The mouth bone is connected to the oesophagus bone, the oesophagus bone is connected to the- wait, these aren't bones. But you should know the general idea: mouth, oesophagus, stomach, duodenum, jejunum, ileum, large intestine, rectum, anus, outside world. For more information you can look at my ANHB2212 posts:
Be familiar with the overall nervous innervation of the GIT
There are two main nervous systems innervating the gut. Extrinsic control of the GIT is achieved via the parasympathetic and sympathetic nervous systems, whereas intrinsic control is achieved via the enteric nervous system which is basically like the gut's own personal nervous system. The enteric nervous system is made up of pacemaker regions, as well as a few plexuses of nerves. These communicate with the effector systems (muscles, exocine/endocrine cells etc.) and the sensory systems (chemoreceptors etc.).
Know the different phases of the digestion process
There are four main phases of the digestive process:
- Interdigestive- between meals
- Cephalic- seeing, smelling and tasting food- gets your stomach rumbling before the food is even in your belly!
- Gastric phase- digesting of food in the stomach
- Intestinal phase- digesting of food in the intestines
The mouth carries out both mechanical and chemical digestion. The teeth grind up food, breaking it down into smaller bits (mechanical digestion). At the same time, salivary glands secrete enzymes such as amylase, which plays some roles in chemical digestion. They also secrete mucus and lysozymes. Secretion of saliva is mainly dictated by the parasympathetic nerves. Sympathetic nerves play a small role- they can modify the saliva composition. (When we're running away from tigers, we probably don't need enzymes that can help digest our food, but we do need to stop our mouths from drying out.)
Have a knowledge of stomach structure and the substances secreted by this organ
The stomach has three main regions: the fundus, body and antrum. Each end of the stomach also has a sphincter- the top one, the cardiac sphincter (named for being close to the heart, not because it is part of the heart or anything), stops food from coming back up, and the pyloric sphincter at the bottom stops the duodenum from becoming overwhelmed with lots of food at once. The fundus is at the top of the stomach and normally contains gas. The body is the main bit of the stomach, and contains cells that secrete a bunch of useful substances. Finally, the antrum is at the bottom which is quite muscular to allow mixing to take place.
As I just mentioned, the body cells secrete lots of useful stuff. These include mucus, HCl, intrinsic factor (which binds B12, aiding in its absorption) and pepsinogen (the precursor for the enzyme pepsin, which breaks proteins down- see my post on proteolytic enzymes for more detail). Essentially, HCl helps to denature and break down proteins, pepsinogen breaks down stuff once activated to pepsin by HCl, intrinsic factor helps absorb vitamin B12 and mucus protects the stomach lining from the harmful effects of the HCl.
Understand the control of acid and pepsinogen secretion during the cephalic, gastric and intestinal phases of digestion.
I'm just going to start out with the diagram that I wish our lecturer started with, because I think it would have made it much easier to follow along in the lecture.
Firstly, I'll talk about the cells themselves and how they work. The body of the stomach has glands containing mucous cells, parietal cells and chief cells. Other important cells are G-cells and ECL (enterochromaffin-like) cells.
Mucous cells aren't on the diagram above, but basically they come in two flavours: mucous surface cells (MSC) and mucous neck cells (MNC), depending on their location. They secrete mucus, hence the name.
Parietal cells secrete HCl and intrinsic factor. They have internal folds called intracellular cannaliculi, which increase the surface area. They also have shitloads of mitochondria because they need lots of energy to pump out those hydrogen ions. And yes, hydrogen ions are simply directly pumped out by a H+/K+ transporter (a.k.a. "proton pump") in the cannalicular membrane. From the diagram above, you can see that they are stimulated by histamine, gastrin and ACh. Of these, histamine has the biggest effect.
There's not much to say about chief cells, other than that they secrete pepsinogen, which gets converted to pepsin when it meets up with HCl. Synergy ftw!
G-cells are gastrin-secreting cells located in the antrum. From the diagram you can see that they are activated by the parasympathetic nervous system. They also have microvilli covered in chemoreceptors that sense peptides and amino acids. This also increases gastrin secretion. Also, from the diagram I'm looking at right now, the cells are triangle-shaped and look kinda like Illuminati logos. Hate to break it to y'all, but this must totally mean that we're all owned by the Illuminati and our digestion is under their control.
I won't talk much about ECL cells either, except that they produce histamine, which as I mentioned above, is pretty much the main activator of parietal cells.
Now let's see how this works during two of the phases of digestion!
The cephalic phase mainly involves nerves, as it's basically the phase that makes you get hungry from simply seeing food. This activates the vagus nerve of the parasympathetic nervous system, which from the diagram above, causes parietal cells to secrete hydrochloric acid (HCl) and intrinsic factor, and chief cells to secrete pepsinogen. The parasympathetic nervous system also acts on histamine-secreting ECL (enterochromaffin-like) cells and gastrin-secreting G-cells to intensify these effects. Overall, the cephalic phase is responsible for 30% of gastric secretions.
The gastric phase is the filling of the stomach. This distends the stomach, activating stretch receptors, which activate afferent vagal nerves travelling to the brain. This causes efferent nerves to intensify the effect of the parasympathetic nervous system, causing more acid and so forth to be released.
The intestinal phase is basically what happens when the food is in the intestines. When food begins to enter the duodenum (the C-shaped curve at the very top of the intestine), the food causes the last 10% of gastric acid to be released. However, any more food begins to inhibit gastric acid secretion in order to protect the duodenum from becoming overloaded. This is due to a whole bunch of nervous reflexes between the duodenum and antrum, thanks to plexuses in the duodenal and antral walls and receptors in the duodenal mucosa that are triggered by distension of the duodenum.
The rest of the intestinal phase of digestion deals with hormones that cause the pancreas to secrete enzymes (to digest the food) and bicarbonate (to neutralise the acidic chyme from the stomach), as well as carry out other functions related to digestion. That'll be covered in my next post!
Part one done! Only two more lectures to cover! I see the light!
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