Sympathetic and Parasympathetic Arousal

The autonomic nervous system runs the body functions you don't consciously control, like heart rate, breathing, and digestion. It has two branches that work like a gas pedal and a brake. When your brain detects a threat, the sympathetic branch floors the gas: heart rate and breathing speed up, pupils dilate, digestion shuts down, blood flow shifts toward your muscles, and the adrenal glands dump epinephrine (adrenaline) into the bloodstream. All of this happens together, in seconds, because one burst of nervous system activity and one wave of hormones reach many organs at once. That's why a sudden scare hits your whole body, not just one part of it.

The parasympathetic branch is the brake, and it is just as active as the gas pedal, not simply its absence. It uses its own nerve pathways and its own neurotransmitter, mainly acetylcholine, to slow the heart, restart digestion, and return breathing to normal. This rebound takes real time, partly because epinephrine lingers in the blood and keeps tissues activated even after the sympathetic nerve signals stop firing. That lag is why your hands can still shake and your heart can still race minutes after a near-miss car accident, even though the danger is long gone.

This same sympathetic surge reappears as the alarm phase of Hans Selye's general adaptation syndrome, the body's first response to any prolonged stressor. Where arousal in this topic describes a quick reaction to one scare, the alarm phase marks the opening stage of a longer stress response that continues into resistance and, if the stressor persists, exhaustion.