All About All Your Sphincters

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Sphincters are special, circular muscles that open and close certain body parts. Most often, the action of a sphincter is to regulate the passage of some type of fluid, such as bile, urine or fecal matter.

The action of sphincters may happen involuntarily through the autonomic nervous system or may be under some voluntary control through the somatic nervous system.

If a sphincter loses muscle tone or has too much tone (spasticity), symptoms and/or illness can follow, such as urinary retention and bladder and fecal incontinence.

Digestive System Sphincters

Upper Esophageal Sphincter

The upper esophageal sphincter (UES), also known as the inferior pharyngeal sphincter, can be found at the end of the pharanx, where it protects the entrance to the esophagus. The UES is resposible for preventing air from getting into the esophagus when we breathe and to prevent food from being aspirated into our respiratory tracts. Becauese of its location, the UES also plays a role in burping and vomiting. A malfunctioning of the UES, as part of gastroesophageal reflux disease, (GERD) can cause acid to back up into the throat or into the airways.

Lower Esophageal Sphincter

The Lower esophageal sphincter (LES), also known as the cardiac sphincter, is located at the bottom of the esophagus where it meets up with the stomach. Its primary functions are to allow food to pass from the esophagus into the stomach, to allow air to escape from the stomach when burping, and to prevent stomach acid from washing back up into the esophagus.

A malfunction of the LES is one of the primary causes of GERD.

Pyloric Sphincter

The pyloric sphincter is located between the stomach and the duodenum, which is the first part of the small intestine. The pyloric sphincter opens to allow partially digested food (chyme) to pass from the stomach into the duodenum for further digestion and absorption of nutrients into the body.

Sphincter of Oddi

Sphincter of Oddi (SO) is located where the common bile duct and the pancreatic duct connect to the duodenum. The SO opens after we have eaten so as to allow bile from the gallbladder, and enzymes from the pancreas, to enter the duodenum so as to break down food components for absorption into the body. Sphincter of Oddi dysfunction (SOD), a relatively rare health disorder, which can cause episodes of pain in the chest area.

Ileocecal Sphincter

The ileocecal sphincter is located where the small intestine and the large instestine meet. There is not much known about this sphincter, other than that it is thought to expel chyme from the end of the small intestine, (the ileum) into the large intestine. The ileocecal sphincter may play a role in preventing bacteria from the large intestine from colonizing in the small intestine, a health condition known as small intestinal bacterial overgrowth.

Anal Sphincter

The anal sphincter is located at the end of the rectum, and therefore at the end of the digestive tract.

The anal sphinter regulates the process of the evacuation of stool. It has both an inner and outer component. The inner sphincter is under involuntary control (and therefore prevents stool from leaking out), while the outer sphincter is predominantly under voluntary control, thus allowing for a bowel movement. A malfunction of the anal sphincter can cause stool leakage, a health condition known as fecal incontinence.

Other Sphincters

Urethral sphincter: Also known as the sphincter urethrae, this sphincter controls the holding and emptying of urine. Like the anal sphincter, the urethral sphincter has both inner and outer muscles, which are respectively primarily under involuntary and voluntary control.

Iris sphincter: Also known as the pupillary sphincter or sphincter pupillae. This sphincter regulates the closing of the pupil in the eye.

Source:

Minocha, A. & Adamec, C. (2011) The Encyclopedia of the Digestive System and Digestive Disorders (2nd Ed.) New York:Facts on File.

"Upper Esophageal Sphincter" Motor Function of the Pharynx, Esophagus, and its Sphincters. Book Excerpt on PubMed.

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