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  • Transcriber: Joseph Geni Reviewer: Camille Martínez

    翻訳: Hiromi Nakai 校正: Natsuhiko Mizutani

  • Can you hear me OK?


  • Audience: Yes.


  • Jim Hudspeth: OK. Well, if you can, it's really amazing,

    (ハドスピス)OK  これは驚ろくべきことです

  • because my voice is changing the air pressure where you sit

    なぜなら私の声は皆さんの 座っている場所の空気圧を

  • by just a few billionths of the atmospheric level,

    大気圧のわずか数十億分の1程度だけ 変化させます

  • yet we take it for granted

    でも あたり前のように

  • that your ears can capture that infinitesimal signal


  • and use it to signal to the brain the full range of auditory experiences:

    ありとあらゆる聴覚体験が 脳へ信号として送られています

  • the human voice, music, the natural world.

    人間の声 音楽 自然界の音もあります

  • How does your ear do that?


  • And the answer to that is:


  • through the cells that are the real hero of this presentation --

    この講演で真の主人公となる ある細胞です

  • the ear's sensory receptors,


  • which are called "hair cells."


  • Now, these hair cells are unfortunately named,

    さて これらの有毛細胞は 不運にもそう名付けられました

  • because they have nothing at all to do with the kind of hair

    不運と言ったのは この頭から 失われつつある普通の毛髪とは

  • of which I have less and less.


  • These cells were originally named that by early microscopists,

    最初にこれらの細胞を名付けたのは 初期の顕微鏡学者でした

  • who noticed that emanating from one end of the cell


  • was a little cluster of bristles.


  • With modern electron microscopy, we can see much better

    現代の電子顕微鏡を使えば もっと詳細に観察でき

  • the nature of the special feature that gives the hair cell its name.

    有毛細胞の名前の由来となった特徴が より鮮明に見られます

  • That's the hair bundle.


  • It's this cluster of 20 to several hundred fine cylindrical rods

    このように 20本から数100本の 微細な棒状の円柱が集結して

  • that stand upright at the top end of the cell.


  • And this apparatus is what is responsible for your hearing me right this instant.

    そしてこの器官によって 今まさに 私の声が聞こえているのです

  • Now, I must say that I am somewhat in love with these cells.

    さて 私はこれらの細胞たちを愛していると 言っておかなければなりません

  • I've spent 45 years in their company --


  • (Laughter)


  • and part of the reason is that they're really beautiful.

    その美しさも 理由の一部です

  • There's an aesthetic component to it.


  • Here, for example, are the cells


  • with which an ordinary chicken conducts its hearing.


  • These are the cells that a bat uses for its sonar.


  • We use these large hair cells from a frog for many of our experiments.

    実験で良く使うのは こんなカエルの大きな有毛細胞です

  • Hair cells are found all the way down to the most primitive of fishes,

    有毛細胞は さらに遡って きわめて原始的な魚にもあります

  • and those of reptiles often have this really beautiful,

    は虫類の有毛細胞は このように本当に美しいものも多く

  • almost crystalline, order.


  • But above and beyond its beauty,


  • the hair bundle is an antenna.


  • It's a machine for converting sound vibrations into electrical responses

    機械のように 音の振動を電気的応答に変換し

  • that the brain can then interpret.


  • At the top of each hair bundle, as you can see in this image,

    この写真のように それぞれの感覚毛の先端には

  • there's a fine filament connecting each of the little hairs,

    小さい毛である 不動毛の1本1本を繋ぐ

  • the stereocilia.


  • It's here marked with a little red triangle.


  • And this filament has at its base a couple of ion channels,

    このフィラメントのつけ根には 2-3個のイオンチャンネルという

  • which are proteins that span the membrane.


  • And here's how it works.


  • This rat trap represents an ion channel.

    このネズミ捕りが イオンチャンネルです

  • It has a pore that passes potassium ions and calcium ions.

    カリウムイオンとカルシウムイオンを 通す穴が開いています

  • It has a little molecular gate that can be open, or it can be closed.

    分子に対して小さなゲートがあって 開けたり閉めたりすることができます

  • And its status is set by this elastic band which represents that protein filament.

    タンパク質のフィラメントを表す このゴム紐でゲートの開閉が決まります

  • Now, imagine that this arm represents one stereocilium

    さてこの腕が 不動毛だとしましょう

  • and this arm represents the adjacent, shorter one


  • with the elastic band between them.


  • When sound energy impinges upon the hair bundle,


  • it pushes it in the direction towards its taller edge.


