Figure 1.

The microbiota-gut-brain axis.

The bidirectional communication between the brain and gut microbiota is mediated by several pathways including the immune system, neuroendocrine system, enteric nervous system (ENS), circulatory system, and vagus nerve. The routes of these pathways contain various neuroactive compounds including microbial-derived metabolites, microbial-derived products, peptides, gut hormones, and neuroactive substances. Upon entering the brain, metabolites can subsequently influence neurodevelopment and neurodegeneration of numerous conditions, such as multiple sclerosis, Parkinson's Disease, Alzheimer's Disease, CNS malignancies, stroke, autism spectrum disorder, depression, anxiety, stress, and schizophrenia.