Anatomy Atlas

Accessory nerve (CN XI)

Nervus accessorius
Also: CN XI · Cranial nerve XI · Spinal accessory nerve
NerveHigh-yielddraftNeuro

Motor nerve to sternocleidomastoid and trapezius, arising from the upper cervical cord. It crosses the posterior triangle superficially, where lymph node biopsy can injure it and cause shoulder droop.

Core anatomy

Origin

Spinal accessory nucleus in the ventral horn of the upper cervical cord (about C1–C5). The rootlets ascend beside the spinal cord through the foramen magnum into the skull. (A small "cranial root" from the nucleus ambiguus joins the vagus and is usually considered part of CN X.)

Course

Exits through the jugular foramen with CN IX and X → descends, often crossing in front of or behind the internal jugular vein → pierces and supplies sternocleidomastoid → crosses the posterior triangle superficially on levator scapulae, just deep to the investing fascia (emerging about a third of the way down the posterior border of SCM, above the nerve point) → supplies trapezius from its deep surface (see triangles of the neck).

Motor

Sternocleidomastoid (turns the face to the opposite side) and trapezius (shrugs, elevates and retracts the scapula, rotates it upward for abduction above 90°). C2–C4 fibers from the cervical plexus add proprioception.

Relations

In the posterior triangle it lies superficially among the occipital and posterior cervical (level V) lymph nodes.

Muscles innervated

  • Sternocleidomastoid — Spinal accessory nerve (CN XI) (motor); C2–C3 ventral rami (proprioception, via the cervical plexus).
  • Trapezius — Spinal accessory nerve (CN XI) for motor; C3–C4 (cervical plexus) for pain and proprioception.

Mentioned in

Generated automatically from links in other entries.

Where the cranial nerves leave the brainstem (ventral view)The front of the brainstem. The optic nerves meet at the chiasm above. The oculomotor nerve emerges from the interpeduncular fossa between the cerebral peduncles of the midbrain. The trochlear nerve is the only one to leave from the back, just below the inferior colliculus, and winds forward around the peduncle. The trigeminal nerve leaves the lateral pons. The abducens leaves at the pontomedullary junction near the midline; the facial and vestibulocochlear nerves leave at the cerebellopontine angle. The glossopharyngeal and vagus nerves leave the postolivary sulcus of the medulla, the hypoglossal leaves the preolivary sulcus between the pyramid and the olive, and the accessory nerve rises from the upper cervical cord.Optic (II)Oculomotor (III)Trochlear (IV) — from the dorsal sideTrigeminal (V)Abducens (VI)Facial (VII)Vestibulocochlear (VIII)Glossopharyngeal (IX)Vagus (X)Accessory (XI) — from C1–C5Hypoglossal (XII)Midbrain (cerebral peduncle)PonsPyramidOliveSpinal cordMedullaVentral view · dashed IV leaves from the back

Swipe sideways to see the whole diagram →

Where the cranial nerves leave the brainstem. Ventral view. III leaves the interpeduncular fossa of the midbrain; IV is the only nerve to leave from the back (dashed); V leaves the lateral pons; VI the pontomedullary junction near the midline; VII and VIII the cerebellopontine angle; IX and X the postolivary sulcus; XII the preolivary sulcus between pyramid and olive; and XI rises from the upper cervical cord. Nuclei follow the same levels: III–IV midbrain, V–VIII pons, IX–XII medulla.
Original schematic, created for this atlas.
The glossopharyngeal, vagus and accessory nerves descending from the skull base in the neck, with the vagus continuing through the thorax to the stomach
The accessory nerve leaving the jugular foramen with CN IX and X.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 793. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Spinal accessory nerve injury PANCE

Mechanism / lesion

Iatrogenic — posterior triangle lymph node biopsy, neck dissection; also penetrating trauma.

Classic presentation

Injury in the posterior triangle spares SCM (supplied higher up) but paralyzes trapezius: shoulder droop, weak shrug, difficulty abducting above 90°, lateral scapular winging (the inferior angle moves laterally — contrast with the medial winging of a long thoracic nerve palsy), shoulder pain.

Proximal (jugular foramen) lesions

Mechanism / lesion

Tumors at the jugular foramen or skull base (with CN IX and X palsies).

Classic presentation

Weak SCM (weak head turning to the opposite side) and trapezius on the same side as the lesion.

High-yield PANCE pearls

  • Shoulder droop and weak shrug after a posterior triangle node biopsy → CN XI injury.
  • Trapezius palsy → lateral scapular winging; long thoracic nerve (serratus anterior) → medial winging.
  • Weak head turning to the left → right SCM / right CN XI.

Imaging anatomy

MRI

MRI of the skull base and neck for unexplained XI palsy; EMG/nerve conduction confirms denervation of trapezius.

Physical exam

Strength / function testing

SCM: turn the head against resistance (turning to the left tests the right SCM) and look/palpate for contraction. Trapezius: shrug the shoulders against resistance; look for asymmetry, wasting and scapular position; abduct the arms above 90°.

Self-quiz

Q1.

Three weeks after excision of a lymph node from the right posterior triangle, a patient cannot shrug his right shoulder fully and has trouble lifting his arm above his head. Which muscle is weak?

Show answer

B

The spinal accessory nerve crosses the posterior triangle superficially and is at risk during node biopsy. Trapezius weakness causes shoulder droop, weak shrug and difficulty abducting above 90°.

Q2.

The spinal accessory nerve arises from which part of the nervous system?

Show answer

B

Its motor neurons lie in the ventral horn of C1–C5; the rootlets ascend through the foramen magnum and leave through the jugular foramen.

Q3.

Through which opening does the spinal accessory nerve leave the skull?

Show answer

C

CN XI rootlets ascend from the upper cervical cord through the foramen magnum, then leave the skull through the jugular foramen with CN IX and X.

Q4.

A patient has weakness turning the head to the left against resistance. Which muscle and nerve are affected?

Show answer

E

Each SCM turns the face to the opposite side, so weak turning to the left points to the right SCM and right CN XI.

Q5.

A tumor at the jugular foramen compresses the accessory nerve. Which other cranial nerves are likely to be affected?

Show answer

B

The jugular foramen transmits CN IX, X and XI with the internal jugular vein, so a lesion there causes a combined palsy (dysphagia, hoarseness, weak SCM and trapezius).

Flashcards

See also