The Nervous System
Understanding the nervous system is essential for effective TRE practice. The tremor mechanism works directly with the autonomic nervous system, and having a clear picture of how this system functions will help you understand what is happening in your body during practice and why self-regulation is so important.
The Autonomic Nervous System
The nervous system has two main divisions: the somatic nervous system, which controls voluntary movement (moving your arm, walking, speaking), and the autonomic nervous system (ANS), which regulates involuntary processes (heart rate, digestion, breathing at rest).
The autonomic nervous system is, in turn, divided into two branches:
The sympathetic nervous system is often called the "fight or flight" system. When activated, it prepares the body for action: heart rate increases, breathing quickens, blood flow is directed to the muscles, and digestion slows. Pupils dilate, alertness increases, and the body is primed to respond to threat.
The parasympathetic nervous system is often called the "rest and digest" system. When dominant, it promotes calm, recovery, and restoration: heart rate slows, breathing deepens, digestion resumes, and the body can repair and regenerate.
In a healthy, well-regulated nervous system, these two branches work together in a dynamic balance. The sympathetic system activates when needed (during exercise, when facing a challenge, in response to danger), and the parasympathetic system brings the body back to baseline when the challenge has passed.
Problems arise when this natural rhythm is disrupted: when the sympathetic system remains chronically activated, or when the nervous system becomes unable to return to a calm baseline after stress.
The Survival Responses
When the nervous system perceives threat, it initiates a cascade of responses designed to ensure survival. These responses evolved over millions of years and are shared with many other animals.
Fight
Mobilisation of aggression to confront the threat. The body prepares to defend itself: muscles tense, jaw clenches, hands form fists. Urge to fight, defend, push away. Feeling powerful.
When completed: Energy is discharged, system returns to baseline.
When incomplete: May lead to chronic anger, irritability, hypervigilance.
Flight
Mobilisation to escape the threat. Energy surges to the legs, the heart races, the body prepares to run. Urge to run, escape. Anxiety, panic, restlessness.
When completed: Escape occurs, system calms.
When incomplete: May lead to chronic anxiety, feeling trapped, compulsive movement.
Freeze
When neither fight nor flight is possible, the nervous system may initiate a freeze response. Mixed sympathetic and parasympathetic activation. Immobility, feeling stuck, unable to move. Time distortion, may include dissociation.
Purpose: When fight or flight are not possible, freeze is a strategy. "Play dead," don't draw attention, wait for an opening.
When incomplete: May lead to feeling stuck in life, chronic freeze states, dissociation.
Collapse (Dorsal Vagal Shutdown)
The final survival strategy when everything else has failed. Parasympathetic dominance (dorsal vagal). Numbing, disconnection. Dissociation, feeling nothing. May include fainting.
Purpose: Consciousness dims to reduce suffering.
When incomplete: May lead to chronic numbing, depression, disconnection from body and emotions.
It is important to understand that these responses are automatic. They are not conscious decisions but survival mechanisms that operate below the level of voluntary control. When someone freezes during a threatening situation, they are not choosing to freeze; their nervous system is doing what it has evolved to do.
Fawn
A more recently recognised response, particularly in relational trauma: appeasing the threat through submission, people-pleasing, or compliance.
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." The energy mobilised for survival remains trapped in the body.
Key insight: Trauma often results when survival energy is activated but cannot complete its intended action. You want to fight back but are physically overpowered. You want to run but are trapped. You freeze but the threat continues beyond what freeze can handle. Your system mobilises energy but you're too small or weak to effectively use it.
Result: The activated survival energy remains in the system, creating ongoing dysregulation. The nervous system stays prepared for a threat that has already passed.
Trauma is frozen, incomplete survival energy. Healing trauma is about safely allowing that energy to complete and discharge.
Polyvagal Theory
Polyvagal theory, developed by Stephen Porges, offers a more nuanced understanding of the autonomic nervous system that has become foundational to trauma-informed somatic work.
Porges identified that the vagus nerve – the primary nerve of the parasympathetic system – actually has two distinct branches with very different functions:
The ventral vagal complex (the "social engagement system") is the newest evolutionary development, found only in mammals. When this system is active, we feel safe, connected, and capable of social engagement. Our facial expressions are animated, our voice is melodic, and we can attune to others. This is the state of calm, connected presence.
The dorsal vagal complex (the "shutdown system") is the oldest evolutionary pathway, shared with reptiles. When threat is overwhelming and neither fight, flight, nor social engagement is possible, this system initiates shutdown: collapse, dissociation, numbness. This is the freeze response at its most extreme.
Porges describes these three states as a hierarchy:
- Social engagement (ventral vagal) – We feel safe and can connect with others
- Mobilisation (sympathetic) – We perceive threat and mobilise to respond
- Shutdown (dorsal vagal) – Threat is overwhelming; we collapse or dissociate
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 is the process by which our nervous system evaluates risk without involving the thinking parts of our brain. We can feel unsafe even when we logically know we are safe, and vice versa.
– Stephen Porges, The Polyvagal Theory
Understanding polyvagal theory helps explain why trauma responses can be so persistent. A person who experienced overwhelming threat may have a nervous system that defaults to shutdown or hypervigilance, even in objectively safe situations. The neuroception has been calibrated by past experience to perceive danger where none exists.
Ongoing Impacts on Nervous System Regulation
Trauma affects how the nervous system functions day-to-day:
Hyperarousal (chronic sympathetic activation): Hypervigilance (always watching for threat), startle easily, difficulty relaxing or sleeping, irritability and anger, racing thoughts, anxiety.
Hypoarousal (chronic shutdown): Numbing and disconnection, low energy and fatigue, depression, difficulty feeling emotions, brain fog, dissociation.
Dysregulation (difficulty moving between states): Swinging between hyperarousal and hypoarousal, difficulty finding balance, either too much activation or too much shutdown, hard to self-regulate.
How TRE Relates to the Nervous System
With this understanding of the nervous system, we can appreciate what TRE is doing:
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Activating the tremor mechanism engages a natural, involuntary process that helps discharge accumulated neuromuscular tension.
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The tremors themselves are a form of controlled activation: mild sympathetic arousal in a safe context.
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Through repeated practice, the nervous system learns that it can move into activation and then successfully return to calm. This builds the capacity for regulation.
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Self-regulation practices allow us to modulate the intensity, staying within our window of tolerance.
The key insight is that TRE is not about achieving maximum release or the most intense tremors. It is about working within your window of tolerance, giving your nervous system the experience of activation followed by successful return to baseline. This is how we build regulatory capacity over time.
If TRE practice takes you outside your window of tolerance – into overwhelm or dissociation – it can actually be counterproductive. The nervous system learns from experience, and repeated experiences of dysregulation do not build capacity. Self-regulation is therefore not optional but essential.