The “fight-or-flight” response evolved in our distant ancestors to prime their hearts, muscles and lungs for action, giving them that vital extra boost to escape ravenous predators or see off aggressive rivals. After the immediate threat had passed, a “relaxation” or “rest and digest” response kicked in to restore their overexcited bodies to a state of calm better suited for more restful activities, such as feeding and grooming.
But in modern humans, technologies such as social media and email can keep our fight-or-flight response constantly switched on by continually presenting us with stimuli that provoke feelings of excitement, fear or aggression.
If checking your emails makes you feel anxious or hostile, there will be strong, sustained activity in a structure in your brain called the amygdala, which processes emotions such as fear and anger. The amygdala operates like a smoke alarm, emitting a shrill alarm in response to signs of danger in the environment.
But unlike a smoke alarm, the amygdala and a neighbouring region called the hippocampus — which encodes and stores memories — can learn from emotionally charged experiences: they begin to associate particular stimuli with unpleasant or painful outcomes. These “conditioned stimuli” then provoke an aggressive or fearful response from the body, even if nothing bad actually happens. In the context of emails, this means that simply seeing a particular word or phrase in the subject line, or reading the source of the message, can trigger strong feelings of fear or hostility.
Chemical distress signal
When the amygdala registers a possible threat in our environment, it sends a chemical distress signal to a gland in the brain called the hypothalamus. This forwards the message to the rest of the body including the adrenal glands, which are the triangular structures that sit on top of the kidneys. The message is forwarded at lightning speed via the sympathetic nervous system – the branch of the involuntary (or “autonomic”) nervous system that primes the body for vigorous action.
Within a fraction of a second, this electrical signal activates the fight-or-flight response.
The adrenal glands respond by flooding the bloodstream with adrenaline and noradrenaline (also known as epinephrine and norepinephrine). As the hormone adrenaline circulates around the body, it brings on a number of physiological changes needed for physical exertion, including increased heart rate, breathing rate and blood pressure. It also diverts blood flow from the digestive system to the muscles and increases blood sugar levels.
Meanwhile in the brain, noradrenaline acts as a neurotransmitter or molecular nerve signal to increase our state of arousal and alertness. This has the benefit of enhancing attention and memory formation, but it also promotes feelings of restlessness and anxiety.
As the initial surge of adrenaline subsides, the pituitary gland and hypothalamus sustain the stress response by signalling to the adrenal glands to release the hormone cortisol, a form of steroid that influences the immune, digestive and reproductive systems. Adrenaline and cortisol also affect our mood.
The downside of self reflection
In nature, the flight-or-flight response helps to ensure that an animal lives to fight another day. But the unique problem that we humans have is our ability to reflect on the past and future, a talent that is made possible by our highly developed prefrontal cortex (PFC) at the front of each hemisphere of our brain.
If this self-reflection involves rumination and worry, it can sustain the activation of our sympathetic nervous system even in the absence of fear or anger-provoking external stimuli. Self-reflection is mediated by a constellation of brain regions in the PFC known as the default mode network.
This chronic (sustained or long-term) stress has widespread consequences. They include:
- Anxiety.
- Depression.
- Problems with memory and concentration.
- Muscle tension and pain.
- Weight gain.
- Trouble sleeping.
Years of chronic stress are also linked to high blood pressure, strokes and heart attacks.
Acute (short-term) stress can increase our state of alertness and promote learning, but chronic stress may impair memory and even shrink the hippocampus.
And while acute stress primes the immune system, chronic stress actually suppresses it.
Mixed blessing
So our recently evolved, highly developed PFC is a mixed blessing. It provides “top-down” regulation of thought, action and emotion – allowing us to adapt and function effectively in our increasingly information-driven, interconnected society.
But the ability to reflect on unpleasant events in the past and future can also keep our stress response switched on. What’s more, these chronic levels of stress can actually impair the PFC’s ability to moderate our habitual responses of fear and aggression to conditioned stimuli.
It’s a vicious circle!
This post was written with James Drever of Careful Digital for week 1 of his 3-week Intentional Technology course.
In the next post, learn how to use focused attention meditation to counteract the debilitating effects of chronic stress
Further reading
Understanding the stress response – Harvard Health Publishing. How chronic activation of the body’s fight-or-flight response affects health.
Adrenaline, part of the Society for Endocrinology’s “You and Your Hormones” series.
Cortisol, part of the Society for Endocrinology’s “You and Your Hormones” series.
Why Zebras Don’t Get Ulcers by Robert Sapolsky explains why wild animals are less susceptible than humans to chronic stress-related disorders such as ulcers, hypertension (high blood pressure), decreased neurogenesis (nerve growth) and shrinkage of the hippocampus.
Kim, J. J. & Diamond, D. M. (2002). The stressed hippocampus, synaptic plasticity and lost memories. Nature Reviews Neuroscience 3: 453-462. (How chronic stress changes the brain in ways that impair learning and memory.)
Kim, E. J. et al (2015). Stress effects on the hippocampus: a critical review. Learning & Memory 22(9): 411-416. (A roundup of the research into how stress affects the “plasticity” or flexibility of nerves in the hippocampus and can even lead to shrinkage of this part of the brain.)
Woo, E. et al (2021). Chronic stress weakens connectivity in the prefrontal cortex: architectural and molecular changes. Chronic Stress https://doi.org/10.1177/24705470211029254 (How chronic stress weakens the ability of the prefrontal cortex to control thoughts, behaviour and emotion.)
McEwen, B. S. (2017). Neurobiological and systemic effects of chronic stress. Chronic Stress; 1: 1-11. (How chronic stress affects not only cognition, decision-making and mood, but also affects our physiology, including the immune system.)
Plastic Brain 