Nerve physiology: cable properties: 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: cable properties

Sample item

Two unmyelinated axons have the same membrane properties, but axon X has a larger diameter than axon Y. Which statement about passive spread of depolarisation is correct?

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

The answer and the reasoning

B. Axon X has a longer length constant and conducts faster

The length constant is the square root of membrane resistance divided by axoplasmic resistance per unit length. Increasing diameter lowers axoplasmic resistance more than it lowers membrane resistance, so the length constant increases roughly with the square root of diameter. Current then spreads further ahead of the active region and conduction velocity rises.

Option by option

  • A. Both have the same length constant because membrane properties are equal

    Equal membrane properties do not make equal cable properties, since axoplasmic resistance depends on diameter.

  • B. Axon X has a longer length constant and conducts faster (correct)

    A larger diameter lengthens the length constant and speeds conduction.

  • C. Axon X conducts slower because internal resistance is higher

    Internal resistance falls with larger cross-sectional area, so it is lower in axon X.

  • D. Axon X has a shorter length constant and conducts faster

    A shorter length constant would slow conduction, and thicker axons do not have one.

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 B, Axon X has a longer length constant and conducts faster. The length constant is the square root of membrane resistance divided by axoplasmic resistance per unit length.

Why is "Both have the same length constant because membrane properties are eq…" not the answer?

Equal membrane properties do not make equal cable properties, since axoplasmic resistance depends on diameter.

Why is "Axon X conducts slower because internal resistance is higher" not the answer?

Internal resistance falls with larger cross-sectional area, so it is lower in axon X.

Why is "Axon X has a shorter length constant and conducts faster" not the answer?

A shorter length constant would slow conduction, and thicker axons do not have one.

What concept does this question test?

It tests Nerve physiology: cable properties in Physiology. Increasing diameter lowers axoplasmic resistance more than it lowers membrane resistance, so the length constant increases roughly with the square root of diameter. Current then spreads further ahead of the active region and conduction velocity rises.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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: cable properties?

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.