Sphincter of Oddi Function
Sphincter of Oddi Wiki
Ampulla of Vatar
The sphincter of Oddi, or otherwise called the sphincter of the hepato-pancreatic ampulla, is located at the second part of the duodenum. As the name implies, it houses the ampulla of Vater, which in turn surrounds the bile duct and pancreatic duct. The key in order to understand the function of the sphincter of Oddi is to discuss first how it looks like. The sphincter of Oddi is composed of smooth muscle both circular and longitudinal fibers. Three divisions of circular muscle were identified: one is the sphincter ampullae which surrounds the common channel formed by the union of the bile duct and pancreatic duct; next is the sphincter pancreaticus which covers the main pancreatic duct; and last is the sphincter choledochus which surrounds the common bile duct. Longitudinal muscle fibers or fasciculi longitudinales are found in between the bile and pancreatic ducts.
The Sphincter of Oddi as a Resistor
The sphincter of Oddi is a high resistance zone. Its tonic contraction creates a basal pressure higher compared to the pressure in the common bile duct and duodenum. These contractions are also rhythmic and wave-like in quality. This is to ensure that contents of the common bile duct and pancreatic duct do not enter the duodenum. Take note that the pancreas secretes digestive enzymes that may be destructive to the mucosa of the gut when uncontrolled by the sphincter of Oddi. This is where the mini-sphincters come into play. When the sphincter is not stimulated, it contracts.
The Sphincter of Oddi as a Pump
This major function of the sphincter of Oddi is under neuroendocrine control. The substance cholecystokinin relaxes the sphincter of Oddi along with the stimulation of pancreatic enzyme secretion and contraction of the gallbladder. It is produced by the mucosal cells that lines the upper small intestine once they detect presence of fats and amino acids. Cholecystokinin is most secreted 30 minutes after a meal when it reaches the duodenum. The longitudinal fibers of the sphincter of Oddi previously mentioned function as promoters of the flow of bile and digestive enzymes into the duodenum.
The Sphincter of Oddi Participates in the Migrating Motor Complex
The migrating motor complex, or the MMC for short, is smooth muscle contractions that are also termed myoelectrical motor complex. This is the response of the gastro-intestinal tract to stimulation by the hormone motilin. These contractions occur inside the intestinal lumen. It has three phases, the third phase of which migrates from the antrum of the stomach down to the ileum of the small intestine. The sphincter of Oddi also responds to motilin, which controls the motility of the digestive tract. The sphincter’s contractions increase in frequency and amplitude shortly before and during the intense contractions of the duodenum. These contractions occur in the sphincter of Oddi during the fasting state.
The Sphincter of Oddi as a Reflux Inhibitor
Another function of the sphincter of Oddi is to prevent entry of contents of the duodenum into the bile duct or pancreatic duct. It is always important to go back to the anatomy of the sphincter in order to understand how it is able to perform all these functions at the same time. Studies on the physiology of the sphincter of Oddi show that the interplay between the small component sphincters and the longitudinal muscle fibers. In between contractions, the sphincter of Oddi allows flow of bile and digestive enzymes into the duodenum. This mechanism, which is termed as trans-sphincteric flow, shows that the sphincter performs its resistor and pump functions simultaneously. Trans-sphinteric flow is diminished by the action of motilin, with which the sphincter of Oddi also responds.
Clinical Correlation of the Functions of the Sphincter of Oddi
Although the sphincter of Oddi is just one part of the extra-hepatic biliary tree, its functions are very vital to maintain order inside the human body. If the sphincter of Oddi fails to perform even one of its functions, it would have devastating effects to digestion and to the quality of life of the person. Because the sphincter of Oddi relaxes after a meal when it is highly stimulated by cholecystokinin, pain from its dysfunction is mostly felt after eating and is called postprandial pain. Because of its function as a pump for bile and pancreatic secretions, its failure to do so frequently causes pancreatitis because the digestive enzymes of the pancreas are not secreted and are instead directed on the organ causing inflammation.
During surgical removal of the gallbladder or even in gallstone removal, iatrogenic trauma may be caused to the sphincter of Oddi. This would cause inflammation to the sphincter which in its process of repair would eventually cause narrowing. Other effects are also paradoxical response of the sphincter of Oddi to cholecystokinin resulting to dyskinesia or non-relaxation.