Editing Esophagus (section)

===Sphincters===<!-- Other articles link here -->
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[[esophageal sphincter]], 
[[esophageal sphincter, upper]], 
[[esophageal sphincter, lower]], 
[[upper esophageal sphincter]], 
[[lower esophageal sphincter]], and 
[[cardiac sphincter]]. 
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The esophagus is surrounded at the top and bottom by two muscular rings, known respectively as the upper esophageal sphincter and the lower esophageal sphincter.<ref name=GRAYS2005 /> These [[sphincter]]s act to close the esophagus when food is not being swallowed. The upper esophageal sphincter is an anatomical sphincter, which is formed by the lower portion of the inferior pharyngeal constrictor, also known as the cricopharyngeal sphincter due to its relation with [[cricoid cartilage]] of the [[larynx]] anteriorly. However, the lower esophageal sphincter is not an anatomical but rather a functional sphincter, meaning that it acts as a sphincter but does not have a distinct thickening like other sphincters.

The upper esophageal sphincter surrounds the upper part of the esophagus. It consists of [[skeletal muscle]] but is not under [[muscle contraction|voluntary control]]. Opening of the upper esophageal sphincter is triggered by the [[Pharyngeal reflex|swallowing reflex]]. The primary muscle of the upper esophageal sphincter is the [[inferior pharyngeal constrictor muscle#Origin and insertion|cricopharyngeal part]] of the [[inferior pharyngeal constrictor]].<ref>{{Cite journal|last1=Mu|first1=L|last2=Wang|first2=J|last3=Su|first3=H|last4=Sanders|first4=I|title=Adult human upper esophageal sphincter contains specialized muscle fibers expressing unusual myosin heavy chain isoforms|journal=J. Histochem. Cytochem.|volume=55|issue=3|pages=199–207|date=March 2007|pmid=17074861|doi=10.1369/jhc.6A7084.2006|doi-access=free}}</ref>

The lower esophageal sphincter, or gastroesophageal sphincter, surrounds the lower part of the esophagus at the [[gastroesophageal junction]] between the esophagus and the stomach.<ref name=GUYTONHALL2005 /> It is also called the cardiac sphincter or cardioesophageal sphincter, named from the adjacent part of the stomach, the ''[[cardia]]''. Dysfunction of the gastroesophageal sphincter causes gastroesophageal [[wikt:reflux#Noun|reflux]], which causes [[heartburn]], and, if it happens often enough, can lead to [[gastroesophageal reflux disease]], with damage of the esophageal mucosa.<ref name="Kahrilas">{{cite journal|author=Kahrilas PJ|title=Gastroesophageal Reflux Disease|journal=[[The New England Journal of Medicine]]|volume=359|issue=16|pages=1700–07|year=2008|pmid=18923172|pmc=3058591|doi=10.1056/NEJMcp0804684}}</ref>