How the Body and Brain Communicate

Our bodies and brains really are not separate things, even though they are typically treated thusly. We tend to think of the body as the meat sack the brain is in charge of hauling around, when in fact they are dynamic and interlinked.

One of the major pathways of brain-body communication is the tenth cranial nerve, otherwise known as the vagus nerve, (historically referred to in Traditional Chinese Medicine as the du channel) which serves as the foundation of polyvagal theory. All methods of body-brain intercommunication appear to organize through this channel.

It’s the nerve that physically creates the mind-body connection, linking your brain and your gut and the rest of you. It’s the body’s communication system. It’s how information, inflammation, and relaxation spread. And because our whole body is involved in our emotional health, understanding the vagus nerve helps us understand both our physical health and our emotions, and how they are connected. We’ll look at trauma, inflammation, stress, and toxicity and how the vagus nerve mediates them all.

The vagus nerve’s information flow is designed to help us stay alive and stay safe. But like every other safety process, it is designed to err on the side of caution by embedding memories of past threats and overcorrecting juuuuuuust in case something might be dangerous. Hence, the prevalence of trauma-informed responses and fuckery galore.

In this blog, we’re going to take some extremely fancy science and boil it down into something a normal-but-nerdy-enough-to-be-reading-this human can understand. If you can understand these ideas, then you’ll have the key to understand so much about how our bodies and brains work, how they get messed up, and how they can heal.


The vagus nerve is the information highway between the emotional-cognitive centers of the brain and the body, including our intestinal functions.

Right off the bat, one of the most important things to know about the vagus nerve is that it swings both ways (heh): It carries information and activates response both from the brain to the body and from the body to the brain. We like to think of the brain as being in charge, but we have four times more messages going up to the brain rather than down from the brain—80% of are body-to-brain (afferent messaging), and only 20% is brain to body (efferent messaging).

Junior high science flashback: Nerves are cells that have special communication functions. Like telephone lines that run all over your body, they communicate throughout the body as bioelectrical signals. Like, what happens if you touch a hot thing? The nerves in your hand send a message of “OW!” to your brain, which then sends a message back saying: “WELL PUT IT THE DOWN THEN! THAT’S THE LITERAL MEANING OF DROP IT LIKE ITS HOT!!!”

Nerves exist throughout our body and serve to transmit electrical impulses for intra-body communication. Our sensory and motor functions in the body are specifically tasked to 12 cranial nerves. Some of these cranial nerves are super specific. Like the olfactory nerve, unsurprisingly transmits information about what we smell to our brain. But number 10 of 12, the vagus nerve, is the longest, weirdest, and most complex of the twelve pairs of cranial nerves. The name itself (vagus) comes from the Latin word for “wandering” because this nerve wanders all around the damn body, sending information to organs and tissues. It wouldn’t be irrational to say that this particular nerve is an information superhighway.

Nerve cells don’t just bop around aimlessly like your useless ex. They are organized into systems. Nervous systems are the networks of the nerve cells and fibers that transmit messages. These systems make the messages transmit more efficiently. So that you will, indeed, drop it like it’s hot when need be.

Now while the vagus nerve is the messanger for an entire nervous system, it is only one of several nervous systems that create the entire human nervous system. So, the human nervous system can be broken up into the central nervous system (brain and spine) and the peripheral nervous system which connects the brain and spine to the rest of the body. The peripheral nervous system can be split again into two parts:

  • The somatic nervous system, which we have voluntary control over
  • The autonomic nervous system which is the part that we do not have voluntary control over, which regulates all of our organs and endeavours to maintain bodily homeostasis

The autonomic system is the one we are focusing on in this book. All autonomic means is self-governing. These nerves work involuntarily and are not under conscious control. So you are not allowed at any time to decide whether or not you are going to drop the hot thing. That’s the kind of shit that will get you killed. Your body is not interested in negotiation, shit’s just getting dropped. You’re starting to feel me on why this is important info, yeah?

The autonomic nervous system has three components: the sympathetic, parasympathetic, and enteric nervous systems. And the vagus nerve is their communication pathway. It’s important to understand how they work in relation to each other.

The sympathetic nervous system governs our fight-flight-freeze instinct, which is critical to our survival. It is activated in the face of a threat or perceived threat, whether it be to our actual, physical safety, the individuals we love and care for, our possessions, or our needs, wants, desires, well-being, and belief systems. Essentially, anything that challenges the core of who and what we are and who or what belongs to us invokes a protective response. It has nothing to do with how mentally or physically strong we are or how much we love the person or people that we are with. That ability to be engaged and connected goes offline.

If we perceive aggression as a means of escape, we will fight. If our instincts tell us we can’t fight effectively but we can escape, we will flee. If neither of these responses are likely going to be effective, we will freeze.

All of these strategies, even fighting, are purely defensive ones. The intent is not to win against or overwhelm the threat, but to survive. The body’s job is to size up the situation and figure out the best means of staying alive. It chooses a strategy based on the information it is receiving and past understanding of the threat. We may move back and forth between strategies as we take in information and gauge our effectiveness at managing the perceived imminent threat. All three responses are part of our defensive, survival instincts.

I’m focusing so much on the freeze response (official science-y term: dorsal-vagal response) because it’s talked about the least. And it’s the one that most human beings are embarrassed about, although it makes just as much sense as a protective strategy as fight and flight do. It’s also the most physically dangerous: the freeze response can actually kill you if it goes on too intensely, too long.

