Anatomy of the vagus nerve (CN X)

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Introduction0:00–0:33

The vagus nerve is the tenth cranial nerve, and its name comes from the latin word vagary, which means indefinite or wandering.
This actually reflects its wide distribution seeing as the vagus nerve is the most extensively distributed cranial nerve; in fact, it innervates structures all the way from the head and neck to parts of the digestive tract!
The vagus nerve has multiple functions such as somatic, visceral and special sensory innervation, as well as somatic and visceral motor innervation.Now, the vagus nerve emerges from the lateral medulla through a group of rootlets that join and leave the cranium through the jugular foramen along with cranial nerves 9 and 11, or the glossopharyngeal and spinal accessory nerves.

CN X0:33–4:55

Both left and right vagus nerves leave the cranium through the jugular foramen on their respective sides, and penetrate the carotid sheath.
Then, they descend within the carotid sheath, posterior to the common carotid artery and medial to the internal jugular vein.
From here, each vagus nerve takes a different course as they enter the thorax. The right vagus nerve crosses the right subclavian artery anteriorly, runs posterior to the superior vena cava and descends posterior to the right main bronchus to contribute to the cardiac, pulmonary and esophageal plexuses.
When it reaches the lower part of the esophagus, it forms the posterior vagal trunk and then enters the abdomen through the esophageal hiatus of the diaphragm.
The left vagus nerve, on the other hand, enters the thorax between the left common carotid artery and the left subclavian artery and descends on the aortic arch.
Then, it runs posterior to the left main bronchus, contributing to the cardiac pulmonary and esophageal plexuses, and finally it continues to form the anterior vagal trunk, and enters the abdomen through the esophageal hiatus.
In the abdomen, the vagal trunks terminate by dividing into branches that contribute to various plexuses which supply the visceral organs of the abdomen up to approximately the last third of the transverse colon.
Over its course, both the right and left vagus nerves give off a number of branches such as the pharyngeal branches, the recurrent laryngeal nerves and the superior laryngeal nerves, which then further divide into the internal and external laryngeal nerves.
All of these branches provide branchial motor innervation to the muscles of the fourth and sixth pharyngeal arches. Remember that the pharyngeal arches are 6 embryological structures, of which only 5 eventually develop into various muscles, arteries, bones and cartilage of the head and neck.
Structures derived from the fourth pharyngeal arch include the cricothyroid muscle, all muscles of the pharynx, except the stylopharyngeus, and all muscles of the palate, except the tensor veli palatini, as well as the thyroid and epiglottic cartilages of the larynx.
The structures of the 4th arch are all innervated by the superior laryngeal nerves and pharyngeal branches of vagus. Structures derived from the sixth pharyngeal arch include all the intrinsic muscles of the larynx, except the cricothyroid muscle; as well as the cricoid and arytenoid cartilages of the larynx.
The structures of the 6th arch are innervated by the recurrent laryngeal nerves. Now, the recurrent laryngeal nerves are called so because they run in the opposite direction from the vagus nerve, meaning that while the vagus descends into the thorax, the recurrent laryngeal nerves ascend back towards the larynx.
Now, the course of the right and left recurrent laryngeal nerves back towards the larynx is slightly different.The left recurrent laryngeal nerve crosses under the arch of the aorta at the ligamentum arteriosum, and ascends between the trachea and the esophagus, while the right recurrent laryngeal nerve hooks around the right subclavian artery and ascends in the neck between the trachea and the esophagus.
Once in the neck, both nerves pass posterior to the thyroid gland and enter the larynx. So, together, the superior laryngeal, pharyngeal and the recurrent laryngeal branches of the vagus nerve provide branchial motor innervation to the majority of the muscles of the palate, pharynx and the larynx which are mainly involved in swallowing and phonation.
The superior laryngeal nerve also provides sensory information specifically via the internal laryngeal nerve from the structures of the larynx located above the vocal folds, such as the epiglottis, the base of the tongue, the epiglottic glands and the mucous membrane surrounding the entrance of the larynx.
Sensory information from all the structures below the vocal folds travels via the recurrent laryngeal nerves.Now let’s look at how the vagus nerve, along with the glossopharyngeal nerve, participate in the gag reflex.

