http://en.wikipedia.org/wiki/Vagus_nerve
The vagus nerve (play /ˈveɪɡəs/ US dict: vā′·gəs), also called pneumogastric nerve or cranial nerve X, is the tenth of twelve (excluding CN0) paired cranial nerves. Upon leaving the medulla between the medullary pyramid and the inferior cerebellar peduncle, it extends through the jugular foramen, then 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. 80-90% of the nerve fibers in the vagus nerve are afferent (sensory) nerves communicating the state of the viscera to the brain.[1]
The medieval Latin word vagus means literally "wandering" (the words vagrant, vagabond, and vague come from the same root). Sometimes the branches are spoken of in the plural and are thus called vagi (/ˈveɪdʒaɪ/, US dict: vā′·jī). The vagus is also called the pneumogastric nerve since it innervates both the lungs and the stomach.
The motor division of the vagus nerve is derived from the basal plate of the embryonic medulla oblongata, while the sensory division originates from the cranial neural crest.
The vagus nerve includes axons which emerge from or converge onto three nuclei of the medulla:
The Dorsal nucleus of vagus nerve - which sends parasympathetic output to the viscera, especially the intestines
The Nucleus ambiguus - which sends parasympathetic output to the heart (slowing it down) and
The Solitary nucleus - which receives afferent taste information and primary afferents from visceral organs
Right and left vagus nerves descend from the cranial vault through the jugular foramina, penetrating the carotid sheath between the internal and external carotid arteries, then passing posterolateral to the common carotid artery. The cell bodies of visceral afferent fibers of the vagus nerve are located bilaterally in the inferior ganglion of the vagus nerve (nodose ganglia).
Vagus nerve - dissection
The right vagus nerve gives rise to the right recurrent laryngeal nerve, which hooks around the right subclavian artery and ascends into the neck between the trachea and esophagus. The right vagus then crosses anteriorly to the right subclavian artery and runs posterior to the superior vena cava and descends posterior to the right main bronchus and contributes to cardiac, pulmonary, and esophageal plexuses. It forms the posterior vagal trunk at the lower part of the esophagus and enters the diaphragm through the esophageal hiatus.
The left vagus nerve enters the thorax between left common carotid artery and left subclavian artery and descends on the aortic arch. It gives rise to the left recurrent laryngeal nerve, which hooks around the aortic arch to the left of the ligamentum arteriosum and ascends between the trachea and esophagus. The left vagus further gives off thoracic cardiac branches, breaks up into pulmonary plexus, continues into the esophageal plexus, and enters the abdomen as the anterior vagal trunk in the esophageal hiatus of the diaphragm.
The vagus nerve supplies motor parasympathetic fibers to all the organs except the suprarenal (adrenal) glands, from the neck down to the second segment of the transverse colon. The vagus also controls a few skeletal muscles, notable ones being:
Cricothyroid muscle
Levator veli palatini muscle
Salpingopharyngeus muscle
Palatoglossus muscle
Palatopharyngeus muscle
Superior, middle and inferior pharyngeal constrictors
Muscles of the larynx (speech).
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 (also known as Alderman's nerve) and part of the meninges. This explains why a person may cough when tickled on the ear (such as when trying to remove ear wax with a cotton swab).
Parasympathetic innervation of the heart is controlled by the vagus nerve. To be specific, the vagus nerve acts to lower the heart rate. The right vagus innervates the sinoatrial node. Parasympathetic hyperstimulation predisposes those affected to bradyarrhythmias. The left vagus when hyperstimulated predisposes the heart to atrioventricular (AV) blocks.
At this location, neuroscientist Otto Loewi first proved that nerves secrete substances called neurotransmitters, which have effects on receptors in target tissues. In his experiment, Loewi electrically stimulated the vagus nerve of a frog heart, which slowed the heart. Then he took the fluid from the heart and transferred it to a second frog heart without a vagus nerve. The second heart slowed down without an electrical stimulation. Loewi described the substance released by the vagus nerve as vagusstoff, which was later found to be acetylcholine. Drugs that inhibit the muscarinic cholinergic receptor (anticholinergics) such as atropine and scopolamine are called vagolytic because they inhibit the action of the vagus nerve on the heart, gastrointestinal tract, and other organs. Anticholinergic drugs increase heart rate and are used to treat bradycardia (slow heart rate). Atropine is no longer indicated for the treatment of pulseless electrical activity (PEA) or asystole per the 2010 ACLS guidelines, as it has not been shown to improve outcomes in these clinical scenarios.
