Digestive system in a human body is not as simple as it sounds. A good digestion is essential for the apt performance of all other systems in the body. Like any other system in the body, digestive tract is under neurological and hormonal control.
The nervous system affecting digestion can be divided into two – the somatic nervous system and the autonomic nervous system. The somatic nervous system enables the body to adapt to stimuli from the external environment like, touch, smell, colour etc. The autonomic system involves the maintenance of internal environment of the body. This is the system that controls the gastrointestinal tract. The autonomic system can be further divided into sympathetic nervous system and the parasympathetic nervous system. When the sympathetic system is stimulated there is a need for large amounts of blood in peripheral tissue such as skeletal muscle. In order to fulfil this need, blood is shunted form Gastro Intestinal Tract, resulting in reduced digestive activity. For the most part salivation ceases and mouth becomes dry. Secretion of saliva is under control of the parasympathetic system. It also stimulates the production of digestive enzymes and the processes of digestion which in return increases peristalsis and secretion in the gut. It is also responsible to dilate blood vessels leading to the GI tract, increasing blood flow. This is important following the consumption of food, due to the greater metabolic demands placed on the body by the gut. The parasympathetic nervous system is active during periods of digestion and rest.
The initial digestion starts by breaking the food mechanically in the mouth by chewing (teeth) and actions of the tongue. In order to carry out chemical breakdown hormones are secreted by a number of ductless-endocrine-glands throughout the body. Several areas of the digestive tract secrete hormones which act as chemical messengers on the areas of the digestive tract to control the process of digestion. Hormones involved in digestion are known as gastrin, secretin, and cholecystokinin . The presence of protein in the stomach stimulates secretion of gastrin. Kahlson (p: 174) advocated that histamine release is an important step in the stimulation of acid secretion. Black and Shankley(p: 486) concluded that cholinergic stimulation releases histamine. Food passing into the duodenum causes the production of secretin and this hormone induces the release of alkaline secretions from the pancreas, stops further passage of food into the intestine until the acid is neutralized. Cholecystokinin (CCK) is released from intestinal epithelium in response to fats and causes the release of bile from the gall bladder and lipase from the pancreas. It is synthesized by I-cells in the mucosal epithelium of the small intestine and secreted in the duodenum, the first segment of the small intestine, and causes the release of digestive enzymes and bile from the pancreas and gallbladder, respectively. Some other hormones also influence digestion of food. “Glucocorticoids from the cortex of the adrenal gland may increase stomach secretions, while epinephrine from the medulla of the adrenal gland inhibits stomach secretions. Thyroid hormones stimulate the motility of the intestines.” (Audrey H Ensminger and M. E Ensminger, p: 279)
Kahlson G, Rosengren E, Svahn D, Thunderberg R, Mobilization and formation of histamine in the gastric mucosa as related to acid secretion. J Physiol (Lond) 1964; 174; 400-16
Black J W, Shankley N P, How does gastrin act to stimulate oxyntic cell secretion? Trends Pharmacol Sci 1987; 8: 486-90
Ensminger M. E, Ensminger Audrey, 1995 ‘The Concise Encyclopedia of Food and Nutrition’ CRC Press