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  • You're standing at the ready inside the goal.

  • When suddenly, you feel an intense itch on the back of your head.

  • We've all experienced the annoyance of an inconvenient itch,

  • but have you ever pondered why we itch in the first place?

  • The average person experiences dozens of individual itches each day.

  • They can be triggered by all sorts of things,

  • including allergic reactions,

  • dryness,

  • and even some diseases.

  • And then there are the mysterious ones that pop up for no reason at all,

  • or just from talking about itching.

  • You're scratching your head right now, aren't you?

  • Anyhow, let's take one of the most common sources: bug bites.

  • When a mosquito bites you,

  • it releases a compound into your body called an anticoagulant

  • that prevents your blood from clotting.

  • That compound, which we're mildly allergic to,

  • triggers the release of histamine,

  • a chemical that makes our capillaries swell.

  • This enables increased blood flow,

  • which helpfully accelerates the body's immune response

  • to this perceived threat.

  • That explains the swelling,

  • and it's the same reason pollen can make your eyes puff up.

  • Histamine also activates the nerves involved in itching,

  • which is why bug bites make you scratch.

  • But the itchy sensation itself isn't yet fully understood.

  • In fact, much of what we do know

  • comes from studying the mechanics of itching in mice.

  • Researchers have discovered that itch signals in their skin

  • are transmitted via a subclass of the nerves that are associated with pain.

  • These dedicated nerves produce a molecule called natriuretic polypetide B,

  • which triggers a signal that's carried up the spinal cord to the brain,

  • where it creates the feeling of an itch.

  • When we scratch, the action of our fingernails on the skin

  • causes a low level pain signal that overrides the itching sensation.

  • It's almost like a distraction, which creates the sensation of relief.

  • But is there actually an evolutionary purpose to the itch,

  • or is it simply there to annoy us?

  • The leading theory is that our skin has evolved to be acutely aware of touch

  • so that we're equipped to deal with risks from the outside world.

  • Think about it.

  • Our automatic scratching response would dislodge anything harmful

  • that's potentially lurking on our skin,

  • like a harmful sting,

  • a biting insect,

  • or the tendrils of a poisonous plant.

  • This might explain why we don't feel itching inside our bodies,

  • like in our intestines,

  • which is safe from these external threats,

  • though imagine how maddening that would be.

  • In some people, glitches in the pathways responsible for all of this

  • can cause excessive itching that can actually harm their health.

  • One extreme example is a psychological condition called delusory parasitosis

  • where people believe their bodies are infested with mites or fleas

  • scurrying over and under their skin,

  • making them itch incessantly.

  • Another phenomenon called phantom itching

  • can occur in patients who've had amputations.

  • Because this injury has so severely damaged the nervous system,

  • it confuses the body's normal nerve signaling

  • and creates sensations in limbs that are no longer there.

  • Doctors are now finding ways to treat these itching anomalies.

  • In amputees, mirrors are used to reflect the remaining limb,

  • which the patient scratches.

  • That creates an illusion that tricks the brain

  • into thinking the imaginary itch has been satisfied.

  • Oddly enough, that actually works.

  • Researchers are also searching for the genes involved in itching

  • and developing treatments to try and block the pathway of an itch

  • in extreme cases.

  • If having an unscratchable itch feels like your own personal hell,

  • Dante agreed.

  • The Italian poet wrote about a section of hell

  • where people were punished by being left in pits to itch for all eternity.

You're standing at the ready inside the goal.

Subtitles and vocabulary

B2 H-INT US itch itching skin creates sensation allergic

【TED-Ed】Why do we itch? - Emma Bryce

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    Jost Lin   posted on 2017/05/10
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