The sympathetic nervous system is one of the two opposing aspects of the autonomic nervous system, which controls our involuntary organisation. The main communicating neurotransmitters of the sympathetic system are noradrenaline and adrenaline. Although active all the time (natural sympathetic tone), the system comes to the fore when we have to survive under sudden threat.
It prepares the body for immediate action by initiating a fight or flight response (sympathico-adrenal response). To do this the sympathetic nervous system increases heart output (fast forceful pulse) and breathing capacity (deeper breathing with airways wide open), narrows certain blood vessels to increase blood pressure while opening up others to bring oxygen where needed (rerouting blood from the skin and gut to the heart, brain and muscles), dilates the pupil and increases tunnel vision (better focus and less peripheral vision), raises hair on the skin (pilo-erection or goose bumps to reduce energy loss), releases glucose and fatty acids (to supply a burst of energy) and enhances the formation of new memory for the danger event in specific areas of the brain. At the same time the sympathetic system reduces those activities which may interfere with self-defence during an emergency such as digestion, secretion of tears and saliva, urinating and defecating, bloodflow to the skin (causing paleness), susceptibility to loud sounds and sensitivity to pain. To amplify and guarantee a proper sympathetic response, the sympathetic nerve system also stimulates the adrenal gland to secrete the hormones adrenaline and noradrenaline into the blood stream which then sustains the sympathetic effects on the body as a back-up system.
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