The Brain Gut Connection in IBS

Anatomical picture of the brain and gut
Getty Images/PIXOLOGICSTUDIO/Science Photo Library

Dysfunction in the connection between the brain and the gut may be a contributing factor in irritable bowel syndrome (IBS).

Some health problems are pretty simple to understand. If you have a sore throat, your doctor will take a tissue sample from your throat and run a test to see if you have a strep infection. A strange looking mole on your skin can be tested to see if it is cancerous. Unfortunately, IBS is far from simple.

Unlike diseases that are visible, to understand what is going wrong in IBS, researchers have found that they need to look beyond the gut and toward the complex communication systems that connect the gut with the brain. To truly appreciate the work that is being done in this area, you would need to have a degree in neuroscience. Even without such a degree, it is helpful to have some basic understanding of the complex connection between the brain and the gut and how this relates to IBS.

Biology Basics

See if any of the following discussions ring a bell from your time spent in high school biology class. Communication among all of the parts of our bodies occurs through the passing of information from nerve to nerve. Here is a simplified description of the various pathways along which this communication takes place:

  • Central nervous system (CNS) –- the brain and the spinal cord
  • Peripheral nervous system (PNS) – nerve pathways that extend beyond the brain and spinal cord.

    The peripheral nervous system is further divided into two parts:

    • Somatic nervous system –- responsible for voluntary control of muscles and reaction to external sensations.
    • Autonomic nervous system – responsible for the motor and sensation responses of our internal organs (viscera).

    Enteric Nervous System

    The enteric nervous system (ENS) is a part of the autonomic nervous system that is responsible for regulating the process of digestion.

    The ENS manages motility (movement of muscles), secretion of fluid and blood flow. The ENS handles so much responsibility on its own that it is sometimes given the name “the little brain.” Given this description, it is easy to see that understanding how the enteric system operates is essential to an understanding of what may be going wrong in a body with IBS.

    Up the Down Staircase

    Communication is a two-way street when it comes to the brain (central nervous system) and the digestive system (enteric nervous system). Complex pathways link the brain and the intestines with information flowing back and forth on a continual basis. This close connection is most clearly seen in our response to stress (perceived threat), which suggests that this complex communication network was very important for our survival as a species.

    Researchers are finding evidence that dysfunction along these up and down pathways may be contributing to the abdominal pain, constipation and/or diarrhea that are the symptoms of IBS.

    Nerves in the gut that are experiencing excessive sensitivity can trigger changes in the brain. Thoughts, feelings, and activation of parts of the brain that have to do with anxiety or arousal, can stimulate exaggerated gut responses. Malfunction may also be found along the many different pathways that connect the brain and gut. For instance, there is evidence that abnormal functioning along two separate pathways in the autonomic nervous system is associated with the symptom of diarrhea vs. the symptom of constipation. In general, it seems that dysfunction in the brain-gut communication system is interfering with the body’s ability to maintain homeostasis, a state in which all systems are working smoothly.

    The Role of Serotonin

    More biology: The means by which one nerve cell communicates with the next is through chemicals called neurotransmitters. An extremely important neurotransmitter for digestive functioning is serotonin (5-HT). It is estimated that up to 95% of the serotonin in the human body is found in the digestive tract. Serotonin is considered to be a vital part of the communication system between the brain and the gut.

    Serotonin seems to play a part in motility, sensitivity, and secretion of fluids. Movement, pain sensitivity and the amount of fluid in the stool – you can see why serotonin has been a focus for IBS researchers!

    Differences have been found in serotonin levels between patients who suffer from diarrhea vs. those who have constipation. Patients with diarrhea had higher than normal levels of serotonin in their blood following a meal, while patients who suffered from constipation had lower than normal levels of serotonin. This difference underlies the efforts to develop medication that either increases or decreases serotonin levels by targeting specific receptor sites (5-HT3 and 5-HT4) to treat IBS. There are two such medications, but both have strict restrictions on their use in order to prevent serious negative side effects:

    • Lotronex -- a 5-HT3 blocker for treatment of diarrhea
    • Zelnorm – a 5-HT4 stimulant for treatment of constipation

      A new IBS research direction is a focus on a class of proteins called serotonin reuptake transporters (SERTs). SERTs are responsible for removing serotonin after it has been released. There is some indication that there are differences in SERT activity when IBS or inflammation is present. One school of thought is that the excess of serotonin interferes with the process of homeostasis, thus preventing the digestive system from functioning in a normal way.

      Knowledge is Power

      How can you translate your new knowledge into helping to better manage your IBS? Obviously, we don’t have the power to directly affect our serotonin levels. However, there are two areas where our actions have a direct impact on the communication system between the brain and the gut. Through the use of relaxation exercises, you can actively work to turn off the stress response, in which gut changes come about in response to thoughts and feelings. You could also consider the gastrocolic reflex in which colon contractions are stimulated by eating a large meal or fatty foods, when deciding what foods to eat. For diarrhea, it would be better to eat smaller meals, while for constipation, a large meal would be preferable to trigger a bowel movement.

      The understanding that the problems in IBS extend way beyond having a “sensitive stomach” can help you to develop a variety of strategies for addressing these very problems.

      Further Reading from Dr. Bolen, Your IBS Guide:

      Sources:

      Aggarwal, A., Cutts, T.F., Abell, T.L., Cardoso, S., Familoni, B., Bremer, J., & Karas, J. “Predominant symptoms in irritable bowel syndrome correlate with specific autonomic nervous system abnormalities.” (1994) Gastroenterology 106:1114-1118.

      Baker, D.E. “ Rationale for using serotonergic agents to treat irritable bowel syndrome” (2005) American Journal of Health-System Pharmacy 62:700-711.

      Mayer, E.A., Tillisch, K. & Bradesi, S. "Modulation of the Brain-Gut Axis as a Therapeutic Approach in Gastrointestinal Disease” (2006) Alimentary Pharmacology & Therapeutics 24:919-933.

      Norton, W. & Drossman, D. “Symposium Summary Report” (2007) Digestive Health Matters 16:4 -7.

      Schmulson, M. J. “Brain-Gut Interaction in Irritable Bowel Syndrome: New Findings of a Multicomponent Disease Model” (2001) The Israel Medical Association Journal 3:104-109.

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