## Resting Membrane Potential Determinants **Key Point:** The resting membrane potential is primarily determined by the K⁺ gradient because the resting neuronal membrane is much more permeable to K⁺ than to Na⁺ or other ions. ### Ion Concentration Gradients | Ion | Intracellular | Extracellular | Equilibrium Potential (E~ion~) | | --- | --- | --- | --- | | K⁺ | 140 mM | 5 mM | –90 mV | | Na⁺ | 10 mM | 145 mM | +60 mV | | Cl⁻ | 10 mM | 110 mM | –70 mV | | Ca²⁺ | 0.0001 mM | 2 mM | +120 mV | ### Why K⁺ Dominates 1. **Selective Permeability:** At rest, the neuronal membrane is ~25× more permeable to K⁺ than to Na⁺ due to open K⁺ leak channels. 2. **Goldman-Hodgkin-Katz Equation:** The resting potential (–70 mV) lies closer to E~K~⁺ (–90 mV) than to E~Na~⁺ (+60 mV), reflecting K⁺'s dominant contribution. 3. **Na⁺–K⁺-ATPase:** This pump maintains the K⁺ gradient by actively extruding 3 Na⁺ and importing 2 K⁺, establishing and sustaining the electrochemical gradient. **High-Yield:** The resting potential is NOT at the K⁺ equilibrium potential (–90 mV) but slightly less negative (–70 mV) because small Na⁺ permeability and Cl⁻ permeability also contribute slightly. **Mnemonic:** **K-REST** — K⁺ determines the RESTing potential because the membrane is highly permeable to K⁺ at rest.
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