Accessory nerve - Brain - Brainstem - e-Anatomy
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Accessory nerve

Nervus accessorius

  • Latin synonym: Nervus cranialis XI
  • Synonym: Cranial nerve XI
  • Related terms: Accessory nerve [XI]

Definition

Muhammad A. Javaid

Number: XI

Name: Accessory (often separated into the cranial accessory and spinal accessory nerves)

Sensory, motor, or both: Mainly motor

Origin/Target: Cranial and Spinal Roots

Nuclei: Nucleus ambiguus, Spinal accessory nucleus

Function: Controls the sternocleidomastoid and trapezius muscles, and overlaps with functions of the vagus nerve (CN X). Symptoms of damage: inability to shrug, weak head movement.

Description:

The accessory nerve, or the 11th cranial nerve (CN XI), is primarily a motor nerve responsible for innervating specific muscles in the neck and shoulder region and contributing to certain movements of the soft palate, pharynx, and larynx, in association with the vagus nerve. It is unique among the cranial nerves as it has both cranial and spinal origins, which together form its two distinct components: the cranial root and the spinal root.

Nuclei and Formation:

Cranial Root Nucleus (nucleus ambiguus)

The cranial root of the accessory nerve originates from the nucleus ambiguus, located within the medulla oblongata.

The nucleus ambiguus receives corticonuclear fibers from both cerebral hemispheres, which allow for coordinated motor control.

Spinal Root Nucleus

The spinal root arises from the spinal nucleus of the accessory nerve, found in the anterior gray column of the spinal cord, within the upper five cervical segments (C1-C5).

The spinal nucleus also receives fibers from corticospinal tracts from both cerebral hemispheres, ensuring central control of its motor outputs.

Emergence and Course:

Cranial Root

The cranial root emerges from the anterior surface of the medulla oblongata, in the groove between the olive and the inferior cerebellar peduncle > It then courses laterally within the posterior cranial fossa, eventually joining with the spinal root. Together, they exit the skull through the jugular foramen > After exiting the skull, the cranial root separates from the spinal root and joins the vagus nerve (CN X). It contributes fibers to the vagus nerve’s pharyngeal and recurrent laryngeal branches:

Pharyngeal branch: Innervates muscles of the pharynx and soft palate (except tensor veli palatini).

Recurrent laryngeal branch: Innervates intrinsic muscles of the larynx (except cricothyroid).

Spinal root

The spinal root arises from motor neurons in the spinal nucleus of the accessory nerve, in the C1-C5 segments of the spinal cord > Fibers emerge between the anterior and posterior roots of the cervical spinal nerves, forming a single nerve trunk > This trunk ascends into the skull, passing through the foramen magnum, and joins the cranial root briefly. Both roots exit together through the jugular foramen, but they soon separate again > The spinal root then travels downward and laterally, entering the sternocleidomastoid muscle on its deep surface to provide motor innervation. The nerve continues through the posterior triangle of the neck and reaches the superior border of the trapezius muscle, which it also supplies.

Sternocleidomastoid muscle: Facilitates head rotation and tilting movements to the opposite side (right accessory nerve causes leftwards rotation and tilting).

Trapezius muscle: Controls shoulder elevation (e.g., shrugging) and assists in certain arm movements, such as pulling or reaching overhead

Pathway summary:

  • Cranial Root: Medulla oblongata → Posterior cranial fossa → Joins spinal root → Exits through jugular foramen → Joins vagus nerve → Contributes to innervate muscles of the soft palate, pharynx, and larynx.

  • Spinal Root: C1-C5 spinal segments → Ascends through foramen magnum → Joins cranial root → Exits jugular foramen → Separates → Supplies the sternocleidomastoid and trapezius muscles.

Actions:

Cranial Root Actions (via vagus nerve contributions):

Ensures coordinated swallowing and phonation (speech) through its role in movements of the palate, pharynx, and larynx.

Spinal Root Actions:

Sternocleidomastoid muscle: Rotates the head to the opposite side.

Trapezius muscle: Elevates the shoulder (e.g., shrugging).

Clinical Dysfunction:

Damage to the accessory nerve may occur due to trauma (e.g., surgical procedures in the posterior triangle of the neck), tumors, or neurological conditions. Lesions can impair motor function of the sternocleidomastoid and trapezius muscles:

  • Weakness of the sternocleidomastoid: Difficulty turning the head to the opposite side of the lesion. Example: A patient may struggle to turn their head to the left if the right accessory nerve is damaged.

  • Weakness of the trapezius: Difficulty elevating the shoulder (e.g., shrugging). Drooping of the shoulder on the affected side.

  • Role in Cranial Dysfunction (Cranial Root): As it contributes to the vagus nerve, accessory nerve lesions may cause hoarseness, difficulty swallowing, or impaired phonation due to dysfunction of the pharyngeal and laryngeal muscles.

  • Clinical Test: To assess spinal root injury, ask the patient to shrug both shoulders against resistance (trapezius function) and turn their head against resistance (sternocleidomastoid function). Weakness on one side suggests accessory nerve damage.

References

  • Snell, R.S. (2010). ‘Chapter 11: The cranial nerve nuclei and their central connections and distribution’, in Clinical Neuroanatomy. (7th ed.) Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, pp. 355-356.

  • Bordoni B, Reed RR, Tadi P, et al. Neuroanatomy, Cranial Nerve 11 (Accessory) [Updated 2023 Jul 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK507722/

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