Which ion is the most common determinant of the resting membrane potential in excitable cells?

Explanation

## Determinants of Resting Membrane Potential **Key Point:** The resting membrane potential (RMP) in excitable cells is approximately −70 mV, and potassium is the primary ion responsible for establishing this voltage. ### Why Potassium Dominates The resting membrane is far more permeable to K⁺ than to other ions due to the high number of open K⁺ channels at rest. Using the **Goldman-Hodgkin-Katz (GHK) equation**, the membrane potential is weighted by both the concentration gradient AND the relative permeability of each ion: $$V_m = \frac{RT}{F} \ln \frac{P_K[K^+]_{out} + P_{Na}[Na^+]_{out} + P_{Cl}[Cl^-]_{in}}{P_K[K^+]_{in} + P_{Na}[Na^+]_{in} + P_{Cl}[Cl^-]_{out}}$$ At rest, $P_K \gg P_{Na}$ and $P_{Cl}$ is low, so the equation simplifies to the **Nernst potential for K⁺** (~−90 mV). The actual RMP is slightly less negative (−70 mV) because of small Na⁺ leak inward and the electrogenic Na⁺/K⁺-ATPase (which pumps 3 Na⁺ out for 2 K⁺ in). ### Ion Concentration Gradients | Ion | Intracellular | Extracellular | Nernst Potential | |-----|---------------|---------------|------------------| | K⁺ | 140 mM | 5 mM | −90 mV | | Na⁺ | 10 mM | 145 mM | +60 mV | | Cl⁻ | 10 mM | 110 mM | −70 mV | **High-Yield:** K⁺ permeability at rest is ~25–30 times greater than Na⁺ permeability, making K⁺ the dominant ion. **Clinical Pearl:** Hyperkalemia (elevated extracellular K⁺) depolarizes the membrane by reducing the K⁺ concentration gradient, increasing excitability initially, then paradoxically decreasing it as the membrane becomes less excitable. Hypokalemia hyperpolarizes the membrane, reducing excitability.