[Paleopsych] NYT: The Other Brain Also Deals With Many Woes

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The Other Brain Also Deals With Many Woes


    Two brains are better than one. At least that is the rationale for the
    close - sometimes too close - relationship between the human body's
    two brains, the one at the top of the spinal cord and the hidden but
    powerful brain in the gut known as the enteric nervous system.

    For Dr. Michael D. Gershon, the author of "The Second Brain" and the
    chairman of the department of anatomy and cell biology at Columbia,
    the connection between the two can be unpleasantly clear. "Every time
    I call the National Institutes of Health to check on a grant
    proposal," Dr. Gershon said, "I become painfully aware of the
    influence the brain has on the gut."

    In fact, anyone who has ever felt butterflies in the stomach before
    giving a speech, a gut feeling that flies in the face of fact or a
    bout of intestinal urgency the night before an examination has
    experienced the actions of the dual nervous systems.

    The connection between the brains lies at the heart of many woes,
    physical and psychiatric. Ailments like anxiety, [4]depression,
    irritable bowel syndrome, ulcers and Parkinson's disease manifest
    symptoms at the brain and the gut level.

    "The majority of patients with anxiety and depression will also have
    alterations of their GI function," said Dr. Emeran Mayer, professor of
    medicine, physiology and psychiatry at the University of California,
    Los Angeles.

    A study in 1902 showed changes in the movement of food through the
    gastrointestinal tract in cats confronted by growling dogs.

    One system's symptoms - and cures - may affect the other.
    Antidepressants, for example, cause gastric distress in up to a
    quarter of the people who take them. Butterflies in the stomach are
    caused by a surge of stress [5]hormones released by the body in a
    "fight or flight" situation. Stress can also overstimulate nerves in
    the esophagus, causing a feeling of choking.

    Dr. Gershon, who coined the term "second brain" in 1996, is one of a
    number of researchers who are studying brain-gut connections in the
    relatively new field of neurogastroenterology. New understandings of
    the way the second brain works, and the interactions between the two,
    are helping to treat disorders like constipation, ulcers and
    Hirschprung's disease.

    The role of the enteric nervous system is to manage every aspect of
    digestion, from the esophagus to the stomach, small intestine and
    colon. The second brain, or little brain, accomplishes all that with
    the same tools as the big brain, a sophisticated nearly self-contained
    network of neural circuitry, neurotransmitters and proteins.

    The independence is a function of the enteric nervous system's

    "Rather than Mother Nature's trying to pack 100 million neurons
    someplace in the brain or spinal cord and then sending long
    connections to the GI tract, the circuitry is right next to the
    systems that require control," said Jackie D. Wood, professor of
    physiology, cell biology and internal medicine at Ohio State.

    Two brains may seem like the stuff of science fiction, but they make
    literal and evolutionary sense.

    "What brains do is control behavior," Dr. Wood said. "The brain in
    your gut has stored within its neural networks a variety of behavioral
    programs, like a library. The digestive state determines which program
    your gut calls up from its library and runs."

    When someone skips lunch, the gut is more or less silent. Eat a
    pastrami sandwich, and contractions all along the small intestines mix
    the food with enzymes and move it toward the lining for absorption to
    begin. If the pastrami is rotten, reverse contractions will force it -
    and everything else in the gut - into the stomach and back out through
    the esophagus at high speed.

    In each situation, the gut must assess conditions, decide on a course
    of action and initiate a reflex.

    "The gut monitors pressure," Dr. Gershon said. "It monitors the
    progress of digestion. It detects nutrients, and it measures acid and
    salts. It's a little chemical lab."

    The enteric system does all this on its own, with little help from the
    central nervous system.

    The enteric nervous system was first described in 1921 by Dr. J. N.
    Langley, a British physician who believed that it was one of three
    parts - along with the parasympathetic and sympathetic nervous systems
    - of the autonomic nervous system, which controls involuntary
    behaviors like breathing and circulation. In this triad, the enteric
    nervous system was seen as something of a tag-along to the other two.

    After Langley died, scientists more or less forgot about the enteric
    nervous system. Years later, when Dr. Gershon reintroduced the concept
    and suggested that the gut might use some of the same
    neurotransmitters as the brain, his theory was widely ridiculed.

    "It was like saying that New York taxi drivers never miss a showing of
    'Tosca' at the Met," he recalled.

    By the early 80's, scientists had accepted the idea of the enteric
    nervous system and the role of neurotransmitters like serotonin in the

    It is no surprise that there is a direct relationship between
    emotional stress and physical distress. "Clinicians are finally
    acknowledging that a lot of dysfunction in GI disorders involves
    changes in the central nervous system," said Gary M. Mawe, a professor
    of anatomy and neurobiology at the University of Vermont.

    The big question is which comes first, physiology or psychology?

    The enteric and central nervous systems use the same hardware, as it
    were, to run two very different programs. Serotonin, for instance, is
    crucial to feelings of well-being. Hence the success of the
    antidepressants known as S.S.R.I.'s that raise the level of serotonin
    available to the brain.

