Why does injury to the trochlear nerve produce weakness of the contralateral superior oblique muscle?
- A Its nucleus lies in the pons and axons cross in the trapezoid body
- B Its fibres cross within the anterior white commissure after synapsing in the oculomotor nucleus
- C It is the only cranial nerve whose fibres decussate completely and exit from the dorsal aspect of the brainstem ✓
- D Its nucleus receives purely ipsilateral cortical input via the posterior limb of the internal capsule
Correct answer: C. It is the only cranial nerve whose fibres decussate completely and exit from the dorsal aspect of the brainstem
Explanation
The trochlear nucleus sits at the level of the inferior colliculus in the midbrain, and every fibre from it crosses in the superior medullary velum before emerging from the dorsal brainstem below the inferior colliculus, making it unique among cranial nerves. Because the crossed fibre innervates the opposite superior oblique, a nuclear or fascicular midbrain lesion weakens the contralateral muscle while a cisternal or cavernous lesion weakens the ipsilateral muscle.
Reference: Gray's Anatomy, 42nd ed.
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