The Second Brain
Did you know that over one half of your nerve cells are located in your intestines in what many refer to as the second brain or enteric nervous system (ENS)? And, just like the brain in your skull, the brain in your intestines is able to learn, remember, and produce emotion-based feelings.
So the expression 'gut-feelings' actually has basis in fact!
The second brain consists of an extensive network of one hundred million neurons - approximately the same number found in the brain and more than in either the spinal cord or the entire peripheral nervous system.
Although much of the second brain's functions are to coordinate the complex processes of digestion including synchronising the release of all the various enzymes and fluids involved, churning and propelling the contents along and controlling expulsion of the waste, the functions of the ENS are thought to have wider implications.
Innervation of the second brain
Whilst the enteric nervous system is capable of functioning autonomously, it receives considerable innervation from the autonomic nervous system (ANS) and thus is often considered a part of the ANS.
Many of the functions of the digestive system are governed by the parasympathetic arm of the autonomic nervous system. The parasympathetic nervous system governs renewal, rest and digestion and functions effectively as the 'brakes' of the human body. Whereas the opposing sympathetic nervous system governs the alarm states of fight or flight and serves as the 'gas' or 'accelerator' of the human biosystem and turns the processes of digestion off.
The parasympathetic input to the second brain is via the the longest and biggest nerve in the body: the vagus nerve. This nerve is one of the cranial nerves which means that it emerges directly from the base of the brain rather than being routed via the spinal cord. Whereas, the sympathetic nervous system input comes via nerves in the spine with local connections known as the prevertebral ganglia.
However, of the fibres in the vagus nerve approximately 90% are carrying sensory information from the gut to the brain with only 10% providing motor nerve feedback from the brain to the ENS. This means that whilst the two brains influence each other, the state of the intestines almost certainly has an effect upon brain function.
The nerves of the enteric nervous system are sandwiched between the different layers of tissue in the alimentary canal. One set of nerves known as the myenteric or Auerbach's plexus is located between the circular and longitudinal muscles of the digestive tract. This plexus controls the muscular movement of the intestines. The other set of nerves are found beneath the intestinal lining and govern the production of digestive fluids and are known as Meissner's or the submocosal plexus.
The two brains
Just like the brain, the second brain has sensory and motor neurons, information processing circuits, and glial cells which act as a diffusion barrier between the circulation and the neurons and function like the blood-brain barrier.
The ENS also employs more than 30 neurotransmitters including acetylcholine, dopamine, nitric oxide, noradrenaline (norepinephrine) and serotonin. In fact, 95% of the body's serotonin is found in the bowels.
The second brain even produces benzodiazepines which are similar to the pharmacological drugs Valium and Xanax and for every neurotransmitter produced there are corresponding receptor sites. Also, just like the brain, the second brain can become addicted to substances such as opiates according to pain management specialist, Dr Michael Loes.
Another way in which the two brains are similar is in having natural 90-minute activity cycles. In the brain slow-wave sleep is interrupted by periods of rapid eye movement (REM) sleep during which dreams occur every 90 minutes. And in the intestines, the gut has corresponding 90-minute cycles of slow-wave muscle contractions interspersed with short bursts of rapid muscle movement.
However, according to Dr Michael Gershon of Columbia University Medical Center, an expert in the emerging field of neurogastroenterology and author of The Second Brain, although the influence of the second brain is far-reaching, it is not the seat of any conscious thought or executive decision-making.
Development of the second brain
Some regard this second brain as a relic of a more primitive nervous system that existed before the evolution of the dominant brain. More like the nervous systems seen in octopi where two-thirds of their nervous system is in their arms and they are able to function effectively even when the nerve(s) from the central 'brain' are severed.
In humans, both brains arise developmentally from the same neural crest tissue and whilst they have evolved to perform different functions, they remain very interconnected via the vagus nerve.
Some researchers suggest that intelligent processing of digestion was delegated to the second brain in order to free-up the operating capacity of the brain, but the ENS appears too complex to have evolved simply for this function.
Mood and the second brain
Of course, everyone recognises that stress can affect digestion causing chronic indigestion, ulcers, spasms and diarrhoea. We also know that cheese can cause nightmares and that indigestion can make us feel grumpy and out-of-sorts.
