Welcome. This is our last lecture on the GI tract. Today, we want to talk about motility and how we move materials through the GI tract. So if you recall, we've been talking about the, the gastrointestinal tract or GI tract as a processing plant where we start in the mouth and we digest the materials in the next compartment, which is in the stomach and in the small intestines. And then, move materials into the lower portion of the intestine where we're absorbing most of our fluids and our nutrients. And then, move into the, to the colon where the large intestinal region, where we are absorbing a lot of the fluids. And so, this, this is a unidirectional movement through the tract. And what's important is we have a timed event. So that there are sufficient amounts of time for digestion to occur, and for reabsorb, for absorption to occur before the materials then exit from the body as the feces, fecal material. A motility then is going to govern this time, this timing as we're moving along the, the tube. So let's consider the first, the general anatomy of the tube. As you recall from our first lecture that in, of this particular series that we have, on the outer most aspect of the gastrointestinal tract or tube, we have two layers of smooth muscle and that's what's shown here. The smooth muscle is said to be in the muscularis externa. The muscularis externa has these two layers the inner most layer is circumferential around the lumen of the tube and the outermost layer is longitudinally-oriented along the long axis of the tube. When we contract this, the circular muscle, the circular layer or the inner layer, if that contacts, it will make the lumen of the tubes smaller, and if it relaxes, then the lumen of the tube increases. In contrast, when we contract, when we contract the outermost layer, as we as we contract the outermost layer, the tube will shorten, and it will shorten in regions or segments along the tube. The from the time that we enter into the esophagus to the time that we leave the tube, most of the, of this muscle is going to be smooth muscle. It starts in the in the lower 1 3rd of the esophagus and moves all the way through to to the end of the large intestine. This is all smooth muscle, and the smooth muscle is the signal, signal unit type smooth muscle, where all of the cells are going to be connected to one another through gap junctions, so they're electrically coupled cells. [COUGH] Excuse me. The activity of the two layers of muscle have to be coordinated and this coordination is done by a local nervous system which is called the enteric system enteric nervous system, and that is what shown here. So, the enteric nervous system is located between the two muscle layers. The enteric nervous system effectively governs the mortality along the tract independent of the central nervous system. The central nervous system can modulate this activity through other branches of the autonomic nervous system and that is the parasympathetic and the sympathetic nervous system. Now, the, there's another factor that we have to consider when we talking about motility of the smooth muscle through this tract. And that is, is that there is located within the tract, within the, within the fundic region of the stomach, within the small intestine and within the large intestine specialized smooth muscle cells which are called pacemakers. And like all pacemakers, they have an unstable resting membrane potential and that's what's shown here. So in our top image, we have the cells are slowly depolarizing. And then they repolarize, and then again, slowly repolarize. As the cells are slowly repolarizing, depolarizing and repolarizing, they're generating what is called the electrical slow-waves. These electrical slow-waves then bring the resting membrane potential of the smooth muscle cells of these pacemaker cells close to threshold, and at that point, we can initiate an action potential. And if the action potential is initiated, it means we've reached the voltage at which the voltage gated calcium channels open. The voltage gated calcium channels open, calcium enters the cells, and action potential is generated. To repolarize the cells, we then close the voltage-gated calcium channels and open a voltage-gated potassium channel, and that then repolarizes the cells. It, within this, the intestines, that is, within the small intestine and the large intestine, every every one of these slow-waves, electrical slow-waves is not associated with an action potential. Only those that reach threshold and will generate will generate the action potential. But those that generate the action potential will be followed by a contraction. So, wherever we have an action potential generated, there would be a contraction of the smooth muscle. Within the stomach, this is not true. So within the stomach, as we have these rhythmic slow-waves, the rhythmic slow-waves actually can give rise to a contraction. So within the stomach, you don't have to generate an action potential, but the threshold of the slow-wave is associated with, with contraction. There's one other thing about these slow-waves, and that is that the slow-wave determines the frequency of the action potential, and therefore, the frequencies of the contraction, but then the slow-waves can be modulated. That is the pacemaker cells. Can, their timing can be modulated by the sympathetic system and by the parasympathetic, parasympathetic system. The sympathetic nervous system will cause a hyperpolarization of these cells and move this and move this resting membrane potential of the pacemaker cell further from threshold. This simply means that it takes a longer time for the cells to be able to reach threshold, and eventually, to fire off an action potential. So the sympathetic nervous system then delays the, the ability of the cell to reach, to reach a threshold and to generate an action potential, and therefore, delays contraction. The converse is true of the parasympathetic system. The parasympathetic system moves the resting membrane potential towards threshold, so it's fast, the cells reach threshold at a faster pace. And by reaching threshold at a faster pace, they can then generate an action potential and, and contraction, so we can speed up contractions by having input from the parasympathetic system. Now, what kinds of motility are we actually talking about? So, in the fed, in the fed state, you have two types of motility. The motility that we're going to talk about first is called segmentation or this is a mixing type of motility. In segmentation, we have a neighboring two neighboring regions of the, of the tract, where the first is relaxed, and we have a food bolus