| Vagus nerve | |
|---|---|
 Plan of the upper portions of theglossopharyngeal, vagus, andaccessory nerves. | |
 Course and distribution of theglossopharyngeal, vagus, andaccessory nerves. | |
| Details | |
| Innervates | Levator veli palatini,salpingopharyngeus,palatoglossus,palatopharyngeus,superior pharyngeal constrictor,middle pharyngeal constrictor,inferior pharyngeal constrictor,viscera |
| Identifiers | |
| Latin | nervus vagus |
| MeSH | D014630 |
| NeuroNames | 702 |
| TA98 | A14.2.01.153 |
| TA2 | 6332 |
| FMA | 5731 |
| Anatomical terms of neuroanatomy | |
The vagus nerve, also known as the tenth cranial nerve (CN X), plays a crucial role in theautonomic nervous system. This nerve carries both sensory and motor fibers and serves as a major pathway that connects the brain to various organs, including the heart, lungs, and digestive tract. As a key part of the parasympathetic nervous system, the vagus nerve helps regulate essential functions like heart rate, breathing, and digestion. By controlling these processes, the vagus nerve contributes to the body's "rest and digest" response, helping to calm the body after stress, lower heart rate, improve digestion, and maintain homeostasis.
The vagus nerve consists of two branches: the right and left vagus nerves. In the neck, the right vagus nerve contains approximately 105,000 fibers, while the left vagus nerve has about 87,000 fibers, according to one source. However, other sources report slightly different figures, with around 25,000 fibers in the right vagus nerve and 23,000 fibers in the left.[1][2]
The vagus nerve is the longest nerve of theautonomic nervous system in the human body, consisting of both sensory and motor fibers. The sensory fibers originate from thejugular andnodose ganglion, while the motor fibers are derived from neurons in the dorsal motor nucleus of the vagus and thenucleus ambiguus.[3] Historically, the vagus nerve was also known as thepneumogastric nerve, reflecting its role in regulating both the lungs and digestive system.
Upon leaving themedulla oblongata between theolive and theinferior cerebellar peduncle, the vagus nerve extends through thejugular foramen, then passes into thecarotid sheath between theinternal carotid artery and theinternal jugular vein down to theneck,chest, andabdomen, where it contributes to the innervation of theviscera, reaching all the way to thecolon. Besides giving some output to various organs, the vagus nerve comprises between 80% and 90% ofafferent nerve fibers conveyingsensory information about the state of the body's organs to thecentral nervous system.[4]
The right and left vagus nerves descend from the cranial vault through the jugular foramina,[5] penetrating the carotid sheath between the internal and external carotid arteries, then passing posterolateral to the common carotid artery. The cell bodies ofvisceral afferent fibers of the vagus nerve are located bilaterally in theinferior ganglion of the vagus nerve (nodose ganglia).
The vagus runs parallel to the common carotid artery and internal jugular vein inside the carotid sheath.
The right vagus nerve gives rise to the rightrecurrent laryngeal nerve, which hooks around the rightsubclavian artery and ascends into the neck between thetrachea andesophagus. The right vagus then crosses anterior to the right subclavian artery, runs posterior to thesuperior vena cava, descends posterior to theright main bronchus, and contributes tocardiac,pulmonary, andesophageal plexuses. It forms theposterior vagal trunk at the lower part of the esophagus and enters thediaphragm through theesophageal hiatus.
The left vagus nerve enters thethorax betweenleft common carotid artery and leftsubclavian artery and descends on theaortic arch. It gives rise to the leftrecurrent laryngeal nerve, which hooks around theaortic arch to the left of theligamentum arteriosum and ascends between the trachea and esophagus. The left vagus further gives off thoracic cardiac branches, breaks up into thepulmonary plexus, continues into the esophageal plexus, and enters the abdomen as theanterior vagal trunk in the esophageal hiatus of the diaphragm.
