The Motility of the Gastrointestinal Tract
James Christensen, M. D.
Peer Review Status: Internally Peer Reviewed
Introduction
The evolution of the stomach came about because of the need of the animal for the digestive tract to prepare food before its delivery to the intestine and for it to be delivered in a controlled manner. The reduction of the large masses of solids that are ingested to the small particles that are delivered to the duodenum is a relatively slow process, and so the capacity for the storage of food before it reaches the grinding process must have developed simultaneously with the grinding mechanism itself. Both these two functions, the storage of solids and their grinding into small particles, require distinctive specializations in the motor functions of the stomach. These features constitute the subject of this chapter.
Regions of the Stomach(Figure 8.1)
The classical way anatomists divide the stomach (fundus, corpus, antrum, and pylorus) makes little sense in terms of motor function. Gastric motor function involves two distinct units, the proximal stomach and the distal stomach. The former includes the fundus and the rostral two-thirds of the corpus, while the latter constitutes the rest of the corpus, the antrum, and the pylorus. This plan for the division of the stomach rests on motor criteria. The distal stomach exhibits rhythmic contractions driven by electrical slow waves, whereas the proximal stomach does not. The distal stomach grinds food, the proximal stomach stores it.
Proximal Stomach
Contractions. The muscle of the proximal stomach generates two kinds of contractions: slow sustained contractions and more rapid phasic contractions. The sustained contractions last 1 to 3 minutes and produce intraluminal pressures up to 50 centimeters of water as measured by the technique of intraluminal manometry. The phasic contractions last only 10 to 15 seconds and give rise to lower intraluminal pressures, 5 to 15 centimeters of water. These two kinds of contractions occur independently. They involve the whole of the proximal stomach at the same time, alternately squeezing the gastric content and then relaxing that compression. That is, the proximal stomach acts as a contractile unit with no apparent progression of the contractile impulse from one region to another.
Regulation of Contractions. Contractions of the proximal stomach result from small, slow, and variable depolarizations of the muscle, without spike potentials. Repolarization of the muscle accompanies the relaxation of contraction. Electrical slow waves do not occur in this region. The principal motor innervation of the proximal stomach constitutes cholinergic nerves, which are excitatory, and inhibitory nerves that seem most likely to act by the release of nitric oxide. A variety of peptide hormones affect the proximal stomach but none has been shown to have a physiologically significant place in the regulation of its movements.
Reflexes constitute the major mechanism for the regulation of function in this region. These are inhibitory reflexes, excited by deglutition, by gastric distension, and by intestinal dilatation. These reflexes provide the means for receptive relaxation, the expansion of this part of the organ that occurs during eating. In the principal stimulus involved in receptive relaxation, that which is associated with deglutition, the proximal stomach relaxes with each swallowed bolus even before the bolus has reached it. The filling of the stomach also activates the reflex, presumably through the action of gastric mechanoreceptors. Receptive relaxation ceases immediately when eating stops, to be replaced by the slow contractions that cause emptying. The mechanisms controlling the time of appearance and the force of these contractions remain unknown.
The emptying of the stomach must be closely regulated both to prevent the duodenum from receiving more fat than the slow process of intestinal lipolysis can handle and to protect the duodenum from the damaging effect of excessive amounts of gastric acid. Reflexes resembling those that induce receptive relaxation inhibit the proximal stomach in order to regulate gastric emptying. These reflexes are initiated by the excitation of duodenal mucosal chemoreceptors to acid, fats, and osmotic pressure in the luminal contents. Dietary fats constitute the main governor of the rate of gastric emptying through these reflexes.
Function of Contractions. The weak phasic contractions of the proximal stomach seem to have little effect on gastric emptying. Instead, they simply induce small local fluid transits about the surface of the mass of stored solids. The tonic contractions, in contrast, clearly regulate the emptying of fluids from the stomach. They force all the content of the proximal stomach toward the distal stomach where the solid components are retained until they have been reduced to small particles. As a result, radioisotopic or radiographic studies of the gastric emptying of liquid meals measure mainly the function the proximal stomach, while studies with solid meals reflect the additional role of the distal stomach.
