Accessory nucleus corresponds to which cranial nerve?

• A. IX
• B. X
• C. XI
• D. XII

Answer: (C)

The main idea is that some cranial nerves have nuclei in the brainstem, while the accessory nucleus is a motor nucleus located in the spinal cord that forms the spinal accessory nerve. This nerve travels up into the skull and ultimately innervates two muscles: the sternocleidomastoid and the trapezius. Those actions—head rotation and shoulder elevation—are what the accessory nerve controls, so the nerve associated with this nucleus is the accessory nerve, cranial nerve XI. Clinically, weakness here shows up as difficulty shrugging the shoulders or turning the head. The other nerves listed have their motor nuclei in the brainstem (not from a spinal accessory nucleus), so they don’t correspond to the accessory nucleus.