Spasm Sphincter a circular muscle that constricts a passage or closes a natural orifice. When relaxed, a sphincter allows materials to pass through the opening. When contracted, it closes the opening.
There are four main sphincter muscles along the alimentary canal that aid in digestion: The cardiac sphincter, between the esophagus and the stomach, opens at the approach of food, which is then swept into the stomach by rhythmic peristaltic waves.
The pyloric sphincter controls the opening from the stomach into the duodenum. It is usually closed, opening only for a moment when a peristaltic wave passes over it. Two anal sphincters, internal and external, control the anus, allowing the evacuation of feces.
In addition, there are sphincters in the iris of the eye, the bile duct (sphincter of Oddi), the urinary tract, and elsewhere in the body. .