Distal Stomach
Contractions. Peristaltic contractions characterize the distal stomach. These begin as shallow circumferential indentations at the junction between the proximal and distal stomach. They migrate through the conical antrum to the pylorus, deepening and accelerating as they progress. They occur at intervals of 20 seconds or integral multiples of 20 seconds. That is, their period exhibits a constant fundamental of 20 seconds. Their force can produce an intraluminal pressure, as measured by intraluminal manometry, of over 100 centimeters of water, when that measurement is made near the pylorus. They deepen as they approach the pylorus, in part because of the underlying conical configuration of the antrum. The simultaneous acceleration and deepening indentation of the peristaltic contraction leads, in the rostral part of the antrum, to the formation of a degree of luminal occlusion ahead of the solid mass that is too large to accommodate the mass. This development forces the large solid masses back toward the proximal stomach in a process sometimes called retropulsion. The peristaltic contractions become fully occlusive some 3 to 4 centimeters rostral to the pylorus and the acceleration is such that this part of the antrum contracts essentially simultaneously throughout its length. This is called the terminal antral contraction. Not all peristaltic contractions meet this description. Some die out before they reach the terminal antrum and pylorus. Also, not all of the peristaltic events begin at the junction between the proximal stomach and the distal stomach. When the stomach is greatly distended, antral peristaltic contractions start farther toward the pylorus.
Regulation of Contractions. Intracellular recordings from the muscle of the distal stomach show regular, slow, stereotyped fluctuations in membrane potential consisting of an depolarization, a plateau, and a repolarization. Extracellular electrodes detect these events as electrical slow waves, the pacesetter potentials of the distal stomach. The signals recur at 20-second intervals at the pacesetter zone, an area on the greater curvature at the level of the junction between the two parts of the stomach, and spread to the pylorus. The magnitude and velocity of spread of the signal increase from the pacesetter zone to the pylorus. These signals pace the peristaltic contractions, determining their frequency, velocity, and direction of migration. The occurrence of a phasic contraction linked to a slow wave cycle depends upon the actions of nerves, excitatory cholinergic nerves and inhibitory nerves that probably act by the release of nitric oxide. Gastrin, an excitatory hormone, also may have some place in the physiological regulation of gastric peristalsis.
Function of Contractions. The peristaltic contractions of the antrum bring about both the grinding of solids and the delivery of liquids to the duodenum. The grinding takes a relatively long time so that antral motor function determines, in part, the rate of emptying of solids from the stomach. As a peristaltic pump, the antrum also delivers the fluid containing suspended solid particles to the duodenum in boluses. The size of each bolus can vary with the volume forced into the antrum by the tonic contractions of the proximal stomach, but the rate of delivery of boluses to the duodenum, three per minute, is constant. In this way, antral contractions smooth the pattern of the outflow from the stomach.
Pylorus
Contractions. No complete luminal occlusion exists at the pylorus when the stomach is quiet. The muscle of the pylorus does, however, show the high degree of myogenic tone characteristic of other sphincters. Thus, it is probably tonically contracted in vivo to some degree but not enough to fully occlude the lumen. The pylorus does, however, close forcefully when a peristaltic contraction reaches it. Because of the acceleration of peristalsis, each wave ends with the terminal antral contraction, as described above. This event includes the pylorus. Thus, the pylorus closes with each terminal antral contraction, remaining closed until after the contraction has ended. Then it opens again after a second or so.
Regulation of Contractions. The electrical slow waves of the distal stomach enter the pylorus, thus coordinating pyloric closure with antral peristalsis. The nerves of the pylorus constitute excitatory cholinergic fibers and inhibitory fibers that probably act by the release of nitric oxide. No physiological role for hormones has been shown in the regulation of pyloric contractions. The opening of the pylorus after it has contracted may represent, in part, the dilating effect of the contraction of the longitudinal muscle layer of the stomach. This suggestion comes from the way a part of the longitudinal muscle layer insert into the pyloric ring.
Function of Contractions. The main function of pyloric closure is to reduce the volume of fluid that refluxes into the distal stomach from the duodenum. The contraction of the pylorus in concert with antral contraction also helps to accomplish the sieving of solid particles by the distal stomach. Pyloric function alone has little to do with the regulation of gastric emptying.
Next Page | Previous Page | Title Page
See related Provider Textbooks about Internal Medicine.
See related Provider Topics Anatomy, Bones, Joints and Muscles, Brain and Nervous System, Digestive Diseases--General, Digestive System, Gastrointestinal or Internal Medicine.
See related Patient Textbooks about Internal Medicine.
See related Patient Topics Bones, Joints and Muscles, Brain and Nervous System, Digestive Diseases--General, Digestive System, Gastrointestinal or Internal Medicine.
Virtual Hospital Home | Virtual Children's Hospital Home | Site Map | Mirror Sites | Search
Provider Health Topics A-Z | Provider Textbooks | Patient Health Topics A-Z | Patient Textbooks
About Us | Continuing Education | Translations | Links | Support Us
Policies | Comments and Questions | E-mail This Page | UI Health Care Home
All contents copyright © 1992-2004 the Author(s) and The University of Iowa. All rights reserved.
http://www.vh.org/adult/provider/internalmedicine/motilitygastro/8.html