It's the first sense you use when you're born. One out of every fifty of your genes is dedicated to it. It must be important, right? Okay, take a deep breath through your nose. It's your sense of smell, and it's breathtakingly powerful. As an adult, you can distinguish about 10,000 different smells. Here's how your nose does it. Smell starts when you sniff molecules from the air into your nostrils. 95% of your nasal cavity is used just to filter that air before it hits your lungs. But at the very back of your nose is a region called the olfactory epithelium, a little patch of skin that's key to everything you smell. The olfactory epithelium has a layer of olfactory receptor cells, special neurons that sense smells, like the taste buds of your nose. When odor molecules hit the back of your nose, they get stuck in a layer of mucus covering the olfactory epithelium. As they dissolve, they bind to the olfactory receptor cells, which fire and send signals through the olfactory tract up to your brain. As a side note, you can tell a lot about how good an animal's sense of smell is by the size of its olfactory epithelium. A dog's olfactory epithelium is 20 times bigger than your puny human one. But there's still a lot we don't know about this little patch of cells, too. For example, our olfactory epithelium is pigmented, and scientists don't really know why. But how do you actually tell the difference between smells? It turns out that your brain has 40 million different olfactory receptor neurons, so odor A might trigger neurons 3, 427, and 988, and odor B might trigger neurons 8, 76, and 2,496,678. All of these different combinations let you detect a staggeringly broad array of smells. Olfactory neurons are always fresh and ready for action. They're the only neuron in the body that gets replaced regularly, every four to eight weeks. Once they are triggered, the signal travels through a bundle called the olfactory tract to destinations all over your brain, making stops in the amygdala, the thalamus, and the neocortex. This is different from how sight and sound are processed. Each of those signals goes first to a relay center in the middle of the cerebral hemisphere and then out to other regions of the brain. But smell, because it evolved before most of your other senses, takes a direct route to these different regions of the brain, where it can trigger your fight-or-flight response, help you recall memories, or make your mouth water. But even though we've all got the same physiological set-up, two nostrils and millions of olfactory neurons, not everybody smells the same things. One of the most famous examples of this is the ability to smell so-called "asparagus pee." For about a quarter of the population, urinating after eating asparagus means smelling a distinct odor. The other 75% of us don't notice. And this isn't the only case of smells differing from nose to nose. For some people, the chemical androstenone smells like vanilla; to others, it smells like sweaty urine, which is unfortunate because androstenone is commonly found in tasty things like pork. So with the sweaty urine smellers in mind, pork producers will castrate male pigs to stop them from making androstenone. The inability to smell a scent is called anosmia, and there are about 100 known examples. People with allicin anosmia can't smell garlic. Those with eugenol anosmia can't smell cloves. And some people can't smell anything at all. This kind of full anosmia could have several causes. Some people are born without a sense of smell. Others lose it after an accident or during an illness. If the olfactory epithelium gets swollen or infected, it can hamper your sense of smell, something you might have experienced when you were sick. Not being able to smell anything can mess with your other senses, too. Many people who can't smell at all also can't really taste the same way the rest of us do. It turns out that how something tastes is closely related to how it smells. As you chew your food, air is pushed up your nasal passage, carrying with it the smell of your food. Those scents hit your olfactory epithelium and tell your brain a lot about what you're eating. Without the ability to smell, you lose the ability to taste anything more complicated than the five tastes your taste buds can detect: sweet, salty, bitter, sour, and savory. So, the next time you smell exhaust fumes, salty sea air, or roast chicken, you'll know exactly how you've done it and, perhaps, be a little more thankful that you can.