within that region. And the second is contracted and has no food within the region. The first will contract and the second relaxes, and that moves the food bolus, then to the second contract, second chamber. And then, as the, as the second chamber contracts, the first then open, relaxes again, and so we have this sloshing of material back and forth. This kind of movement or segmentation or mixing movement is very critical for mixing the materials the food substances with the enzymes and with the buffers, and all of the secretions that we have added to the lumens of the tract. In addition to that, it's also mixing the material with the surfaces of the epithelial cells that are lining this track and, and enhancing the absorption of the nutrients that we, that we have generated. So, the smooth, the small amino acids, the small sugars, and so forth are now being delivered to the surfaces of these cells, and they can then be absorbed by these cells. The sloshing back and forth between segments or segmentation, is using the inner the inner muscle mass of this, of the tract. So it's the inner muscle mass that constricts, closing the first, the lumen of the first chamber, and then its the inner muscle mass that relaxes and allows the second chamber to recieve the food bolus. And so the sloshing back and forth then is due to the inner muscle masses changes in inner muscle masses. The second type of movement is called peristalsis, and that's where we're moving from one region of the, of the tube to the second region of the tube. And then, to a third region of the tube, and so forth. In order to do this, again, we're going to have a contraction and a relaxation event occurring. But we're, now, we're going to contract the outermost muscle mass. The outermost muscle mass shortens, and the innermost muscle mass has to relax, so that, as we shorten the tube, the tube will get fatter. This will occur and in segments as we're going as we precede from from the beginning towards the anus. So from the mouth towards the anus, we will have peristalsis or this segments segmental movement of the food bolus. It's a progressive movement as we go along the entire, of the entire tract. And again, the waves of, of motility for both the segments segmentation and peristalsis is going to be set not only by the interact, activity of the enteric nervous system, but also by the pacemakers. And so, it's the combination of those two signals which will generate our timing for the movement of the material as we move along the tract. Now, in the fasting state, we also have a type of peristalsis, but it's different in timing from that which we see when food, is present within, within, the lumen of the gastrointestinal tube. This particular type of movement is propulsive movement. It's going to be moving materials from one region that is from the stomach towards the anus. and it's called the migratory motor complex. Migratory motor complex is sort of a good housekeeping type of movement. And in this case, what it's doing is, is sweeping clean the entire tract during the fasting period. So, it's sweeping clean any food food debris or nutrients that may have been left behind. This is a very important type of activity, because if these materials are left within the tract, you can have bacterial growth or overgrowth within the tract, and that can lead to painful, to a painful situation. It could lead to diarrhea, it could lead to nausea, it could lead to vomiting. These migratory motor complexes are initiated, not by our pacers, pacemaker cells, but by the hormone motilin. This hormone is secreted in the absence of food within the lumen of the stomach. It's also secreted by the duodenum and by the intestine the jejunum of the small intestines in the absence of food. Motilin then will initiate the contract, the contractal events, both within the stomach, as well as within the intestines. The migratory motor complex usually starts in the stomach, but it can initiate in the duodenum and in the and or in the jejunum. And the frequency of contractions, as I said, is going to be different from this frequency of contractions that we saw with the peristalsis that, that was present when we had food present within the lumen. Now, there are regions which are, are isolated from one another and, as you know, we isolate the stomach, which has a very caustic acidic acid hydrochloric acid in it from the duodenum and from the esophogus, because these latter two regions are not ca-, are not protected to the caustic actions of the acid. These regions that are isolated by sphincters. These sphincters are not under voluntary control. These are sphincters which are under involuntary control. And it's due to the tonic inhibition of the smooth muscle. And is, again, the the inner circular layer of the smooth muscle, the muscularis externis. So when we contracted that smooth muscle layer, we can include the lumen of the tube. We open it at very small amounts between the the pylorus region of the stomach, and the duodenum, to allow very small amounts of our acidic chyme to enter into the duodenum. So we have a regulated opening of that region. And so, that means that we have to, for relaxation of the sphincter, we have to release the inhibition that the sphincter is contracted under an inhibitory activity. We release that inhibition and allow the sphincter to relax, so that this, so that the there's is a small lumen through which the acidic chyme can exit. This is a regulated relaxation. And it's, by turning off this tonic inhibition that we're then able to then open the sphincter. Very small amounts are ejected from the stomach into the duodenum. And this is really important, because if we have a very large amount of acidic chyme or, or, or for instance, a hype a hypotonic solution coming from the stomach into the duodenum. You can then get water moving into the duodenum, from osmotic action, and what that will do is stretch the duodenum, and that's going to be very painful. So this is a regulated exiting of material at this point. We also have regulated openings of these sphincters. There's a second sphincter, not only at the pipe, pipe, there's a sphincter between the esophagus and the stomach, and between the stomach and the duodenum, but there's also one between the ileum and the colon. and these, these regulated openings then are allowing the receiving section region of the tube to be able to handle the material that's being that's being delivered. We have regulated openings in respond to the migrating motor complex as well. And that is between meals, so that when we're sweeping clean the these, the intestine of any debris or any nutrients, then, at that time, we