Regarding the ionic basis of resting membrane potential (RMP) in a mammalian neuron, all of the following statements are correct EXCEPT:

Question

Accepted Answer

D. Chloride ions contribute to RMP maintenance by equilibrating across the membrane at a potential more negative than the K⁺ equilibrium potential. ## Analysis of Resting Membrane Potential Statements

**Key Point:** The resting membrane potential is determined by the unequal distribution of ions across the neuronal membrane and the selective permeability of the membrane to different ions.

### Evaluation of Each Statement

| Statement | Correctness | Reason |
|-----------|-------------|--------|
| Na⁺/K⁺-ATPase stoichiometry | ✓ Correct | Pumps out 3 Na⁺, imports 2 K⁺; electrogenic (net outward positive charge) |
| K⁺ permeability dominance | ✓ Correct | At rest, P_K >> P_Na (ratio ~40:1), so RMP approaches K⁺ equilibrium potential (~−90 mV); actual RMP ~−70 mV due to small Na⁺ leak |
| Chloride contribution | ✗ **INCORRECT** | Cl⁻ equilibrium potential is approximately **−70 to −80 mV**, which is MORE negative than or equal to RMP, NOT more negative than K⁺ equilibrium potential (−90 mV). Cl⁻ is typically at or near electrochemical equilibrium and does NOT actively drive RMP; it follows the potential set by K⁺ and Na⁺ gradients |
| Goldman-Hodgkin-Katz equation | ✓ Correct | The GHK equation incorporates all three ions and their permeabilities: $V_m = \frac{RT}{F} \ln \frac{P_K[K^+]_{out} + P_{Na}[Na^+]_{out} + P_{Cl}[Cl^-]_{in}}{P_K[K^+]_{in} + P_{Na}[Na^+]_{in} + P_{Cl}[Cl^-]_{out}}$ |

**High-Yield:** Chloride is a **passive follower** of the membrane potential; it does not actively establish RMP. The K⁺/Na⁺ gradient and their permeabilities are the primary determinants.

**Clinical Pearl:** In neurons, Cl⁻ is often maintained at electrochemical equilibrium by the NKCC1 cotransporter (in immature neurons, Cl⁻ is elevated intracellularly, making GABA depolarizing). Understanding Cl⁻ distribution is crucial for interpreting inhibitory neurotransmission.

### Why the Correct Answer is Option 3

The statement incorrectly claims that Cl⁻ equilibrium potential is more negative than the K⁺ equilibrium potential. In reality, E_Cl (~−70 mV) is **less negative** (i.e., more depolarized) than E_K (~−90 mV). This is a common source of confusion in physiology exams.

Answer

A. The Goldman-Hodgkin-Katz equation accounts for all three major ions (K⁺, Na⁺, and Cl⁻) and their relative permeabilities to predict RMP

Answer

B. The resting membrane potential is approximately −70 mV because K⁺ permeability is much higher than Na⁺ permeability at rest

Answer

C. The Na⁺/K⁺-ATPase pump maintains the concentration gradients by extruding 3 Na⁺ ions and importing 2 K⁺ ions per ATP hydrolyzed

Regarding the ionic basis of resting membrane potential (RMP) in a mammalian neuron, all of the following statements are correct EXCEPT:

Explanation

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