Danger! Danger!
In normal operation, the enteric nervous system directs the muscles surrounding the small intestine to contract in an organized fashion. This moves food down the pipe and gives the mixing motion that digestive chemicals need to do their decomposing thing. The nervous system also tells glands in the intestines to secrete chemicals needed to make amino acids from Kentucky Fried Chicken.

But if you happen to see a swarm of killer bees -- or somebody trying to collect a serious debt -- the gut is going to get a stress signal from the brain through the vagus nerve connecting them.

spiny-plexus! A neuron from a guinea pig's small intestine, filled with a marker substance to make it easier to see. This type of neuron is responsible for the continued flow of information about conditions in the upper gut to the regions below, like the large intestine.
Image courtesy of The Autonomic Neuroscience Group. © Paul P. Bertrand.

The result of this signal will depend on the genetics of the individual enteric nervous system receiving it. In some people, the result is a command to stop the digestive behavior, and the gut will kind of freeze in place. In others, it becomes a command to "Let 'er rip," and a gush of diarrhea will head south. In the most unlucky among us, the nerves might mutter "barftime," leaving a gush of vomit to head north.

Like we said, this is a highly individual thing, but what's common, says Jackie Wood, an Ohio State University neurophysiologist who has studied the enteric nervous system since 1968, is the ability of the enteric nervous system to run set "programs," specifying certain actions such as cramp, run, or toss cookies. In this way, he says, the gut's brain resembles simple invertebrate (defined), including nervous systems, which also repeatedly run simple programs. Only difference: Invertebrate programs say things like "wingbeat," or "sniff around for food without looking any stupider than a cockroach's got to look."

Aside from its role in helping to convert food into available nutrients, the major role of the enteric nervous system seems to be protecting the body from external threats. Emptying the digestive system, by hook or by crook, as we just saw, is one way it prepares the body for fight or flight, both of which are easier on an empty stomach.

However, the enteric nervous system can be more subtle. It turns out that a danger signal from the brain in the skull causes the immune system to kick into gear and help protect the body from modern hazards, like switchblades and bad situation comedies (not).

Could you be more specific?
I'll try. When the brain signals danger, it tells so-called mast cells in the lining of the small intestine and/or colon to release histamine (defined) and other chemicals. These chemicals trigger an inflammatory response inside the small intestine, attracting immune cells from the bloodstream into the area. Voila, the body is ready for trauma, which often introduces dirty -- infectious -- material into the colon.

Believe it or not, a knife to the gut is serious business, since the contents of the digestive tract are full of bacteria that can be deadly in short order. If the body is prepared by the presence of large numbers of inflammatory cells called neutrophils, Wood says, it has a better chance of controlling the infection and surviving the stabbing.

Thus one ancestral function of the enteric nervous system was to protect eohippus, the primordial horse, against infection resulting from the bite of a hungry saber-toothed cat.

But when this inflammatory response gets out of whack, it can cause no end of problems -- things like irritable colon, a spasmodic condition that plagues its victims with diarrhea, gas and constipation; and ulcerative colitis, an inflammatory and disabling disease of the colon.

The enteric nervous system can explain other problems that are sometimes considered to be psychological in origin, like difficulty swallowing, ulcers and chronic abdominal pain.

Let's talk therapeutics
The point of understanding the enteric nervous system is not just to brag about how elegant evolution is, but to cure disease. Scientists have figured out that a substance called substance P triggers mast cells to release histamine, which in turn attracts immune cells. And substance P links to a receptor called NK1 receptor on the mast cells. Knowledge of this mechanism has prompted several pharmaceutical companies to try to devise drugs to block the chain of events that lead to ulcerative colitis.

Final question: Is this thing downstairs in the gut really a brain?
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