Three brains are tucked inside your head

We don’t have one brain in our heads; we have three. We started with a “lizard brain” to keep us breathing, then added a brain like a cat’s, and then topped those with the thin layer of Jell-O known as the cortex—the third, and powerful, “human” brain. 

The prefrontal cortex is only the newest addition to the brain. Three brains are tucked inside your head, and parts of their structure took millions of years to design. (This “triune theory of the brain” is one of several models scientists use to describe the brain’s overarching structural organization.) Your most ancient neural structure is the brain stem, or “lizard brain.” This rather insulting label reflects the fact that the brain stem functions the same in you as in a gila monster. The brain stem controls most of your body’s housekeeping chores. Its neurons regulate breathing, heart rate, sleeping, and waking.

Sitting atop your brain stem is what looks like a sculpture of a scorpion carrying a slightly puckered egg on its back. The Paleomammalian brain appears in you the same way it does in many mammals, such as house cats, which is how it got its name. It has more to do with your animal survival than with your human potential. Most of its functions involve what some researchers call the “four F’s”: fighting, feeding, fleeing, and … reproductive behavior.

Large neural highways run overhead these two brains, combining with other roads, branching suddenly into thousands of exits, bounding off into the darkness. Neurons spark to life, then suddenly blink off, then fire again. Complex circuits of electrical information crackle in coordinated, repeated patterns, then run off into the darkness, communicating their information to unknown destinations.

Arching above like a cathedral is your “human brain,” the cortex. Latin for “bark, the cortex is the surface of your brain. It is in deep electrical communication with the interior. This “skin” ranges in thickness from that of blotting paper to that of heavy-duty cardboard. It appears to have been crammed into a space too small for its surface area. Indeed, if your cortex were unfolded, it would be about the size of a baby blanket

Brain Rules: 12 Principles for Surviving and Thriving at Work, Home, and School by John Medina