The Neurobiology of Trauma

In recently published articles we defined and looked at the aetiology of trauma and discussed the prevalence, incidence, and risk factors for ASD and PTSD. In this article we shift our focus to the neurobiological side of things i.e. what happens to the brain during the course of trauma.

Trauma, stress, and the Autonomic Nervous System

With exposure to stress or trauma, the Autonomic Nervous System (ANS) comes under direct fire. It is comprised of the Sympathetic Nervous System (SNS) and the Parasympathetic Nervous System (PNS). Under normal “at rest” conditions – meaning, when not under stress – the PNS is tasked with maintaining normal physiological activity: decreasing the heart and breathing rates, while increasing the blood flow to the digestive system (thus it is said to be involved with “rest and digest” functions).

The SNS, meanwhile, is the adaptive system. Designed to protect the body when there is a perceived threat to survival or body integrity, the SNS activates the “fight, flight, or freeze” response. When this response is activated:

  1. Adrenalin and cortisol levels increase dramatically, producing hypervigilance and hyperarousal
  2. The body is prepared to fight or run from danger through:
    1. Constricted blood vessels and pupils
    2. Decreased blood flow to the digestive system
    3. Increased perspiration
    4. Increased heart and breathing rates
  3. If fleeing or fighting is impossible, the body is prepared to “freeze” through dissociation:
    1. The individual feels numb and disengaged
    2. The individual may faint
    3. Time is suspended
    4. Derealisation may occur, theoretically preparing the body for camouflage, increasing chances for survival through compliance with the attacker, or creating conditions in which death is not as painful if the attacker does not go away (Bicknell-Hentges & Lynch, 2009)

Clearly, the SNS stress response is critical for survival. The problem comes when the SNS is stimulated too often; harmful effects occur to the body. Because the autonomic stress response is triggered in the same way by both physical and emotional pain, people who continually perceive danger in their environments will engender a constant autonomic response of alertness. This could range from mere vigilance to terror. Whether the individual has actually been exposed to a traumatic event or is “only” experiencing an ongoing stress does not matter; either way the ANS response causes serious damage to the individual’s health (Bremner, 2002).

Hippocampal damage to memory formation

The hormones of adrenalin and cortisol are released during stressful times, soaking the parts of the brain involved in memory and response to stress. Receiving the hormones mobilises these brain systems to respond to danger, and so is critical for survival, but excessive or repetitive activation of this response can result in long-term changes to just these same areas of memory and stress response. For example, studies have shown that cortisol may cause damage to the hippocampus, impairing memory formation (Sapolsky, 1996).

Increased physiological response to all stimuli

Individuals exposed to intense or prolonged trauma may have a more intense physiological response to all stress. In a study comparing individuals with PTSD to non-traumatised controls, those with PTSD responded to reminders of trauma with significant increases in blood pressure, heart rate, and skin conductance (Pitman, Orr, Forgue, De Jong & Claiborn, 1987). Studies have also demonstrated that chronic SNS arousal increases other physiological responses, including irritability in the limbic system. This irritability means that almost any excessive stimulus – such as loud noises, strong odours, or flashes of light – stimulates the fight-or-flight response. Each time such an individual is re-stressed, the sensitivity of the response is increased, as is the individual’s vulnerability to anxiety and depression (Scaer, 2005).

Chronic SNS arousal can negatively impact higher brain functions

Self-soothing disrupted

During the first three years of life, brain growth is predominantly in the right hemisphere of the brain, which processes nonverbal signals and communications, including facial expression of feelings, perception of emotion, and regulation of the autonomic nervous system. If a child is exposed to significant stress or PTEs during this time, the critical function of self-soothing (regulated by the right hemisphere) is unable to develop properly (Siegel, 2003).

Cortical functions unable to modulate lower brain responses

In normal individuals, brain growth and increased organisation means that the higher, more complex areas of the brain begin to control the more reactive, primitive-functioning parts of the lower brain. The sophisticated cortex enables the individual to increasingly modulate his or her impulses and behavioural responses to strong emotions. Chronic arousal of the SNS, however, impacts the development of those higher brain functions, making it difficult for the person to think logically, plan, and problem-solve before reacting. Language is also affected. Looked at another way, the more complex the survival network becomes (i.e., the more traumatised the person is), the more difficult it is for the person’s higher cortical functions to subdue the primitive parts during learning, concentration, and recall (Perry, 2001).

States become traits

Finally, persistent SNS stimulation also increases the risk that characteristics of the state of arousal become more stable traits. Chronic hyperarousal may permanently alter the SNS adrenalin system. Persistent dissociation may alter the opioid (that is, endorphins, the body’s natural morphine) system (Bicknell-Hentges & Lynch, 2009).

Trauma behaviour makes sense

Just imagine if you were lost in the forest somewhere with a group of children. As the adult, you realise that when night falls, it is you who must watch out for marauding wild animals and any human predators. The children go to sleep, but you stay up 24/7 in order to do the lookout work. Yet you cannot sleep during the day as you must use daylight to try to find your way out of the forest. The next night you face the same task, and sometimes you do have to deal with animals. It is frightening and exhausting.

Obviously, after some time of needing to be totally awake and hypervigilant, you would begin to feel anxious. As body and mind became tired and burned out from the guard duty and attempts to guide the group safely out, your memory, normally so reliable, would be struggling to remember simple things. You might begin to do increasingly bizarre things because brain changes brought on by the stress of constant vigilance would mean that your ability to regulate your behaviour and impulses would be steadily decreasing. At some stage, it would all be too much. If you continued to be saddled with ensuring safety in an unsafe environment, you might become full of despair. Certainly all of these responses, plus addictions and self-harming, could await you after the event, when you were all back safely home. And trauma experts would say that these behaviours would make sense as responses to the potentially traumatising ordeal you were enduring.

As we have described, exposure to PTEs does not bring an automatic sentence of PTSD, but the resultant over-stimulation of the autonomic nervous system does put traumatised individuals at much higher risk for constrained memory formation, decreased cortical modulation of lower brain impulses and behaviours, and eventual transitioning of temporary traits into stable (unwanted) traits. As mental health professionals, we are called upon to understand this basic neuroscience in order to better inform and guide our clients’ treatment.

This article was adapted from the upcoming Mental Health Academy’s CPD course “Working with Trauma”. Learn more:


  • Bicknell-Hentges, L, & Lynch, J.J. (2009, March). Everything counselors and supervisors need to know about treating trauma. Paper based on a presentation at the American Counseling Association Annual Conference and Exposition, Charlotte, N.C.
  • Bremner, J.D. (2002). Understanding trauma-related disorders from a mind-body perspective: Does stress damage the brain? New York: W.W. Norton & Company.
  • Perry, B.D. (2001). The neurodevelopmental impact of violence in childhood. In Schetky, D., & Benedek, E.P. (Eds.), Textbook of child and adolescent forensic psychiatry, (221-238). Washington, D.C.: American Psychiatric Press, Inc.
  • Pitman, R.K., Orr, S.P., Forgue, D.F., de jong, J., & Claiborn, J.M. (1987). Psychophysiologic assessment of posttraumatic stress disorder imagery in Vietnam combat veterans. Archives of General Psychiatry, Vol 44, 970-975.
  • Sapolsky, R. M. (1996). Why stress is bad for your brain. Science, Vol 273, 749-750.
  • Scaer, R.C. (2005). The trauma spectrum: Hidden wounds and human resiliency. New York: W.W. Norton & Co.