Digestive System

In general, the digestive system is controlled by both nerves and hormones.   The nerves that control digestion are in the submucosa layer of the tissue and in between the muscle layers of the walls of the organ. These nerves are part of the autonomic nervous system, a part of the nervous system that controls involuntary activity. The autonomic nervous system consists of two branches called the parasympathetic nervous system and the sympathetic nervous system. In general, the activity of the digestive system increases if the parasympathetic nervous system is aroused and decreases if the sympathetic nervous system is aroused. The sympathetic nerves that regulate digestion are called the greater splanchnic nerves, the lumbar splanchnic nerves, the lumbar splanchnic nerves, and the sacral splanchnic nerves. After crossing ganglia, messages travel through these nerves to the spinal cord and ultimately the brain.  The parasympathetic nerves that regulate digestion for the liver, stomach, pancreas , small intestine, and part of the large intestine are vagus nerves (the tenth cranial nerve). Unlike the splanchnic nerves, these nerves carry messages directly to the brain. The third sacral nerve regulates the other part of the large intestine. This nerve is largely involved in defecation reflexes.

Too much arousal of either branch can lead to problems. For instance, if the sympathetic nervous system is too aroused, food will not move through the digestive tract and mucus will not be released. However, food will move through the digestive tract too fast if the parasympathetic nervous system is too aroused.  As a result, not enough nutrients may be absorbed into the body. Thus, both branches work together to attain the correct balance so that food moves through the tract at a suitable pace.

The hormones that control digestion are produced and secreted by various organs along the tract. These hormones cause the various chemicals that aid in digestion to be released or prevented from being released. For instance, gastrin, which is secreted by the stomach, causes gastric juice to be released into the stomach. Similarly, the duodenum releases gastric inhibitory peptide, secretin, and cholecystokinin. In addition to preventing the stomach from releasing gastric juice, gastric inhibitory peptide causes the pancreas to release insulin in response to the presence of glucose in the duodenum. Secretin causes the pancreas to secrete water and sodium bicarbonate (baking soda) and the liver to release bile into the duodenum.  It also inhibits the activity of the stomach. Cholecystokinin causes the pancreas to release pancreatic juice and the gallbladder to release bile. Cholecystokinin also inhibits the stomach. Therefore, nerves and hormones work together to ensure that food moves through the tract at an appropriate pace and is adequately digested.