Welcome to Vagus, Baby

Have you ever had butterflies in your stomach when something made you nervous? Or a gut feeling when you were anxious? Or have you ever been so stressed you have trouble going to the bathroom?

If so, then you’ve already been introduced to your gut-brain axis, a strong connection that scientists have used to understand everything from stomach aches to trauma disorders. Unsurprisingly, the gut-brain axis involves your gut and your brain, and they like talking to each other. But there is an additional player in this relationship, a very vocal yet underappreciated third-wheeler: gut microbiota. Gut microbiota refers to the trillions of tiny non-human cells sitting inside your gut that, collectively, behave like another organ. This “organ” has such an influential role in the relationship between your brain and gut that scientists have officially added it to the group chat, now called the microbiota-gut-brain axis (MGBA).

These microscopic cells (microbes) have lived in our intestines since before we were born. So how do they send messages all the way up to the brain?

It turns out these microbes release chemical signals, little pouches of information that can travel throughout the body through the blood. To do this, these molecular packages first have to wiggle through the walls of the intestine and reach a blood vessel. Once inside the bloodstream, these packages will take a long journey throughout the whole body, likely making a pit stop at the liver and kidney before eventually making it to the brain.

But microbes don’t always have time for snail mail. Sometimes they just have to send a text. Let’s talk about the vagus nerve.

The vagus nerve is named after the Latin word for “wandering” because, of the nerves that stick directly out of the brain, the vagus nerve travels the furthest down the body. And just like using your phone to send a text, nerves rely on electricity to communicate through their branch-like structures. Your brain is a promiscuous texter, so it talks not only with the gut through the vagus nerve, but also to vital organs like your heart, lungs, kidneys, and spleen.

So instead of jumping into a blood vessel, the chemical packages released by gut microbes can go high-five one of the branches of the vagus nerve that sit just outside the intestinal walls. These high-fives tell the vagus nerve to send an electrical signal up to the brain to let it know what the microbes are saying in milliseconds.

So gut microbes send messages to the brain. But isn’t communication a two-way street? It certainly is! In fact, the vagus nerve contains 75% of the pathways that the brain uses to control your “rest and digest” response, which is the opposite of “fight or flight”. This “rest and digest” response is your brain’s way of tellingl your body to relax and take a breath, which may prevent your gut microbes from getting stressed out too.

The specific effects of the vagus nerve on gut microbe health, however, are largely a mystery. Playing detective, scientists can trigger the vagus nerve using an external electrical pulse and then see what happens to gut microbiota before and after the pulse. This pulse, called vagus nerve stimulation, is a type of therapy for multiple gastrointestinal and anxiety disorders, yet we still do not fully understand its effects on gut microbiota.

More detective work is needed to investigate how the tiny friends in our intestines react to the vagus nerve’s messages. Soon we will further decode this exchange of notes between the world that lives in our gut and the world we create in our heads.

Monica Tschang is a neuroscience PhD student studying how the world of microbes inside our guts plays a role in how we respond to stress. She studies a pretty severe example of stress, as she is working to understand how gut microbes might drive symptoms of post-traumatic stress disorder in people exposed to explosives in areas of conflict.