Why do we itch? - Emma Bryce

譯者: Lilian Chiu 審譯者: Helen Chang

You're standing at the ready inside the goal

你正擺好姿勢守著球門，

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

突然，你感到後腦勺很癢。

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

我們都曾歷過惱人的難搔之癢，

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

但你是否曾想過為什麼我們會癢呢？

The average person experiences dozens of individual itches each day.

一般人每天會發癢數十次。

They can be triggered by all sorts of things,

各種東西都有可能引發癢感，

including allergic reactions,

包括過敏性反應、

dryness,

乾燥，

and even some diseases.

甚至某些疾病。

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

還有些時候，發癢是沒有原因的，

or just from talking about itching.

或只因為說到癢，就癢起來了。

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

你現在正在搔你的頭，對嗎？

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

總之，我們來談其中一種 最常見的原因：蚊蟲叮咬。

When a mosquito bites you,

蚊子咬你時

it releases a compound into your body called an anticoagulant

會將一種叫做「抗凝血劑」的 複合物注入你體內，

that prevents your blood from clotting.

它會讓你的血液無法凝固。

That compound, which we're mildly allergic to,

我們對那種複合物有輕微的過敏，

triggers the release of histamine,

它會造成身體釋放「組織胺」，

a chemical that makes our capillaries swell.

一種化學物，會讓毛細管腫脹。

This enables increased blood flow,

血液流量就會增加，

which helpfully accelerates the body's immune response

這能協助身體更快對於

to this perceived threat.

感知到的威脅產生免疫反應。

That explains the swelling,

這是腫脹的原因，

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

這也就是花粉能導致你的 眼睛腫起來的原因。

Histamine also activates the nerves involved in itching,

組織胺還會觸發與癢有關的神經，

which is why bug bites make you scratch.

這就是為什麼蚊蟲叮咬 會讓你想抓癢。

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

但瘙癢感還沒有被完全了解。

In fact, much of what we do know

事實上，我們對它的了解，

comes from studying the mechanics of itching in mice.

大多來自探討老鼠瘙癢機制的研究。

Researchers have discovered that itch signals in their skin

研究者發現，牠們皮膚中的瘙癢信號

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

是透過與疼痛相關的神經來傳輸的。

These dedicated nerves produce a molecule called natriuretic polypetide B,

這些專用神經會產生 一種叫做「多肽Ｂ」的分子，

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

它會觸發一個信號， 透過脊髓傳輸到大腦，

where it creates the feeling of an itch.

大腦就會產生瘙癢的感覺。

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

當我們搔癢時，指甲在皮膚上的動作

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

會造成一個低度的疼痛訊號， 蓋過瘙癢的感覺。

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

這就像是分散注意力 能造成放鬆的感覺一樣。

But is there actually an evolutionary purpose to the itch,

但，瘙癢是否真有進化上的目的？

or is it simply there to annoy us?

或者它單純只是種惱人的存在？

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

主要的理論是，我們的皮膚已經 演化成能夠敏銳感受到觸碰，

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

讓我們能準備好 處理來自外界的風險。

Think about it.

試想看看。

Our automatic scratching response would dislodge anything harmful

我們的自動搔癢反應，

會除去潛藏在皮膚上的 任何有害物質。

that's potentially lurking on our skin,

like a harmful sting,

就像有害的螫刺、

a biting insect,

正在叮咬的昆蟲，

or the tendrils of a poisonous plant.

或有毒植物的捲鬚。

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

這或能解釋為什麼我們體內不會癢，

like in our intestines,

比如在腸道內，

which is safe from these external threats,

在那裡不會有這些外在的威脅，

though imagine how maddening that would be.

不過，可以想像一下， 體內瘙癢會多讓人抓狂。

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

某些人身上，負責這機制的 通路會發生小故障，

can cause excessive itching that can actually harm their health.

造成過度瘙癢，甚至 危害到他們的健康。

One extreme example is a psychological condition called delusory parasitosis

有一個極端的例子，是種 心理疾病，叫做「寄生蟲妄想症」，

where people believe their bodies are infested with mites or fleas

這種病患會相信他們 身上有小蝨或跳蚤寄生，

scurrying over and under their skin,

並在他們的皮膚上、皮膚下亂竄，

making them itch incessantly.

讓他們不斷瘙癢。

Another phenomenon called phantom itching

另一種現象叫做幻覺瘙癢，

can occur in patients who've had amputations.

會發生在截肢病人身上。

Because this injury has so severely damaged the nervous system,

因為截肢傷害嚴重破壞了神經系統，

it confuses the body's normal nerve signaling

混淆了身體的正常神經訊號發送，

and creates sensations in limbs that are no longer there.

製造出已截之肢肢體上的感覺。

Doctors are now finding ways to treat these itching anomalies.

醫生們現在正在想辦法 治療瘙癢異常的現象。

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

針對被截肢者，可以用鏡子 來反映殘缺的肢體，

which the patient scratches.

也就是病人抓癢之處。

That creates an illusion that tricks the brain

這就能製造出幻覺來欺騙大腦，

into thinking the imaginary itch has been satisfied.

讓大腦認為想像的 瘙癢感已經被解決了。

Oddly enough, that actually works.

說來很奇怪，那方式真的可行。

Researchers are also searching for the genes involved in itching

研究者也在尋找與瘙癢相關的基因，

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

並發展治療方式，針對極端案例，

試圖阻擋瘙癢感的通路。

in extreme cases.

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

如果有抓不到的癢 會讓你感覺像活在地獄，

Dante agreed.

但丁也同意。

The Italian poet wrote about a section of hell

這位意大利詩人筆下的地獄，

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

其中有一區的懲罰就是把人 丟入瘙癢深坑，永遠不能出來。