Nutrient tasting and signaling mechanisms in the gut II. The intestine as a sensory organ: neural, endocrine, and immune responses

The lining of the gastrointestinal tract is the largest vulnerable surface that faces the external environment. Just as the other large external surfa ce, the skin, is regarded as a sensory organ, so should the intestinal muco sa. In fact, the mucosa has three types of detectors: neurons, endocrine ce lls, and immune cells. The mucosa is in immediate contact with the intestin al contents so that nutrients can be efficiently absorbed, and, at the same time, it protects against the intrusion of harmful entities, such as toxin s and bacteria, that may enter the digestive system with food. Signals are sent locally to control motility, secretion, tissue defense, and vascular p erfusion; to other digestive organs, for example, to the stomach, gallbladd er, and pancreas; and to the central nervous system, for example to influen ce feeding behavior. The three detecting systems in the intestine are more extensive than those of any other organ: the enteric nervous system contain s on the order of 10(8) neurons, the gastroenteropancreatic endocrine syste m uses more than 20 identified hormones, and the gut immune system has 70-8 0% Of the body's immune cells. The gastrointestinal tract has an integrated response to changes in its luminal contents, When this response is maladju sted or is overwhelmed, the consequences can be severe, as in cholera intox ication, or debilitating, as in irritable bowel syndrome. Thus it is essent ial to obtain a full understanding of the sensory functions of the intestin e, of how the body reacts to the information, and of how neural, hormonal, and immune signals interact.