WEBVTT

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You're standing at 
the ready inside the goal

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

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

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

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The average person experiences
dozens of individual itches each day.

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They can be triggered by all sorts
of things,

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including allergic reactions,

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dryness,

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and even some diseases.

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

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or just from talking about itching.

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

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

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When a mosquito bites you,

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it releases a compound into your body
called an anticoagulant

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that prevents your blood from clotting.

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That compound, 
which we're mildly allergic to,

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triggers the release of histamine,

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a chemical that makes 
our capillaries swell.

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This enables increased blood flow,

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which helpfully accelerates 
the body's immune response

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to this perceived threat.

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That explains the swelling,

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

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Histamine also activates the nerves
involved in itching,

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which is why bug bites make you scratch.

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

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In fact, much of what we do know

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comes from studying 
the mechanics of itching in mice.

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Researchers have discovered that
itch signals in their skin

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

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These dedicated nerves produce a molecule
called natriuretic polypetide B,

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

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where it creates the feeling of an itch.

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

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

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

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But is there actually an evolutionary
purpose to the itch,

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or is it simply there to annoy us?

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

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

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Think about it.

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Our automatic scratching response
would dislodge anything harmful

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that's potentially lurking on our skin,

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like a harmful sting,

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a biting insect,

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or the tendrils of a poisonous plant.

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

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like in our intestines,

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which is safe from these external threats,

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though imagine how 
maddening that would be.

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

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can cause excessive itching
that can actually harm their health.

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One extreme example is a psychological
condition called delusory parasitosis

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where people believe their bodies
are infested with mites or fleas

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scurrying over and under their skin,

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making them itch incessantly.

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Another phenomenon 
called phantom itching

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can occur in patients 
who've had amputations.

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Because this injury has 
so severely damaged the nervous system,

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it confuses the body's normal 
nerve signaling

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and creates sensations in limbs
that are no longer there.

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Doctors are now finding ways
to treat these itching anomalies.

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

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which the patient scratches.

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That creates an illusion 
that tricks the brain

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into thinking the imaginary 
itch has been satisfied.

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Oddly enough, that actually works.

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Researchers are also searching
for the genes involved in itching

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

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in extreme cases.

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

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Dante agreed.

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The Italian poet wrote about
a section of hell

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