Polyvagal theory

The Polyvagal theory (gr. 'polus', “‘many’” + 'vagal', "'vagus nerve'") specifies two functionally distinct branches of the vagus, or tenth cranial nerve. It serves to identify the relationship between visceral experiences and the vagus nerve's parasympathetic control of the heart, lungs, and digestive tract. The theory was introduced in 1994 by Dr. Stephen Porges, director of the Brain-Body Center at the University of Illinois at Chicago. According to the theory and its subsequent proof,[1] the automatic nervous system is interconnected with and sensitive to influences that flow from the body toward the brain, called afferent influences. This effect has been observed and demonstrated by adaptive reactivity dependent on the neural circuits' phylogenical development. It builds on the study of what Charles Darwin referred to as the “pneumogastric nerve." The polyvagal theory claims that humans have physical reactions,[1] such as cardiac and digestive changes, associated with their facial expressions. Porges argues this theory with observations from both evolutionary biology and neurology.

The branches of the vagal nerve serve different evolutionary stress responses in mammals: the more primitive branch elicits immobilization behaviors (e.g., feigning death), whereas the more evolved branch is linked to social communication and self-soothing behaviors. These functions follow a phylogenetic hierarchy, where the most primitive systems are activated only when the more evolved functions fail. These neural pathways regulate autonomic state and the expression of emotional and social behavior. Thus, according to this theory, physiological state dictates the range of behavior and psychological experience. Polyvagal theory has many implications for the study of stress, emotion, and social behavior, which has traditionally utilized more peripheral indices of arousal, such as heart rate and cortisol level. The measurement of vagal tone in humans has become a novel index of stress vulnerability and reactivity in many studies of populations with affective disorders, such as children with behavioral problems and those suffering from borderline personality disorder.

Phylogenetic subsystems/stages

The vagus nerve is a primary component of the autonomic nervous system. Polyvagal theory outlines the structure and function of the two distinct branches of the vagus, both of which originate in the medulla.[2] More specifically, each branch is associated with a different adaptive behavioral strategy, both of which are inhibitory in nature via the parasympathetic nervous system. The vagal system is in opposition to the sympathetic-adrenal system, which is involved in mobilization behaviors. According to polyvagal theory, these opposing systems are phylogenetically arranged.[2]

The dorsal vagal complex

The dorsal branch of the vagus originates in the dorsal motor nucleus and is considered the phylogenetically older branch.[3] This branch is unmyelinated and exists in most vertebrates. This branch is also known as the “vegetative vagus” because it is associated with primal survival strategies of primitive vertebrates, reptiles, and amphibians.[3] Under great stress, these animals freeze when threatened, conserving their metabolic resources.

The dorsal vagal complex (DVC) provides primary control of subdiaphragmatic visceral organs, such as the digestive tract. Under normal conditions, the DVC maintains regulation of these digestive processes. However, prolonged disinhibition can be lethal for mammals, as it results in apnea and bradycardia.[2]

The ventral vagal complex

With increased neural complexity seen in mammals (due to phylogenetic development) evolved a more sophisticated system to enrich behavioral and affective responses to an increasingly complex environment.[2] The ventral branch of the vagus originates in the nucleus ambiguus and is myelinated to provide more control and speed in responding.[2] This branch is also known as the “smart vagus” because it is associated with the regulation of sympathetic “fight or flight” behaviors in the service of social affiliative behaviors.[3] These behaviors include social communication and self-soothing and calming.[2] In other words, this branch of the vagus can inhibit or disinhibit defensive limbic circuits, depending on the situation. The VVC provides primary control of supradiaphragmatic visceral organs, such as the esophagus, bronchi, pharynx, and larynx. The VVC also exerts important influence on the heart. When vagal tone to the heart’s pacemaker is high, a baseline or resting heart rate is produced. In other words, the vagus acts as a restraint, or brake, limiting heart rate. However, when vagal tone is removed, there is little inhibition to the pacemaker, and so rapid mobilization (“fight/flight”) can be activated in times of stress, but without having to engage the sympathetic-adrenal system, as activation comes at a severe biological cost.[2]

Vagal tone: a physiological marker of stress

In order to maintain homeostasis, the central nervous system responds constantly, via neural feedback, to environmental cues. Stressful events disrupt the rhythmic structure of autonomic states, and subsequently, behaviors. Since the vagus plays such an integral role in the peripheral nervous system via regulation of heart rate, it follows that the amplitude of respiratory sinus arrhythmia (RSA) is a good index of parasympathetic nervous system activity via the cardiac vagus.[4] That is, RSA is a measurable, noninvasive way to see how the vagus modulates heart rate activity in response to stress. This method is useful to measure individual differences in stress reactivity.

