The Motility of the Gastrointestinal Tract
James Christensen, M. D.
Peer Review Status: Internally Peer Reviewed
Introduction
Movements of the walls of the small intestine must help that organ to reduce the 6 to 12 liters of fluid it receives daily to the 1 to 2 liters it delivers daily to the colon. Digestion and absorption in the intestine require not only the constant mixing and antegrade propulsion of the intestinal content but also its local microcirculation across the absorbing surface of the epithelium. These three kinds of flows result from the combined contractions of the three muscular layers, the longitudinal and circular layers of the tunica muscularis and the muscle of the mucosa.
Although the structure of the small intestine looks the same all along its length, several observations suggest that its motility varies quantitatively from one region to another. Transit slows greatly from the duodenum to the ileum. The volume of the fluid load also falls greatly. Both the viscosity and the chemical composition of the contents change. These changes correlate with quantitative differences in motility.
Separate Actions of the Three Muscle Layers
The two layers of the tunica muscularis seem to act sometimes together and sometimes independently. Contractions of both layers move the contents of the intestine. Theoretical considerations suggest that contractions of the circular layer principally cause antegrade transit while those of the longitudinal muscle layer induce mixing movements. The usual assumption of symmetry, both radial and axial symmetry, in the peristaltic contractions of the intestine does not accord with the facts. Both radial and axial asymmetries characterize the contractions of these muscle layers in the intestine. They undoubtedly affect flow patterns. These asymmetries result both from the nature of the coordination of the two muscle layers and from the patterns of spread of excitation within the muscle layers.
The function of the muscle of the mucosa may not be entirely independent of the actions of the other muscle layers, but the consequences of its action are distinctly different. This layer shifts the pliable mucosa back and forth over the somewhat stiffer tunica muscularis. Also, the fingers of mucosal muscle that extend into the mucosal villi make them move rhythmically. The controls for these pumping contractions of the intestinal villi remain to be discovered.
Peristalsis Considered as a Two-Dimensional Event
The intestine can be viewed as series of overlapping functional units. Contraction, considered as an event at a single unit, may be radially concentric or eccentric, occlusive or variably nonocclusive, and variable in duration. The variety of possible shapes in contractions cannot be verified because of the inadequacy of the available methods to visualize them in vivo. The durations of contractions vary much less: most contractions are rhythmic phasic events lasting about 5 seconds in the human duodenum. Tonic contractions, smaller in force than phasic contractions, last much longer, several minutes. Tonic and phasic contractions frequently occur together.
Peristalsis Considered as a Three-Dimensional Event
Some contractions of the circular muscle layer spread smoothly along the intestine in the pattern called peristalsis. Others involve the simultaneous contraction of short segments of the intestine that are separated by uncontracted segments of similar length, a pattern called segmentation. A third pattern of contractions, called pendular movements, manifests itself as swaying motions seen in isolated loops of intestine, probably reflecting rhythmic contractions confined to the longitudinal layer of muscle.
Contractions as Inferred from Intraluminal Pressures
Manometry, the recording of intraluminal pressures from a bundle of small, perfused, open tipped catheters placed in the intestinal lumen, reveals transient peaks in pressure of stereotyped form, lasting about 5 seconds and recurring at apparently irregular intervals. Plots of the frequency distribution of the lengths of these intervals, however, reveal that they are multiples of a fundamental 5-second period. When records are made from a series of catheters with their tips located at close intervals along the axis of the intestine, some pressure peaks can be seen to spread at a uniform velocity through the domain under study, while others do not. The sets of progressive peaks are interpreted as records of peristaltic contractions. No manometric pattern specific for segmentation has been identified. It seems unlikely that pendular movements can even be detected manometrically.
Patterns of Contractions
Manometry, the only objective method capable of showing the distributions of intestinal contractions simultaneously in time and space, reveals that different patterns occur in the fed state and in the fasted state. In the fed state, contractions appear to occur with a quasi-random distribution in space and time at multiples of a fundamental period, 5 seconds in the duodenum and about 8 seconds in the ileum. In the fasted state, however, a pattern of periodic activity prevails. This pattern, the migrating motor complex, is described in Chapter VI.
Regulation of Contractions
The pacesetting electrical slow waves of the intestine, described in Chapter V, account both for the constant fundamental period of rhythmic contractions and for the progression of peristaltic contractions. The signals, recurring continuously at a constant frequency at any one point along the intestine, exhibit a decline in frequency along the intestine. The decline occurs in steps, frequency plateaus being tens of centimeters long. Within a frequency plateau, the slow wave spreads or migrates antegrade, toward the colon, at a constant velocity. Contractions may accompany these slow waves, locked to them in phase. Thus, if a contraction starts with one slow wave, it must follow that slow wave, taking both its direction and its velocity. Contractions cannot follow one another more closely than the slow wave frequency allows, about 12 contractions per minute in the human duodenum.
Nerves seem to regulate contractions by establishing the likelihood of their development with any given slow wave cycle. Thus, nervous factors could determine when a peristaltic contraction occurs, where it begins, and how far it goes. The responsible nerves appear to include both excitatory cholinergic fibers and inhibitory fibers that probably release nitric oxide.
The control mechanism outlined here explains peristalsis, but it does not explain segmentation. There is no satisfactory explanation of a way the control system operates that could bring about this pattern of contraction.
Functions of Contractions
Contractions of the longitudinal muscle layer must shorten the intestine. Theoretical considerations suggest that the rhythmic alternated shortening and lengthening of the intestine promote the mixing of the content. Peristaltic contractions of the circular muscle layer, in contrast, mainly produce forward transit with relatively little mixing. Since the two kinds of contractions occur together most of the time, the flows they induce, though highly complex, must be of both kinds.
The function of the contractions of the muscle of the mucosa cannot be easily investigated directly. Both the shifting of the elastic mucosa over the stiffer tunica muscularis, a motion permitted by the thick and gelatinous submucosa, and the pumping motions of the mucosal villi must mix the layer of fluid at the absorbing epithelial surface. Since diffusion through this layer is the rate-limiting step in the intestinal absorption of many nutrients, the mucosal muscle, by stirring this otherwise unstirred layer of fluid, may have a critical place in nutrition.
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