The Digestive System part 1: The Mouth, Oesophagus and Stomach

And now we're onto some more notes on the digestive system! This will be in 3 parts, the first being on the mouth, oesophagus and stomach, the second on the pancreas, liver and biliary tree (whatever that is) and the third on the small intestine, colon and rectum.

1. Detail the safeguards that prevent food from entering the airway during swallowing.

There are several safeguards that prevent food from entering the airway. The first involves the uvula (the thing that dangles down at the back of the throat) and the soft palate (the roof of the mouth). When the uvula detects food, the soft palate rises to stop food from going into the nose. The second safeguard involves the epiglottis and the larynx. During swallowing, the larynx moves up, causing the epiglottis to block the opening of the trachea so that food cannot enter.

2. How is gastrin different from the rest of the gastric secretions listed in the lecture (salivary amylase, salivary lipase, gastric lipase etc.)?

Salivary amylase, salivary lipase, gastric lipase and so on are all enzymes, as denoted by the -ase suffix at the end of their names. Enzymes are proteins that catalyse certain chemical reactions in the body- in this case, enzymes help molecules to break down into smaller components. The digestive enzymes are released directly into the lumen (cavity) of the mouth, stomach, intestines etc., where they do their job.

Gastrin, on the other hand, is a hormone. Hormones are distributed through the endocrine system- that is, they travel through the blood to get to their target cells rather than being released directly into the lumen of organs. Rather than assisting in breaking down molecules, the function of gastrin is to stimulate parietal cells in the fundus of the stomach to release hydrochloric acid. (The fundus is an area of the stomach located around the top of the stomach.)

3. Describe the cephalic phase of gastric secretion. What is the mediator in this process?

The cephalic (neural) phase of gastric secretion is mediated by the vagus nerve (cranial nerve X). It is a parasympathetic nerve, meaning that it usually has inhibitory effects on organs- except for those in the digestive tract, on which it has a stimulatory effect. During the cephalic phase, the vagus nerve stimulates muscular contraction and secretion of hydrochloric acid, pepsinogen and mucus. No food has to be in the stomach for this to occur- only thinking about food is required (which is probably why you can feel hungry when thinking about food).

4. Describe the intestinal phase of gastric secretion. Contrast it to the other two phases.

This is the area that I'm a bit hazy on. From what I understand the intestinal phase runs pretty much at the same time as the other two phases (in fact reading over my notes I see that there is a lot of overlap between the three phases). The intestinal phase mainly inhibits the action of the other two phases- while the cephalic and gastric phases are both stimulating contraction and trying to push food out of the stomach (particularly in the gastric phase), the intestinal phase tries to slow these processes down in order to give the intestines more time to prepare for food. Also, unlike the first two phases, which are mediated somewhat by the vagus nerve (the cephalic phase more directly and the gastric phase more indirectly, as the latter is initiated by stretching of the stomach which is mediated by the vagus nerve), the intestinal phase appears to be mediated more through the hormones that are released by the entry of chyme into the duodenum. These hormones include GIP (gastric inhibitory hormone), which inhibits gastric secretion and motility, secretin, which inhibits gastric secretion and CCK (cholecystokinin), which inhibits gastric emptying.