Study shows entire brain responds to gut through neuronal connections

The strange feeling in the gut is considered as an innate intuition that emerges from deep within the belly and helps guide the actions of people if allowed.

It is also a metaphor that scientists refer to as the “gut-brain axis”—a biological reality where the gut and the microbes in it transmit signals to the brain, and vice versa.

Image Credit: anatomy insider/Shutterstock.com

It is well known that the brain responds to signals in the gut, triggering motor functions involved with digestion. These types of fundamental biological actions are mostly automatic and induced by the brainstem.

A question arises: what happens if the higher brain—the thinking, emotional centers—were also influenced by signals in the gut. A new study on rats at the University of Illinois demonstrates that the entire brain responds to the gut, particularly the small intestine, through neuronal connections.

Scientists introduced neuron-loving viruses into the small intestine of rats to map the connections and monitored the viruses as they migrated from neuron to neuron along the Vagus and spinal nerves and through the entire brain. The concept was that the movement of viruses simulated the movement of normal signals through neurons from the gut to the brain, and vice versa.

We saw a lot of connections in the brainstem and hindbrain regions. We knew these regions are involved in sensing and controlling the organs of the body, so there weren’t any big surprises there. But things got more interesting as the viruses moved farther up into parts of the brain that are usually considered emotional centers or learning centers, cognitive places. They have all these multifaceted functions. So thinking about how information from the small intestine might be nudging those processes a little bit is really cool.”

Coltan Parker, Study Lead Author and Doctoral student, University of Illinois

The study has been published in Autonomic Neuroscience: Basic and Clinical.

The study presents the first-ever comprehensive map of neuronal connections between small intestine—which were referred to as “underloved” part of the digestive system by Parker and colleagues—and the whole brain. The involvement of emotional and cognitive centers indicates how the thinking brain at times overrides the feeling of being full, providing fuel to explore relationships between depression and digestive troubles, and much more.

Now we’re actually finding the neuro-anatomy that might be involved in that ‘feeling in your gut.”

Megan Dailey, Study Co-Author and Program Administrator, College of Agricultural, Consumer and Environmental Sciences, University of Illinois at Urbana-Champaign

The study also identified a rare characteristic feature of the neurons themselves apart from illustrating how wide-ranging the connections are between the brain and the small intestine.

For a long time, scientists have predicted sensations from the gut, or anywhere in the body, traveled to the brain along a single set of neurons (the sensory neurons), where instructions from the brain traveled back along a different set of neurons (the motor neurons). However, as part of the mapping study, the Illinois team has identified that a few of the neurons—around half—transmit both sensory and motor signals. They could cross-talk inside the same neuron.

According to Elizabeth Davis, “From the cortex to the brainstem, in pretty much every region we investigated, there was that 50% overlap of sensory-motor signals. It was throughout the brain, consistently.” Davis is a graduate of the Illinois Neuroscience Program and is currently a postdoctoral scholar at the University of Southern California. She is also the co-author of the study.

A similar pattern, that is, 50% of neurons with both sensory and motor signaling capabilities, had been proved just once as part of a study mapping neuronal connections between fat tissue and the brain. According to the researchers, new evidence of a similar crosstalk pattern could hint at a general architecture of neuronal networks between the body and brain.

This study shows that sensorimotor feedback loops are abundant across all levels of the brain. Up until now, it has really been unknown how information in the small intestine, about nutrients or anything else, can get up to the brain and affect cognitive-emotional processes, and then how those processes can come back down and affect the gut. With more research, we may finally begin to understand how hunger makes us ‘hangry’, or how a stressful day becomes an irritable bowel.”

Coltan Parker, Study Lead Author and Doctoral Student, University of Illinois