How do we smell? - Rose Eveleth

嗅觉是怎么回事? - Rose Eveleth

翻译人员: Rong Han 校对人员: Bighead Ge

它是你出生时第一个使用的感觉。

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

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

你每五十个基因之中

就有一个用于它。

It must be important, right?

它肯定很重要，对不对？

Okay, take a deep breath through your nose.

好吧，做个深呼吸

从你的鼻子吸气。

It's your sense of smell,

它就是你的嗅觉，

and it's breathtakingly powerful.

它的强大简直要让你倒吸一口凉气。

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

一个成人可以区分出

大约一万种不同的气味。

你的鼻子是这样做到的：

Here's how your nose does it.

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

开始时你把空气中的分子

从鼻孔吸入。

95% of your nasal cavity

你的鼻腔中95%的部分

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

用来过滤刚吸入的空气

然后这些空气才进入你的肺里。

But at the very back of your nose

但是在你的鼻子的最后面

is a region called the olfactory epithelium,

有一个区域叫做嗅上皮，

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

是一小块皮肤

它是你能闻到东西的关键。

The olfactory epithelium has a layer of olfactory receptor cells,

嗅上皮是一层

嗅觉受体细胞组成的，

special neurons that sense smells,

它们是一群能感觉到气味的特殊神经元

like the taste buds of your nose.

就好像你鼻子里的味蕾。

When odor molecules hit the back of your nose,

当气味分子撞到你的鼻子后部，

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

他们就会被困在一层粘液里，

这层粘液就盖在嗅上皮表面。

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

气味分子被（粘液）溶解之后，

就结合到嗅觉受体细胞上，

这些细胞再产生和发送出信号

which fire and send signals through the olfactory tract

通过嗅束

up to your brain.

传到你的大脑。

As a side note, you can tell a lot

一个题外话，

你可以

about how good an animal's sense of smell is

根据一种动物嗅上皮的大小

by the size of its olfactory epithelium.

来判断它的嗅觉有多好。

A dog's olfactory epithelium

狗的嗅上皮

比人类的小小嗅上皮

is 20 times bigger than your puny human one.

要大20倍。

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

不过关于这一小群细胞

我们还有很多不知道的呢。

比如说，我们的嗅上皮是带色素的，

For example, our olfactory epithelium is pigmented,

and scientists don't really know why.

科学家们还不知道这是为什么。

But how do you actually tell the difference between smells?

但你究竟是如何区分

不同的气味的呢？

It turns out that your brain has

事实证明你的大脑里有

4千万个不同的嗅觉受体神经元，

40 million different olfactory receptor neurons,

so odor A might trigger

所以 气味A可能会触发神经元 3、 427、 和 988，

neurons 3, 427, and 988,

and odor B might trigger

而气味 B 可能触发神经元 8、 76 和 2,496,678。

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

All of these different combinations

所有这些不同的组合

可以让你闻出多到惊人的各种气味。

let you detect a staggeringly broad array of smells.

Olfactory neurons are always fresh and ready for action.

而且，你的嗅觉神经元总是精神抖擞

并随时准备行动。

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

他们是身体里唯一

被定期更新的神经元，

every four to eight weeks.

每四到八个星期更新一次。

Once they are triggered, the signal travels through a bundle

一旦这些神经元被触发，

信号通过一束

称为嗅束的线路传播

called the olfactory tract

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

直到遍布你的大脑里的它们目的地。

信号会在扁桃体，

the thalamus, and the neocortex.

丘脑

和大脑皮层都有停留。

This is different from how sight and sound are processed.

这种传播方式

和视觉及声音信号的处理方式不同。

这些信号都先要

Each of those signals goes first to a relay center

到达一个中继中心，

位于大脑半球的中央，

in the middle of the cerebral hemisphere

然后再输出到大脑的其他区域。

and then out to other regions of the brain.

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

但是嗅觉，它比

其他感官更早进化出来，

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

它是通过一条直接的途径

进入大脑的不同区域。

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

它在这些区域可以触发你的战逃反应，

帮助你回忆，

help you recall memories, or make your mouth water.

或者让你流口水。

但即使我们都有

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

相同的生理结构，

两个鼻孔和数以百万计的嗅觉神经元，

two nostrils and millions of olfactory neurons,

不是每个人都能闻到同样的气味。

not everybody smells the same things.

One of the most famous examples of this

这方面最著名的例子之一

就是能够闻到所谓的"芦笋尿"。

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

For about a quarter of the population,

人群中大约四分之一的人，

urinating after eating asparagus

在吃了芦笋后排尿时，

means smelling a distinct odor.

会嗅到一种特定的气味。

The other 75% of us don't notice.

而另外75%的人就闻不到。

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

这还不是唯一一种

不同的鼻子闻到不同的气味的例子。

For some people, the chemical androstenone smells like vanilla;

对某些人来说，

化合物雄甾酮闻起来像香草 ；

to others, it smells like sweaty urine,

而对其他人来说，闻起来就像汗尿。

which is unfortunate

这些人太不幸了，

because androstenone is commonly found in tasty things like pork.

因为雄甾酮常见于

猪肉这一类好吃的东西里。

So with the sweaty urine smellers in mind,

所以考虑到这些闻到汗尿味的人，

pork producers will castrate male pigs

猪肉生产商们会将公猪阉割，

to stop them from making androstenone.

使它们不能产生雄甾酮。

The inability to smell a scent is called anosmia,

不能闻到某种气味

被称为嗅觉障碍，

and there are about 100 known examples.

大约已有100 个已知的例子。

People with allicin anosmia can't smell garlic.

大蒜素嗅觉障碍的人闻不到大蒜味。

丁香酚嗅觉障碍的人闻不到丁香味。

Those with eugenol anosmia can't smell cloves.

And some people can't smell anything

有些人什么味

at all.

也闻不到。

This kind of full anosmia could have several causes.

这种完全性的嗅觉障碍

可能有几个成因。

Some people are born without a sense of smell.

有些人天生没有嗅觉，

Others lose it after an accident or during an illness.

有些人在发生意外之后

或患病期间失去嗅觉。

If the olfactory epithelium gets swollen or infected,

如果嗅上皮肿涨或受了感染，

it can hamper your sense of smell,

会影响你的嗅觉。

something you might have experienced when you were sick.

你可能已经在

生病时有过这种经历。

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

而且闻不到气味的时候，

你的其他感官也会受到影响。

Many people who can't smell at all

很多闻不到任何味道的人

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

尝到的味道也和我们

不一样。

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

其实东西吃起来的味道

和它闻起来的味道是密切相关的。

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

你咀嚼食物的时候，

空气被推入你的鼻腔

carrying with it the smell of your food.

夹带着你的食物的味道。

Those scents hit your olfactory epithelium

这些气味接触到你的嗅上皮，

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

告诉你的大脑很多

关于你在吃的东西的信息。

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

如果不能闻，

你也就不能品尝到

anything more complicated than the five tastes

任何比简单的五味

复杂一点的味道了。

your taste buds can detect:

你的味蕾可以尝到这五味：

sweet, salty,

甜，

咸，

bitter, sour, and savory.

酸，

苦，

和薄荷味。

So, the next time you smell exhaust fumes,

所以下次你闻到尾气油烟，

咸咸的海洋空气，

salty sea air, or roast chicken,

或者烤鸡

you'll know exactly how you've done it

你就知道你到底是怎么闻到的了，

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

而且，或许，对于你可以闻到味多一点点感激的心。