Fig. 1

Pathways linking gut microbial function to changes in the CNS. a Macro-scale pathways: The enteric nervous system (ENS) intrinsic to the gut is connected to the central nervous system directly via the vagus nerve. As such, any microbe-derived metabolite that accesses the ENS has the potential to travel to and impact the brain and spinal cord. Likewise, an extensive network of blood capillaries collects nutrients absorbed from the gut for transfer around the body. Microbial metabolites that access the bloodstream can impact any part of the body, though still need to breach the blood-brain-barrier (BBB) to access the CNS. b Transit across the intestinal epithelium: In a healthy gut with functional tight junctions, selective uptake of contents of the intestinal lumen occurs across the epithelial cells (route “i”). Dysbiosis of the gut microbiota can damage the structural integrity of the epithelial barrier allowing uncontrolled transit of metabolites and other luminal contents to pass into the body (route “ii”)