Gut-Brain Axis

We already know that the brain regulates all the ongoing mechanisms within the body. Interestingly, the gut-brain link is unique since the gut can also influence the brain. This article will discuss the brain and GI tract connection-but from a unique viewpoint.

A wealth of studies discovered a constant crosstalk between our gut and brain, and researchers named it Gut-Brain Axis 1.

The gut-brain axis has been a significant interest of research in recent years. It is a complex two-way communication that not only regulates normal functioning and stability(homeostasis) within the GI tract and digestive functions but also impacts higher cognitive (thinking and reasoning processes) brain functions and motivation 2.

It also means that the brain could affect gut health, and likewise, the gut could affect brain health. The classic example would be that mental health disorders, such as anxiety, and depression, also cause GI symptoms, such as nausea, diarrhoea/constipation and stomach-ache. Similarly, a problematic gut could alter brain behaviour, such as our ability to respond to stress3.

The gut-brain axis usually communicates through various routes, such as the Enteric Nervous System (ENS), Autonomic Nervous System (ANS), neuroendocrine hormones and immune cells1.

However, the vagus nerve plays a central role in brain-gut communication.

Vagus Nerve:

It is a 10th cranial nerve (nerve arising directly from the brain) and a crucial part of the parasympathetic nervous system (already discussed in the introduction to the GI tract article that controls all those functions, which needs no conscious effects. The Vagus nerve regulates various bodily functions, from heartbeat and immune response to mood regulation and digestion 4. It also controls several reflexes, including swallowing, sneezing and coughing 5.

Vagus is a paired nerve, meaning that it comprises:

  • The nerves travel up from the GI tract to the brain, known as sensing or afferent neurons.
  • The neurons going down from the brain to the gut are called motor or efferent neurons.

The vagus nerve is the most direct link between the brain and the gut.

Research has found vagus nerve involvement in hunger, stomach fullness and stress and its anti-inflammatory role through motor(efferent)nerves 5.

A study published in the journal of Frontiers in Psychiatry found that depending on the location of sensing neurons, the vagus nerve can perceive tension in the intestinal wall, gut hormones and chemical signals from the gut microbiota(bacteria) 4.

According to a research article published in the journal Nature Reviews Gastroenterology & Hepatology, the vagus nerve connects the cognitive and emotional centres of the brain with the peripheral intestinal functions 6. It explains why we perceive nausea, hunger, fullness and pain, and these inputs also overlap with emotional symptoms.

Vagus nerve improper functioning has been linked with obesity, stress, mood and inflammatory disorders (such as inflammatory bowel disease), as revealed by the journal article published in Gastroenterology 5.

Another study published in the journal of Psychiatry Research found that disruption in gut-related vagal communication was associated with adult neurological health problems, including stress, anxiety and excessive fear disorders, which caused brain changes comparable to as observed in psychiatric issues.

Similarly, a French study (human-based) reported IBS (irritable bowel disease) and Crohn’s disease as a result of gut-brain dysfunction due to reduced functioning by the vagus nerve. It also revealed high anxiety and depression symptoms in the same patients7.

The gut microbiota also greatly affects the brain-gut axis. Although there are various pathways through which gut microbiota can influence the brain, their favourite one is the direct route to the brain via the vagus nerve.

An Irish study published in the journal Neuron discovered that the chemicals released by gut microbes(bacteria) activate vagus sensor neurons to communicate with the brain and use it to alter behaviour1.

The gut microbiota also acts on emotional and cognitive centres in the brain (both directly and indirectly), where any disruption in gut microbiota is linked to changes within gut-brain axis communication8.

A research review published in the journal PLoS Pathogens mentioned that most GI problems, due to unfavourable changes in gut microbiota, often have associated psychiatric disorders 9.

The ability of the microbiota to interact with the brain and bring changes in behaviour has emerged as an exciting concept in medical studies 3. Hence, more research has now focused on treating mental health disorders and GI tract issues through gut microbiota using the gut-brain axis via the vagus nerve.

Probiotics and Gut-Brain Axis:

Probiotics are live bacteria which are very health-friendly if eaten and appear to be a great source to expand the gut microbiota to strengthen brain-gut communication and health.

Lactobacilli is one of many bacteria found in probiotics which could benefit mental health by provoking anti-anxiety and anti-depression effects. Hence, it also illustrates how microbiota supports brain health 10.

A research study published in Biological Sciences found lactobacilli stopped showing anti-anxiety effects once the vagal nerve was detached in the experimental animals. Thus, this study further supported that the preferred route of microbiota to benefit the brain is the vagus nerve10.

Take Home Message:

A wealth of research found that the brain could affect gut health and vice versa through the brain-gut axis. It is a complex two-way communication that regulates homeostasis (within the gut) and digestive and brain cognitive functions.

Gut microbiota(bacteria) play a significant role in supporting the brain-gut axis. Similarly, the vagus nerve has a central role in brain-gut communication. It is also a preferred route through which gut flora act on the brain.

Based on evident research, disruption in gut microflora not only cause gastrointestinal symptoms but also affect brain health. However, recent research has supported probiotics to boost gut flora to improve gut-brain health and communication.

References:

  1. Fülling, C., Dinan, T. G., & Cryan, J. F. (2019). Gut Microbe to Brain Signaling: What Happens in Vagus…. Neuron, 101(6), 998–1002. https://doi.org/10.1016/j.neuron.2019.02.008
  2. Rhee, S. H., Pothoulakis, C., & Mayer, E. A. (2009). Principles and clinical implications of the brain–gut–enteric microbiota axis. Nature Reviews Gastroenterology & Hepatology, 6(5), 306–314. https://doi.org/10.1038/nrgastro.2009.35
  3. Cryan, J. F., & O’Mahony, S. M. (2011). The microbiome-gut-brain axis: from bowel to behavior. Neurogastroenterology & Motility, 23(3), 187–192. https://doi.org/10.1111/j.1365-2982.2010.01664.x
  4. Breit, S., Kupferberg, A., Rogler, G., & Hasler, G. (2018). Vagus Nerve as Modulator of the Brain–Gut Axis in Psychiatric and Inflammatory Disorders. Frontiers in Psychiatry, 9(44). https://doi.org/10.3389/fpsyt.2018.00044
  5. Browning, K. N., Verheijden, S., & Boeckxstaens, G. E. (2017). The Vagus Nerve in Appetite Regulation, Mood, and Intestinal Inflammation. Gastroenterology, 152(4), 730–744. https://doi.org/10.1053/j.gastro.2016.10.046
  6. University of Colorado Anschutz Medical Campus. (2022, July 28). Better insight into the vagus nerve’s link to brain. ScienceDaily. https://www.sciencedaily.com/releases/2022/07/220728134052.htm
  7. Pellissier, S., Dantzer, C., Mondillon, L., Trocme, C., Gauchez, A.-S., Ducros, V., Mathieu, N., Toussaint, B., Fournier, A., Canini, F., & Bonaz, B. (2014). Relationship between vagal tone, cortisol, TNF-alpha, epinephrine and negative affects in Crohn’s disease and irritable bowel syndrome. PloS One, 9(9), e105328. https://doi.org/10.1371/journal.pone.0105328
  8. Appleton, J. (2018). The Gut-Brain Axis: Influence of Microbiota on Mood and Mental Health. Integrative Medicine: A Clinician’s Journal, 17(4), 28–32. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6469458/
  9. Lyte, M. (2013). Microbial Endocrinology in the Microbiome-Gut-Brain Axis: How Bacterial Production and Utilization of Neurochemicals Influence Behavior. PLoS Pathogens, 9(11), e1003726. https://doi.org/10.1371/journal.ppat.1003726
  10. Bravo, J. A., Forsythe, P., Chew, M. V., Escaravage, E., Savignac, H. M., Dinan, T. G., Bienenstock, J., & Cryan, J. F. (2011). Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve. Proceedings of the National Academy of Sciences, 108(38), 16050–16055. https://doi.org/10.1073/pnas.1102999108