Which cranial nerve is the only one that emerges from the dorsal aspect of the brainstem and is also the only one whose fibres cross completely (decussate) within the CNS?

• A Trochlear nerve (CN IV) ✓
• B Oculomotor nerve (CN III)
• C Abducens nerve (CN VI)
• D Hypoglossal nerve (CN XII)

Explanation

The trochlear nerve arises from the trochlear nucleus at the level of the inferior colliculus and its fibres decussate completely in the superior medullary velum before emerging from the dorsal midbrain just below the inferior colliculus. It then wraps around the cerebral peduncle. This dual distinction, complete decussation plus dorsal exit, is unique to CN IV. CN III and CN VI emerge ventrally and do not decussate, while CN XII emerges from the preolivary sulcus.

Reference: Gray's Anatomy for Students, 4th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.