Boost Your Brain's Performance

Browse our selection of books, supplements, and natural remedies to maximize your brain's health and performance potential

Image 1 Image 2 Image 3 GAMES BOOKS SUPPLEMENTS

Accessory Nerve

In our daily routine, we use many gestures as part of communicating. These would not be possible without the accessory nerve. These gestures include nodding of the head (alternating up and down movement), which shows acceptance or agreement with someone’s statement, and head shaking (alternating sideways head movement), which shows no or a negative response. Others are frowning, surprised or scared feeling, etc. An important one is shrugging of the shoulders. This gesture shows ignorance, indifference, or passive acceptance. The word “whatever” can best describe this gesture. Shrugging of shoulders is done by contraction of a muscle of the neck and back named the trapezius. This muscle is supplied by the spinal root of the eleventh cranial nerve, known as the accessory nerve. In the following words, we shall look at the origin, course, classification, and functional components of the accessory nerve. We shall discuss different nuclei related to it and its functions. Last but not least, we shall have a look at the diseases caused by the damage to the accessory nerve and the ways to check for its different lesions.

An illustration showing the muscles connected to the accessory nerve
An illustration showing the muscles connected to the accessory nerve

Accessory Nerve

The nerves which originate directly from the brain are known as cranial nerves, as they originate inside the cranium. They are 24 in number (12 pairs), and their area of supply is restricted to the head and neck region except for the vagus nerve, which not only innervates the structures in the head and neck region but also supplies different structures in the chest as well as abdominal cavities. The accessory nerve is the eleventh cranial nerve (CN XI). It is a motor nerve and supplies different muscles in the head area (i.e., pharyngeal, laryngeal, and palatal muscles (discussed below)) and in the neck area (i.e., sternocleidomastoid and trapezius).

Origin and Course of the Accessory Nerve

The accessory nerve is formed by joining two roots that differ in their origin and course as well as the area of innervation; cranial root and spinal root.  

Read more about the Functions of the Autonomic Nervous System

Cranial Root

The cranial root is thought to be a part of the vagus nerve. The cranial root of the accessory nerve emerges as small rootlets from the medulla oblongata at its anterior aspect. These rootlets join each other and travel laterally in the posterior cranial fossa to join the spinal root. Both the roots in the form of a single nerve leave the cranial cavity via the jugular foramen along with the glossopharyngeal nerve, vagus nerve, and internal jugular vein and come to the neck. In the neck, both the roots get separated again. The cranial root joins the vagus nerve, and the further course is along the vagus nerve branches. The fibers that innervate the muscles of the soft palate (except tensor vali palatini) go to the pharyngeal plexus via the pharyngeal branch of the vagus nerve. The same route contains fibers that supply the muscles of the pharynx except for the stylopharyngeus muscle. As it is considered part of the vagus nerve, the fibers that supply intrinsic muscles of the larynx via the recurrent laryngeal branch of the vagus nerve are thought to come from the cranial root of the accessory nerve. 

Spinal Root

The spinal root emerges from the 1st five cervical segments of the spinal cord at its anterior aspect in the form of a series of rootlets. These rootlets join, ascend upward, leave the neck area and enter the skull via the foramen magnum. In the cranial cavity, it joins the cranial root and again leaves the cranial cavity via the jugular foramen. Both the roots get separated, and the spinal root descends down in the neck to supply first the sternocleidomastoid muscle and then the trapezius muscle.

Classification and Functional Components

The accessory nerve is a purely motor nerve with no sensory fibers. It contains special visceral efferents that provide motor information to different muscles. These fibers provide motor innervation to these muscles. Both the roots of the accessory nerve contain the same type of fibers.

