How do we smell? - Rose Eveleth

嗅覺是怎麼一回事？ - Rose Eveleth

譯者: Jessie Lee 審譯者: Bernice Huang

嗅覺是你出生時頭一個使用的感官

It's the first sense you use when you're born.

One out of every fifty of your genes is dedicated to it.

你每五十個基因裡

就有一個基因致力為嗅覺工作

It must be important, right?

嗅覺一定很重要，對吧？

Okay, take a deep breath through your nose.

好，來個深呼吸

用你的鼻子吸氣

It's your sense of smell,

這就是你的嗅覺

and it's breathtakingly powerful.

而且它強大地令人屏息

As an adult, you can distinguish about 10,000 different smells.

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

Here's how your nose does it.

Smell starts when you sniff molecules from the air into your nostrils.

嗅覺從你聞到從空氣中

進入鼻孔的氣味分子開始

95% of your nasal cavity

在那分子到達肺部之前

is used just to filter that air before it hits your lungs.

95% 的鼻腔

僅用來過濾空氣

But at the very back of your nose

但在你鼻子的最後方

is a region called the olfactory epithelium,

有個叫「嗅覺上皮」的區域

a little patch of skin that's key to everything you smell.

那片小小的皮膚

是你聞到氣味的關鍵

The olfactory epithelium has a layer of olfactory receptor cells,

嗅覺上皮有一層

嗅覺受體細胞

special neurons that sense smells,

可以感受氣味的特殊神經元

like the taste buds of your nose.

就像似你鼻子的味蕾

When odor molecules hit the back of your nose,

當氣味分子達到你鼻子的後方

they get stuck in a layer of mucus covering the olfactory epithelium.

附著在那包覆在

嗅覺上皮的黏液層上

As they dissolve, they bind to the olfactory receptor cells,

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

which fire and send signals through the olfactory tract

傳送訊號

up to your brain.

至你的大腦

As a side note, you can tell a lot

附帶一提

你可以很容易

about how good an animal's sense of smell is

依動物嗅覺上皮的大小

by the size of its olfactory epithelium.

來判斷牠們的嗅覺有多靈敏

A dog's olfactory epithelium

狗的嗅覺上皮

比人類的大上二十倍

is 20 times bigger than your puny human one.

比人類的大上二十倍

But there's still a lot we don't know about this little patch of cells, too.

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

For example, our olfactory epithelium is pigmented,

and scientists don't really know why.

而科學家們並不十分清楚為什麼

But how do you actually tell the difference between smells?

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？

It turns out that your brain has

其實，你腦中有

四千萬個不同的嗅覺受體神經元

40 million different olfactory receptor neurons,

so odor A might trigger

氣味 A 可以觸發神經元 3、427 和 988

neurons 3, 427, and 988,

and odor B might trigger

而氣味 B 可以觸發神經元 8、76 和 2,496,678

neurons 8, 76, and 2,496,678.

All of these different combinations

所有各式不同的組合

讓你聞出數量驚人的各種氣味

let you detect a staggeringly broad array of smells.

Olfactory neurons are always fresh and ready for action.

加上你的神經元總是常保清新

且隨時準備行動

They're the only neuron in the body that gets replaced regularly,

它們是我們身上唯一

定時更新的神經元

every four to eight weeks.

每四到八周更新一次

Once they are triggered, the signal travels through a bundle

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」

called the olfactory tract

to destinations all over your brain, making stops in the amygdala,

傳達到分佈在你整個大腦

停留在扁桃腺、

the thalamus, and the neocortex.

丘腦、

和腦皮質

This is different from how sight and sound are processed.

這有別於

視覺與聽覺訊號的傳遞

每個視覺與聽覺訊號

Each of those signals goes first to a relay center

會先停留在一個

位於大腦半球中央的中繼中心

in the middle of the cerebral hemisphere

然後分送到大腦的其他區域

and then out to other regions of the brain.

