By Sandra Blakeslee
The New York Times
Ever wonder why people get “butterflies” in the stomach before going on stage? Or why an impending job interview can cause an attack of intestinal cramps? And why do antidepressants targeted for the brain cause nausea or abdominal upset in millions of people who take such drugs?
The reason for these common experiences, scientists says, is because each of us literally has two brains, – the familiar one encased in our skulls and a lesser-known but vitally important one found in the human gut. Like Siamese twins, the two brains are interconnected, when one gets upset, the other one does, too.
The gut’s brain, known as the enteric nervous system, is located in sheaths of tissue lining the esophagus, stomach, small intestine and colon. Considered a single entity, it is packed with neurons, neurotransmitters and proteins that zap messages between neurons, support cells like those found in the brain proper; and a complex circuitry that enables it to act independently, learn, remember and, as the saying goes, produce gut feelings.
The brain in the gut plays a major role in human happiness and misery. But few people know it exists, said Dr. Michael Gershon, a professor of anatomy an cell biology at Columbia Presbyterian Medical Center in New York. For years, people who had ulcers, problems swallowing or chronic abdominal pain were told that their problems were imaginary, emotional, simply all in their heads. Gershon said. They were shuttled to psychiatrists for treatment.
Doctors were right in ascribing these problems to the brain, Gershon said, but they blamed the wrong one. Many gastrointestinal disorders such as colitis, and irritable bowel syndrome originate from problems within the gut’s brain, he said. And the current wisdom is that most ulcers are caused by a bacterium, not by hidden anger at one’s mother.
Symptoms stemming from the two brains get confused, Gershon said. “Just as the brain can upset the gut, the gut can upset the brain. If you were chained to the toilet with cramps, you’d be upset, too,” he said. Details of how the enteric nervous system mirrors the central nervous system have been emerging in recent years, said Gershon, who is considered one of the founders of a new field of medicine called neurogastroenterology.
Nearly every substance that helps run and control the brain has turned up in the gut, Gershon said. Major neurotransmitters such as serotonin, dopamine, glutamate, norepinephrine and nitric oxide are there. Two dozen small brain proteins, called neuropeptides, are in the gut, as are major cells of the immune system. Enkephalins, one class of body’s natural opiates, are in the gut.
And in a finding that stumps researchers, the gut is a rich source of benzodiazepines – the family of psychoactive chemicals that includes such popular drugs as Valium and Xanax.
In evolutionary terms, it makes sense that the body has two brains, said Dr. David Wingate, a professor of gastrointestinal science at the University of London and a consultant at Royal London Hospital. The first nervous systems were in tubular animals that stuck to rocks an waited for food to pass by, Wingate said. The limbic system is often referred to as the “reptile brain”.
As life evolved, animals needed a more complex brain for finding food and sex and so developed a central nervous system. But the gut’s nervous system was too important to put inside the newborn head with long connections going down to the body, Wingate said. Offspring need to eat and digest food at birth.
Therefore, nature seems to have preserved the enteric nervous system as an independent circuit inside higher animals. It is only loosely connected to the central nervous system and can mostly function alone, without instructions from topside.
This is indeed the picture seen by developmental biologists. A clump of tissue called the neural crest forms early in embryogenesis, Gershon said. One section turns into the central nervous system. Another piece migrates to become the enteric nervous system. Only later are the two nervous systems connected via a cable called the vagus nerve.
Until relatively recently, people thought that the gut’s muscles and sensory nerves were wired directly to the brain and that the brain controlled the gut through two pathways that increased or decreased rates of activity, Wingate said. The gut was simply a disgusting tube with simple reflexes.
Trouble is, no one bothered to count the nerve fibers in the gut. When they did, he said, they were surprised to find, that the gut contains 100 million neurons – more than the spinal cord has. Yet the vagus nerve only sends a couple of thousand nerve fibers to the gut.
The brain sends signals to the gut by talking to a small number of “command neurons”, which in turn send signals to gut interneurons that carry messages up and down the pike, Gershon said. Both command neurons and interneurons are spread throughout two layers of gut tissue called the myenteric plexus and the submuscosal plexus. (“Solar plexus” is actually a boxing term that refers simply to nerves in the abdomen.)
Command neurons control the pattern of activity in the gut, Gershon said. The vagus nerve only turns the volume by changing its rates of firing.
Â Reprinted at:
The Denver Post, January 23, 1996