Think Twice: How the Gut's 'Second Brain' Influences Mood and Well-BeingScientific American: Technically known as the enteric nervous system, the second brain consists of sheaths of neurons embedded in the walls of the long tube of our gut, or alimentary canal, which measures about nine meters end to end from the esophagus to the anus.
The second brain contains some 100 million neurons, more than in either the spinal cord or the peripheral nervous system... Thus equipped with its own reflexes and senses, the second brain can control gut behavior independently of the brain...
'The system is way too complicated to have evolved only to make sure things move out of your colon,' says Emeran Mayer... For example, scientists were shocked to learn that about 90 percent of the fibers in the primary visceral nerve, the vagus, carry information from the gut to the brain and not the other way around...
'A big part of our emotions are probably influenced by the nerves in our gut,' Mayer says. Butterflies in the stomach -- signaling in the gut as part of our physiological stress response... is but one example. Although gastrointestinal (GI) turmoil can sour one's moods, everyday emotional well-being may rely on messages from the brain below to the brain above...
The enteric nervous system uses more than 30 neurotransmitters, just like the brain, and in fact 95 percent of the body's serotonin is found in the bowels... It's little wonder that meds meant to cause chemical changes in the mind often provoke GI issues as a side effect. Irritable bowel syndrome... also arises in part from too much serotonin in our entrails, and could perhaps be regarded as a 'mental illness' of the second brain...
Scientists are learning that the serotonin made by the enteric nervous system also play a role in more surprising diseases... 'It was totally unexpected that the gut would regulate bone mass to the extent that one could use this regulation to cure -- at least in rodents -- osteoporosis,' says Gerard Karsenty...
Serotonin seeping from the second brain might even play some part in autism... The same genes involved in synapse formation between neurons in the brain are involved in the alimentary synapse formation. 'If these genes are affected in autism,' says [Michael Gershon, author of the 1998 book, The Second Brain], 'it could explain why so many kids with autism have GI motor abnormalities.'... Cutting edge research is currently investigating how the second brain mediates the body's immune system response; after all, at least 70 percent of our immune system is aimed at the gut to expel and kill foreign invaders...
Mayer is doing work on how the trillions of bacteria in the gut 'communicate' with enteric nervous system cells (which they greatly outnumber). His work with the gut's nervous system has led him to think that in coming years psychiatry will need to expand to treat the second brain in addition to the one atop the shoulders... It may well behoove us all to pay more heed to our so-called 'gut feelings' in the future.
