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Can you literally "die of fright?" Turns out, you can! In this episode of SciShow Hank explores the mechanisms in your body that activate when you get scared, and how they can sometimes get out of hand.
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Sources:
http://www.scientificamerican.com/article.cfm?id=scared-to-death-heart-attack
http://science.howstuffworks.com/life/scared-to-death.htm
http://online.wsj.com/news/articles/SB10001424052970203400604578072900187957988
http://www.bostonmagazine.com/health/blog/2013/10/24/can-really-scared-death/
http://io9.com/5919137/the-science-of-being-scared-to-death
http://articles.timesofindia.indiatimes.com/2010-12-06/hyderabad/28230672_1_horror-films-residential-school-student
http://www.hopkinsmedicine.org/asc/faqs.html
OK, I don't want to freak you out here, mainly because, if I do, I'm worried that I might accidentally kill you. The truth is, the idea of being scared to death is actually more than just a figure of speech. You've probably heard of or even seen this happen with other animals. Small mammals and birds often die of shock when they're captured. In human terms, that's pretty much dying of fright.

And even though you'd might like to think of yourself as being tougher than a tree sparrow or a shrew, even a perfectly fit person could be one good scare away from game over.

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Martin A. Samuels, the chair of neurology at Boston's Brigham and Women's hospital, has collected hundreds of reports of totally healthy people dying from pure fright: kids on extreme amusement park rides, victims of break-ins and muggings, and folks in what would have been completely non-fatal car accidents.

Rates of mysterious heart failures have been found to rise during traumatic events like earthquakes and riots, and in 2010, a student in India actually died of shock after watching a series of scary movies.  This fear-induced death is actually a kind of sudden heart failure called stress cardiomyopathy.  

Say you just had the ever-living poo scared out of you by a house fire or a cobra in your underwear drawer or a masked man with a gun.  Your freaked-out brain immediately starts activating your trusty fight-or-flight response in a big, big way.  This response is a function of the autonomic, or involuntary, nervous system, which goes bonkers when you're suddenly confronted with a life-threatening situation.  When this happens, the adrenal glands on top of your kidneys produce a tsunami of adrenaline and other stress hormones called catecholamines that boost your blood pressure, get your heart hammering, tense up your muscles and basically wind you up to either punch that cobra in its fanged little face or run away fast.  Evolutionarily speaking, it's a pretty good survival response, but there is a downside.  

In high doses, adrenaline can become toxic, damaging the lungs, liver, kidneys, and especially the heart, which is at the most immediate risk.  When all that adrenaline slams into your heart, it floods the special muscle and nerve tissue that keep the pace of your heart steady, the adrenaline triggers special receptors on those cells that let lots of calcium ions rush in, and they in turn activate those nerve and muscle cells.  Together, those ion signals make your muscle fibers contract, and if the adrenaline keeps surging, they don't relax, and if they don't relax, your heart can't regain the steady rhythm it needs to survive.  

This erratic, fear-induced heart rhythm is called ventricular fibrillation.  It basically causes the lower chambers of your heart to vibrate in a way that interferes with the pumping of blood throughout your body.  All of that offbeat contracting can sometimes actually distort the shape of the heart, too, ballooning it out in certain areas and collapsing it in others.  

The Japanese physicians who first identified this condition apparently thought those poor misshapen hearts looked like traditional Japanese octopus traps, so they named the condition Takotsubo Syndrome, after those urn-like devices, but some westerners know the condition by the more romantic name of "Broken Heart Syndrome".  

If a person is already at risk for a heart attack, a sudden intense fear reaction can also give them a classic heart attack.  When this happens, all that adrenaline basically serves to destabilize the sticky plaque that's in the arteries, causing them to become fully blocked. Researchers are starting to look into how a history of anxiety and possible genetic variations might contribute to how your heart responds to an adrenaline bomb, but right now, doctors don't know how to tell who might be more vulnerable to sudden death by fear. Until then, just remember, while you certainly don't want to live in fear of dying of fear, you may want to think twice about scaring your grandpa too badly.

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