One of the most fascinating things I saw during my surgery rotation in medical school was watching the intestines continue to wiggle (called peristalsis) while a person was completely under deep anesthesia. I remember a surgeon showing me by touching the intestines; the intestines responded like they were independent of the unconscious person they belonged to.
The intestines have a large bundle of neurons called the enteric nervous system (ENS). The ENS has the second largest accumulation of nerve cells in the body (the brain has the most). If you isolate the gut from the rest of the body it will still be able to perform its most important functions, such as contracting and secreting substances, because it works independently. It has as many neurons as the entire spinal cord and an abundance of hormones that are often attributed to emotional responses, hormones like serotonin and dopamine. In fact, 95% of the body’s serotonin is found in the gut. That feeling of butterflies in the stomach is likely related to this emotional responsiveness of the gut.
The enteric nervous system is often referred to as a mini-brain or second brain. This “second brain” is thought to be the primitive brain that evolutionarily came first. Primitive humans had to focus primarily on finding food for survival, underlining the value of this smart gut. Our higher thinking areas likely evolved from this primitive gut brain assuring that our responses to our environment come from signals from this primitive gut. For example, motion sickness can make people vomit, which doesn’t seem very productive. However, poison can give the same sensation as motion sickness, so the body doesn’t take any chances and empties the stomach. Even though you know you haven’t ingested any poison, that instinct to vomit is still there.
The ENS forms when cells migrate from the neural crest (the structure that also forms the brain and spinal cord) to settle in the gut. There is so much overlap between our brains and our ENS that some say it’s hard to tell where one ends and where another begins. Some propose that the process of communication between the gut and the brain occurs largely without conscious awareness and plays an important role in emotional function. This unconscious emotional response of the gut, may have an influence on decision-making. Perhaps this can help explain what we would call intuition or a premonition, sometimes called a “gut feeling”.
Some posit that irritable bowel syndrome (IBS) may have some connection to an interruption in communication between the central and enteric nervous systems. In fact, antidepressant medications which modulate serotonin levels are on the list of treatment options for IBS.
Now, when you have a “visceral reaction” or an “instinct” about something, you can remember that this is your second brain at work and realize that it’s smarter than you think.