FIGURE 2.

The effect of gut microbiota on the cognitive brain function. The gut microbiota and probiotic bacteria produce neurotransmitters, some neurotransmitter precursors, neuromodulators, and various metabolites. These bioactive factors underlie the cognitive processes of the brain including learning and memory. They reach the brain through blood vessels, cross the BBB, and directly affect the brain function. However, since the BBB is impermeable to polar molecules, it is suggested that some productions of the bacteria impact the brain action indirectly via modulating the ENS and the vagus nerve. ENS, enteric nervous system; n, nerve; NT, neurotransmitter; Ach, acetylcholine; NM, neuromodulators; NH, neurohormones; Mets, metabolites; SCFAs, short chain fatty acids.