  • The sliding of the stereocilia puts tension in the link

    不動毛どうしがスライドすることで このリンクが引っ張られ

  • until the channels open and ions rush into the cell.

    チャンネルが開くと イオンがセルへ流入します

  • When the hair bundle is pushed in the opposite direction,


  • the channels close.


  • And, most importantly,


  • a back-and-forth motion of the hair bundle,


  • as ensues during the application of acoustic waves,

    音波があたっている間は 引き起こされて

  • alternately opens and closes the channel,


  • and each opening admits millions and millions of ions into the cell.

    開くたびに数百万個のイオンが 細胞に流入します

  • Those ions constitute an electrical current


  • that excites the cell.


  • The excitation is passed to a nerve fiber,


  • and then propagates into the brain.


  • Notice that the intensity of the sound


  • is represented by the magnitude of this response.


  • A louder sound pushes the hair bundle farther,

    大きな音で押されると 感覚毛は大きく動き

  • opens the channel longer,


  • lets more ions in


  • and gives rise to a bigger response.


  • Now, this mode of operation has the advantage of great speed.

    さて この動作方式は とても速い というのが長所です

  • Some of our senses, such as vision,

    視覚など 私たちの感覚の中には

  • use chemical reactions that take time.

    化学反応を利用し 時間のかかるものもあります

  • And as a consequence of that,


  • if I show you a series of pictures at intervals of 20 or 30 per second,

    もし私が皆さんに一連の写真を 1秒に20枚から30枚の割合で見せたら

  • you get the sense of a continuous image.

    連続したイメージという 感覚を得るでしょう

  • Because it doesn't use reactions,

    有毛細胞は 反応を利用しないので

  • the hair cell is fully 1,000 times faster than our other senses.

    他の感覚器官に比べ 優に1000倍もの早さです

  • We can hear sounds at frequencies as great as 20,000 cycles per second,

    私たちは音を毎秒2万回もの 高周波数域まで聞くことができます

  • and some animals have ever faster ears.


  • The ears of bats and whales, for example, can respond to their sonar pulses

    例えばコウモリやクジラの耳は 毎秒15万回という彼らの超音波パルスを

  • at 150,000 cycles a second.


  • But this speed doesn't entirely explain why the ear performs so well.

    しかし耳の能力が高いことは このスピードだけでは説明できません

  • And it turns out that our hearing benefits from an amplifier,

    「アクティブプロセス」と呼ばれる 増幅器が私たちの聴力に

  • something called the "active process."

    大いに役立っていることが わかっています

  • The active process enhances our hearing


  • and makes possible all the remarkable features that I've already mentioned.

    すでに述べたような全ての特徴を 可能にしているものです

  • Let me tell you how it works.


  • First of all, the active process amplifies sound,

    まず アクティブプロセスは 音を増幅します

  • so you can hear, at threshold, sounds that move the hair bundle

    最小の音だと 感覚毛の動きは 10分の3ナノメートルほどですが

  • by a distance of only about three-tenths of a nanometer.


  • That's the diameter of one water molecule.


  • It's really astonishing.


  • The system can also operate


  • over an enormously wide dynamic range.

    ものすごく広い ダイナミックレンジを有します

  • Why do we need this amplification?


  • The amplification, in ancient times, was useful


  • because it was valuable for us to hear the tiger before the tiger could hear us.

    なぜなら虎が私たちに気づく前に 私たちが虎に気づくことが重要だったからです

  • And these days, it's essential as a distant early warning system.

    その時代の 早期遠距離警報システムでした

  • It's valuable to be able to hear fire alarms

    今日では火災警報とか 消防車やパトカーなどの緊急車両の

  • or contemporary dangerous such as speeding fire engines or police cars or the like.

    サイレンなど危険信号に 気づけることが重要です

  • When the amplification fails, our hearing's sensitivity plummets,

    増幅ができなくなると 聴覚が劇的に低下します

  • and an individual may then need an electronic hearing aid

    そうなった人は 生物的な聴覚支援の代わりに

  • to supplant the damaged biological one.


  • This active process also enhances our frequency selectivity.

    このアクティブプロセスは私たちの 周波数に対する選択性も強化します

  • Even an untrained individual can distinguish two tones

    訓練していない人でも 周波数が 0.2%違うだけの

  • that differ by only two-tenths of a percent,


  • which is one-thirtieth of the difference between two piano notes,

    その差は ピアノで隣り合う音の 30分の1の違いです

  • and a trained musician can do even better.

    訓練された音楽家なら もっとよく聞き分けられます

  • This fine discrimination is useful


  • in our ability to distinguish different voices


  • and to understand the nuances of speech.

