The beautiful, mysterious science of how you hear | Jim Hudspeth

Summary

In this enlightening talk, Jim Hudspeth explores the intricacies of human hearing, focusing on the remarkable hair cells within the ear. These cells, named for their bristle-like structures, convert sound vibrations into electrical signals that the brain interprets. Hudspeth delves into the speed and sensitivity of these cells, highlighting the active process that amplifies sound to an astonishing degree, allowing us to hear an extensive range of frequencies. He also touches on the cochlea's role in frequency separation and the potential for future research to regenerate hair cells, offering hope for combating hearing loss.

Takeaways

• 👂 The human ear is capable of detecting minute changes in air pressure, translating them into a wide range of auditory experiences.
• 🌟 The 'hair cells' in the ear are the sensory receptors responsible for hearing and are named for the bristle-like structures on one end of the cell.
• 🔬 Modern electron microscopy has revealed the intricate structure of hair cells, particularly the hair bundle, which is essential for converting sound vibrations into electrical signals.
• 💖 The speaker expresses a deep admiration for hair cells, not only for their function but also for their aesthetic beauty across different species.
• 🔊 The hair bundle acts as an antenna, responding to sound vibrations and triggering ion channels to open and close, generating an electrical current that is sent to the brain.
• 📈 The intensity of sound is represented by the magnitude of the response in the hair cells, with louder sounds causing a greater ion flow and a stronger signal to the brain.
• ⚡ The ear's sensory response is incredibly fast, much faster than other senses, allowing us to perceive high-frequency sounds up to 20,000 cycles per second.
• 🔊 The 'active process' in the ear amplifies sound, enabling us to hear incredibly faint sounds and operate over a wide dynamic range, which is crucial for early warning systems and environmental awareness.
• 🎼 The cochlea acts as an 'acoustic prism,' separating complex sounds into their component frequencies, allowing us to distinguish between different musical instruments and voices.
• 🔍 The active process also enhances our frequency selectivity, enabling us to discern very close frequencies, which is vital for understanding speech and music.
• 👂🔧 The hair cells' active process is self-regulated, adjusting sensitivity and amplification based on the acoustic environment, and can even emit sounds in ultraquiet conditions.

Q & A

How do hair cells in the ear contribute to our ability to hear?

Hair cells, the sensory receptors in the ear, are responsible for converting sound vibrations into electrical responses through the hair bundle, which is a cluster of bristles at the top of the cell. The movement of these bristles in response to sound waves opens and closes ion channels, creating an electrical current that is interpreted by the brain as sound.

What is the aesthetic appeal of hair cells according to the speaker?

The speaker finds hair cells aesthetically pleasing due to their beauty and the intricate order found in various species, such as the almost crystalline arrangement in reptiles.

How does the hair bundle function as an antenna for sound?

The hair bundle acts as an antenna by converting sound vibrations into electrical signals. When sound energy hits the hair bundle, it causes the stereocilia to slide against each other, opening ion channels and allowing ions to flow into the cell, which generates an electrical current.

What is the role of the active process in our hearing?

The active process amplifies sound, allowing us to hear extremely faint sounds and operate over a wide dynamic range. It enhances our hearing sensitivity, frequency selectivity, and the ability to tolerate a broad range of sound intensities.

How does the cochlea act as an acoustic prism?

The cochlea separates complex sounds into their component frequencies, much like an optical prism separates white light into different colors. It represents each frequency at a different position along its length, allowing the brain to identify different sounds by analyzing the nerve signals from the hair cells.

Why is the active process in hearing considered faster than other senses?

The active process in hearing is faster because it does not rely on chemical reactions that take time, unlike vision. Hair cells can respond to sound frequencies up to 20,000 cycles per second, making hearing significantly faster than other senses.

What is the significance of the hair cell's instability in hearing?

The instability of the hair cell is crucial for the active process. It allows the hair bundle to tremble and oscillate, which enhances the signal of even weak sounds, amplifying them and improving our frequency selectivity.

