You Have Two Brains

As do I. It was described by Byron Robinson in The Abdominal and Pelvic Brain in 1907 and named the enteric nervous system (ENS) by Johannis Langley.1 About the same time it was found that the ENS can act autonomously. When its main connection with the brain, the vagus nerve, is severed the ENS still is capable of coordinating digestion. Interest in this gut brain dropped until the field of neurogastroenterology was born in the 1990’s. It has since been learned that about 90% of the signals passing along the vagus nerve come not from the brain above, but from the ENS.2

How do these two brains compare? Both have barriers restricting blood flows to their respective brains and are supported by glial cells. The first brain consists of about 85 billion neurons; the second brain has about 500,000 neurons. 100 neurotransmitters have been identified for the first brain; 40 neurotransmitters have been identified for the second brain. Each brain produces about half of the body’s dopamine. The first brain produces 5% of all serotonin. The second brain produces 95% of all serotonin. This final comparison is quite telling. Serotonin is best known as the “feel-good” molecule. It is involved in preventing depression and in regulating sleep, appetite, and body temperature. Serotonin produced in the gut gets into the bloodstream, where it plays a role in repairing damaged cells in the liver and lungs. Moreover, it is important for the normal development of the heart, as well as in the regulation of bone density by inhibiting bone formation.

Serotonin produced in the ENS affects mood by stimulating the vagus nerve. Research has shown that stimulation of the vagus nerve can be an effective treatment for chronic depression that has failed to respond to other treatments.3 These gut to brain signals via the vagus nerve might also explain why fatty foods make us feel good. Brain scans of volunteers given a dose of fatty acids directly into the gut had a lower response to pictures and music designed to make them feel sad that a control group given saline. The fatty acid group also reported being only about half as sad as the control group.4

Stress leads the gut to increase its production of ghrelin. Ghrehlin is a hormone that makes you feel hungrier as well as reducing anxiety and depression. It stimulates the release of dopamine in the brain both directly, by directly triggering pleasure and reward pathways, and indirectly by signals triggered via the vagus nerve. At one time during our evolutionary past, the stress-busting effect of ghrelin might have been useful, but today the result of chronic stress or depression can be chronically elevated ghrelin leading to obesity.

The second brain has also been implicated in a variety of first brain disorders. In Parkinson’s disease the problems with movement and muscle control are caused not only by loss of dopamine producing cells in the first brain, but also by dopamine producing cells in the second brain due to Lewy bodies. It is even suspected that the disease starts in the second brain as the result of some trigger such as a virus, and then spreads to the brain via the vagus nerve. Similarly the characteristic plaques and tangles found in the first brains of people with Alzheimer’s are present in their second brains also.

Cells in the second brain could be used as the basis for treatments. One experimental intervention for neurodegenerative diseases involves transplanting neural stem cells into the first brain to replenish lost neurons. Harvesting these cells from the brain or spinal cord is difficult. Neural stem cells have been found in the second brain of human adults.5 These cells could be harvested using a simple endoscopic gut biopsy. This could provide a ready source of neural stem cells. One research team is planning toed them to treat diseases including Parkinson’s.

1Young, E. (2012). Alimentary thinking. New Scientist, 15 December, 39-42.

2American Journal of Gastrointestinal and Liver Physiology, vol 283, p G217.

3The British Journal of Psychiatry, vol 189, p.282.

4The Journal of Clinical Investigation, vol 121, p. 3094.

5Cell Tissue Research, vol 344, p.217.

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