The resting membrane potential of a typical neuron is approximately −70 mV. Which ion gradient is primarily responsible for maintaining this potential?

Explanation

## Ion Gradients and Resting Membrane Potential **Key Point:** The resting membrane potential (RMP) of approximately −70 mV in neurons is maintained by the **Na⁺/K⁺-ATPase pump** and the **selective permeability of the cell membrane** to different ions. ### Ionic Composition | Ion | Intracellular | Extracellular | Concentration Gradient | |-----|---------------|---------------|------------------------| | K⁺ | ~140 mM | ~5 mM | 28:1 (high inside) | | Na⁺ | ~12 mM | ~145 mM | 1:12 (low inside) | | Cl⁻ | ~4 mM | ~110 mM | 1:27 (low inside) | | Ca²⁺ | ~0.0001 mM | ~2 mM | 1:20,000 (very low inside) | ### Mechanism 1. **Na⁺/K⁺-ATPase** pumps 3 Na⁺ OUT and 2 K⁺ IN (electrogenic — creates −10 to −15 mV contribution) 2. **Selective membrane permeability** at rest: - Membrane is ~25× more permeable to K⁺ than Na⁺ - K⁺ diffuses OUT down its concentration gradient - This outward K⁺ movement leaves behind negative anions (proteins, phosphates) - Results in negative internal charge **High-Yield:** The **Nernst equation** for K⁺ alone predicts RMP ≈ −90 mV; the actual −70 mV reflects the small but significant Na⁺ leak inward. $$E_K = 61 \log \frac{[K^+]_{out}}{[K^+]_{in}} = 61 \log \frac{5}{140} ≈ −90 \text{ mV}$$ **Mnemonic:** **K-IN, Na-OUT** — K⁺ concentration gradient (high inside) is the PRIMARY determinant of RMP; Na⁺ gradient (high outside) opposes it slightly.