Mechanisms of Stimulation of the Digestive Glands

Each of the enzyme-secreting glands must be stimulated to secrete its product at the proper time. Continuous secretion of enzymes by the glands would be unnecessary and even harmful. Coordination of the secretion of digestive juices with the presence of food is achieved in two ways - with the help of the nervous system and hormones.

Hormones are chemical substances secreted in one part of the body and carried by the blood to another part, where they exert their specific effects. The function of the salivary glands is entirely regulated by the nervous system. The smell or taste of food irritates nerve cells in the nasal or oral cavity, causing them to send impulses to the salivary center located in the medulla oblongata; these impulses are transmitted to the salivary glands, causing the secretion of saliva.

The mere presence of tasteless, odorless objects in the mouth, such as pebbles, stimulates other cells in the oral mucosa, which in the same way cause salivation. In addition, impulses can come from higher centers of the brain: just the sight of food or the thought of it can cause the secretion of saliva. Thus, the salivary glands respond to chemical, mechanical and mental stimuli.

We owe a significant part of our knowledge about the mechanism regulating the secretion of gastric juice to the Russian physiologist I.P. Pavlov, who developed many experimental methods and conducted many subtle experiments. One of these experiments consisted of cutting the esophagus of a dog and bringing the two resulting ends to the surface of the neck, so that when the dog fed, the food, instead of going into the stomach, came out through an opening in the neck.

Although no food was used, this “mock feeding” resulted in the secretion of gastric juice in an amount equal to about a quarter of normal. This quarter of normal juice secretion is stimulated by nerve impulses originating in the taste buds or the eye and traveling to the brain, from where they are sent to the stomach.

When the nerves leading to the stomach are cut, juice secretion is completely turned off. When food is inserted into the part of the esophagus that leads to the stomach that is cut, so that the dog cannot see or smell or taste the food, the entry of food into the stomach causes the secretion of about half the normal amount of juice. This juice secretion occurs even when the nerves leading to the stomach are cut, although less juice is released.

Consequently, the secretion of juice depends partly on the nervous stimulation of the gastric glands by impulses from cells located in the gastric mucosa, and partly on the action of a hormone called gastrin. The hormone gastrin is released by the cells of the pyloric mucosa of the stomach whenever partially digested food comes into contact with these cells.

The existence and action of this hormone was finally proven in cross-circulation experiments, in which the circulatory system of one dog was connected to the circulatory system of another. When food was introduced into the pyloric region of the stomach of one dog, the gastric glands of the other began to secrete.

Some secretion of gastric juice is caused by the presence of food in the intestines. It is possible that this is due to the action of amino acids absorbed into the blood from the small intestine, or perhaps some still unknown reflex or hormone.

The participation of so many different mechanisms allows the stomach to provide the proper amount of juice in accordance with the amount and nature of food taken.

The pancreas is stimulated by the hormone secretin, secreted by the mucosal cells of the upper small intestine. In turn, these cells are stimulated by the acidity of food entering the intestines from the stomach. Under normal conditions, acidic chyme entering the small intestine stimulates cells in its walls, causing them to release secretin into the intestinal blood vessels. This hormone, carried by the blood throughout the body, eventually reaches the pancreas and causes it to secrete the hormones it synthesizes.