Another crucial physiological component in trauma is the autonomic nervous system (ANS), which governs involuntary bodily functions and modulates states of arousal and rest. The ANS is divided into two primary branches: the sympathetic nervous system (associated with fight-or-flight responses) and the parasympathetic nervous system (linked to rest, digestion, and repair). Trauma causes profound dysregulation in this system, often pushing individuals into prolonged defensive states that alternate between hyperarousal (sympathetic overdrive) and hypoarousal (parasympathetic shutdown) (van der Kolk, 2014).
Stephen Porges’ Polyvagal Theory has significantly advanced our understanding of how trauma affects the ANS, primarily through the vagus nerve. This tenth cranial nerve interfaces with the heart, lungs, digestive tract, and brainstem. According to Porges (2011), the vagus nerve operates through two pathways: the ventral vagal complex, which promotes social engagement and emotional regulation, and the dorsal vagal complex, which governs immobilization or “freeze” responses. In safe conditions, the ventral vagal system supports connection, calm, and resilience. However, in the presence of an overwhelming threat, trauma survivors often shift into dorsal vagal dominance, resulting in collapse, numbness, or dissociation.
This autonomic imbalance contributes to a wide range of trauma symptoms: insomnia, digestive issues, shallow breathing, cold extremities, derealization, and emotional blunting. Moreover, trauma survivors often show low vagal tone and reduced heart rate variability (HRV), both indicators of poor parasympathetic flexibility and an impaired capacity to return to homeostasis after stress (Thayer & Lane, 2009).
From a therapeutic standpoint, understanding ANS dysregulation is essential. Symptoms such as shutdown, dissociation, or “freeze” responses are not psychological resistance but biological survival adaptations. These autonomic responses can be tracked and regulated through bottom-up interventions that support ventral vagal engagement, such as paced breathing, safe touch, rhythmic movement, vocalization, and co-regulation with an attuned other (Dana, 2018).