Gastric Emptying

Anatomic Considerations

Gastric contractions result from activity of smooth muscle cells that are arranged in three layers: an outer longitudinal layer, a middle circular layer, and an inner oblique layer. The longitudinal layer is absent on the anterior and posterior surfaces of the stomach. The circular layer is the most prominent and is present in all areas of the stomach except the paraesophageal region. The oblique layer , the least complete, is formed from two bands of muscle lying on the anterior and posterior surfaces. These two bands meet at the gastroesophageal sphincter and fan out to fuse with the circular muscle layer in the caudad part of the stomach. Both the circular and the longitudinal muscle layers increase in thickness toward the duodenum.

The stomach is richly innervated with both intrinsic and extrinsic nerves. The intrinsic nerves lie in various plexuses, the most prominent being the myenteric plexus, which lies in a three-dimensional matrix between the longitudinal and circular muscle layers and throughout the circular muscle layer. The myenteric plexus receives nerve endings from other intrinsic plexuses, as well as from extrinsic nerves. Axons from neurons within the myenteric plexus synapse with the muscle fibers and with glandular cells of the stomach. Extrinsically the stomach is innervated by branches of the vagus nerves and by fibers originating in the celiac plexus of the sympathetic nervous system.

The pylorus , or gastroduodenal junction, is characterized by a thickening of the circular muscle layer of the distal antrum. Separating this bundle of muscle from the duodenum is a connective tissue septum; however, some of the longitudinal muscle fibers pass from the antrum to connect with muscle cells of the duodenum. The pylorus is richly innervated with both extrinsic and intrinsic nerves, and nerve endings within the thickened circular muscle layer are abundant. Many of these endings contain neuropeptides, especially enkephalin, and many produce and release nitric oxide (NO).

The anatomy of the proximal duodenum is similar to that of the rest of the intestine (described in Chapter 5 ). A significant difference is the larger number of intrinsic nerves present in this area compared with the rest of the small bowel. These nerves may be involved in the regulation of gastric emptying (described in a later section).

In addition to the muscle cells and nerves, interstitial cells of Cajal (ICCs) also are prominent in all regions and appear to play a prominent role in regulating motility. Many ICCs form gap junctions with smooth muscle cells, whereas others appear to create a bridge between nerve endings and smooth muscle cells.

Contractions of the Orad Region of the Stomach

As detailed in the previous chapter, the predominant motor activity of the orad region of the stomach is the accommodation of ingested material. The musculature of the orad stomach is thin, contractions are weak, and during the remainder of the digestive state, pressures are essentially equal to intraabdominal pressure, with superimposed tonic pressure changes. These pressure changes are predominantly of low amplitude and have durations of 1 minute or more. The contractions that produce these pressure changes reduce the size of the stomach as the stomach empties. These tonic contractions result in accommodation of the remaining gastric contents and propulsion of those contents into the caudad stomach. A consequence of this minimal contractile activity is that little mixing of ingested contents occurs in the orad stomach. Contents often remain in relatively undisturbed layers for an hour or more after eating. As a result, salivary amylase (see Chapter 7 ), which is inactivated by gastric acid, can digest a significant portion of the starch present in a meal. Little is known about the regulation of contractions in the orad stomach during digestion. Both gastrin and cholecystokinin (CCK) decrease contractions and increase gastric distensibility. However, only the effect of CCK appears to be physiologic.