We have all have experienced a variety of
gut feelings and gut reactions associated with different emotions, including anxiety,
anger,
and sadness. The English language if full of expression like
“butterflies in the stomach,” “gut wrenching decisions,” “knots in the
stomach,” “pain in the butt” and “scared s***less” are just a few
examples. Similarly, almost everybody has made gut-based decisions in
their lives, often when a lot was at stake, or when the decision was of
great personal importance. And there is hardly a day on which a
prominent politician, athlete or entertainment celebrity does not make
reference to their gut feelings. But despite this tight link between our
subjective emotional experiences and the gut in our language, most
people have never seriously thought that there may be a neurobiological
basis underlying these associations. Even more outlandish and
provocative is the recently proposed idea that the 100 trillion microbes
living in our gut may have a word to say in this mind-gut conversation!
However, unnoticed by the media and the awareness of the general
public, a lot of science has accumulated over the past 25 years that
clearly demonstrates a close bidirectional interactions between the
brain and the gut. (Mayer.
Nature Reviews Neuroscience,
2011 ). When we experience an emotion, the brain talks to the gut, and
the gut responds back to the brain. The top down signaling can be viewed
as the yang, the bottom up signaling as the yin of brain-gut
interactions. Emotional responses of distinct brain networks result in
autonomic nervous system activity, which is sent to multiple targets in
the gut and adjust their activities: The blood supply, the enteric
nervous system, the gut-based immune system and
hormone containing
cells in the gut. Within this assembly of gut-based cells, these nerve
signals from the brain generate a mirror image of every emotion at the
gut level, in a similar way as each emotion generates a distinct pattern
of activities of our facial muscles, creating the characteristic facial
expressions. The only difference is that while everybody can read an
emotional facial expression, we are completely unaware of the
emotion-related events going on in our gut.
On the other hand, a complex array of sensory mechanisms encodes this
gut activity during emotional experiences and in response of the
dramatic changes that occur in the lumen of the gut when we eat, when we
are hungry and even when we
sleep.
Sensory nerve endings, hormone containing cells and immune cells in the
gut layer that extend their tentacles into the gut lumen. Much of this
sensory information about the state of our gut is transmitted back to
the brain either through the bloodstream or the vagus nerve, into the
same brain circuits that generated the emotion in the first place. Why
does the brain need this “detour” through the messy world of the gut in
order to generate our so cherished human emotions? Would a brain in a
jar or the supercomputers of the future still be able to generate human
emotions? The fact that a close link between the gut, the nervous system
and responses to the
environment
has existed for billions of years of evolution, all the way back to the
first primitive marine animals living in the vastness of the oceans,
suggests this interactions must have a highly adaptive value.
The brain-gut loop and the reverberating activity within it is
probably also responsible for the characteristic long lasting
experiences of certain emotions, which often outlast the initial
trigger. The engagement of the brain-gut loop may also play a role in
the encoding of complex emotional
memories
each time we respond to a salient event in our environment. A lifetime
of such memory encoding generates a vast database in our brain which we
can access within seconds to make an intuitive decision based on our gut
feelings. This database has been referred to as somatic markers by
Antonio Damasio (better referred to as visceral markers), and the
mechanism involved in such intuitive decisions as “as if loops,” not
requiring the slow loop through the gut (Damasio.
Descartes’ Error,
Penguin Books, 2005). Searching for the right decision in this vast
database can be compared to the activity of the search engines from
Google and Amazon, which can assist in identifying personally relevant
choices and making decisions (Mayer.
The Mind-Gut Connection, Harper Wave, 2016).
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If the gut were a simple pump as the heart, or a pipe like the blood
vessels or a mechanical device like our muscles, this view of emotions
as subjective experiences generated within the brain-body axis would be
simple to comprehend and fairly straightforward. However, the gut is
unique: Having the surface area of a basketball court, being studded
with an enormous number of sophisticated sensors, being flooded with all
kinds of foods which we ingest, it is our biggest interface with the
external world. And most intriguingly, being the home of some 100
trillion microorganisms which live in synergy with our intestine most of
the time, it receives an amazing amount of information from an
invisible superorganism living inside of us from the day we are born to
the time we die. As I discuss in my book
The Mind-Gut Connection, we are at the beginning of our
understanding
how our food and these microorganisms influence the ongoing dialogue
between our brain and the gut, and how this microbial influence affects
our emotions and our cognitions.
While much of the current media hype and exaggerated claims about
health
promoting diets and food supplements targeted at the gut microbiota is
not founded on solid scientific evidence, the realization of the
intricate connections between our gut, its microbial inhabitants and our
brain has the potential to revolutionize our concepts about the nature
of emotions in health and diseases of the brain, including
depression, anxiety, Parkinson’s disease and
autism spectrum disorders.