    言葉のニュアンスを理解するのに 役立ちます

  • And, again, if the active process deteriorates,

    繰り返しになりますが アクティブプロセスが劣化すれば

  • it becomes harder to carry out verbal communication.

    言語によるコミュニケーションが より難しくなります

  • Finally, the active process is valuable in setting the very broad range

    最後に アクティブプロセスは

  • of sound intensities that our ears can tolerate,

    耳が許容する音の大きさを 拡大するのに役立っています

  • from the very faintest sound that you can hear, such as a dropped pen,

    聞くことのできる最も微かな音 たとえば ペンの落ちたときの音から

  • to the loudest sound that you can stand --


  • say, a jackhammer or a jet plane.

    たとえば 削岩機やジェット機まで

  • The amplitude of sounds spans a range of one millionfold,


  • which is more than is encompassed by any other sense


  • or by any man-made device of which I'm aware.

    私の知る限り あらゆる人工のデバイスよりも 大きな値です

  • And again, if this system deteriorates,

    繰り返しになりますが もしこのシステムが損傷すると

  • an affected individual may have a hard time

    その影響によって 最も微かな音を聴きにくくなったり

  • hearing the very faintest sounds

    最も大きい音に 耐えられなくなったりするかもしれません

  • or tolerating the very loudest ones.

    さて 有毛細胞の働く仕組みを理解するために

  • Now, to understand how the hair cell does its thing,

    耳の中で有毛細胞の周りはどうなっているか 見てみましょう

  • one has to situate it within its environment within the ear.

    聴覚器は カタツムリのような らせん型の渦巻管だと 学校で学びます

  • We learn in school that the organ of hearing


  • is the coiled, snail-shaped cochlea.

    それは頭蓋の両側面の骨に 埋め込まれたようになっています

  • It's an organ about the size of a chickpea.


  • It's embedded in the bone on either side of the skull.

    周波数が異なっていて 私たちに別の色として見える光に

  • We also learn that an optical prism


  • can separate white light into its constituent frequencies,


  • which we see as distinct colors.

    渦巻管は 複雑な音を周波数で分解する―

  • In an analogous way,


  • the cochlea acts as sort of an acoustic prism


  • that splits apart complex sounds into their component frequencies.


  • So when a piano is sounded,


  • different notes blend together into a chord.

    音を分けて それぞれ別の場所に届けます

  • The cochlea undoes that process.

    この図では ピアノの中央のCと 両端の音 あわせて3つが

  • It separates them and represents each at a different position.


  • In this picture, you can see where three notes --

    最も低い周波数は渦巻管の最先端まで 伝わっていって取り出され

  • middle C and the two extreme notes on a piano --

    最も高い周波数である 20,000ヘルツの音は

  • are represented in the cochlea.


  • The lowest frequencies go all the way up to the top of the cochlea.

    そして他の周波数は この間のどこかで取り出されます

  • The highest frequencies, down to 20,000 Hz,


  • go all the way to the bottom of the cochlea,

    音階で隣り合う音の高さは 渦巻管の表面において

  • and every other frequency is represented somewhere in between.


  • And, as this diagram shows,

    さて この周波数の分離は

  • successive musical tones are represented a few tens of hair cells apart

    違った音を聞き分ける能力において 重要な鍵となります

  • along the cochlear surface.


  • Now, this separation of frequencies


  • is really key in our ability to identify different sounds,

    それぞれに違った高さの音が 固有の集まりを作っているのです

  • because very musical instrument,


  • every voice,


  • emits a distinct constellation of tones.


  • The cochlea separates those frequencies,


  • and the 16,000 hair cells then report to the brain

    何の音を聴いているのかを 判断します

  • how much of each frequency is present.

    ただ 私が説明したいと思っている全てが これで説明できるわけではありません

  • The brain can then compare all the nerve signals


  • and decide what particular tone is being heard.


  • But this doesn't explain everything that I want to explain.


  • Where's the magic?

    そして 最初にお伝えした 驚くべき特徴が実現されるのでしょうか

  • I told you already about the great things that the hair cell can do.


  • How does it carry out the active process

    私たちはかつて 感覚毛は刺激された時以外は

  • and do all the remarkable features that I mentioned at the outset?

    じっとしている 受動的な存在だと考えていました

  • The answer is instability.

    しかし実は 感覚毛はアクティブな機構です

  • We used to think that the hair bundle was a passive object,

    機械的に活動して聴力を増強するために 感覚毛は内部でずっと

  • it just sat there, except when it was stimulated.


  • But in fact, it's an active machine.

