Contact of Food with the Epithelium Stimulates Secretion—Function of Enteric Nervous Stimuli
The presence of food in a particular segment of the gastrointestinal tract usually stimulates the glands of that region and 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 activates the enteric nervous system of the gut wall. The types of stimuli that activate this system 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 Increases the Alimentary Tract Glandular Secretion Rate. Stimulation of the parasympathetic nerves to the alimentary tract almost invariably increases the rates of alimentary glandular secretion. This increased secretion rate is especially true of the glands in the upper portion of the tract (innervated by the glossopharyngeal and vagus parasympathetic nerves) such as the salivary glands, esophageal glands, gastric glands, pancreas, and Brunner’s glands in the duodenum. It is also true of some glands in the distal portion of the large intestine, which are innervated by pelvic para sympathetic 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 Has a Dual Effect on the Alimentary Tract Glandular Secretion Rate. Stimulation of the sympathetic nerves going to the gastrointestinal tract causes a slight to moderate increase in secretion by some of the local glands. However, sympathetic stimulation also results in constriction of the blood vessels that supply the glands. Therefore, sympathetic stimulation can have a dual effect: (1) sympathetic stimulation alone usually slightly increases secretion and (2) if para sympathetic or hormonal stimulation is already causing copious secretion by the glands, superimposed sympathetic stimulation usually reduces the secretion, some times significantly so, mainly because of vasoconstrictive reduction of the blood supply.
Regulation of Glandular Secretion by Hormones. In Secretion the stomach and intestine, several different gastrointestinal hormones help regulate the volume and character of the secretions. These hormones are liberated from the gastrointestinal mucosa in response to the presence of food in the lumen of the gut. The hormones are then absorbed into the blood and carried to the glands, where they stimulate secretion. This type of stimulation 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 poly peptides or polypeptide derivatives and will be discussed in more detail later.
