Peristalsis is a moving ring of contraction that propels material along the GI tract. It involves neurally mediated contraction and relaxation of both muscle layers. Peristalsis occurs in the pharynx, esophagus, gastric antrum, and the small and large intestine.
Segmental contractions produce narrow areas of contracted segments between relaxed segments. These movements allow mixing of the luminal contents with GI tract secretions and increase exposure to the mucosal surfaces where absorption occurs. Segmentation occurs predominantly in the small and large intestine.
There are also characteristic pathological patterns of motility. During spasm, maximal contractile activity occurs continuously in a dysregulated manner. In ileus, contractile activity is markedly decreased or absent; it often results from irritation of the peritoneum, such as occurs in surgery, peritonitis, and pancreatitis.
Functional Anatomy of the Stomach
As discussed, the stomach is divided into two functional regions—proximal and distal, with sphincters at either end. The LES and cardia (defined as the region of the stomach immediately surrounding the LES) have important functions. Relaxation of the LES and cardia allows entry of food from the esophagus into the stomach and the release of gas, called belching. By maintaining tone, reflux of contents from the stomach back into the esophagus is prevented.
The proximal part of the stomach (the fundus together with the corpus or body) produces slow changes in tone compatible with its reservoir function. It is important for receiving and storing food and for mixing the contents with gastric juice (
Table 29.2 ). Generation of tone in the proximal portion of the stomach is also an important driving force in the regulation of gastric emptying. Low tone and consequently low intragastric pressure are associated with delayed or slow gastric emptying, and an increase in tone in this region is required for gastric emptying to occur.
The distal part of the stomach is important in the mixing of gastric contents and for propulsion through the pylorus and into the duodenum. The muscle layers in the region of the gastric antrum are much thicker than in the more proximal regions of the stomach, and thus the antrum is capable of producing strong phasic contractions. Contractions initiated by the slow wave begin in the midportion of the stomach and move toward the pylorus. The strength of these contractions varies during the postprandial period. In the gastric phase of the meal the pylorus is usually closed, and these antral contractions serve to mix the gastric contents and reduce the size of solid particles (grinding). However, eventually these antral contractions are also important in emptying the stomach of its contents.
The pyloric sphincter is the gastroduodenal junction and is defined as an area of thickened circular muscle. This is a region of high pressure generated by tonic smooth muscle contraction. It is important in regulating gastric emptying.
Control of Gastric Motility in the Gastric Phase
Gastric motility is highly regulated and coordinated to perform the functions of storage and mixing. Regulation of emptying of contents into the small intestine, an important part of gastric motor function, will be considered in detail in the discussion of the duodenal phase of the meal, because the controls are generated in the duodenum.