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 simultaneously: the brainstem and the spinal cord.
- Cranial nucleus (cerebral part) is called the nucleus ambiguus. It is located in the dorsal part of the medulla oblongata. It is important to note that this structure does not exclusively belong to the eleventh pair. It is a shared motor nucleus that serves three pairs of cranial nerves at once: the glossopharyngeal (IX), vagus (X), and accessory (XI) nerves. The axons of the cells in this nucleus form the cranial roots of the accessory nerve.
- Spinal nucleus is the nucleus of the accessory nerve (nucleus n. accessorii). Unlike the cranial nucleus, it extends much further inferiorly and is located in the anterior horns of the spinal cord gray matter. The extent of this nucleus spans segments C1 to C5 (sometimes reaching the C6 level). This is precisely where the spinal part of the accessory nerve originates.
Formation of roots and the nerve trunk
The process of forming the single trunk of the accessory nerve is a complex topographical pathway involving two groups of roots emerging from different parts of the nervous system.
- Cranial roots (radices craniales, or pars vagalis) leave the brainstem, specifically the medulla oblongata. Their point of emergence onto the surface is the postolivary sulcus.
- Spinal roots (radices spinales, or pars spinalis) emerge from the lateral funiculus of the spinal cord.
The subsequent pathway of the spinal roots is quite specific: after emerging from the spinal cord, they course superiorly, ascend to the base of the skull, and enter the cranial cavity through the foramen magnum. Once inside the cranial vault, the spinal roots meet the cranial roots. 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 bony canal, it does not pass alone, but in close proximity to the glossopharyngeal (IX) and vagus (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 entirely incorporated into it. The specific feature of this branch is that it predominantly carries fibers that functionally belong to the vagus nerve, supplementing its motor capacity.
- External branch (r. externus). This branch is the main conductor of motor impulses to the neck muscles. It runs inferiorly and posteriorly, passes deep to the sternocleidomastoid muscle (m. sternocleidomastoideus), and provides its innervation. Then the nerve emerges into the lateral cervical triangle, crosses it, and enters the substance of the trapezius muscle (m. trapezius), which it also successfully innervates.
Briefly: answers to topical questions
From which sulcus of the medulla oblongata do the cranial roots of the accessory nerve emerge?
The cranial roots of the accessory nerve emerge from the postolivary (dorsolateral) sulcus of the medulla oblongata, posterior to the olive. The roots of the glossopharyngeal and vagus nerves also emerge from this same sulcus. Subsequently, the cranial roots join with the spinal roots to form the common trunk of the accessory nerve.
What clinical symptoms are observed in peripheral paralysis (lesion) of the accessory nerve?
A lesion of the accessory nerve results in peripheral paresis, accompanied by weakness and atrophy of the sternocleidomastoid and trapezius muscles. Drooping of the shoulder and displacement of the scapula are observed on the affected side. The patient is unable to raise the arm above the horizontal level (above 90°) and cannot turn the head to the side opposite the lesion. Atrophy primarily affects the upper part 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 spinal efferent fibers of segments C2–C4.
To avoid confusing the emergence of the roots: the cranial roots emerge POSTERIOR to the olive, while the spinal roots ascend through the FORAMEN MAGNUM to subsequently exit together through the JUGULAR foramen.
Class and exam questions
In which parts of the CNS are the nuclei of the accessory nerve located?
They are located in two zones: the cranial nucleus (nucleus ambiguus) is situated in the dorsal part of the medulla oblongata, while the spinal nucleus is located in the anterior horns of the spinal cord at the C1–C5(6) level.
With which nerves does the accessory nerve share a cranial nucleus?
The nucleus ambiguus is shared by the accessory (XI), glossopharyngeal (IX), and vagus (X) nerves.
Which muscles are innervated by the XI pair of cranial nerves?
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 common trunk of the accessory nerve leaves the cranial cavity through the jugular foramen, passing there alongside cranial nerves IX and X.
What else to study on this topic
- Syntopy of the accessory nerve in the jugular foramen with CN IX and X
- Detailed course of the external branch in the lateral cervical triangle
- Functional significance of the internal branch fibers within the vagus nerve
- Relationship of the external branch with the sternocleidomastoid muscle
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