You’ve most likely heard of the term "fight-or-flight” system when talking about anxiety and stress. Sometimes it's referred to as the "fight-or-flight response” or the "fight-flight-freeze system". But what does it really mean?
During a stressful situation such as preparing to write an exam, or bungy jumping for the first time, or speaking in front of an audience, the body reacts by firing the fight-or-flight response which results in a cascade of hormones that cause physiological changes in the body. This stress response starts in the brain when our eyes or ears detect a stressor and activates the limbic system.
This system sits on top of the brainstem and is made up of three main brain regions (the amygdala, the hippocampus, and the hypothalamus) and a couple of other nearby brain regions. It plays a role in emotional regulation, formation of memories and intuitive responses to our environment.
If the amygdala interprets the images and sounds as a threat, it immediately sends a distress signal to the hypothalamus (aka the command centre of the brain). The hypothalamus maintains homeostasis in the body and communicates through the autonomic nervous system with the rest of the body. It sends out signals to control involuntary body functions such as heart beat, pupil dilation, blood pressure, and digestive processes.
The autonomic nervous system can be subdivided into two further systems
After the hypothalamus receives the distress signal, it sends a signal through the sympathetic nervous system to the adrenal glands to release the hormone epinephrine (aka adrenaline) into the bloodstream. As this hormone makes it way through the body, some significant physiological changes occur that will help the body to either fight, flee, or freeze in response to the stressor.
The heart rate increases to speed up the blood flow through the body, so the muscles get more oxygen to be ready for muscle movements. At the same time, the person breathes more rapidly, blood flows faster to the lungs, dilating the bronchioles, which in turn allows more oxygen to be sent to the brain to heighten alertness. A bunch of other physiological changes occur at the same time, making our body a perfectly functioning machine.
As the initial surge of epinephrine dwindles, the hypothalamus activates a second stress response that consists of the Hypothalamus-Pituitary Glands-Adrenal Glands Axis (HPA axis). The HPA axis makes sure that the sympathetic nervous system remains activated and involves the release of the stress hormone cortisol. With cortisol rushing through our bloodstream, the body stays on active mode and remains alert.
During the fight-or-flight response, the prefrontal cortex receives less blood, resulting in a diminished ability for rational thoughts, cause-and-effect analysis, an inability to focus on small tasks, and incapability of engaging in meaningful relationships.
Once the ears and eyes realize that the threat is over, cortisol levels in the bloodstream decrease and the parasympathetic nervous system sends signals to start a body relaxation reaction. This part of the nervous system is regulated by the cranial, spinal and vagus nerves. The main neurotransmitter released by the parasympathetic nervous system is acetylcholine which helps to reduce the stress response. By reducing this response, the body flows back into homeostasis, meaning that the heart rate and breathing slow down, thus allowing the to body to return to a resting state.
When the parasympathetic nervous system is engaged, we regain access over our brain, including the prefrontal cortex. This reinstates the ability for planning, conscious thought, and socialization.