Friday, February 4, 2011

Vagus Nerve (CN X)

Also known as cranial nerve X (CN X), the vagus nerve is one of the twelve paired cranial nerves. Consisting of both motor and sensory fibers, the vagus nerve emerges from the medulla oblongata in the groove between the olive and the inferior peduncle. Then, it projects through the jugular foramen, passing into the carotid sheath between the internal carotid artery and the internal jugular vein down below the head, to the neck, chest and abdomen, where it contributes to the innervation of the viscera. Besides output to the various organs in the body the vagus nerve conveys sensory information about the state of the body's organs to the central nervous system. Between 80 and 90% of the fibers that make up the vagus nerve are sensory nerves communicating the state of the viscera to the brain.

The vagus nerve also supplies motor parasympathetic fibers to all the organs except the suprarenal glands, from the neck down to the second segment of the transverse colon. This means that the vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech (via the recurrent laryngeal nerve) and keeping the larynx open for breathing, via action of the posterior cricoarytenoid muscle, the only abductor of the vocal folds. It also has some afferent fibers that innervate the inner (canal) portion of the outer ear, via the Auricular branch, which also known as Alderman's nerve, and part of the meninges. This explains why a person may cough when tickled on their ear (such as when trying to remove ear wax with a cotton swab).