Motor system: upper motor neurone lesion: 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 · Motor system: upper motor neurone lesion

Sample item

A man six weeks after a stroke has a spastic arm, brisk reflexes and an upgoing great toe on the same side. Which mechanism most directly accounts for the spasticity?

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

The answer and the reasoning

B. Loss of descending inhibitory drive, which disinhibits the stretch reflex

Lesions of the upper motor neurone remove corticoreticular and other descending inhibitory influences, so gamma-motor neurone and stretch reflex gain increase, causing velocity-dependent hypertonia, hyperreflexia, clonus and the extensor plantar response. Lower motor neurone lesions cause flaccidity, atrophy and fasciculations. Spasticity may be delayed after the initial flaccid stage.

Option by option

  • A. Degeneration of anterior horn cells

    Loss of anterior horn cells would produce flaccid weakness with wasting.

  • B. Loss of descending inhibitory drive, which disinhibits the stretch reflex (correct)

    Correct. Disinhibition of spinal reflex circuits produces spasticity.

  • C. Destruction of muscle spindles

    Spindle loss would reduce, not increase, reflex gain.

  • D. Block of neuromuscular transmission

    A junctional block would cause weakness without hyperreflexia.

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, Loss of descending inhibitory drive, which disinhibits the stretch reflex. Lesions of the upper motor neurone remove corticoreticular and other descending inhibitory influences, so gamma-motor neurone and stretch reflex gain increase, causing velocity-dependent hypertonia, hyperreflexia, clonus and the extensor plantar response.

Why is "Degeneration of anterior horn cells" not the answer?

Loss of anterior horn cells would produce flaccid weakness with wasting.

Why is "Destruction of muscle spindles" not the answer?

Spindle loss would reduce, not increase, reflex gain.

Why is "Block of neuromuscular transmission" not the answer?

A junctional block would cause weakness without hyperreflexia.

What concept does this question test?

It tests Motor system: upper motor neurone lesion in Physiology. Lower motor neurone lesions cause flaccidity, atrophy and fasciculations. Spasticity may be delayed after the initial flaccid stage.

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 Motor system: upper motor neurone lesion?

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.