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The fight-or-flight response (also called the fight-or-flight-or-freeze response, hyperarousal, or the acute stress response) is a physiological reaction that occurs in response to a perceived harmful event, attack, or threat to survival. . It was first described by Walter Bradford Cannon. The reaction is activated by the autonomic nervous system, which primes the animal for fighting or fleeing.

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

Autonomic Nervous System
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 flight or fight response and it's role is mediated by two different components.

Sympathetic Nervous System

The Sympathetic Nervous System originates in the spinal cord and it's 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.

Parasympathetic Nervous System

The Parasympathetic Nervous System originates in the spinal cord and medulla and works in concert with the sympathetic nervous system. It's 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.

Reaction
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. 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. Catecholamine hormones, such as adrenaline or noradrenaline, 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
 * Shaking

Function of Physiological Changes
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.
 * The pupils dilate to help see with increased clarity.
 * Increased perspiration to prevent over-heating due to the increased metabolic rate.

Evolutionary Perspective
An Evolutionary Psychology explanation is that early humans had to react to threatening stimuli quickly and did not have time to psychologically and physically prepare themselves. The fight or flight response provided prehistoric humans with the mechanisms to rapidly respond to threats against survival.

Examples
A typical example of the stress response is a grazing zebra. 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. Note that the zebra and cat still maintain homeostasis in all states.

Varieties of Responses
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.

Other animals have alternative self-protection methods. Some species of cold-blooded animals 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 either in a physical or 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 either fight or flee against something that is threatening, such as a hungry lion, or fight for or fly towards something that is needed, such as the safety of the shore from 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.

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 a threat.

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.

Emotion Regulation
In the context of the fight or flight response, emotional regulation is used to proactively avoid threats of stress or to control the level of emotional arousal.

Emotional Reactivity
During the reaction, the intensity of emotion that is brought on by the stimulus will also determine the nature and intensity of the behavioral response. Individual's with higher levels of emotional reactivity may be prone to anxiety and aggression, which further illustrates the further implications of appropriate emotional reaction in the fight or flight response.

Content Specificity
The specific components of cognitions in the fight or flight response seem to be largely negative. These negative cognitions may be characterized by: attention to negative stimuli, the perception of ambiguous situations as negative, and the recurrence of recalling negative words. There are also may be specific negative thoughts associated with emotions commonly seen in the reaction.

Perception of Control
Perceived control relates to an individual's thoughts about control over situations and events. Perceived control should be differentiated from actual control because an individual's beliefs about their abilities may not reflect their actual abilities. Therefore, overestimation or underestimation of perceived control can lead to anxiety and agression.

Social Information Processing
The social information processing model proposes a variety of factors that determine behavior in the context of social situations and preexisting thoughts. The attribution of hostility, especially in ambiguous situations, seems to be one of the most important cognitive factors associated with the fight or flight response because of it's implications towards aggression.

Negative Effects of the Stress Response in Humans
The stress response temporarily suppresses various biological processes including: sexual responses and digestive mechanisms. This is in an effort to focus on the stressor situation. While the fight or flight response is an adaptive reaction, prolonged increases in stress can cause a variety of negative physiological and psychological negative effects, including:

Physiological Effects
 * Headaches
 * Muscle tension and pain
 * Chest pain
 * Fatigue
 * Changes in sex drive
 * Upset stomach
 * Problems with sleeping

Psychological Effects
 * Anxiety
 * Restlessness
 * Lack of motivation or focus
 * Irritability or anger
 * Depression

Behavioral Effects
 * Overeating or undereating
 * Drug or alcohol abuse
 * Social withdrawal

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 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 panic disorder, in which the stress response is activated by the catastrophic misinterpretations of bodily sensations.

For potential positive meanings, reference can be made to Hans Selye's psychological concept of eustress or good, healthy stress.