Some researchers are now coming to understand that a complex relationship exists between digestion and mental states and that the second brain plays a key role in many psychological disorders.
For example, some investigators are starting to regard irritable bowel syndrome (IBS) as being a mental illness of the second brain involving as it does excessive blood levels of serotonin. Unrelated illnesses such as osteoporosis and autism too have also been associated with elevated intestinal blood levels of serotonin. And, as you are already aware, mercury is a giant spanner in the works of all the biochemical systems of the body and may account for the derangements seen in all these disorders.
Dysfunctional serotonin metabolism may also explain why many depressed people also have digestive problems as a consequence of which they may also be overweight. Conversely, some researchers have also successfully treated depression using electrical stimulation of the vagus nerve.
Just as with the brain, the nerves of the ENS can be damaged by a poor or intermittent blood supply which can be caused by the deposition of fat around the intestines and this can become a vicious circle as the circulation and nervous supply slowly become compromised.
This also explains why people taking antidepressant medications such as Prozac often experience digestive problems as a side-effect. These drugs are part of the family of pharmaceuticals known as selective serotonin re-uptake inhibitors (SSRIs) and cause an increase in free brain serotonin which is associated with an elevation in mood, which, as we have already seen may be associated with IBS-type symptoms in the second brain.
The solar plexus and the second brain
I remember watching a programme in which subjects were shown rapidly flashed images, some of which were neutral or pleasant, and some of which were disturbing whilst researchers monitored their biological responses. The researchers were floored and at a loss to explain how it was that the body was responding before the mind and sometimes before the image was even shown!
Different possible explanations present themselves. One is that nature has equipped us to be prepared and any system that pre-empts danger obviously has survival value. Another explanation is that processing does not occur in the brain as is assumed but intelligence exists in every cell and is holistic. Alternatively, it is possible that the second brain has roles beyond those that science is currently willing to recognise.
The ancient and indigenous peoples all speak of the seat of the being as rooted in the solar plexus chakra or Dan Tian in Traditional Chinese Medicine, and we know that our gut feelings rarely serve us badly. Whilst your solar plexus governs the processes of digestion, it also reacts to and digests your inward and outward 'realities'.
However, sometimes the 'gut feelings' we are getting are hard to reconcile with our five sensory input and we tend to trust what we perceive to be more tangible in preference to our instincts until they have been proved justified enough times. Sometimes also, even if we trust our gut instincts it can be hard to know what to do with the information we receive in this way.
The second brain and immune function
The second brain also plays a key role in immune function, with at least 70 percent of the immune system located in the gut. We are like doughnuts, just with a long convoluted tube through our middle! And this long tube represents our greatest exposure to our environment since it also has to serve as a membrane across which nutrients can be absorbed. So in terms of defence, the intestines represent the gates to the fort.
Bacteria play an important role in the ecology of our body. We are, in fact, colonies of cells with 90% of the cells in our body being bacteria! In health the balance will be maintained between our bacterial populations and our own cells, but stress, toxins and diet can all alter the ecology rapidly as bacteria multiply every 20 minutes or so.
This fact also gives the lie to the notion that we can somehow sterilise our environments and ourselves and that this is in any way desirable. Obviously, we seek to avoid pathogenic bacteria and other microorganisms, but the whole of the Universe (including us) is teeming with life!
Reflections on the second brain
These new insights into the similarity of the two brains demonstrate why drugs for digestive problems potentially affect mood and why mood-altering drugs may also adversely affect digestion. It also suggests that mental health is highly dependent upon a healthy digestive system and is also affected by the foods that we eat.
According to the naturopathic view, nearly all health problems are not only associated with digestive dysfunction, but are actually ultimately caused by digestive problems although this link may not be obvious. This explains why many practitioners and naturopaths will seek to address the processes of digestion and diet first in order to re-establish a good foundation for health.
Finally, these new insights into the interconnectedness of the mind and body illustrate just one more way in which the old paradigms are breaking down.
Further resources
The Second Brain by Dr Michael Gershon in available from Amazon UK and US.
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