Exposure to Cholera Toxin, can result in diarrhoea resulting in severe dehydration due to excessive water loss. Why might this be detrimental to neuronal homeostasis and what is the consequence?

Question

BlackTom AI · Accepted Answer

Loss of extracellular water concentrates ions; a rise in extracellular K+ ([K+]o) is particularly disruptive to neuronal membrane potentials. Key points: resting membrane potential is largely set by the K+ gradient (inside ~140 mM, outside ~3–4 mM) and the Nernst potential for K+. During an action potential, voltage-gated K+ channels open and K+ efflux repolarises the membrane; the driving force for that efflux depends on the electrochemical gradient for K+.

Step-by-step reasoning:
1. Use the Nernst equation for K+ at physiological temperature (approximate form): E_K ≈ -61.5 log([K+]i/[K+]o).
2. Baseline example: [K+]i = 140 mM, [K+]o = 4 mM → log(140/4) = log(35) ≈ 1.544 → E_K ≈ -61.5 × 1.544 ≈ -95 mV.
3. If dehydration raises [K+]o to 8 mM (example doubling): log(140/8) = log(17.5) ≈ 1.243 → E_K ≈ -61.5 × 1.243 ≈ -76.5 mV.
4. E_K becomes ~19 mV less negative, so the driving force for K+ efflux during repolarisation is reduced. With less K+ leaving, repolarisation is slowed and the membrane potential is less negative (depolarised). This impairs the normal return to resting potential after depolarisation and can disrupt neuronal signaling.

Why the correct option is correct: it explicitly states increased extracellular K+ reduces the K+ electrochemical gradient and decreases repolarisation — matching the Nernst-based reasoning and physiological role of K+ in repolarisation.

Why other options are incorrect:
- Options invoking increased extracellular Na+ focus on increasing Na+ gradient/excitability. Moderate changes in [Na+]o have less effect on AP amplitude (Na+ reversal potential ~+60 mV) and neuronal excitability is primarily determined by K+-mediated resting potential; dehydration’s most critical effect is on K+ handling.
- The last option correctly identifies decreased K+ gradient but wrongly claims increased K+ conductance; elevated [K+]o does not intrinsically increase K+ conductance and severe hyperkalaemia can inactivate Na+ channels, reducing excitability.

Conclusion: The correct answer is: "Increases extracellular K+ ion concentration → decreasing the electrochemical gradient for K+ → decrease repolarisation of membrane potential following depolarisation." This matches the provided answer.

Exposure to Cholera Toxin, can result in diarrhoea resulting in severe dehydration due to excessive water loss. Why might this be detrimental to neuronal homeostasis and what is the consequence?Single choice

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