Gut–brain axis

The gut–brain axis, also described as gut–brain interaction, is the two-way biochemical signaling that takes place between the gastrointestinal tract (GI tract) and the central nervous system (CNS). The term "microbiota–gut–brain axis" highlights the putative role of gut microbiota interacting with brain functions, according to preliminary research. Broadly defined, the gut–brain axis includes the central nervous system, neuroendocrine system, neuroimmune systems, the hypothalamic–pituitary–adrenal axis (HPA axis), sympathetic and parasympathetic arms of the autonomic nervous system, the enteric nervous system, vagus nerve, and the gut microbiota. The history of ideas about a relationship between the gut and the mind dates from the nineteenth century. Chemicals released by the gut microbiome can influence brain development, starting from birth. A review from 2015 states that the gut microbiome influences the CNS by "regulating brain chemistry and influencing neuro-endocrine systems associated with stress response, anxiety and memory function". Various factors influence the human gut microbiota composition, accounting for the vast variability found amongst humans. Factors such as diet, medication exposure such as antibiotics, genetics, and environment all affect the framework of the human gut microbiota. These differences across populations may affect how the gut-brain axis is interpreted and studied. The diversity in the gut microbiomes across individuals has led to inconsistencies in this field of research. The bidirectional communication may involve immune, endocrine, humoral and neural connections between the gastrointestinal tract and the central nervous system. A 2019 review of laboratory research suggests that the gut microbiome may influence brain function by releasing chemical signals, possibly including cytokines, neurotransmitters, neuropeptides, chemokines, endocrine messengers and microbial metabolites, such as "short-chain fatty acids, branched chain amino acids, and peptidoglycans". These chemical signals are then transported to the brain via the blood, neuropod cells, nerves, endocrine cells, where they may impact different metabolic processes. The first of the brain–gut interactions shown, was the cephalic phase of digestion, in the release of gastric and pancreatic secretions in response to sensory signals, such as the smell and sight of food. This was first demonstrated by Pavlov through Nobel prize winning research in 1904. As of October 2016, most of the work done on the role of gut microbiota in the gut–brain axis had been conducted in animals, or on characterizing the various neuroactive compounds that gut microbiota can produce.

Enteric nervous system

The enteric nervous system is one of the main divisions of the nervous system and consists of a mesh-like system of neurons that governs the function of the gastrointestinal system; it has been described as a "second brain" for several reasons. The enteric nervous system can operate autonomously. It normally communicates with the central nervous system (CNS) through the parasympathetic (e.g., via the vagus nerve) and sympathetic (e.g., via the prevertebral ganglia) nervous systems. However, vertebrate studies show that when the vagus nerve is severed, the enteric nervous system continues to function. In vertebrates, the enteric nervous system includes efferent neurons, afferent neurons, and interneurons, all of which make the enteric nervous system capable of carrying reflexes in the absence of CNS input. The sensory neurons report on mechanical and chemical conditions. Through intestinal muscles, the motor neurons control peristalsis and churning of intestinal contents. Other neurons control the secretion of enzymes. The enteric nervous system also makes use of more than 30 neurotransmitters, most of which are identical to the ones found in CNS, such as acetylcholine, dopamine, and serotonin. More than 90% of the body's serotonin lies in the gut, as well as about 50% of the body's dopamine; the dual function of these neurotransmitters is an active part of gut–brain research. The first of the gut–brain interactions was shown to be between the sight and smell of food and the release of gastric secretions, known as the cephalic phase, or cephalic response of digestion.

Gut microbiota

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