The fight-or-flight response (also called the acute stress response) was first described by Walter Bradford Cannon. His theory states that animals react to threats with a general discharge of the sympathetic nervous system, priming the animal for fighting or fleeing. This response was later recognized as the first stage of a general adaptation syndrome that regulates stress responses among vertebrates and other organisms.
Upon sensing a threat the brain stimulates the hypothalamus to secrete corticotropin-releasing hormone that induces adrenocorticotropic hormone from the pituitary to stimulate the release of cortisol from the adrenal cortex to increase blood sugar levels in preparation for fight or flight.
Simultaneously, the adrenal gland also releases catecholamine hormones, such as adrenaline or noradrenaline, into the blood stream. Numerous hormone receptors exist around the body that allow for an immediate, systemic physiological response that can 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 the blood vessels in many parts of the body.
- Liberation of nutrients (particularly fat and glucose) for muscular action.
- Dilation of the blood vessels for muscles.
- Inhibition of the lacrimal gland (responsible for tear production) and salivation.
- Dilation of the pupil (mydriasis).
- Relaxation of the bladder.
- Inhibition of an erection.
- Auditory exclusion (loss of hearing).
- Tunnel vision (loss of peripheral vision).
- Disinhibition of spinal reflexes.
- Shaking.
The fight-or-flight response
A diagrammatic representation of the fight-or-flight response.
The stress response halts or slows down various processes, such as sexual responses and digestive systems, to focus on the stressor situation. This typically causes negative effects like constipation, anorexia, erectile dysfunction, difficulty urinating, and difficulty maintaining sexual arousal. These are functions that are controlled by the parasympathetic nervous system and are therefore suppressed by sympathetic arousal.
Prolonged stress responses may result in chronic suppression of the immune system, leaving the body open to infections. However, a short boost to the immune system shortly after the fight-or-flight response is activated has been described. Some think that this may have filled an ancient need to fight the infections in a wound that one may have received during interaction with a predator.
Stress responses are sometimes a result of mental disorders, such as post-traumatic stress disorder (in which the individual shows a stress response when remembering a past trauma) and in panic disorder (in which the stress response is activated by the catastrophic misinterpretations of bodily sensations).