From which sulcus of the medulla oblongata do the cranial rootlets of the accessory nerve emerge?

The cranial rootlets of the accessory nerve emerge from the posterolateral (dorsolateral) sulcus of the medulla oblongata, posterior to the olive. The rootlets of the glossopharyngeal and vagus nerves also emerge from this sulcus. The cranial rootlets subsequently join the spinal rootlets to form the common accessory nerve trunk.

What clinical symptoms are observed in peripheral paralysis (lesion) of the accessory nerve?

An accessory nerve lesion results in peripheral paresis, accompanied by weakness and atrophy of the sternocleidomastoid and trapezius muscles. On the affected side, shoulder drop and scapular displacement are observed. The patient is unable to elevate the arm above the horizontal level (above 90°) and cannot turn the head toward the side opposite the lesion. Atrophy primarily affects the upper portion of the trapezius muscle.

With which branches of the cervical plexus does the external branch of the accessory nerve anastomose?

The external branch of the accessory nerve anastomoses with somatic efferent spinal fibers from cervical segments C2–C4.

In which parts of the CNS are the accessory nerve nuclei located?

They are located in two regions: the cranial nucleus (nucleus ambiguus) is in the dorsal medulla oblongata, while the spinal nucleus is in the anterior horns of the cervical spinal cord at levels C1–C5(6).

Which nerves share a cranial nucleus with the accessory nerve?

The nucleus ambiguus is a shared motor nucleus for the accessory (CN XI), glossopharyngeal (CN IX), and vagus (CN X) nerves.

Which muscles are innervated by the eleventh cranial nerve?

The accessory nerve provides motor innervation to two muscles: the sternocleidomastoid and the trapezius (via its external branch).

Home › Human Anatomy › Accessory Nerve

Accessory Nerve

Nervus accessorius

For medical students2 min readUpdated 2026-10-10

The accessory nerve is the eleventh cranial nerve (CN XI) and is strictly a motor nerve. Its primary function is to provide complete motor innervation to two large muscles: the sternocleidomastoid and the trapezius.

Fiber TypeExclusively motor nerve

Target MusclesSternocleidomastoid and trapezius

Cranial EntryForamen magnum (spinal rootlets)

Cranial ExitJugular foramen (along with CN IX and X)

Nuclei of the Accessory Nerve

The anatomical uniqueness of the accessory nerve lies in the fact that its motor nuclei are located in two isolated anatomical zones of the central nervous system: the brainstem and the spinal cord.

Cranial nucleus (cerebral part) — known as the nucleus ambiguus. It is located in the dorsal medulla oblongata. It is important to note that this structure does not belong exclusively to the eleventh cranial nerve. It is a shared motor nerve nucleus serving three pairs of cranial nerves: the glossopharyngeal (CN IX), vagus (CN X), and accessory (CN XI) nerves. Axons of neurons from this nucleus form the cranial rootlets of the accessory nerve.
Spinal nucleus — the accessory nerve nucleus (nucleus n. accessorii). Unlike the cranial nucleus, it extends much lower, located in the anterior horns of the spinal cord gray matter. This nucleus spans from spinal segments C1 to C5 (sometimes reaching C6). This is the origin of the spinal part of the accessory nerve.

Formation of Rootlets and Nerve Trunk

The formation of the single trunk of the accessory nerve is a complex topographic pathway involving two groups of rootlets emerging from different parts of the nervous system.

Cranial rootlets (radices craniales, or pars vagalis) emerge from the brainstem, specifically the medulla oblongata. Their surface exit point is the post-olivary sulcus (posterior to the olive).
Spinal rootlets (radices spinales, or pars spinalis) emerge from the lateral funiculus of the spinal cord.

The subsequent path of the spinal rootlets is very specific: after exiting the spinal cord, they ascend toward the skull base and enter the cranial cavity through the foramen magnum. Once inside the cranial vault, the spinal rootlets meet the cranial rootlets. Fusing together, they form the common trunk of the accessory nerve — truncus n. accessorii.

Exit from the Cranial Cavity and Branching

The formed trunk of the accessory nerve leaves the cranial cavity through the jugular foramen. In this narrow osseous channel, it travels alongside the glossopharyngeal (CN IX) and vagus (CN X) nerves. Immediately after exiting the jugular foramen, the trunk of the eleventh pair divides into two functionally and topographically distinct branches.

Internal branch (r. internus). Almost immediately after its formation, it joins the vagus nerve (CN X) and becomes incorporated into it. The key feature of this branch is that it carries fibers functionally belonging to the vagus nerve, supplementing its motor potential.
External branch (r. externus). This branch is the main conductor of motor impulses to the neck muscles. It travels downward and backward, passes deep to the sternocleidomastoid muscle (m. sternocleidomastoideus) to innervate it. The nerve then emerges into the posterior triangle of the neck, crosses it, and enters the substance of the trapezius muscle (m. trapezius), which it also successfully innervates.

Mnemonic

To remember rootlet trajectories: cranial rootlets exit POSTERIOR to the olive, while spinal rootlets ascend through the FORAMEN MAGNUM to subsequently exit together through the JUGULAR foramen.

Frequently asked questions

From which sulcus of the medulla oblongata do the cranial rootlets of the accessory nerve emerge?: The cranial rootlets of the accessory nerve emerge from the posterolateral (dorsolateral) sulcus of the medulla oblongata, posterior to the olive. The rootlets of the glossopharyngeal and vagus nerves also emerge from this sulcus. The cranial rootlets subsequently join the spinal rootlets to form the common accessory nerve trunk.
What clinical symptoms are observed in peripheral paralysis (lesion) of the accessory nerve?: An accessory nerve lesion results in peripheral paresis, accompanied by weakness and atrophy of the sternocleidomastoid and trapezius muscles. On the affected side, shoulder drop and scapular displacement are observed. The patient is unable to elevate the arm above the horizontal level (above 90°) and cannot turn the head toward the side opposite the lesion. Atrophy primarily affects the upper portion of the trapezius muscle.
With which branches of the cervical plexus does the external branch of the accessory nerve anastomose?: The external branch of the accessory nerve anastomoses with somatic efferent spinal fibers from cervical segments C2–C4.
In which parts of the CNS are the accessory nerve nuclei located?: They are located in two regions: the cranial nucleus (nucleus ambiguus) is in the dorsal medulla oblongata, while the spinal nucleus is in the anterior horns of the cervical spinal cord at levels C1–C5(6).
Which nerves share a cranial nucleus with the accessory nerve?: The nucleus ambiguus is a shared motor nucleus for the accessory (CN XI), glossopharyngeal (CN IX), and vagus (CN X) nerves.
Which muscles are innervated by the eleventh cranial nerve?: The accessory nerve provides motor innervation to two muscles: the sternocleidomastoid and the trapezius (via its external branch).
How does the nerve exit the cranial cavity?: The main trunk of the accessory nerve leaves the cranial cavity through the jugular foramen, passing alongside cranial nerves IX and X.

Go deeper

Syntopy of the accessory nerve in the jugular foramen with CN IX and X
Detailed course of the external branch in the posterior triangle of the neck
Functional significance of internal branch fibers within the vagus nerve
Relationship of the external branch to the sternocleidomastoid muscle