Cranial nerve X: anatomy, nerve passage, fiber composition, nuclei in the medulla, functions, effects of damage and evaluation.
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Voice by : Marty Henne
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The vagus nerve, or cranial nerve X, is the longest cranial nerve with diverse functions, many of which are critical. It consists of both sensory, or afferent; and motor, or efferent, fibers; and involves 4 nuclei in the medulla. It passes through two ganglia - superior and inferior - shortly after exiting the skull via the jugular foramen.
- Motor neurons that originate from the nucleus ambiguus contain special visceral efferent fibers. They control most muscles of the pharynx, larynx, and some muscles of the soft palate and tongue, and thus play an important role in swallowing and speech;
- Fibers from the dorsal vagal motor nucleus are general visceral efferent fibers. They provide parasympathetic innervation to cardiac, pulmonary, and esophageal muscles; as well as the glands of the gastrointestinal tract.
- Sensory neurons that end in the spinal trigeminal nucleus have their cell bodies in the superior ganglion. They contain general somatic afferent fibers that convey sensation from the outer ear and tympanic membrane.
- Neurons that terminate in the solitary nucleus have their cell bodies in the inferior ganglion. They provide general visceral afferent fibers that conduct sensory impulses from the carotid and aortic bodies. There are also special visceral afferent fibers that convey taste sensation from the pharynx, palate, and epiglottis.
The vagus nerve descends within the carotid sheath together with carotid arteries and internal jugular vein. In the neck, it gives out several branches:
- The pharyngeal branch carries both motor and sensory fibers to most muscles of the pharynx and palate, as well as the palatoglossus muscle of the tongue; and conveys sensory information from the carotid body.
- Next is the superior laryngeal nerve which shortly divides into internal and external branches. The internal branch supplies sensation to the mucosa - from the epiglottis to the level just above the vocal folds. The external branch controls the cricothyroid muscle of the larynx.
- There are also superior cardiac branches which descend and merge with other cardiac branches to form cardiac plexuses.
The right vagus nerve then continues downward, anteriorly to the subclavian artery, at which point, the right recurrent laryngeal nerve branches off and loops upward to enter the larynx. On the other side, left recurrent laryngeal nerve loops around the aortic arch to ascend to the larynx. These recurrent laryngeal nerves control all muscles of the larynx except for the cricothyroid muscle. They also carry sensory information from the level of the vocal folds and below.
As it continues to the thorax and abdomen, the vagus nerve gives rise to more branches, contributing to the formation of cardiac, pulmonary, esophageal, gastric and celiac plexuses.
Damage to vagus nerve results in hoarseness or loss of voice, difficulty swallowing/speaking; impaired gag reflex, reduced gastrointestinal motility, increased heart rate and other parasympathetic problems. The effect is fatal if both nerves are damaged.
Vagus nerve is usually evaluated together with glossopharyngeal nerve. In addition to observing any speech or swallowing problems, patients are tested for symmetry of the gag reflex and symmetry of uvula elevation when saying "ah". The uvula often deviates away from the affected side. Hoarseness or loss of voice in combination with a normal uvula elevation and normal gag reflex typically indicates a lesion of the recurrent laryngeal nerve.
30 окт 2022
