membrane potential

Meaning: Membrane potential is the electrical potential difference across the plasma membrane, inside minus outside. A resting neuron sits near −70 mV. Ions are unevenly distributed (K⁺ high inside, Na⁺ high outside), and the resting membrane is far more permeable to K⁺ than to Na⁺. K⁺ leaks out through open channels, leaving negative charge behind, so the potential is pulled toward the K⁺ equilibrium potential. The Na⁺/K⁺ ATPase (3 Na⁺ out, 2 K⁺ in) maintains the gradients and adds a small direct negative contribution. Depolarization makes the potential less negative; hyperpolarization makes it more negative.

Equation: Eion = (RT/zF) ln([ion]out/[ion]in), where Eion is the equilibrium potential of one ion (V), R = 8.31 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. The actual membrane potential is a permeability-weighted blend of these values (the Goldman equation).

On the MCAT: A passage alters ion concentrations or channels in a neuron or muscle cell, and you predict the direction of change. Raising extracellular K⁺ makes EK less negative, so the cell depolarizes. Opening Na⁺ channels moves the potential toward ENa (positive); opening K⁺ channels moves it toward EK (more negative). You may also read voltage-versus-time graphs of an action potential.

Common confusion: Students credit the pump with creating the resting potential. The pump builds and maintains the gradients, but the K⁺ leak sets most of the value, so blocking the pump changes the potential only slowly.