Stress and the brain

In this post I am going to be talking about the brain, specifically its role in the fight or flight response when it is activated by a real or perceived threat, known as a stressor.

Whilst you have most definitely heard of stress and perhaps even experienced it for yourself, like many others, you may not understand why it is happening, what is going on in the brain (and body), and how it affects you psychologically when it is. I think it is important to educate my clients in how the brain works, the stress response and how it can affect us. Having a deeper understanding of what is going on for you it is my hope that it will allow you to feel that you have the ability to take control of your situation, give you hope, as well as the realisation that you are not going mad.

I am a bit of an anatomy and physiology geek so I’m going to go in to a reasonable level of detail about the process and the parts of the brain involved. I have simplified things a little but not dumbed it down completely because I feel that with a proper understanding of what is happening in your brain and body it helps to take a layer of anxiety away as you recognise that what is happening is completely normal, even if you don’t feel it.

I’m going to start by pointing out the parts of the brain that are involved in the stress response so you know what I’m talking about later on. So, here is a brain.

This is what we normally think of when we think of our brain. But these areas labelled here don’t get involved with keeping us safe, with the exception of the prefrontal cortex there at the front of the brain which does play a small role. No. What we’re interested in is those parts deeper inside, at the base of the brain. The hypothalamus, the hippocampus, the amygdala, and the pituitary. We also have the adrenal glands, not a part of the brain I know, but they also play a big part in the stress response so we can’t leave them out. They sit atop of your kidneys. So now you know all the players involved, we can move on to how they all work together when responding to stress.

The stress response happens so quickly, even before we become consciously aware of what the danger is. This is very reassuring to know, that our brain and body responds as quickly as possible to ensure our survival.

The stress response starts, at a physiological level, from the moment the body realises the presence of something it perceives to be a danger (also known as a stressor). This might be a chemical or biological agent (cigarette smoke), environmental condition (overcrowded shop), external stimulus (a dog barking), a thought, or an event (a car accident). The purpose of the stress response is protection, keeping us safe and allowing us to eliminate, reduce or cope with the stress.

So I want you to imagine for a moment that you’re out for a walk and you encounter a wild animal. The wild animals we have here in the UK aren’t exactly dangerous, so you might want to switch continents in your mind where there are bears or lions or something else with big teeth and sharp claws.

There are two strands, for want of a better word, to the stress response. When we encounter that dangerous animal, the eyes and/or ears send the information to the amygdala which processes the information and triggers both strands of the stress response by releasing corticotropin-releasing factor (CRF). The first strand involves the amygdala sending a distress signal to the hypothalamus, which in turn sends out messages through the sympathetic nervous system (SNS), producing adrenaline and involuntary body functions to occur that are conducive to allowing us to fight, flee, or hide from the danger.

So we experience noticeable changes in our heart rate (beating faster than normal), pulse rate, blood pressure because our body requires more blood to be pushed in to the muscles, in particular our legs and arms, our heart and other vital organs. Our breathing changes, becoming faster and/or shallower, so that our lungs can take in as much oxygen as possible making us as alert as we can be. Our senses, especially sight and hearing, become sharper so that we can keep tabs on the danger. We become more alert. All of these physiological changes help us to be in the optimal physical state required to respond to the danger.

The second strand of the stress response involves the HPA axis. The CRF that the amygdala released activates the hypothalamus causing it to produce its own CRF. This activates the pituitary gland causing it to produce Adrenocorticotropic hormone (ACTH) in to the blood. This stimulates the adrenal cortex to release cortisol into the bloodstream. Cortisol, known as a glucocorticoid, releases blood sugar (glucose) and fats in order to supply energy to all parts of the body, necessary to deal with a prolonged attack. Cortisol travels to the prefrontal cortex, hippocampus, hypothalamus, and the pituitary which all have glucocorticoid receptors, which the glucocorticoids can attach to. The cortisol stimulates the prefrontal cortex and the hippocampus which in turn send signals to inhibit the activity of the hypothalamus. Cortisol also has a direct inhibitory effect on the activity of the hypothalamus and the pituitary. As a result of this inhibitory effect, CRF and ACTH release starts to decline and then cortisol production declines too. Cortisol is alo broken down in the bloodstream by enzymes. So you can see that after the danger has gone, cortisol levels return to normal fairly quickly.

However, if that wild animal hangs around for a prolonged period of time or we convince ourselves that to be the case, then the HPA axis is constantly being triggered to produce cortisol, prolonging the stress response. The level of cortisol in the blood increases significantly and the enzymes that naturally break it down cannot keep up with it. These levels of cortisol and the consistency with which it is present has a toxic effect on the glucocorticoid receptors resulting in the reduction of inhibition on the hypothalamus and pituitary so that CRF and ACTH continues to be produced and so the adrenal cortex is stimulated to produce yet more cortisol. And the cycle repeats. After a while, this has an effect on the body that can contribute to the health problems associated with chronic stress.

In part 2, I will discuss the psychological effects of stress.