You're standing at the ready inside the goal when suddenly, you feel an intense itch on the back of your head. We've all experienced the annoyance of an inconvenient itch, but have you ever pondered why we itch in the first place? The average person experiences dozens of individual itches each day. They can be triggered by all sorts of things, including allergic reactions, dryness, and even some diseases. And then there are the mysterious ones that pop up for no reason at all, or just from talking about itching. You're scratching your head right now, aren't you? Anyhow, let's take one of the most common sources: bug bites. When a mosquito bites you, it releases a compound into your body called an anticoagulant that prevents your blood from clotting. That compound, which we're mildly allergic to, triggers the release of histamine, a chemical that makes our capillaries swell. This enables increased blood flow, which helpfully accelerates the body's immune response to this perceived threat. That explains the swelling, and it's the same reason pollen can make your eyes puff up. Histamine also activates the nerves involved in itching, which is why bug bites make you scratch. But the itchy sensation itself isn't yet fully understood. In fact, much of what we do know comes from studying the mechanics of itching in mice. Researchers have discovered that itch signals in their skin are transmitted via a subclass of the nerves that are associated with pain. These dedicated nerves produce a molecule called natriuretic polypetide B, which triggers a signal that's carried up the spinal cord to the brain, where it creates the feeling of an itch. When we scratch, the action of our fingernails on the skin causes a low level pain signal that overrides the itching sensation. It's almost like a distraction, which creates the sensation of relief. But is there actually an evolutionary purpose to the itch, or is it simply there to annoy us? The leading theory is that our skin has evolved to be acutely aware of touch so that we're equipped to deal with risks from the outside world. Think about it. Our automatic scratching response would dislodge anything harmful that's potentially lurking on our skin, like a harmful sting, a biting insect, or the tendrils of a poisonous plant. This might explain why we don't feel itching inside our bodies, like in our intestines, which is safe from these external threats, though imagine how maddening that would be. In some people, glitches in the pathways responsible for all of this can cause excessive itching that can actually harm their health. One extreme example is a psychological condition called delusory parasitosis where people believe their bodies are infested with mites or fleas scurrying over and under their skin, making them itch incessantly. Another phenomenon called phantom itching can occur in patients who've had amputations. Because this injury has so severely damaged the nervous system, it confuses the body's normal nerve signaling and creates sensations in limbs that are no longer there. Doctors are now finding ways to treat these itching anomalies. In amputees, mirrors are used to reflect the remaining limb, which the patient scratches. That creates an illusion that tricks the brain into thinking the imaginary itch has been satisfied. Oddly enough, that actually works. Researchers are also searching for the genes involved in itching and developing treatments to try and block the pathway of an itch in extreme cases. If having an unscratchable itch feels like your own personal hell, Dante agreed. The Italian poet wrote about a section of hell where people were punished by being left in pits to itch for all eternity.
你正擺好姿勢守著球門, 突然,你感到後腦勺很癢。 我們都曾歷過惱人的難搔之癢, 但你是否曾想過為什麼我們會癢呢? 一般人每天會發癢數十次。 各種東西都有可能引發癢感, 包括過敏性反應、 乾燥, 甚至某些疾病。 還有些時候,發癢是沒有原因的, 或只因為說到癢,就癢起來了。 你現在正在搔你的頭,對嗎? 總之,我們來談其中一種 最常見的原因:蚊蟲叮咬。 蚊子咬你時 會將一種叫做「抗凝血劑」的 複合物注入你體內, 它會讓你的血液無法凝固。 我們對那種複合物有輕微的過敏, 它會造成身體釋放「組織胺」, 一種化學物,會讓毛細管腫脹。 血液流量就會增加, 這能協助身體更快對於 感知到的威脅產生免疫反應。 這是腫脹的原因, 這也就是花粉能導致你的 眼睛腫起來的原因。 組織胺還會觸發與癢有關的神經, 這就是為什麼蚊蟲叮咬 會讓你想抓癢。 但瘙癢感還沒有被完全了解。 事實上,我們對它的了解, 大多來自探討老鼠瘙癢機制的研究。 研究者發現,牠們皮膚中的瘙癢信號 是透過與疼痛相關的神經來傳輸的。 這些專用神經會產生 一種叫做「多肽B」的分子, 它會觸發一個信號, 透過脊髓傳輸到大腦, 大腦就會產生瘙癢的感覺。 當我們搔癢時,指甲在皮膚上的動作 會造成一個低度的疼痛訊號, 蓋過瘙癢的感覺。 這就像是分散注意力 能造成放鬆的感覺一樣。 但,瘙癢是否真有進化上的目的? 或者它單純只是種惱人的存在? 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, 讓我們能準備好 處理來自外界的風險。 試想看看。 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。 就像有害的螫刺、 正在叮咬的昆蟲, 或有毒植物的捲鬚。 這或能解釋為什麼我們體內不會癢, 比如在腸道內, 在那裡不會有這些外在的威脅, 不過,可以想像一下, 體內瘙癢會多讓人抓狂。 某些人身上,負責這機制的 通路會發生小故障, 造成過度瘙癢,甚至 危害到他們的健康。 有一個極端的例子,是種 心理疾病,叫做「寄生蟲妄想症」, 這種病患會相信他們 身上有小蝨或跳蚤寄生, 並在他們的皮膚上、皮膚下亂竄, 讓他們不斷瘙癢。 另一種現象叫做幻覺瘙癢, 會發生在截肢病人身上。 因為截肢傷害嚴重破壞了神經系統, 混淆了身體的正常神經訊號發送, 製造出已截之肢肢體上的感覺。 醫生們現在正在想辦法 治療瘙癢異常的現象。 針對被截肢者,可以用鏡子 來反映殘缺的肢體, 也就是病人抓癢之處。 這就能製造出幻覺來欺騙大腦, 讓大腦認為想像的 瘙癢感已經被解決了。 說來很奇怪,那方式真的可行。 研究者也在尋找與瘙癢相關的基因, 並發展治療方式,針對極端案例, 試圖阻擋瘙癢感的通路。 如果有抓不到的癢 會讓你感覺像活在地獄, 但丁也同意。 這位意大利詩人筆下的地獄, 其中有一區的懲罰就是把人 丟入瘙癢深坑,永遠不能出來。