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 gastrointestinal tract or GI tract as a processing plant, where we start in the mouth and we digest the materials and the next compartment, which is in the stomach and in the small intestine. 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 colon or the large intestinal region, where we are absorbing a lot of the fluids. And so this is a unidirectional movement through the tract. And what's important is that we have a timed event so that there's sufficient amounts of time for digestion to occur and for absorption to occur before the materials then exit from the body as the fecal material. And motility then is going to govern this timing as we're moving along the tube. So let's consider the general anatomy of the tube. As you recall from our first lecture that of this particular series that we have on the outermost aspect of the gastrointestinal tract or the tube. We have two layers of smooth muscle, and that's what's shown here. This smooth muscle is said to be in the muscularis externa. The muscularis externa has these two layers. The innermost 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 circular muscle, the circular layer or the inner layer, if that contracts, it will make the lumen of the tube smaller, and if it relaxes, then the lumen of the tube increases. In contrast, when we contract the outermost layer, as we contract the outermost layer, the tube will shorten. And it will shorten in regions or segments along the tube. From the time that we enter into the esophagus to the time that we leave the tube, most of this muscle is going to be smooth muscle. It starts in the lower one-third of the esophagus and moves all the way through to the end of the large intestine. This is all smooth muscle. And the smooth muscle is the single 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 nervous system, and that's what's shown here. The enteric nervous system is located between the two muscle layers. The enteric nervous system effectively governs motility 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 there's another factor that we have to consider when we're talking about motility of the smooth muscle through this tract. And that is that there is located within the tract, 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 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 that we've reached the voltage at which the voltage-gated calcium channels can open. The voltage-gated calcium channels open. Calcium enters the cells, and then 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. Within the intestines, that is, within the small intestine and the large intestine, every one of these slow waves, electrical slow waves, is not associated with an action potential, only those that reached threshold 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 will 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 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 the slow waves can be modulated, that is, the pacemaker cells, their timing can be modulated by the sympathetic system and by the parasympathetic system. The sympathetic nervous system will cause a hyperpolarization of these cells 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 ability of the cell to reach a threshold and to generate an action potential and, therefore, delays contraction. The converse is to the parasympathetic system. The parasympathetic system moves the resting membrane potential towards threshold so the cells reach threshold at a faster pace. And by reaching threshold at faster pace, they can then generate an action potential and contractions. So we can speed up contractions by having input from the parasympathetic system. And what kinds of motility are we actually talking about? So 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 two neighboring regions 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 chamber. And then, as the second chamber contracts, the first then 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 or the food substances. With the enzymes and with the buffers and all of these secretions that we have added to the lumens of the track. In addition to that, it's also mixing the material with the surfaces of the epithelial cells that are lining this track and enhancing the absorption of nutrients that we have generated. So the smooth, the small amino acids, the small sugars and so forth. They're 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 muscle mass of the track. So, it's the inner muscle mass that constricts closing the lumen of the first chamber. And then, it's the inner muscle mass that relaxes, and allows the second chamber to receive the food bolus. And so the sloshing back and forth then is due to the inner muscle masses, changes in the inner muscle masses. The second type of movement is called peristalsis, and that's where we're moving from one region 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 for now we're going to contract the outer most muscle mass, the outer most muscle mass shortens and the inner most muscle mass has to relax, so that as we shorten the tube the tube will get fatter. This will occur in segments as we proceed from the beginning towards the anus. So from the mouth towards the anus, we will have peristalsis, or this segmental movement of the food bolus. It's a progressive movement as we go along the entire tract. And again, the waves of motility for both the segments, a segmentation and peristalsis is going to be set not only by the activity 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 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 or when food is present 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. In this case, what its doing is it's sweeping clean the entire track, during fasting period. Sweeping clean any 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 track. You can have bacterial growth or overgrowth within the tract and that can lead 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 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 jejunum of the small intestines in the absence of food. Motilin then will initiate 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/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 is present when we have food present within the lumen. Now there are regions which are isolated from one other and as you know, we isolate the stomach which has a very caustic hydrochloric acid in it. From