<P> The sympathetic nervous system originates in the spinal cord and its main function is to activate the physiological changes that occur during the fight - or - flight response . This component of the autonomic nervous system utilises and activates the release of norepinephrine in the reaction . </P> <P> 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 . Its main function is to activate the "rest and digest" response and return the body to homeostasis after the fight or flight response . This system utilises and activates the release of the neurotransmitter acetylcholine . </P> <P> 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 . 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 (epinephrine) or noradrenaline (norepinephrine), facilitate immediate physical reactions associated with a preparation for violent muscular action and: </P> <Ul> <Li> Acceleration of heart and lung action </Li> <Li> Paling or flushing, or alternating between both </Li> <Li> Inhibition of stomach and upper - intestinal action to the point where digestion slows down or stops </Li> <Li> General effect on the sphincters of the body </Li> <Li> Constriction of blood vessels in many parts of the body </Li> <Li> Liberation of metabolic energy sources (particularly fat and glycogen) for muscular action </Li> <Li> Dilation of blood vessels for muscles </Li> <Li> Inhibition of the lacrimal gland (responsible for tear production) and salivation </Li> <Li> Dilation of pupil (mydriasis) </Li> <Li> Relaxation of bladder </Li> <Li> Inhibition of erection </Li> <Li> Auditory exclusion (loss of hearing) </Li> <Li> Tunnel vision (loss of peripheral vision) </Li> <Li> Disinhibition of spinal reflexes </Li> <Li> Shaking . </Li> </Ul>

Fight-or-flight response resides in what part of the brain
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