The gut has a mind of its own

Operating like the cranial brain and looking uncannily similar to it, the gut brain is continuously active, whether we're aware of it or not, writes CHANTAL OUIMET

Ever wonder why you get cramps when you're stressed? Or why you get "butterflies" in your stomach before a job interview? And why your gut tells you not to trust a certain person?

Scientists say it's because the body has two brains -- the familiar one encased in our skull and another more obscure one in our gut.

This "second brain," known as the enteric nervous system, is located in our digestive tract and holds about 100-million nerve cells -- more than in our spinal cord.

Less complex and smaller than our cranial brain, this "second brain," which contains between 70 to 85 per cent of the body's immune cells, is an independent data-processing centre handling a complicated circuitry of neurons, neuromodulators and neurotransmitters.

"Every neurotransmitter that exists in our brain, also exists in the gut without exception. The brain in the gut is simply the brain gone south," says Dr. Michael Gershon.

In 1899, anatomists and physiologists studying dogs found that, unlike any other reflex, the continuous push of material through the digestive system continued after nerves linking the brain to the intestines were severed. In other words, they discovered the gut had a mind of its own.

Operating like our brain and looking uncannily similar to it, the gut brain responds to stimulus and is continuously active whether we're aware of it or not. But it doesn't think or feel. Feeling is held in the cerebral cortex of the brain. This "second brain" performs a different role.

"The brain in the head deals with the finer things in life: religion, philosophy, appreciation of art and music, creativity, etc.," says Dr. Gershon. "Whereas the brain in the gut deals with this dirty, messy and disgusting business of digestion. The brain in the head doesn't have to get its hands dirty with that kind of thing since it has delegated the job."

They may have different roles but our two brains are interconnected. One thousand to 2,000 nerve fibres connect them and enable the two to talk. When one gets upset, the other one does too.

"I don't think we could have made that statement a few years ago. . . . We've been finding out that the nerves in the gut independently regulate gut function, but do so in a dialogue with the nerves in our head. It's a nerve-to-nerve discussion," says Keith Sharkey, physiology and biophysics professor at the University of Calgary.

Interest in the gut brain resurfaced in the early 1980s after new technology became available.

"For the first time, we were able to see in elegant and exquisite detail the specific way that nerves went from A to B," explains Prof. Sharkey. "That gives you a chance to ask questions that could not or had not been asked before."

There are approximately 250 research laboratories now studying the enteric nervous system around the world. This new breed of neuroscientists is not only fuelling the present renaissance in the field of neurogastroenterology (study of the nerves entrenched in the lining of the esophagus, stomach, small intestine and colon), but offering insights into malfunctions of both brains.

Scientists have discovered that the gut brain may be involved in gastrointestinal disorders like ulcerative colitis, Crohn's disease and irritable bowel syndrome -- a condition that affects between 15 and 20 per cent of the population.

"Back in the days when . . . I was a medical student, I was taught that these diseases were psychosomatic. . . . But they're a real thing. Your gut can literately drive your brain crazy," notes Dr. Gershon. "If you are walking around with a burning sensation in your upper belly and it feels terrible, you can get pretty anxious. Likewise, if you've got aches and you're on the toilet with diarrhea every five minutes, it can change your personality. But it's more than that."

For many years, individuals (mostly women) with irritable bowel syndrome, a functional disorder characterized by abdominal pain, bloating, flatulence, diarrhea and/or constipation, suffered in silence. Doctors believed the illness was imagined -- all in the head.

"We now know IBS is not psychosomatic. There is an element of the brain controlling the gut which has to be born in the mind. But we now understand that there is also an organic and physiological basis for the functional changes in the little brain," declares Prof. Sharkey.

Dr. Nicholas Diamant, a gastroenterologist at Toronto Western Hospital and an emeritus professor of medicine and physiology at the University of Toronto, agrees that both brains are involved in the disorder.

"The brain sends signals down to the little brain via the spinal cord which acts as a gate for the pain signals," Dr. Diamant says. "The brain may not be closing this gate adequately to modulate and regulate the signals coming up from the gut. Therefore, the brain is letting more signals come up than it normally would."