Nuclei of Accessory Nerve

The nuclei of cranial nerves are the collection of cell bodies of neurons forming that cranial nerve present in the central nervous system. Note that outside the CNS, these collections are known as ganglions. There are two nuclei associated with the accessory nerve. These include the cranial nucleus and the spinal nucleus. These nuclei are briefly discussed below:

Cranial Nucleus

The cranial nucleus is actually a part of the nucleus ambiguus present in the reticular formation of the medulla oblongata. The nucleus ambiguus is actually the motor nucleus of the vagus nerve. So, this provides evidence for the above-said statement that the cranial root of the accessory nerve is actually a part of the vagus nerve. Another fact is the fibers originating from this nucleus are distributed via the branches of the vagus nerve. This nucleus receives impulses from both the cerebral hemispheres via the corticonuclear fibers (fibers that take motor information from the motor cortex etc., to the cells of the motor nuclei of the cranial nerves). Efferents go to the pharyngeal, laryngeal, and palatal muscles.

Spinal Nucleus

The spinal nucleus is present in the anterior grey column of the upper five cervical segments of the spinal cord. It is composed of cell bodies of lower motor neurons that innervate the trapezius and sternocleidomastoid muscles. This nucleus receives afferents from both cerebral hemispheres via corticonuclear fibers. 

Functions

Motor innervation to the constrictor muscles of the pharynx. The superior and middle constrictors receive their nerve supply from the fibers of the cranial accessory nerve (CN XI) through the pharyngeal branch of the vagus nerve. The inferior constrictor receives its nerve supply from the above-mentioned nerve as well as from the external and recurrent laryngeal nerves, which are the branches of the vagus nerve. As the name shows, these muscles constrict the pharynx and are involved in the swallowing process.

All the intrinsic muscles of the larynx are supplied by the motor fibers of the accessory nerve, which run through the recurrent laryngeal branch of the vagus nerve, except the cricothyroid muscle, which is innervated by the external laryngeal branch of the vagus. These muscles are involved in phonation and voice production. 

As we discussed, the spinal root supplies the sternocleidomastoid and trapezius. The sternocleidomastoid muscle tilts the head towards the shoulder when acting alone. When the SCMs of both sides contract, they cause downward movement of the head (flexion at the cervical spine). They also maintain the level of the head at a horizontal position. 

Multiple nerves supply the trapezius muscle. The upper part of the muscle receives its nerve supply from the accessory nerve, while the lower part is thought to receive its innervation via the thoracic roots etc. The trapezius causes shrugging of the shoulders and helps in the rotation of the neck sideways and in the extension of the neck. 

Diseases of Accessory Nerve

There are many diseases that affect the nervous system. Some of them are systemic, which affect the nervous system as a whole, and some are localized to particular nerves. We will limit our discussion to the diseases of the accessory nerve and lesions of the brain stem that affect the nuclei of the accessory nerve.

Damage to the accessory nerve can be caused by neuritis, ischemic injury, trauma to the skull resulting in crush injury of the nerve, tumors (i.e., tumors of the brain in the posterior cranial fossa), jugular foramen syndrome (i.e., nasopharyngeal carcinoma or glomus tumors, etc.) stab wounds, gunshot wounds, exposure to toxins or drugs, multiple sclerosis and diabetic neuropathy, etc. 

Drooping of the shoulder

If the accessory nerve gets damaged along its course, it will result in paralysis of the trapezius and sternocleidomastoid muscles. Due to trapezius paralysis, the shoulder on the damaged side will droop due to the weight of the upper limb. There will be difficulty in raising the upper limb above the horizontal. 

Drooping of the shoulder due to unsupported weight of the upper limb in trapezius muscle paralysis may result in intractable neuralgia owing to the traction of the nerves forming the brachial plexus.  

Hoarseness of voice

As discussed above, all the intrinsic muscles of the larynx except the cricothyroid are supplied by the accessory nerve fibers via the recurrent laryngeal branch of the vagus nerve. In case of lesions of the vagus nerve, lesions of the accessory nerve, or damage to the recurrent laryngeal branch of the vagus nerve due to any surgery in the neck area, trauma to the neck, stab or gunshot wounds, or jugular foramen syndrome (which includes nasopharyngeal carcinoma or glomus tumors, etc.) can lead to hoarseness of voice. In severe cases, there may be a complete absence of voice. 

