Accessory nerve root (CN XI)

Radix nervi accessorii

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What it is
Its cranial rootlets leave the lower medulla behind the olive, where the spinal root, which climbs from the upper cervical cord through the foramen magnum, joins them. Shown as a short stub on both sides; the nerve continues in the peripheral nervous system.
Function
Motor to sternocleidomastoid and trapezius (spinal part); the cranial part joins the vagus for the larynx.
Size
Stub about 1 cm; root 1.6 mm thick
Clinical note
Often injured in posterior-triangle neck surgery, causing a drooping shoulder.

Nearby nervous structures Abducens nerve root (CN VI), C1 spinal nerve, Facial nerve root (CN VII), Fourth ventricle, Glossopharyngeal nerve root (CN IX), Hypoglossal nerve root (CN XII), Medulla oblongata, Pons, Vagus nerve root (CN X), Vestibulocochlear nerve root (CN VIII)