The gut–brain axis refers to the biochemical signaling taking place between the gastrointestinal tract and the nervous system, often involving intestinal microbiota, which have been shown to play an important role in healthy brain function.
Our gut microbiota play a vital role in our physical and psychological health via its own neural network: the enteric nervous system (ENS), a complex system of about 100 million nerves found in the lining of the gut. The ENS is sometimes called the “second brain,” and it actually arises from the same tissues as our central nervous system (CNS) during fetal development. Therefore, it has many structural and chemical parallels to the brain.
Our ENS doesn’t wax philosophical or make executive decisions like the gray shiny mound in our skulls. Yet, in a miraculously orchestrated symphony of hormones, neurotransmitters, and electrical impulses through a pathway of nerves, both “brains” communicate back and forth. These pathways include and involve endocrine, immune, and neural pathways.
It has been shown that colonization by gut microbiota impacts mammalian brain development by initiating signaling mechanisms that affect neuronal circuits involved in motor control and anxiety behavior. Using measures of motor activity and anxiety-like behavior, scientists demonstrate that GF mice display increased motor activity and reduced anxiety, compared with SPF mice with a normal gut microbiota. This behavioral phenotype is associated with altered expression of genes known to be involved in second messenger pathways and synaptic long-term potentiation in brain regions implicated in motor control and anxiety-like behavior.
At this point in time, even though the research is ongoing and complex, it is clear that the brain and gut are so intimately connected that it sometimes seems like one system, not two.