Your Second Brain

Sometimes people joke about there being two brains: a female one inside the head and a male one inside the trousers. (The same comparison is made of women’s and men’s hearts, too). If you think that in this chapter I’m going to talk about this supposed second brain – the pride and vanity hidden inside your underwear - then you’re mistaken. Although I do understand why that kind of ‘brain’ is famous: its decision-making power is so strong that it isn’t affected by the mental ‘filters’ of cool rationality, common sense and logic.
What I’m going to talk about in this book is another brain: our digestion. It is not as glam- orous and interesting as our sex drive but can be just as wild and unpredictable; and we use it far more because our digestive brain goes into action several times every day.
The intestine is not the part of our anatomy we are most passionate about or one that in- creases our pulse rate. No famous poet has written an ode to it and normally artists are not in- spired by the ‘beauty’ of the digestive system. Quite the opposite in fact. The most common view of the gut is that it is an ugly body part which looks a bit like a snake, smells badly and sometimes makes socially unacceptable and embarrassing noises.
However, I promise you that we have a true brain in our intestine and its neuronal function is very similar to the brain in our heads which has the capacity to perceive beauty. The digestive system has its own extensive network of neurons, which are located between the two muscular layers on its walls. The structure of these digestive neurons is identical to that of the cerebral neurons: both release the same neurotransmitters, hormones and chemical molecules.
I want to introduce you to the enteric nervous system (ENS), our ‘second brain’. This is not a metaphor; it is the official name accepted by the medical profession.
The importance of the ENS was only demonstrated relatively recently with the publication of the work of Professor Michael Gershon, chairman of the Department of Anatomy and Cell Biol- ogy at Columbia University, New York, and the forerunner of the new science of neurogastroen- terology. This new area of speciality studies the symptoms of psychosomatic upsets which have a gastrointestinal expression and relates them to the central nervous system. Doctor Gershon has spent thirty years of his scientific career on an in-depth study of the attitude and behaviour of the human guts, and he has confirmed that our nervous digestion system has its own cerebral activity and intelligence. His book published in 1999 was a significant step forward in informa- tion on the ENS compared with the existing medical and scientific understanding of the subject at the time.
According to new data, the figure for the total number of neurons found in the small intestine is around a hundred million. This figure represents a considerably higher number than the neu- rons in the spinal cord. The brain in our gut is the main production line responsible for produc- ing and storing the chemical substances called neurotransmitters, most of which are identical to those found in the central nervous system (CNS), such as acetylcholine, dopamine and sero- tonin. These substances regulate our moods and our emotional and psychological well-being; and form a group of essential substances ensuring correct communication between the neurons and the body’s warning system. They represent the ‘words’ in the neuronal language.
The presence of such a wide variety of neurotransmitters in our intestines is a clear indication of the complexity of the rich digestive language and its ability to carry out neuronal functions and express its own emotions.

Gershon revealed that 90% of serotonin (the famous ‘happiness’ or ‘feel-good’ hormone ) is produced and stored in the intestine where it regulates peristaltic movements and sensory trans- mission. Only the remaining 10% of the body’s serotonin is synthesised in the neurons of the central nervous system, i.e. in the higher brain.
This minimum amount of cerebral serotonin is of vital importance for human beings, and per- forms various functions, including regulating our mood (the calm ‘feel-good’ sensation), ap- petite, sleep and muscular contractions, and it intervenes in cognitive functions such as memory and learning. Serotonin is the ‘messenger of happiness’ and thanks to it the neurons can communicate with each other, releasing it and capturing it again as needed .
Before this revelation, the scientific world did not pay much attention to this aspect of the in- testines and did not appreciate the nerve network which runs through them. The general view was that the decisions made by the upper brain were dominant and that its influence on the di- gestive system was one-way i.e. that the process was directed downwards from the brain. The scientific observations of Professor Gershon, however, now lead us to think that influence travels in both directions; that there is constant communication between the two brains : the one inside our skull and its twin down there in our gut.
I can assure you that the relationship between the two brains, which involves the hormonal, metabolic and emotional levels, is very complex and we could even call it “intellectual”; it is also normally quite democratic and mutually respectful.