How do the emissions from the ear demonstrate the activity of hair cells?

Emissions from the ear, such as otoacoustic emissions, occur when the active process in the hair cells goes unstable in a quiet environment, causing the ears to emit sounds. This demonstrates the hair cells' ability to actively generate sound.

What are the future research questions the speaker wants to address regarding hair cells?

The speaker is interested in understanding the molecular motor responsible for hair cell amplification, how the amplification is adjusted in different acoustic environments, and finding ways to address the deterioration of human hearing and possibly regenerate hair cells.

Why don't human ears emit sounds like a public address system when the amplification is turned up?

While the active process in the ear can cause emissions, the ear has a self-regulating mechanism that controls its sensitivity and amplification based on the environment, preventing it from emitting sounds like an over-amplified PA system.

How do hair cells in nonmammalian animals differ from those in humans in terms of regeneration?

Nonmammalian animals can replace their hair cells through cell division, maintaining normal hearing throughout their lives. In contrast, human hair cells do not regenerate when they die, leading to potential hearing loss.

Outlines

00:00

👂 The Marvel of Hearing: Hair Cells in Action

This paragraph introduces the intricate process of how we hear, focusing on the ear's sensory receptors known as 'hair cells.' These cells, named for the bristle-like hair bundle at one end, are responsible for converting sound vibrations into electrical signals that the brain can interpret. The hair bundle, composed of stereocilia connected by a filament with ion channels, is highlighted as a critical component for auditory perception. The explanation includes how sound waves cause the stereocilia to move, opening ion channels and generating an electrical current that signals the brain. The paragraph also emphasizes the speed and sensitivity of this process, which is significantly faster than other senses, allowing us to perceive a wide range of frequencies and sound intensities.

05:04

🔊 The Active Process: Amplifying Auditory Experiences

The active process within the ear is explored in this paragraph, detailing how it amplifies sound to an extraordinary degree, allowing us to hear even the slightest movements of the hair bundle. This amplification was crucial for early humans to detect predators and remains important for detecting modern dangers like fire alarms. The active process also enhances our ability to discern different frequencies, which is vital for understanding speech and recognizing voices. The paragraph explains how the cochlea acts as an 'acoustic prism,' separating complex sounds into their component frequencies, and how hair cells report these frequencies to the brain. The active process is also linked to the hair bundle's instability, which is essential for its amplification capabilities.

10:05

🎶 Tuning In to Sound: The Active Role of Hair Cells

This paragraph delves into the active role of hair cells in tuning into specific frequencies, illustrating how each hair cell amplifies a particular frequency while ignoring others. The demonstration of a sound system amplifying a specific frequency in the speaker's voice serves as a metaphor for how hair cells function, highlighting their ability to enhance and selectively report frequencies. The paragraph also discusses the potential issues with amplification, such as instability leading to unwanted emissions, and how the ear can control its sensitivity based on the acoustic environment. The phenomenon of otoacoustic emissions, sounds emitted by the ear itself, is introduced as evidence of the ear's active process.

15:07

🔬 The Future of Hearing: Research and Regeneration

The final paragraph addresses future research directions in the field of hearing, focusing on three main questions. The first is to uncover the molecular motor behind the hair cell's amplification, which operates at remarkable speeds. The second is to understand how the amplification is adjusted in response to different acoustic environments. The third and most pressing issue is the deterioration of hearing and the potential for regeneration of hair cells, as seen in nonmammalian animals. The paragraph concludes with a hopeful note on the possibility of decoding the molecular signals for hair cell regeneration in humans, which could revolutionize the treatment of hearing loss.

Keywords

💡Hair Cells

Hair cells are the sensory receptors in the ear responsible for converting sound vibrations into electrical signals that the brain can interpret. They are named for the hair-like projections, or stereocilia, that respond to sound waves. In the video, hair cells are described as the 'real hero' and are central to the theme of how the ear processes auditory information. The script explains that these cells are found in various animals, from primitive fishes to humans, and their structure and function are crucial for understanding the complexity of hearing.