The vagus nerve includesaxons which emerge from or converge onto fournuclei of the medulla:
Themotor division of the glossopharyngeal nerve is derived from thebasal plate of theembryonicmedulla oblongata, while thesensory division originates from thecranial neural crest.[6]
The vagus nerve supplies motorparasympathetic fibers to all the organs (except theadrenal glands) from theneck down to the second segment of thetransverse colon. The vagus also controls a fewskeletal muscles, including:
This means that the vagus nerve is responsible for such varied tasks asheart rate, gastrointestinalperistalsis,sweating, and quite a few muscle movements in the mouth, includingspeech (via therecurrent laryngeal nerve). It also has some afferent fibers that innervate the inner (canal) portion of theouter ear (via the auricular branch, also known asArnold's or Alderman's nerve) and part of themeninges.[7] The vagus nerve is also responsible for regulating inflammation in the body, via theinflammatory reflex.[8]
Efferent vagus nerve fibers innervating the pharynx and back of the throat are responsible for thegag reflex. In addition,5-HT3 receptor-mediated afferent vagus stimulation in the gut due togastroenteritis is a cause ofvomiting.[9] Stimulation of the vagus nerve in thecervix uteri (as in some medical procedures) can lead to avasovagal response.
The vagus nerve also plays a role in satiation following food consumption.[10] Knocking out vagal nerve receptors has been shown to causehyperphagia (greatly increased food intake).[11] NeuroscientistIvan De Araujo and colleagues have shown that the vagus nerve transmits reward signals from the body to the brain,[12][13] potentially explaining how stimulation of the nerve leads to emotional changes.
Parasympathetic innervation of the heart is partially controlled by the vagus nerve and is shared by thethoracic ganglia. Vagal and spinal ganglionic nerves mediate the lowering of theheart rate. The right vagus branch innervates thesinoatrial node. In healthy people, parasympathetic tone from these sources is well-matched to sympathetic tone. Hyperstimulation of parasympathetic influence promotesbradyarrhythmias. When hyperstimulated, the left vagal branch predisposes the heart toconduction block at theatrioventricular node.
At this location, neuroscientistOtto Loewi first demonstrated that nerves secrete substances calledneurotransmitters, 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 without electrical stimulation. Loewi described the substance released by the vagus nerve asvagusstoff, which was later found to beacetylcholine.
Drugs that inhibit themuscarinic receptors (anticholinergics) such asatropine andscopolamine, 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 treatbradycardia.
Excessive activation of the vagal nerve duringemotional stress, which is a parasympathetic overcompensation for a strongsympathetic nervous system response associated with stress, can also causevasovagal syncope due to a sudden drop incardiac output, causingcerebral hypoperfusion. Vasovagal syncope affects young children and women more than other groups. It can also lead to temporaryloss of bladder control under moments of extreme fear.
Research has shown that women having had completespinal cord injury can experienceorgasms through the vagus nerve, which can go from theuterus andcervix to the brain.[14][15]
Insulin signaling activates theadenosine triphosphate (ATP)-sensitive potassium (KATP) channels in thearcuate nucleus, decreases AgRP release, and through the vagus nerve, leads to decreased glucose production by the liver by decreasing gluconeogenic enzymes:phosphoenolpyruvate carboxykinase,glucose 6-phosphatase.[16][17]
 | This section needs to beupdated. Please help update this article to reflect recent events or newly available information.(January 2023) |
Vagus nerve stimulation (VNS) therapy via aneurostimulator implanted in the chest has been used to controlseizures inepilepsy patients and has been approved for treating drug-resistantclinical depression.[18] Several noninvasive VNS devices that stimulate an afferent branch of the vagus nerve are available. GammaCore is recommended byThe National Institute for Health and Care Excellence (NICE) for cluster headaches.[19]
VNS may also be achieved by one of thevagal maneuvers: holding the breath for 20 to 60 seconds, dipping the face in cold water, coughing, humming or singing, or tensing the stomach muscles as if to bear down to have a bowel movement.[20] Patients withsupraventricular tachycardia,[20]atrial fibrillation, and other illnesses may be trained to perform vagal maneuvers (or find one or more on their own).[citation needed]
