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. It 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. More specifically, the adrenal medulla produces a hormonal cascade that results in the secretion of catecholamines , especially norepinephrine and epinephrine . The hormones estrogen , testosterone and cortisol , and the neurotransmitters dopamine and serotonin , also affect how organisms react to stress.
The sympathetic nervous system is one of the two main divisions of the autonomic nervous system , the other being the parasympathetic nervous system . The autonomic nervous system functions to regulate the body's unconscious actions. The sympathetic nervous system's primary process is to stimulate the body's fight-or-flight response . It is, however, constantly active at a basic level to maintain homeostasis . The sympathetic nervous system is described as being complementary to the parasympathetic nervous system which stimulates the body to "rest-and-digest" or "feed and breed".
In physiology , a stimulus (plural stimuli ) is a detectable change in the internal or external environment . The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor , it normally elicits or influences a reflex via stimulus transduction . These sensory receptors can receive information from outside the body, as in touch receptors found in the skin or light receptors in the eye, as well as from inside the body, as in chemoreceptors and mechanorceptors . An internal stimulus is often the first component of a homeostatic control system . External stimuli are capable of producing systemic responses throughout the body, as in the fight-or-flight response . In order for a stimulus to be detected with high probability, its level must exceed the absolute threshold ; if a signal does reach threshold, the information is transmitted to the central nervous system (CNS), where it is integrated and a decision on how to react is made. Although stimuli commonly cause the body to respond, it is the CNS that finally determines whether a signal causes a reaction or not.
A stressor is a chemical or biological agent , environmental condition, external stimulus or an event that causes stress to an organism .
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