In this video, we're going to examine the four basic layers or tunics of the elementary canal. The elementary canal tunics have specific subdivisions within these layers that emphasize one type of tissue over another based on the specific function of that region of the GI tract. So these layers are the mucosa, the sub mucosa, the muscularis externa, and the sarosa. And we're going to dive a little bit deeper into each of these four tunics. So let's talk about the mucosa layer. So the mucosa layer lines the lumen. Remember the lumen is the space. So here's that mucosa layer sitting right over here. this sort of light brownish color and sort of this uh slightly darker brownish color. When we look at the mucus mucosa, the layers within the mucosa will perform one of three or sometimes all of the following functions. It will help to secrete mucus uh to protect the elementary canal from its own digestive enzymes. It secretes digestive enzymes. it can secrete hormones. Um, it's also involved in the absorption of the end products of digestion. So, monomers uh from the macroolelecules that are being broken down. And lastly, it helps to protect against infectious disease. Sometimes embedded within the mucosa you have specialized uh lymphatic tissues uh called malts or mucosa associated lymphatic tissues where you have a higher cluster of white blood cells that allows you to sample the types of pathogens that you might be consuming in your food. So there are three basic sub layers within the mucosa. The epithelium. So as we're going from deeper to superficial, the epithelium, the laminaropria and the muscularis mucosa. So let's discuss the epithelium. So for the most part, as you progress down the GI tract, the epithelia will be simple columnular cells and mucus secretreting cells. So this is in the majority of the tract. The exceptions here are going to be the mouth, the esophagus, and the anus. Basically, these are the areas where there's potential for a lot of abrasion. So you tend to have stratified epithelia here where those squamus cells can sloth off very easily when they're araided against. So the simple columnular epithelia here primarily serves uh to secrete mucus and this mucus has a two-fold function. it protects the organs or the tissues within the organs from digestive enzymes and second it kind of creates a slippery surface so it eases the passage of food down the tract. Now they can secrete enzymes and hormones and we'll discuss this more so specifically in the videos focusing on the stomach and the small intestine. So again, just to highlight, if we're looking at the epithelium, the mucosal epithelium, it's this sort of lighter brown colored uh tissue here. I'm just going to highlight it in green. So it's this area here that we're talking about. That's where we have our columnular cells. So you can see if you look at other areas, the cells are sort of columnular in shape. And I had mentioned the malts. So here's the mucosa associated lympoid tissues that's going to be embedded within the laminina propria which coincidentally we're going to talk about next. So the lamina propria consists of loose areolar connective tissue. It has some capillaries uh for the purpose of nutrient absorption as well as to provide nutrients to the columnular epithelial cells. This is where you have these follicles of lymphatic tissue. Um we mentioned the mucosa associated lymphatic tissues or malt for short. allows you to sample the types of pathogens that you are consuming because obviously when we're consuming foods that food is not completely sterile. So it allows us to sort of preemptively mount an immune defense before these pathogens might increase in number. So the last component of the mucosa and I don't have a separate little slide uh for this one is the muscularis mucosa. So that's going to be responsible for the local movements that occur within the mucosa that consists of smooth muscle. Okay. So again in this particular image here we have our epithelium. We have this lighter brown color where the malts are and various glands. Um excuse me uh where you have the malts not the glands. Uh that's the lamina propria. And then in this redder shade here, that's the muscularis mucosa. All right, so that is our first tunic, the epithelial tunic. Let's move on to the second one, which is the sub mucosa. So the sub mucosa consists of areolar connective tissue. There's a lot uh a higher density of blood vessels and lymphatic vessels here. Um you do have clusters of neurons that serve locally the tissue. We call these uh nerve plexus. Um so specifically there's the plexus of misner. uh this is responsible for regulating certain secretions, hormone secretions as well as sort of detecting the presence of specific food items and then obviously then uh generating secretions or um yeah generating secretions. So in light of our image here, here's the sub mucosa. So you see this higher density of blood vessels right over here. Excuse me. So our third tunic then consists of the muscularis externa. So the muscularis externa is where you have the bulk of the smooth muscle. uh and it is this region that is going to be responsible for segmentation and paristtoalsis the motility that we discussed in a previous video. So there are two types of arrangements of the smooth muscle there on the inner track the smooth muscle is circular uh and then the outer tract they are longitudinal. Now in certain areas where the circular thickens these regions give rise to what we call sphincters. So you'll hear us discussing various sphincters like the cardiac sphincter that is at the interface of the esophagus and the stomach or the pyloric sphincter which is at the interface of the stomach and the dadum. Uh so within the muscularis uh externa uh you do have another nerve plexus. This is the mayenteric nerve plexus. So it's between the inner and the outer excuse me the inner circular and the outer longitudinal layers. Uh the mayenteric nerve plexus is also known as the plexus of ourbach. uh and it's going to be responsible for motility which already we've established this is where segmentation and paristtoalsis is going to happen uh as well as the rhythm and the force of contraction. So our final layer then is the sorosa. So the sorosa is going to be sort of a continuation of the visceral paritinium. Uh in most cases it's going to be an aerolar connective tissue uh that also has a misothelial or a single epithelial layer on top. uh in the thoracic cavity as opposed to the abdominal pelvic cavity where the esophagus is the aerial connective tissue is actually replaced by a more fibrous connective tissue uh that facilitates the anchorage of the esophagus uh to the dorsal wall and we also see this also in the retroparitinal organs. So those are the organs that are behind the paritum uh the paritinium excuse me. Um they will have both an adventicia as well as a sorosa. So the last thing we want to touch on and we've mentioned it uh numerous times is the nervous system within the gut. Uh it is very elaborate. It is very complicated. So, we're just going to give you a brief overview. Excuse me. So, just a reminder, a nerve plexus is basically just a bunch of nerves that serve a common area, the same area. Okay? It's not a brain. It's not a ganglen. It's just a cluster of nerves that serve one area. So the major nerve supplies to the GI tract help to control motility. So the two major nerve plexi that we have there are we've already mentioned the mayenteric nerve plexus that's going to be controlling our motility. Just remember my myo muscle. And then you have the sub mucosal nerve plexus. Uh recall the sub mucosa are where you might have and if we kind of just uh hop back here that's where you have uh blood lymph vessels uh and such. But in the sub mucosal nerve plexus we're going to be regulating glands as well as the smooth muscle in the mucosa. Uh so the glands uh to promote secretion and whatnot. So this is going to be linked to uh the central nervous system. So to the brain and the spinal cord and we're also going to be involving long ANS which stands for autonomic nervous system. So this is your involuntary nervous system. And the autonomic nervous system is going to be broken down into two major groups, the sympathetic, which is usually the sympathetic nervous system is usually involved in like the flight or flight response or when you're physically active. And then the parasympathetic nervous system. This is usually going to be involved when you're resting, studying, relaxing. Um, so we are involving both. Um, the effect that we have is how much uh we're not signaling in the sympathetic versus how much we're signaling in the parasympathetic. They're never completely shut off. So, it's kind of like driving with one foot on the gas and one foot on the brake. You can't take your foot off either, but whether you're stopped or you're moving forward depends on how much pressure you take off the brake and how much pressure you apply on the gas. Okay? So, the sympathetic impulses are going to inhibit digestive activities. Obviously, fight or flight. When you're running for your life, digestion is a very low priority um uh process. And then parasympathetic impulses are going to stimulate digestive activity. So the secretions of hormones and enzymes and the motility that occurs in the GI tract.