Why does fear make your heart pound and your hands shake?
Before you've even decided what to do, a network of nerves has sped up your heart, widened your pupils and shut down your digestion.
▶ Start the storyYour heart pounds and your hands shake because a part of your nervous system, the sympathetic nervous system, is getting your body ready to fight or run. It works mostly without your permission: the moment you sense a threat, it speeds up your heart, raises your blood pressure, widens your airways and your pupils, and makes you sweat. Your adrenal glands join in, flooding the blood with adrenaline.
Every one of these changes has a job. Blood is pulled away from your stomach and sent to your muscles and brain, which is why fear kills your appetite. Your liver releases sugar and fat as fuel. Your muscles tense for speed and strength, and that tension is what makes you tremble. Your pupils open to let in more light. Even your blood gets ready to clot faster, in case you get hurt.
The physiologist Walter Cannon described this reaction in 1914 and called it the "necessities of fighting or flight". The logic is ancient: a zebra that spots a lion has no time to think it over. Today biologists know the response also includes freezing or fainting, so they often call it the acute stress response. When the danger passes, the opposite system, the parasympathetic one, takes over to "rest and digest".
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Recap
The sympathetic system revs you up to fight or flee; the parasympathetic system calms you down to rest and digest.
Surprising fact · Trembling with fear comes from muscles tensing up to give you extra speed and strength.
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No source, no claim. Every fact in this lesson (20 claims) cites at least one of these.