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. If you recall, we've been talking about the gastrointestinal tract or GI tract as a processing plant. Where we start in the mouth. Then digest the food materials in the next compartments, which are the stomach and small intestine. Then the food materials move into the lower portion of the intestine, where we absorb most of our fluids and nutrients. The waste products then move to the colon, or large intestine, where much of the remaining fluid is absorbed. This is a unidirectional movement through the tract. 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 waste material exits the body as the fecal material. Motility governs this timing as we're moving along the tube. Let's consider the general anatomy of the tube. As you recall from our first lecture of this particular system that we have on the outermost aspect of the gastrointestinal tract or the tube. two layers of smooth muscle. That is what is shown here. This smooth muscle is said to be in the muscularis externa. The muscularis externa has two layers of muscle. The innermost layer is circumferential around the lumen of the tube. 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, it makes the lumen of the tube smaller. If it relaxes, then the lumen of the tube increases. In contrast, when we contract the outermost muscle layer, as we contract the outermost layer, the tube shortens. It shortens 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. Smooth muscle starts in the lower one-third of the esophagus and remains through to the end of the large intestine. This is all smooth muscle. The smooth muscle is the single unit type of smooth muscle, where all of the cells are connected to one another through gap junctions. They are electrically coupled cells. [COUGH] Excuse me. The activity of the two layers of muscle have to be coordinated. This coordination is done by a local nervous system, called the enteric nervous system. That is 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 branches of the autonomic nervous system, the parasympathetic and the sympathetic nervous systems. Now there's another factor that we have to consider when we're talking about motility of the smooth muscle throughout this tract. That is there are located within the tract, within the fundic region of the stomach, within the small intestine and within the large intestine, specialized smooth muscle cells called pacemakers. Like all pacemakers, they have an unstable resting membrane potential. That is what is shown here. In our top image, the cells are slowly depolarizing, and then they repolarize, and then again slowly depolarize. As the cells are slowly depolarizing and repolarizing, they generate what is called the electrical slow waves. These electrical slow waves bring the resting membrane potential of the smooth muscle cells, these pacemaker cells close to threshold. At that point, we can initiate an action potential. If the action potential is initiated, it means that we've reached the voltage at which the voltage-gated calcium channels open. The voltage-gated calcium channels open. Calcium enters the cells, and the action potential is generated. To repolarize the cells, we close the voltage-gated calcium channels and open a voltage-gated potassium channel. That action 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 reach threshold generate the action potential. Those that do generate the action potential will be followed by a contraction. Wherever we have an action potential generated, there will be a contraction of the smooth muscle. Within the stomach, this is not true. Within the stomach, we have these rhythmic slow waves. But here, each rhythmic slow wave can give rise to a contraction. So within the stomach, you don't have to generate an action potential. The threshold of the slow wave is associated with contraction. There's one other thing about these slow waves. The slow wave determine the frequency of the action potential, and therefore, the frequency of the contraction. The slow waves can be modulated. That is, the pacemaker cells, their timing can be modulated by the sympathetic nervous system and by the parasympathetic system. The sympathetic nervous system will cause hyperpolarization of these cells It acts to move the 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. It therefore, delays contraction. The converse is the parasympathetic system. The parasympathetic system moves the resting membrane potential towards threshold. So the cells reach threshold at a faster pace. By reaching threshold at faster pace, they can then generate an action potential and contractions faster. So we can speed up contractions by having input from the parasympathetic system. What kinds of motility are we actually talking about? In the fed state, you have two types of motility. The motility that we'll consider first is called segmentation. This is a mixing type of motility. In segmentation, we have two neighboring regions of the tract. The first is relaxed. A food bolus is located within that region, The second adjacent region is contracted. There is no food within this region. The first region contracts as the adjacent second regions relaxes. That moves the food bolus into to the second adjacent region. Then, the second region contracts, as the first region relaxes again. So we have a sloshing of material back and forth. This kind of movement, segmentation or mixing movement, is very critical for mixing the engested materials, the food substances with the enzymes and buffers. All of the secretions that were added to the lumen of the track. In addition segmentation mixing the food material with the surfaces of the epithelial cells that line this tract. This enhances the absorption of nutrients that were generated. THat includes the small amino acids, small sugars, and so forth. They are delivered to the surfaces of these cells where 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 tract. It's the inner muscle mass that constricts closing the lumen of the first region. And then, it's the inner muscle mass that relaxes, and allows the second region to receive the food bolus. The sloshing back and forth is due to the inner muscle masses. Changes in the inner muscle masses contracted state. The second type of movement is called peristalsis. This involves 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 propulisive motion contraction and relaxation events occur. Now it is the outermost muscle mass that undergoes contraction and relaxation. As the the outermost muscle mass shortens, the innermost muscle mass relaxes. Such that as the tube shortens, it widens. The tube get fatter. This occurs in repeating segments as we proceed towards the anus. So from the mouth towards the anus, we will have peristalsis. This is 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 segmentation and peristalsis is set not only by the activity enteric nervous system but also by the pacemakers. It is the combination of those two signals which generates the timing for movement of material as we proceed along the tract. Now in the fasting state, there is also a type of peristalsis but it's different in timing from that observed when food is present within the lumen of the gastrointestinal tube. This particular type of movement is propulsive movement. It does move materials from one region that is from the stomach towards the anus. This process is called the migratory motor complex. Migratory motor complex is sort of a good housekeeping type of movement. In this case, it is sweeping clean the entire tract during fasting periods. 