The resting potential of a neuron is negative because _____________.

Question

BlackTom AI · Accepted Answer

1. Relevant knowledge points and concepts:
Resting membrane potential arises from ionic concentration gradients (mainly K+ and Na+), selective membrane permeability (ion channels), and the electrogenic Na+/K+ ATPase. Intracellular proteins and organic anions are largely impermeant and contribute fixed negative charge. The Nernst equation gives the equilibrium potential for a single ion; the Goldman–Hodgkin–Katz equation gives the membrane potential when multiple ions with different permeabilities contribute.

2. Step-by-step reasoning:
Because the membrane at rest has many K+ “leak” channels, its permeability to K+ (PK) is much greater than to Na+ or other ions. K+ is concentrated inside the cell while Na+ is concentrated outside. K+ tends to diffuse out down its concentration gradient, leaving behind negatively charged, impermeant anions and creating a negative intracellular potential. The membrane potential settles where the electrical gradient opposes further K+ efflux — near the K+ equilibrium potential.

3. Calculations (intermediate steps using the Nernst equation):
For monovalent ion at 37°C: E = (61.5 mV) · log10([out]/[in]).
Typical values: [K+]in ≈ 140 mM, [K+]out ≈ 4 mM.
E_K = 61.5 · log10(4/140) = 61.5 · log10(0.02857) = 61.5 · (−1.544) ≈ −95 mV.
For Na+ (for comparison): [Na+]out ≈ 145 mM, [Na+]in ≈ 12 mM → E_Na ≈ +66 mV.
Because PK >> PNa, the resting potential is weighted toward E_K (e.g., ≈ −70 mV typical), producing a negative resting potential.

4. Why the correct answer is correct:
Option d states there is an excess of K+ inside and the membrane is permeable mainly to K+. That matches the biophysical basis: a high intracellular K+ concentration combined with dominant K+ permeability produces net K+ efflux and an interior negative voltage — the principal cause of the negative resting potential.

5. Why the other options are incorrect:
a) “Equally permeable to all ions” is false: if permeabilities were equal, Vrest would be an average of equilibrium potentials, not strongly negative as observed. b) Claims K+ is higher outside — incorrect direction of gradient. c) Places organic anions outside — in reality impermeant anions are inside and contribute negative charge. e) Emphasizes Cl– permeability: although Cl– distribution matters, resting permeability is dominated by K+ leak channels; stating membrane is mainly permeable to Cl– misidentifies the major contributor.

6. Conclusion:
Because intracellular K+ is high and the resting membrane is mainly permeable to K+, the resting potential is negative. This confirms answer d).

The resting potential of a neuron is negative because _____________.单项选择题

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The resting potential of a cell is negative because

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