Rediscovery of the Gut Brain: the Role of Serotonin
One of the more interesting aspects of Dr. Gershon’s work is that his discovery of the enteric nervous system was actually a rediscovery. One hundred years ago researchers had classified the nervous system of the gut as distinctly different from the rest of the peripheral nervous system.
In 19th century England two researchers Bayliss and Sterling, working with dogs, identified what they called the “law of the intestine” now known as peristaltic reflex. They showed that this reflex continued even when all connection to the central nervous system was eliminated. They concluded that since all reflex is associated with nervous system response that the gut must function with what they called a “local nervous system” of the gut.
It was also known that there was a significant quantity of nervous tissue in the gut. A German scientist, Auerbach, had identified a complex network of nerve cells and fibers (the myenteric plexus) wedged between the two layers of muscle that encircle the gut. Later there was an additional discovery of a smaller plexus in the submucosa (submucosa plexus) of the gut. Both of these discoveries were prior to the work of Bayliss and Sterling, thus they had reason to attribute their findings to a “local nervous system”
The groundbreaking aspect of these early discoveries was that the gut worked independently of any direction from the head brain/central nervous system. All organs have connection to the central nervous system and all but the gut cease to function when that connection is cut.
So what happened? Why was it necessary for Gershon and others to rediscover the gut brain? This is actually discussed in Gershon’s book The Second Brain. Essentially the description of the enteric nervous system, the gut brain that was contained in early texts literally disappeared as the neurotransmitter for first the parasympathetic nervous system, acetylcholine, and later the neurotransmitter for the sympathetic nervous system, norepinephrine, were identified.
This orderly system of two aspects of the peripheral nervous system with two identified neurotransmitters led to the gradual assumption that the gut brain was simply part of the parasympathetic nervous system. This of course was completely dismissive of what earlier researchers had discovered, that compared to the number of nerve cells in the myenteric plexus and the submucosa plexus (the tissues of the gut brain), the number of motor nerve fibers connecting the brain or spinal cord to the gut is incredibly small. It appears that there are some two thousand nerve fibers heading from the vagus nerve to the gut and yet there are over one hundred million nerve cells in the human small intestine. This aspect of the enteric nervous system was completely out of sync with the conditions that allowed for the classifications sympathetic and parasympathetic.
The advancements in understanding how the nervous system communicates with itself and the rest of the body provided the foundations on which Gershon would later identify serotonin as the principal neurotransmitter of the gut brain. His work on serotonin and its role in the gut brain lead to the rediscovery of the enteric nervous system and his rightful place as the father of neurogastroenterology.
In my next post we will begin to understand that serotonin is likely the link that combines IBS and migraine. It is clear that serotonin is playing a role in migraine as the triptan drugs provide relief by altering serotonin levels. It is also clear that selective serotonin reuptake inhibitors like Prozac can have an impact on depression and mood. And yet the gut brain research shows that some 95% of the body’s serotonin is made in the gut…95%! It is indeed the principal neurotransmitter of the gut brain and plays many roles.
Is there any evidence that other diseases that affect the head brain also have impact in the gut brain? The answer is a resounding yes and it is probably the part of Gershon’s book that literally had my mouth drop wide open. Doctors performing autopsies on Parkinson’s patients and Alzheimer’s patients find lesions in the brains of these patients indicative of each disease. Identical lesions are found in the tissues of the gut brain of these same patients. Nobody knows if these diseases are starting in the head brain and moving to the gut, starting in the gut and moving to the brain or starting both places at the same time.
This research is clearly showing that conditions previously considered to only affect the brain are also affecting the digestive system. It certainly opens the door of possibility that migraine and ibs both related to serotonin could be the same disease occurring in different locations. At the very least we have to be willing to recognize that if you have a problem in your head, you very well may have a problem in your gut.
Thanks for reading, I look forward to your comments and questions,
Tom
One of the more interesting aspects of Dr. Gershon’s work is that his discovery of the enteric nervous system was actually a rediscovery. One hundred years ago researchers had classified the nervous system of the gut as distinctly different from the rest of the peripheral nervous system.
In 19th century England two researchers Bayliss and Sterling, working with dogs, identified what they called the “law of the intestine” now known as peristaltic reflex. They showed that this reflex continued even when all connection to the central nervous system was eliminated. They concluded that since all reflex is associated with nervous system response that the gut must function with what they called a “local nervous system” of the gut.
It was also known that there was a significant quantity of nervous tissue in the gut. A German scientist, Auerbach, had identified a complex network of nerve cells and fibers (the myenteric plexus) wedged between the two layers of muscle that encircle the gut. Later there was an additional discovery of a smaller plexus in the submucosa (submucosa plexus) of the gut. Both of these discoveries were prior to the work of Bayliss and Sterling, thus they had reason to attribute their findings to a “local nervous system”
The groundbreaking aspect of these early discoveries was that the gut worked independently of any direction from the head brain/central nervous system. All organs have connection to the central nervous system and all but the gut cease to function when that connection is cut.
So what happened? Why was it necessary for Gershon and others to rediscover the gut brain? This is actually discussed in Gershon’s book The Second Brain. Essentially the description of the enteric nervous system, the gut brain that was contained in early texts literally disappeared as the neurotransmitter for first the parasympathetic nervous system, acetylcholine, and later the neurotransmitter for the sympathetic nervous system, norepinephrine, were identified.
This orderly system of two aspects of the peripheral nervous system with two identified neurotransmitters led to the gradual assumption that the gut brain was simply part of the parasympathetic nervous system. This of course was completely dismissive of what earlier researchers had discovered, that compared to the number of nerve cells in the myenteric plexus and the submucosa plexus (the tissues of the gut brain), the number of motor nerve fibers connecting the brain or spinal cord to the gut is incredibly small. It appears that there are some two thousand nerve fibers heading from the vagus nerve to the gut and yet there are over one hundred million nerve cells in the human small intestine. This aspect of the enteric nervous system was completely out of sync with the conditions that allowed for the classifications sympathetic and parasympathetic.
The advancements in understanding how the nervous system communicates with itself and the rest of the body provided the foundations on which Gershon would later identify serotonin as the principal neurotransmitter of the gut brain. His work on serotonin and its role in the gut brain lead to the rediscovery of the enteric nervous system and his rightful place as the father of neurogastroenterology.
In my next post we will begin to understand that serotonin is likely the link that combines IBS and migraine. It is clear that serotonin is playing a role in migraine as the triptan drugs provide relief by altering serotonin levels. It is also clear that selective serotonin reuptake inhibitors like Prozac can have an impact on depression and mood. And yet the gut brain research shows that some 95% of the body’s serotonin is made in the gut…95%! It is indeed the principal neurotransmitter of the gut brain and plays many roles.
Is there any evidence that other diseases that affect the head brain also have impact in the gut brain? The answer is a resounding yes and it is probably the part of Gershon’s book that literally had my mouth drop wide open. Doctors performing autopsies on Parkinson’s patients and Alzheimer’s patients find lesions in the brains of these patients indicative of each disease. Identical lesions are found in the tissues of the gut brain of these same patients. Nobody knows if these diseases are starting in the head brain and moving to the gut, starting in the gut and moving to the brain or starting both places at the same time.
This research is clearly showing that conditions previously considered to only affect the brain are also affecting the digestive system. It certainly opens the door of possibility that migraine and ibs both related to serotonin could be the same disease occurring in different locations. At the very least we have to be willing to recognize that if you have a problem in your head, you very well may have a problem in your gut.
Thanks for reading, I look forward to your comments and questions,
Tom
posted by Tom Stav at 8:11 AM
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