Nerve
Accessory nerve (CN XI)
Motor supply for head turning and shoulder support
High-yield
Its superficial course in the posterior triangle makes it vulnerable during neck procedures. Trapezius weakness can cause shoulder droop, reduced shrug strength and impaired upward rotation of the scapula.
Overview
The spinal accessory nerve begins in the upper cervical spinal cord, ascends through the foramen magnum and leaves the skull through the jugular foramen. In the neck it supplies sternocleidomastoid and then crosses the posterior triangle toward trapezius.
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Reference facts
- Origin
- Spinal accessory nucleus in the upper cervical anterior horn, usually C1–C5 with possible C6 contribution. Descriptions of a separate cranial root differ; associated pharyngeal and laryngeal fibers are distributed with CN X.
- Skull exit
- Enters through the foramen magnum and exits through the jugular foramen
- Course
- Ascends from the cervical cord into the posterior cranial fossa, leaves via the jugular foramen, reaches sternocleidomastoid and crosses the posterior triangle to the deep surface of trapezius
- Motor
- Principal motor supply to sternocleidomastoid and trapezius, supporting head rotation and scapular movement
- Sensory
- No defined skin territory. Cervical nerve connections carry much of the muscle sensory input; human studies also identify sensory fibers within CN XI.
Clinical correlation
Injury after the sternocleidomastoid branch may weaken trapezius while sparing head turning. More proximal damage can affect both muscles. At the jugular foramen, combined IX, X and XI deficits form the classic Vernet syndrome pattern.
Frequently asked questions
Where does the accessory nerve (CN XI) arise?
Spinal accessory nucleus in the upper cervical anterior horn, usually C1–C5 with possible C6 contribution. Descriptions of a separate cranial root differ; associated pharyngeal and laryngeal fibers are distributed with CN X.
What muscles does the accessory nerve (CN XI) supply?
The accessory nerve (CN XI) supplies the principal motor supply to sternocleidomastoid and trapezius, supporting head rotation and scapular movement.
What is the sensory distribution of the accessory nerve (CN XI)?
No defined skin territory. Cervical nerve connections carry much of the muscle sensory input; human studies also identify sensory fibers within CN XI.
Where does the accessory nerve (CN XI) leave the skull?
Enters through the foramen magnum and exits through the jugular foramen.