    外部からのインプットが全くない 休止状態のときでも

  • It's constantly using internal energy to do mechanical work

    アクティブな感覚毛は 絶え間なく振動しています

  • and enhance our hearing.

    絶え間なく前後に 動いています

  • So even at rest, in the absence of any input,


  • an active hair bundle is constantly trembling.

    その音を捕まえてその音と同期して 巧妙に動き始めます

  • It's constantly twitching back and forth.

    そうすることで シグナルを何千倍にも増強させます

  • But when even a weak sound is applied to it,

    この同じ不安定性がまた 周波数の選択性を高めます

  • it latches on to that sound and begins to move very neatly

    ある感覚毛は 刺激のないときに

  • in a one-to-one way with it,

    振動しているいつもの周波数で 最も大きく振動するようになっています

  • and by so doing, it amplifies the signal about a thousand times.

    つまり この組織は素晴らしく鋭い聴覚を もらたすだけでなく

  • This same instability also enhances our frequency selectivity,


  • for a given hair cell tends to oscillate best

    ではここで ちょっとしたデモンストレーションを

  • at the frequency at which it normally trembles


  • when it's not being stimulated.


  • So, this apparatus not only gives us our remarkably acute hearing,

    ある特定の周波数だけ 感度を高くしてもらいます

  • but also gives us the very sharp tuning.

    有毛細胞がある一つの周波数に 同調しているのと同じで

  • I want to offer you a short demonstration

    増幅器が私の声の 特定の周波数を強調します

  • of something related to this.

    背景となる音と比べてある高さの音だけが くっきりと浮かび上がる様子がわかりますか

  • I'll ask the people who are running the sound system


  • to turn up its sensitivity at one specific frequency.

    それぞれの有毛細胞はある特定の周波数だけを 増幅して伝える一方で

  • So just as a hair cell is tuned to one frequency,


  • the amplifier will now enhance a particular frequency in my voice.

    そして一連の有毛細胞は 一つのグループとして

  • Notice how specific tones emerge more clearly from the background.

    聞こえた音にどの周波数が存在したか 脳に伝えます

  • This is exactly what hair cells do.

    そして脳は 何のメロディを聴いているのか

  • Each hair cell amplifies and reports one specific frequency

    また何を意図したスピーチなのかを 判断できます

  • and ignores all the others.


  • And the whole set of hair cells, as a group, can then report to the brain


  • exactly what frequencies are present in a given sound,


  • and the brain can determine what melody is being heard


  • or what speech is being intended.


  • Now, an amplifier such as the public address system

    不思議なのは アクティブプロセスが 同じことにならない理由です

  • can also cause problems.


  • If the amplification is turned up too far,

    その答えは 「音を出す」です

  • it goes unstable and begins to howl

    適当な静かな環境下で 健常者の7割は

  • or emit sounds.

    耳から1つ以上の音を 出しています

  • And one wonders why the active process doesn't do the same thing.


  • Why don't our ears beam out sounds?


  • And the answer is that they do.

    健常者の耳からは 高い周波数で

  • In a suitably quiet environment, 70 percent of normal people


  • will have one or more sounds coming out of their ears.

    背景の雑音も 識別できるかもしれません

  • (Laughter)


  • I'll give you an example of this.

    ゴボゴボいう胃の音、心音や 衣服のこすれ音などです

  • You will hear two emissions at high frequencies


  • coming from a normal human ear.


  • You may also be able to discern background noise,


  • like the microphone's hiss,


  • the gurgling of a stomach, the heartbeat, the rustling of clothes.

    それぞれの耳は違っています だから私の左右の耳も異なります

  • (Hums, microphone hiss, dampened taps, clothes rustling)


  • This is typical.

    でも耳は 劣化しなければ

  • Most ears emit just a handful of tones,

    何年間も あるいは何十年間も

  • but some can emit as many as 30.


  • Every ear is unique, so my right ear is different from my left,


  • my ear is different from your ear,

    耳は感度すなわち増幅率を 自分で調整できることがわかりました

  • but unless an ear is damaged,

    だからスポーツイベントや コンサートのような

  • it continues to emit the same spectrum of frequencies


  • over a period of years or even decades.


  • So what's going on?


  • It turns out that the ear can control its own sensitivity,


  • its own amplification.


  • So if you're in a very loud environment, like a sporting event

    ただ音響システムが 大半の増幅作用を受け持っています

  • or a musical concert,


  • you don't need any amplification,


  • and the system is turned down all the way.

    この機能はほぼ最大限の 増幅をする状態です

  • If you are in a room like this auditorium,

    さらに無響室のように 極めて静かな部屋では

  • you might have a little bit of amplification,