Thanksgiving may well be the year's biggest bonanza for your brain — all of them. This famous feast doesn't just satisfy the survival instinct of your rudimentary reptilian brainstem. The gathering of family and friends also serves up the emotional interaction craved by your mammalian limbic brain.
Celebrating this historic holy day does more than fulfill the spiritual longing of your time traveling cerebral cortex, that human thinking cap and vault of heaven where you envision and give thanks-be to a higher power. The thanksgiving meal is also is a sensuous source of activity for another one of your brains, the one in your gut known as the "enteric nervous system."
Derived from the Greek word for within (and opposite of dysentery), your enteric nervous system is located throughout the lining of your gastrointestinal tract. This highly organized network of nerve cells is now recognized as a complex integrative brain in its own right, with a hundred-million neurons (more than the spinal cord has).
Like the central nervous system, the enteric nervous system has sensory and motor neurons, glial cells, and information processing circuits. The intricate circuitry in this remarkable brain enables it to perceive and respond, learn and remember, even to produce feelings — although mostly bad ones, like when you eat too much at thanksgiving.
Please Pass the Tryptophan
Thanksgiving has another reputation: the classic consequences of contentment and sleepiness. Turkey as well as the nuts in the stuffing and many of the other foods served are high in tryptophan, the essential amino acid you need to make serotonin. Serotonin is the inhibitory neurotransmitter important to the maintenance of good mood. It promotes calmness and is responsible for normal sleep. (Prozac blocks the reuptake of serotonin.)
Serotonin is needed to counterbalance the excitatory neurotransmitters, dopamine and norepinephrine, and has a role in regulating eating behavior and in producing satiety after eating. Cells located in several brainstem clusters called the "raphe nuclei" synthesize serotonin from tryptophan in the presence of vitamins B1, B3, B6, and folic acid — which your thanksgiving meal should provide plenty of.
This is Your Brain on Dessert
Although turkey is high in tryptophan, that's not why you might feel sleepy after a large thanksgiving dinner. Tryptophan must compete with the other amino acids in a high-protein meal to get into your brain, and tryptophan has a lower priority. But, because the thanksgiving meal is also loaded with carbohydrates and sugars (did somebody say pie?), your blood glucose levels rise, causing insulin to be released. This tends to pull all the amino acids out of your bloodstream, except tryptophan, which can then cross your blood-brain barrier and help synthesize serotonin so you can conk out on the couch.
Pain the the Gut
The enteric nervous system (ENS) uses the same major neurotransmitters as the central nervous system, including acetylcholine, norepinephrine, dopamine, and serotonin. That's why drugs that target the brain can cause severe reactions in the gastrointestinal tract. When doctors attribute nervous disorders of the stomach and intestines to psychological problems in the brain, they are only partly right. These problems don't originate with the cranial brain, but with the abdominal brain.
Symptoms emanating from the two brains often get confused. "Just as the brain can upset the gut, the gut can also upset the brain. If you were chained to the toilet with cramps, you'd be upset, too," says Michael D. Gershon, M.D., author of the 1998 book, The Second Brain.
Dr. Gershon is chairman of the Department of Anatomy and Cell Biology at Columbia University's College of Physicians and Surgeons. His thirty years of pioneering research into the enteric nervous system has ushered in the new medical field of neurogastroenterology, which promises rapid advances in the management of functional bowel disease. "Although we still are unable to relate complex behaviors such as gut motility and secretion to the activity of individual neurons, work in that area is proceeding briskly." (Hosp Pract, July 1999)
Internal Vigilance
The idea of a brain in the gut is not new. In his 1907 book, The Abdominal and Pelvic Brain, a respected physician and researcher named Byron Robinson wrote:
"In the abdomen there exists a brain of wonderful power maintaining eternal, restless vigilance over its viscera. It presides over organic life. . . . The abdominal brain is a receiver, a reorganizer, an emitter of nerve forces. It has the power of a brain. . . . The abdominal brain is not a mere agent of the [cerebral] brain and cord; it receives and generates nerve forces itself; it presides over nutrition. It is the center of life itself. . . . The abdominal brain can live without the cranial brain, which is demonstrated by living children being born without cerebrospinal axis. On the contrary, the cranial brain cannot live without the abdominal brain."
Surely You Digest
Acting independently of your cranial brain, the ENS coordinates behavior in your esophagus, stomach, small intestine, and colon — with particular emphasis on mixing food with digestive enzymes and propelling it through the intestine. The ENS helps control the absorption of nutrients into your bloodstream and protects against the possible intrusion of harmful bacteria and other toxins that enter the digestive system with food.
The ENS also has a major role in protecting you from external threats. If the danger is minor, your ENS may just shut down your digestive system. If fight or flight is imminent, it can rapidly empty your digestive tract from whichever end is more convenient — by tossing your cookies or by scaring you **itless.
Serious Protective Work
When your head alerts your gut to danger, histamine is released from mast cells in the lining of the small intestine and colon. This triggers a protective inflammatory response that attracts immune cells from the bloodstream into the area. Your body is then ready for trauma, such as an animal bite (evolutionarily speaking) that would introduce infectious material into your colon. You now have a better chance of controlling the infection and surviving the bite — or the knife or bullet (contemporarily speaking).
Master of its Own Domain
Dr. David Wingate, a professor of gastrointestinal science at the University of London, sees the logic of having two brains:
"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. 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." (NY Times, Jan. 23, 1996)
Therefore, the ENS was not replaced. It was connected. During fetal development, a clump of tissue called the "neural crest" is formed and then divides. Part of it becomes your central nervous system and another migrates south to form your enteric nervous system.
These two nervous systems are then later connected by your vagus nerve, which is composed of only a few thousand nerve fibers. Because your gut brain is on-location and master of its own domain, it is able to take second-to-second control without the need for a thick cable linking it to your head.
Your Inner Skin
The coordinated intelligence of your enteric nervous system is not only essential for digestion (hence survival), but for the quality of life itself — as anyone can testify who has ever suffered from irritable bowel syndrome or simply from indigestion or cramps, diarrhea or constipation.
Your gastrointestinal tract is basically a sensory organ that has a profound impact on your sense of well being — not unlike your skin. It evaluates situations based on sensory input, then decides how the digestive tract will respond in order to best ensure your survival.
Like your skin: when all is well, you don't even know it's there. But when it hurts, you hurt.
Professor J.B. Furness and his fellow researchers at the University of Melbourne describe the lining of the gastrointestinal tract as "the largest vulnerable surface that faces the external environment. Just as the other large external surface, the skin, is regarded as a sensory organ, so should the intestinal mucosa." He elaborates:
"The three detecting systems in the intestine are more extensive than those of any other organ: the enteric nervous system contains on the order of 10(8) neurons, the gastroenteropancreatic endocrine system uses more than 20 identified hormones, and the gut immune system has 70-80% of the body's immune cells. . . . Signals are sent locally to control motility, secretion, tissue defense, and vascular perfusion; to other digestive organs, for example, to the stomach, gallbladder, and pancreas; and to the central nervous system, for example to influence feeding behavior." (Am J Physiol, Nov. 1999)
The Plexus at the Nexus
It's comforting to know we have such a smart lining to that complex tube that tunnels all the way through our body. When one considers that nutrient digestion is our fundamental interface with the environment — and that nutrition is the basic evolutionary driving force that shapes our development as a species and our growth as an individual — then the role of the enteric nervous system becomes paramount to our existence and our future.
This Thanksgiving Day, when tryptophan is coursing through your veins and brains, and you are letting kith and kin, God and country, know how much you appreciate them, also take a moment to give your tummy a warm rub of appreciation. It likes that.
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