What is the “fight-or-flight” response?

Following up on my neuromodulators topic (have a look at my previous posts about serotonin and oxytocin), today I will talk about the “stress” or “fight-or-flight” hormone: adrenaline.

Experiencing a stressful event usually triggers the release of adrenaline, the “stress hormone”, that in turn produces well-orchestrated physiological changes. The stress response begins in the brain, but, unlike the previous neuromodulators I wrote about, adrenaline is not released directly by neurons in the brain, but it is secreted by the adrenal gland, that sits above the kidneys. The emotional processing of a stressful situation starts in the amygdala, a collection of different nuclei of neurons found deep in the brain: when this area perceives a potential threat, it instantly sends a distress signal to the hypothalamus. The hypothalamus then acts as a command center and communicates with the rest of the body through the autonomic nervous system, that controls involuntary body functions (such as breathing, blood pressure, heartbeat and so on) in order to trigger the release of adrenaline from the adrenal glands (see picture below).

Schematic of the brain areas involved in the release of adrenalin and the adrenal glands, the actual site from which adrenaline is secreted after receiving the input from the brain. Modified from

The autonomic nervous system has two components: the sympathetic nervous system, that triggers the “fight-or-flight” response, acting like a gas pedal in a car, and the parasympathetic nervous system, that promotes the “rest-and-digest” response, calming the body down after the threats have passed. The activation of the sympathetic nervous system  stimulates the adrenal glands above the kidneys, that respond by releasing noradrenaline into the bloodstream.

The release of adrenaline into the body occurs very quickly, usually within few seconds after the perceived threat, and it goes away as soon as the threat disappears. Even though it is a very fast process, so fast that it got the name of adrenaline rush, the release of adrenaline triggers many changes in the body, such as increasing the heart rate, decreasing the body’s ability to feel pain, a temporary increase of strength and a sharpening of mental focus (see picture below for more details on the physical changes triggered by adrenaline).

Summary of all the steps of adrenaline release and its effects on the body. Modified from

All of these physical effects happen so quickly that people are not even aware of them. Indeed, the wiring is so efficient that the amygdala and hypothalamus start the communication that will eventually lead to the adrenaline release in the adrenal glands even before the brain’s visual areas have had a chance to fully process what is going on. That is the reason why we are able to jump out of the path of an oncoming car even before we realise what we are doing. This fast communication evolved as a survival mechanism, enabling us humans and other mammals to react quickly to life-threatening situations: this near-instantaneous sequence of hormonal changes and physiological responses helped and keeps on helping individuals to fight the threat off or flee to safety. 

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