the duodenum and from the esophagus because this lateral two regions 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 the sphincters which are under involuntary control. And it's due to the tonic inhibition of the smooth muscle and is again the inner circular layer of the smooth muscle the muscularis externas. So when we contract that smooth muscle layer you can occlude the lumen of the tube. We open it at very small amounts between the pylorus region of the stomach and the duodenum to allow very small amounts of our acidic con to enter into the duodenum. So we have a regulated opening of that region and so that means that for relaxation of the sphincter we have to release the inhibition that the sphincter is contracted it's under an inhibitory activity. We release that inhibition and allow the sphincter to relax so that there's a small lumen through which the acidic can exit. This is a regulated relaxation, and it's by turning off this tonic inhibition that we're then able to open the sphincter. Very small amounts are rejected from the stomach into the duodenum, and this is really important, because if we have a very large amount of acidic chyme. Or for instance, a hypertonic 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 regulating exiting of material at this point. We also have regulated openings of these sphincters. There's a second sphincter not only at the, 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 regulated openings then are allowing the receiving region of the tube to be able to handle the material that's being delivered. We have regulated openings and respond to the migratory motor complex as well. And that is between meals, so that when we're sweeping clean the intestine of any debris or any nutrients. Then at that time we will open periodically the sphincters and to allow the material to pass and then the sphincters will close again. There is no time at which the sphincters area all patently open along the track. So what about mass movements? Now mass movements, is where we have waves of contraction that are occurring within the large intestine. And here, we're now propelling the fecal material through the large intestine toward the rectum and the anus. These waves of contractions are not two peristaltic waves, in the sense that they are not using 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 when we have contraction, then we're essentially contracting a part of the chamber. We contract it 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 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 moving the materials to the anal canal. The anal canal has an involuntary sphincter and we have an external sphincter at the anus The, which is voluntary. The Mass Movement you all are familiar with is you have the sensation that you need to go to the bathroom. 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, is simply expelling the fecal material from the body. And it occurs with the 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. All right, 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 spend some time talking about vomiting. I'm pretty sure that everybody has experienced vomiting at some point in 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 material from the stomach. The stomach content being expelled being back out through the mouth. These can also be triggered by either delayed emptying of the stomach or by rapid emptying of the stomach. Where in the delayed emptying of the stomach, we have material that stays in the stomach let's say for example from a very fatty meal. And it's retained, and retained, and retained every several hours and eventually the body just expels it through the mouth. And the other one is were we have this very rapid dump emptying of the stomach content into the duodenum. And again as I said, then you can have a very rapid swelling of the duodenal region due to the movement of water coming into this region to triturate the hypotonic material that's coming from the stomach. This actually has a name and that's called the dumping syndrome of the stomach. The vomiting occurs is the expulsion of the chyme from the stomach, but it can be that you can, are eliminating material from the beginning of the duodenum. And in, under those conditions, then the vomitist will have a color, it's sort of a greenish color, and that's due to the bile which has been delivered from the gal bladder to the duodenum. Vomiting is controlled by the vomiting center, and that center is located in the medulla at the brain stem. 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-thoracic cavity and that helps to move the material then out from the stomach and out of the mouth. All right, so what's our general concepts? So the first of the general concepts is that we have movement of which involves coordinated activities of the two outer muscle masses of the wall of the gastrointestinal tract. And the second general concept is that the movement and activities are coordinated by this local nervous system which is called the enteric nervous system. And that this can be modulated from input from both the sympathetic nervous system and the parasympathetic nervous system. The third is that there's pacemaker 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 action potential contraction can follow.and this is true throughout the intestine,but in the stomach itself that the slow waves, the electrical slow waves 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 food bolus along the tract. In the fasted state, the migrating motor complex sweeps the tract clean and this is a propulsive type of movement along the tract. It has very different timing from the peristalsis movement 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 the second. We have to have a relaxation of the sphincter, a slight opening of the sphincter to allow the material to exit from segment one to segment two. The tonic contractions of the sphincters are going to occur when there is no motility within the tract. And so, they will open when there's food within the tract at designated times and they will also open during the migratory motor complex as we're sweeping the extra nutrients from the tract. But that they will be closed at all the other times and in particular, when there's no motility within the GI tract. So this then ends our discussion of the GI tract and the next time when we meet, we're going to talk about the urinary system. So see you then, bye-bye.
