Vagus nerve


Cranial nerve X is the longest of the cranial nerves.

Extends from the brain stem through organs in the neck, thorax and abdomen.

The vagus nerve is the tenth cranial nerve and arises from a series of rootlets in the medulla; it carries both afferent (80%) and efferent (20%) fibers. 

It has the longest and widest distribution of all the cranial nerves and functions as a bidirectional link between the brain and peripheral organs. 

Afferents from the vagus nerve project to the nucleus tractus solitarii which subsequently communicates with other regions of the brain including the dorsal raphe nucleus, locus coeruleus, amygdala and other areas.

The vagus nerve does not participate in cranial ganglia, as most of its fibers provide an array of ganglia on or near the thoracic viscera (esophagus, trachea, heart, lungs) and the abdominal viscera (stomach, pancreas, liver, kidneys). 

The vagus nerve travels all the way down to the midgut/hindgut junction, which occurs just before the splenic flexure of the transverse colon.

Exits the brain stem through rootlets in the medulla that are caudal to the rootlets for the ninth cranial nerve, and forms the tenth cranial nerve and exits the cranium via the jugular foramen.

There are two sensory ganglia associated with the vagus nerve, they are the superior and inferior vagal ganglia.

The branchial motor component of the vagus nerve originates in the medulla in the nucleus ambiguus.

The nucleus ambiguus contributes to the vagus nerve as three major branches which leave the nerve distal to the jugular foramen.

The pharyngeal branch of the vagus nerve travels between the internal and external carotid arteries and enters the pharynx at the upper border of the middle constrictor muscle.

The pharyngeal branch supplies the all the muscles of the pharynx,including the three constrictor muscles, levator veli palatini, salpingopharyngeus, palatopharyngeus and palatoglossal muscles,and soft palate except the stylopharyngeas and tensor palati.

The superior laryngeal nerve branches distal to the pharyngeal branch and descends lateral to the pharynx where it divides into an internal and external branches.

The internal branch of the superior laryngeal nerve is purely sensory, and the external branch innervates the cricothyroid muscle.

The third branch is the recurrent branch of the vagus nerve and it has a different path on the left and right sides of the body.

On the right side the recurrent branch leave the vagus anterior to the subclavian artery and wraps back around the artery to ascend posterior to it in a groove between the trachea and esophagus.

The left recurrent branch leaves the vagus nerve on the aortic arch and loops posterior to the arch to ascend through the superior mediastinum along a groove between the esophagus and trachea.

Both recurrent branches enter the larynx and supply intrinsic muscles of larynx, excluding the cricothyroid.

The visceromotor or parasympathetic component of the vagus nerve originates from the dorsal motor nucleus of the vagus in the dorsal medulla.

The visceromotor part of the vagus innervates ganglionic neurons which are located in or adjacent to each target organ.

In the head-neck innervation includes the pharynx and larynx (via the pharyngeal and internal branches).

The thorax branches go to the lungs for bronchoconstriction, to the esophagus to influence peristaltic activity and to the heart for slowing the rate.

Abdominal branches go to the stomach, pancreas, small intestine, large intestine and colon for secretion and constriction of smooth muscle.

The vagus nerve has receptors in the abdominal viscera, esophagus, heart and aortic arch, lungs, bronchia and trachea.

Sensation from the mucous membranes of the epiglottis, base of the tongue, aryepiglottic folds and the upper larynx are carried by the internal laryngeal nerve.

The presence of gastrointestinal bloating, indigestion, and hiccoughs can stimulate the vagus nerve and cause palpitations due to branches of the vagus nerve innervating the G.I. tract, diaphragm, and lungs are present.

Sensation below the vocal folds of the larynx is carried by the recurrent laryngeal nerves.

Sensation of the tenth cranial nerve conducts sensations from the larynx, pharynx, skin the external ear and external auditory canal, external surface of the tympanic membrane, and the posterior cranial fossa meninges.

Sensation from the larynx travels via the recurrent laryngeal and internal branches of the vagus to reach the inferior vagal ganglion.

Elevated levels of cortisol and adrenaline seen instress and anxiety can interfere with the normal functioning of the parasympathetic nervous system resulting in overstimulation of the vagus nerve.

Vagus nerve induced heart palpitations are felt as a thud, a hollow fluttery sensation, or a skipped beat, depending on at what point during the heart’s normal rhythm the vagus nerve fires. 

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