1.5.3 The Neurobiology of Trauma

Topic Progress:

In his book ‘The Body Keeps the Score’ Besel Van der Kolk digs deeper into how the fight or flight response works biologically to keep us safe and why this sometimes goes wrong causing ongoing post-traumatic stress symptoms.3

We take in information through our five senses – touch, sight, sound, smell and taste. Part of the limbic brain called the thalamus receives these impressions and mixes them together to form an overall story – “what is happening to me?” (see fig. 2). This is only a cursory overview which is designed to make us respond quickly, because during our evolution animals that responded quickly to danger survived at a greater rate than those who did not, and so organisms developed threat detection systems that reacted quickly rather than accurately. A more accurate assessment can be made later, and either confirmed or denied. This quick summary of what is happening is then passed on along two different brain pathways which Van der Kolk calls the ‘low road’ and the ‘high road’.

The Threat Detection System (‘Low Road’)

The low road goes to the amygdala in the limbic brain. The amygdala has to work out whether or the summary given by the thalamus represents a real threat. It is helped in this regard by a nearby brain area called the hippocampus. The hippocampus relates new experiences to old experiences and asks “have I seen something like this before?” or more specifically, “have I seen something BAD like this before?” If both the amygdala and the hippocampus agree that “yes, we have seen something like this before and it was BAD” – then the amygdala sends a reply message back to the Hypothalamus (an extension of the thalamus) saying “Confirmed! Threat detected.” One of the jobs of the hypothalamus is to then activate the nervous system to release stress chemicals like adrenaline.

Fig. 2 The brain’s threat detection system. (‘Low road’)

The bodies nervous system is the most important body function in terms of post-traumatic stress.

The nervous system extends from the brain through the spinal cord and out into all the extremities of the body. It has has two complimentary systems which are controlled largely by the limbic brain. They are:

  • The sympathetic nervous system (SNS)
  • The para-sympathetic nervous system (PNS)

The SNS controls arousal (including the fight-flight response). It is called sympathetic because it works with the emotions. It moves blood to muscles and uses adrenal glands to release adrenaline to make us responsive and athletic. For this reason Van der Kolk refers to it as the body’s ‘accelerator’.4

The parasympathetic nervous system (PNS) is so-called because it works against the emotions. It controls self-preservation mechanisms like healing, digestion and rest, and reverses arousal by releasing acetylcholine. Acetylcholine is a neurotransmitter that is found in the body as well as the brain. It helps to relax muscles and return our breathing to normal after exertion or stress. For this reason van der Kolk likens it to the body’s ‘brake’.5

Together, the accelerator (SNS) and brake (PNS) help us to constantly return to a state of homeostasis (balance). Humans, like other animals need to constantly switch between states of arousal and periods of rest, and the bodies nervous system helps us to accomplish that.

So in summarizing the nervous system we can say:

  • Our nervous system is heavily involved in the regulation of stress.
  • Our sympathetic nervous system (SNS) works like an accelerator, which enables us to be responsive and athletic
  • Our parasympathetic nervous system (PNS) works like a brake to bring us back to a state of rest.
  • When we perceive a potential threat the amygdala and hippocampus get activated. Their job is to fetch up negative memories that help create a ‘safety bias’.

Usually this safety bias works to keep us safe. However, people who suffer post-traumatic stress seem to continue this negative bias in a pathological way. One ofthe other jobs of the Hippocampus is to process and store short term memory into long term memory. However, due to the inability to resolve the traumatic situation (fight or flee) this traumatic event gets stuck as ‘working memory’. The end result is that the traumatic memories can continue to feel real as though they were happening now.

The Threat Detection System (High Road)

The ‘high road’ of the threat detection system goes from the thalamus to the medial prefrontal cortex (PFC) and takes slightly longer. It’s job is to conduct a final and much more detailed review of the threat. The emotional brain areas are incapable of doing this because they only look at the emotional tone of the situation or the ‘big picture’ and ask “where have I seen this before?”

If the PFC takes a more careful look and decides that this is not a real threat after all then it can send a message back to the hypothalamus that the stress chemicals are not required and the PNS will secrete calming chemicals (such as acetylcholine) to restore balance.

In PTSD (and to a lesser extent in other small ‘t’ traumas) the limbic system is hyper-active which effectively creates a disconnect with the ‘sensible’, rational frontal areas of the brain (and in particular the PFC). This hampers the individuals ability to correctly diagnose the level of threat and calm down. In particular, people with trauma tend to read social situations and cues (such as facial gestures) wrongly. Consequently the traumatized person is almost always in collision with something or someone.

People who are traumatized have a harder time working out what is a real threat and what isn’t due to having a hyper-vigilant threat detection system. So this becomes a vicious cycle whereby the threat detection circuit over-emphasizes threats and continues to pump out more stress hormones like adrenaline. Constantly elevated stress levels take a terrible toll on the body over many years. This causes knock-on emotional and physical health problems in the short term such as; sleep disorders, memory and attention problems and constant irritability, and may also cause serious long term and chronic health problems such as heart disease.

To an extent, our threat detection systems are also determined by serotonin levels, meaning that people with low levels of serotonin may be more susceptible to the symptoms of traumatic stress. For example, animals with low serotonin are known to be hyper-reactive to stressful stimuli such as loud noises. One of the things that we know can contribute to low serotonin levels is perceiving oneself to be of low status (within a given hierarchy).