Gag reflex4:55–5:49

The gag reflex prevents foreign objects in the oral cavity from unintentionally entering the throat and also helps prevent choking.
This reflex has an afferent limb supplied by the glossopharyngeal nerve, and an efferent limb supplied by the vagus nerve.
Now, this pathway starts with stimulation of the posterior pharyngeal wall, the tonsillar area, or the base of the tongue which sends signals via the glossopharyngeal nerve that carries this information to the superior glossopharyngeal ganglion located in the jugular foramen.
Then, these first order neurons send fibers to the nucleus ambiguus from which efferent nerve fibers travel through the vagus nerve to the pharyngeal muscles which results in bilateral contraction of the posterior pharyngeal muscles.

Quiz5:49–6:00

Let’s take a quick break and try to remember which structures are derived from the fourth versus the sixth pharyngeal arches!Now that we understand the general pathway of the vagus nerve as a whole, let’s look more specifically into the course of the different motor and sensory fibers of the vagus nerve, starting off with somatic or branchial motor fibers.

Somatic motor innervation6:00–6:36

This pathway begins in the cerebral cortex where axons travel and synapse in the nucleus ambiguus located in the medulla which is where the cell bodies of the lower motor neurons in this motor pathway are found.
Efferent fibres arising from the nucleus ambiguus then emerge from the medulla and travel through the vagus nerve and its branches to the muscles of the fourth and sixth pharyngeal arches.
The vagus nerve also has somatic sensory fibers which provide general sensory information to the mucosa of the larynx, some of the dura mater surrounding the posterior cranial fossa, a small section of skin of the external ear as well as the posteroinferior aspect of the external auditory canal and tympanic membrane.

Somatic sensory innervation6:36–8:41

This pathway starts with somatic sensory fibers from these structures traveling through the vagus nerve, including the auricular branch of the vagus nerve for the structures of the ear, to the inferior or superior vagal ganglion.
Now, the cell bodies of the first order neurons for the general sensation from the larynx are found in the inferior vagal ganglion, while the cell bodies of the first order neurons for the general sensation from the ear and dura mater are found in the superior vagal ganglion.
From here, the fibers project to and synapse in the spinal trigeminal nucleus located in the medulla, which is where the cell bodies of the second order neurons in the vagal nerve general sensory pathway are found.Next up, is the visceral sensory fibres, which receive sensory information from the mucosa of the laryngopharynx, the larynx, the aortic body chemoreceptors, as well as the thoracic and abdominal viscera.
These fibres, which are first order neurons in the vagal nerve visceral sensory pathway, travel through the inferior vagal ganglion where their cell bodies are found.
They then project towards and eventually synapse in the nucleus of the solitary tract which is where the cell bodies of the secondary neurons are found.
Now don't forget that the vagus nerve also carries special sensory information to the brain. This pathway starts with special sensory fibers in the epiglottic region and the laryngopharynx, that travel via the vagus nerve to the inferior vagal ganglion where the cell bodies of these first order neurons are found.
Then, they exit the jugular foramen and reach the brainstem at the level of the rostral medulla. From here, the fibers travel in the solitary tract and synapse in the gustatory part of the nucleus of the solitary tract where the cell bodies of second order neurons in this pathway are found.Finally, the vagus nerve also carries parasympathetic fibers which provide visceral motor innervation to the involuntary muscles and glands of the tracheobronchial tree, the esophagus, the heart and the digestive tract as far as the last third of the transverse colon.

Parasympathetic innervation8:41–9:22

This pathway starts with preganglionic fibers that originate from the dorsal motor nucleus of the vagus nerve, which is located in the medulla.
Then, the fibers travel through the vagus nerve and synapse at various ganglia located close to or in the walls of the different thoracic and abdominal target organs.
Finally, postganglionic fibers will travel and synapse directly onto the target organs.Alright, as a quick recap. The vagus nerve is the 10th and most widely distributed cranial nerve in the body.

Review9:22–10:20

It is composed of a variety of different motor and sensory fibers. First off, the vagus nerve, specifically the superior laryngeal, pharyngeal and recurrent laryngeal branches provide branchial or somatic motor innervation to the muscles of the fourth and sixth pharyngeal arches which include the majority of the muscles of the palate, pharynx and larynx.
The vagus nerve also carries somatic sensory information from the larynx, some of the dura mater surrounding the posterior cranial fossa, and a small section of skin of the external ear; Visceral sensory information (is carried) from the mucosa of the laryngopharynx and the larynx, the aortic body chemoreceptors, as well as the thoracic and abdominal organs; and Special sensory or taste information (comes) from the epiglottic region and the laryngopharynx.
Finally, the vagus nerve has parasympathetic fibers that innervate the