the 100 billion neurons in your skull conduct electrical signals to communicate and carry

messages between them, ultimately creating, well — YOU... and everything you do.

It's the most complex, essential and identifiable organ in the body, acting as the center of

the nervous system in all vertebrates. But what if how you think and feel isn't just

controlled by the brain and central nervous system, but in fact, a second brain that you

didn't know about?

The brain uses the vagus nerve to connect and communicate with many parts of the body,

including the vocal cords, heart, lungs and most of the digestive tract. It also uses

this nerve to convey "fight or flight" messages to the body, which explains why you

might experience a cracking voice, racing heart, shortness of breath and a knot in your

stomach when you're really stressed or nervous. But while this extensive network is constantly

sending signals from the brain to your body, scientists have noted that 80-90% of the nerve

fibers in the enteric nervous system are actually going from the gut to the brain.

In fact, the enteric nervous system, which is an extensive mesh-like network of neurons

that controls your entire digestive tract from the esophagus to the anus, doesn't

even require the brain at all. When the vagus nerve is severed, the enteric nervous system

is capable of organizing and initiating its own reflexes for digestion because it has

such a complex collection of neurons. It is your second brain, and controls you far

more than you realize.

From an evolutionary perspective — before modern food safety protocols — our biggest

choices were about eating. Is it safe? Will it make me sick? Will it give me energy? This

is why we would need a direct line of communication between the gut and the brain. For example,

fat and sugar are good sources of fuel for the body, so we have evolved mechanisms to

notify the brain to release the "feel-good" neurotransmitter dopamine when we eat foods

high in fats and sugars, which make us continually seek them out.

But the enteric nervous system becomes even more interesting when you realize that the

microbes in your gut actually produce over 50% of the dopamine found in your body, and

90% of the serotonin; two neurotransmitters that drastically affect your mood, happiness

and pleasure. And these microbes can manipulate your cravings and eating behaviours for their

own survival. Your diet affects which bacteria thrive, but as a result, the flourishing bacteria

in your gut can then send signals to the brain via the vagus nerve to tell you to keep eating

the foods they love. This is why some scientists have suggested that treatment with probiotics

or fecal transplants could be an effective therapy for overeating; by getting rid of

certain bacteria that generate intense cravings for high-fat and high-sugar foods.

In fact, current research is pointing to a gut with healthy microbiomes as a strong predictor

of emotional and mental health. Mice raised in sterile environments, where no bacteria

could colonize their gut, exhibit social traits similar to those seen in humans on the autism spectrum.

When researchers studied their brains, they also found significant changes in the

levels of serotonin and specific proteins related to learning and memory formation.

But after feeding the mice with specific probiotics to help cultivate healthy microbiomes,

the symptoms were alleviated. Their gut controlled their behaviour!

A 30-day study comparing people who ate probiotic yogurt found a decrease in the levels of anxiety

and depression compared to people who consumed milk with no bacteria.

And another study found that mice fed a specific bacteria had less stress-related hormones

in their blood and performed better on tests of learning and memory. But when researchers

severed the vagus nerve, all of these improvements disappeared. Again, their guts were in control

of their emotions!

Studies have even shown that stimulating the vagus nerve at different frequencies, by implanting

a small pacemaker, can help patients with treatment-resistant depression, by mimicking

vagal nerve activity like a person with a healthy gut.

And the essential role of the gut doesn't end there. Given that large social groups

increase the potential of one's genes being passed on — for both us and our bacteria —

researchers now believe that the function of the microbiome is partly to promote pro-social behaviours.

Healthy gut bacteria can lead to a higher resilience of negative emotions from others,

which makes you more forgiving and better at fostering strong social connections, ultimately

improving your shot at reproduction.

All thanks to your second brain!

Special thanks to Audible for supporting this episode

to give you a free 30 day trial at audible.com/asap.

This week, we wanted to recommend the book "How Not To Be Wrong:

the power of mathematical thinking", which helped us create our

previous episode, "Are good looking people jerks?"

It's an awesome read. You can get a free 30-day trial at audible.com/asap, and choose from massive selection.

We love them as they're great when you're on the go. And subscribe for more weekly science videos!