The Vagus Nerve: From Animal Spirits to Poly Vagal Theory

This article traces the development of our understanding of the ANS and the vagus nerve in particular, from the Greeks, to the modern era of Gellhorn, Selyes, Levine and Porges. It will appear in the September issue of Advanced Clinical Education

The notion that the body was fundamentally divided into two systems, an animal (somatic) and organic (autonomic) originated with the ancient Greeks. (Clarke E, 1987).  In the second century AD, Galen was able to follow the vagus nerve from the cranium, exiting the jugular foramina and travelling to the organs. (Clarke E, 1987)The Latin word vagus means wandering or rambling which is the type of journey the two branches of that nerve make through the thoracic and abdominal cavities. Galen also identified the sympathetic trunks as crossing the ribs and communicating with the organs via plexuses. He was able to identify most of the cranial nerves but incorrectly grouped the glossopharyngeal, vagus and accessory nerves within the sympathetic trunk, his sixth cranial nerve. Galen saw the nerves as pipes, allowing “animal spirits” to pass between the organs, a process he termed “sympathy”. (Webber, 1978)

Over a millennium later the renowned Italian anatomist Vesalius improved upon Galen’s work although he left unaltered the notion of the sixth nerve as being the source of the sympathetic trunk. (Ackerknecht, 1974) As an interesting side bar, it’s worth noting that the cadavers the anatomists of the time had to work with were for the most part victims of capital punishment, via either decapitation or hanging, which made study of the cranial nerves connection to the body quite challenging!! It wasn’t until the French anatomist Jean Riolan in the early 1600s, that the sympathetic trunk was said to originate from spinal nerves rather than from a branching off from the vagus. (French, 1971) Other contributions of note regarding the autonomic nervous system came from: Willis (nomenclature of the cranial nerves especially “wandering” vagus); Winslow (the first to document the gray and white rami communicantes); du Petit (conducted sympathectomies to investigate relationship of sympathetic nerves to cranial ones); and especially, Bichat. (Oakes, 2016). Exploration of the nervous system was limited to gross histology and embryology which meant that without microscopic detail, the enlarging understanding of anatomy was not matched by a corresponding increase in knowledge of physiology.

With the advent of the scientific age, studies of anatomy and physiology became increasingly more precise. The functions of the nervous system were no longer attributed to animal spirits and “sympathy” moving in the body. Still, it’s worth noting and appreciating the very prescient statement of Johnstone in 1795, when he stated:

“In the next age, consequences of these truths will be unveiled to an extent not now to be conceived but of the greatest importance in the healing art: in the meantime, what is here proposed will furnish some direction to a sagacious searcher into the seat and nature of internal diseases”. - (Clarke E, 1987)

It would be another hundred and fifty years before this insight into the relationship between the nervous system and the “nature of internal diseases” was understood and most fully articulated by Selye. More about him shortly.

In 1898, Langley introduced the term autonomic nervous system (ANS) which he saw as a more accurate term than the one that was in usage at the time, the visceral nervous system. The term autonomic implies a certain amount of autonomy but under the influence of a higher power – in Greek auto means self and nomos means higher law. Of course, this is the term we continue to use today and it accurately describes the relationship of how the higher centres in the brain influence the function of the non-somatic nerves in the periphery. Langley also coined the terms sympathetic and parasympathetic and observed that these two systems produce opposing effects in the digestive tract, heart, vasculature and lungs.

According to many stress researchers, as well as historians, modern biological formulations of stress can be traced back to Hans Selye and an article written in 1939 where he speculated that there was a characteristic triphasic pattern of nonspecific physiological responses to injury that he termed the General Adaptation Syndrome or Selye’s Syndrome. (Selye, 1936)  The three stages were an initial alarm phase, followed by a stage of resistance or adaptation, and where the stress was unrelenting or additional stresses were added that overwhelmed the adaptation response, exhaustion and death. He defined this adaption syndrome as the “the sum of all non-specific, systemic reactions of the body which ensue upon long continued exposure to stress”. Selye’s emphasis was largely on the adrenocortical mediation of the stress response and he made only modest contributions to understanding the role of ANS in his proposed process of adaptation. His research on adaptation and stress was reported in national newspapers, magazines, and clinical journals across the world. Time magazine, for example, ran a number of cover stories on the multiple manifestations of stress in the lives of contemporary Americans, often focusing on Selye’s theories. (Jackson, 2014).