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 tract. bacterial growth or overgrowth may occur within the tract. This state 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 initiates 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. The frequency of contractions, as I said, is different from the frequency of contractions that is observed in peristalsis when food is present. When food is present within the lumen. Now recall that specific regions are isolated from one other. As you know, we isolate the stomach which has a very caustic hydrochloric acid in it from the duodenum and from the esophagus. This is important because this two regions are not protected to the caustic actions of the acid. These regions are isolated by sphincters. These sphincters are not under voluntary control. These sphincters are under involuntary control. And it's due to the tonic inhibition of the smooth muscle, the inner circular layer of the smooth muscle of the muscularis externas. When we contract that smooth muscle layer, you can occlude the lumen of the tube. We do open it so that very small amounts of chyme leave the pylorus region of the stomach to enter the duodenum. THis allows very small amounts of acidic chyme to enter into the duodenum. Thisis a regulated opening of that sphincter. To relax the sphincter we have to release the inhibition that the keeps the sphincter 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. It is 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. 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 then water moves rapidly into the duodenum by osmotic action. THat process will stretch the duodenum. That's going to be very painful. So there is regulated exiting of material at this point. We also have regulated openings of other sphincters. There's a second sphincter between the esophagus as well as between the stomach and the duodenum. There's also one between the ileum and the colon. These regulated openings then allow the receiving region of the tube to be able to handle the material that's being delivered. We have regulated openings that respond to the migratory motor complex as well. That occurs between meals, when we are sweeping clean the intestine of any debris or any nutrients. Then at that time we will open periodically the sphincters to allow the material to pass and then the sphincters close again. There is no time at which the sphincters are all patently open along the tract. So what about mass movements? Now mass movements, is where there is a wave of contraction occurring within the large intestine. Here, we're propelling the fecal material through the large intestine toward the rectum and the anus. These waves of contractions are not true peristaltic waves. They do not use the outer layer of smooth muscle That in the muscularis externis. Instead, mass movement uses the inner layer, the circumferentially-oriented muscle layer of the muscularis externis. When these contraction occur, then essentially a part of the reion contracts. It contracts and the adjacent region relaxes to receive it. Mass movement is almost like the segmentation type of movement but it is propulsive. This has a special name, it's called Mass Movements. It occurs about 1 to 3 times per day under normal circumstances. It moves fecal material to the anal canal. The anal canal has an involuntary sphincter and an external sphincter at the anus. This one is under voluntary control. The Mass Movement you all are familiar with is simply the sensation that you need to go to the bathroom. But it may not be appropriate and so you don't. Nothing happens. You are able to retain the fecal material until you can get to the bathroom and then eliminate it. You can eliminate it under voluntary control. The elimination of the fecal material from the body is called defecation. Defecation, there's an a in there, is simply expelling the fecal material from the body. It occurs with the relaxation of the voluntary anal sphincter, that's the external sphincter. There is 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. if you bring some type of toxic material into the body, then the body expels the material from the stomach. The stomach content is expelled out through the mouth. Vomiting reflexes can also be triggered by either delayed emptying of the stomach or by rapid emptying of the stomach. In delayed emptying of the stomach, material stays in the stomach let's say for example from a very fatty meal. It is retained, and retained, and retained every several hours. Eventually the body just expels it through the mouth. The other situation is where we have a very rapid dump or emptying of the stomach content into the duodenum. This causes a very rapid swelling of the duodenal region due to the movement of water coming into this region to triturate the hypertonic material that's coming from the stomach. This actually has a name. It is called the dumping syndrome of the stomach. Vomiting occurs to expel the chyme from the stomach, but it can also be material from the beginning of the duodenum. Under these conditions, then the vomitist will have a color, it's sort of a greenish color. This is due to the bile which has been delivered from the gal bladder to the duodenum. Vomiting is controlled by the vomiting center. That center is located in the medulla of the brain stem. 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. Instead, you change the pressure within the inter-thoracic cavity. This helps to move the material 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 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. These are called electrical slow waves. These slow waves can lead to an action potential which fires at the peak of the depolarization as the pacemakers approach threshold. At that point, if an action potential occurs, then contraction can follow. This is true throughout the intestine, but in the stomach itself 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. which allows the material to slosh back and forth in order to have optimal absorption of the materials. We also have peristalsis. Peristalsis is the propulsive movement of the food bolus along the tract. In the fasted state, the migrating motor complex sweeps the tract clean. This is a propulsive type of movement along the tract. It has very different timing from the peristalsis, the movement associated with feeding. The fifth general concept is that tonic contractions can occur at the sphincters. This closes the sphincters. In order to have material moving from one segment of the tract to the second. there has to be a relaxation of the sphincter. This makes a slight opening of the sphincter to allow the material to exit from segment one to segment two. The tonic contractions of the sphincters occur when there is no motility within the tract. The sphincters open when there's food within the tract only at designated times and they also open during the migratory motor complex as the extra nutrients are swept from the tract. The sphincters will be closed at all the other times. In particular, when there's no motility within the GI tract. So this then ends our discussion of the GI tract. The next time when we meet, we'll discuss the urinary system. So see you then, bye-bye.
