Nerve physiology: compound action potential: 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: compound action potential

Sample item

A mammalian mixed nerve is stimulated supramaximally. The compound action potential recorded 10 cm from the stimulating electrode is lower and broader than the one recorded 2 cm away. What is the explanation?

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

The answer and the reasoning

A. Fibres with different conduction velocities arrive at different times, so the summed signal disperses

A compound action potential is the algebraic sum of many fibres with different velocities. Over a longer distance, fast fibres arrive earlier and slow ones later, so the peak flattens and widens (temporal dispersion). Individual action potentials are regenerated at each node and do not decay.

Option by option

  • A. Fibres with different conduction velocities arrive at different times, so the summed signal disperses (correct)

    Correct. Temporal dispersion of fibres with different velocities explains the change.

  • B. The Na+-K+ pump progressively fatigues along the nerve

    Pump fatigue does not occur over a single brief volley and would not selectively broaden the waveform.

  • C. Distant axons have a higher threshold and are not recruited

    A supramaximal stimulus recruits every fibre, so threshold differences do not explain the difference.

  • D. Each action potential loses amplitude as it propagates along the axon

    Action potentials propagate without decrement because they are regenerated along the axon.

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.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, Fibres with different conduction velocities arrive at different times, so the summed signal disperses. A compound action potential is the algebraic sum of many fibres with different velocities.

Why is "The Na+-K+ pump progressively fatigues along the nerve" not the answer?

Pump fatigue does not occur over a single brief volley and would not selectively broaden the waveform.

Why is "Distant axons have a higher threshold and are not recruited" not the answer?

A supramaximal stimulus recruits every fibre, so threshold differences do not explain the difference.

Why is "Each action potential loses amplitude as it propagates along the axon" not the answer?

Action potentials propagate without decrement because they are regenerated along the axon.

What concept does this question test?

It tests Nerve physiology: compound action potential in Physiology. Over a longer distance, fast fibres arrive earlier and slow ones later, so the peak flattens and widens (temporal dispersion). Individual action potentials are regenerated at each node and do not decay.

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: compound action potential?

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.