The "fight-or-flight response", also called the "fight-or-flight-or-freeze response", the "fright, fight or flight response", "hyperarousal" or the "acute stress response", was first described by Walter Cannon in 1929.
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.
Normally, when a person is in a serene, unstimulated state, the "firing" of neurons in the locus coeruleus is minimal. A novel stimulus (which could include a perception of danger or an environmental stressor such as elevated sound levels or over-illumination), once perceived, is relayed from the sensory cortex of the brain through the hypothalamus to the brainstem.
That route of signaling increases the rate of noradrenergic activity in the locus coeruleus, and the person becomes alert and attentive to the environment. Similarly, an abundance of catecholamines at neuroreceptor sites facilitates reliance on spontaneous or intuitive behaviors often related to combat or escape.
If a stimulus is perceived as a threat, a more intense and prolonged discharge of the locus ceruleus activates the sympathetic division of the autonomic nervous system. This activation is associated with specific physiological actions in the system, both directly and indirectly through the release of epinephrine (adrenaline) and to a lesser extent norepinephrine from the medulla of the adrenal glands. The release is triggered by acetylcholine released from preganglionic sympathetic nerves. The other major factor in the acute stress response is the hypothalamic-pituitary-adrenal axis.
A typical example of the stress response is a grazing zebra, calmly maintaining homeostasis. If the zebra sees a lion closing in for the kill, the stress response is activated. The escape requires intense muscular effort, supported by all of the body’s systems. The sympathetic nervous system’s activation provides for these needs. A similar example involving fight is of a cat about to be attacked by a dog. The cat shows accelerated heartbeat, piloerection (hair standing on end, normally for conservation of heat), and pupil dilation, all signs of sympathetic arousal (Gleitman et al., 2004).
Though Cannon, who first proposed the idea of fight-or-flight, provided considerable evidence of these responses in various animals, it subsequently became apparent that his theory of response was too simplistic. Animals respond to threats in many complex ways. Rats, for instance, try to escape when threatened, but will fight when cornered. Some animals stand perfectly still so that predators will not see them. Many animals freeze or play dead when touched in the hope that the predator will lose interest.
Others have more exotic self-protection methods. Some species of fish change color swiftly, to camouflage themselves. These responses are triggered by the sympathetic nervous system, but in order to fit the model of fight or flight, the idea of flight must be broadened to include escaping capture in either a physical way or in a sensory way. Thus, flight can be disappearing to another location or just disappearing in place. And often both fight and flight are combined in a given situation.
The fight or flight actions also have polarity - the individual can fight or fly against or away from something that is threatening, such as a hungry lion, or fight or fly for or towards something that is needed, such as the safety of the shore of a raging river.
A threat from another animal does not always result in immediate fight or flight. There may be a period of heightened awareness, during which each animal interprets behavioral signals from the other. Signs such as paling, piloerection, immobility, sounds, and body language communicate the status and intentions of each animal. There may be a sort of negotiation, after which fight or flight may ensue, but which might also result in playing, mating, or nothing at all. An example of this is kittens playing: each kitten shows the signs of sympathetic arousal, but they never inflict real damage.
In prehistoric times when the fight or flight response evolved, fight was manifested in aggressive, combative behavior and flight was manifested by fleeing potentially threatening situations, such as being confronted by a predator. In current times, these responses persist, but fight and flight responses have assumed a wider range of behaviors. For example, the fight response may be manifested in angry, argumentative behavior, and the flight response may be manifested through social withdrawal, substance abuse, and even television viewing (Friedman & Silver 2007).
Males and females tend to deal with stressful situations differently. Males are more likely to respond to an emergency situation with aggression (fight), while females are more likely to flee (flight), turn to others for help, or attempt to defuse the situation – 'tend and befriend'. During stressful times, a mother is especially likely to show protective responses toward her offspring and affiliate with others for shared social responses to threat (Taylor et al., 2000).
The stress response halts or slows down various processes such as sexual responses and digestive systems to focus on the stressor situation and typically causes negative effects like constipation, anorexia, erectile dysfunction, difficulty urinating, and difficulty maintaining sexual arousal.. These are functions which are controlled by the parasympathetic nervous system and therefore suppressed by sympathetic arousal
Prolonged stress responses may result in chronic suppression of the immune system, leaving the body open to infections, however there is a short boost of the immune system shortly after the fight or flight response has been activated. This is due to 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 panic disorder, in which the stress response is activated by the catastrophic misinterpretations of bodily sensations.
This article incorporates public domain material from the United States Government document "http://www.surgeongeneral.gov/library/mentalhealth/chapter4/sec2_1.html".