When most people think about the autonomic nervous system (ANS), they often reduce it to two categories: fight or flight (sympathetic nervous system) and rest and digest (parasympathetic nervous system). While this simplified model offers a useful starting point, modern neuroscience – particularly Polyvagal Theory —reveals a far richer and more nuanced picture. Each branch of the ANS has distinct subdivisions that shape how we respond to stress, safety, and social connection.
The Sympathetic Nervous System: More Than Just Fight or Flight
The sympathetic nervous system (SNS) is traditionally known as the fight or flight”system, responsible for our body’s immediate response to danger or stress. However, the SNS governs arousal and energy mobilization more broadly, which includes positive and playful activation, not just fear or threat responses.
Consider the example of a child running and laughing during play. Their SNS is active, but this is not a stress response – it is a joyful, engaged activation that supports:
- Increased heart rate and breathing to fuel physical movement
- Heightened sensory awareness and alertness
- Energised muscles ready for action
- Positive emotional engagement and social bonding
This illustrates that the sympathetic nervous system also supports playfulness, exploration, and connection. It is a system finely tuned to modulate arousal based on context and perception, rather than simply switching on in “stress” mode.
Anatomically, the SNS ganglia are housed along the spine, spanning from the C8 to L2 vertebrae, orchestrating this mobilisation response throughout the body.
The Parasympathetic Nervous System: Rest, Digest, and Beyond
The parasympathetic nervous system (PNS) is often simplified as the “rest and digest” system, promoting relaxation, digestion, and recovery. But Polyvagal Theory expands this view by identifying two distinct branches within the parasympathetic system:
Ventral Vagal Complex
- (Myelinated, Newer Evolutionarily)
- Supports social engagement, calm alertness, and feelings of safety and connection.
- Helps regulate facial expression, vocalisation, and the ability to be present with others
- Enables a relaxed, connected state conducive to healing and well-being
Dorsal Vagal Complex (Unmyelinated, Older Evolutionarily)
- Governs immobilization, but with two very different expressions:
- Healthy dorsal vagal activation – Enables deep rest, restorative sleep, and digestive processes – the classic “rest and digest”.
- Dysregulated dorsal vagal activation: Leads to collapse, freeze, shutdown, numbness, withdrawal, and overwhelm. This protective response can occur during trauma or extreme stress, when the body essentially “shuts down” to conserve energy and protect itself.
Recognising these two modes helps us differentiate between healthy stillness and harmful shutdown — a crucial distinction for trauma-sensitive therapies and somatic practices.
Dynamic Modulation: Beyond the Binary of Stress and Rest
The autonomic nervous system is not a simple on/off switch between stress and relaxation. Instead, it dynamically modulates arousal and regulation depending on factors such as:
- Safety and environmental cues
- Social connection and engagement
- Internal bodily states and past experiences
This dynamic regulation explains why some sympathetic activation feels playful and energising, while other times it feels anxious or overwhelming. Similarly, parasympathetic activity can support both deep healing and, when dysregulated, maladaptive shutdown.
Why This Matters for Healing and Therapy
Understanding the complexity of the autonomic nervous system enriches approaches to healing, trauma recovery, and somatic therapy. It highlights the importance of supporting:
- Safe social engagement (ventral vagal activation)
- Balanced arousal states, including positive sympathetic activation
- Recognising and gently moving out of dorsal vagal shutdown
- Practices such as SomaSensing breathwork, mindful movement, and somatic awareness can help retrain the nervous system, promoting resilience, connection, and well-being.