 Selye situated his work primarily within studies of traumatic shock and toxicology although his research took place within a wide contemporary interest in adaption in general. Concerns about the impact of social and technological change on human health and on the limits of adaptability were clearly shaped not only by increased knowledge of physiology but also by fears about increasing levels of mental stress, fatigue, and traumatic neurosis among industrial workers and returning soldiers. (Rosenberg, 1998) Notable among these who studied these psychosocial developments was Francis Crookshank (1873-1939) who observed in 1923 that:

 “The fact is that, at the present time, social change—industrial, economic, and the like—is everywhere modifying the conditions of life more rapidly than we care to admit; — far more rapidly for the exercise of the natural powers of adaptation of the human race.” (Crookshank, 1923)

Some authors say that it was Walter Cannon’s description of what he termed “social homeostasis” that actually provided the blueprint for Selye’s subsequent construction of a “natural philosophy of life,” which was put forward to aid people in maintaining or restoring individual happiness, and social stability. Cannon also coined the term fight or flight and, as an interesting side bar, discovered, along with other notable researchers of the time that saline infusions could be used to treat shock. (Cannon, 2017 )

In the modern age, the importance of the autonomic nervous system is viewed not only in terms of the physical health of heart, vasculature, digestion etc. but also as a significant determinate of our personal subjectivities – our sense of who we are, how we feel and how we behave. Giving attention to these subjectivities – our sense of self - is a relatively new phenomena in the West. By way of contrast, in the East explorations of the inner landscape of mind and body via meditation and yogic practices have been a central part of Hinduism and Buddhism for at least four thousand years. Borrowing a term from the renowned Harvard psychologist, William James, – who, incidentally,  praised the ability of yoga to stabilize the nervous system and restore or unleash “unused reservoirs of power” in a Presidential address to the American Philosophical Society in 1906 (Ramsden, 2014) –  Alan Wallace calls this method of enquiry into personal experience, “radical empiricism”. Stillness in the mind and body can lead to higher knowledge (insight) as well as heightened states of consciousness that transcend the “noise” of everyday states of mind, or discursiveness. (Wallace, 2015)

In the 1960s research into the ANS by Gellhorn led him to propose a neurophysiological explanation for these states of calmness and insight as being associated with a concordant increase in the activity of the parasympathetic branch of the ANS. (Gellhorn, 1967) Like Langley, he observed that the SNS and PNS operated in a process of reciprocal inhibition whereby an increase in the activity in one branch would lead to a suppression of the other. Gellhorn observed that the ANS could become “tuned” in one direction or another and rather than exhibit a sinusoidal wave of SNS and PNS activity throughout the day, the ANS would get stuck, usually in SNS arousal and develop a new baseline there. Access to feelings of calmness, and high levels of reflective thought is lost when the ANS is tuned in this way. Self-regulation and emotional well-being become increasingly difficult as the fight or flight processes associated with the SNS are in a constant state of “on”.

The capacity of certain kinds of manual therapy to “unstick” this static condition was well illustrated in Cottingham’s study into the effects of the Rolfing method of myofascial treatment and structural alignment (Cottingham J, 1988)  His research made use of a Vagal Tone Monitor (VTM), used before, during and after Rolf style myofascial release. The VTM monitors fluctuations in the Respiratory Sinus Arrhythmia (RSA) to determine the outflow of the Vagus nerve. The initial study demonstrated a reflexive spike in PNS activity during a procedure and a follow up study showed that the baseline of PNS activity could be shifted as a result of a series of ten, Rolf style MFR, soft-tissue treatments. Forty years on this dynamic of tuning and the associated loss of what Cottingham termed “autonomic flexibility” is now being taken up in the context of heart rate variability and its relationship to health, fitness and rehabilitation. (Cottingham J, 1988)

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Another important contribution from Gellhorn is his research that shows muscle tone, hypothalamic and emotional excitability are positively correlated. What does this mean in practical terms? Basically, it shows that willed changes in muscle tone may be used to control emotions although at the conclusion of the same article he acknowledged that this is difficult to achieve in situations that are arousing negative or in other ways uncomfortable states. (Gellhorn, 1970)

Gellhorn’s research also looked at the ANS in terms of its integrations, or lack thereof, with the somatic systems. Ergotropic (SNS) and Trophotropic (PNS) tuning were investigated at the level of the cerebral cortex as well as the effects they had on hypothalamic temperature. (Gellhorn, 1967)  His work was seminal and created a framework for the complex reappreciation of the ANS that is at the heart of Polyvagal theory.

