The vagus nerve (cranial nerve X) is a mixed nerve, meaning that it contains both sensory and motor components. Specifically, it has an extensive motor and sensory distribution to thoracic and abdominal viscera, as well as to structures in the pharynx and larynx. It emerges from the brainstem as a bundle of small rootlets. From there, it wanders inferiorly below the head and neck to enter the chest and abdomen. This is how it received its Latin term, vagus, which means “wandering.” There are two major roles of the vagus nerve. First, it functions as the parasympathetic motor output of the autonomic nervous system; and second, it functions to relay information about the viscera to the brain via sensory neurons. These sensory neurons are called primary visceral sensory neurons. In fact, most of the nerve fibers in the vagus nerve are sensory in nature.
Anatomy and Physiology
The vagus nerve originates in the medulla oblongata from four main nuclei: the nucleus solitarius, nucleus ambiguus, dorsal nucleus of the vagus nerve, and spinal trigeminal nucleus. The fibers from the nucleus solitarius receive sensory taste information along with sensory input from organs. The nucleus ambiguus fibers send parasympathetic output to the heart, which is used to lower heart rate. Fibers from the dorsal nucleus of the vagus nerve send parasympathetic output to abdominal organs, which results in increased secretory activity of glands and increased rates of peristalsis. Finally, the spinal trigeminal nucleus receives sensory information from the outer ear and the mucus layer of the larynx. It travels a long distance from its point of origin in the brainstem to the various organs of the neck, thorax, and abdomen that it innervates.
In its journey, cranial nerve X exits the skull through the jugular foramen along with cranial nerves IX and XI. The vagus nerve then descends in a covering (called the carotid sheath) with the internal jugular vein and internal carotid artery, inferiorly through the neck and eventually into the chest and abdomen. Along the way, the vagus nerve gives off several branches on both sides of the body to innervate various organs and/or skeletal muscles. It has two enlargements: the superior ganglion—which is near the opening of the foramen that receives general sensory information and connects with neurons from cranial nerves IX and XI—and the inferior ganglion—which receives sensory input from various organs. Because the vagus nerve carries so many different afferent fibers from so many different visceral organs, it is a very large and thick nerve that is easily identified within the neck, thorax, and abdominal cavity.
In the thorax, the right and left vagus nerves take different paths on their way to the abdomen. The left vagus nerve is closely aligned with the arch of the aorta and gives off a branch, the left recurrent laryngeal nerve. This branch then hooks around a ligament under the arch before ascending up into the neck to supply motor function and sensory sensation to the larynx. On the right side, the right recurrent laryngeal nerve comes off the right vagus nerve much higher up, hooking around the right subclavian artery before ascending up the neck.
The recurrent laryngeal nerves innervate the intrinsic muscles of the larynx, such as the thyroarytenoid, posterior and lateral cricoarytenoid, and arytenoid muscles. Because the left recurrent laryngeal nerve passes under the aortic arch, it is longer than the right recurrent laryngeal nerve. Along the way, both recurrent branches give off smaller branches to the heart, esophagus, trachea, and pharyngeal constrictor muscles before reaching their destination, the larynx.
The vagus nerves enter the abdomen through a natural hole in the diaphragm called the esophageal hiatus, which allows the esophagus, vagus nerve, and a few vessels to pass through the diaphragm. This section of cranial nerve X supplies motor parasympathetic fibers to all organs (except the adrenal glands) from the neck down to the latter third of the transverse colon. It also innervates some muscles in the larynx that are involved in speech, the throat, and the palate. Based on the location of the vagus nerves braches, cranial nerve X is responsible for a variety of tasks that include the control of heart rate and blood pressure, gut peristalsis, speech, and breathing.
Clinical Symptoms and Disease
Isolated lesions of the vagus nerve are uncommon, but damage to the recurrent nerves can occur. Cancers of the larynx or thyroid glands or thyroid surgery can injury these nerves. The result is decreased movement of the vocal fold on the damaged side, causing hoarseness. Bilateral damage to the recurrent branch can result in difficulty in swallowing, reduced gag reflexes, and dysarthria (problems with speaking due to the lack of muscle control). To test which vagus nerve is damaged, an easy clinical diagnostic assessment is to see if the patient’s uvula deviates to one side. If it does, the uvula will move away from the side of the lesion. In addition, patients will not be able to elevate their palate.
Robin Michaels and Jennifer L. Hellier
See also: Autonomic Nervous System; Cranial Nerves; Nerves; Peripheral Nervous System; Seizures
Liang, Barbara. (2012). The 12 cranial nerves. Retrieved from http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP11504
Moore, Keith L., Anne M. R. Agur, & Arthur F. Dalley (Eds.). (2010). Essential clinical anatomy (4th ed.). Baltimore, MD: Williams and Wilkins.
Yale University School of Medicine. (1998). Cranial nerves. Retrieved from http://www.yale.edu/cnerves/