The Vagus Nerve : Parasympathetic Functions: In the thorax and abdomen, the vagus nerve is the main parasympathetic outflow to the heart and gastro-intestinal organs. The Heart Cardiac branches arise in the thorax, conveying parasympathetic innervation to the sino-atrial and atrio-ventricular nodes of the heart (For more heart anatomy, see here). These branches stimulate a reduction in the resting heart rate. They are constantly active, producing a rhythm of 60 – 80 beats per minute. If the vagus nerve was lesioned, the resting heart rate would be around 100 beats per minute. Gastro-Intestinal System The vagus nerve provides parasympathetic innervation to the majority of the abdominal organs. It sends branches to the the oesophagus, stomach and most of the intestinal tract – up to the splenic flexure of the large colon. The function of the vagus nerve is to stimulate smooth muscle contraction and glandular secretions in these organs. For example, in the stomach, the vagus nerve increases the rate of gastric emptying, and stimulates acid production. ....................................................................... Clinical Relevance: Disorders of the Vagus Nerve Cardiovascular Many pharmacological agents can be used to potentiate vagal tone on the heart therefore slowing the heart rate. Beta-blockers, muscarinic agonists and cardiac glycosides such as Digoxin are just a few that can be used. Vasovagal syncope can ensue during a period of emotional stress for example causing a sudden drop in blood pressure and heart rate. Further to this a carotid massage can compress the carotid sinus leading to the perception of a high blood pressure. This will cause CN X to increase its firing leading to a decreased activity of the SA node and AV node. Overall a decreased rate and strength of contraction will ensue and the person may experience syncope. Many congenital heart defects such as a patent ductus ateriosus can irritate the left recurrent laryngeal nerve, leading to dysphonia (hoarse voice). Gastro-Intestinal Lesions to the CN X are rare. A lesion to the pharyngeal branches can lead to dysphagia (difficulty swallowing) due to the involvement with the muscles of the pharynx. As CN X innervates the Palatopharyngeus and Salpingopharyngeus muscles a lesion here will cause the Palatoglossal arch to drop leading to Uvula deviation away from the affected side. The CN IX is sensory to the oropharynx and laryngopharynx with CN X being the motor efferents involved in the Gag reflex therefore a lesion in this area will cause a loss of the Gag reflex. Once upon a time a Vagotomy could be done to reduce excess stomach acid production. However with advancements in pharmacological therapy this is no longer necessary. Other As stated above a lesion to one of the RLN’s will cause dysphonia. A lesion to both RLN’s will cause aphonia (loss of voice) and a stridor (inspiratory wheeze). Paralysis of the RLN’s usually occur due to cancer of the larynx or thyroid gland or due to surgical complications.
Posted on: Sat, 06 Dec 2014 13:32:32 +0000
Trending Topics
Recently Viewed Topics
© 2015