A&P of the Gastrointestinal System - Stomach wall layers, Regions of stomach, Secretory cell types (mucous, parietal, chief, enteroendocrine), Control of gastric secretions and motility (neural and hormonal, cephalic, gastric and intestinal phases, vagovagal reflex, enterogastric reflex).

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The stomach has 4 regions: the cardia, fundus, main body, and the pylorus above the pyloric sphincter. Different regions differ in gastric secretions.
The wall of the stomach consists of four layers: mucosa, submucosa, a muscle layer called muscularis externa, and serosa.
The muscularis externa has 3 sublayers: outer longitudinal, middle circular, and inner oblique muscle. The oblique muscle is unique to the stomach, as compared to other digestive organs. It enables the churning motions required for mechanical digestion.
The mucosa is lined with a surface epithelium which folds into the underlying connective tissue - the lamina propria – to form tiny holes called gastric pits. At the bottom of these pits are gastric glands producing various secretions. There are four major secretory cell types in these glands:
- Mucous cells secrete an alkaline mucus that protects the epithelium from gastric acid. This is one of several mechanisms that prevent the stomach from digesting itself. Other protective measures include tight junctions between epithelial cells to block gastric juice from seeping into the underlying tissue, and a rapid turnover of gastric epithelial cells to quickly replace injured cells.
- Parietal cells secrete hydrochloric acid and intrinsic factor. Hydrochloric acid has several functions: it breaks up food, denatures proteins, kills ingested bacteria, and activates the protein-digesting enzyme pepsin. Intrinsic factor is required for the absorption of vitamin B12 in the intestine.
- Chief cells producing pepsinogen, the inactive form of pepsin, which becomes activated by the acid produced by parietal cells. In infants, chief cells also produce some other enzymes for digestion of milk.
- Enteroendocrine cells secreting various hormones into the bloodstream. Notable hormones include: gastrin produced by G-cells and somatostatin by D-cells in the pylorus region; and histamine by ECL-like cells in the fundus. Gastrin increases secretions by gastric glands, promotes gastric emptying, and stimulates intestinal motility. Histamine increases gastric acid production by parietal cells. Somatostatin inhibits the release of gastrin, reducing gastric secretions and motility. It also inhibits a large number of other hormones.
Gastric activity is under both neural and hormonal control; and occurs in 3 phases, which overlap and can happen simultaneously:
- The cephalic phase starts before food reaches the stomach. It is triggered by the thought, smell, sight, or taste of food. These inputs act on the hypothalamus, which relays the information to the medulla. Vagus nerve fibers from the medulla then stimulate the enteric nervous system to prepare the stomach for digestion.
- The gastric phase is triggered by the arrival of food in the stomach. As the stomach fills, stretch receptors in stomach wall activate the parasympathetic system to release acetylcholine, which stimulates gastric secretions and motility. There is also a local reflex mediated via the myenteric plexus. At the same time, partially digested proteins and the rising pH stimulate the release of gastrin, which increases gastric acid production and stimulates gastric motility. A negative feedback loop exists to wind down this phase as the stomach is emptying.
- The intestinal phase is triggered by the arrival of chyme in the duodenum. Initially, the stretching and presence of partially digested food in the duodenum further stimulate gastric activity via the vagovagal reflex and production of intestinal gastrin. Soon however, distension of the duodenum inhibits gastric secretions and slows down stomach emptying via the enterogastric reflex and several intestinal hormones, thus allowing time for the intestine to process chyme.