You're standing at the ready inside the goal when suddenly, you feel an intense itch on the back of your head.
你站在球门前，准备就绪，突然，后脑勺一阵奇痒难耐。
We've all experienced the annoyance of an inconvenient itch, but have you ever pondered why we itch in the first place?
人人都有因抓不到痒而无比烦躁的时候，但你有没有想过为什么会痒？
The average person experiences dozens of individual itches each day.
普通人一天会感受到数十次瘙痒。
They can be triggered by all sorts of things, including allergic reactions, dryness, and even some diseases.
原因更是五花八门，包括过敏反应、干燥甚至某些疾病。
And then there are the mysterious ones that pop up for no reason at all, or just from talking about itching.
更有一些原因不明的发痒，或只是因提到“痒”而莫名感到痒。
You're scratching your head right now, aren't you?
你现在是不是正在挠头抓痒？
Anyhow, let's take one of the most common sources: bug bites.
不管怎样，我们先聊聊最常见的发痒原因：被虫咬。
When a mosquito bites you,
蚊子在叮人的时候，
it releases a compound into your body called an anticoagulant that prevents your blood from clotting.
会向人体释放一种被称为“抗凝血剂”的化合物，它防止人体内血液凝结。
That compound, which we're mildly allergic to,
人体会对这种化合物轻微过敏，
triggers the release of histamine, a chemical that makes our capillaries swell.
进而释放出组胺，而组胺会导致毛细血管肿胀。
This enables increased blood flow, which helpfully accelerates the body's immune response to this perceived threat.
这促使血液流量增加，有助于加快人体对该已知威胁的免疫反应。
That explains the swelling, and it's the same reason pollen can make your eyes puff up.
这解释了皮肤红肿的原因，花粉过敏导致眼睛肿胀也是同样的原理。
Histamine also activates the nerves involved in itching, which is why bug bites make you scratch.
组胺也会触发与痒相关的神经，这就是我们会因蚊虫叮咬而挠痒的原因。
But the itchy sensation itself isn't yet fully understood.
尽管如此，瘙痒感本身却尚未得到充分的认识。
In fact, much of what we do know comes from studying the mechanics of itching in mice.
事实上，我们所知的大部分来源于对老鼠的瘙痒机制进行的研究。
Researchers have discovered that itch signals in their skin
研究人员发现，老鼠的皮肤组织里的痒信号
are transmitted via a subclass of the nerves that are associated with pain.
是通过属于疼痛类的某些神经传输的。
These dedicated nerves produce a molecule called natriuretic polypetide B,
这些专属神经会产生一种被称为“利钠多肽B”的分子，
which triggers a signal that's carried up the spinal cord to the brain, where it creates the feeling of an itch.
它会发出一个信号，由脊髓传送至大脑，造成瘙痒的感觉。
When we scratch, the action of our fingernails on the skin causes a low level pain signal that overrides the itching sensation.
当我们抓痒时，指甲触碰肌肤的动作会触发一种盖过瘙痒感的低水平疼痛信号。

It's almost like a distraction, which creates the sensation of relief.
从而分散注意力，舒缓瘙痒感。
But is there actually an evolutionary purpose to the itch, or is it simply there to annoy us?
但瘙痒感是否具有革命性作用，还是纯粹为了惹人烦躁？
The leading theory is that our skin has evolved to be acutely aware of touch
主流理论认为，人类的皮肤通过不断进化，最终具备敏锐地感知触碰的能力，
so that we're equipped to deal with risks from the outside world. Think about it.
这样更有利于人们就能抵御外界风险。想想看。
Our automatic scratching response would dislodge anything harmful that's potentially lurking on our skin,
人类的自主抓挠反应可以驱除潜伏在皮肤上的有害物质，
like a harmful sting, a biting insect, or the tendrils of a poisonous plant.
例如毒虫刺伤、昆虫叮咬或者有毒植物的藤蔓。
This might explain why we don't feel itching inside our bodies, like in our intestines,
这就可以解释为什么我们不会觉得身体内部发痒，例如肠子里，
which is safe from these external threats, though imagine how maddening that would be.
因为内脏不会遇到外部威胁，尽管如此，设想下内脏发痒还是挺让人抓狂的。
In some people, glitches in the pathways responsible for all of this can cause excessive itching that can actually harm their health.
在某些人身上，过度瘙痒甚至会损害健康，这往往是由于神经通路的故障。
One extreme example is a psychological condition called delusory parasitosis
一个极端的例子就是被称为“寄生虫妄想症”的心理疾病，
where people believe their bodies are infested with mites or fleas scurrying over and under their skin, making them itch incessantly.
患者固执地认为自己感染了螨虫或跳蚤，虫子在皮上和皮下乱窜，让他们瘙痒不停。
Another phenomenon called phantom itching can occur in patients who've had amputations.
另一种情况是幻肢瘙痒，发生在经历过截肢的病人身上。
Because this injury has so severely damaged the nervous system,
这种损伤会严重损害神经系统，
it confuses the body's normal nerve signaling and creates sensations in limbs that are no longer there.
迷惑身体的正常神经信号，并造成在不再存在的肢体上的感觉。
Doctors are now finding ways to treat these itching anomalies.
如今，医生们已经找到了治疗这种瘙痒异常的方法。
In amputees, mirrors are used to reflect the remaining limb, which the patient scratches.
在截肢者身上，用镜子反射尚存的肢体，代替已截肢的肢体让患者抓痒。
That creates an illusion that tricks the brain into thinking the imaginary itch has been satisfied.
这个办法制造了一个幻象来欺骗大脑，让它相信幻想的瘙痒已经得到满足了。
Oddly enough, that actually works.
奇怪的是，这真的行得通。
Researchers are also searching for the genes involved in itching
研究人员也在寻找跟痒相关的基因，
and developing treatments to try and block the pathway of an itch in extreme cases.
并且正在开发一种在极端病例里能阻断痒的神经通路的治疗方案。
If having an unscratchable itch feels like your own personal hell, Dante agreed.
如果痒而不能挠会让你觉得犹如身在地狱，但丁也有同感。
The Italian poet wrote about a section of hell where people were punished by being left in pits to itch for all eternity.
这位意大利诗人描写的地狱中，就有人们被罚身陷坑中，受无尽瘙痒的折磨。