Your Nervous System
and Trauma
A series on the neuroscience of trauma
Understanding how your body responds to threat, and why those responses can become stuck long after the danger has passed.
Your body has an automatic alarm system
Every human being is born with a sophisticated internal alarm system designed to protect them from danger. This system operates far beneath conscious awareness, faster than thought and faster than language. When your brain detects a threat, your body responds instantly and automatically, mobilising every available resource to help you survive.
This alarm system is governed by the autonomic nervous system (ANS), a vast network of nerves that regulates heart rate, breathing, digestion, blood pressure, and dozens of other functions. It runs continuously in the background without any conscious input from you.
The autonomic nervous system has two main branches that work in balance with one another. Think of them as the accelerator and brake pedal in a car. In a healthy, regulated nervous system, they work in smooth coordination, activating when action is needed and returning to rest when it is not.
In trauma, this coordination breaks down. Understanding how these two branches work is central to understanding how psychological trauma affects the body, and why the effects can persist for years after the original events.
The Accelerator
“Fight or flight”
Activates rapidly in response to perceived threat. Mobilises energy, increases alertness, and prepares the body for action. Designed to be powerful but short-lived.
The Brake
“Rest and digest”
Restores calm after threat has passed. Conserves energy, promotes recovery, and supports the body’s long-term maintenance functions.
Fight or flight
When your brain detects danger, or something that resembles past danger, the sympathetic nervous system fires rapidly, triggering a cascade of physiological changes. This response is ancient, evolutionary, and extraordinarily efficient. It does not wait for your conscious mind to assess the situation. It acts first.
The fight-or-flight response is not a malfunction. It is the body doing exactly what it was designed to do, keeping you alive in the face of threat.
This state is meant to last minutes, not hours, days, or years. Once the threat has passed, the nervous system is designed to return to baseline. In trauma, this completion cycle is interrupted. What happens inside the body during this activation, and what that interruption costs, is what the next sections describe.
Rate increases
Blood is pumped more rapidly to muscles, preparing the body for physical action.
Airways dilate
Bronchial tubes widen and breathing quickens to deliver more oxygen to the muscles.
Pupils dilate
Wider pupils allow more light in, improving peripheral vision and threat detection.
Adrenaline surges
Adrenaline and cortisol flood the bloodstream, sharply elevating alertness and energy.
Suppressed
Non-essential processes like digestion and immune function are temporarily shut down.
Tense and ready
Skeletal muscles receive increased blood flow and tense in preparation for movement.
Sweating increases
Sweat glands activate to cool the body during exertion and improve grip under physical stress.
Clotting primed
Platelets become more active, preparing the body to limit blood loss in the event of injury.
Rest and recovery
When threat has passed and safety is re-established, the parasympathetic nervous system takes over, most powerfully through the vagus nerve. The heart slows, breathing deepens and steadies, digestion resumes, and the muscles soften. A nervous system that cannot return to this state carries a significant long-term cost to health, mood, and cognitive function.
Rate slows
Heart rate and blood pressure return to resting levels as the body shifts out of emergency mode.
Deepens and slows
Breathing becomes slower and more diaphragmatic, activating the calming vagal response.
Resumes
Digestive activity increases; stomach acid and gut motility return to normal.
Restored
The immune system, suppressed during threat, resumes its protective and repair functions.
Improves
The body’s capacity for restorative sleep and tissue repair is only available in parasympathetic states.
Access restored
Higher cognitive functions, including reflection, learning, and connection, become available again.
Becomes possible
Safe intimacy and genuine presence with others return when the nervous system is no longer scanning for threat.
Stabilises
Emotional regulation improves as the brain’s regulatory capacity is no longer suppressed by chronic alarm.
How the two systems compare
| Body system | Sympathetic (threat) | Parasympathetic (safety) |
|---|---|---|
| Heart rate | Increases; beats harder and faster | Slows to resting rate |
| Breathing | Rapid, shallow, chest-based | Slow, deep, diaphragmatic |
| Pupils | Dilated for wider threat scanning | Constricted; normal focused vision |
| Muscles | Tense; increased blood supply | Relaxed; blood returns to organs |
| Digestion | Suppressed; dry mouth | Stimulated; saliva and gut motility resume |
| Stress hormones | Adrenaline and cortisol released | Hormone levels normalise |
| Cognitive access | Threat-focused; narrow attention | Broad; reflective and creative thinking |
| Immune function | Suppressed during acute stress | Restored; repair and maintenance active |
| Sleep capacity | Disrupted; light or fragmented | Restorative sleep becomes accessible |
| Social engagement | Reduced; withdrawal or hypervigilance | Safe social connection becomes possible |
When the cycle gets stuck
In a well-regulated nervous system, the two branches cycle naturally, activating under pressure and returning to rest when pressure subsides. Trauma disrupts this cycle at a neurobiological level. The threat response activates but never fully completes. The memory of what happened remains stored in an unprocessed, emotionally raw state, with the body’s alarm still ringing.
A triggering event occurs
An overwhelming experience activates the sympathetic nervous system. The body mobilises for survival.
The threat response is interrupted
Because action is impossible, prevented, or the threat is too prolonged, the stress response cycle cannot complete normally.
The memory becomes stuck
The experience is stored in a neurologically isolated state, incomplete, unintegrated, and linked to the original physiological alarm. Reminders can instantly reactivate the full threat response, even years later.
Processing restores balance
With appropriate therapeutic support, the nervous system can complete what was interrupted, allowing the memory to be stored as something that happened in the past rather than something still happening now.
The role of triggers. One of the hallmarks of unresolved trauma is triggering: a present-day stimulus activating the full emotional and physiological alarm of a past event. A sound, a smell, a tone of voice can serve as a sensory bridge between present and past, bypassing rational thought entirely. This is not a character weakness. It reflects the way unresolved trauma memories are stored, without a clear timestamp and with the body’s threat response still attached.
Freeze and shutdown. Not all trauma responses involve activation. When threat is perceived as inescapable, or when the nervous system has been overwhelmed repeatedly, a third response becomes possible: freeze, or in its more extreme form, shutdown. This can manifest as emotional numbness, dissociation, physical heaviness, or profound disconnection from one’s own body and surroundings.
Your experience makes sense
If you are living with the effects of trauma, understanding the nervous system can be genuinely reorienting. Many of the experiences that feel most confusing make complete sense when understood through this lens.
Your reactions are not irrational
Trauma responses are logical adaptations to past threat. They were protective once. The difficulty is that they continue to operate as though the threat is ongoing.
Your body is not your enemy
The symptoms you experience are the nervous system’s attempt to keep you safe. Working with your body, rather than against it, is central to recovery.
Change is neurologically possible
The nervous system is not fixed. Evidence-based treatments can facilitate new learning at a neurobiological level, allowing the alarm system to recalibrate.
The goal is balance, not suppression
Recovery is not about eliminating the stress response. It is about restoring the natural capacity to activate when needed, and return to rest when the threat has passed.
Ready to take
the first step?
If you recognise your experience in what you have read here, EMDR therapy may be able to help.
No referral required.