This article describes different aspects of the gut-brain axis. Here, learn about the relations between the gut and the brain, the benefits of microbes in the human body, and more…
Keywords: gut-brain axis, neuroendocrine, microbiota, mutualistic, autoimmune, gut, brain.
Table of contents
| 1. | Introduction |
| 2. | Types of microbe-host relationships |
| 3. | From brain to gut |
| 4. | The effects of stress on the gut |
| 5. | Links |
Introduction:
The gut-brain axis is the biochemical signaling between the GIT and the CNS. This complex communication system ensures the proper maintenance of gastrointestinal homeostasis and multiple effects on motivation and higher cognitive functions.
The term gut-brain axis refers to the role of the gut flora in this axis, but ‘the microbiota-gut-brain axis ‘may be a better term. Its role is to monitor and integrate gut functions and to link emotional and cognitive centers of the brain with peripheral intestinal functions and mechanisms such as immune activation, intestinal permeability, enteric reflex, and entero-endocrine signaling.
The components of the gut-brain axis are the CNS, neuroendocrine and neuroimmune systems, including the hypothalamic-pituitary-adrenal axis, sympathetic and parasympathetic division of ANS, including the enteric nervous system, vagus nerve, and the gut microbiota.
The microbiome describes either the collective genomes of the microorganisms that reside in the environmental niche or the microorganisms themselves. The microbes are present in the stomach and small and large intestines. In the distal gut, microbes are about 100 trillion.
This mass is mainly formed by symbiotic microbes. It has been called a ‘forgotten organ. Approximately 1014 microorganisms are present in the human gut, about 10-fold more cells in the human body.
The microbiome’s genetic material is about 150 times more than the human genome. Therefore some scientists label the microbiome as a ‘Superorganism.’
Microbiota are ecological communities of commensal, symbiotic, and pathogenic microorganisms found in and on all multicellular organisms. And it is a collection of microbiomes and includes bacteria, fungi, and viruses.
They are essential for their host’s immunological, hormonal, and metabolic homeostasis. In the large intestine, over 700 species of bacteria with multiple functions and fungi and protozoa are present. The gut metagenome is the aggregate of all the genomes of gut microbiota.
Types of microbe-host relationships:
a)Commensalism-where the microbiota colonize a host in non-harmful coexistence.
b) Mutualistic=symbiosis when both are benefited in this coexistence.
c) Parasitic when coexistence is harmful to the host.
The gut-brain axis is responsible for the cephalic phase of the salivary gland secretion and gastric secretion in response to sensory signals, for example, sight, smell, or thought. This was shown by Pavlov.
The gut flora in the gut-brain axis appears to produce psychiatric and neurological conditions. Therefore, psychiatric and neurological disorders will also affect the gut flora.
The gut flora is established one to two years after birth in humans. By that time, the intestinal epithelium and the intestinal mucosal barrier that it secretes have co-developed in a way that it is tolerant to, and even supportive of, the gut flora, which also provides a barrier to pathogenic organisms.
Although each person’s microbiota profile is different, the relative presence of these bacterial phylotypes is similar among healthy individuals.
In humans, the gut microbiota has the largest quantity of bacteria and the most significant number of species. The two more prominent phyla are Firmicutes and Bacteroides. They form 75% of the microbiome population.
The gut metagenome is the aggregate of all the genomes of gut microbiota. There is a mutualistic relationship between humans and gut flora. The gut flora derives energy from the fermentation of undigested carbohydrates and the subsequent absorption of short-chain fatty acids, acetate, butyrate, and propionate.
The gut flora also synthesizes vit. K and vit. B, and they metabolize bile acids, sterols, and xenobiotics. So the gut flora works like an endocrine organ. The gut flora change is seen with various inflammatory and autoimmune conditions. It has been called a ‘forgotten organ.
The composition of human gut flora changes over time when the diet, drugs, or disease correlate with changes in levels of circulating cytokines, some of which can affect brain function and overall health changes. The gut flora also releases molecules that can directly activate the vagus nerve, which informs about the state of the intestines to the brain.
Likewise, stressful situations activate the hypothalamic-pituitary-adrenal axis causing changes in the gut flora, intestinal epithelium, and systemic effects.
The enteric nervous system includes efferent, afferent, and interneurons –capable of carrying reflexes without CNS input. The enteric nervous system uses more than 30 neurotransmitters, most of which are used in the CNS, for example, acetylcholine, dopamine, and serotonin.95% Serotonin is contained in the gut. It is present in the mucosa and nerve terminals of the enteric nervous system.
Serotonin is also known as the ‘Feel good hormone.’ It may be remembered that the ‘love hormone is oxytocin. The dual functions of these neurotransmitters are an active part of the gut-brain axis.
The gut-brain axis is a bidirectional neurohormonal communication system. It is essential for maintaining homeostasis and is regulated by CNS, HPA Axis, the Enteric nervous system, and the neural, endocrine, immune, and metabolic pathways.
The gut flora can produce many neuroactive molecules, such as acetylcholine, catecholamines, histamine, melatonin, and serotonin.
From brain to gut :
Different psychological stressors modulate the composition and total biomass of the enteric microbiota. These effects may be mediated through the parallel neuroendocrine output efferent system (ANS and HPA ), directly via host-enteric microbiota signaling, and indirectly via changes in the intestinal milieu.
The effects of stress on the gut: Decreased metabolism, Decreased enzyme output, Decreased absorption, Decreased blood flow, and oxygen supply to the gut. These efferent neural pathways, associated with pain-modulator endogenous pathways, constitute the ‘Emotional motor system.’
The direct influence is mediated by the secretion under the regulation of the brain. There are neurotransmitter receptors in bacteria. For example, a high affinity for the GABA system has been reported in Pseudomonas fluorescent, with binding properties similar to a brain receptor.
The microbiome may cause disease when a delicate relationship is disturbed. For example, Diabetes mellitus, Obesity. Metabolic syndromes, Stress, Anxiety, Heart disease, Allergic disorders, IBD, cancers, etc. The use of the microbiome is under trial for treating some diseases.
A healthy gut is essential for a happy life and leads to a healthy brain.
This article is intended for an international audience of medical care providers and learners.
This activity aims for learners to better apply the latest scientific knowledge.
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