Which of the following best distinguishes the resting membrane potential (RMP) from the equilibrium potential for potassium (E_K)?

Explanation

## Distinguishing RMP from E_K ### Resting Membrane Potential (RMP) - The **actual measured voltage** across the neuronal membrane at rest (approximately −70 mV in neurons) - Determined by the **relative permeability** of the membrane to all ions, not just one - Reflects the combined contribution of Na+, K+, Cl−, and other ions weighted by their conductance - Maintained by the Na+/K+ ATPase and selective ion permeability ### Equilibrium Potential for Potassium (E_K) - The **theoretical voltage** at which the concentration gradient for K+ exactly balances the electrical gradient - Calculated using the **Nernst equation**: $E_K = \frac{RT}{zF} \ln \frac{[K^+]_{out}}{[K^+]_{in}}$ ≈ −90 mV - At E_K, there is **zero net K+ flux** across the membrane - A **hypothetical value** — the membrane is never actually at E_K under physiological conditions ### Key Relationship | Feature | RMP | E_K | |---------|-----|-----| | **Nature** | Actual measured voltage | Theoretical equilibrium voltage | | **Value** | ~−70 mV | ~−90 mV | | **Determinants** | All ions (weighted by permeability) | K+ gradient alone | | **Meaning** | Voltage at which net current = 0 | Voltage at which K+ flux = 0 | | **Physiological state** | Steady-state condition | Never actually achieved | **Key Point:** The RMP is **more positive (less negative)** than E_K because the membrane is slightly permeable to Na+, which depolarizes the cell away from the K+ equilibrium potential. **High-Yield:** RMP ≈ −70 mV; E_K ≈ −90 mV; E_Na ≈ +60 mV. The RMP lies between E_K and E_Na, weighted toward E_K because K+ permeability >> Na+ permeability at rest.