gut brain

[Guest guest]

It occurred to me that some folks may not be familiar with the gut brain and

its functions, so here's an article that summarizes some of what's known

about it.

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(from BioMedNet)

Gut Thoughts

by Maia Szalavitz

Though few know about it, humans have a second brain that handles most of

the body's digestive functions. Study of the enteric nervous system is a

rapidly growing specialty, offering insight into malfunctions of the " gut

brain " as well as the more complex cranial brain.

Digestion is such a prosaic function that most people prefer not to think

about it. Fortunately, they don't have to - at least not with the brain in

their heads. Though few know about it, humans (and other animals) have a

second brain that handles most digestive functions.

Deep in your gut lies a complex self-contained nervous system containing

more nerve cells than the spinal cord, and indeed more neurons than all the

rest of the peripheral nervous system. There are over 100 million nerve

cells in the human small intestine alone.

Malfunctions of this " gut brain " may be involved in irritable bowel syndrome

(IBS), a condition that affects an estimated 20 percent of the U.S.

population and is believed to be responsible for $8 billion in health care

costs alone in the United States each year, according to the International

Foundation for Functional Gastrointestinal Disorders. Patients with IBS

suffer bouts of chronic diarrhea, constipation, or sometimes both

alternately. IBS is the most common diagnosis made by gastroenterologists.

The study of the enteric nervous system is a rapidly growing specialty known

as neurogastroenterology.

" What the gut has to do is extremely complicated, " says Gershon,

chair of the department of anatomy and cell biology at the Columbia

University College of Physicians and Surgeons and author of The Second Brain

(Harper Perennial, 1999). " If the brain had to control that, it would have

to run huge cables and have a huge number of cells devoted solely to that

purpose. It makes great evolutionary sense to [separate these functions] and

essentially use a microcomputer that is independent rather than a central

processing unit. "

In fact, researchers believe that the gut brain evolved first - because

digestion came before locomotion in multicellular creatures. In mammals, the

two systems originate near each other in the outer layer of the early

embryo.

Like many poorly understood organs, the gut brain was discovered by

classical anatomists in the 19th century and then ignored. " No one knew what

it did, " says Wingate, emeritus professor of gastrointestinal science

at Queen , University of London. " When you'd ask what it was for in

medical school, they'd say, 'Let's move on.' "

In 1899, physiologists studying dogs found that unlike any other reflex, the

continuous push of material through the digestive system (now called the

peristaltic reflex) continued when nerves linking the brain to the

intestines were cut.

By the 1970s, a society for the study of gastrointestinal motility had been

set up - but how this motility was controlled remained unclear. The vagus

nerve, for example, sends some fibers from the brain to the gut; however, it

connects directly with only a tiny minority of cells there.

In 1965, Gershon published a paper in Science suggesting that serotonin

might act as a neurotransmitter in the gut. At the time, acetylcholine and

norepinephrine were accepted as transmitters in the peripheral nervous

system, but serotonin was seen as a centrally acting transmitter used by

some nerves to modulate the action of others.

The peripheral nervous system wasn't supposed to use such controls - only

the brain and spinal cord were believed to process information through

" interneurons " such as those containing serotonin.

At a meeting of the Society for Neuroscience in 1981, however, Gershon and

others marshaled enough data to finally convince skeptics that serotonin was

indeed a key transmitter in the gut.

In fact, it is now known that 95% of the body's serotonin is used by the

gut - and the enteric nervous system contains every neurotransmitter and

neuromodulator found so far in the brain.

" We now know quite a lot about the library of programs run by the [gut

brain], " says Jackie Wood, professor of physiology and cell biology and of

internal medicine at Ohio State University. " For example, when the bowel is

empty, one particular program runs. " Called the migrating motor complex

(MMC), this involves a series of movements running from the stomach to the

end of the small intestine, which is believed to function in keeping the

potentially dangerous bacteria stored in the colon from moving upwards

rather than out.

At least 500 different species of deadly bacteria have been found to inhabit

a person's colon at any given time; " traveler's diarrhea " often results when

this mix is changed through exposure to new pathogens. If this happens, the

gut runs a program designed to expel as much of its contents as quickly as

possible - unpleasant for the vacationer, but much better than a fatal

infection.

