Walter Cannon was a scientist whom Hans Selye  regarded as his immediate predecessor in terms of thinking.

Walter Bradford Cannon, M.D. (October 19, 1871 – October 1, 1945) was an American physiologist, professor and chairman of the Department of Physiology at Harvard Medical School.

  In 1915, he coined the term fight or flight to describe an animal's response to threats in Bodily Changes in Pain, Hunger, Fear and Rage: An Account of Recent Researches into the Function of Emotional Excitement., and he expanded on Claude Bernard's concept of homeostasis. He popularized his theories in his book The Wisdom of the Body, first published in 1932.

The fight-or-flight response (also called the fight, flight, freeze, or fawn response [in PTSD], hyperarousal, or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival.[1] It was first described by Walter Bradford Cannon.[a][2] His theory states that animals react to threats with a general discharge of the sympathetic nervous system, priming the animal for fighting or fleeing.[3] More specifically, the adrenal medulla produces a hormonal cascade that results in the secretion of catecholamines, especially norepinephrine and epinephrine.[4]  Amit Sood, Professor of Medicine at Mayo Clinic College of Medicine discusses the classic fight or flight response and states that estrogen and testosterone are also hormones that affect how we react to stress, as are the neurotransmitters dopamine and serotonin.[5]

This response is recognized as the first stage of a "general adaptation syndrome" that regulates stress responses among vertebrates and other organisms.

The autonomic nervous system is a control system that acts largely unconsciously and regulates heart rate, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system is the primary mechanism in control of the fight-or-flight response and its role is mediated by two different components.

The sympathetic nervous system originates in the spinal cord and its main function is to activate the physiological changes that occur during the fight or flight response. This component of the autonomic nervous system utilizes and activates the release of norepinephrine in the reaction.

The parasympathetic nervous system originates in the spinal cord and medulla and works in concert with the sympathetic nervous system. Its main function is to activate the "rest and digest" response and return the body to homeostasis after the fight or flight response. This system utilizes and activates the release of the neurotransmitter acetylcholine

The reaction begins in the amygdala, which triggers a neural response in the hypothalamus. The initial reaction is followed by activation of the pituitary gland and secretion of the hormone ACTH. The adrenal gland is activated almost simultaneously and releases the neurotransmitter epinephrine. The release of chemical messengers results in the production of the hormone cortisol, which increases blood pressure, blood sugar, and suppresses the immune system.[10] The initial response and subsequent reactions are triggered in an effort to create a boost of energy. This boost of energy is activated by epinephrine binding to liver cells and the subsequent production of glucose. Additionally, the circulation of cortisol functions to turn fatty acids into available energy, which prepares muscles throughout the body for response.[6] Catecholamine hormones, such as adrenaline (epinephrine) or noradrenaline (norepinephrine), facilitate immediate physical reactions associated with a preparation for violent muscular action. These include the following:

Acceleration of heart and lung action
Paling or flushing, or alternating between both
Inhibition of stomach and upper-intestinal action to the point where digestion slows down or stops
General effect on the sphincters of the body
Constriction of blood vessels in many parts of the body
Liberation of metabolic energy sources (particularly fat and glycogen) for muscular action
Dilation of blood vessels for muscles
Inhibition of the lacrimal gland (responsible for tear production) and salivation
Dilation of pupil (mydriasis)
Relaxation of bladder
Inhibition of erection
Auditory exclusion (loss of hearing)
Tunnel vision (loss of peripheral vision)
Disinhibition of spinal reflexes

The physiological changes that occur during the fight or flight response are activated in order to give the body increased strength and speed in anticipation of fighting or running. Some of the specific physiological changes and their functions include:

Increased blood flow to the muscles activated by diverting blood flow from other parts of the body.
Increased blood pressure, heart rate, blood sugars, and fats in order to supply the body with extra energy.
The blood clotting function of the body speeds up in order to prevent excessive blood loss in the event of an injury sustained during the response.
Increased muscle tension in order to provide the body with extra speed and strength.

Adrenaline, Cortisol, Norepinephrine: The Three Major Stress Hormones:

Adrenaline is a hormone produced in the medulla of the adrenal glands, from where it is released into the bloodstream. It has many different actions depending on the type of cells it is acting upon. However, the overall effect of adrenaline is to prepare the body for the ‘fight or flight’ response in times of stress, ie, for vigorous and/or sudden action. Key actions of adrenaline include increasing the heart rate, increasing blood pressure, expanding the air passages of the lungs, enlarging the pupil in the eye (see figure), redistributing blood to the muscles and altering the body’s metabolism, so as to maximise blood glucose levels (primarily for the brain). A closely related hormone, noradrenaline, is released mainly from the nerve endings of the sympathetic nervous system (as well as in relatively small amounts from the adrenal medulla). There is a continuous low level of activity of the sympathetic nervous system resulting in release of noradrenaline into the circulation, but adrenaline release is only increased at times of acute stress.

Stress also stimulates the release of adrenocorticotropic hormone from the pituitary gland which promotes the production of the steroid hormone cortisol from the cortex of the adrenal glands. This steroid hormone is more important in altering the body’s metabolism (ie, raising plasma glucose) under conditions of longer-term, ongoing (chronic), rather than acute stress.

  1. Cannon, Walter (1932). Wisdom of the Body. United States: W.W. Norton & Company. ISBN 0393002055.
  2. Walter Bradford Cannon (1929). Bodily changes in pain, hunger, fear, and rage. New York: Appleton-Century-Crofts.
  3. Jansen, A; Nguyen, X; Karpitsky, V; Mettenleiter, M (27 October 1995). "Central Command Neurons of the Sympathetic Nervous System: Basis of the Fight-or-Flight Response". Science Magazine 5236 (270).
  4. Walter Bradford Cannon (1915). Bodily Changes in Pain, Hunger, Fear and Rage: An Account of Recent Researches into the Function of Emotional Excitement. Appleton-Century-Crofts
  5. "Adrenaline, Cortisol, Norepinephrine: The Three Major Stress Hormones, Explained". Hufflington Post. April 19, 2014. Retrieved 16 August 2014.
  6. "HOW CELLS COMMUNICATE DURING THE FIGHT OR FLIGHT RESPONSE". University of Utah. Retrieved 18 April 2013.