Figure 1.

Gut-brain Axis. The gut-brain axis is the bidirectional communication between the gut and the brain, and its communication is associated by different pathways with the autonomic nervous system, the enteric nervous system, the neuroendocrine system, and the immune system. Trillions of gut microbes and their metabolites reside in the lumen of the gastrointestinal tract and influence the local mucosal system (visualized in the inset for the barrier in the colon). Signaling pathways between the gut microbiota and the brain include neural pathways such as the vagus nerve, immune pathways, and humoral pathways. Gut microbiota influence host metabolism through local interactions, peripheral systems, and bidirectional communication with the liver. Schematic created with BioRender.com.