by Anna Fouks
It is a common misconception that different organ systems function separately from each other with few connections. However, contrary to popular belief, significant relations have been found between systems that seem to control very different aspects of our complex bodies. Among the most surprising is the connection between the brain and the bacteria within the digestive system.
The main system behind this phenomenon is the microbiota-gut-brain (MGB) axis. This is a communication structure that includes the following: the central nervous system (comprised of the brain and spinal cord), autonomic nervous system (responsible for involuntary body functions and maintaining physiological homeostasis), enteric nervous system (part of the autonomic nervous system that governs the gastrointestinal tract), and hypothalamic pituitary adrenal (HPA) axis that is part of the body’s hormonal stress response.
The MGB axis helps activate appropriate responses to external stress. Within the HPA axis, the hypothalamus and pituitary gland in the brain and the adrenal glands above the kidneys communicate with each other through hormones, ultimately releasing cortisol, a major stress hormone. Meanwhile, the central nervous system releases signals to the enteric nervous system, muscle cells, and gut mucosa to control the body’s immunity, permeability, and secretion of mucus carried out by the ethereal nervous system, muscle cells, and the gut mucosa. The microbiota (various microorganisms) within the gut also affect gastrointestinal processes by being connected in communications among the brain, gut, and other parts of the intestine.
There are more than 100 trillion bacteria entities in the healthy adult intestine, with approximately 200 strains including more than 100 different bacterial species. More than 90% of the existing human microbiome belongs to the Bacteroidetes and the Fermicutes phyla. Bacteria in the intestine can help perform certain digestive functions that their host cannot do, such as the synthesis of vitamins and the breakdown of carbohydrates for energy sources, while also developing and promoting our immune system. The microbiota profile has shown to have similar distribution and action patterns in the gastrointestinal tract among healthy individuals. Some divergences from these patterns are associated with health issues, such as abnormally high Fermicutes to Bacteroidetes (F/B) ratios have been found to be correlated with adult obesity.
Since the brain and gut microbiota are connected through the MGB axis, researchers are interested in how changes in the bacterial patterns may affect an individual’s psychology. For example, “leaky gut” syndrome (or intestinal permeability) refers to when the typically impermeable wall around the intestines weakens and allows toxins and bacteria in the gut pass into the bloodstream. Leaky gut symptoms have been associated with depression and fatigue. More research is needed to examine this connection. However, it is theorized that leaky gut can cause autoimmune responses like inflammation, which affect various areas of the body including neuronal tissue, and so can induce or exacerbate exacerbate depressive symptoms. One study that analyzed blood samples from depressed individuals even found that 35% of the participants showed signs of intestinal permeability.
Animal research has also found connections between the brain and gut. A study done at McMaster University stressed mice by separating them from their mothers for three hours every day when they were 3 to 21 days old. One group of mice had no microbiota in their gut (called “germ-free” mice) and were kept in sterile conditions, while a second group had gut microbiota and were not raised in a sterile environment. In a follow-up after the maternal separation, researchers conducted a series of behavioural tests to measure the mice’s anxious and depressive symptoms. They found that mice with microbiota had elevated levels of the stress hormone corticosterone and exhibited symptoms of depression and anxiety. The germ-free mice had physiological and HPA axis changes, but did not present the same anxious and depressive symptoms. However, when microbiota was from control mice (with no maternal separation) were transferred into the germ-free mice, they began to show signs of anxiety and depression-like behaviour.
Researchers are continuously trying to learn more about the relationship between the brain and gut, and so much of this link is still a mystery. That being said, these studies are reminders to us that the various parts of our organisms are more connected to each other than we think