Nerve and muscle: differential nerve block: 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 and muscle: differential nerve block

Sample item

During the onset of a spinal anaesthetic with bupivacaine, a patient first feels warmth in the legs and can no longer feel a pinprick, yet can still move the toes. What best explains why motor function is lost last?

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

The answer and the reasoning

A. Large myelinated motor fibres need a longer exposed length of nerve blocked and so are blocked last

Small fibres, including preganglionic sympathetic B fibres, then A-delta and C pain and temperature fibres, are blocked first. Large myelinated Alpha motor fibres need several nodes of Ranvier exposed to the anaesthetic, so they are blocked last and recover first. Sympathetic block explains the early warmth and vasodilatation.

Option by option

  • A. Large myelinated motor fibres need a longer exposed length of nerve blocked and so are blocked last (correct)

    Correct. Fibre diameter and the number of nodes to be blocked set the order.

  • B. The local anaesthetic does not reach the anterior nerve roots

    Anterior and posterior roots lie in the same cerebrospinal fluid and are bathed by the same drug.

  • C. Motor fibres are unmyelinated and conduct too slowly to be blocked

    Motor fibres are heavily myelinated, not unmyelinated.

  • D. Motor neurones lack voltage-gated sodium channels

    Motor neurones depend on sodium channels for conduction, and the same channel is blocked.

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, Large myelinated motor fibres need a longer exposed length of nerve blocked and so are blocked last. Small fibres, including preganglionic sympathetic B fibres, then A-delta and C pain and temperature fibres, are blocked first.

Why is "The local anaesthetic does not reach the anterior nerve roots" not the answer?

Anterior and posterior roots lie in the same cerebrospinal fluid and are bathed by the same drug.

Why is "Motor fibres are unmyelinated and conduct too slowly to be blocked" not the answer?

Motor fibres are heavily myelinated, not unmyelinated.

Why is "Motor neurones lack voltage-gated sodium channels" not the answer?

Motor neurones depend on sodium channels for conduction, and the same channel is blocked.

What concept does this question test?

It tests Nerve and muscle: differential nerve block in Physiology. Large myelinated Alpha motor fibres need several nodes of Ranvier exposed to the anaesthetic, so they are blocked last and recover first. Sympathetic block explains the early warmth and vasodilatation.

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 and muscle: differential nerve block?

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.