    But 95 percent of the body's serotonin is housed in the gut, where it
    acts as a neurotransmitter and a signaling mechanism. The digestive
    process begins when a specialized cell, an enterochromaffin, squirts
    serotonin into the wall of the gut, which has at least seven types of
    serotonin receptors. The receptors, in turn, communicate with nerve
    cells to start digestive enzymes flowing or to start things moving
    through the intestines.

    Serotonin also acts as a go-between, keeping the brain in the skull up
    to date with what is happening in the brain below. Such communication
    is mostly one way, with 90 percent traveling from the gut to the head.

    Many of those messages are unpleasant, and serotonin is involved in
    sending them. Chemotherapy drugs like doxorubicin, which is used to
    treat breast [6]cancer, cause serotonin to be released in the gut,
    leading to nausea and vomiting. "The gut is not an organ from which
    you wish to receive frequent progress reports," Dr. Gershon said.

    Serotonin is also implicated in one of the most debilitating gut
    disorders, irritable bowel syndrome, or I.B.S., which causes abdominal
    pain and cramping, bloating and, in some patients, alternating
    diarrhea and constipation.

    "You can run any test you want on people with I.B.S., and their GI
    tracts look essentially normal," Dr. Mawe said. The default assumption
    has been that the syndrome is a psychosomatic disease.

    But it turns out that irritable bowel syndrome, like depression, is at
    least in part a function of changes in the serotonin system. In this
    case, it is too much serotonin rather than too little.

    In a healthy person, after serotonin is released into the gut and
    initiates an intestinal reflex, it is whisked out of the bowel by a
    molecule known as the serotonin transporter, or SERT, found in the
    cells that line the gut wall.

    People with irritable bowel syndrome do not have enough SERT, so they
    wind up with too much serotonin floating around, causing diarrhea.

    The excess serotonin then overwhelms the receptors in the gut,
    shutting them down and causing constipation.

    When Dr. Gershon, whose work has been supported by Novartis, studied
    mice without SERT, he found that they developed a condition very much
    like I.B.S. in humans.

    Several new serotonin-based drugs - intestinal antidepressants, in a
    way - have brought hope for those with chronic gut disorders.

    Another mechanism that lends credence to physiology as the source of
    intestinal dysfunctions is the system of mast cells in the gut that
    have an important role in immune response.

    "During stress, trauma or 'fight or flight' reactions, the barrier
    between the lumen, the interior of the gut where food is digested, and
    the rest of the bowel could be broken, and bad stuff could get
    across," Dr. Wood said. "So the big brain calls in more immune
    surveillance at the gut wall by activating mast cells."

    These mast cells release histamines and other inflammatory agents,
    mobilizing the enteric nervous system to expel the perceived
    intruders, and causing diarrhea.

    Inflammation induced by mast cells may turn out to be crucial in
    understanding and treating GI disorders. Inflamed tissue becomes
    tender. A gut under stress, with chronic mast cell production and
    consequent inflammation, may become tender, as well.

    In animals, Dr. Mawe said, inflammation makes the sensory neurons in
    the gut fire more often, causing a kind of sensory hyperactivity. "I
    have a theory that some chronic disorders may be caused by something
    like attention deficit disorder in the gut," he said.

    Dr. Gershon, too, theorizes that physiology is the original culprit in
    brain-gut dysfunctions. "We have identified molecular defects in the
    gut of everyone who has irritable bowel syndrome," he said. "If you
    were chained by bloody diarrhea to a toilet seat, you, too, might be

    Still, psychology clearly plays a role. Recent studies suggest that
    stress, especially early in life, can cause chronic GI diseases, at
    least in animals. "If you put a rat on top of a little platform
    surrounded by water, which is very stressful for a rat, it develops
    the equivalent of diarrhea," Dr. Mayer said.

    Another experiment showed that when young rats were separated from
    their mothers, the layer of cells that line the gut, the same barrier
    that is strengthened by mast cells during stress, weakened and became
    more permeable, allowing bacteria from the intestine to pass through
    the bowel walls and stimulate immune cells.

    "In rats, it's an adaptive response," Dr. Mayer said. "If they're born
    into a stressful, hostile environment, nature programs them to be more
    vigilant and stress responsive in their future life."

    He said up to 70 percent of the patients he treats for chronic gut
    disorders had experienced early childhood traumas like parents'
    divorces, chronic illnesses or parents' deaths. "I think that what
    happens in early life, along with an individual's genetic background,
    programs how a person will respond to stress for the rest of his or
    her life," he said.

    Either way, what is good for one brain is often good for the other,
    too. A team of researchers from Penn State University recently
    discovered a possible new direction in treating intestinal disorders,
    biofeedback for the brain in the gut.

    In an experiment published in a recent issue of Neurogastroenterology
    and Motility, Robert M. Stern, a professor of psychology at Penn
    State, found that biofeedback helped people consciously increase and
    enhance their gastrointestinal activity. They used the brains in their
    heads, in other words, to help the brains in their guts, proving that
    at least some of the time two brains really are better than one.



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