Which cranial nerve contains fibers that cross after leaving the midbrain to affect extraocular movements?

Which cranial nerve contains fibers that cross after leaving the midbrain to affect extraocular movements?

• A. Cranial Nerve II
• B. Cranial Nerve III
• C. Cranial Nerve IV
• D. Cranial Nerve VI

Answer: (B)

This question focuses on how some extraocular motor fibers cross in the brainstem. The nerve that carries fibers which cross to reach the target muscle is the trochlear nerve. Its fibers originate in the trochlear nucleus in the midbrain, cross to the opposite side, and then exit dorsally to innervate the superior oblique muscle of the contralateral eye. This crossing explains why weakness of the superior oblique affects the opposite eye and why patients get vertical diplopia that changes with gaze direction. The other cranial nerves controlling the eye muscles do not cross in this same way.

This question focuses on how some extraocular motor fibers cross in the brainstem. The nerve that carries fibers which cross to reach the target muscle is the trochlear nerve. Its fibers originate in the trochlear nucleus in the midbrain, cross to the opposite side, and then exit dorsally to innervate the superior oblique muscle of the contralateral eye. This crossing explains why weakness of the superior oblique affects the opposite eye and why patients get vertical diplopia that changes with gaze direction. The other cranial nerves controlling the eye muscles do not cross in this same way.