Acccessory nerve

Motor function

Motor innervation of sternocleidomastoid and trapezius muscles.

Anatomy

1. Intra-axial segment

The bulbar motor fibers arise from nucleus ambiguus. The bulbar fibers emerge from the lateral medulla in the postolivary sulcus, caudal to glossopharyngeal and vagus nerves.

The spinal motor fibers arise from the spinal nucleus of the accessory nerve. The spinal fibers emerge from lateral aspect of the cervical spinal cord between the ventral and dorsal roots. The fibers combine and enter the skull through foramen magnum.

2. Cisternal segment

The bulbar fibers course through the basal cistern together with the glossopharyngeal and vagus nerves. They join with spinal fibers ascending through foramen magnum.

3. Skull base segment

The accessory nerve exits the skull through pars vascularis portion of the jugular foramen accompanied by vagus nerve and the jugular bulb.

4. Extracranial segment

Distal to the jugular foramen, the nerve enters the carotid space.The bulbar motor fibers cross into vagus nerve and they supply muscles of pharynx and larynx.

Only the spinal motor fibers remain in the extracranial accessory nerve. They descend medial to the sternocleidomastoid muscle. They innervate the sternocleidomastoid and the trapezius muscle.

Clinical implications

• The most common cause of accessory nerve palsy is iatrogenic injury from radical neck dissection.
• Initial symptoms are downward and lateral rotation of the scapula and shoulder droop caused by trapezius weakness. Atrophy of ipsilateral sternocleidomastoid and trapezius muscles and compensatory hypertrophy of ipsilateral levator scapulae appear several months after the initial injury.