Poly Vagal Theory

To appreciate the theory, developed by Dr Stephen Porges, a little update on neuro anatomy will be helpful. Porges has detailed how the neural regulation of the ANS has changed through evolution. In mammals, such as we humans, the ventral (front) side of the vagus nerve has evolved to respond to cues of safety in our environment and interactions. This branch of the vagus originates in an area of brain stem known as the nucleus ambiguous and travels primarily to organs above the diaphragm, and interacts within the brain stem with structures regulating the striated muscles of the face and head. This relationship, of vagus nerve to striated muscle fibres associated with expression, is central to appreciating how this branch of the vagus has become known as the social nervous system. It supports feelings of physical and emotional safety and creates emotional connection to others in our social environment. This emergent social engagement system provides the mechanism for co-regulation of physiological state, as mammals can convey cues of safety and danger—via vocalizations, head gestures, and facial expressions—to conspecifics and other species as well - such as our pets! To survive and prosper mammals need to be able to consistently and successfully down regulate primitive defence mechanisms (Porges & Dana, 2018)

The dorsal (back) side of the vagus nerve responds to cues of danger. We have this branch in common with all vertebrates and it functions to down-regulate the body into a low energy consuming state of immobility or freezing. In humans and other mammals, it manifests as death feigning for survival – play dead and they’ll leave me alone - and the trauma-driven responses of syncope and dissociation (Porges & Dana, 2018)

The brain does not have to consciously evaluate for risk or safety. Porges proposes a new term, neuroception, for the subconscious activity in the brain that monitors the environment and alerts us to either what and whom is unsafe, or to situations that are safe and nurturing. Other authors have reported that the brain can get stuck in a negative bias where the neuroceptors perceive almost every situation and interaction as dangerous with associated aggression, high anxiety and other forms of emotional distress. Furthermore, in some instances, neuroception appears to switch off almost entirely, leaving the person with an inability to sense real threats to safety. (Walker, 2022)

Here’s a summary of this perspective on the autonomic nervous system:

Parasympathetic / Ventral Vagal state — our centred “true self” state, where all authentic social interaction, connection and cognition occurs. Feeling safe.

Sympathetic state — sensing threat or danger via neuroception, and feeling the need to either “fight” or “flee” from a situation to seek safety

Dorsal Vagal state — we sense our safety is so immediately threatened that we shut down and collapse. “Freezing” is a hybrid state between this Dorsal Vagal state and the sympathetic nervous system

The dynamic interplay between these three is well illustrated here:


polyvygal chart 1024x597

Used with permission: From Ruby Jo Walker LCSW, SEP, CHT


Perhaps what’s most exciting about Poly Vagal theory is that it not only provides a comprehensive description of the vagus nerve and its various functions but it’s also prescriptive, if you like, of approaches that remedy autonomic, and sensory-motor, dysfunctions. The approach is largely “bottom up”, or through the body, rather than “top down” which is more the domain of psychology and psychiatry via talk therapy and drugs. At unite, and the associated group iLs (Integrated Listening Systems), both of which work to make Porges’s Poly Vagal approaches widely available, there is acknowledgement that top down has its place but also a recognition of the ways in which those approaches in isolation have failed many people with PTSD, trauma, learning difficulties and sensory motor disorders . (Porges, 2022).

Bottom up approaches include the Somatic Experiencing (SE) work developed by Peter Levine, a non-invasive listening therapy called the Safe and Sound Protocol (SSP), Dr Bessel van der Kol’s trauma resolution exercises (as well as the other therapies he recommends like Structural Integration and yoga) and Pat Ogden’s Sensorimotor Psychotherapy, plus many others.

In conclusion, it seems important to place these theories and practices regarding the restoration of more adequate and fulfilling autonomic functions, and the lessening of the actions and attitudes that drive dysfunction, into societal contexts. Numerous authors – Cannon, Crookshank, Selye, James, Hannah, Rolf – have argued, from their different places in history, that the pace of life and the values developed in capitalist economies lead to nervous system overwhelm generally, with exhaustion, breakdown and disassociation the outcome for many individuals. Capital has shown that it has no conscience. Remedies for these corrosive effects of modern life are not easy to come by and it would be naïve to think that someday there will be a world without violence, exploitation and sociopaths in management, and public office. For example, here in Australia one in 6 Australian women and 1 in 16 men have been subjected, since the age of 15, to physical and/or sexual violence by a current or previous cohabiting partner. This is now, 2022.  If we are to put this knowledge of the nervous system to its best use, then let’s hope the skilful methods developed by Porges, Levine, Ogden and so on are not just about more navel gazing for the already privileged but find their way into the nitty gritty world of community health, veteran’s health as well as to the survivors of torture, domestic and institutional abuse.


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