Gut Feelings=Brain Feelings: Discovery of Actual Neuronal Pathways

Today’s post is about an amazing piece of research out of the University of Pittsburgh, published in the Proceedings of the National Academy of Sciences.[i]

We’re all familiar with “gut feelings” and know that our brain can influence our digestive system.  Think of those butterflies in your stomach when you’re nervous.  At this point, I’m sure all of you are also familiar with the fact that the gut can influence the brain – I talk about it all the time.

There are a lot of questions though about how this all works.  What exact neural pathways are involved in connecting the brain to the gut?

To figure this out, these researchers injected a strain of rabies virus into the stomach of rats.  They then traced the progress of the virus, as it made its way to the brain.  It jumped from neuron to neuron, thereby eventually revealing the pathway from the stomach to the brain.  They found that the parasymptathic nervous system (the “rest and digest” side) goes from the stomach to a brain area called the rostral insula, which is responsible for visceral sensation and emotion regulation.  Says one of the authors of the paper, “The stomach sends sensory information to the cortex, which sends instructions back to the gut…That means our ‘gut feelings’ are constructed not only from signals derived from the stomach, but also from all the other influences on the rostral insula, such as past experiences and contextual knowledge.”[ii]

The sympathetic nervous system pathways, those that regulate our “fight or flight” response which kick in when we are stressed, trace from the stomach to the primary motor cortex, which directs the movements of the muscles.  That makes sense, actually:  when you are about to be eaten by a tiger, your body does not waste energy on digestion (which actually takes a huge amount of energy).  Often, in fact, under acute dire circumstances, people evacuate food through either vomiting or defecation.  (Scaring the sh-t out of someone is an expression for a reason.)  Energy is directed to the muscles, to run or fight.

Why is this particularly interesting?  “Our results suggest that the gut–brain axis should also be viewed from another perspective; that is, how signals from the brain influence the gut microbiome. As we noted here, the balance of activation in the two autonomic drives to the stomach can tune the gastric microenvironment. Stomach content has a strong influence on the composition of the microbiome that is passed on to more distal regions of the gastrointestinal tract.”

For example, stress is a primary cause of gastritis (inflammation of the lining of the stomach) and stomach ulcers.  The latter, we’ve known for many years, is associated with the growth of the bacteria, Helicobacter pylori (H. pylori).  This is a remarkable paragraph from the paper:

 “Ulcer formation provides one concrete example of the interaction between central signals and the stomach’s microbiome. For more than a century, every increase in unemployment and its associated stress was accompanied by an increase in death rates from stomach ulcers. We now know that a proximal cause of ulcer formation is often infection by Helicobacter pylori. However, the growth conditions for this bacterium can be influenced by parasympathetic command signals communicated by the vagus nerve…Our current finding of direct cerebral control over parasympathetic output to the stomach elucidates a mechanism for a significant psychosomatic contribution to this problematic disease.”

That is, signals from the brain’s cerebral cortex (higher thinking) may “…influence the bacteria’s growth by adjusting gastric secretions to make the stomach more or less hospitable to invaders.”

As much as stress can cause disease then, controlling it can be a cure.  As these researchers say (highlights added by me), “Our results provide cortical targets for brain-based therapies for functional gastrointestinal disorders. This could involve altering stomach function and/or the microbiome through the engagement of specific cortical areas, using noninvasive transcranial stimulation alone or combined with cognitive-, behavioral-, and movement-based therapies.”  Considering the potential side effects of so many of the medications used to treat GI issues, that sounds awfully good to me.  And I’ll tell you the other thought that occurs to me:  the next time I “feel it in my gut,” I will forever more know that this means that I am really registering something in my brain, even if I haven’t yet consciously processed it.

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[i] Levinthal, DJ and Strick, PL. Multiple areas of the cerebral cortex influence the stomach.  Proceedings of the National Academy of Sciences of the United States of America. 2020. https://doi.org/10.1073/pnas.2002737117

[ii] https://www.sciencedaily.com/releases/2020/05/200518154939.htm