嗅覺是你出生時頭一個使用的感官 你每五十個基因裡 就有一個基因致力為嗅覺工作 嗅覺一定很重要,對吧? 好,來個深呼吸 用你的鼻子吸氣 這就是你的嗅覺 而且它強大地令人屏息 成人可辨認出約一萬種不同的氣味 成人可辨認出約一萬種不同的氣味 你的鼻子是這麼作用的 嗅覺從你聞到從空氣中 進入鼻孔的氣味分子開始 在那分子到達肺部之前 95% 的鼻腔 僅用來過濾空氣 但在你鼻子的最後方 有個叫「嗅覺上皮」的區域 那片小小的皮膚 是你聞到氣味的關鍵 嗅覺上皮有一層 嗅覺受體細胞 可以感受氣味的特殊神經元 就像似你鼻子的味蕾 當氣味分子達到你鼻子的後方 附著在那包覆在 嗅覺上皮的黏液層上 當氣味分子溶解時 會與嗅覺受體細胞結合 燃燒並透過嗅覺神經束 傳送訊號 至你的大腦 附帶一提 你可以很容易 依動物嗅覺上皮的大小 來判斷牠們的嗅覺有多靈敏 狗的嗅覺上皮 比人類的大上二十倍 比人類的大上二十倍 但我們對於這一小塊的細胞群 仍有許多的未知 例如,我們的嗅覺上皮是帶有色素的 而科學家們並不十分清楚為什麼 但實際上,你如何分辨不同的氣味呢? 但實際上,你如何分辨不同的氣味呢? 其實,你腦中有 四千萬個不同的嗅覺受體神經元 氣味 A 可以觸發神經元 3、427 和 988 而氣味 B 可以觸發神經元 8、76 和 2,496,678 所有各式不同的組合 讓你聞出數量驚人的各種氣味 加上你的神經元總是常保清新 且隨時準備行動 它們是我們身上唯一 定時更新的神經元 每四到八周更新一次 一旦這些神經元被觸發 氣味訊號就會經由 一個叫做「嗅覺神經束」 傳達到分佈在你整個大腦 停留在扁桃腺、 丘腦、 和腦皮質 這有別於 視覺與聽覺訊號的傳遞 每個視覺與聽覺訊號 會先停留在一個 位於大腦半球中央的中繼中心 然後分送到大腦的其他區域 但嗅覺呢,因為它的演化 早於你大部分的其他感官 嗅覺是直接進入 腦中不同的區域 這些區域是可以觸發戰鬥或逃跑反應、 幫助你喚起記憶、 或讓你分泌唾液 但即便我們有一樣的生理構造── 但即便我們有一樣的生理構造── 兩個鼻孔和數百萬的嗅覺神經元 卻不是每個人聞到的氣味都一樣 其中最著名的一個例子是 嗅出所謂的「蘆筍尿」的能力 大約 1/4 的人們 吃了蘆筍後排尿時 會聞到尿液裡的特殊異味 但其他 3/4 的人卻聞不出來 而這還不是 每個鼻子聞到的氣味不同的唯一例子 對某些人而言 化學物質「雄甾酮」聞起來像香草 別人聞起來卻像汗尿味 這很可惜 因為雄甾酮通常存在像豬肉這樣好吃的東西裡 因為雄甾酮通常存在像豬肉這樣好吃的東西裡 因顧慮到這些會聞到尿汗味的人 豬肉供應商將公豬去勢 來防止公豬製造雄甾酮 無法聞到某種氣味 稱為嗅覺喪失症 而且已知的例子大概有一百種 有大蒜素嗅覺喪失症的人聞不出大蒜味 有丁香酸嗅覺喪失症的人聞不出丁香味 而有些人則完全聞不出任何氣味 而有些人則完全聞不出任何氣味 這種完全嗅覺喪失症 可能有幾個肇因 有些人天生就沒有嗅覺 有些人是意外傷害後 或是患病時喪失嗅覺 如果嗅覺上皮腫脹或感染 會對你的嗅覺造成阻礙 當你生病時你可能有這樣的經驗 當你生病時你可能有這樣的經驗 然而聞不到任何氣味 也會影響你其他的感官 許多完全聞不到氣味的人 嚐到的味道也和我們不盡相同 嚐到的味道也和我們不盡相同 事實證明,東西嚐起來的味道 和聞起來的氣味是息息相關的 當你咀嚼食物時 夾帶食物氣味的空氣 被推上你的鼻腔 那氣味到達你的嗅覺上皮 告訴你的大腦許多 關於你正在進食的東西的訊息 沒了嗅覺的能力 你喪失了品嚐任何 比五味更複雜的味道 比五味更複雜的味道 你的味蕾可以分辨 甜、 鹹、 酸、 苦、 香 所以,下次你聞到排出的煙、 海的鹹味、 或烤雞的香味 你就知道你到底怎麼辦到的