Figure 1.

Communication pathways of the microbiota-gut-brain axis. Multiple hard-wired or direct (e.g. ENS and vagus nerve) and indirect (e.g. neurotransmitters, SCFAs, cytokines) communication pathways of the gut-brain axis are modulated by gut microbiota. These routes include the neural pathway (e.g. vagus nerve, ENS, neurotransmitters and neuroactive metabolites such as the SCFA butyrate), immune pathway (e.g. cytokines), and neuroendocrine pathways (e.g. gut hormone secretion such as peptide YY, neuropeptide Y, and glucagon-like peptide-1; cortisol secretion via the hypothalamus pituitary adrenal (HPA) axis). Neuroactive dietary and microbially-produced metabolites modulate the microbiome-gut-brain axis to affect gut-barrier function, hormone secretion from enteroendocrine cells (EECs), neurotransmitter production by gut epithelium and microbiota, and enteric glial signaling which are relevant to neurodegenerative disease. Image was generated with BioRender.