Testing the Accessory nerve

The accessory nerve can be tested by asking the patient to shrug his shoulders against resistance. Contraction of the trapezius can be felt in the case of an intact nerve. Another method is to ask the patient to turn his face sideways against resistance. In the case of an intact nerve, the contraction of the opposite sternocleidomastoid muscle can be demonstrated.

Lesions of the brain stem affecting the Accessory nerve nucleus

As the cranial nucleus of the accessory nerve is present in the medulla oblongata, lesions of the lower part of the brain stem may involve these nuclei. This results in signs and symptoms the same as in the case of damage to the accessory nerve along its course. Raised intracranial pressure (RIP), as in the case of tumors of the brain in the posterior cranial fossa, may cause herniation of the brain stem through the foramen magnum. Pressure over the nuclei and traction of the cranial nerves results in paralysis of these nerves. Mostly, the last four cranial nerves, glossopharyngeal, vagus, accessory, and hypoglossal nerves, are all involved in such lesions. Individual lesions are less common. The same happens in the case of Arnold-Chiari malformation, in which there is herniation of the cerebellar tonsils along with the medulla oblongata through the foramen magnum. 

Summary

The accessory nerve (CN XI) is the eleventh cranial nerve. It is a pure motor nerve and contains only special visceral efferents as its functional components. The accessory nerve is formed by the two roots, which differ in their origin and course. The cranial root is thought to be a part of the vagus nerve. It emerges as small rootlets from the medulla oblongata. These rootlets join each other and travel laterally in the posterior cranial fossa to join the spinal root, leave the cranial cavity via the jugular foramen along with the glossopharyngeal nerve, vagus nerve, and internal jugular vein, and come to the neck. In the neck, both the roots get separated again. The cranial root joins the vagus nerve, and the further course is along the vagus nerve branches. The fibers that innervate the muscles of the soft palate (except tensor vali palatini) and pharyngeal muscles (except the stylopharyngeus) go to the pharyngeal plexus via the pharyngeal branch of the vagus nerve. Motor supply to the intrinsic muscles of the larynx is via the recurrent laryngeal branch of the vagus nerve. The spinal root emerges from the 1st five cervical segments of the spinal cord in the form of a series of rootlets. These rootlets join, ascend upward, leave the neck and enter the skull via the foramen magnum. It joins the cranial root and again leaves the cranial cavity via the jugular foramen. Both the roots get separated, and the spinal root descends down in the neck to supply the sternocleidomastoid and trapezius. The accessory nerve has two nuclei, the cranial nucleus, which is a part of the nucleus ambiguus, and the spinal nucleus. Both nuclei receive motor information from both the cerebral hemispheres via corticonuclear fibers. The accessory nerve is involved in phonation (laryngeal muscles innervation), swallowing (pharyngeal muscles), and movements caused by the palatal muscles. Diseases related to the accessory nerve include hoarseness of voice, complete voice absence, and drooping of the shoulder. The accessory nerve can be tested by asking the subject to shrug his shoulders against resistance. Damage to the accessory nerve can be caused by ischemic injury, trauma to the skull resulting in crush injury of the nerve, tumors (i.e., tumors of the brain in the posterior cranial fossa), jugular foramen syndrome (i.e., nasopharyngeal carcinoma or glomus tumors, etc.) stab wounds, gunshot wounds, exposure to toxins or drugs, multiple sclerosis, etc. Lesions of the brain stem (tumors of the brain stem, Arnold-Chiari malformation, etc.) involving the accessory nerve nucleus result in the same signs and symptoms. Treatment involves the solution of the underlying cause by using either medications or surgical interventions. 

References

Illustrated Anatomy of the Head and Neck, Fehrenbach and Herring, Elsevier, 2012

Blumenfeld H. Neuroanatomy Through Clinical Cases. Sinauer Associates, 2002

Ropper, AH, Brown RH. Victor’s Principles of Neurology, 8th ed. McGraw-Hill, 2005

Standring S (ed.) Gray’s Anatomy, 39th edition. Elsevier Churchill Livingstone, 2005Image source: Muscles controlled by the accessory nerve