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. == Gut microbiota ==

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. Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolizing bile acids, sterols, and xenobiotics. The composition of human gut microbiota changes over time, when the diet changes, and as overall health changes. == Gut–brain integration ==

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). 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. These pathways include neural signaling through the vagus nerve, endocrine signaling through stress hormones, and immune signaling mediated by cytokines. The pathways described are thought to work together simultaneously rather than function independently. This process is thought to be regulated via the gut microbiota, which ferment indigestible dietary fibre and resistant starch; the fermentation process produces short chain fatty acids (SCFAs) such as propionate, butyrate, and acetate. SCFA’s produced by microbes are critical for proper gut-brain axis modulation and brain health, as they help maintain blood-brain barrier integrity and suppress neuroinflammation. Because of these properties, SCFA’s are being explored as therapeutics for Alzheimer’s disease, Parkinson’s disease, and other neurodegenerative conditions, as well as for stroke and other forms of brain injury. Their ability to cross the blood-brain barrier enables them to exert direct effects within the central nervous system. Supporting this, mice treated with live Clostridium butyricum, a butyrate-producing bacterium, show increased levels of butyric acid in the brain, and treatment was observed to support healing of cerebral ischemia-associated injury. While probiotic-based interventions such as this one are theoretically straightforward, they depend on whether administered bacteria can successfully integrate into the host gut microbiome, which remains a major challenge and limits the development of targeted probiotic therapeutics in humans. As a result, less targeted but potentially more replicable approaches, such as dietary modification, are also being explored. In particular, precision nutrition research is investigating how varying levels of prebiotics like dietary fiber can promote SCFA-producing microbes and thereby support brain health. ==Gallery==

File:Bifidobacterium adolescentis Gram.jpg|Bifidobacterium adolescentis File:Lactobacillus sp 01.png|Lactobacillus sp 01 == References ==