Gastric Motility

Activity of the smooth muscle of the stomach carries out three principal functions: (1) The muscle relaxes to accommodate large volumes ingested during a meal; (2) contractions mix ingested material with gastric juice, facilitating digestion, solubilizing some constituents, and reducing the size of the particles; and (3) contractions propel material into the duodenum at a rate regulated to provide optimal time for intestinal digestion and absorption.

Structure and Innervation of the Stomach

Unlike the rest of the GI tract, the stomach has three layers of smooth muscle cells: an outer longitudinal layer, a middle circular layer, and an inner oblique layer. The longitudinal layer is incomplete, being absent from the anterior and posterior surfaces, whereas the circular layer is the most prominent. The oblique layer is formed from two bands of muscle that radiate from the lower esophageal sphincter over the anterior and posterior surfaces to fuse with the circular muscle in the caudad portion of the stomach. The thickness of the circular and longitudinal layers increases as one moves distally toward the pylorus (Fig. 6).

The innervation of the stomach is similar to that described in the previous chapter for the GI tract in general. Although both plexuses are present, the myen-teric is the most prominent and receives extrinsic innervation from the vagus and from fibers originating in the celiac plexus of the sympathetic nervous system. The myenteric plexus also receives innervation from other intrinsic plexuses. Axons from neurons of the myenteric plexus synapse with smooth muscle cells and with secretory and endocrine cells of the mucosa.

As shown in Fig. 6, the stomach can be divided into two regions based on patterns of motility: the orad or proximal region is responsible for accommodating an

FIGURE 6 Anatomic and functional divisions of the stomach. To consider motility, the stomach is divided into an orad (proximal) region and a caudad (distal) region. Fundus, body, and antrum are anatomic designations for regions of the stomach.

Antrum

FIGURE 6 Anatomic and functional divisions of the stomach. To consider motility, the stomach is divided into an orad (proximal) region and a caudad (distal) region. Fundus, body, and antrum are anatomic designations for regions of the stomach.

ingested meal, and the caudad or distal region is responsible for the contractions that lead to mixing and propulsion into the duodenum.

Motility of the Orad Area

As indicated earlier, the most important motor activity of the orad region of the stomach is accommodation to ingested material. The musculature of the orad stomach is thin, and during the remainder of the digestive process, contractions are weak and infrequent. Therefore, ingested material is deposited in layers and may remain relatively unmixed for up to an hour. This allows salivary amylase, which is denatured when mixed with gastric acid, to digest a significant proportion of the carbohydrates present. The weak contractions of the orad region reduce the size of the stomach as its contents empty. It is uncertain whether the musculature contracts tonically to maintain pressure on the remaining contents or whether contractions propel the contents into the caudad stomach. Little is known about the regulation of these contractions. In addition to facilitating accommodation, CCK inhibits these contractions, making the stomach more distensible and ultimately decreasing the rate of gastric emptying.

Motility of the Caudad Area

In the fasted state, for the greater part of the time, the stomach is quiescent. However, vigorous contractions may occur at the same time in the caudad stomach and the upper part of the small intestine. In humans these contractions occur at intervals of approximately 90 min and are referred to as the migrating motility complex or migrating myoelectric complex. The contractions begin in the midportion of the stomach and move distally over the caudad area. These periods of cyclical contractions, which last 3-5 minutes, propel any residue from the previous meal and accumulated mucus into the duodenum. Eating abolishes the migrating motility complex. After a meal, contractions are more frequent and of varying strength, depending on the nature of the material ingested. These contractions are peristaltic and originate in the midstomach. As they move distally toward the pylorus, both velocity and force of contraction increase (Fig. 7). In humans, each contraction lasts from 2-20 seconds, and contractions may occur at rates of 3-5 per minute. This is the primary form of motility of the caudad stomach and both mixes food with digestive juices and propels it into the duodenum. As the peristaltic contraction approaches the pylorus, its velocity actually increases so that it overtakes most of the gastric contents moving in front of it. Some of the contents are "squirted" into the duodenum, but most

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