The gut has been referred to as the “second brain,” because of direct connections found between the brain, our nervous system and the gut. There is growing evidence that microbiota can influence mind and behavior, and conversely, that mind and behavior can influence microbiota.
The vagus nerve is a cranial nerve that is routed directly from the brain to the gut. Also, various nerves in the sympathetic nervous system are routed directly from the spine to the gut. These direct pathways may allow for a direct signaling between the brain and the gut, and the gut and the brain.
Microbiota interact with our body in complex biochemical ways with complex mechanisms that we still don’t understand. For example, research has shown that gut microbiota can trigger cytokine reactions. Cytokines are involved in the host inflammatory response of the immune system, and cause “activation of microbial metabolite sensor receptors that are more highly expressed on inflammatory and intestinal epithelial cells.” [1]
Microbiota have also been shown to trigger the release of corticosteroids and neurotransmitters which can then travel to the brain systemically. They can cause biochemical changes in the brain.
It is also believed that microbiota can be influenced by our brain and behavior. Neurotransmitter activity has the potential to influence the behavior of microbiota through the nervous system pathways, including the vagus nerve and various sympathetic nerves. Examples of what they influence include heart rate, breathing, sleep cycles, digestion, mood, concentration, appetite, and muscle control. They are involved in the body’s organs, psychological functions, and physical functions. [2]
Neurotransmitters can be influenced or changed by behavior. Because of the direct connection between the nervous system and gut microbiota, the potential for microbiota to be influenced by neurotransmitter signaling is very possible.
This is a very new area of study, one with a lot of controversy among professional peers in the field. The thought is that altering gut microbiomes may have an effect on mental health, possibly positively affecting or curing some mental illnesses and mental states.
Mechanistically, most of the interaction between gut microbiota and the nervous system takes place at the epithelial cell layer of the intestine. A healthy gut will have a healthy symbiotic relationship with the brain, but a dysbiotic gut, or one with an unhealthy amount of pathogenic bacteria, will have a disruption in the epithelium of the intestine, which has been shown in research to affect the behavior of laboratory mice in some studies. [3]
Some research has shown that when mice are given probiotics, it can reverse changes that were made to their microbiota after they were put in stressful environments.
Scientists are also noting a high correlation between disorders of the gastrointestinal system and mental conditions such as autism.
This is a very new area of study, one with a lot of controversy among professional peers in the field. The thought is that altering gut microbiomes may have an effect on mental health, possibly positively affecting or curing some mental illnesses and mental states. There is much research to be done, and the mechanisms of how the microbiome and the brain interact are still not well understood, but the implications of being able to manipulate behavior and mental states through a person’s microbiome could be a game changer in our understanding of human psychology and neurology.
Citations:
- https://pubmed.ncbi.nlm.nih.gov/31013453/
- https://www.medicalnewstoday.com/articles/326649#:~:text=Neurotransmitters%20are%20chemical%20messengers%20in,heart%20rate
- Thaiss, C. A., Zeevi, D., Levy, M., Zilberman-Schapira, G., Suez, J., Tengeler, A. C., Abramson, L., Katz, M. N., Korem, T., Zmora, N., Kuperman, Y., Biton, I., Gilad, S., Harmelin, A., Shapiro, H., Halpern, Z., Segal, E., & Elinav, E. (2014). Transkingdom control of microbiota diurnal oscillations promotes metabolic homeostasis. Cell, 159(3), 514–529.
