While scratching can feel pretty good, we all know what happens next.

儘管抓癢很舒服，但我們都知道接下來會怎樣，

When considering scratching an itch, it may be better to follow the old Yiddish proverb:

當你想要抓癢的時候，聽從這句猶太俗諺也許比較好：

"A quarrel is like an itch; the more you scratch, the more it itches."

「吵架就像抓癢一樣；你越用力，事情會越發不可收拾。」

There are plenty of things that cause that irritating, unpleasant, itchy sensation.

造成這種惱人、不舒服的癢感的原因有很多，

But why do we itch in the first place?

但起初為什麼我們會有癢的感覺呢？

The itch sensation, also known as pruritus, is not the most pleasant thing,

這種癢的感覺又稱搔癢，雖然不是那麼舒服，

but it evolved for a good reason.

卻是一種有益的演化，

Stretched out, your skin covers about 20 square feet, the biggest organ you have.

如果將你的皮膚全部攤開，面積大約有 20 平方英尺，是你身上最大的器官，

Your other organs on the inside are protected by your immune system, but skin is your first line of defense,

雖然你體內的其他器官是由免疫系統來保護，但皮膚是你身體的第一道防線，

the wall that guards the human realm, exposed to the elements,

就像一道牆，保護會接觸到各種東西的人體，

and it's got a unique way to deal with different types of threats.

它還能以一種特別的方式應付各式各樣的威脅，

Where there's an itch, there's a desire to scratch, often an unconscious desire.

只要身體有癢處，我們常常會下意識地想去抓，

This reflex likely prompted our outdoor-dwelling ancestors to remove dead skin or parasitic threats

這種反射動作使我們居住野外的祖先，可以去除身上的死皮或寄生蟲，

and other irritating things like thorns and allergens.

以及惱人的刺和過敏原，

And much like yawning, being itchy is thought to be contagious,

和打呵欠很像，大家覺得搔癢會傳染，

as some of you may be noticing right about now.

就像現在，也許你們有些人有注意到，

We're social animals, so watching some infested tribe mate get their scratch on

因為我們是群居動物，所以看到同伴抓癢的時候，

could have given us a head start to get rid of those parasites.

也會讓我們開始想去掉身上的寄生蟲，

So, we know the purpose of itch.

我們現在知道搔癢的目的了，

But what happens inside our bodies that makes it feel different from other sensations like pressure or heat?

但我們體內發生什麼變化，讓搔癢和其他像按壓、溫度之類的感覺不一樣？

Turns out, we don't know all the pieces to the itch puzzle yet.

但結果是，我們還沒完全參透關於搔癢的所有謎題，

Up until about a decade ago scientists thought itch was just a dialed-down, less intense version of pain.

直到大約 10 年前，科學家才認為搔癢只是程度比較輕微的疼痛，

When something makes contact with our bodies, nerve endings in the epidermis, the outermost layer of our skin,

當有東西接觸到我們的身體，我們皮膚最外層的表皮，裡面的神經末梢

relay this information through electrical and chemical messages, up the spine and to the brain.

就會將資訊藉由電化學脈衝，轉達至脊椎和大腦，

Different stimuli activate different nerve pathways and cause different sensations in our brains.

不同的刺激會觸發不同的神經傳導路徑，大腦就會有不同的感受，

Light touch might feel nice.

輕碰也許感覺還不賴，

A punch to the face will probably hurt.

而往臉上揍一拳大概會很痛，

We now know that there's special itch-specific circuitry in our nervous systems,

現在我們知道，在我們的神經系統裡，傳遞搔癢有特定的電路，

involving its own chemicals and cells.

以及化學物質、細胞，

And we share one universal response to itching: scratching.

對於搔癢，我們普遍的反應是：抓癢，

And boy, does it feel good!

而抓癢的確很舒服！

But, why?

但為什麼？

Scratching is technically a form of pain.

嚴格來說，搔抓也會造成疼痛，

I mean, the name of a small wound that breaks the skin is a scratch.

我的意思是，搔抓會在皮膚上製造小小的傷口，

Scratching causes a low-level pain signal to the brain that masks and overrides the itch signal,

搔抓造成輕微的疼痛訊號傳至大腦，覆蓋了搔癢的訊號，

which is why slapping, pinching, and pressing itchy regions also gives you relief.

這也是為什麼拍打、擠壓癢處，會讓你覺得比較舒服，

Remember that the next time your mom tells you not to scratch.

記住這點，下次你媽叫你不准抓癢的時候就可以用，

Your brain responds to pain by triggering the release of chemicals at the irritated site,

疼痛會觸發大腦，釋放化學物質到發炎位置，

which tunes down the painful sensation.

