Neurology Explained - The Accessory nerve (Cranial Nerve XI)
I must guess that this series is getting longer than you expected, but do not worry, we are gradually coming to an end just a little more cranial nerve post, and we are done. In case you haven't read my previous post on the cranial nerves, I will put the link at the end of the post. Today, I will be discussing the Accessory nerve (Spinal Accessory Nerve) which is the cranial Nerve XI. To understand this cranial nerve, I will be writing about the origin of the nerve nucleus, the nerve fibers that innervates the structures, and the structures it will be supplying or innervating.
The Accessory Nerve has its nucleus in the medulla of the brain. The accessory nerve has two part, it has both the cranial part and the spinal part. Starting with the origin, like I said that that the origin of the accessory nerve is in the medulla. Did you remember me talking about the Nucleus ambiguus? The Nucleus Ambiguss is a nucleus origin for the glossopharyngeal nerve, the Vagus nerve, as well as the cranial part of the Accessory nerve.. The Nucleus The nerve fiber of the cranial nerve XI comes out of the Nucleus Ambiguus, and the fiber goes into the Jugular foramen. Along with the accessory nerve fiber are also fibers of the Vagus nerve, glossopharyngeal nerve, and fibers of cranial nerve 12. Along with the Vagus nerve which supplies the pharynx muscles, part of the cranial accessory nerve fiber goes into the carotid sheath. Other nerves that runs through the carotid sheath are the Glossopharyngeal Nerve, and the Hypoglossal Nerve to form the pharyngeal plexus, which supplies the muscles of the pharynx, and the soft pallet muscles.,. The spinal accessory nerve fiber originates from the lateral funiculus of the spinal cord , then it goes into the foramen magnum where it then goes into the posterior cranial fossa where it then goes into the jugular foramen., . Remember I said the accessory nerve fiber splits into two after exiting the jugular foramen, where one goes with the vagus nerve fiber, to form the pharyngeal plexus, that nerve fiber branch is known as the internal branch, while the other breach that doesn't go towards the pharynx is known as the external branch..
The internal branch of the accessory nerve fibres moves through the internal carotid artery, beneath the styloid process to the sternocleidomastoid muscle and the upper fibers of trapezius muscles. The sternocleidomastoid muscle runs from the mastoid process to the starnum.,. The Trapezius muscle holds the scapula against the thoracic wall. It is important to know that the trapezius is getting nerve branches from C3 and C4, while the sternocleidomastoid muscle gets its nerve branches from C2 and C3.. It is important to know that the spinal part of the accessory nerve starts from the lateral funiculus of the spine, where it goes through the foramen magnus, and then to the jugular foramen. This joins the external branch of the accessory nerve fiber coming from the Nucleus Ambiguus. This goes to attach to the sternocleidomastoid muscle to give an innervation and then goes to supply the trapezius muscles. In the precentral gyrus, there are neurons known as the primary somatomotor cortex, which sends its branch downwards to connect to nucleus ambiguus and the will continue to supply the spinal C2, C3, and C4 either contralateral (trapezius) or ipsilateral (sternocleidomastoid muscle).,
Lesions in the peripheral part, the nucleus ambiguus, primary somatomotor cortex, and the spinal regions as a result of tumor, abscess, polio, amyotrophic lateral sclerosis, intraspinal tumors, syringomyelia, and so on, can lead to transient, impairment of functions of the nerve, and can cause conditions such as in cases of Wallenberg's syndrome. Lsions of the spinal accessory branch alone is very rare, but can be caused by injuries from surgeries. Exposure to radiation can also cause injury to the spinal branch of the accessory nerve, other causes can be during love play, such as in cases of love bite, and exercises.
To quickly summarize what I have been writing, the assessory nerve (cranial nerve 11) has two part, the cranial part which originates from the nucleus ambiguss, and runs along with the vagus nerve to reach the pharyngeal muscle, and the spinal part of the accessory nerve which originates from the spinal portion, which is situated in the lateral funiculus. The fibers go through the foramen magnum, then leaves through the jugular foramen, where it descends to supply the sternocleidomastoid muscle, and trapezius muscle.
You can read my previous posts here