There’s a model called the Three-Part Brain. The idea is that the brain developed in three evolutionary stages. This concept first came about in the 1960’s, and it’s a useful way of thinking about how the brain develops. The model is that the brainstem is the oldest part, at the bottom of the brain, and developed when the reptiles evolved. Then the limbic system, in the center of our heads, developed when mammals came into being. And the prefrontal cortex, the outer layer at the front of our brain, developed last with the primates and humans. We each have all three parts of the brain, as a testament to our evolutionary past.
But not only did the three parts of the brain evolve bottom up across millions of years, they develop in us as individuals in the same order. The brainstem develops first, then the limbic system, then the prefrontal cortex, mimicking our ancestral history.
Reptilian Brain: The Brainstem
The brainstem, the part of the brain we have in common with the reptiles, is between the base of our brain and the top of our spinal cord.
It’s responsible for the basic housekeeping that keeps us alive and functioning.
This is our life support system. Our hearts beat, we breathe, we digest, we survive thanks to our brainstems. They take care of things that are mostly outside our conscious control, and often also outside our awareness. We’re never aware of our blood pressure until something goes wrong.
Things like arousal, sleep-wakefulness, hunger-fullness, and the delicate chemical balance that keeps us alive are all regulated by the brainstem. And it’s easy to recognize that we have these things in common with most of the animals, such as lizards, fish, birds, and cats.
This part of the brain develops in the womb and is functional by the time we’re born. It’s the first of the three parts of the brain to develop, and it has to be. It’s far more important that your body be able to take your first breath and circulate your blood than it is to regulate your emotions or write an email to your mother. The other two parts have plenty of time to fully develop.
Mammalian brain: The Limbic System
Millions of years after animals had fully functioning brainstems, the mammals evolved. Mammals are social creatures with an emotional life. So with mammals came a well-developed limbic system. This part of the brain sits on top of the brainstem in the center of our heads. It coordinates our emotions, social relationships, and motivation.
The limbic system, our mammalian brain, helps us make emotional relevance out of events.
Your dog gets happy when you get out the leash. He’s associated the leash with walks and has positive emotions about going on a walk around the neighborhood with you.
While other animals may have emotions (birds can definitely sound angry when you disrupt them at a birdfeeder) mammalian emotions and emotional memories are more developed and have a much broader range. And the limbic system is critically important in the formation of long-term memory. We categorize memories by their emotions. More emotionally intense memories last longer and are often more readily recalled.
Not only did it evolve after the brainstem, the limbic system develops second in humans. We are not born with fully functioning limbic systems. Just look at a newborn. He can’t control his emotions. He doesn’t even smile. Sure, he makes simple associations between feelings and events (hungry = upset, full = calm), but he certainly can’t manage all those emotions yet. The brain structures that make up the limbic system aren’t fully developed until about age six. Even then, we continue to develop this part of the brain as we use it. Trauma can have a huge impact on how the limbic system functions. Not only do survivors of trauma often find their emotions (such as fear, anger, and anxiety) are out of whack, their social relationships can suffer, memories are often all-too-present – or completely out of reach.
Human Brain: Prefrontal Cortex
The final part of the brain to evolve was the prefrontal cortex. It became fully developed with the emergence of the higher primates, and particularly with human beings. The term “cortex” literally means “bark,” and the cortex is the thin outer layer of gray matter, like the bark of a tree. “Prefrontal” just means that it’s at the very front. So this is the thin, wrinkly layer of cells on the outside of your brain, just behind your forehead.
And this is a very important part of the brain.
This is the part that is so much more complicated in humans than it is in other animals. It gives us the ability to think and reason.
It makes us rational creatures. It gives us language and empathy. Its key roles are anticipating what is about to happen and planning ahead. We wouldn’t be who we are without this part of the brain.
And as anyone who remembers being a child or teenager realizes, this part of the brain takes a long time to develop. It starts early, but it certainly takes its time. While toddlers learn language, elementary school kids understand cause-and-effect, simple logic, and empathy, the prefrontal cortex undergoes massive reconstruction during adolescence, which in brain development terms lasts from age twelve until about age twenty-four. So the prefrontal cortex isn’t done with its initial development until we’re in our early to mid-twenties.
How the Three-Part Brain Relates to Anxiety
Anxiety is trying to protect us from a very wide range of danger. These threats can loosely be seen as connected to any of the three parts of the brain: survival, emotional-social, or rational.
The survival threats of harm can be from physical sources: immediate ones such as a bookcase that’s precariously leaning over, or a chronic threat like a violent ex that just might show up at your door one day.
The brainstem is responsible for the shut-down response – the freeze response – that reptiles use as a first line of defense under threat. We carry an ancient survival skill of playing dead – either holding perfectly still or going completely limp – and often shutting down our basic awareness of what is going on around us. Under extreme duress, we go into shut-down as a last-ditch effort to survive.
Anxiety can alert us to social threats, too. Back in the early days of humanity, being part of a social group was necessary to survival. Exile from the group was certain death, so your ancestor had better find a way to fit in.
We face that threat of not belonging to our social circles in a dizzying variety of ways today. And even if we might not physically die if we are rejected, it can feel like we might. That’s partly because it isn’t just a legacy from ages past.
As babies and small children, we would die if we were rejected. First, not getting our physical needs met means starving to death. But even in orphanages, they’ve found that when workers who are overwhelmed and understaffed manage to meet babies’ physical needs but not hold, soothe, and pay attention to them, some infants will simply die, even though they had enough to eat.
Belonging to a group is truly life-or-death from the beginning of our lives, and it’s no surprise we continue to be anxious about social rejection.
We face threats on a rational or thinking brain level as well. The primary tasks of our prefrontal cortex are anticipation and interpretation. This leads directly to the human ability to worry. Your dog might tear up the trash can if you come home late to feed him, but he doesn’t worry that you’re near the bottom of the dogfood bag and don’t get paid until next week.
On the one hand, the ability to reason, plan, and anticipate are wonderful blessings that enable us to thrive and live longer. On the other hand, the ability to worry about the future is one of the doorways to much human suffering.
Recruiting the Different Parts
Each of our three brain parts has a different way of causing and responding to anxiety.
One thing that can be helpful to do when we’re anxious and can’t easily calm ourselves is to notice which part of our brain is most anxious.
If it’s mainly in our reptilian brain, you might feel it physically – either your heart is racing and you’re breathing fast or you are feeling spaced out, numb, and sluggish. Or both at the same time – like one foot on the gas and one foot on the brake. If this is the strongest part of your anxiety, try to engage another part of your brain. Connect with another person to turn on your mammalian brain, or recite boring, logical facts to yourself to engage your human rational brain. Even pull out a book and make yourself read a couple of paragraphs about anything.
If your mammalian brain is anxious – you’re overwhelmed with emotions or feeling abandoned or rejected, then engage the human brain or the reptilian brain. This is where breathing exercises, regular exercises, taking a walk or a hot bath can engage your brainstem and recruit it to calm you down.
Likewise, if your human, rational brain is worrying non-stop, it’s time to get out of your thoughts and use your mammalian or reptilian brains. Just focusing on your body, your emotions, or another person – or maybe listening to music – can bring down the intensity of the fear and worry.
The idea of learning about the three-part brain model to help with anxiety is to meet each part’s needs where it is, and to recruit other parts to help when one gets overwhelmed. It takes a little practice, but even the act of practicing can remove you from the anxiety and give you something else to do with your three-part brain.