Polyvagal Theory: Understanding the Nervous System and Trauma

Polyvagal Theory, founded by Dr. Stephen Porges, Ph.D., provides a groundbreaking understanding of the relationship between the autonomic nervous system (ANS) and behavioural responses, as well as the impact of chronic stress on physical health. Before the development of this theory, the nervous system was oversimplified, seen as a binary system that was either “on” or “off” in response to stress. However, Porges revealed a more complex picture, introducing a three-part hierarchical model and highlighting the crucial role of the vagus nerve in the autonomic system’s communication and connection.1

What Is Polyvagal Theory? Sympathetic and Parasympathetic Activation

To understand the polyvagal theory, it is important to have a basic understanding of the sympathetic and parasympathetic nervous systems and the vagus nerve. The vagus nerve belongs to the parasympathetic nervous system, often referred to as the rest-and-digest system, which promotes a state of calmness in the body. Porges identified a hierarchy of responses within the autonomic nervous system, including ventral vagal social engagement, sympathetic nervous system activation, and dorsal vagal shutdown.

The ventral vagal social engagement is the state in which humans can connect and relate to others. It is characterised by openness, compassion, joy, mindfulness, and curiosity. This state enhances various bodily processes such as digestion, immunity, circulation to the extremities, and the ability to form relationships while reducing defensive responses.

Sympathetic nervous system activation, which evolved approximately 400 million years ago, triggers a state of mobilisation known as fight-or-flight. In this state, the body prepares to confront a threat by increasing arousal and activating various physiological responses. Heart rate, blood pressure, adrenaline levels, and blood circulation all increase, while digestion, immunity, relational ability, and fuel storage decrease.

However, sympathetic activation cannot be sustained indefinitely, as it would be detrimental to health. The body self-regulates and eventually adopts dorsal vagal shutdown, a more primal state characterised by freezing, lethargy, hopelessness, and reduced basic bodily functions. This shutdown state decreases heart rate, blood pressure, temperature regulation, and immune response, and it also affects social functioning, such as eye contact, facial expressions, and intonation.

According to Polyvagal Theory, the ANS forms the foundation for all human experiences. It explains how individuals engage with the world through various states of activation, including connection, disconnection, and attunement. However, some people may experience a mismatch, perceiving the environment as dangerous even when it is safe.

Polyvagal Theory helps us understand how stress is stored and processed in the body. Chronic stress can alter the brain’s perception of stimuli, leading to increased sensitivity to threats and the interpretation of non-threatening input as dangerous. This heightened perception of danger can be emotionally and physically exhausting, affecting various aspects of life, including work, relationships, hobbies, and studies. It can also impact social interactions, causing individuals to perceive benign stimuli as threatening, which can strain relationships. Recognising the different states of activation and understanding individuals’ needs in each state is crucial for supporting their well-being.2

Polyvagal Theory and Trauma

Polyvagal Theory is particularly relevant in trauma recovery. Trauma, whether experienced in early childhood or later in life, disrupts the autonomic nervous system’s regulation and can lead to hypersensitivity to perceived threats. Dysregulation of the autonomic nervous system can result in a range of physical and psychological symptoms, such as chronic pain, digestive issues, depression, anxiety, PTSD, and substance use disorders. Traumatic experiences can also impact future relationships, as individuals may develop a fear of intimacy and connection due to past experiences of danger or uncertainty with primary caregivers or trusted individuals.

By understanding the dysregulation of the autonomic nervous system (ANS) caused by trauma, we can better comprehend the physical and emotional consequences individuals may experience.

Trauma and chronic stress, particularly in early childhood, can lead to ANS dysregulation. A dysregulated ANS may overreact to perceived threats or dangers that do not actually exist. Living in a constant state of mobilisation or immobilisation can be debilitating, leading to chronic pain, digestion issues, and difficulties in connecting and socialising. Maladaptive coping mechanisms like substance use, gambling, and unhealthy eating habits may develop as a result.

Trauma, which is understood as an experience rather than an event, can arise from various causes such as childhood neglect, abuse, parental drug use or incarceration, and adverse childhood experiences (ACEs) like living in dangerous or impoverished neighbourhoods. These traumatic experiences can lead to relational or developmental trauma. Research shows that individuals who experience three or more ACEs are at increased risk of asthma, heart disease, depression, anxiety, and obesity.3 The compounded stress and trauma affect the central nervous system, disrupting ANS functioning and communication through the vagus nerve, which connects the brain to the digestive system and heart. This chronic dysregulation of the ANS results in physical and emotional symptoms as the system continuously searches for threats. Digestive disorders, autoimmune diseases, chronic fatigue, depression, anxiety, PTSD, and substance use disorders are among the common health issues associated with trauma or chronic stress.

Furthermore, early childhood experiences significantly impact future relationships, as the ANS becomes overprotective in adulthood due to chronic feelings of danger or uncertainty around primary caregivers. Adults who experienced violence, neglect, or inconsistent and inattentive parenting may reject intimacy and connection, viewing them as dangerous despite their deep desire for love and connection. The need for connection is often replaced by a need for protection.

Although adverse childhood experiences have profound effects on development and health, trauma at any stage of life can disrupt the autonomic nervous system. Accidents, assaults, natural disasters, and chronic stress are some examples of events that can cause ANS dysregulation. For individuals with a history of trauma, the ANS reacts not only to the perceived safety or danger in their immediate environment but also to the internal interaction between the present environment and the fear or stress triggered by past life events.4

The Polyvagal Theory provides valuable insights into therapy and community support. By recognising the importance of safety, understanding defence mechanisms, and adapting interventions based on the individual’s autonomic responses, therapists can effectively assist clients in their healing journey. Additionally, by incorporating the principles of the Polyvagal Theory into community practices, we can foster a more supportive and empathetic society better equipped to assist individuals dealing with stress and trauma.


  1. Porges S. W. (2009). The polyvagal theory: new insights into adaptive reactions of the autonomic nervous system. Cleveland Clinic journal of medicine, 76 Suppl 2(Suppl 2), S86–S90. https://doi.org/10.3949/ccjm.76.s2.17
  2. Porges, S. W. (2011). The polyvagal theory: Neurophysiological foundations of emotions, attachment, communication, and self-regulation. W.W. Norton.
  3. Herzog, J. I., & Schmahl, C. (2018). Adverse Childhood Experiences and the Consequences on Neurobiological, Psychosocial, and Somatic Conditions Across the Lifespan. Frontiers in psychiatry, 9, 420. https://doi.org/10.3389/fpsyt.2018.00420
  4. Steimer T. (2002). The biology of fear- and anxiety-related behaviours. Dialogues in clinical neuroscience, 4(3), 231–249. https://doi.org/10.31887/DCNS.2002.4.3/tsteimer