RSA is the widely used measure of the amplitude of heart rate rhythm associated with rate of spontaneous breathing.[5] Research has shown that amplitude of RSA is an accurate indicator of the efferent influence of the vagus on the heart.[5] Since inhibitory effects of the VVC branch of the vagus allow for a wide range of adaptive, prosocial behaviors, it has been theorized that individuals with greater vagal tone are able to exhibit a greater range of such behaviors. On the other hand, decreased vagal tone is associated with illnesses and medical complications that compromise the CNS.[5] These complications may reduce one's capacity to respond to stress appropriately.

Clinical applications of polyvagal theory and vagal tone

Vagal tone has been used in medical and psychological research to better understand the physiological underpinnings of various disorders.

Clinical applications in the human fetus

Healthy human fetuses have a high variability in heart rate, which is mediated by the vagus.[6] On the other hand, heart rate decelerations, which are also mediated by the vagus, are a sign of fetal distress. More specifically, prolonged withdrawal of vagal influence on the heart creates a physiological vulnerability to the influence of the Dorsal Vagal Control, which in turn produces bradycardia (very low heart rate). However, the onset of this deceleration is commonly preceded by transitory tachycardia, which is reflective of the immediate effects of Ventral Vagal Control withdrawal.

Results of Porges' Theory

As described by Bessel van der Kolk, professor of psychiatry at the Boston University School of Medicine:[7]

The Polyvagal Theory provided us with a more sophisticated understanding of the biology of safety and danger, one based on the subtle interplay between the visceral experiences of our own bodies and the voices and faces of the people around us. It explained why a kind face or a soothing tone of voice can dramatically alter the way we feel. It clarified why knowing that we are seen and heard by the important people in our lives can make us feel calm and safe, and why being ignored or dismissed can precipitate rage reactions or mental collapse. It helped us understand why focused attunement with another person can shift us out of disorganized and fearful states. In short, Porges’s theory made us look beyond the effects of fight or flight and put social relationships front and center in our understanding of trauma. It also suggested new approaches to healing that focus on strengthening the body’s system for regulating arousal.

See also

References

  1. 1 2 Porges, Stephen W (April 2009). "The polyvagal theory: New insights into adaptive reactions of the autonomic nervous system". Cleveland Clinic Journal of Medicine. 76 (Supplement 2): S86–S90. doi:10.3949/ccjm.76.s2.17. ISSN 1939-2869. PMC 3108032. PMID 19376991.
  2. 1 2 3 4 5 6 7 Porges, Stephen W (October 2001). "The polyvagal theory: phylogenetic substrates of a social nervous system". International Journal of Psychophysiology. Elsevier. 42 (2): 123–146. doi:10.1016/S0167-8760(01)00162-3. ISSN 0167-8760. PMID 11587772.
  3. 1 2 3 Beauchaine, Theodore P; Gatzke-Kopp, Lisa; Mead, Hilary K (February 2007). "Polyvagal Theory and developmental psychopathology: Emotion dysregulation and conduct problems from preschool to adolescence". Biological Psychology. Elsevier. 7 (2): 176. doi:10.1016/j.biopsycho.2005.08.008. ISSN 0301-0511. PMC 1801075. PMID 17045726.
  4. ↑ Porges, Stephen W (2011). The Polyvagal Theory: Neurophysiological Foundations of Emotions, Attachment, Communication, and Self-regulation. W. W. Norton & Company. ISBN 978-0-3937-0700-7.
  5. 1 2 3 Porges, Stephen W (2011). The Polyvagal Theory: Neurophysiological Foundations of Emotions, Attachment, Communication, and Self-regulation. W. W. Norton & Company. p. 69. ISBN 978-0-3937-0700-7.
  6. ↑ Reed, Shawn F; Ohel, Gonen; David, Rahav; Porges, Stephen W (September 1999). "A neural explanation of fetal heart rate patterns: A test of the polyvagal theory". Developmental Psychobiology. Wiley. 35 (2): 109. doi:10.1002/(SICI)1098-2302(199909)35:2<108::AID-DEV4>3.0.CO;2-N. ISSN 1098-2302. PMID 10461125.
  7. ↑ Van Der Kolk, Bessel (2014). The body keeps the score: brain, mind, and body in the healing of trauma. New York: Viking Penguin. p. 81. ISBN 9780670785933. Retrieved 3 February 2018.
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