But smell, because it evolved before most of your other senses,

但嗅覺呢，因為它的演化

早於你大部分的其他感官

takes a direct route to these different regions of the brain,

嗅覺是直接進入

腦中不同的區域

where it can trigger your fight-or-flight response,

這些區域是可以觸發戰鬥或逃跑反應、

幫助你喚起記憶、

help you recall memories, or make your mouth water.

或讓你分泌唾液

但即便我們有一樣的生理構造──

But even though we've all got the same physiological set-up,

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

two nostrils and millions of olfactory neurons,

卻不是每個人聞到的氣味都一樣

not everybody smells the same things.

One of the most famous examples of this

其中最著名的一個例子是

嗅出所謂的「蘆筍尿」的能力

is the ability to smell so-called "asparagus pee."

For about a quarter of the population,

大約 1/4 的人們

urinating after eating asparagus

吃了蘆筍後排尿時

means smelling a distinct odor.

會聞到尿液裡的特殊異味

The other 75% of us don't notice.

但其他 3/4 的人卻聞不出來

And this isn't the only case of smells differing from nose to nose.

而這還不是

每個鼻子聞到的氣味不同的唯一例子

For some people, the chemical androstenone smells like vanilla;

對某些人而言

化學物質「雄甾酮」聞起來像香草

to others, it smells like sweaty urine,

別人聞起來卻像汗尿味

which is unfortunate

這很可惜

because androstenone is commonly found in tasty things like pork.

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

So with the sweaty urine smellers in mind,

因顧慮到這些會聞到尿汗味的人

pork producers will castrate male pigs

豬肉供應商將公豬去勢

to stop them from making androstenone.

來防止公豬製造雄甾酮

The inability to smell a scent is called anosmia,

無法聞到某種氣味

稱為嗅覺喪失症

and there are about 100 known examples.

而且已知的例子大概有一百種

People with allicin anosmia can't smell garlic.

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

Those with eugenol anosmia can't smell cloves.

And some people can't smell anything

而有些人則完全聞不出任何氣味

at all.

而有些人則完全聞不出任何氣味

This kind of full anosmia could have several causes.

這種完全嗅覺喪失症

可能有幾個肇因

Some people are born without a sense of smell.

有些人天生就沒有嗅覺

Others lose it after an accident or during an illness.

有些人是意外傷害後

或是患病時喪失嗅覺

If the olfactory epithelium gets swollen or infected,

如果嗅覺上皮腫脹或感染

it can hamper your sense of smell,

會對你的嗅覺造成阻礙

something you might have experienced when you were sick.

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

Not being able to smell anything can mess with your other senses, too.

然而聞不到任何氣味

也會影響你其他的感官

Many people who can't smell at all

許多完全聞不到氣味的人

also can't really taste the same way the rest of us do.

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

It turns out that how something tastes is closely related to how it smells.

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

As you chew your food, air is pushed up your nasal passage,

當你咀嚼食物時

夾帶食物氣味的空氣

carrying with it the smell of your food.

被推上你的鼻腔

Those scents hit your olfactory epithelium

那氣味到達你的嗅覺上皮

and tell your brain a lot about what you're eating.

告訴你的大腦許多

關於你正在進食的東西的訊息

Without the ability to smell, you lose the ability to taste

沒了嗅覺的能力

你喪失了品嚐任何

anything more complicated than the five tastes

比五味更複雜的味道

比五味更複雜的味道

your taste buds can detect:

你的味蕾可以分辨

sweet, salty,

甜、

鹹、

bitter, sour, and savory.

酸、

苦、

香

So, the next time you smell exhaust fumes,

所以，下次你聞到排出的煙、

海的鹹味、

salty sea air, or roast chicken,

或烤雞的香味

you'll know exactly how you've done it

你就知道你到底怎麼辦到的

and, perhaps, be a little more thankful that you can.

也或許，為你可以聞到那些氣味有多一點點的感激