- Living within the human body—and vastly outnumbering genes—are trillions of bacteria collectively known as “microbiota.”
- Research shows that the microbiome influences emotions as well as psychiatric conditions such depression and anxiety.
- The vagus nerve is key to understanding the gut-brain axis and why one often feels things in the gut and heart.
A typical human cell consists of about 20,000-25,000 genes neatly packed in the DNA. Not only do those genes define who we are—but they also translate directly into our personalities, preferences, emotional experiences, pathologies, and behaviors.
Living within us, and vastly outnumbering our human genes, are trillions of bacteria, fungi, viruses, and parasites, collectively known as our “microbiota.” Our microbiota and their genes are known as our “microbiome.”
Our bodies harbor about 150 times the number of microbiota genes compared to human genes, which helps to explain the interest in exploring the role our microbiome plays in supporting and maintaining both our physical as well as mental health and of course, indirectly, our behavior. In case you are curious, there are more foreign bacterial cells inside of us than there are human cells. Yes, you read that correctly.
Which brings us to this question: Why are two authors who specialize in human behavior writing about our microbiota and our intestines? For this simple reason—there is now ample empirical evidence that many of our behaviors are influenced by our microbiome. It can no longer be ignored. If we want to understand ourselves and others and why we do the things we do, many of the answers reside in our guts.
The impact of the microbiome on mood and physical needs
Presently, massive studies of these little lifeforms that inhabit us are ongoing. From the surface of our skin (you should see what lives inside your navel) to our nose, mouth, genitals, stomach, and especially our intestines—all are being examined to learn the effects of our microbiome and its impact on our behavior.
We have all experienced that stomach bug that had us dashing to the bathroom to throw up, or dealt with diarrhea leaving us weak, lethargic, and disinterested in planned activities. There is obviously a direct correlation between a healthy body and a healthy microbiome; however, we want to understand beyond this superficial relationship. We know that microbes help us deal with everyday pathogens, digestion, and the extraction of vitamins from food; we want to know how they influence emotions, as well as psychiatric conditions such as depression and anxiety.
We cannot take the microbiome for granted, because inflammation and disruption to our microbiota colony can have serious and dangerous consequences. Cancers, inflammatory bowel diseases (IBD and Crohn's disease), autoimmune diseases, allergies (ranging from mild to life-threatening), psoriasis, and even brain and mood disorders are all possible consequences of microbiota disruption. Fortunately, we can regulate gut bacteria not only for our physical needs but also to have a more positive effect on mood and anxiety.
The role of the vagus nerve
But how could the little bacterial bugs living inside our guts do all of this? It is called the gut-brain axis and it comes together thanks to that wondrous Vagus nerve, also known as the 10th cranial nerve or CN X, the longest nerve of the autonomic nervous system.
The vagus nerve is connected to the brain stem (medulla oblongata) bilaterally and it descends via the neck innervating (connecting, threading through) no less than the pharynx, larynx, heart, lungs, diaphragm, liver, spleen, bladder, large intestine, and the small intestine. That’s quite the list. The functions of the vagus nerve are extraordinarily impressive and complex, with the most important one being its ability to intimately bring together information from your inner organs to your brain at over 150 mph via afferent fibers.
Imagine your gut, liver, heart, and lungs in a meeting with your brain, with the vagus nerve serving as the meeting host. Information flows back and forth because, as it turns out, we also have neurons in the strangest of places such as in the stomach and the heart.
Let us repeat that. Yes, you have neurons in the heart and the stomach—the same kind of neurons that we have in the brain, and these neurons are collecting information about the organs and transmitting it back to the brain via the vagus nerve. It is all intertwined like one giant switchboard.
This explains why when you are nervous—maybe you realize you made a huge mistake at work, your stomach feels noxious, your chest feels tight, perhaps you find yourself doing a very noticeable dry swallow, you feel choked up as you ponder how to fix this mess—this is all because from your gut to your brain you are super-connected via the vagus nerve.
When you are hungry, when you feel not quite right, perhaps you are short-tempered and irritable, it is the stomach communicating via the vagus nerve—up the chain, through the lungs and heart to the brain—exactly how you feel. When bad news makes you feel like you got kicked in the stomach or you experience the loss of a loved one, that broken heart you feel is not imaginary; you are feeling those sentiments because of the interconnectedness of the brain-gut matrix.
It is at these times that all that information going back and forth between the brain and the organs of the body serves to help self-regulate the stress response, by the effect it can have on the vagus nerve to control breathing and modulation of tension throughout its long tract. This explains why when we are stressed there is so much massaging and stroking of the neck, which kickstarts the vagus nerve to get us back to a state of homeostasis.
Gut bacteria and serotonin
But let’s get back to our microbiome. Take for example serotonin, that wonderful little neurotransmitter/hormone that helps keep us happy. You probably assume that serotonin would be made in the brain and it just stays there—that seems sensible, right? Actually, our gut bacteria manufacture about 95 percent of our body’s supply of serotonin, and most of the serotonin in our body resides outside the brain. And that’s just the start of it.
While we think of serotonin as only affecting the brain and our temperament, it also affects the heart, blood pleasure, and the overall vascular system as well as many other organs. That is why we have reached this point where we have to take note that what is inside of us affects our behavior. That tension you may feel—that short fuse you sometimes express, that anger or depression you may experience, that lack of energy and drive you to go through on a given day—is a lot more complex than you ever thought. The brain-mind-body matrix helps to explain so much of what we often feel or see when we observe others.
Although there is still so much to learn about the microbiome (we have provided an ample list of valuable resources for you to explore), much has already been written. From this, we know that it makes no sense to look at behavior as though the brain had no connection to the rest of the body; that model just does not carry any weight anymore. What we eat, how we eat, and what we carry in and on us has consequences. Carl Sagan once said, “We are not a single organism but an array of about 10 trillion beings, and not all of the same kind.”
So the next time you have an itch, a slight discoloring of the skin, feel nervous and find yourself clearing your throat a lot more, or are emotionally “gutted” and have to sit for a moment, that is no accident—just because you don’t see them or how they interact with your body does not mean your microbiota is not influencing you and your behavior.
Co-authored by Abbie Maroño and Joe Navarro
Abbie Maroño is the director of Behavioural Research in Communications(BRINC) and is on the faculty at the University of Northampton (UK).
Copyright © 2022, Joe Navarro and Abbie Maroño.
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