Gastrointestinal Function part 2

Second last post for PHYL2001!

Understand the structure and function of the gastric mucosal layer.

I've talked about the function before- the mucous protects the stomach against HCl. It does this because it has bicarbonate ions in it, which neutralise HCl. Additionally, the gel-like consistency of the mucous prevents rapid diffusion of H⁺ down its concentration gradient from the lumen to the stomach wall (and prevents bicarbonate from rapidly moving in the opposite direction).

Be familiar with the basic anatomy of the exocrine pancreas.
Know the substances secreted by the pancreas and how they play a role in acid neutralisation and the digestion process.

The exocrine pancreas is the part of the pancreas that secretes digestive hormones. (It doesn't secrete insulin or glucagon- that's the endocrine pancreas' job). It is made up of duct cells, which secrete bicarbonate ions to neutralise acidic chyme coming out of the stomach, and acinar cells, which secrete the digestive enzymes. These enzymes include proteases, lipases, amylases and nucleic acid enzymes, so that pretty much everything gets broken up.

Understand the basic anatomy of the bilary system.

Bile is produced in the liver, and then travels down the hepatic ducts (left and right hepatic ducts which join to form a common hepatic duct) and then down the cystic duct to the gallbladder. The gallbladder stores the bile until it needs to be used. When stimulated by cholecystokinin (CCK), a hormone you're going to hear a lot about, the gallbladder contracts, squeezing bile back out of the cystic duct and down the common bile duct. The common bile duct meets up with the pancreatic duct, and together they leave through the major duodenal papilla into the duodenum. Guarding this opening is the Sphincter of Oddi, which helps to control the release of biliary secretions.

Know the control pathways responsible for pancreatic enzyme secretion (cholecystokinin/CCK via the vagus afferents).
Understand the role of CCK RF and trypsin in the control of CCK release and the role of CCK on the gall bladder and the sphincter of Oddi.

As I just mentioned, CCK is important in gallbladder contraction. CCK also causes the sphincter of Oddi to relax, and inhibits gastric secretion and emptying. Its major role, however, is to stimulate the acinar cells to secrete pancreatic enzymes.

CCK release is a bit unusual. CCK RF (CCK releasing factor) and trypsin are released continuously. Between meals, trypsin eats up the CCK RF and so it doesn't reach its target. During the digestion of food, however, trypsin is busy eating up food rather than the CCK RF. This allows CCK RF to bind to target cells in the duodenum, triggering release of CCK.

CCK does not actually stimulate the pancreas directly- instead, it stimulates vagal afferent nerves, which go up to the brain and stimulate vagal efferent nerves. It is these nerves that go down to the pancreas and stimulate secretion of enzymes.

Know the control pathways responsible for pancreatic bicarbonate secretion (secretin directly & via vagus nerve) 

Secretin is a bit more simple than CCK in terms of its secretion. It is released from duodenal cells called enterocytes in response to acid (remember, the food coming out of the stomach is acidic as it is mixed with stomach acid). Secretin acts in two ways: most of it travels through the blood to affect the pancreas, while the rest of it stimulates vagal nerves like CCK does. The end result is that the duct cells of the pancreas produce bicarbonate in order to neutralise the stomach acid.

One more to go!