Corpus: Accessory nerve

The accessory nerve is the 11th cranial nerve. It is divided into an internal branch (ramus internus) originating in the brainstem and an external branch (ramus externus) originating in the spinal cord. It is a motor nerve that carries somatic motor and primarily visceral motor fibers.

The fibers of the smaller internal branch arise from the nucleus ambiguus at the base of the rhomboid fossa and leave the medulla oblongata as the cranial root (radix cranialis).

The fibers of the external branch originate from the spinal cord segments C1-C5. The corresponding core area is called the accessory nucleus (nucleus nervi accessorii). The axons of the motor neurons located here leave the spinal cord laterally as the spinal root (radix spinalis).

3.1. Radix cranialis

The fibers of the cranial root leave the medulla oblongata below the roots of the vagus nerve (cranial nerve X) in the posterolateral sulcus. Like the glossopharyngeal nerve, facial nerve, mandibular nerve, and vagus nerve, they carry special visceral motor fibers. This portion forms the internal branch of the accessory nerve, also known as the vagal part due to its core area. In the cranial cavity, it combines with the fibers of the spinal root to form the accessory nerve trunk (truncus nervi accessorii).

3.2. Radix spinalis

The fibers of the spinal root emerge from the spinal cord between the anterior and posterior roots of the spinal nerves. They ascend cranially along the spinal cord in the subarachnoid space and enter the cranial cavity through the foramen magnum. In the posterior cranial fossa, they join the accessory nerve trunk as the spinal part (pars spinalis).

The accessory nerve trunk, containing fibers from both roots, exits the cranial cavity through the jugular foramen. Before exiting, it sends some fibers to the vagal nerve at the jugular ganglion. Extracranially, the nerve splits into its two parts, now called the internal and external branches.

4.1. Internal ramus

The internal branch carries the special visceral motor fibers of the cranial root. It joins the vagus nerve trunk between the superior and inferior ganglia. Together with vagus fibers, it forms the recurrent laryngeal nerve, whose terminal branch (inferior laryngeal nerve) supplies the internal laryngeal muscles. It also helps supply the pharyngeal muscles via the superior laryngeal nerve.

4.2. External ramus

The external branch usually runs ventrally, or more rarely dorsally, to the internal jugular vein. It then moves to the sternocleidomastoid muscle, innervates it, and continues to the trapezius muscle. Along its course, it picks up fibers from the cervical plexus, forming a nerve plexus from which the trapezius muscle is innervated.

The connections between the external branch and the anterior rami of the cervical spinal nerves are sometimes referred to as the accessory-cervical plexus by anatomists.

Recent research (as of 2022) increasingly supports describing the cranial root of the accessory nerve as part of the vagus nerve. The remaining spinal root is then considered the actual accessory nerve. To avoid confusion, the term "proper accessory nerve" (nervus accessorius proprius) has been proposed, though it is not yet widely accepted.

The accessory nerve can be damaged by surgical procedures in the neck area or by blunt or sharp trauma. In a radical neck dissection, a lesion of the accessory nerve is sometimes accepted to remove all lymph nodes.

The resulting neurological deficits from accessory nerve palsy depend on the exact site of damage. Lack of innervation leads to muscle atrophy of the sternocleidomastoid and trapezius muscles. This can cause the shoulder girdle to drop, the scapula to protrude, and the arm to be less able to abduct.