Neurophysiology: decerebrate rigidity: 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 · Neurophysiology: decerebrate rigidity

Sample item

A patient has a brainstem lesion below the red nucleus, with extension of all four limbs and the neck arched back. Which motor outflow is acting unopposed?

ALateral corticospinal tractBRubrospinal tractCMedullary reticulospinal tractDVestibulospinal and pontine reticulospinal tracts

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

The answer and the reasoning

D. Vestibulospinal and pontine reticulospinal tracts

Transection below the red nucleus separates the brainstem extensor-facilitating nuclei from inhibitory descending influences, so lateral vestibulospinal and pontine reticulospinal activity dominates and drives extensor tone. This is decerebrate rigidity. Lesions above the red nucleus allow rubrospinal flexion and give decorticate posture.

Option by option

A. Lateral corticospinal tract
The corticospinal tract is interrupted and cannot drive the rigidity.
B. Rubrospinal tract
The rubrospinal tract facilitates flexors and is cut off from its source in this lesion.
C. Medullary reticulospinal tract
The medullary reticulospinal tract inhibits extensor tone but is deprived of cortical drive, so it cannot oppose extension.
D. Vestibulospinal and pontine reticulospinal tracts (correct)
Correct. These pathways excite extensor muscles and lose their opposition.

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 D, Vestibulospinal and pontine reticulospinal tracts. Transection below the red nucleus separates the brainstem extensor-facilitating nuclei from inhibitory descending influences, so lateral vestibulospinal and pontine reticulospinal activity dominates and drives extensor tone.

Why is "Lateral corticospinal tract" not the answer?

The corticospinal tract is interrupted and cannot drive the rigidity.

Why is "Rubrospinal tract" not the answer?

The rubrospinal tract facilitates flexors and is cut off from its source in this lesion.

Why is "Medullary reticulospinal tract" not the answer?

The medullary reticulospinal tract inhibits extensor tone but is deprived of cortical drive, so it cannot oppose extension.

What concept does this question test?

It tests Neurophysiology: decerebrate rigidity in Physiology. This is decerebrate rigidity. Lesions above the red nucleus allow rubrospinal flexion and give decorticate posture.

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 Neurophysiology: decerebrate rigidity?

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.