"Brain in Butt" Study Leads to Misunderstanding About Gut Nervous System

The brain and spinal cord may control most of the body, but some parts handle themselves just fine without their input, thank you very much! The enteric nervous system, which functions independently of the brain and spine, controls the crucial motions of the digestive system to move food and poop through all those tubes. Some refer to it as the “brain in the butt,” but as scientists write in the journal JNeurosci, it’s a little more complicated than that.

In the paper, published Monday, a team of researchers led by Nick Spencer, Ph.D., a professor of medicine and public health at Flinders University, describe new findings about the neurons controlling the gut. The neurons in isolated mouse digestive systems, they found, fire in predictably rhythmic patterns. These patterns, in turn, are thought to cause muscle cells in the gut to depolarize together, which causes the muscles to contract at once, pushing food along from the stomach to the colon. Referring to those gut-controlling neurons as the “brain in the butt” is a fun way to describe this phenomenon, but it oversimplifies a system that has major evolutionary significance.

While the enteric nervous system, referred to by some scientists as a “second brain,” is a highly developed system of neurons that function independently of the brain, it’s not actually a brain (and it’s not actually in the butt, though it is butt-adjacent). The enteric nervous system actually seems to have first developed in organisms without central nervous systems, leading researchers to theorize that it could actually predate brains as we know them. Before this study, scientists had already determined that the enteric nervous system controls blood flow in the digestive tract, regulates secretion and absorption of substances in the gut, and influences the movement of food.

The new paper adds to our understanding of the enteric nervous system by revealing “a novel pattern of rhythmic coordinated neuronal firing in the ENS that has never been identified,” write the study’s authors. In other words, they now know which patterns of neuron activation match up with the food-pushing rhythms of the contracting gut. Using high-resolution neuronal imaging, the team captured the activity of individual neurons, as well as groups of neurons, as pellets of food got pushed through the gut through the normal process of peristalsis. By doing so, they observed rhythmic patterns of nerve cells firing that had never been characterized before.

Noting that these patterns corresponded with the peristaltic motion of the mouse guts in the form of smooth muscle cell depolarization, the team concludes rhythmic nerve firing patterns must be responsible for the muscle contractions that push food and waste through the gut.

It’s not certain whether these findings will also apply to humans, but the particular systems and mechanisms in this study are shared across different mammals. Though there may not be any immediate medical applications for this understanding, these never-before-observed patterns of enteric nervous system coordination contribute to a richer understanding of a basic animal process. They also confirm that, though there’s no “brain” in the butt, the tubes leading to the butt are pretty smart indeed.