- Understand how the motility functions of the stomach contribute to the integrated response to a meal
- Describe the muscle layers and their connections to the enteric nervous system that subserve gastric motility
- Define the motility patterns that characterize movement of the stomach under fed and fasted conditions and their control mechanisms
- Describe receptive relaxation and mixing/grinding patterns of motility and their regulation
- Understand how the stomach is emptied, and how this is coordinated with the function of downstream segments
- Understand the pathophysiology of disease states where gastric motility is abnormal, including gastroparesis, pyloric stenosis, and vomiting
As we have learned from previous chapters, the stomach is a segment of the gastrointestinal tract in which important aspects of digestion and secretory function are initiated. However, in addition to these functions, which are largely dependent on gastric secretory function, the stomach also plays critical roles that depend on its motility properties.
First, the stomach can be considered as a homogenizer, mechanically breaking down ingested food into an emulsion of small particles with a vastly increased total surface area, thereby amplifying the effects of digestion. Second, the stomach is a critical contributor to the matching of food delivery to the digestive and absorptive capacity of more distal segments of the gut. Under normal circumstances, the stomach allows the delivery of approximately 200 kcal/h into the small intestine, although this may vary somewhat depending on the physical form of the meal (solid vs. liquids) and the nutrient(s) of which it is comprised, as will be described further. The stomach thus serves as a reservoir, allowing food particles to move only slowly into the duodenum to maximize their chances for assimilation. To accomplish this function, the stomach exhibits remarkable pressure/volume characteristics, which are vital in accommodating the volume of the meal without allowing significant reflux of the gastric contents back into the esophagus, or forcing them prematurely into the duodenum. Distention of the stomach also delivers important information to downstream segments of the gastrointestinal tract, as well as contributing to the signaling of satiety. These latter features likely underlie the effectiveness of gastric-reduction surgery for the treatment of morbid obesity, since the small remaining stomach reservoir only allows the patient to take small meals comfortably.
Finally, the stomach possesses distinct motility functions during the fasted state. Most importantly, it has developed mechanisms whereby ingested solids that cannot be digested or mechanically dispersed can be expelled from the stomach under normal conditions. This can be considered a “housekeeping” function, which sweeps undigested materials or ingested foreign objects along the length of the entire gastrointestinal tract, beginning at the stomach. This housekeeping function, mediated by a specific motility pattern known as the migrating motor complex or MMC, accounts for the fact that coins or similar objects that are swallowed by small children will eventually be passed in the feces.
From a motility standpoint, ...