In 1915, a young Harvard Physiologist named Walter Cannon described “the necessities of fighting or flight” in his now classic book, Bodily Changes in Pain, Hunger, Fear and Rage[i]. With the turn of a phrase, Cannon began the process of ingraining the now famous “fight or flight” instinct into our consciousness. As often occurs with scientific breakthroughs, the now common concept was a result not of deliberate examination but one of serendipity.

As a young medical student, Cannon used the newly discovered X-ray to investigate the mechanism of swallowing; did food reach the stomach because of muscles in the mouth or esophagus? Starting with geese and then moving to cats, Cannon X-rayed animals as they swallowed a variety of liquids and foods. The results were a success, as Cannon discovered how peristalsis helped move food through digestion. While putting the cats through his experiment, Cannon noticed a peculiarity. When the cats got scared or alarmed, the movement of the food through the gastrointestinal tract came to a halt. Even stranger, if the cats were consoled and comforted, GI tract movement resumed.[ii]

From this simple observation of digestion, Cannon and his colleagues at the Harvard lab researched the wider impact of what occurred when animals were alarmed. After a series of wide-ranging experiments focusing on what happens to different organs when animals experienced alarm, Cannon centered on the sympathetic nervous system, and the hormone that it triggered, adrenaline. With the administration or uptick of adrenaline through emotional disturbance, the animals body transformed, as if preparing for battle. He saw all of the now classic signs; increases in heart rate, breathing rate, and glucose mobilization, as well as the redistribution of blood flow. With these observations, Cannon was convinced that in response to any acute emergency, the body had a singular response, and it was all because of adrenaline; Prepare to fight or flight.

Cannon wasn’t finished with stress. In 1932 in his book The Wisdom of the Body, he outlined the other side of the stress coin, how one returns to normal. Influenced by the ideas of Claude Bernard decades earlier, Cannon sought to discover how stability in the body was maintained. How did breathing rates, heart rate, and blood pressure return to normal after a stress inducing disturbance in these systems? He believed that the body had a natural control system, that was based on the stability of the internal environment. Through innovative animals experiments looking at hunger, thirst, Cannon outlined the concept of homeostasis, or the maintenance of a relatively stable environment.

Cannon had his grand theories. In response to emergencies, adrenaline was the key. When faced with a disturbance, the body sought to prepare and then return itself to balance, regardless of what type of stressor it was. In other words, the system was reactive, the emotional or physical trigger caused a downstream reaction. With the two major concepts of stress, fight or flight and homeostasis, in place, Cannon changed our understanding of emergencies and emotional disturbance forever. But it took another scientist to bring the ideas to the mainstream.

Hans Selye and the popularization of Stress

Like his predecessor, Hans Selye didn’t set out to understand stress, he was after something far different–a new sex hormone. He began his experimentation by injecting ovarian extract into rats, hoping to discover the new hormone being released as a reaction to the extract. With each reaction, he saw a distinct response, but it wasn’t his sought after hormone. Instead, their was a signature response, the adrenal cortexes enlarged and the immune system reacted. Frustrated and downtrodden by his lack of hormone discovery, Selye began to think about this standard response in a different way.

He tried injecting different substances, shocking the rats, and a whole slew of other activities, and each time, the same signature response occurred. Selye believed that in response to any noxious stimulus, the body had a generalized response. Selye termed the stimulus a stress.

In his work, Selye built upon Cannon’s research and found that it wasn’t only adrenaline that caused the body to be ready for battle. The Hypothalamic-Pituitary Axis (HPA) played a large role. The HPA axis results in the release of Cortisol and other hormones that we now commonly refer to as ‘stress hormones.’ Selye believed the body went through an alarm stage (similar to Cannon’s fight or flight stage) in which it martialled a reaction to an stimulus. Cortisol acted as the main trigger, which resulted in the signature response that Selye kept observing. Crucially, Selye believed that this was a nonspecific reaction. No matter what the stressor–be it burns, cold, infection or trauma– the body responded in the same way. And if it occurred too frequently, the body would eventually become ‘exhausted.’

With the believed universality of Selye’s concept he called the General Adaptation Syndrome, Selye began applying it to everything. Any environmental stressor had to cause the same reaction. He took to popular writing to explain his concept, and the harm that could potentially occur. Too much stress and your organs could be damaged or even fail. Despite calls for Selye to term this negative reaction as something other than “stress” by the journal Nature, Selye’s choice phrase grew into an empire. Stress was negative, and it was a singular reaction.

