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The gut–brain axis is the two-way biochemical signaling that takes place between the gastrointestinal tract and the central nervous system. The term "gut–brain axis" is occasionally used to refer to the role of the gut microbiota in the interplay as well. The "microbiota–gut–brain axis" explicitly includes the role of gut microbiota in the biochemical signaling events that take place between the GI tract and the CNS. Broadly defined, the gut–brain axis includes the central nervous system, neuroendocrine system, neuroimmune systems, the hypothalamic–pituitary–adrenal axis , sympathetic and parasympathetic arms of the autonomic nervous system, the enteric nervous system, vagus nerve, and the gut microbiota.
Chemicals released in the gut by the microbiome can vastly influence the development of the brain, starting from birth. A review from 2015 states that the microbiome influences the central nervous system by “regulating brain chemistry and influencing neuro-endocrine systems associated with stress response, anxiety and memory function”. The gut, sometimes referred to as the “second brain”, functions off of the same type of neural network as the central nervous system suggesting why it plays a significant role in brain function and mental health.
The bidirectional communication is done by immune, endocrine, humoral and neural connections between the gastrointestinal tract and the central nervous system. More research suggests that the gut microorganisms influence the function of the brain by releasing the following chemicals: cytokines, neurotransmitters, neuropeptides, chemokines, endocrine messengers and microbial metabolites such as "short-chain fatty acids, branched chain amino acids, and peptidoglycans”. The intestinal microbiome can then divert these products to the brain via the blood, neuropod cells, nerves, endocrine cells and more to be determined. The products then arrive at important locations in the brain, impacting different metabolic processes. Studies have confirmed communication between the hippocampus, the prefrontal cortex and the amygdala , which acts as a key node in the gut-brain behavioral axis.
While IBS is the only disease confirmed to be directly influenced by the gut microbiome, many disorders have been linked to the gut-brain axis as well. The impact of the axis, and the various ways in which one can influence it, remains a promising research field which could result in future treatments for psychiatric, age-related, neurodegenerative and neurodevelopmental disorders. For example, according to a study from 2017, “probiotics have the ability to restore normal microbial balance, and therefore have a potential role in the treatment and prevention of anxiety and depression”.