Vagus nerve blocking (VBLOC) therapy is similar to VNS but used only during the day. In a six-monthopen-label trial involving three medical centers in Australia, Mexico, and Norway, vagus nerve blocking helped 31 obese participants lose an average of nearly 15 percent of their excess weight. As of 2008[update], a yearlongdouble-blind,phase II trial had begun.[21]
Vagotomy (cutting of the vagus nerve) is a now obsolete therapy that was performed forpeptic ulcer disease and now superseded by oral medications, including H2 antagonists, proton pump inhibitors and antibiotics. Vagotomy is currently being researched as a less invasive alternative weight-loss procedure togastric bypass surgery.[22] The procedure curbs the feeling of hunger and is sometimes performed in conjunction with putting bands on patients' stomachs, resulting in an average of 43% of excess weight loss at six months with diet and exercise.[23]
One serious side effect of vagotomy is avitamin B12 deficiency later in life – perhaps after about 10 years – that is similar topernicious anemia. The vagus normally stimulates the stomach'sparietal cells to secrete acid and intrinsic factor.Intrinsic factor is needed to absorb vitamin B12 from food. The vagotomy reduces this secretion and ultimately leads to deficiency, which, if left untreated, causes nerve damage, tiredness, dementia, paranoia, and ultimately death.[24]
Researchers fromAarhus University and Aarhus University Hospital have demonstrated that vagotomy prevents (halves the risk of) the development ofParkinson's disease, suggesting that Parkinson's disease begins in the gastrointestinal tract and spreads via the vagus nerve to the brain.[25] Or giving further evidence to the theory that dysregulated environmental stimuli, such as that received by the vagus nerve from the gut, may have a negative effect on the dopamine reward system of thesubstantia nigra, thereby causing Parkinson's disease.[26]
The sympathetic and parasympathetic components of the autonomic nervous system (ANS) control and regulate the function of various organs, glands, and involuntary muscles throughout the body (e.g., vocalization, swallowing, heart rate, respiration, gastric secretion, and intestinal motility). Hence, most of the signs and symptoms of vagus nerve dysfunction, apart from vocalisation, are vague and non specific. Laryngeal nerve palsy results in paralysis of an ipsilateral vocal cord and is used as a pointer to diseases affecting the vagus nerve from its origin down to termination of its branch of the laryngeal nerve.
The hypersensitivity of vagal afferent nerves causes refractory or idiopathic cough.
Arnold's nerve ear-cough reflex, though uncommon, is a manifestation of a vagal sensory neuropathy and this is the cause of a refractory chronic cough that can be treated withgabapentin. The cough is triggered by mechanical stimulation of the external auditory meatus and accompanied by other neuropathic features such as throat irritation (laryngeal paresthesia) and cough triggered by exposure to nontussive triggers such as cold air and eating (termed allotussia). These features suggest a neuropathic origin to the cough.[27]
Pathology of the vagus nerve proximal to the laryngeal nerve typically presents with symptom hoarse voice and physical sign of paralysed vocal cords. Although a large proportion of these are the result of idiopathic vocal cord palsy but tumours especially lung cancers are next common cause. Tumours at the apex of right lung and at the hilum of the left lung are the most common oncological causes of vocal cord palsy. Less common tumours causing vocal cord palsy includes thyroid and proximal oesophageal malignancy.
TheLatin wordvagus means literally "wandering" (the wordsvagrant,vagabond,vague, anddivagation come from the same root). Sometimes the right and left branches together are spoken of in the plural and are thus calledvagi (/ˈveɪdʒaɪ/VAY-jy). The vagus was also historically called thepneumogastric nerve since it innervates both the lungs and the stomach.
Dr. Camilleri says a follow-up double-blinded study, which will involve up to 300 patients at multiple medical centers including a limited number from Mayo Clinic, will be important for gauging the device's true effectiveness.
