BIOLOGY Volume 1 - A Guide to General Biology - 2004

8. HETEROTROPHIC NUTRITION

8.4. Nervous and hormonal regulation of digestive gland functions

8.4.2. Gastric juice

Gastric juice secretion occurs in three phases. The first of these is the nervous phase. The presence of food in the Oral Cavity and the act of swallowing trigger a Nerve Impulse, which is transmitted via the Vagus nerve through the Brain to The Stomach. The sight, smell, taste, and even the thought of food can trigger a similar reflex. All of these stimuli prompt the gastric glands to secrete gastric juice, with secretion beginning even before the food reaches the stomach, thereby preparing the stomach to receive food. The nervous phase lasts approximately one hour.

The second, gastric phase takes place when food directly or indirectly stimulates the gastric mucosa. Upon entering the stomach, food stretches it and irritates the receptors in the stomach wall. These receptors send nerve impulses to Meissner's nerve plexus (Fig. 8.11) in the submucosa, from which nerve impulses are in turn directed to the gastric glands, stimulating the secretion of gastric juice. The distension of the stomach and the presence of food within it also activate specialized endocrine Cells in the mucosa, resulting in the release of the hormone gastrin. Via the bloodstream, it reaches the gastric glands and stimulates them to produce gastric juice enriched with Hydrochloric acid. This phase lasts for about four hours.

The third phase, the intestinal phase, takes place in the Small Intestine. When acidic chyme enters the duodenum and comes into contact with its walls, nervous and hormonal responses are triggered. The presence of food stimulates receptors in the small intestine, but Reflexes passing through the brain inhibit the secretion of gastric juice and slow down the release of chyme from the stomach. This prevents too large a bolus of food from being dumped into the small intestine all at once. In addition, the duodenal mucosa produces two Hormones—cholecystokinin (CCK) and secretin. (CCK is also known as pancreozymin, though only one of these names should be used, preferably CCK). These two hormones are delivered to the stomach, Pancreas, and Liver via the bloodstream. In the stomach, secretin inhibits gastric juice secretion, while CCK slows down the emptying of gastric contents.