The gastrointestinal tract is arranged linearly in the following sequence: mouth, esophagus, stomach, small intestine (including the duodenum, jejunum, and ileum), large intestine, and anus. Other structures of the gastrointestinal tract are the salivary glands, pancreas, liver, and gallbladder, all of which serve secretory functions.
The wall of the gastrointestinal tract has two surfaces, mucosal and serosal. The mucosal surface faces the lumen, and the serosal surface faces the blood (Fig. 8-1). The layers of the gastrointestinal wall are as follows, starting from the lumen and moving toward the blood: A mucosal layer consists of a layer of epithelial cells, a lamina propria, and a muscularis mucosae. The epithelial cells are specialized to carry out absorptive and secretory functions. The lamina propria consists primarily of connective tissue, but it also includes blood and lymph vessels. The muscularis mucosae consists of smooth muscle cells; contraction of the muscularis mucosae changes the shape and surface area of the epithelial cell layer. Beneath the mucosal layer is a submucosal layer, which consists of collagen, elastin, glands, and the blood vessels of the gastrointestinal tract. Motility of the gastrointestinal tract is provided by two layers of smooth muscle, circular muscle and longitudinal muscle, which are interposed between the submucosa and the serosa. The longitudinal muscle layer is thin and contains few nerve fibers, whereas the circular muscle layer is thick and more densely innervated. Neurons do not make true synapses on the gastrointestinal smooth muscle fibers; rather they release transmitters from varicosities along the length of their axons. Two plexuses, the submucosal plexus and the myenteric plexus, contain the nervous system of the gastrointestinal tract. The submucosal plexus (Meissner’s plexus) lies between the submucosa and the circular muscle. The myenteric plexus lies between the circular muscle and the longitudinal muscle.
Figure 8–1 The structure of the wall of the gastrointestinal tract.