In less than one-tenth of a second
Your eyes spot a snake (or your ears pick up on footsteps behind you in an alley) and shoot a signal to the amygdala, a bundle of brain cells that acts as the hard drive of human emotion. It instantly triggers the adrenal glands--and just like that, your adrenaline rush is on, lending you added strength and speed.
The amygdala then prompts the already-busy adrenal glands to churn out cortisol, a stress hormone. High levels of cortisol can impede insulin, causing a rise in blood sugar--i.e., a little extra fuel for a fight-or-flight situation.
Within three seconds
Thanks to the adrenaline, you're breathing faster (to take in extra oxygen), your heart is racing (to get that oxygen to your muscles), your appetite stalls (the energy your body would use for digestion is diverted toward survival), you've started to sweat (to keep from overheating), and your pupils are dilated (to better ID an enemy).
The cortisol has saturated your bloodstream and feeds back into the amygdala to boost the perception of danger. It also reinforces your memory of the initial fright, so you may feel a little jumpy for the next few days.
Within five seconds
The rest of your brain is now fully tuned in to the threat. Nerve cells start to release endorphins, morphinelike natural painkillers that can combat the effects of physical stress (just in case you get injured by the scary thing that's revved you up).
Your brain is also releasing dopamine, the neurotransmitter best known for ushering in a "feel good" sensation akin to what you'd get from some narcotics. Hence, that thrill-packed delight some people may feel while watching horror movies.
Within five minutes
If the terror has passed, your body starts to simmer down (you can help it along by taking slow, measured breaths). The thinking center of your brain, the frontal cortex, was drowned out by all the fuss but is now able to unleash nerve signals that quiet down the amygdala and talk sense into the rest of your head.
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