A Product of their times

The evolution of the concept of stress didn’t occur in a physiologist vacuum. Ideas grow from the context of their times. And in many ways the evolution of the concept of stress is the result of the simultaneous growth and battle of physiology versus psychology. In the start of Bodily Changes in Pain, Hunger, Fear and Rage, Cannon’s Darwinian influence can be seen:

“the doctrine of human development from sub-human antecedents has done much to unravel the complex nature of man. As a means of interpretation this doctrine has been directed chiefly towards the solving of puzzles in the peculiarities of anatomical structure…expressive actions and gestures– the facial appearance in anger, for example–observed in children and in widely distinct races, are found to be innate, and are best explained as the retention in human beings of responses which are similar in character in lower animals.”

Cannon saw himself as extending the explosion of Darwinian explanation in the physical sciences to the world of emotions and what we now call stress. He believed that we could best understand these reactions as a simple response coming from our ancient ancestry.

During the middle of the 20th century, with Ivan Pavlov and B.F. Skinner’s influence, the concept of Behaviorism took root in the understanding of psychology. The theory posited that behaviors could best be understand as a reflex or a reaction to our environment. Whenever a stimulus was presented, we had a response that was either ingrained, or conditioned. It shouldn’t come as a surprise that Cannon was a friend of Pavlov, the man who popularized the notion of conditioning, which relied on ingraining a response to a stimulus. Cannon and Pavlov exchanged correspondence over Pavlov’s ideas of a conditioned reflex. As Cannon outlined his theories and Selye then built upon them, it’s little doubt that the ideas that held a prominent place in the conceptual framework of psychology at the time, influenced these thinkers. Whether it was Darwin or the Behaviorist, the framework at the time, provides a glimpse of the lens that most scientist saw during their time. During this unique time, the physiology and the psychology were on the same page, reactive response driven systems dominated.

A Simple View

Scientific rigor or acceptance doesn’t guarantee translation to the lay public. To make that jump requires something else, a capturing of the publics imagination. The concept needs to invade our mental space, capturing part of our collective conscious just as any viral meme would today. And that’s what separates the work of Selye and Cannon apart. Their legacy in popular culture can be seen in the words and phrases we still use today. Anytime we invoke the powers of adrenaline, be it in pushing a football player to extraordinary performance in the final moments of a game, or an adventurer saving himself in a life or death situation, we have Cannon to thank. Similarly, when we blame stress for our recent trip to the doctors or even our greying hair, it’s a clear reflection of the lasting impact of Hans Selye.

Their joint ideas of a non-specific response became a fixture of both scientific and popular understanding of stress. Whenever we were faced with a stressful event or alarming situation, our bodies initiated a cascade of events to prepare us. Whether it was through the GPA axis and cortisol or the Sympathetic Nervous System and adrenaline, both Cannon and Selye believed the response was the same. We had a general stress response, with both Cannon and Selye leaving only a little room for minor adjustments based on the specific stressor we encountered.

Furthermore, our one general response system was reactive. We needed the stressor or stimulus to trigger this cascade. The ideas were so fervently implanted in our minds that it wasn’t until fifty years after Selye introduced his concept of a General Adaptation Syndrome, that his idea of non-specificity was scrutinized with experimental testing (Pacak et al. 1998). By this time, it was too late, the concept of stress, as Selye envisioned it, was firmly planted in our conscious minds. Like a catchy theme song that wormed it’s way into our minds, once ideas have taken hold, it’s difficult to let them go, even if their flaws later become clear.

If Selye and Cannon’s general reactive response is correct, then we all would experience the same adrenaline or cortisol driven response. Yet, individuals facing similar circumstances, experience different reactions and display different behaviors. A few years ago I was with a group of six runners who came upon a meth camp on a run through the woods. We all didn’t sprint away. Some froze, others ran, a few searched for more information. But, the behaviors weren’t the only things that differed. If you asked each individual, his or her internal experiences were just as varied. Some were scared and frightened, others filled with excitement, and a few were calm, cool and collected. Did the same general response cause all of these varied feelings and behaviors? Or did each individual experience their own distinct stress response, guided by a unique combination of nervous system activation and hormonal output?

As is often the case, our body is smarter than we like to give it credit for.

This is part of a serialization for the HPW Scholar program. This is Part 1 of 5 that will appear there. The rest of the series is reserved for HPW Scholars. For more information check out the HPW Scholar Program



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The Origins of Stress- Understanding our bodies response

2 thoughts on “The Origins of Stress- Understanding our bodies response

  • June 26, 2018 at 9:18 am

    Hi Steve!
    thanks for the great articles on stress and good to see new articles on running. Have read tons of great stuff in your blog, specially older articles specific about running. Have you read “How Emotions Are Made” by Lisa Feldman Barrett? If not, take a look. Think her theory would match your interests and add to your research.

    • July 2, 2018 at 4:51 pm

      Yes! I love that book.


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