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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