The important role of digestion and absorption of food is managed by our second brain – a nine-metre tract of neurons living in our guts, and the source of that 'gut feeling'.
One of Ayatollah Khomeini’s odder statements was, “If a fly gets into the throat of one who is fasting, it is not necessary to pull it out.” What he wouldn’t have known was that the fly mentioned would be profoundly interfering with the human body’s second brain, a nine-metre long collection of neurons (brain cells), neurotransmitters and proteins that function very similarly to the brain in our heads.
At least the Ayatollah left the option to remove the fly (even though it’s not necessary). And yes, biologically-speaking, we really do have two brains in our bodies.
This second brain is called the Enteric Nervous System (ENS), and resides in the tissues lining the oesophagus, stomach, small intestines, large colon, and ending in the anus. So you're actually insulting two brains simultaneously when you say someone’s stupid as an a*****e.
The ENS has around 100 million neurons but no concept of consciousness, although it can think (in a certain fashion) and even store memories. Certainly it does have the ability to make you feel good or bad – it is the source of those gut feelings that you’re not well or nervous.
There’s a theory about why the ENS exists.
On primordial Earth, the earliest nervous systems were reactive systems; i.e. they served sea organisms which simply waited for food to float in. As such, they had simple autonomic nervous systems (ANS). As animals evolved, they became proactive and started to hunt for food or sex. This required a more complex nervous system as they had to manage locomotion with their new limbs – and process information from newly-evolved eyes, ears, tongues and other sensory organs.
However, the older functions of the ANS involved with body regulation, digestion and waste control were still required. With simpler animals, a new kind of brain (a basic limbic system) evolved to augment the ANS – this meant that they could develop and learn techniques to escape predators and seek food and sex – and importantly still digest and excrete without having to spend time thinking about that too.
This new brain is called the Central Nervous System (CNS) and it enabled animals to interact with their environment – and this profoundly enhanced their survival chances. As animals grew larger, both the CNS and ANS grew more complex and some portions, particularly those related to digestion evolved to become the ENS in vertebrates.
In the case of Palaeolithic man, the CNS (including the limbic system) became our cranial brain and spinal cord. The human limbic system still has a lot to do with survival – it is here that we feel emotions and learn not to touch hot kettles or poke eyes and stuff. The rest of the CNS allows us to taste, see, hear, feel, smell, learn, think and at the same time, breathe and pump the heart unconsciously.
The human ENS then took over the role of digestion and regulating some important hormones. The ENS in humans has more neurons than the spinal cord so it really should be able to do more than just controlling digestion and secreting hormones. What more it can do, however, is unclear at the moment.
What we do know is that the ENS manages peristalsis (intestinal movements), digestive enzymes secretions and the release of hormones regulating blood sugars – these are all managed by chemical sensors in the ENS which monitors readings from the food being digested. Additionally, the ENS plays an enormous part in how we feel.
Stress induces the “butterflies in the stomach” sensation – this is caused by signals from the CNS, which in turn triggers the production of hormones such as serotonin, which then agitates the sensory nerves in the ENS around the stomach. Stress-induced serotonin can also overstimulate the intestines and induce diarrhoea (and hence the joke about why French soldiers wear brown trousers). Excess serotonin can have the same excitatory effect on the oesophagus, causing a choking sensation.
The digestive system also shuts down temporarily during stress to divert energy towards tackling any impending threats. So it seems that the main reason for having the ENS is the division of labour between two bodily systems that require a lot of complex management.
The CNS controls everything we do consciously and allows us to manage our environments. It also handles some of the subconscious autonomous functions – and it is free to do all of this because the important role of digestion and absorption of food is managed by our second brain.
If we liken our body to a car, the driver is the main brain, able to steer and buy fuel for the car, and the second brain is actually the car engine itself – providing the power to go where it’s needed.
However, this division of labour comes at a price. The ENS communicates to the brain via the vagus nerve and the traffic is overwhelmingly one way, from the ENS to the CNS – and generally the news is negative, like intestinal discomfort or the need to visit the toilet.
No news from the ENS is good news indeed. But it's not only about watching our diet. What we think and feel in our first brain affects our second brain profoundly and therefore we need to be aware of this rebound effect. Basically, if we feel bad, the ENS can make us feel even worse. This anxiety served our Palaeolithic ancestors by freeing up energy for flight while keeping them alert and sensitive to dangers.
However, in modern humans, such stress is often not unlike a driver wildly accelerating while steering a car – the impact of any mistake will simply be amplified by the resulting car crash.