Guts

Just last Sunday, I talked about the hara (腹), the "center of being" just below the stomach and in the general vicinity of the diaphragm.  Coincidentally, today I come across a very interesting article in New Scientist by Emma Young about the so-called "other brain," the one that resides in the gut.

The human body includes a separate nervous system from that of the brain, one that is around 30 feet long and stretches from the esophagus to the anus.  This "other brain," the enteric nervous system (ENS), consists of a widely distributed network of neurons spread throughout two layers of gut tissue. It is part of the autonomic nervous system, the network of peripheral nerves that control visceral functions.  The "other brain" shares many features with the first. It contains an estimated 500 million neurons - about five times as many as in the brain of a rat - and also has its own version of a blood-brain barrier to keep its physiological environment stable.

The ENS is actually the original nervous system, emerging in the first vertebrates over 500 million years ago and becoming more complex as vertebrates evolved - possibly even giving rise to the brain itself.  Biologist Richard Dawkins points out that nervous systems have generally evolved as a long trunk cable running from front to rear, either along the dorsal side, like our spinal cord, or along the ventral side, in which case it is often double with a ladder of connections between the left and right sides, as in worms and arthropods.  "In a typical Earth creature," Dawkins writes, "the main longitudinal trunk cable has side nerves, often paired in segments repeated serially from front to rear.  And it usually has ganglia, local swellings which, when sufficiently large, are dignified with the name of brain."

It seems that we are blessed with both options, a dorsal nervous system crowned with a ganglia of epic proportions, and the ENS, a complex system of nerves and neurons, but without a ganglia worthy of the title "brain."  It is almost as if we have evolved to consist of an amalgam of two organisms, a large-brained biped that carries around a second, worm-like organism in its abdomen.  We can only wonder which organism in this symbiotic relationship is the chicken and which is the egg - that is, are we bipeds carrying around a helpful sort of worm, or are we worms that managed to get a helpful biped to carry us around?

Without the gut, there would be no energy to sustain life.  Its vitality and healthy functioning is so critical that the brain needs to have a direct and intimate connection with the gut.  The main connection between the gut and the brain is the vagus nerve.  However, about 90 per cent of the signals passing along the vagus nerve come not from the brain, but from the ENS.  If the vagus is severed, the ENS remains quite capable of coordinating digestion on its own.

The ENS is capable not only of autonomy, but it can also influence the brain.  It produces a wide range of hormones and around 40 neurotransmitters of the same classes as those found in the brain. In fact, neurons in the gut are thought to generate as much dopamine as those in the head. Intriguingly, about 95 per cent of the serotonin present in the body at any time is in the ENS.

What are neurotransmitters doing in the gut? In the brain, dopamine is a signalling molecule associated with pleasure and the reward system. It acts as a signalling molecule in the gut too, transmitting messages between neurons that coordinate, for example, the contraction of muscles in the colon.  Serotonin - best known as the "feel-good" molecule involved in preventing depression and regulating sleep, appetite and body temperature - also transmits signals  in the ENS.  Serotonin produced in the gut gets into the blood, where it is involved in repairing damaged cells in the liver and lungs.  It is also important for normal development of the heart, as well as regulating bone density by inhibiting bone formation.

The feeling of "butterflies" we sometimes experience in the stomach is the result of blood being diverted away to muscles as part of the fight-or-flight response instigated by the brain. However, stress also leads the gut to increase its production of ghrelin, a hormone that, as well as making one feel more hungry, reduces anxiety and depression. Ghrelin stimulates the release of dopamine in the brain both directly, by triggering neurons involved in pleasure and reward pathways, and indirectly by signals transmitted along the vagus nerve.

In Parkinson's disease, the problems with movement and muscle control are caused by a loss of dopamine-producing cells in the brain. However, the protein clumps that do the damage, called Lewy bodies, also show up in dopamine-producing neurons in the gut. In fact, judging by the distribution of Lewy bodies in people who have died from Parkinson's, the disease may actually start in the gut as the result of an environmental trigger such as a virus, and then spreads to the brain via the vagus nerve.

Likewise, the characteristic plaques or tangles found in the brains of people with Alzheimer's are present in neurons in their guts too. And people with autism are prone to gastrointestinal problems, which are thought to be caused by the same genetic mutation that affects neurons in the brain.

Although scientists are only just beginning to understand the interactions between the two brains, the gut already offers a window into the pathology of the brain.