resting potential
Summary
The resting potential is the steady membrane voltage of an unstimulated excitable cell, about −70 mV (inside negative) in a typical neuron. It arises mainly because K⁺ leaks out through open channels while the membrane is far less permeable to Na⁺; the Na⁺/K⁺ pump maintains the gradients.
Detail
Meaning: The resting potential is the voltage across the plasma membrane (inside relative to outside) when a neuron is not firing, typically about −70 mV. K⁺ leak channels are open at rest, so K⁺ diffuses out down its concentration gradient and leaves the inside negative. Because Na⁺ permeability is low but not zero, the resting value sits near the K⁺ equilibrium potential (roughly −90 mV) and is pulled slightly toward positive by Na⁺ leaking in. The Na⁺/K⁺ ATPase moves 3 Na⁺ out and 2 K⁺ in per ATP, which restores the gradients that leak would otherwise dissipate.
Equation: Eion = (RT/zF) ln([ion]out/[ion]in), where Eion is the Nernst equilibrium potential (V), R = 8.314 J/(mol·K), T is temperature (K), z is the ion's charge (unitless), F = 9.65 × 10⁴ C/mol, and concentrations are in mol/L.
On the MCAT: Passages give ion concentrations or a voltage-versus-time trace and ask what happens when something changes. Raising extracellular K⁺ makes the resting potential less negative (depolarization). Opening more K⁺ channels moves it more negative (hyperpolarization). The resting potential is the baseline the cell returns to after an action potential.
Common confusion: Students credit the pump alone for the voltage. The pump is slightly electrogenic, but the main source is K⁺ leaving through leak channels. Blocking the pump changes the potential slowly, as gradients run down, while changing K⁺ permeability or extracellular K⁺ changes it quickly.
Sources
- Campbell Biology
- Principles of Neural Science
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.