Figure 1.

The brain-gut axis and the bi-directional system of communication. The brain-gut axis is a bi-directional system of communication between the brain and the gastrointestinal tract. This links emotional and cognitive centres of the brain with peripheral control and function of the gut and its resident microbiota. Serotonin is a key element of this axis, acting as a neurotransmitter in the CNS and in the enteric nervous system that is present in the wall of the gut. A. Neural communication between the gut and brain is via the vagus (stomach and rectum) and dorsal root ganglia (DRG-small and large intestine), via projections from the enteric nervous system to sympathetic ganglia and parasympathetic innervation of the gut. B. Humeral communication is via release of bacterial factors, production of cytokines and circulating hormones. An important advance for future studies will be testable models of a potential mechanism of action (e.g., cutting the vagus can block some effects of changing the gut microbiota in rodent models).