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Sympathetic activation triggers the fight-or-flight response by redirecting blood flow to skeletal muscles, increasing heart rate and breathing, and mobilizing energy reserves through glycogen breakdown. The sympathetic nervous system also stimulates the reticular activating system to enhance mental alertness, allowing the body to respond effectively to threatening or challenging situations.

The sympathetic nervous system releases norepinephrine at peripheral synapses and distributes epinephrine and norepinephrine through the bloodstream via the adrenal medullae. These neurotransmitters produce widespread physiological effects that are longer-lasting than parasympathetic responses because norepinephrine lingers in the synaptic cleft longer than acetylcholine.

Sympathetic postganglionic axons diverge extensively, allowing simultaneous activation of multiple tissues and organs. Additionally, norepinephrine persists longer in the synaptic cleft than acetylcholine, and epinephrine and norepinephrine released from the adrenal medullae intensify and prolong sympathetic responses throughout the body.

The sympathetic nervous system regulates blood pressure by constricting and dilating blood vessels in response to pressure changes. It also promotes renin release from the kidneys, an enzyme that controls blood pressure. These mechanisms work together to maintain cardiovascular stability during physical activity and stress.

During sympathetic activation, the digestive and urinary systems reduce their activity as blood flow redirects to skeletal muscles. Simultaneously, metabolic rate increases and glycogen breaks down rapidly in the liver and muscles to provide energy. Adipose tissue also releases lipids to fuel the body's heightened energy demands.

When body temperature rises, sympathetic nerves dilate skin blood vessels and activate sweat production to release excess heat. Conversely, when temperature drops, skin blood vessels constrict to retain heat. These opposing responses allow the sympathetic nervous system to maintain thermal homeostasis during varying conditions.

Sympathetic activation increases mental alertness by stimulating the reticular activating system, creating a feeling of being on edge. Individuals experience a surge of energy and euphoria, often accompanied by temporary insensitivity to pain and disregard for danger. Muscle tone elevates, and the person may appear tense or shiver.