A Brief Summary of Polyvagal Theory

A few years ago, I read the full-size version of Stephen Porges book ‘The Polyvagal Theory: Neurophysiological Foundations of Emotions, Attachment, Communication and Self-regulation.’ To say that book is densely written is an understatement. Every time I read it, I could feel new neurons sprouting in my brain from all the learning, which is kind of cool, but mostly it was just highly exhausting. Some of the content stuck, but I was left with a lot of holes in my understanding of the bigger picture.

There are 2 main parts to understanding the Polyvagal Theory: the anatomy/physiology part and the theory part. I felt like I got lost in the details on anatomy/physiology while reading the full version and finished the book without a coherent sense of the theory. Luckily for me, Porges later wrote a condensed version of his book. So, with some trepidation, I embarked on reading, "The Pocket Guide to the Polyvagal Theory: The Transformative Power of Feeling Safe' with hopes of rectifying my swiss cheese understanding of the Polyvagal Theory.  

The Anatomy/Physiology

There are 12 paired cranial nerves that emerge from the brain, all with different jobs to do. The 10th cranial nerve is called the Vagus nerve, and it originates from the brainstem and ends in the peripheral organs of the body. The Vagus nerve is like a thick electrical cord that has many fibers within it. Some fibers are sending information from the brain to the body (motor fibers), and other fibers are sending information from the body’s organs to the brain (sensory fibers).

The vagus nerve has two main branches; a myelinated branch (located ventrally) and an unmyelinated branch (located dorsally). Myelination refers to a protective lipid sheath that wraps around a nerve to protect it and to speed transmission of signals. A myelinated nerve sends signals faster than an unmyelinated nerve.

Traditionally, we are taught that the autonomic nervous system has two components, the sympathetic and parasympathetic systems, which work to achieve equilibrium. The sympathetic nervous system is the gas pedal which activates the body in ways that are energizing (movement, alertness, exuberance) and ways that support a stress response (fight/flight/freeze). 

The parasympathetic nervous system is in charge of slowing things down (rest & digest). It is composed of the unmyelinated vagus nerve, which connects to organs below the diaphragm such as the gut. However, a small number of unmyelinated nerve fibers also go to the heart and can cause an abnormally slow heart rate.

In his polyvagal theory, Porges explains that an important 3rd component to the autonomic nervous system is the myelinated vagus nerve, which is located above the diaphragm and has branches to the face, head, lungs and heart.

Myelinated Vagus Nerve: The myelinated branch of the vagus nerve connects to our heart and lungs and is also responsible for the expressions of the upper portion of the face, our prosody (intonation) of voice, and our middle ear muscles. Mammals developed a myelinated vagus nerve to meet our need to be parented and protected, and to communicate safety in order to mobilize, socialize, and reproduce.

When we feel safe, our myelinated vagus nerve runs the show, and we can engage with others because we feel safe enough to do so. In this safe physiologic state, we can relax and be expressive with our facial expressions and can pick up and focus on sounds that are in the range of soft, high-frequency harmonics of the human voice. Our own vocalizations are also calm and full. In this state of safety, the myelinated vagus nerve modulates our heart rate, protecting it from both the overexcitement of the sympathetic nervous system (tachycardia) and from the parasympathetic unmyelinated vagus nerve which can cause our hearts to slow down too much (bradycardia).

When we feel unsafe, our face may become expressionless and still, our middle ear muscles lose their tone, and we become very sensitive to low-frequency predator sounds. This loss in tone in the middle ear muscles makes it difficult to hear human voices, and background noise can become overwhelming. If you have worked with clients who get overwhelmed in shopping malls or crowds, the loss of tone in the middle ear muscles, resultant difficulties hearing distinct voices, and increased hypersensitivity to sound may all be contributing.

The Theory 

The theory part of the Polyvagal Theory relates to the idea of there being a hierarchy in terms of our physiological responses, from the top featuring our newer social engagement system, down to fight/flight, and finally, to our last and oldest line of defense, the collapse response. Newer circuits inhibit older circuits.