" Another program involves a flood of serotonin throughout the entire

circuit, which produces the digestive pattern that mixes and stirs the

contents, " says Wood.

Because the gut brain is smaller and more accessible than the brain itself,

understanding it could offer insights about how to parse the more complex

organ. " [That idea] was what lead me to begin my research when I was a

fledgling neuroscientist, " says Gershon. " I looked at the brain and found it

daunting, and I still do, so I looked for a simpler nervous system to

study. " He adds, " 'Simple nervous system,' of course, turned out to be an

oxymoron. "

Unlike the cranial brain, however, the gut brain doesn't seem to be

conscious - or at least, in health, it doesn't impinge much on

consciousness. " The gut is not an organ from which you like to receive

frequent progress reports, " says Gershon. For most digestive processes, no

news is good news.

The problem in IBS, in fact, may be that the enteric nervous system becomes

overly sensitive to normal functioning and reports to the brain when it

shouldn't. Or, the brain may overreact to normal bowel signals.

Normally, the brain may avoid conscious awareness of most gut activity. But

in IBS, says Wingate, one theory is that " the barrier to information being

projected into consciousness is lowered. "

As in many heterogeneous conditions defined by symptoms rather than specific

pathology, different subgroups of patients may have different causes or

varying levels of contributions by different factors.

In some cases, IBS may be an autoimmune problem - something like multiple

sclerosis of the gut, where immune cells attack nervous tissue. " If you

catch it early enough, " says Wood, " You can use steroids to treat it [in

such cases]. " High doses of steroids shut down immune activity and prevent

immune cells from causing harm, but they don't help once damage has been

done.

The gut is, in fact, a major immune organ, containing more immune cells than

the rest of the body combined. The enteric nervous system interacts

intimately with the immune system, and can affect mood and behavior by

signaling the central nervous system.

Further, the gut brain may in fact be the only system that can refuse

central signals. Says Gershon, " The gut brain can say no to the big brain,

absolutely. In fact, there are nerve fibers that project towards the CNS,

and if the [bowel] doesn't like the message, it can turn it off or cancel

it. "

Indeed, the vagus nerve mostly carries information from the enteric nervous

system to the brain - for every one message sent by the brain to the gut,

about nine are sent in the other direction. And recent research has found

that stimulating this nerve can have antidepressant and even

learning-enhancing effects - so " gut feelings " could genuinely be more than

just a metaphor.

The similarities between the two nervous systems may also mean that they are

vulnerable to similar toxins and disease processes. For example, in both

Parkinson's disease and Alzheimer's, the degenerative processes seen in

brain nerve cells are also seen in the neurons of the enteric system.

This link could also help explain the connection between psychological

problems and gut problems - and could put to rest the myth that problems

such as IBS are simply " neuroses " because they so often occur in people with

other psychological disorders.

It may be that the real reason that bowel disorders often accompany

psychological problems is that both brain and gut neurons are suffering

simultaneously - in addition to the fact that having to spend a significant

portion of one's life attending to bathroom functions is in itself

depressing.

Simultaneous effects of drugs on both systems also account for the

gastrointestinal " side effects " of Prozac and other drugs that act on

serotonin metabolism - which actually may have more effect on the bowel than

on the brain, because serotonin predominates in the bowel and the drug moves

through the digestive system before reaching the brain.

Fortunately, in most people, the bowel quickly develops tolerance to these

drugs, and gastrointestinal side effects usually subside within a few days

or weeks of the start of treatment. In fact, low doses of SSRI (selective

serotonin reuptake inhibitor) drugs may actually help patients with IBS. And

since different serotonin receptors predominate in the brain and in the gut,

new drugs may be developed to affect certain subtypes but not others.

" What's exciting, " says Wingate, " is getting away from essentially anecdotal

ways of categorizing patients by symptoms and being able to study [their

problems] in a very systematic biological way. "

Maia Szalavitz is a health/science journalist who has written for the New

York Times, the Washington Post, Newsday, New York Magazine, Salon, and

other major publications.

Suze Fisher

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