From In an Unspoken Voice: How the Body Releases Trauma and Restores Goodness by Peter A. Levine, PhD

“Tummy Talk -

It has long been known that the brain can influence our internal organs. When this process goes awry, one becomes the unfortunate bearer of what has been referred to as psychosomatic illness. The principal idea of the one-way effect of mind over body evolved as the “psychosomatic paradigm” of the 1930s through 1950s. Today, it remains conventional wisdom, and few doctors deny that an overwrought mind and unsettled emotions affect the human body in the form of “functional” disorders, which include high blood pressure, gastrointestinal symptoms, chronic pain, fibromyalgia and migraines, as well as a multitude of other, so-called idiopathic diseases. In 1872, however, long before the rise of psychosomatic medicine, the amazing Charles Darwin realized that there was a vital two-way connectivity between brain and body: ‘When the heart is affected it reacts on the brain; and the state of the brain reacts through the pneumo-gastric on the heart; so under any excitement, there will be much mutual action and reaction between these, the two most important organs in the body.’

The “pneumo-gastric” nerve Darwin speaks of is none other than the vagus nerve described in Steven Porges’s polyvagal theory. The primitive (unmyelinated) vagus nerve of the immobilization system connects the brain with most of our internal organs. This enormous nerve is the second largest nerve in our body, comparable in size to the spinal cord. In particular, this nerve largely serves the gastrointestinal system, influencing ingestion, digestion, assimilation and elimination. It also significantly affects the heart and lungs, as Darwin clearly recognized. Furthermore, embedded within the lining of the gastrointestinal wall itself there is a massive plexus of nerves. This complex network of sensory, motor and interneurons (those nerve cells that connect between the sensory and motor neurons) integrates the digestive and eliminative organs so that they function coherently.

This intricate system has about the same number of neurons and white matter as does a cat’s brain. Because of this complexity, it has sometimes been called the second or enteric brain; the other three are the reptilian (instinctual), the paleomammalian (limbic/emotional) and the primate (enlarged, rational) neocortex. The enteric nervous system is our oldest brain, evolving hundreds of millions of years ago. It produces many beneficial hormones, including 95% of the serotonin in the body, and thus is a primary natural medicine factory and warehouse for feel-good hormones.

Amazingly, as much as 90% of the vagus nerve that connects our guts and brains is sensory! In other words, for every one motor nerve fiber that relays commands from the brain to the gut, nine sensory nerves send information about the state of the viscera to the brain. The sensory fibers in the vagus nerve pick up the complex telecommunications going on in the gut and relay them, first up to the (mid) brain stem and then to the thalamus. From there, these signals virtually influence the entire brain, and subliminal “decisions” are made that profoundly influence our actions. Many of our likes and dislikes, our attractions and repulsions, as well as our irrational fears, are the result of these implicit computations in our internal states.

It can be said that humans have two brains: one in the gut (the enteric brain) and the “upstairs brain,” sitting within the vaulted dome of the cranium. These two brains are in direct communication with each other through the hefty vagus nerve. And if we go with the numbers - nine sensory/afferent nerves to every one motor/efferent nerve - our guts apparently have more to say to our brains (by a ratio of 9:1) than our brains have to say to our guts!

Let’s look more deeply at the functions of this massive nerve, which not only connects organs and brain, but functions primarily in the direction of gut to brain. Why is it even important for the body to talk to the brain in the first place? From the perspective of evolution (and the general parsimony of nature), it is unlikely that such a myriad of nerve fibers would be allotted to making bidirectional communication possible if that linkage weren’t vitally important.

Most of us have experienced butterflies in our stomach when asked to make a public speech. On the other hand, some people are known for “having gall,” while others are quite “bitter” or “bilious.” And then too, at times we may have “knots in our guts” and are “twisted up inside.” Or we may be “heavyhearted” or nursing a “heartache.” And blessed are the times when we have surrendered to the pure mirth of a spontaneous “belly laugh.” Or, again, we may be “openhearted and filled with warmth in our bellies,” feeling an inner peace and love for the whole world. On the occasions when we have accomplished notable achievements, our chests may “swell with pride.” Such is the variety of poignant messages emanating from our viscera.

When aroused to fight or flight (sympathetic arousal), our guts tighten, and the motility of the gastrointestinal system is inhibited. After all, there is no sense in spending a lot of metabolic energy on digestion when it is best used to speed up the heart’s rhythm and to strengthen its contraction, as well as to tense our muscles in readiness for impending action. When we are mortally threatened, or when the threat is internal (say, from the flu or from eating a bacterially infected food), our survival response is to vomit or to expel the contents of our intestines with diarrhea, and then to lie still so as to conserve energy. It seems possible that prey animals also resort to this reaction when a predator suddenly springs on it from within striking distance. In this case, the violent expulsion of the animal’s intestinal contents may actually lighten its weight and give it a better chance of escaping. This fraction-of-a-second advantage could mean the difference between life or death.

The powerful effects of both the sympathetic and the vagus nerves on the viscera serve critical survival functions. The activation of these two systems is meant to be brief in response to acute emergency. When they become stuck (in either sympathetic overdrive or vagal overactivity), the survival function is drastically subverted: one may end up suffering from a painfully knotted gut, as in the case of persistent sympathetic hyperarousal, or be tormented by spasms of twisting cramps and disruptive diarrhea in chronic vagal hyperactivity. When equilibrium is not restored, these states become chronic, and illness ensues.

Together, these complex systems (the vagus and the enteric plexus), not unlike a great marriage, put gut and brain in either blissful harmony or in dreadful unending battle. When there is a coherent balance between the two, the hedonic (concerned with pleasure or pleasurable sensations) fulcrum is tipped toward heaven; when the regulatory relationship is disordered, the gates of hell are opened wide like the great maw of misery.”