 The Social Engagement System (mammalian):

The key to the polyvagal theory is understanding the importance of the social engagement system, which is mediated by the myelinated, supradiaphragmatic (above the diaphragm) branch of the vagus nerve. The Polyvagal Theory explains how what’s happening in our bodies (our physiological state) influences our behaviour and feeling states, and thus, our ability to interact with other people.

When we are interacting in the world, we are continuously, both consciously and unconsciously, processing information or ‘cues’ from other people that tell us if they are safe or potentially dangerous. Our nervous system interprets these cues via information from seeing non-verbal signs, including other people’s facial expressions, the intonation of their voice, body movements, gestures, and eye contact. Without consciously processing these cues, our bodies react to other people on a physiological level by either keeping us calm or activated. As mammals, safety is essential to reproduce, digest, and socially connect. However, when we don’t feel safe, our social engagement system via our myelinated vagus is functionally retracted, and our older defense systems take over.

Social play is an opportunity to exercise the social engagement system (largely using face-to-face cues) to override the sympathetic nervous system. With social play, we shift between activating the myelinated vagus nerve (social connection) and the sympathetic nervous system (excitement). If we think about how many of our clients never had the opportunity to play as children due to abuse and lack of safety, it then makes a lot of sense that their ability to regulate their fight/flight/collapse responses could be compromised.

Fight/Flight (sympathetic nervous system): If we feel unsafe or in danger, our social engagement system goes ‘offline’ and then we are left in a defensive state of fight/flight. This involves the traditional sympathetic nervous system, actively pumping out stress hormones. When we are in fight/flight mode, we have an urge to mobilize and either defend or flee.

In trauma and PTSD, people may have a down-regulated social engagement system that keeps them in a state of hyperarousal (sympathetic nervous system), disconnection (reptilian unmyelinated vagus) and in a mode of coping (avoidance), all in an attempt to simply survive and optimize safety. A person’s nervous system may be primed to interpret and react to cues based on past traumatic experiences that don’t always fit with the present-day situation.

Collapse/Shutdown/Dissociate (reptilian): If we are unable to move, defend ourselves or flee because we are confined or held down, then we shift into a collapse/shutdown/dissociative state. This is thanks to our ancient unmyelinated vagal nerve. Why is this? Let’s look at our ancient ancestors; reptiles exemplify the two systems that we are traditionally taught. The sympathetic nervous system increases activity, and the parasympathetic (unmyelinated vagus nerve) decreases activity. This system works just fine since reptiles can defend themselves by immobilizing for extended periods, thus requiring lower oxygenation needs. When we are unable to defend ourselves via our fight/flight response, we physiologically shift into our last line of defense via our unmyelinated vagus nerve. We then mimic our reptilian ancestors by shutting down to conserve energy, increasing pain thresholds, and altering our consciousness level.

Summary:

The guidebook version is a great place to start to learn about the Polyvagal Theory, or if you simply need a refresher. Porges has a lot of interesting things to say in the guidebook about people with autism, sound hypersensitivities, psychiatric diagnostic categories, and our approach to mental health in general. Also, if you want more of the nitty-gritty science, the full book version goes into a lot of detail around things like heart rate variability and respiratory sinus arrhythmia.

Finally, I want to highlight that for me, the most important statement in the entire guidebook is the following: “The feeling of safety is the treatment." If we forget everything about Porges’ Polyvagal Theory and only remember that statement, I think we’ll be okay. We’ll be walking away with the heart of a theory that clearly illustrates how a sense of safety and social connection is a central component of healing.

Dana Ross, MD, MSc, FRCPC is a psychiatrist in the Trauma Therapy Program at Women’s College Hospital in Toronto, Canada. The Trauma Therapy Program specializes in treating those who have experienced childhood interpersonal trauma. She practices individual and group therapy and is an assistant professor in the Division of Psychotherapy, Humanities, & Psychosocial Interventions in the Department of Psychiatry at the University of Toronto. She is the co-founder of traumaedessentials.com which offers online educational content for healthcare providers on topics related to trauma. For other book reviews by Dr. Ross, see traumaedessentials.com/book-reviews.