💡Hair Bundle

The hair bundle is a cluster of fine cylindrical rods, or stereocilia, that protrude from the top of hair cells. It is the key structure that enables hearing by detecting sound vibrations. The video emphasizes the hair bundle's role in capturing even the smallest changes in air pressure, such as those produced by a human voice. The script provides an analogy of a rat trap to illustrate how the hair bundle works, with the stereocilia sliding against each other to open ion channels and generate an electrical response.

💡Ion Channels

Ion channels are proteins that span the cell membrane and allow specific ions, such as potassium and calcium, to pass through. In the context of the video, these channels are critical for the function of hair cells, as they open and close in response to the movement of the hair bundle, allowing ions to flow into the cell and create an electrical current. The script uses the analogy of a rat trap to describe the mechanism of ion channels, highlighting their importance in the process of hearing.

💡Active Process

The active process refers to the amplification of sound within the ear, which enhances our ability to hear faint sounds and increases the frequency selectivity of our hearing. The video explains that this process is not just passive but involves the hair cells actively using internal energy to perform mechanical work. The script describes how the active process allows us to hear sounds that move the hair bundle by a tiny distance, comparable to the diameter of a water molecule, and how it contributes to the ear's remarkable sensitivity and dynamic range.

💡Cochlea

The cochlea is a snail-shaped, coiled organ in the inner ear that acts as an acoustic prism, separating complex sounds into their component frequencies. The video script describes the cochlea's role in the process of hearing, explaining how it separates different frequencies of sound and represents them at different positions along its length. This separation is essential for our ability to identify and distinguish between different sounds, as each frequency is detected by specific hair cells along the cochlear surface.

💡Frequency Selectivity

Frequency selectivity is the ability of the ear to distinguish between sounds of different frequencies. The video highlights how the active process enhances this selectivity, allowing even untrained individuals to differentiate between tones that differ by a very small percentage. The script explains that this fine discrimination is useful for understanding speech and recognizing different voices, and it is achieved through the specific activation of hair cells that are sensitive to particular frequencies.

💡Stereocilia

Stereocilia are the individual hair-like structures that make up the hair bundle on hair cells. They are arranged in tiers of varying heights and play a crucial role in the mechanotransduction of sound. The video script describes how the movement of stereocilia against each other, in response to sound vibrations, opens ion channels and triggers the process of hearing. The term is used to illustrate the intricate structure of hair cells and their function in detecting sound.

💡Auditory Experiences

Auditory experiences refer to the full range of sounds that humans can perceive, including speech, music, and natural environmental sounds. The video script discusses how the ear's ability to detect minute changes in air pressure allows for a wide array of auditory experiences. This concept is central to the video's theme, as it sets the stage for explaining the complex process by which the ear translates sound waves into electrical signals that the brain can interpret.

💡Dynamic Range

Dynamic range is the ratio between the loudest and softest sounds that the ear can detect. The video script emphasizes the ear's extraordinary dynamic range, which is much broader than that of any other sense or man-made device. This wide range is made possible by the active process and is essential for hearing faint sounds, like a whisper, as well as extremely loud ones, such as a jet engine.

💡Oscillation

Oscillation, in the context of the video, refers to the natural trembling or twitching motion of the hair bundle even in the absence of sound. This instability is actually a feature that enhances the ear's sensitivity and frequency selectivity. The script explains how this oscillation is harnessed by the hair cells to amplify weak sound signals and to resonate at specific frequencies, contributing to the sharp tuning of our hearing.

💡Hearing Aid

A hearing aid is an electronic device that supplements the biological hearing process when it is impaired. The video script mentions hearing aids in the context of the active process failing, which can lead to a decrease in hearing sensitivity. Hearing aids are used to amplify sound for individuals with hearing loss, helping them to better perceive speech and other auditory cues. The concept is related to the video's theme as it represents a technological attempt to replicate or enhance the natural amplification provided by the ear's active process.

