The Stress Resilient Mind Blog
Publication date: 03 February 2012
Recently I came across the work of Dr Stephen Porges, who developed the polyvagal theory as a more complete model of the Autonomic Nervous System (ANS). The ANS is a critical part of the physiology of stress and anxiety. The polyvagal theory makes a lot of sense and can potentially explain some findings that are anomalous from the point of view of the traditional understanding of the ANS. It also has something to say about how biofeedback and mindfulness help transform the physiology of stress.
The ANS is made up of two branches, the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PSNS), which between them control much of the body's automatic functioning such as heart rate, digestion, metabolic activity and blood sugar. The traditional understanding is that the two have opposite and antagonistic effects on the body's arousal level. The SNS ramps it up while the PSNS calms it down again. They're like the accelerator and brake in a car, or like the two arms of a set of scales insofar as they mutually inhibit each other.
The classic view of stress is that in the acute phase at least, the sympathetic is dominant. The sympathetic response is commonly known as the fight or flight response because it prepares the body for action. The parasympathetic is sometimes known as the rest and digest response, because it calms us down and re-engages the digestive system (which the SNS tends to switch off).
It's not that this picture is wrong – just too simplistic or incomplete. For instance it can't explain that asthma is typically made worse by stress, yet physiologically it is a state of parasympathetic dominance (the PSNS restricts the air ways in the lungs).
The polyvagal theory derives its name from the vagus nerve, a complex and multi-branched pathway which carries parasympathetic signals from the brain to the viscera, but which also carries sensory data in the other direction, from the viscera to the brain. Dr Porges saw that the vagal system could be subdivided into two, based on function, anatomy and phylogeny. PSNS signals originate from at least two brain nuclei. Dr Porges notes that the two vagal subdivisions arose at different points in our evolutionary past. Let's call them the old and the new vagal systems. Very old species such as sharks have only the old vagal system. The SNS appears to have evolved separately again. So what we have is a hierarchy of three systems.
At the lowest level is the old vagal, which seems to have evolved as a defence against scarcity by ramping down metabolism to conserve resources. It's still present in mammals and humans, where it manifests its effects as a freezing response, involving a loss of muscle tone and a blanking of the mind, even fainting. This can be catastrophic, even resulting in death at times. Next up we have the SNS, which is also a defence but this time it ramps up metabolism to prioritise resources for action. The SNS inhibits the old vagal system.
At the highest level we have the new vagal system, which is found only in mammals. It differs from the old vagal system in using myelinated nerves which work much faster than the old vagal's unmyelinated nerves. The new vagal system acts as a brake on the SNS. Anatomically it stimulates the heart and lungs, and also the face. The muscles of the face allow us to express emotions, particularly positive and social emotions.
This new understanding helps us see how stress and anxiety in the form of sympathetic dominance put us at such a disadvantage: we become hyper-vigilant and our social systems are disengaged. The new vagal enables us to re-engage our social systems. In the most extreme cases of stress, such as in trauma, the old vagal system may come to dominate.
I've certainly got a lot more to learn about the polyvagal theory. I've order Dr Porges' book.
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