Which CN 3 fibers are known to cross after leaving the midbrain?

• A. Inferior oblique
• B. Superior rectus
• C. Inferior rectus
• D. Medial rectus

Answer: (B)

The ability to move both eyes together in vertical gaze is supported by some oculomotor pathways that cross the midline after leaving the midbrain. Specifically, fibers destined for the superior rectus are known to decussate, or cross, to innervate the opposite eye’s superior rectus. This crossing helps coordinate upgaze by aligning the action of the superior rectus muscles in both eyes. The other muscles innervated by the oculomotor nerve—the inferior rectus, medial rectus, and inferior oblique—generally receive fibers that stay on the same side, without crossing. So, among the options, the superior rectus fibers are the ones known to cross after exiting the midbrain.