Jan 11 2007

Gut Reaction to the Second Brain

Dr. Michael Gershon, a neuroscientist at Columbia University, has written a book entitled the Second Brain, about his specialty – the enteric nervous system (the nervous system of the gut). Dr. Gershon is an excellent scientist, and I applaud his attempt to popularize his work (something that more scientists should do), but I must say I think his characterization of the enteric nervous system as a second brain is unfortunate and confusing.

First some background. The vertebrate nervous system is divided anatomically into the central nervous system (CNS – the brain and spinal cord) and the peripheral nervous system (PNS – everything outside the brain and spinal cord, mainly nerves, but also including some neurons – nerve cell bodies). Functionally the peripheral nervous system can be divided into the sensory system, the motor system, and the autonomic system. The central nervous system has these components as well, but also has many more systems and functions. The autonomic nervous system can be functionally further broken down into the parasympathetic system, the sympathetic system, and the enteric system.

The basic function of the autonomic nervous system is to control the function of the organs and glands of the body. For example, the autonomic system can speed up or slow down the heart, cause sweating, or salivation. The gastrointestinal (GI) system has a very robust autonomic nervous system, including a complex of neurons and nerves intrinsic to the organs of the GI system – the enteric nervous system. Networks of nerves in the periphery are often called a plexus, and the enteric nervous system has two – the myenteric plexus that controls smooth muscle activity and the submucous plexus that controls blood flow, secretions, and absorption.

Dr. Gershon’s point is that the enteric nervous system, in addition to being very large, is far more complex than was previously recognized, which is certainly true, but also generically true of all aspects of neurobiology. He also points out that this system uses many of the same neurotransmitters (chemicals neurons use to talk with each other) as does the central nervous system, and that it contains a complex network on interneurons. This means that a great deal of neuronal processing can occur within the enteric nervous system, independent of the CNS. He also points out that the brain can affect the enteric nervous system (as when emotional stress can cause GI symptoms) and likely the enteric nervous system can affect the brain.

All of this is accurate and very interesting – I just don’t think it adds up to labeling the enteric nervous system a “second brain.” I have read several blogs and articles, and have received many questions that seem to indicate this has caused confusion.

It does raise the interesting question of what makes a brain a “brain.” Mere complexity, I would argue, is not enough. If this were the case then the best candidate for a second brain would be the cerebellum. Although this is already considered part of the brain or the CNS, it is really an independent structure that contains more neurons and connections than the cortex, carries out complex processing, and has its own memory. But the cerebellum does not think. It has no independent awareness or consciousness. Its processing, although complex, is dedicated to a very focused task – that of coordinating motor movement.

The cortex, on the other hand, evolved to be aware and to carry out cognitive tasks – so-called higher functions. It is not merely complex, the type of processing, memory, and activity that occurs creates the epiphenomenon of consciousness. Admittedly this definition runs into problems at the primitive end of the spectrum (if consciousness is an emergent property of the brain, when did it emerge in evolution?). Are pre-conscious brains, brains? Does the cluster of ganglia at the center of the nervous system of an insect count as a brain? Perhaps we can think of a brain as a command center, regardless of how primitive it is.

Also, this continuum of complexity argues more against the “second brain” designation than for it. Dr. Gershon is simply calling the most complex cluster of neurons outside the brain a second brain, but what about the next most complex cluster down the line of complexity? Do we have a third and fourth brain? Any argument for the specialness of the enteric nervous system over other parts of the nervous system could also be used to argue for the specialness of the brain over the enteric nervous system, creating a logical dilemma.

The enteric nervous system is complex, but its complexity is focused, like the cerebellum. It regulates the function of the GI system – speeding up or slowing down contractions, secretions, blood flow, and absorption. It does integrate sensory input, and it does both react to hormones and secret hormones. But it is fairly diffusely distributed throughout the GI system. Further, there is no emergent property of the overall functioning of this system. It does not think.

Some of the similarities with the CNS are an indication only of evolutionary history, not current function. For example, the fact that the same neurotransmitters are used by the enteric nervous system as the brain is a result of the fact that all neurons likely share a common ancestor. The same neurotransmitters are also used in other parts of the peripheral nervous system. Acetycholine is used for memory function in the brain, and by nerves to signal the muscles to contract. So what?

A related misconception that is quite common is that large dinosaurs had second brains in their tails to help coordinate their back ends. Actually, what they had were ganglia, and they were placed at the base of the spine near the hips, not at the end of the tail. A ganglion is a cluster of neurons in the peripheral nervous system, and they give rise typically to either sensory or autonomic nerves (the neurons that give rise to motor nerves live in the spinal cord, not ganglia). Dinosaurs likely had motor ganglia to help coordinate their hind limbs, since otherwise the electrical signals would take too long to get from their brain all the way to their hind limbs.

I admit I am nitpicking. Dr. Gershon’s book is otherwise excellent and I am quibbling a bit about his characterizations. I further admit that there is no clean definition of “brain” and part of my problem with the term “second brain” is aesthetic. But I do think this issue highlights the need for scientists to be careful about how they communicate to the public. The phrase “second brain” makes for a catchy book title, and certainly has seemed to have its intended effect. But I think it generates misconceptions in the public’s understanding of neuroanatomy and function, rather than illuminating by analogy. Some articles on this topic, for example, have called irritable bowel syndrome a “mental illness of the gut.” The lay press is also all too happy to write pithy phrases about thinking with our guts. It also glosses over some of the most interesting concepts of neuroscience about how brain activity causes mental function, and how that is different from mere processing or relaying.

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