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動画の字幕をクリックしてすぐ単語の意味を調べられます!
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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.
コツ:単語をクリックしてすぐ意味を調べられます!

読み込み中…

【TED-Ed】Why do we itch? - Emma Bryce

15423 タグ追加 保存
Jost Lin 2017 年 7 月 27 日 に公開
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