The nervous system
Your heart racing before a difficult conversation. The wave of calm after a long exhale. That jolt when you hear a sudden loud noise.
None of this is random. It’s your nervous system responding to the world, moment by moment. This is where trauma takes hold, and where healing begins.
The stress response
The part of the nervous system most relevant to trauma is the autonomic nervous system. It runs in the background, regulating everything you don’t consciously control: your heart rate, digestion, and breathing at rest.
When your nervous system perceives threat (whether through sensory input, memories, or even thoughts), it triggers a stress response designed to keep you alive. This is coordinated by the hypothalamic-pituitary-adrenal (HPA) axis:
- The hypothalamus, which links the nervous system to the hormonal system, signals danger
- The pituitary gland releases hormones into the bloodstream
- The adrenal glands release stress hormones: adrenaline for immediate response, and cortisol for sustained alertness
The result is activation of the sympathetic nervous system, often called ‘fight or flight’: your heart rate goes up, breathing quickens, blood flows to your muscles, and digestion pauses. Your body is preparing for action. This state of heightened physiological activation is called hyperarousal.
Once the threat passes, the parasympathetic nervous system (sometimes called ‘rest and digest’) brings everything back to baseline. Cortisol drops, heart rate normalises, digestion resumes. The body recovers.
In a well-regulated system, these two modes work in balance: activation when needed, then return to calm. Problems arise when your system becomes chronically activated and the alarm never fully switches off.
Polyvagal theory
The sympathetic/parasympathetic model is useful, but it doesn’t tell the whole story. Polyvagal theory, developed by Dr Stephen Porges, offers a more nuanced understanding that has become foundational to trauma-informed work.
The key insight is that the vagus nerve, the main nerve of the parasympathetic system, has two distinct branches with very different functions:
- The dorsal vagal complex (or shutdown system) is the evolutionarily older branch, shared with reptiles. When threat is overwhelming and there is no way to fight, flee, or get help, this system takes over: collapse, dissociation, numbness. This state of reduced physiological activation is called hypoarousal.
- The ventral vagal complex (or social engagement system) is newer and found only in mammals. When active, we feel safe and connected. Our face is expressive, our voice has warmth, and we can connect with others. This is the state of calm, connected presence.
This means that instead of two states (activated or calm), there are actually three: social engagement, mobilisation, and shutdown.
The defence cascade
These three states form a hierarchy. When we perceive threat, we move through them in a predictable sequence:
- Social engagement (ventral vagal) — We first try to connect: call for help, negotiate, seek support
- Mobilisation (sympathetic) — If that fails, we fight or flee
- Shutdown (dorsal vagal) — If that fails too, we collapse into immobilisation
This sequence is not a choice; it is automatic, built into our biology. The nervous system tries each strategy in order, falling back to older systems when newer ones fail.
Neuroception
The nervous system moves through these states based on its assessment of safety: what Porges calls neuroception. This assessment happens automatically, below conscious awareness. Our nervous system is constantly scanning the environment and making split-second determinations about safety.
Neuroception works in two directions: top-down, from our thoughts and interpretations, and bottom-up, from signals inside the body (what we call interoception). This is why we can feel unsafe even when we logically know we are safe, and vice versa.
‘Our neuroception is our personal threat-detection system. If we have a severe trauma history, the nervous system is potentially returned into chronic states of defense, with a very low threshold to detect threat. If our nervous system is more buffered, we are more resilient to transitory challenges. If we have safety in our lives and good relationships with opportunities to co-regulate, the threshold to react is elevated and we become more resilient.’
— Stephen Porges, Polyvagal Perspectives
Why shutdown is problematic for mammals
An important insight from polyvagal theory concerns why the shutdown response is particularly challenging for humans and other mammals.
For reptiles, this response works well. Reptiles have small brains that need little oxygen, so they can immobilise and hold their breath for extended periods without harm. On the other hand, mammals have large brains that require massive amounts of oxygen. Dorsal vagal shutdown (when our heart rate drops, breathing becomes shallow, and we dissociate) is far more disruptive to our physiology.
This helps explain why trauma that involves freeze or shutdown responses can be so debilitating. The mammalian nervous system is not well-designed for prolonged immobilisation. When we get stuck in these states, or when they become our default response to stress, the effects ripple through our entire being.
The survival responses
With polyvagal theory in mind, we can understand the survival responses as expressions of different nervous system states.
Mobilisation
When the sympathetic nervous system activates, energy is mobilised for action:
- Fight is mobilisation of aggression to confront the threat. The body prepares to defend itself: muscles tense, jaw clenches, hands form fists. The urge is to fight, defend, push away.
