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The Polyvagal Theory and why should we care?


I came across the polyvagal theory during my Kinesiology studies. It has completely changed my perspective on what I thought I knew about our nervous system (acquired through my Pharmaceutical studies). It is a breakthrough work offering different understanding of human behaviours and trauma and I thought it was really worth a blog.

The polyvagal theory from Dr. Stephen W Porges revisits the anatomy and physiology of one the most important actor of the mind-body connection: the vagus nerve.

It presents new ways of approaching mental health, such as anxiety, depression, recovery from early trauma which can commonly keep individuals living in perpetual fight-or-flight state or in freeze state (emotionally shutdown or disconnected). It has also further application when dealing with children’s social engagement or behavioural issues. To me, it does reinforce again the importance of a mind-body approach in promoting health and wellbeing focusing on the importance of neuro-regulation.

FIRST A LITTLE BIT ABOUT THE VAGUS NERVE

Vagus is latin for “wandering” which is a perfect name considering that it is the longest of the 12 cranial nerves and that it has the widest distribution in the body.

It originates in the brainstem (at the back of the head) and regulate many organs: oesophagus, heart, lung, gut.

  • 80% of its fibers are sensory. This means they collect information from our internal organs and send the information to the brain stem and to higher structure.

  • 20% of its fibers are motor and regulated by the brain cause our heart to go faster or slower.

In its tonic state, the vagal nerve has an inhibitory function, that works like a brake on the heart’s pacemaker and allows us to calm down (commonly called the vagal brake).

It is said that 80% of the parasympathetic nervous system (PNS) neurology is carried through the vagus nerve. The PNS has a calming effect promoting health, growth and repair.

YOUR NERVOUS SYSTEM IS NOT BINARY

The autonomic nervous system (ANS) is the part of the nervous system that acts largely unconsciously, without you having to command it and regulates bodily functions such as the heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal.

In many textbooks (including the way it was initially taught to me) the ANS is described in 2 parts: sympathetic nervous system (SNS), which is often considered the "fight or flight" system, while the parasympathetic nervous system (PNS) is often considered the "rest and digest" or "feed and breed" system. These 2 systems are seen having "opposite" actions where one system activates a physiological response and the other inhibits it.

Dr. Stephen Porges theory revolutionizes this idea showing that the Autonomic Nervous System (ANS) is not solely a paired antagonistic system (like PNS and SNS) but a hierarchical system in which newly evolved circuits inhibits older circuits.

Through studying the neuroanatomy of the vagus nerve, he shows that even though there is only 1 vagus nerve, mammals have in fact 2 vagal circuits:

  • an unmyelinated* shared with reptiles and retained through evolution.

  • a uniquely mammalian circuit that is myelinated.

*Myelinated means it is surrounded by a layer of fat.

The myelinated portion only represents 3%. These 2 vagal circuits originate in different areas of the brainstem.

“We have 2 circuits for defense, the popular fight-flight and a more ancient, reptilian one that causes “shut down”, “immobilisation”, “freeze”, “death feigning” and this one manifests in people who are restrained and have no options to get out (like for example in situations of abuse or trauma).

Reptilian style response, playing dead, stop breathing was adaptively useful for them (made it more difficult to be detected by predators) and they could do it for a very long time they don’t need much oxygen. But that response, which has been retained through evolution is not very adequate for mammals who required lots of oxygen. Freeze and shutting down respiration and heart rate can quickly lead to death”.

Interestingly Dr. Porges shows that this newest myelinated circuit appearing in mammalians is responsible for controlling facial & laryngeal muscles. Hence, controls vocalisations, facial expressivity, and the muscles in the middle ear that controls listening.

For example, studies show if myelinisation is not working people can get auditory hypersensitive and difficulties to understand human voice in background noise.

He called this circuit the Social Nervous System (SocNS), to acknowledge its importance for social interaction. Through evolution, mammals chose to live in groups, needed to communicate and bond in order to survive.

HIERARCHY OF BASIC SURVIVAL RESPONSES

According to Dr. Porges the hierarchy to the basic survival responses is as below :

  • Absence of danger, safety cues: we can engage in social behaviour, bond (Social Nervous System)