sympathetic nervous system
The sympathetic nervous system is the body's accelerator pedal. It is the branch that fires up during stress, fear, or exertion, producing the classic fight-or-flight response: pounding heart, wide pupils, sweaty palms, and a body primed for action.
Its nerves originate from the thoracic and lumbar regions of the spinal cord. Short preganglionic fibers release acetylcholine onto nicotinic receptors in ganglia, then long postganglionic fibers travel to target organs and mostly release norepinephrine onto adrenergic receptors. A special case is the adrenal medulla, which acts like a giant sympathetic ganglion and dumps adrenaline directly into the bloodstream.
Pharmacologically, the sympathetic system is targeted from many angles. Sympathomimetic drugs amplify its effects, while sympatholytics, beta blockers, and alpha blockers dampen them. Because sympathetic tone strongly influences blood pressure and heart rate, these drugs are central to treating hypertension, heart disease, and shock.
An epinephrine injection for anaphylaxis directly mimics massive sympathetic discharge: it raises blood pressure through alpha receptors and reopens constricted airways through beta-2 receptors.
Adrenaline as a chemical sympathetic surge.
Mnemonic for fight-or-flight: heart up, lungs open, eyes wide, gut down, sweat on. Almost every sympathetic effect makes sense once you picture an animal running from danger.