Most of us think of the brain is the nervous system and when we think more clearly we would add the sense organs and their nerves, the spinal cord and the other nerves coming in and going out of the brain. But wait, there are other nerves and neurons – another three systems. The sympathetic, the parasympathetic and the enteric nervous systems. This last one was named the ‘second brain’ in 1996 by Gershon who studied it.
In the embryo, the little clump of cells that is destined to be nervous tissue splits and part becomes the brain’s system and the other part becomes the gut’s system. They are connected later by the vagus nerve. The gut ends up with 100 million neurons, over half of the neurons residing outside the brain. It seems to make a good control system to manage digestion.
The sympathetic system is a series of ganglions (like tiny brains) along the spine and connected to the spinal cord. Nerves from the ganglions go to almost every organ in the body and helps keep them working at the right level. When there is stress or danger, the sympathetic system creates the conditions for the body to react with ‘fight or flight’, such as shutting down digestion and increasing heart rate.
If the sympathetic system is the accelerator than the parasympathetic system is the brake. The parasypathetic ganglions are near or in the organs they effect, and are connected to the spinal cord or the cranial nerves. The system is thought of as the rest and digestion control. Working somewhat opposite to the sympathetic system, the organs of the body are put in maintenance mode with digestion being promoted and the heart rate decreased.
The brain controls the sympathetic and parasympathetic systems but this control never becomes conscious. So we may know we are angry from our bodies (pounding fast heart, sweat including palms, unclear vision, hot, clenched teeth, pacing etc.) but we may or may not know the cause as it may or may not have risen to consciousness. And we are not aware of the signals to the body, only the body’s response. It is not surprising that ancient people thought of the heart, stomach and so on as the seat of emotion and even thought. This cognitive embodiment is old news.
The second brain is new news; its study has only recently begun. The brain has much less control of the enteric nervous system. Here is part of a Scientific American article by Adam Hadhazy on the subject:
Thus equipped with its own reflexes and senses, the second brain can control gut behavior independently of the brain, Gershon says. We likely evolved this intricate web of nerves to perform digestion and excretion “on site,” rather than remotely from our brains through the middleman of the spinal cord. “The brain in the head doesn’t need to get its hands dirty with the messy business of digestion, which is delegated to the brain in the gut,” Gershon says. He and other researchers explain, however, that the second brain’s complexity likely cannot be interpreted through this process alone.
The second brain informs our state of mind in other more obscure ways, as well. “A big part of our emotions are probably influenced by the nerves in our gut,” Mayer says. Butterflies in the stomach—signaling in the gut as part of our physiological stress response, Gershon says—is but one example. Although gastrointestinal (GI) turmoil can sour one’s moods, everyday emotional well-being may rely on messages from the brain below to the brain above. For example, electrical stimulation of the vagus nerve—a useful treatment for depression—may mimic these signals, Gershon says.
Given the two brains’ commonalities, other depression treatments that target the mind can unintentionally impact the gut. The enteric nervous system uses more than 30 neurotransmitters, just like the brain, and in fact 95 percent of the body’s serotonin is found in the bowels. Because antidepressant medications called selective serotonin reuptake inhibitors (SSRIs) increase serotonin levels, it’s little wonder that meds meant to cause chemical changes in the mind often provoke GI issues as a side effect. Irritable bowel syndrome—which afflicts more than two million Americans—also arises in part from too much serotonin in our entrails, and could perhaps be regarded as a “mental illness” of the second brain.
We may think all this is ‘only emotion’ not really thinking, not actual cognition. However, emotion is extremely important to thinking. It supplies the purpose, the value criteria and the significance to thought.
This is the fourth in a series on embodied cognition. There will be future ones still to come.
Here are the first three in the series: