leak channels

Meaning: Leak channels are ion channels open at rest, so ions pass down their electrochemical gradients without a voltage change or ligand. In neurons, K⁺ leak channels matter most, and Na⁺ leak channels are fewer. Resting permeability to K⁺ is much higher than to Na⁺, so the resting potential (about −70 mV in a typical neuron) lies near the K⁺ equilibrium potential. The Na⁺/K⁺ ATPase (3 Na⁺ out, 2 K⁺ in) uses ATP to maintain the gradients the leak runs down.

Equation: Vm = (RT/F) ln[(PK[K⁺]out + PNa[Na⁺]out) / (PK[K⁺]in + PNa[Na⁺]in)], where Vm is membrane potential (V), R is the gas constant (J/mol·K), T is temperature (K), F is the Faraday constant (C/mol), P is permeability (m/s), and brackets are concentrations (mol/m³). Cl⁻ is omitted here.

On the MCAT: Passages ask what happens to resting potential if K⁺ leak permeability rises (hyperpolarization toward the K⁺ equilibrium potential), if Na⁺ leak rises (depolarization), or if the pump is blocked (gradients slowly dissipate).

Common confusion: Students credit the pump with setting the resting potential. The pump is slightly electrogenic, but the K⁺ leak mainly sets the value. Leak channels are passive and use no ATP.