A 24-year-old man develops extension of all four limbs, opisthotonus, and exaggerated stretch reflexes after a brainstem hemorrhage. MRI shows a lesion between the superior and inferior colliculi of the midbrain. The rigidity in this condition results primarily from:
- A Unopposed tonic activity of gamma motor neurons driven by the lateral vestibulospinal and reticulospinal pathways ✓
- B Loss of rubrospinal facilitation, leaving unchecked pontine reticulospinal drive to flexors
- C Denervation hypersensitivity of alpha motor neurons after corticospinal interruption
- D Loss of cerebellar output causing hypotonia that later becomes spastic
Correct answer: A. Unopposed tonic activity of gamma motor neurons driven by the lateral vestibulospinal and reticulospinal pathways
Explanation
Decerebrate rigidity occurs when the transection separates the red nucleus from the vestibular nuclei, removing cortical and rubral influence while leaving the lateral vestibulospinal and pontine reticulospinal pathways intact. These tonically drive gamma motor neurons, sensitizing muscle spindles and producing extensor hypertonus through the gamma loop. The distractor C fails because rigidity persists after dorsal root section abolishes spindle input, proving it is gamma-driven rather than a denervation phenomenon.
Reference: Ganong's Review of Medical Physiology, 27th ed.
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