Highlights

The human ear can detect infinitesimal changes in air pressure caused by sound, thanks to the ear's sensory receptors called 'hair cells'.

Hair cells, despite their name, are unrelated to hair and got their name from the bristle-like structures observed under early microscopes.

Modern electron microscopy reveals the hair bundle, a cluster of cylindrical rods that play a crucial role in hearing.

The speaker expresses admiration for hair cells, having spent 45 years studying them and appreciating their beauty and function.

Hair cells convert sound vibrations into electrical responses through a mechanism involving stereocilia and ion channels.

The intensity of sound is represented by the magnitude of the hair cell's response, with louder sounds causing a greater response.

Hair cells operate much faster than other senses, enabling humans to hear frequencies up to 20,000 cycles per second.

The 'active process' in the ear acts as an amplifier, enhancing hearing sensitivity and allowing detection of extremely faint sounds.

The active process is essential for hearing in various environments, from ancient times detecting predators to modern uses like hearing fire alarms.

When the active process fails, hearing sensitivity drops, and individuals may require hearing aids.

The cochlea acts as an 'acoustic prism', separating complex sounds into their component frequencies.

The cochlea's separation of frequencies is key to identifying different sounds, with each frequency represented at a different position.

Hair cells are not just passive; they are active machines that use internal energy to enhance hearing.

Demonstration of how a hair cell amplifies a specific frequency, similar to how an amplifier enhances certain tones in sound systems.

Most people's ears emit sounds in a quiet environment, demonstrating the active nature of hair cells.

The ear can control its own sensitivity and amplification, adjusting to the acoustic environment.

Future research aims to understand the molecular motor of hair cell amplification and how to regenerate hair cells in humans.

The speaker concludes by emphasizing the importance of addressing hearing deterioration and the potential for restoring hair cell function.

Transcripts

Transcriber: Joseph Geni Reviewer: Camille Martínez

Can you hear me OK?

Audience: Yes.

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

because my voice is changing the air pressure where you sit

by just a few billionths of the atmospheric level,

yet we take it for granted

that your ears can capture that infinitesimal signal

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

the human voice, music, the natural world.

How does your ear do that?

And the answer to that is:

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

the ear's sensory receptors,

which are called "hair cells."

Now, these hair cells are unfortunately named,

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

of which I have less and less.

These cells were originally named that by early microscopists,

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

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

That's the hair bundle.

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

that stand upright at the top end of the cell.

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

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

I've spent 45 years in their company --

(Laughter)

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

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.

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

and those of reptiles often have this really beautiful,

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

that the brain can then interpret.

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

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

the stereocilia.

It's here marked with a little red triangle.

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

which are proteins that span the membrane.

And here's how it works.

This rat trap represents an ion channel.

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

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

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

Now, imagine that this arm represents one stereocilium

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

until the channels open and ions rush into the cell.

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,

alternately opens and closes the channel,

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

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,

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.

Some of our senses, such as vision,

use chemical reactions that take time.

And as a consequence of that,

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

you get the sense of a continuous image.

Because it doesn't use reactions,

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

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

and some animals have ever faster ears.

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

at 150,000 cycles a second.

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

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

something called the "active process."

The active process enhances our hearing

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

Let me tell you how it works.

First of all, the active process amplifies sound,

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

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.

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.

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

It's valuable to be able to hear fire alarms

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

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

and an individual may then need an electronic hearing aid

to supplant the damaged biological one.

This active process also enhances our frequency selectivity.

Even an untrained individual can distinguish two tones

that differ by only two-tenths of a percent,

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

and a trained musician can do even better.

This fine discrimination is useful

in our ability to distinguish different voices

and to understand the nuances of speech.

And, again, if the active process deteriorates,

it becomes harder to carry out verbal communication.

