The "Fight or Flight" response
The flight or fight response, also called the "acute stress response" was first described by Walter Cannon in the 1920s as a theory that animals react to threats with a general discharge of the sympathetic nervous system.
 In response to acute stress, acetylcholine is released from preganglionic sympathetic nerves that innervate the chromaffin cells of the adrenal medulla. As a consequence, the chromaffin cells secrete the hormone epinephrine (adrenaline) into the general circulation. This provides readily available sources of energy by forming glucose from glycogen depots and [[free fatty acid]s from the triglyceride stores of adipose tissue; it quickens the pulse and raises arterial blood pressure, but also accelerates blood coagulation and thereby protects against blood loss in the event of injury.
More recently, ethologists working with nonhuman primates have established four distinct fear responses that proceed sequentially in response to increasing threat. The sequence begins with "the freeze response" - "stop, look, and listen" . Next comes an attempt to flee, then an attempt to fight. Finally, comes tonic immobility. ("playing dead"). Thus, "freeze, flight, fight, or fright" may be a more complete and nuanced alternative to "fight or flight."  The hypothalamo-pituitary-adrenal axis
Any demand made upon the body - whether physical or emotional, whether from internal or external causes, produces a nonspecific stimulus (a "stressor"). This is converted into nervous signals that may be carried by any of many different neural pathways in the brain, but eventually it acts upon certain neuroendocrine cells in the paraventricular nucleus of the hypothalamus. These cells transform the nervous signals into a humoral messenger, (corticotrophin releasing hormone, CRH), which is secreted from nerve endings into portal blood vessels which carry it to the anterior pituitary gland. There, CRH stimulates the corticotroph...
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