Gut-Brain Axis
The bidirectional highway between your microbiome and your nervous system
Definition
The gut-brain axis is the bidirectional communication network between the central nervous system, the enteric nervous system (the 'second brain' with 500 million neurons in the intestinal wall), and the gut microbiota. Communication occurs via four main pathways: vagus nerve (80% of its fibres are afferent), microbial metabolites (SCFAs such as butyrate, propionate, acetate), neurotransmitters produced by bacteria (serotonin, GABA, dopamine), and immune cytokines.
Detailed explanation
95% of the body's serotonin is produced in the gut (enterochromaffin cells, modulated by microbiota). The vagus nerve transmits viscerosensory information to the nucleus of the solitary tract, regulating mood, anxiety, satiety, and gastrointestinal motility. Vagus nerve stimulation (VNS) has established clinical application in refractory depression and epilepsy.
Growing clinical evidence: human studies show that specific probiotics (Lactobacillus rhamnosus JB-1, Bifidobacterium longum 1714) reduce anxiety and salivary cortisol. Use of psychobiotics — strains with neuropsychological effects — is an emerging field in preventive psychiatry. People with major depression show dysbiosis with reduced abundance of Faecalibacterium and Coprococcus.
Neuroinflammation from intestinal dysbiosis (via endotoxemic LPS crossing an altered intestinal barrier → activated microglia → cerebral neuroinflammation) is a proposed mechanism in Alzheimer's, Parkinson's, and refractory depression. The presence of α-synuclein (Parkinson's protein) in the enteric plexus years before clinical diagnosis supports the 'intestinal origin' hypothesis of several neurodegenerations.
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