Nerve physiology: accommodation: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Nerve physiology: accommodation

Sample item

In a nerve preparation, a stimulating current is increased extremely slowly. The membrane is depolarised beyond the usual threshold potential, yet no action potential is generated. Which mechanism explains this accommodation?

AChloride influx hyperpolarises the membrane below thresholdBSodium channels inactivate and potassium channels open during the slow depolarisationCSodium channels are permanently locked open by the stimulusDVoltage-gated calcium channels open and clamp the potential

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. Sodium channels inactivate and potassium channels open during the slow depolarisation

A slow ramp of depolarisation gives time for the inactivation gates of sodium channels to close and for delayed-rectifier potassium channels to open. The effective threshold rises, so no spike fires. A sudden step of the same size would fire a spike.

Option by option

A. Chloride influx hyperpolarises the membrane below threshold
Chloride movement is not responsible; the membrane is depolarised, not hyperpolarised.
B. Sodium channels inactivate and potassium channels open during the slow depolarisation (correct)
Correct. Sodium inactivation with potassium activation raises the threshold.
C. Sodium channels are permanently locked open by the stimulus
Sodium channels that stayed open would produce depolarisation block, not a failure to start a spike during a gradual ramp.
D. Voltage-gated calcium channels open and clamp the potential
Calcium channels play no clamping role in the axonal action potential.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is B, Sodium channels inactivate and potassium channels open during the slow depolarisation. A slow ramp of depolarisation gives time for the inactivation gates of sodium channels to close and for delayed-rectifier potassium channels to open.

Why is "Chloride influx hyperpolarises the membrane below threshold" not the answer?

Chloride movement is not responsible; the membrane is depolarised, not hyperpolarised.

Why is "Sodium channels are permanently locked open by the stimulus" not the answer?

Sodium channels that stayed open would produce depolarisation block, not a failure to start a spike during a gradual ramp.

Why is "Voltage-gated calcium channels open and clamp the potential" not the answer?

Calcium channels play no clamping role in the axonal action potential.

What concept does this question test?

It tests Nerve physiology: accommodation in Physiology. The effective threshold rises, so no spike fires. A sudden step of the same size would fire a spike.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Nerve physiology: accommodation?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.