Peristalsis, a moving ring of contraction, is responsible for propelling material along the GI tract. It involves the contraction and relaxation of both muscle layers, which are controlled by neural mechanisms. Peristalsis occurs in several parts of the GI tract, including the pharynx, esophagus, gastric antrum, and the small and large intestine.

Segmental contractions create narrow areas of contracted segments between relaxed segments. These contractions are primarily seen in the small and large intestine and serve to mix the contents of the GI tract with secretions and increase exposure to the mucosal surfaces for absorption.

Pathological patterns of motility can also occur. Spasms involve dysregulated, continuous maximal contractile activity. Ileus, on the other hand, is characterized by markedly decreased or absent contractile activity. Ileus often arises from irritation of the peritoneum, such as during surgery, peritonitis, or pancreatitis.

The stomach can be divided into two functional regions - proximal and distal - each with sphincters at their ends. The lower esophageal sphincter (LES) and the region surrounding it, known as the cardia, play important roles. Relaxation of the LES and cardia allows food to enter the stomach from the esophagus and facilitates the release of gas through belching. By maintaining tone, these structures prevent the reflux of stomach contents into the esophagus.

The proximal part of the stomach, consisting of the fundus and corpus (body), undergoes slow changes in tone that align with its function as a reservoir. This region is responsible for receiving, storing, and mixing food with gastric juice. The tone in the proximal portion of the stomach also plays a crucial role in regulating gastric emptying. Low tone results in delayed or slow gastric emptying, while increased tone is necessary for proper emptying to occur.

The distal part of the stomach is essential for mixing gastric contents and propelling them through the pylorus and into the duodenum. The muscle layers in the gastric antrum, which is located in this region, are thicker compared to the more proximal areas of the stomach. Consequently, the antrum can generate strong phasic contractions. These contractions, initiated by slow waves, start in the middle portion of the stomach and move towards the pylorus. Their strength varies during the postprandial period. In the gastric phase of a meal, the pylorus is typically closed, and antral contractions serve to mix the gastric contents and break down solid particles. However, these antral contractions also play a role in emptying the stomach eventually.

The pyloric sphincter, located at the gastroduodenal junction, consists of a thickened circular muscle. This region generates high pressure through tonic smooth muscle contraction and is crucial in regulating gastric emptying.

During the gastric phase, the motility of the stomach is tightly regulated and coordinated to fulfill its storage and mixing functions. The regulation of emptying the stomach contents into the small intestine, an important aspect of gastric motor function, will be further discussed in the duodenal phase, as it involves controls originating from the duodenum.
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.