During microsurgical dissection near the floor of the fourth ventricle, the surgeon must avoid a critical landmark located at the level of the facial colliculus. Which structure underlies the facial colliculus that, if damaged, causes facial palsy?

• A Facial nerve nucleus directly beneath the eminence
• B Motor nucleus of trigeminal nerve
• C Dorsal cochlear nucleus
• D Nucleus of abducens nerve with genu of facial nerve fiber looping around it ✓

Explanation

The facial colliculus is a surface elevation on the floor of the fourth ventricle caused by the genu of the facial nerve (CN VII) fibers curving around the abducens nucleus (CN VI). Surgical injury at this point damages the internal genu of CN VII, causing ipsilateral facial palsy; it also damages the abducens nucleus causing ipsilateral gaze palsy. The facial nerve nucleus itself lies more ventrolaterally in the tegmentum.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.