Gut-brain axis

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.

The gut microbiota is the complex community of microorganisms that live in the digestive tracts of humans and other animals. The gut metagenome is the aggregate of all the genomes of gut microbiota. The gut is one niche that human microbiota inhabit.

In humans, the gut microbiota has the largest quantity of bacteria and the greatest number of species, compared to other areas of the body. In humans, the gut flora is established at one to two years after birth; by that time, the intestinal epithelium and the intestinal mucosal barrier that it secretes have co-developed in a way that is tolerant to, and even supportive of, the gut flora and that also provides a barrier to pathogenic organisms.

The relationship between gut microbiota and humans is not merely commensal (a non-harmful coexistence), but rather a mutualistic relationship. Human gut microorganisms benefit the host by collecting the energy from the fermentation of undigested carbohydrates and the subsequent absorption of short-chain fatty acids (SCFAs), acetate, butyrate, and propionate. Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolizing bile acids, sterols, and xenobiotics. The systemic importance of the SCFAs and other compounds they produce are like hormones and the gut flora itself appears to function like an endocrine organ; dysregulation of the gut flora has been correlated with a host of inflammatory and autoimmune conditions.

The composition of human gut microbiota changes over time, when the diet changes, and as overall health changes. In general, the average human has over 1000 species of bacteria in their gut microbiome, with Bacteroidetes and Firmicutes being the dominant phyla. Diets higher in processed foods and unnatural chemicals can negatively alter the ratios of these species, while diets high in whole foods can positively alter the ratios. Additional health factors that may skew the composition of the gut microbiota are antibiotics and probiotics. Antibiotics have severe impacts on gut microbiota, ridding of both good and bad bacteria. Without proper rehabilitation, it can be easy for harmful bacteria to become dominant. Probiotics may help to mitigate this by supplying healthy bacteria into the gut and replenishing the richness and diversity of the gut microbiota. …

The gut–brain axis, a bidirectional neurohumoral communication system, is important for maintaining homeostasis and is regulated through the central and enteric nervous systems and the neural, endocrine, immune, and metabolic pathways, and especially including the hypothalamic–pituitary–adrenal axis (HPA axis). That term has been expanded to include the role of the gut microbiota as part of the "microbiome-gut-brain axis", a linkage of functions including the gut microbiota.

Interest in the field was sparked by a 2004 study (Nobuyuki Sudo and Yoichi Chida) showing that germ-free mice (genetically homogeneous laboratory mice, birthed and raised in an antiseptic environment) showed an exaggerated HPA axis response to stress, compared to non-GF laboratory mice.

The gut microbiota can produce a range of neuroactive molecules, such as acetylcholine, catecholamines, γ-aminobutyric acid, histamine, melatonin, and serotonin, which are essential for regulating peristalsis and sensation in the gut. Changes in the composition of the gut microbiota due to diet, drugs, or disease correlate with changes in levels of circulating cytokines, some of which can affect brain function. The gut microbiota also release molecules that can directly activate the vagus nerve, which transmits information about the state of the intestines to the brain.

Multiple pathways are implicated in the gut-brain axis. These pathways include neural signaling through the vagus nerve, endocrine signaling through stress hormones, and immune signaling mediated by cytokines. Gut barrier function and immune activity is influenced by metabolites such as short-chain fatty acids, which in turn affect the signaling received by the central nervous system. The pathways described are thought to work together simultaneously rather than function independently.

Likewise, chronic or acutely stressful situations activate the hypothalamic–pituitary–adrenal axis, causing changes in the gut microbiota and intestinal epithelium, and possibly having systemic effects. Additionally, the cholinergic anti-inflammatory pathway, signaling through the vagus nerve, affects the gut epithelium and microbiota. …