Which of the following statements concerning the neuronal membrane at rest is correct?

Question

BlackTom AI · Accepted Answer

Intracellular potassium concentration is higher than extracellular, so the chemical gradient drives K+ out of the neuron at rest.

Relevant concepts:
- Resting membrane potential (~−60 to −80 mV) arises mainly from K+ permeability and the electrochemical gradients for ions.
- The Na+/K+ ATPase maintains concentration differences: typically [K+]inside ≈ 140 mM and [K+]outside ≈ 3–5 mM; [Na+]outside ≈ 140–145 mM and [Na+]inside ≈ 10–15 mM.
- The net movement of an ion depends on both its concentration (chemical) gradient and the electrical gradient; at rest the electrical gradient (negative inside) partially opposes K+ efflux.
- Excitability relates to how close the membrane potential is to threshold for action potential initiation; depolarization (less negative) increases excitability, hyperpolarization (more negative) decreases it.

Step-by-step reasoning:
1. Compare concentrations: K+ is much more concentrated inside the cell than outside (≈140 mM inside vs ≈4 mM outside). By Fick’s law, diffusion moves K+ from high to low concentration — i.e., out of the cell.
2. At the same time the inside-negative membrane potential attracts positively charged K+ ions inward. Resting potential is near the K+ Nernst potential because membrane permeability to K+ is high; equilibrium is reached when outward chemical flux equals inward electrical flux.
3. Sodium shows the opposite gradient (high outside, low inside), so Na+ tends to diffuse into the cell, not out.
4. Chloride is typically more concentrated outside than inside (so the statement that it is greatest inside is false).
5. If the membrane becomes more negative (hyperpolarized), the cell is farther from threshold, so it is less excitable, not more.

Why the correct answer is correct:
Because intracellular K+ concentration is markedly higher than extracellular, the chemical (concentration) gradient favors K+ efflux. The resting membrane potential reflects a balance between this outward chemical drive and the inward electrical attraction; hence the correct statement is that the concentration gradient for potassium tends to diffuse K+ out of the cell.

Why other options are incorrect:
- Extracellular sodium concentration is greater, not less, than intracellular. 
- Chloride concentration is generally greater outside the cell. 
- A more negative resting potential (hyperpolarization) makes the neuron less excitable.

Final confirmation: The correct answer is “The concentration gradient for potassium is such that it tends to diffuse out of the cell.”

Which of the following statements concerning the neuronal membrane at rest is correct?单项选择题

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The resting potential of a cell is negative because

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