Basic Mechanisms of Stimulation of the Alimentary Tract Glands
Effect of Contact of Food with the Epithelium—Function of Enteric Nervous Stimuli. Themechanical presence of food in a particular segment of the gastrointestinal tract usually causes the glands of that region and often of adjacent regions to secrete moderate to large quantities of juices. Part of this local effect, especially the secretion of mucus by mucous cells, results from direct contact stimulation of the surface glandular cells by the food.
In addition, local epithelial stimulation also acti-vates the enteric nervous system of the gut wall. The types of stimuli that do this are (1) tactile stimulation, (2) chemical irritation, and (3) distention of the gut wall. The resulting nervous reflexes stimulate both the mucous cells on the gut epithelial surface and the deep glands in the gut wall to increase their secretion.
Autonomic Stimulation of Secretion Parasympathetic Stimulation. Stimulation of theparasympathetic nerves to the alimentary tract almost invariably increases the rates of alimentary glandula secretion. This is especially true of the glands in the upper portion of the tract (innervated by the glos-sopharyngeal and vagus parasympathetic nerves) such as the salivary glands, esophageal glands, gastric glands, pancreas, and Brunner’s glands in the duode-num. It is also true of some glands in the distal portion of the large intestine, innervated by pelvic parasym-pathetic nerves. Secretion in the remainder of the small intestine and in the first two thirds of the large intestine occurs mainly in response to local neural and hormonal stimuli in each segment of the gut.
Sympathetic Stimulation. Stimulation of the sympa-thetic nerves going to the gastrointestinal tract causes a slight to moderate increase in secretion by some of the local glands. But sympathetic stimulation also results in constriction of the blood vessels that supply the glands. Therefore, sympathetic stimulation can have a dual effect: First, sympathetic stimulation alone usually slightly increases secretion. But, second, if parasympathetic or hormonal stimulation is already causing copious secretion by the glands, superimposed sympathetic stimulation usually reduces the secretion, sometimes significantly so, mainly because of vaso-constrictive reduction of the blood supply.
Regulation of GlandularSecretion by Hormones. In the stomach and intestine, several different gastrointestinalhormones help regulate the volume and character ofthe secretions. These hormones are liberated from the gastrointestinal mucosa in response to the presence of food in the lumen of the gut. The hormones then are absorbed into the blood and carried to the glands, where they stimulate secretion. This type of stimula-tion is particularly valuable to increase the output of gastric juice and pancreatic juice when food enters the stomach or duodenum.
Chemically, the gastrointestinal hormones are polypeptides or polypeptide derivatives.