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

of sound intensities that our ears can tolerate,

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

to the loudest sound that you can stand --

say, a jackhammer or a jet plane.

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.

And again, if this system deteriorates,

an affected individual may have a hard time

hearing the very faintest sounds

or tolerating the very loudest ones.

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

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

We learn in school that the organ of hearing

is the coiled, snail-shaped cochlea.

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

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

We also learn that an optical prism

can separate white light into its constituent frequencies,

which we see as distinct colors.

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.

The cochlea undoes that process.

It separates them and represents each at a different position.

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

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

are represented in the cochlea.

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

The highest frequencies, down to 20,000 Hz,

go all the way to the bottom of the cochlea,

and every other frequency is represented somewhere in between.

And, as this diagram shows,

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

along the cochlear surface.

Now, this separation of frequencies

is really key in our ability to identify different sounds,

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

how much of each frequency is present.

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?

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

How does it carry out the active process

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

The answer is instability.

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

it just sat there, except when it was stimulated.

But in fact, it's an active machine.

It's constantly using internal energy to do mechanical work

and enhance our hearing.

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

an active hair bundle is constantly trembling.

It's constantly twitching back and forth.

But when even a weak sound is applied to it,

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

in a one-to-one way with it,

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

This same instability also enhances our frequency selectivity,

for a given hair cell tends to oscillate best

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,

but also gives us the very sharp tuning.

I want to offer you a short demonstration

of something related to this.

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

to turn up its sensitivity at one specific frequency.

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

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

Notice how specific tones emerge more clearly from the background.

This is exactly what hair cells do.

Each hair cell amplifies and reports one specific frequency

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

can also cause problems.

If the amplification is turned up too far,

it goes unstable and begins to howl

or emit sounds.

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.

In a suitably quiet environment, 70 percent of normal people

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

(Laughter)

I'll give you an example of this.

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.

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

This is typical.

Most ears emit just a handful of tones,

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,

but unless an ear is damaged,

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

or a musical concert,

you don't need any amplification,

and the system is turned down all the way.

If you are in a room like this auditorium,

you might have a little bit of amplification,

but of course the public address system does most of the work for you.

And finally, if you go into a really quiet room

where you can hear a pin drop,

the system is turned up almost all the way.

But if you go into an ultraquiet room such as a sound chamber,

the system turns itself up to 11,

it goes unstable

and it begins to emit sound.

And these emissions constitute a really strong demonstration

of just how active the hair cell can be.

So in the last minute, I want to turn to another question that might come up,

which is: Where do we go from here?

And I would say that there are three issues

that I would really like to address in the future.

The first is: What is the molecular motor

that's responsible for the hair cell's amplification?

Somehow, nature has stumbled across a system

that can oscillate or amplify at 20,000 cycles per second,

or even more.

That's much faster than any other biological oscillation,

and we would like to understand where it comes from.

The second issue is how the hair cell's amplification is adjusted

to deal with the acoustic circumstances.

Who turns the knob to increase or decrease the amplification

in a quiet or in a loud environment?

And the third issue is one that concerns all of us,

which is what we can do about the deterioration of our hearing.

Thirty million Americans,

and more than 400 million people worldwide,

have significant problems on a daily basis

with understanding speech in a noisy environment

or over the telephone.

Many have even worse deficits.

Moreover, these deficits tend to get worse with time,

because when human hair cells die,

they're not replaced by cell division.

But we know that nonmammalian animals can replace their cells,

and those creatures' cells are dying and being replaced throughout life,

so the animals maintain normal hearing.

Here's an example from a little zebra fish.

The cell at the top will undergo a division

to produce two new hair cells.

They dance for a little bit,

and then settle down and go to work.

So we believe that if we can decode the molecular signals that are used

by these other animals to regenerate their hair cells,

we'll be able to do the same thing for humans.

And our group and many other groups are now engaged in research

trying to resurrect these amazing hair cells.

Thank you for your attention.

(Applause)