The Brain Gut Connection in IBS

Irritable bowel syndrome (IBS) is far from simple, and researchers are looking beyond the gut to try and understand the condition. Dysfunction in the complex communication systems between the brain and the gut may be a contributing factor in IBS.

Biology Basics

Communication among all of the parts of our bodies occurs through the passing of information between nerves.

This communication takes place along the following pathways:

• 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 the 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 digestive muscles), secretion of fluid, and blood flow. The dysfunction of the enteric system is a major factor in IBS.

Bidirectional Communication

Communication is a two-way street when it comes to the brain and the digestive system. Several 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 communication network is important for our survival.

Researchers are finding evidence that dysfunction along these back-and-forth pathways may contribute to the abdominal pain, constipation, and/or diarrhea that are the symptoms of IBS. A malfunction may be found along the many different pathways that connect the brain and gut.

• Nerves in the gut that are excessively sensitive 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.

Dysfunction in the brain-gut communication system interferes with the body’s ability to maintain homeostasis, a state in which all systems are working smoothly.

The Role of Serotonin

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 percent 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 (movement), sensitivity, and secretion of fluids.

Differences have been found in serotonin levels between patients who have diarrhea vs. those who have constipation.

In one research study:

• Patients with diarrhea had higher than normal levels of serotonin in their blood following a meal.
• Patients with constipation had lower-than-normal levels of serotonin.

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.

Serotonin and IBS Treatment

This difference underlies the efforts to develop a medication that either increases or decreases serotonin levels by targeting specific receptor sites (5-HT3 and 5-HT4) to treat IBS. There are several new medications in this category to treat IBS-D and IBS-C.

A newer 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.

A Word From Verywell

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.

Through the use of relaxation exercises, you can actively work to reduce your body's physical stress response that triggers gut changes in response to thoughts and feelings. You could also consider the gastrocolic reflex when deciding what foods to eat since colon contractions are stimulated by eating a large meal or fatty foods. For diarrhea, it would be better to eat smaller meals, while for constipation, a large meal would be preferable to trigger a bowel movement.