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

突然，後腦感到一陣強烈的癢

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

我們都曾因為當下不方便處理的癢而感到煩躁不已

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

但首先，你有仔細想過為何我們會覺得癢嗎？

The average person experiences dozens of individual itches each day.

每天人們平均會感受到數十次癢的感覺

They can be triggered by all sorts of things,

許多事物都能觸發癢的感覺

including allergic reactions,

包括過敏反應

dryness,

乾燥

and even some diseases.

甚至是某些疾病

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

另外，還有一種神祕的情況是它毫無原因的突然出現

or just from talking about itching.

或僅僅是談論癢也能觸發癢的感覺

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

你現在正在抓你的頭，沒錯吧？

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

無論如何，我們先從最常見的觸發原因之一開始探討：昆蟲咬傷

When a mosquito bites you,

當蚊子咬你時

it releases a compound into your body called an anticoagulant

牠的口器會釋放名為「抗凝血劑」的混和物到你的身體中

that prevents your blood from clotting.

避免你的血液凝結

That compound, which we're mildly allergic to,

由於我們的身體對這個混和物輕微過敏

triggers the release of histamine,

因此它會觸發組織胺的釋放

a chemical that makes our capillaries swell.

組織胺是一種會讓我們的微血管膨脹的化學物質

This enables increased blood flow,

微血管膨脹使得血流增加

which helpfully accelerates the body's immune response

會加速當身體察覺到威脅時

to this perceived threat.

所做出的免疫反應

That explains the swelling,

這解釋了蚊子咬的地方會腫脹的原因

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

而花粉會讓你的眼周腫脹也是相同的原因

Histamine also activates the nerves involved in itching,

組織胺也會刺激癢覺相關的神經

which is why bug bites make you scratch.

這也是為什麼蟲咬會讓你想抓癢

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

但是目前癢覺本身還沒被完全理解

In fact, much of what we do know

事實上，我們所知很多有關癢覺的知識

comes from studying the mechanics of itching in mice.

都來自於老鼠體內癢覺機制的研究

Researchers have discovered that itch signals in their skin

研究人員發現老鼠皮膚中的癢覺信號

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

是從掌管痛覺相關的子神經轉換而來

These dedicated nerves produce a molecule called natriuretic polypetide B,

這些專門的神經產生了一種叫"Natriuretic polypetide B"的分子結構

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

這個分子會觸發訊號，經由脊髓傳到大腦中

where it creates the feeling of an itch.

因此大腦就會產生癢的感覺

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

當我們搔抓時，指甲在皮膚上的動作

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

會產生可以壓制癢覺的低程度痛覺訊號

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

它就像是干擾一樣，使得癢的感覺得到舒緩

But is there actually an evolutionary purpose to the itch,

然而，癢這件事和演化目的有關嗎？

or is it simply there to annoy us?

還是它僅僅是一個讓我們煩躁的現象呢？

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

目前主流的理論是我們的皮膚已經進化到可敏銳地感知觸覺

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

使得我們能夠處理來自外界的危險

Think about it.

試著想想看

Our automatic scratching response would dislodge anything harmful

我們對癢的自動搔抓反應可以驅逐任何可能

that's potentially lurking on our skin,

對我們的皮膚有潛在傷害的事物

like a harmful sting,

像是傷人的刺

a biting insect,

正在咬我們的昆蟲

or the tendrils of a poisonous plant.

或是有毒植物的卷鬚

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

這或許解釋了為何我們不覺得身體內部會癢

like in our intestines,

像是我們的腸子

which is safe from these external threats,

它們不會受到這些外在威脅

though imagine how maddening that would be.

想想看，如果腸子會受到這些外在威脅，那會是多讓人惱怒的事情

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

對某些人來說，負責傳送感覺的神經路徑上的干擾

can cause excessive itching that can actually harm their health.

可能會導致過度搔癢進而影響健康

One extreme example is a psychological condition called delusory parasitosis

其中一個極端的例子就是名為「寄生蟲妄想症」的心理現象

where people believe their bodies are infested with mites or fleas

它會讓人覺得他們的身體佈滿大量的蟎蟲和跳蚤

scurrying over and under their skin,

並且在皮膚底下亂竄

making them itch incessantly.

讓他們不斷的發癢

Another phenomenon called phantom itching

另外一種現象則稱為「幻肢搔癢」

can occur in patients who've had amputations.

這個現象常發生在曾經做過截肢手術的病人身上

Because this injury has so severely damaged the nervous system,

由於他們所受的傷害對該神經系統造成嚴重損壞

it confuses the body's normal nerve signaling

使得身體中正常的神經訊號產生混淆

and creates sensations in limbs that are no longer there.

並讓早已不存在的肢幹產生感覺

Doctors are now finding ways to treat these itching anomalies.

醫生們也在尋找這種異常癢覺的治療方法

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

對於已截肢的人，會利用鏡子將其仍存在的肢幹反射到已被截肢的那側

which the patient scratches.

並且搔抓那仍存在的肢幹

That creates an illusion that tricks the brain

那樣做產生了欺騙大腦的幻象

into thinking the imaginary itch has been satisfied.

讓大腦認為幻肢的癢覺已經滿足了

Oddly enough, that actually works.

說也奇怪，這個方法確實有用

Researchers are also searching for the genes involved in itching

研究員們也正在研究和癢覺有關的基因

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

並發展在某些極端狀況下阻止癢覺

in extreme cases.

的治療方法

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

如果你認為不能抓的癢是地獄的話

Dante agreed.

但丁是同意這個觀點的

The Italian poet wrote about a section of hell

這位義大利詩人描寫的其中一個地獄類型

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

就是人們被留在一個深坑內受著永恆的搔癢作為處罰