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 K+ 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，K+ 将向哪个方向跨膜移动？

Question

BlackTom AI · Accepted Answer

E_K (the K+ Nernst potential) is strongly negative (around −90 to −95 mV) under standard intracellular/extracellular concentrations, so at a membrane potential of +58 mV K+ will flow out.

1) Relevant concepts
- Nernst potential (E_ion) gives the membrane potential at which there is no net movement of that ion. 
- Driving force for an ion = V_m − E_ion. For a positively charged ion, a positive driving force corresponds to net outward movement (efflux).
- Typical physiological concentrations: [K+]_in ≈ 140 mM, [K+]_out ≈ 4 mM.

2) Step-by-step reasoning and calculations
- Compute E_K using the Nernst equation (at ~37°C, simplified factor ≈ 61.5 mV):
  E_K = 61.5 mV × log10([K+]_out / [K+]_in)
  = 61.5 × log10(4 / 140)
  = 61.5 × log10(0.02857)
  log10(0.02857) ≈ −1.5441
  E_K ≈ 61.5 × (−1.5441) ≈ −95 mV (approximately).
- Driving force = V_m − E_K = (+58 mV) − (−95 mV) = +153 mV.
- A positive driving force for a cation means net outward movement. Thus, if K+-selective channels open, K+ will leave the cell.

3) Why this is correct
- Chemically, K+ is higher inside than outside so diffusion tends to drive K+ out. Electrically, the inside being strongly positive (+58 mV) repels positive K+ ions outward. Both components of the electrochemical gradient therefore favor efflux, so opening K+ channels produces K+ movement out of the cell until V_m approaches E_K.

4) Why other choices are wrong
- “No change”: incorrect because the membrane is far from E_K; opening channels will cause current flow. 
- “In”: incorrect because for K+ to move inward the membrane potential would need to be more negative than E_K (to attract K+ in); +58 mV is much more positive than E_K, so K+ is driven out.

Conclusion: K+ will move Out, which matches the provided correct answer.

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