How Trauma Impacts the Brain and Body | Trillium Counselling

Trauma changes how the brain and body respond to threat by keeping the nervous system's fight, flight, and freeze reactions switched on after the danger passes. These reactions are automatic survival responses driven by the amygdala and the HPA axis, the body's stress-hormone system. When a threat has ended but those systems stay activated — sometimes for months or years — everyday cues can register as danger and trigger the same alarm. Regulating this pattern is what trauma therapy is designed to do.

Last updated: July 2026 · Written by Christopher Hein, Co-Founder, Trillium Counselling

Fight, flight, and freeze: your body's automatic threat response

When your brain detects danger, it doesn't wait for you to think. A small almond-shaped region called the amygdala scans incoming information and — if it registers threat — triggers a cascade of physical changes in under a second. This is the fight-or-flight response, and it's controlled by the sympathetic branch of your autonomic nervous system.

Your heart rate climbs, breathing quickens, muscles tense, digestion slows, and stress hormones (adrenaline and cortisol) flood your bloodstream. Your body is preparing you to either confront the threat or escape it. This response is described by the American Psychological Association and the National Institute of Mental Health as the foundational physiology behind acute stress and trauma reactions.

A third response — freeze — kicks in when neither fighting nor fleeing feels possible. Instead of activating, the body shuts down. Muscles go slack or rigid, heart rate drops, breathing becomes shallow, and time can feel like it's slowing. Freeze is governed by a different branch of the nervous system (the dorsal vagal complex of the parasympathetic system), and it's especially common in situations where escape isn't an option — including many childhood traumas, medical trauma, and interpersonal violence.

None of these responses are choices. They are older than conscious thought, and they happen faster than you can decide.

How trauma changes brain structure and function

When a threat passes, most people's nervous systems settle back into a calm baseline. But when trauma is severe, prolonged, or happens during childhood — or when the freeze response was the only option — the brain can start treating "not dangerous" as "not safe yet." Three brain regions are most affected, and researchers have documented these changes across decades of neuroimaging studies (see this peer-reviewed review in Dialogues in Clinical Neuroscience on the effects of traumatic stress on the brain):

• Amygdala — the alarm system. Becomes hyperactive and more sensitive. It starts flagging things as threatening that objectively aren't — a raised voice, a specific smell, an unexpected touch. This is where triggers come from.
• Hippocampus — the memory organizer. Shrinks in volume and function. Its job is to file memories with time and place, so you can tell that a memory belongs to the past. When it's impaired, traumatic memories often feel like they're happening now, not then.
• Prefrontal cortex — the thinking brain. Becomes underactive relative to the amygdala. This is the part of the brain that would normally say "I'm safe, this is fine." When it's offline, logic and self-talk don't reach the alarm system.

At Trillium, all of our trauma therapists are Registered Social Workers (RSWs) or Registered Psychotherapists (RPs) with additional trauma-specific training — you can see the current team on our trauma therapy page. Our approach is built around what the research on trauma neurobiology actually shows: talk therapy alone often isn't enough, because the parts of the brain that respond to trauma sit below the parts that respond to conversation.

The stress response system: HPA axis and cortisol

Behind fight, flight, and freeze is a longer-running hormonal system called the HPA axis (hypothalamus–pituitary–adrenal axis). When the amygdala fires, it signals the hypothalamus, which triggers the pituitary, which triggers the adrenal glands to release cortisol — the body's main stress hormone.

Cortisol is helpful in short bursts: it sharpens focus, mobilizes energy, and dampens processes that aren't needed for immediate survival (like digestion and immune response). But when the HPA axis stays activated long-term, chronic cortisol elevation is linked to a range of physical health problems, including sleep disruption, digestive issues, chronic inflammation, cardiovascular strain, and immune dysfunction. The Centre for Addiction and Mental Health (CAMH) lists these somatic symptoms as common components of trauma and post-traumatic stress in clinical presentation.

This is why trauma is never "just in your head." The physical symptoms are real, measurable, and mediated by the same nervous-system pathways that generate the emotional ones.

Common emotional and physical signs of a trauma response

Because trauma affects the whole nervous system, symptoms show up in both the mind and the body. Not everyone experiences all of these, and severity varies widely.

Emotional and cognitive signs:

• Intrusive memories, flashbacks, or nightmares
• Feeling on edge, easily startled, or hypervigilant
• Emotional numbness or disconnection
• Trouble concentrating or feeling "foggy"
• Irritability, mood swings, or feeling out of control
• Avoidance of people, places, or situations that feel connected to the trauma

Physical signs:

• Sleep problems — trouble falling asleep, staying asleep, or waking unrefreshed
• Chronic muscle tension, headaches, or jaw clenching
• Digestive issues (nausea, appetite changes, stomach pain)
• Fatigue that doesn't lift with rest
• Racing heart, shallow breathing, or a sense of not being in your body
• Frequent illness or slow recovery from illness

If several of these feel familiar, it doesn't mean anything is wrong with you. It means your nervous system is doing its job — it just hasn't gotten the signal that the threat is over.

Why professional trauma therapy matters

Because trauma changes the physical structure of the brain and the tuning of the nervous system, healing usually needs more than talking about what happened. The evidence-based trauma therapies — EMDR, trauma-focused cognitive behavioural therapy (TF-CBT), and somatic approaches — all work by reaching parts of the brain and nervous system that ordinary conversation doesn't reach.

EMDR (Eye Movement Desensitization and Reprocessing), for example, uses bilateral stimulation to help the brain re-file traumatic memories so they feel like they happened in the past rather than the present. Trauma-focused CBT works with the thought patterns that developed around the trauma. Somatic approaches work directly with the body's stored stress response. All three are recognized by the American Psychological Association and reviewed in the trauma-treatment literature.

A trained trauma therapist can help you understand your specific nervous system, teach you skills to calm the alarm response, and — when you're ready — process the trauma itself so it stops running the show in the background. If you want to see what approaches Trillium's therapists use and how the matching process works, our trauma therapy page lays it all out.

Healing and recovery: what change actually looks like

Recovery from trauma isn't a straight line, and it isn't about forgetting what happened. It's about your nervous system learning — through repeated experiences of safety — that the threat is over. That process can involve several kinds of work, often layered together over time:

• Stabilization first. Before processing traumatic memories, most therapists help you build tools for regulating your nervous system — breathwork, grounding, sleep hygiene, and calming the body when it activates.
• Processing the trauma itself. Once stabilization is in place, structured methods like EMDR or TF-CBT help the brain re-file the traumatic memory so it stops feeling like it's happening now.
• Rebuilding what trauma disrupted. Relationships, work, self-image, and the sense of safety in your own body all often need attention as the trauma response quiets down.
• Physical health. Sleep, exercise, nutrition, and social connection all support nervous-system recovery. They're not the whole answer, but they compound with therapy.

Most people don't heal from trauma alone, and there's no reason you have to. Working with a trauma therapist can shorten what could be significant struggle, and can help you build a life where fight, flight, and freeze stop running in the background. If you'd like help figuring out where to start, you can read more about how trauma therapy works at Trillium.

If you or someone you love is in crisis or having thoughts of suicide, please call or text 9-8-8 — Canada's Suicide Crisis Helpline, available 24/7.