A study by the Mayo Foundation published this August in Gastroenterology, the official journal of the American Gastroenterological Association, suggested there is genetic determinant that predicts the response to medication of IBS patients with diarrhea-predominant symptoms.

"It has to do with how the body inactivates the [neuro]transmitter serotonin. In some patients, the body inactivation is more efficient and the patient therefore responds better to the medication," explains Dr. Michael Carmilleri, professor of medicine and physiology at the Mayo Clinic in Rochester and one of the authors of the report.

"It's a landmark paper. . . . We are starting to relate changes in the signalling in the 'second brain' to real diseases based not only on functional changes but on genetic studies as well," says Prof. Sharkey.

This July, a drug came on the Canadian market to treat patients (women only) with constipation-predominant IBS. Experts say Tegaserod, known commercially as Zelnorm, is only effective in 60 to 65 per cent of people.

"IBS is defined by a series of symptoms," says Prof. Sharkey. "It's a multiple disease entity. . . . We don't understand it well enough to ever consider a miracle-type cure because it's too complicated for that."

In the case of Crohn's disease and ulcerative colitis (both autoimmune diseases), Dr. Carmilleri says the gut brain may play a role. "There is some interaction between the immune cells in the intestines and the 'second brain.' "

The discovery of the brain-gut connection also allowed scientists to learn what is at the heart of the most visceral human emotions. A gut feeling, for example, isn't just a poetic image used to convey intuition. It arises from the interplay between our two brains.

"It's a "body loop" which is activated every time we are being challenged or stressed. From a lifetime of activating this "body loop" during good or bad situations, we learn to interpret this preverbal feedback as good or bad," says Emeran Mayer, professor of medicine and physiology at the University of California -- Los Angeles.

Butterflies are minor indicators of pain and another example of this close relationship. Prof. Mayer says when one is faced with an anxiety-ridden situation, the brain in our skull sends urgent messages to our "second brain" and throws it off balance. "The big brain also becomes more sensitive to signals from the gut and amplifies them to unpleasant conscious sensations," he wrote. Therefore, one reads this response as gurgling or "butterflies" in the belly.

Stomach cramps, heartburn, diarrhea or constipation due to stress are again an illustration of the gut rising to the level of conscious perception. "The dialogue between the brain in our head and the brain in our gut sometimes goes awry. . . . in such a way that the brain in our gut responds inappropriately to stimulus," says Prof. Sharkey. In turn, the nerves tell the muscles to contract more or less or make the glands secrete more or less fluid.

Not all of the signals sent from the "second brain" to the cranial brain are bad news. "Some of the information that is being sent from the gut to the brain can establish how well the brain in the head works. . . . Your gut doesn't think for you but if it's behaving well, it can contribute to your mood being good," says Dr. Gershon. As a result, this interaction plays a role in dictating behavior and in creating human joy as well as sadness.

Scientists affirm the brain constantly communicates and listens to our "second brain." Its functions are then taken over by the brain with respect to the activation of major emotions such as fear, anxiety, anger, sadness or happiness. There is no direct proof but a lot of suggestive evidence.

"Chemicals released from cells within the 'second brain' . . . can activate vagal sensory neurons (cells high in the neck) which signal back to the brain. These vagal sensory neurons play a prominent role in many emotional processes and it is certainly conceivable that such signals play a role in generating happiness or a sense of well-being. The newborn gets its first sense of well-being from stimulation of the gut and release of chemicals through milk," wrote Prof. Mayer.

Dr. Diamant, a specialist in the mind-body connection, says each person's "second brain" reacts differently. "If you think everybody feels the same thing when the gut does something, you are in deep trouble. You have to consider the whole person and all the baggage they carry. The gut's reaction may be based on many experiences as well as the individual's genetic makeup."

It's an exciting time for scientists because research in the field of neurogastroenterology is still evolving. Even medical students are generally shocked by the "second brain's" complexity. "That hasn't really made it to the textbooks yet," says Prof. Sharkey. "It takes a few years for emerging knowledge to become dogma."

Updated July 2003