- Flight is mobilisation to escape the threat. Energy surges to the legs, the heart races, the body prepares to flee. The urge is to run, escape, get away.
Immobilisation
When mobilisation is not possible or has failed, the dorsal vagal system initiates immobilisation. This can take two forms:
- Freeze (also called tonic immobility) is a hybrid state: the sympathetic system is still activated, but dorsal vagal immobilisation has been layered on top. The body is rigid and tense, but cannot move. There is intense internal activation (the heart pounds, stress hormones flood the system) creating the terrible sensation of being trapped, paralysed, unable to act despite desperate internal urgency.
- Collapse (also called flaccid immobility) is full dorsal vagal shutdown. The body goes limp and slack. When everything else has failed, the system shuts down entirely: numbing, disconnection, dissociation, feeling nothing. Consciousness dims to reduce suffering. This may include fainting.
These responses are automatic. They are not conscious decisions but survival mechanisms that operate below the level of voluntary control. When someone freezes or collapses during a threatening situation, they are not choosing to do so; their nervous system is doing what it has evolved to do.
Hybrid states
While it’s useful to think of the three polyvagal states as distinct, in reality they often blend together. These hybrid states combine elements of different branches of the autonomic nervous system.
Play
Play combines sympathetic activation with ventral vagal safety. The heart races and energy surges, but there’s no fear, only exhilaration. This is the state of sports, dancing, flirting, or play-fighting with children. Play allows us to experience intensity without threat, building capacity for activation while staying connected.
Stillness
Stillness combines dorsal vagal immobility with ventral vagal safety. The body is still and quiet, but we feel present and connected rather than shut down. This is the state of deep rest, meditation, or the quiet intimacy of lying with a loved one. It’s the difference between dissociative numbness and the profound rest that comes from feeling safe enough to let go.
Freeze
Freeze, as described above, combines sympathetic activation with dorsal vagal immobilisation. This is why trauma often leaves people feeling ‘stuck’: they are literally caught between mobilisation and immobility.
Appeasement
Appeasement (sometimes called fawn) combines ventral vagal activation with an underlying threat state. Rather than fighting, fleeing, or freezing, the nervous system attempts to neutralise the threat by placating it.
The ventral vagal system, normally used for genuine connection, becomes hijacked for survival: making yourself smaller, smiling, nodding, agreeing, and prioritising the needs of others. The person appears calm and socially engaged but, internally, the nervous system is in a survival state.
Appeasement typically develops when mobilisation is not an option, and the threat is a person you depend on. Children who cannot escape abusive or unpredictable caregivers often develop appeasement as their primary strategy. The logic is: ‘if I can make them happy, I will be safe.’
Signs of an appeasement pattern include:
- Difficulty saying no or setting boundaries
- Automatically prioritising others’ needs over your own
- People-pleasing, even at significant personal cost
- Difficulty knowing what you actually want or feel
- Fear of conflict or disapproval
- Tendency to over-apologise
- Feeling responsible for others’ emotions
Because appeasement often becomes a chronic relational pattern rather than a one-time response, it can be hard to recognise it as a trauma response at all. Yet, it represents a nervous system stuck in survival mode, constantly scanning for signs of displeasure and adjusting to keep others happy.
Trauma as incomplete activation
In an ideal scenario, the threat passes, and the body returns to equilibrium. Animals in the wild demonstrate this beautifully: after escaping a predator, a gazelle will often shake and tremor, discharging the mobilised energy before returning to grazing. The stress response completes its natural cycle.
For humans, this natural completion is often interrupted. Social conditioning teaches us to suppress physical expression of fear and stress. We cannot shake or cry at work; we cannot run from our stressors; we must ‘hold it together’.
Trauma often results when survival energy is activated but cannot complete its intended action: you want to fight back but are overpowered; you want to run but are trapped; you freeze but the threat continues. The energy mobilised for survival remains trapped in the body, creating ongoing dysregulation: the nervous system loses its ability to move fluidly between states, becoming stuck in chronic activation or shutdown, or swinging between the two.
Each survival response, when incomplete, can create characteristic patterns:
| Response | When incomplete, may lead to… |
|---|---|
| Fight | Chronic anger, irritability, hypervigilance |
| Flight | Chronic anxiety, feeling trapped, compulsive movement |
| Freeze | Feeling stuck in life, chronic freeze states, dissociation |
| Collapse | Chronic numbing, depression, disconnection from body and emotions |
| Appeasement | People-pleasing, difficulty with boundaries, loss of self |
Trauma is frozen, incomplete survival energy. Healing trauma is about safely allowing that energy to complete and discharge.