The Polyvagal Theory
The autonomic nervous system is traditionally described as consisting of two antagonistic and balanced branches, the sympathetic and the parasympathetic system. The sympathetic system or the “fight or flight” arousal system kicks into gear when we perceive threats. When that happens your blood pressure increases, your muscles tense, your heart beats faster, and digestion slows down.
The parasympathetic system has been thought to be responsible for “rest-and-digest”. Say it is a nice, sunny day and you decide to relax and take in the nice weather in a comfortable chair. In that situation your “rest and digest” response should cause your blood pressure to decrease, pulse rate to slow, and digestion to start.
Something else, however, is happening in this video.
In the video the possum immobilizes and looks dead, but it’s not engaging in a voluntary behavior, or a ‘fight or flight’ or ‘rest and digest’ response.
It is using a defense mechanism called immobilization which causes a drastic slowing of the metabolic system. The observation of this ancient and primitive system, which is frequently used by reptiles and other primitive animals, helped prompt Dr. Stephen Porges to develop “The Polyvagal Theory” for humans.
Three (Not Two) Autonomic Nervous Systems
The Polyvagal Theory splits the autonomic nervous system into three hierarchical systems.
Oldest System – The oldest system is mediated by unmyelinated vagus nerves that originate in the dorsal motor nucleus of the vagus nerve and extend to the viscera below the diaphragm. In humans this system serves two purposes: In a “threat” context, this system is characterized by immobilization, fainting, and dissociating. If you’ve ever walked into a room and suddenly been immobilized with fear, your body had probably invoked the unmyelinated vagal defense system. This is the primitive defensive mechanism frequently observed in reptiles and small rodents and in the opossum in the video.
However, when humans are in “safe” contexts, the system works to support the subdiaphragmatic organs (the gut) that promote health, growth, and restoration via the classic “rest and digest” mechanisms.
Next Oldest System - The next oldest system, the familiar sympathetic-adrenal nervous system, actively inhibits the older vagal unmyelinated nerve defense system. When triggered this system also stops digestion and mobilizes energy resources resulting in hypervigilance, increased blood pressure, and tension in the muscles for the “fight or flight” response.
Newest System – The most recently evolved autonomic system is unique to mammals. The myelinated vagus system originates in the Nucleus Ambiguus and is linked to the adrenal system, the heart, and muscles of the face. Usually, the myelinated vagus system calms us and actively inhibits the sympathetic-adrenal system and reduces inflammation while processing our moment-to-moment cardiovascular and metabolic needs.
Putting the Brakes On
Dr. Porges says that we are unconsciously always checking to see whether or not we are in a “safe” social environment.
The myelinated vagus system inhibits the high states of arousal associated with the sympathetic nervous system via something called “the vagal brake“. The vagal brake can be thought of as the pacemaker of our heart. Our normal heart rate would be quite high were it not for the myelinated vagus nerve to inhibit it.
Imagine that one foot is pushing the accelerator pedal almost to the floor of your car and the other foot is on the brake of your car. You modulate the system by simply letting your foot up off the brake or pushing it down a bit. It is as though our autonomic system is always primed to take off and the vagal brake is modulating this.
The vagal brake supports the metabolic requirements for mobilization and communication by instantaneously modulating our heart rate while rapidly engaging and disengaging with objects and other individuals within our social environment. This allows us to adjust to the metabolic demands of our moment-to-moment social environment without engaging the sympathetic-adrenal (fight or flight) system.
Dr. Porges says that our system is unconsciously always checking to see whether or not we are in a “safe” social environment. In fact, Dr. Porges names this whole process “The Social Engagement System“. When we are in a “safe” environment the myelinated vagal system has a calming effect on the autonomic system.
The Polyvagal Theory says that our bodies attempt to use these systems in a hierarchal manner. When stress is not present, the most recently evolved system, the myelinated vagus, actively orchestrates our moment-to-moment autonomic functions so long as we are functioning in a “safe” environment. This dynamic interaction is reflected in our heart rate variability, which is considered a key indicator of autonomic system health.
We are unconsciously always appraising our social environment by listening and visually scanning the body movements and especially the faces of people within a given social context. Our nervous systems are also continuously subconsciously monitoring our physiological environment. (This subconscious process has been termed ‘neuroperception’.)
When our social and physiological environments are appraised as being safe, the defensive limbic structures (the sympathetic-adrenal and unmyelinated vagal systems) are inhibited, and calm visceral states emerge. When a “threat” is first perceived, the myelinated vagus system withdraws the vagal brake and the sympathetic adrenal system instantaneously dominates. Dr. Porges does not like to think of this as an on/off system, but rather as a dynamic interaction. On rare occasions in humans, a life threat is encountered which engages the primitive unmyelinated vagus system, and the result is a system shutdown (think immobilization with dissociation or fainting).
When the Brakes Fail!
Dr. Porges’s Polyvagal Theory is much more elegant and complex than portrayed here. On a gross scale it ties together physiology and psychology (psychophysiology). It is upon the basis of this mostly unconscious “safe or threat” evaluation that humans engage socially. Those of us who, for whatever reason, cannot feel “safe” within our social or physiological environments are actively engaging the sympathetic-adrenal system or even the primitive unmyelinated vagus system, and if we live on a long-term basis with these systems engaged, we suffer the consequences.
While most of Dr. Porges’s research and theory covers the physiology of psychological disorders, he does have a few comments about physical disorder.
The inability to distinguish a “safe” environment from a “threat” environment causes these more primitive “threat” systems to be constantly engaged without our consciously knowing it. There can be many causes of this inability to recognize a “safe” environment. It can be due to very early childhood experiences or trauma or, in some cases, physiological disorders.
Dr. Porges also believes these “threat” states are inherently pain-inducing. The sensory fibers associated with the unmyelinated vagus nerve modulate pain when operating within a “safe” environment, but if a “threat” environment is present, then the pain modulation does not occur and the pain signals are amplified.
The Polyvagal Theory presents a different way to view the autonomic functions of humans. I encourage you to read further as the theory extends into many areas of human development and behavior. The Polyvagal Theory extends into the areas of mother-child bonding, PTSD and trauma, vagal tone, heart rate variability, meditation, music therapy, autism, and personality disorders.