Expert: John Locke - 4/5/2007

Question
what is the link between digestive reactions and nerve impulses

Answer
Marcus:

Thank you for writing to AllExperts; I'll be happy to answer your question. The digestive and nervous systems are linked in a two-way fashion; that is, the act of eating can affect the nervous system, and the nervous system can affect hunger and other actions associated with the digestive system. Let's examine how the nervous system affects the digestive system, first.

The stomach is stimulated by the autonomic nervous system, specifically the parasympathetic branch (this is often called the "rest and digest" half of the nervous system because it promotes digestion instead of exertion; it's also part of the nervous system that you have no voluntary control over). The body will react to a fall in blood sugar--among other signals--by producing the feeling of hunger, which happens via the central nervous system. Stimulation of the stomach by the CNS causes the stomach to produce acid, which helps digestion once food is actually eaten. Beyond the stomach, the CNS helps control blood flow through the intestines, where the absorption of food actually occurs, and the elimination of wastes via the rectum. The nervous system is directly connected to almost all functions of the digestive system, which you can read about here in great detail:

http://www.lib.mcg.edu/edu/eshuphysio/program/section6/6ch2/6ch2line.htm

Please note that the website is from a medical school reference library, so its level of detail will be very high.

This whole process can also work in reverse, where the digestive system affects the nervous system. As the stomach stretches to take in food, it activates monitors on its surface. These send a signal back to the brain that promotes  the feeling of satiety, or fullness. Stretching the stomach beyond its normal capacity (about one liter) results in pain receptors being activated; not surprisingly, these promote the feeling of pain because of the excess stretching.

To put it briefly, a number of functions of the digestive system are controlled by the central nervous system, along with feelings like hunger that relate to digestion. The digestive system can directly stimulate the nervous system as well. The back-and-forth of these two regulates the human appetite, which (as you might have guessed by now) is not nearly as simple as it seems!