Fight-or-flight response

The fight-or-flight or the fight-flight-freeze-or-fawn (also called 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 in 1914 to which he referred to as "the necessities of fighting or flight" in 1915. His theory states that animals react to threats with a general discharge of the sympathetic nervous system, preparing 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, as well as the neurotransmitters dopamine and serotonin, also affect how organisms react to stress. The hormone osteocalcin might also play a part.

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

Originally understood as the "fight-or-flight" response in Cannon's research, the state of hyperarousal results in several responses beyond fighting or fleeing. This has led people to calling it the "fight, flight, freeze" response, "fight-flight-freeze-fawn" or "fight-flight-faint-or-freeze", among other variants.

The wider array of responses, such as freezing, flop, faint, flee and fright, has led researchers to use more neutral or accommodating terminology such as "hyperarousal" or the "acute stress response".

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 and the parasympathetic nervous system.

The sympathetic nervous system originates in the spinal cord and its main function is to activate the arousal responses that occur during the fight-or-flight response. The sympathetic nervous system transfers signals from the dorsal hypothalamus, which activates the heart, increases vascular resistance, and increases blood flow, especially to the muscle, heart, and brain tissues. It activates the adrenal medulla, releasing catecholamines that amplify the sympathetic response. Additionally, this component of the autonomic nervous system utilizes and activates the release of norepinephrine by the adrenal glands in the reaction.

The parasympathetic nervous system originates in the sacral spinal cord and medulla, physically surrounding the sympathetic origin, and works in concert with the sympathetic nervous system. It is known as the calming portion of the autonomic nervous system. While the sympathetic nervous system is activated, the parasympathetic nervous system decreases its response. Efferent vagal fibers originating from the nucleus ambiguous fire in parallel to the respiratory system, decreasing the vagal cardiac parasympathetic tone. After the fight or flight response, the parasympathetic system's main function is to activate the "rest and digest" response and return the body to homeostasis. 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, via the sympathetic nervous system, and releases the hormone 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.