So why does the body do something so dangerous? Peter Levine (until Porges, he was one of the few theorists to talk about the body’s survival mechanism and trauma) states that there are four potential evolutionary survival benefits to the freeze response:

• Most predatory animals won’t eat an animal they believe is already dead unless they are really hungry. Most animals have encoded information that meat that is already dead may be spoiled and therefore is a risk to eat.

• It is more difficult for predators to detect prey that is not moving. Immobilization shuts down all movement responses. Even if we are trying to be still and quiet it is difficult to do so unless we have become biochemically immobilized.

• When one animal collapses in a group, this distracts the predator from the rest of the group, allowing their escape.

• The freeze response releases a numbing agent in the body that makes the pain of attack more bearable.

Now obviously, it makes sense that the sympathetic nervous system should not be in charge all the time. It would make us fall apart at the seams pretty quickly (and this is why people who don’t have conscious ways to cope with chronic stress do fall apart at the seams pretty quickly).

Our second nervous system, the parasympathetic nervous system, is the one associated with social relationships and bonding. We’re pack animals. We need to connect to other human beings for long-term survival, not just for mating season, and be able to navigate the world without feeling batshit crazy and threatened by our surroundings all the time. Being relaxed and alert is how we best engage with the world around us. This means that when the parasympathetic system is in charge, we are able to connect and communicate and form social relationships because we feel safe to do so.

The foundation of Porges’ research on the vagal system is the finding that our nervous systems work in ranked order (a hierarchy). Safety and connection and relationships—the functions governed by the autonomic nervous systems— rank highest on the hierarchy. But because these processes are managed outside of conscious thought, our ability to remain calm and engaged and relational is the first thing we lose when our bodies perceive threat. In order to be our best selves, to live and work in cooperation and build relationships, our bodies have to perceive us as being safe.

But when something challenges our sense of safety at a bodily level, the parasympathetic system goes offline. This works because the sympathetic nervous system is myelinated, while the parasympathetic nervous system is not. I know, I know with the science textbook words, but this is actually an important point. Myelination provides insulation, which speeds up the sending of the messages. So the nervous system that is in charge of our stress response works faster than the one that is in charge of our chill response.

Both systems operate involuntarily. And the sympathetic nervous system has to be turned off for the parasympathetic nervous system to work, meaning it has to determine that there is no stress or threat and that you are allowed to go about your day. But even when it’s not running the show, the sympathetic nervous system is still humming in the background, watching for danger. And it works faster than the chill system, so it will shut shit down on a moment’s notice.

Any of us with unresolved trauma histories are far more likely to have our parasympathetic systems go offline, putting us into survival mode. We’ll get more into this in the trauma chapter coming up next.

The stomach is called the second brain for a reason. The third part of the autonomic nervous system is the enteric nervous system, which resides in the gastrointestinal tract and is the largest of the three autonomic nervous systems and has its own unique microcircuits. It receives feedback from the other two systems but is also able to operate independently of them. There are people out there with severed vagus nerves and their enteric nervous systems work just fine. This is some zombie science shit.

Because many neurotransmitters, signalling pathways, and anatomical properties are common to the enteric nervous system and the central nervous system, it’s safe to say that the vagus nerve is actually messaging our “gut feelings” to the brain.

And those gut feelings are loud. Our internal bodily state is communicating continuously with the brain far more than the brain is communicating back with the body (4 to 1, remember?). The brain is dependent on the physiology of the body to manage our breathing, our heart rate, whether or not we are hungry or full, and the nervous system activity related to our emotional states. This isn’t something we think about (pun intended), which is why Dr. Candace Pert, the neuroscientist who first became known as the person who discovered the opiate receptor in the brain as a freakin’ undergrad, stated that the entirety of the human body operates as the unconscious mind.

If the vagal system is the body’s information superhighway, how exactly does that relate to stuff like being scared, activated, or experiencing a trauma trigger? We know that memories of past events are stored in the amygdala, but how exactly is that memory file being pulled up? Is it just within the brain or is it also in the body?

Dr. Pert’s research described and explained how human moods, feelings and emotions are mediated and experienced through peptides. Peptides are made up of amino-acids (the organic compounds that combine to form proteins in the body) but can be best thought of as information substances because their common function in any form is to distribute information throughout the body.

Yes, there is a high concentration of peptide receptors in the amygdala. But there are also high concentrations of them at the back of the spinal cord (vagus!) and any place in the body where information about the five senses enter the body (vagus vagus vagus!). Peptides are what activate the vagal system in order to send and receive messaging. Our body-mind is using peptide cues to choose or discard both emotions and behaviors. And the vagal system is the mechanism for how the information travels.

This is how, as Dr. Pert put it, traumatic emotional events put down deep roots in the body, not just as episodic-autobiographical memories. She noted that “while we may not remember something consciously, the psychosomatic network in the body does.”

And that, smart cookie, is exactly why we’re talking about this in so much scientific depth. Your feelings are real. It’s not a made-up discomfort, it’s biochemical and bioelectrical messaging happening between your body and mind on a constant basis. And the system is wired to err on the side of caution to keep you safe. So if you have been through some deep shit, your whole body becomes wired with a hair-trigger invader detection system. That doesn’t mean healing is impossible. It’s completely fucking possible and I see people do it every day, which is why I love my job so much. But we have to understand what healing entails for the entirety of the body, how to change our relationship with past events in our lives, and how to move forward as healing survivors.

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