|
|
|
|
Vagus nerve
|
| |
|
| |
The vagus nerve (VA-g?s) (also called pneumogastric nerve or cranial nerve X) is the tenth of twelve paired cranial nerves, and is the only nerve that starts in the brainstem (within the medulla oblongata) and extends, through the jugular foramen, 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.
Discussion
Ask a question about 'Vagus nerve' Start a new discussion about 'Vagus nerve' Answer questions from other users |
Encyclopedia
The vagus nerve (VA-g?s) (also called pneumogastric nerve or cranial nerve X) is the tenth of twelve paired cranial nerves, and is the only nerve that starts in the brainstem (within the medulla oblongata) and extends, through the jugular foramen, 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.
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 (VA-gi). The vagus is also called the pneumogastric nerve since it innervates both the lungs and the stomach.
Branches
Innervation
Both right and left vagus nerves descend from the brain in the carotid sheath, lateral to the carotid artery.
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 lower part of esophagus and enters diaphragm through 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, namely:
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. It also receives some sensation from the outer ear, via the Auricular branch (also known as Alderman's nerve) and part of the meninges.
The vagus nerve and the heart
Parasympathetic innervation of the heart is mediated by the vagus nerve. 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 Otto Loewi first proved that nerves secrete substances called neurotransmitters which have effects on receptors in target tissues. Loewi described the substance released by the vagus nerve as vagusstoff, which was later found to be acetylcholine.
The vagus nerve has three nuclei in the CNS associated with cardiovascular control, the dorsal motor nucleus, the nucleus ambiguus and the solitary nucleus. The parasympathetic output to the heart comes mainly from neurons in the nucleus ambiguus and to a lesser extent from the dorsal motor nucleus. The solitary nucleus receives sensory input about the state of the cardiovascular system, being an integrational hub for the baroreflex.
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) and asystole, which is when the heart has no electrical activity. Anticholinergic drugs relax the detrusor muscle and cause constipation which again involves the vagus nerve.
Medical treatment involving the vagus nerve
Vagus nerve stimulation (VNS) therapy using a pacemaker-like device implanted in the chest is a treatment used since 1997 to control seizures in epilepsy patients and has recently been approved for treating drug-resistant cases of clinical depression. A convenient, non-invasive VNS device that stimulates an afferent branch of the vagus nerve is also being developed and will soon undergo trials.
VNS may also be achieved by one of the vagal maneuvers: holding the breath for a few seconds, dipping the face in cold water, coughing, or tensing the stomach muscles as if to bear down to have a bowel movement (valsalva maneuver). Patients with supraventricular tachycardia, atrial fibrillation, and other illnesses may be trained to perform vagal maneuvers (or find one or more on their own).
Vagus nerve blocking (VBLOC) therapy, similar to VNS but only used during the day, has caused 31 obese participants in a six month open label trial involving three medical centers in Australia, Mexico and Norway to lose an average of nearly 15 percent of their excess weight. A one year 300 participant double blind phase II trial has begun.
Vagotomy (cutting of the vagus nerve) is a now-obsolete therapy that was performed for peptic ulcer disease. Vagotomy is currently being researched as a less invasive alternative weight loss procedure to gastric bypass surgery. The procedure curbs the feeling of hunger and is sometimes performed in conjunction with putting bands on patients' stomachs, resulting in average weight loss of 43% at six months with diet and exercise. Five pencil-sized scars are the result of the procedure.
See also:
- Porphyria This rare disorder can cause seizures and damage to the vagal nerve. Diagnosis, in some cases, may require DNA testing.
Physical and emotional effects
Activation of the vagus nerve typically leads to a reduction in heart rate, blood pressure, or both. This occurs commonly in the setting of gastrointestinal illness such as viral gastroenteritis or acute cholecystitis, or in response to other stimuli, including carotid sinus massage, Valsalva maneuver, or pain from any cause, particularly having blood drawn. When the circulatory changes are great enough, vasovagal syncope results. Relative dehydration tends to amplify these responses.
Excessive activation of the vagal nerve during emotional stress, which is a parasympathetic overcompensation of a strong sympathetic nervous system response associated with stress, can also cause vasovagal syncope because of a sudden drop in blood pressure and heart rate. Vasovagal syncope affects young children and women more often. It can also lead to temporary loss of bladder control under moments of extreme fear.
Research has shown that women who have complete transection of the spinal cord can experience orgasms through the vagus nerve, which can go from the uterus, cervix and probably the vagina to the brain.
Effects of vagus nerve lesions
The patient complains of hoarse voice, difficulty in swallowing (dysphagia) and choking when drinking fluid. There is also loss of gag reflex. Uvula deviates away from the side of lesion and there is failure of palate elevation.
Additional images
External links
|
| |
|
|