藉此減低疼痛的感覺，

One of those relief-providing chemicals is serotonin, a neurotransmitter.

其中一種緩解疼痛的化學物質是血清素，是一種神經傳導物質，

But that serotonin also makes it easier for the itch signal to be re-triggered.

但血清素也會更容易，反覆觸發搔癢的訊號，

So, the itch nerves fire again and you get even itchier!

因此，神經會重複發出搔癢訊號，你就會覺得更癢！

Scientists call this constant loop of itching and scratching "the itch-scratch cycle,"

科學家將這種抓癢的無限循環，稱作「抓癢循環」，

very clever, and it can be maddening.

真是天才，但這循環會讓人發狂，

In extreme cases, damage to itch-sensing nerves can cause itching without an actual stimulus.

一些極端的病例中，感知搔癢的神經受損後，即使沒有實際的刺激，也會造成搔癢，

This is the kind of itch that scratching can't help.

這種搔癢就不是抓一抓就能解決的，

People who have recovered from shingles, a virus infection that affects your nervous system,

有些得過帶狀皰疹的病患，因為病毒感染影響了神經系統，

can be left with an unexplainable, never-ending itch once the rash recedes.

即使疹子消退，仍會留下難以形容、無止盡的搔癢，

One women had such an itchy scalp after healing from shingles.

一位曾患帶狀皰疹的女性，就因為覺得頭皮非常癢，

she scratched straight through to her brain.

竟然直接抓到頭破血流。

What?

什麼？

So, what can the rest of us do to avoid scratching and itch?

那其餘的我們可以做什麼，來防止越抓越癢呢？

Itchy ailments have been documented throughout history,

歷史上記載了不少搔癢的相關疾病，

and humans have found some pretty clever ways to find relief.

而人類也發現了一些緩解症狀的妙方，

The Greeks and Romans had mineral baths and animal fat.

希臘、羅馬人利用礦泉水澡和動物脂肪，

The Persians used silver.

波斯人用銀器，

Menthol soothed itchy skin in ancient China, and camphor, a chemical from Evergreen trees

中國古代利用薄荷醇減緩皮膚搔癢，而歷史上用來製造炸藥的化學物質，

historically used to make explosives, has been soothing itchy skin since the 13th century.

長青樹所產生的樟腦，自 13 世紀開始用來減緩皮膚搔癢，

Today we also have anesthetics, which numb the skin of all feeling,

如今我們還有麻醉劑，能麻痺皮膚的所有感覺，

counterirritants like capsaicin, extracted from chili peppers,

抗刺激劑，像是從辣椒中萃取的辣椒素，

and antihistamines and steroid creams, just to name a few.

抗組織胺和類固醇乳膏等等，不勝枚舉。

But since itchiness can be caused by so many things, there isn't a one-size-fits-all remedy.

但由於搔癢的成因有很多種，所以治療的方式要依情況而定，

Luckily, with so many treatments to choose from, at least you don't have to start from scratch.

幸好治療方式有很多種，你至少不必從抓癢開始試，

If you're itchy, just remember: unlike beauty, itch doesn't run only skin deep.

如果你覺得很癢，只要記住：搔癢跟美貌不同，它並不「膚淺」，

It's a sensation that reaches from the epidermis to the brain.

而是從表皮傳達到大腦的一種感覺，

The origins of that itch you might be feeling right now extend way back on the evolutionary tree,

你可能現在正感受到的搔癢，從演化樹上來看，它的起源在很久以前，

and we've only scratched the surface of knowing how it all works.

所以我們距離完全參透它，還有一段很長的距離，

Stay curious!

保持你的好奇心！

Oh, and here is a video challenge for you: re-watch this without itching yourself uncontrollably.

最後要給你們一個挑戰：重看一次影片，而身體沒有癢到受不了。

Hey guys, I wanted to tell you about another great channel. It's part of the PBS Digital Studios family.

嗨大家，我想要推薦你們另一個很棒的頻道，它是美國公共電視台的節目，

Check out Above the Noise. It's a channel that cuts through the hype about the news

去看看《Above the Noise》，這個頻道打破新聞的炒作手法，

so that you can understand the facts behind it and how they relate to your life.

如此一來，你就能了解事實的真相，以及事實與你生活的關聯，

I recently helped them out on an episode about why only some countries have nuclear weapons.

最近我參與了他們的節目，討論為何只有部分國家擁有核武，

We'll leave a link down in the description, so you can check them out.

我們會把連結放在下方介紹，你可以去看看。