will open periodically the sphincters and to allow the materials to pass, and then the sphincters will close again. There is no time in which the sphincters are all patently open along, along the tract. So what about mass movements? Now, mass movements is where we have waves of contractions that are occurring within this, the large intestine. And here, we're now propelling the the fecal material through the large intestine toward the, toward the rectum and the anus. The, this waves of contraction are not two peristaltic waves in the sense they are not using those outer the outer layer of smooth muscle that we were talking about in the muscularis externis. But instead, it's using the inner layer of the circumferentially-oriented muscle layer of the muscularis externis. So we have contraction, then we're essentially contracting a part of the chamber. We've contracted, and then, we relax to receive it. And again, it's sort of almost like the segmentation type of movement, but it's propulsive. This is, has a special name. It's called mass movements. It occurs about 1 to 3 times per day under normal circumstances. and that it's then moving the materials to the anal canal. The anal canal has an, has an involuntary sphincter. And we have an external, an external sphincter at the anus, the which is voluntary. The mass movement, you are all of, are familiar with is, is that you have the sensation that you need to go to the bathroom, but but it may not be appropriate. And so you don't nothing happens and you're able to retain the fecal material until you can get to the bathroom, and then eliminate it, and you're eliminating it under voluntary control. The elimination of the fecal material from the body is called defecation, and defecation there's an A in there. It's simply expelling the fecal material from the body. And it occurs with relaxation of the voluntary anal sphincter. That's the external sphincter and then, also an increase in the intra-abdominal pressure to push the material out from the body. Alright, so one thing that we haven't talked about is vomiting. We talked about diarrhea in the last lecture. And here, I wanted to just to spend some time talking about vomiting. I'm pretty sure everybody has experienced vomiting at some point on time. Vomiting is a protective function where if you're bringing in some type of toxic material into the body. Then the body expels the, the material from the stomach, the stomach content being expelled back out through the mouth. This can also be triggered by either delayed empyting of the stomach or by rapid emptying of the stomach, where in the delayed emptying of the stomach we have material that sort of stays in the stomach, let's say for instance from a very fatty meal. And it's retained and retained and retained for several hours, and eventually the body just expels through the mouth. And this, the other one is where we have this very rapid dump emptying of the, of the stomach content into the duodenal and again as I said, then you can have a, you can have a very rapid swelling. Of the duodenal region due to the movement of water coming into this region to titrate the hypertonic material that is coming from the stomach. This actually has a name and that's called the Dumping Syndrome of the stomach. The vomiting of course, is the expulsion of the chyme from your, from the stomach, but it can be that you can are eliminating material from the beginning of the duodenal. And, and under those conditions, then the vomit it will have a color sort of a greenish color and that's due to the bowel which has been which has been delivered from the gallbladder to, to the duodenal. Vomiting is controlled by the vomiting center, and it's, that center is located in the medulla at the brain stem. The and it is actually controlling the respiratory muscles and the abdominal muscles. It is not causing a reversed peristalsis of the smooth muscles of the muscularis externus, but instead you are changing the pressure within the inter, the inter-thoracic cavity. And then help to to move the, the material, then out from the stomach and out, out of the mouth. Alright, so what's our general concepts? So, the first of the general concepts is that we have movement which involves coordinated activities of the two outer muscle masses of the wall of the gastrointestinal tract. And this move, the second general concept is that movement and activities are coordinated by this local nervous system which is called the enteric nervous system. And that this, this can be modulated from input from both the sympathetic nervous system and the parasympathetic nervous system. The third is that there's pacemakers cells which generate a spontaneous electrical activity and these are called electrical slow waves. And that these can lead to an action potential which fires at the peak of the depolarization, as they're coming up to threshold. And at that point, if you're an action potential contraction can follow. And this is true throughout the intestine but in the stomach itself. That the, that the slow waves, the electrical slow waves can can be associated with contractions directly. The fourth concept is that in the fed state we have two types of motility. We have segmentation or mixing. and this is allowing the material to slosh back and forth in order to have optimal absorption of the materials. And then we also have peristalsis. And peristalsis is the propulsive movement of the, of the food bolus along the tract. In the fasted state, the migratory, the migrating motor complex sweeps the tract clean and this is the propulsive type of movement along the tract. It has very different timing from the peristalsis movement that you, that is associated with feeding. The fifth general concept is that we have tonic contractions can occur at the sphincters and this closes the sphincters. And in order to have material moving from one segment of the tract to a second, we have to have a relaxation of the sphincter, a slight opening of the sphincter to allow the material to to, to exit from ss-, segment one to segment two. The tonic contractions of the, of the sphincters are going to occur when there's no motility within the tract. And so the, they will open when there's food within the tract at designated times. and they will also open during the migratory the migratory motor complex, as we're sweeping the debris the extra nutrient from the tract. But that they will be closed at all the other times, and in particular when there is no motility within the GI tract. So this, then, ends our discussion of the GI tract. And the next time we meet, we're going to talk about the urinary system. So see you then. Bye-bye.
