Sphincter of Oddi

Ampulla of Vatar

Sphincter of Oddi Dysfunction

The Anatomy of the Sphincter of Oddi
In the discussion of the accessory organs of the gastro intestinal tract, a lot of organs should be included along with their anatomic relations and the relations of their ducts. One example of which is the sphincter of Oddi. Labelling them as “accessory” organs is actually an understatement as they cause a lot of trouble if one of them is dysfunctional. However, during digestion and absorption of food, their roles are minor compared to the small and large intestines and other major components of the alimentary tract.
What is the sphincter of Oddi?
The sphincter of Oddi is part of the extra hepatic biliary tree. It is otherwise termed as the sphincter of the hepato pancreatic ampulla.  It is composed of circular and longitudinal layers of smooth muscle that surrounds the ampulla of Vater or the hepato pancreatic ampulla. The ampulla of Vater surrounds the distal part of the bile duct from the gallbladder (thus hepato) and the pancreatic duct from the pancreas. The sphincter of Oddi is actually a high pressure zone that measures up to 6 to 10 mm.
What are the parts of the sphincter of Oddi?
There are three main components to the sphincter of Oddi: First, the sphincter ampullae (ampullary sphincter) which is a small segment that surrounds the common duct from the union of the bile duct and pancreatic duct, although it is not always present in all humans. The second component is another small segment, the sphincter pancreaticus (pancreatic sphincter) that from its name covers the beginning of the primary pancreatic duct. The largest component of the sphincter of Oddi is the sphincter choledochus (bile duct sphincter) that surrounds the distal bile duct.
The sphincter of Oddi is also composed of the fasciculi longitudinales or the longitudinal fascicles which are the longitudinal smooth muscle component. They are bundles of muscle that are found in the intervals between the bile and pancreatic ducts.
What are the anatomic relations of the sphincter of Oddi?
The sphincter of Oddi is found in the second part of the duodenum, the part of the small intestine immediately after the stomach. Visualized through endoscopy, the sphincter of Oddi can be located by first identifying the major and minor duodenal papilla. The major duodenal papilla extends approximately 1 cm into the lumen of the duodenum. The diameter of its opening is approximately 1 mm.  The major duodenal papilla is where the ampulla of Vater or the hepato pancreatic ampulla opens. The minor duodenal papilla is located 20 to 30 mm proximal and medial to the major duodenal papilla with a tiny opening that is very difficult to identify.
As mentioned, not all humans would have their bile duct and pancreatic duct unite distally. The most common would be the union of the ducts at the level of the ampulla of Vater or the sphincter of Oddi. However, separate ducts may exist but still enter the duodenum at the same site.

What are the other important characteristics of the sphincter of Oddi?
Grossly, the muscles of the sphincter of Oddi are embedded in the second part of the duodenum. Although it is also composed of circular and longitudinal muscles histologically, it is functionally separate from the duodenum.
The sphincter of Oddi is innervated by the coelomic ganglion according to a study conducted by Wyatt, AP in 1966. Yet the same exact involvement of the ganglion named cannot be found in current textbooks. What is currently written is that the innervation of the sphincter of Oddi is under neuro endocrine control which involves both the sympathetic and parasympathetic autonomic nervous system and most importantly the enteric nervous system. The mechanism behind the reflex activity of the sphincter of Oddi is further discussed in its functions.
What is the eponym of the sphincter of Oddi?
The sphincter of Oddi is named after Rugero Oddi, a surgeon and physiologist from Perugia, Italy. The sphincter was already described by Glisson in the 17th century, 200 years before Oddi found it. He “discovered” the sphincter of the ampulla of Vater, which is later named sphincter of Oddi after him, when he was a medicine student in his fourth year at the Department of Physiology of the Faculty of Medicine and Surgery (Facoltà di Medicina e Chirurgia) where he studied in Perugia.
What are the functions of the sphincter of Oddi?
It is worth emphasizing that the sphincter of Oddi is a high pressure zone. Its primary function in the human body is as a resistor that limits bile flow during fasting periods. In layman’s terms, it is closed when there is none to digest by tonic contraction.
Another function of the sphincter of Oddi is to pump bile into the duodenum from the bile duct through rhythmical contractions making it a housekeeper of the distal bile duct. It also participates in the contractions of the duodenum, giving it a part in the migrating motor complex during digestion. During these duodenal contractions the contractions of the sphincter also increase in frequency and amplitude thereby preventing reflux of duodenal contents into the bile duct and pancreatic duct.
What are the physiologic mechanisms behind the functions of the sphincter of Oddi?
The sphincter of Oddi plays a major role in biliary secretion. All neurotransmitters involved in the enteric nervous system are also involved in the neuro endocrine control of the sphincter of Oddi. One identified hormone that plays a major role in its function is Cholecystokinin or CCK for short.
As previously mentioned, the sphincter of Oddi is tonically closed when there is no intake of food. As soon as the upper gastro intestinal tract begins digestion and especially when fat and amino acids reach the duodenum 30 minutes after a meal, cholecystokinin enters the blood stream from the duodenal mucosa. This would then stimulate the gallbladder to empty its contents, bile, through rhythmical contractions of its wall. The same hormone would also stimulate increased secretions of digestive enzymes from the pancreas. However, effective emptying would not be completed without CCK decreasing the resistance of the sphincter of Oddi and simultaneously releasing bile and digestive enzymes into the duodenum.