Neuron X has standard resting permeabilities, and standard intracellular and extracellular concentrations of sodium, potassium, chloride and calcium. When depolarised to +58mV by a strong, excitatory synaptic input, what direction will Na+ move across the membrane if its selective channel were opened?
神经元 X 具有标准的静息膜通透性，以及标准的细胞内和细胞外钠、钾、氯和钙浓度。当受到强烈的兴奋性突触输入而去极化至 +58mV 时，如果其选择性通道打开，Na0 将在膜上向哪个方向移动？

(there are only 3 options in this question)
（本题只有 3 个选项）

Question

Neuron X has standard resting permeabilities, and standard intracellular and extracellular concentrations of sodium, potassium, chloride and calcium. When depolarised to +58mV by a strong, excitatory synaptic input, what direction will Na+ move across the membrane if its selective channel were opened?
神经元 X 具有标准的静息膜通透性，以及标准的细胞内和细胞外钠、钾、氯和钙浓度。当受到强烈的兴奋性突触输入而去极化至 +58mV 时，如果其选择性通道打开，Na0 将在膜上向哪个方向移动？

(there are only 3 options in this question)
（本题只有 3 个选项）

BlackTom AI · Accepted Answer

The Nernst equilibrium potential for Na+ determines the membrane voltage at which there is no net Na+ movement, so if the membrane is at that value Na+ flux is zero.

1) Relevant knowledge points and concepts
- Nernst equation gives the equilibrium potential for a single ion: Eion = (RT/zF) ln([out]/[in]).
- When membrane potential (Vm) equals an ion’s equilibrium potential (Eion), the electrical and chemical (concentration) driving forces balance and net movement of that ion is zero.
- Typical physiological concentrations produce ENa around +58 to +61 mV.

2) Complete reasoning process step by step
- Use the Nernst equation for Na+ (z = +1).
- At physiological temperature (approx. 37°C) it is convenient to use the log10 form: ENa ≈ 61.5 mV · log10([Na+]out/[Na+]in).
- Plugging standard concentrations (typical textbook values): [Na+]out ≈ 145 mM, [Na+]in ≈ 15 mM.

3) Calculations with intermediate steps
- Ratio = 145 / 15 = 9.6667.
- log10(9.6667) ≈ 0.985.
- ENa ≈ 61.5 mV × 0.985 ≈ 60.6 mV.
- Many references use slightly different standard intracellular values (e.g., 16 mM) which give ENa ≈ +58 mV. Thus the commonly quoted ENa ~ +58 mV is consistent with “standard” conditions.

4) Why the correct answer is correct
- If the neuron is depolarized to +58 mV, and ENa is approximately +58 mV under standard conditions, then Vm = ENa. At this voltage the electrochemical gradient on Na+ is zero, so opening a Na+-selective channel produces no net Na+ movement (no inward or outward flux).

5) Why the other options are incorrect
- Inward movement would require Vm to be more negative than ENa so the electrical/chemical forces favor Na+ entry; at Vm = ENa this is not the case.
- Outward movement would require Vm to be more positive than ENa so Na+ would be driven out; again not true when Vm = ENa.

6) Conclusion confirming the final answer
- Therefore, with the membrane at +58 mV (the equilibrium potential for Na+ under standard conditions), Na+ will show no net movement. This matches the provided correct answer: No movement (静止不动).

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