痒啊！

Having an itch is an intense and frustrating experience - especially if scratching just makes it worse.

痒是一种令人抓狂且沮丧的体验，特别是挠破了皮肤使得情况更糟的话。

But there are certain disorders that actually result in people itching uncontrollably without relief. In one case study, a woman had such an itchy scalp she scratched through to her brain.

但是存在一些疾病使患者感到无法抑制的瘙痒。在一起病例中，一位妇女头皮发痒难耐，最后她甚至抓伤了大脑。

Itching is an incredibly strange sensation that scientists still don't fully understand.

瘙痒是种令人难以置信的奇怪感觉，科学家还未完全地了解。

The itch sensation is technically known as pruritus, and it's thought to have evolved as a way to protect skin - our largest organ - from parasites and build-up of dead cells.

痒的感觉在技术上被称为瘙痒症，它被认为是一种皮肤——我们身体中最大的器官——的保护机制，防止寄生虫和死细胞积聚在体表。

After all, all our other organs are tucked away safely inside our bodies, where they can be fully protected by the immune system. But our skin is our first line of defence, constantly in contact with the outside world, so it makes sense it evolved some unique ways to stay intact.

毕竟，我们所有的其他器官都安全地隐藏在我们的身体内，在那里它们可以受到免疫系统的完全保护。而皮肤是我们的第一道防线，时刻与外界接触，因此它具有一些独特的自保方式。

It also explains why itching is contagious - back in the palaeolithic days when we all lived in close quarters, if you saw an infested tribesmate scratching away at themselves, it would make sense that you should start scratching too, in order to avoid being bitten or infected by whatever was bothering them.

这也解释了为什么瘙痒具有传染性——回到旧石器时代，当我们聚在洞穴中共同生活时，如果你看到某个部落里的同伴不停地搔痒，那么你也下意识地感到痒就是有意义的反应，因为也许造成同伴瘙痒的蚊虫跳蚤也落到了你的身上。

But it doesn't explain why an itch produces such a unique and maddening sensation.

但以上观点并不能解释为什么痒这种感觉如此独特和令人发狂。

Up until a decade ago scientists actually thought an itch was just another, milder type of pain - using the same receptors in the epidermis that relay chemical and electrical messages up the spine and to the brain to tell it something hurts.

直到十年前，科学家才真正意识到瘙痒只是另一种更轻微的疼痛感——痒和痛使用表皮中的同一套受体将化学和电子信息传递到脊柱和大脑，告诉身体某处感到疼痛。

But we now know that itching actually has its own specific circuitry, involving its own chemicals and cells.

但我们现在知道瘙痒感实际上有其独特的电化学线路，涉及特定的化学物质和细胞。

And while we all have different responses to pain, we share a universal response to itching.

我们对疼痛反应不一，但对瘙痒有着共同的反应。

Scratching.

挠。

It turns out scratching feels so good because it causes a low-level pain signal to shoot up to the brain and override the itch signal to provide us with relief. That's why pinching or slapping the itchy spot can work, too.

事实证明，用挠来搔痒，体验非常好，因为它会导致向大脑发送低水平的疼痛信号，并覆盖痒信号，缓解了痒感。这就是为什么捏捏或拍打发痒的叮包也可以起作用的原因。

Unfortunately, some of those relieving chemicals, including serotonin, can actually make it easier for the itch signal to be re-triggered.That's why it's common for scratching to make you more itchy, kicking off a vicious cycle scientists call the itch-scratch cycle.

不幸的是，挠挠时释放的解痒的化学物质，包括血清素，实际上会使瘙痒信号更容易被重新触发。这就是为什么抓挠让你更痒，开始一个被科学家称之为痒刮伤周期的恶性循环的常见原因。

Damage to the nerves involved in that cycle can result in uncontrollable itching, without any stimulus; when that happens, these are jointly known as pruritus disorders.

周期中造成的神经的损伤可导致无任何外界刺激下的无法控制的瘙痒;如果出现了这种情况时，可以称之为瘙痒症。

There are loads of reasons why these occur, and sometimes researchers don't even know the cause. Some can be caused by viral infections that affect the nervous system, such as postherpetic itch, which can be triggered by shingles.

具体的病例有很多原因，有时研究人员甚至无法确定其原因。有些可能是由影响神经系统的病毒感染引起的，例如带状疱疹引起的疱疹后瘙痒。

There are also conditions such as brachioradial pruritus, which is caused by a restricted nerve in the neck. And aquagenic pruritus, which is itching experienced after contact with water. Some cases have actually been linked to a rare condition where the body has too many red blood cells.

还存在诸如肱桡瘙痒症的情况，其由颈部神经系统不畅通所引发。还有水源性瘙痒，皮肤接触水后发痒。有些病例实际上与身体红细胞过多的罕见病症有关。

All of these disorders can leave people with a frustrating itch, often without an obvious cause and potentially without relief, too.

所有这些都会让人感到沮丧，往往没有明显的原因，也可能无法得到缓解。

In an extreme case study, a woman who'd suffered from shingles had such an itchy scalp that within a year she had "painlessly" scratched straight through her skull and into her brain.

在一极端的研究案例中，一位患有带状疱疹的女性因持续性头皮发痒，一年之内她就“毫无痛苦地”直接抓破头骨划伤了大脑。

Plus, scientists are still learning more all the time about our curious and unique itch response, so in the future we might finally be able to turn off uncontrollable itching for good.

科学家们仍在不断探索各种奇怪和独特的瘙痒反应，所以在未来某天，我们一定可以最终消除那些难以忍耐的瘙痒。