — it is an actively maintained state that gives the neuron the "loaded spring" it needs to fire.
Two forces set the resting value. First, ion gradients: sodium (Na+) and chloride (Cl-) are far more concentrated outside the cell, while potassium (K+) and large organic anions are far more concentrated inside. Second, selective permeability: the resting membrane leaks K+ far more easily than it leaks Na+, so the membrane potential settles close to the equilibrium potential for K+ rather than halfway between the two ions.
| Ion | Concentration | Resting permeability |
|---|---|---|
| Na+ | High outside | Low |
| K+ | High inside | High |
| Cl- | High outside | Moderate |
| A- (organic anions) | High inside | Impermeant |
Left alone, K+ would keep leaking out and Na+ would keep leaking in until the gradients collapsed. The sodium-potassium pump prevents that: using one molecule of ATP per cycle, it exports three Na+ ions for every two K+ ions it imports. Because it moves three positive charges out for only two in, the pump is itself slightly electrogenic — it adds a small extra negativity on top of the diffusion potential.
Common pitfall: treating the resting potential as "the membrane has no charge." It is not neutral — it is a steady mV, held there by continuous active transport. Cut off the ATP supply and the resting potential does not vanish instantly, but it decays as the pump stops fighting the leaks.
Membrane cross-section: resting ion gradient shown as dot-density outside (Na+/Cl-) versus inside (K+/A-) the membrane; the Na+/K+ pump animates one cycle, 3 accent dots crossing out and 2 crossing in.