The Gastrointestinal System

Although the detailed structure ofthe gastrointestinal tract varies from region toregion, there is a common basic structure, outlined in the cross-section shown in Fig. 18.1. It is surrounded by a number of heavily innervated muscle layers, arranged both circularly and longitudinally. Contraction of these muscle layers can mix the contents of the tract and move food in a controlled manner in the appropriate direction. Beneath the muscle layer is the submucosa, consisting mostly of connective tissue, and beneath that is a thin layer of smooth muscle called the muscularis mucosae. Finally, there is the lamina propria, a layer of connective tissue containing capillaries and many kinds of secreting glands, and then a layer of epithelial cells, whose nature varies in different regions of the tract.In addition to the muscle layers, there are two principal layers of neurons; the myenteric plexus, between the longitudinal and circular muscle layers, and the sub-mucosal plexus, which lies in the submucosa. In general, stimulation of the myenteric plexus increases the rate and intensity of the rhythmic contractions of the gut, and increases the velocity of conduction of waves of excitation along the gut wall. The sub-mucosal plexus is mainly sensory, receiving signals from stretch receptors in the gut wall, and from the gut epithelium. The gastrointestinal tract is also heavily innervated, which can control the activity of the entire gut, or part of it.