The Spark of Life: Electricity in the Human Body (27 page)

Making Waves

The most important part of the ear – the bit that actually senses sounds – is tucked safely away inside our skull. This is the cochlea, a fluid-filled sac that is coiled up tightly like a snail shell to fit inside the temporal bone (hence its name – cochlea is Latin for ‘snail’). About the size of a pea when coiled up, it is 35 millimetres long when unrolled and split longitudinally into three compartments by two membranes. Around 16,000 specialized sensory cells called hair cells are arranged along the length of the lower (basilar) membrane in four rows: three rows of outer hair cells and one of inner hair cells. On their upper surface the hair cells bear bundles of stiff eponymous hairs, known as stereocilia, that reach up toward the tectorial membrane.

The region of the cochlear that enables us to hear, shown in the resting state (
left
) and in the active state (
right
). Displacement of the basilar membrane (
right
) causes the hairs on the tip of the hair cells to be displaced. In the case of the inner hair cells, this causes the sensation of sound.

Sound waves induce oscillations in the cochlear fluid on either side of the basilar membrane, causing it to vibrate. By experimenting on human cadavers, a Hungarian engineer, Georg von Békésy, showed that sound moves along the basilar membrane as a travelling wave (like that seen when a whip is cracked), building up in amplitude to reach a peak at a particular point along the length of the membrane and then rapidly dissipating. Where this peak occurs depends on the frequency of the sound: high-frequency sounds move the base of the basilar membrane most, and low frequency sounds produce the greatest deflections at the tip of the cochlea. The tiny movement of the basilar membrane is transferred to the hair cells, causing their stereocilia to be displaced backwards and forwards, so producing a mechanical deflection that opens specialized ion channels.

These mechanically gated ion channels are at the heart of hearing for it is they that convert sound waves into electricity – or more precisely mechanical energy into electrical energy. The molecular identity of the channel is not yet known, but the way in which they are opened has been elucidated and it is remarkable. The stereocilia the hair cells bear are organized in rows of decreasing height and are connected to one another at their tips by a tiny thin stiff rod known as a ‘tip link’. One end of the tip link is also connected to the mechanosensitive channels that sit at the tip of the stereocilia. As the basilar membrane moves up and down, the tip links are stretched or compressed, pulling opening, or forcing shut, the ion channels, respectively. When they open, positively charged ions rush into the cell and alter the voltage gradient across the hair cell membrane. This electrical change has different effects depending on whether the cell is an inner or an outer hair cell.

Picking up Good Vibrations

The inner hair cells are responsible for converting sound waves into electrical impulses and forwarding them to the auditory nerve. The change in voltage across the inner hair cell membrane produced by sound of a specific frequency triggers the release of a chemical transmitter. This stimulates impulses in the terminals of the auditory nerve and so sends signals to your brain. Inner hair cells at different positions along the basilar membrane respond to different frequencies, with those at the base of the cochlea detecting high-pitched sounds and those at the tip responding to low-pitched notes. This frequency discrimination simply reflects the magnitude of the movement of the basilar membrane – recall that high-pitched sounds have their greatest effect at the base of the cochlea. Nerve fibres coming from different regions of the basilar membrane are therefore tuned to specific frequencies, enabling the brain to discriminate different tones on the basis of which fibres are active. This complex bit of molecular machinery is in action inside your head right now, as you listen to the sounds around you.

Dancing Hair Cells

The outer hair cells are far more numerous than the inner ones. Although they play little, if any, role in signalling sounds to the brain, they are essential for normal hearing as they mechanically amplify sound vibrations by ‘dancing’ in time to the beat. This amplification is critical for detecting low-intensity, high-frequency noises because the vibrations of the sound waves are damped down as they pass through the fluid-filled canals of the inner ear.
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Without magnification of the signal, the inner hair cells are not stimulated sufficiently to activate the auditory nerves. The cochlear amplifier, as it is known, also sharpens the ability of the ear to discriminate different frequencies. It may have evolved to enable the first mammals to hear the faint high-pitched cries of their young; now it helps us hear the squeak of a bat.

The existence of a natural amplifier in the ear was first proposed in 1948, but the idea was rejected and it was not until the late 1970s that its validity was established. The fact that hair cells could be seen to ‘dance’ provided the clue to how it worked. An outer hair cell will twitch in time to music piped directly into the cell via an electrode connected to an amplifier. I’ve not forgotten the time I visited Jonathan Ashmore’s laboratory at University College London and looking down the microscope was amazed to see a tiny hair cell bopping away to ‘Rock around the Clock’. It kept perfect time. The contractions of the hair cell are powered by prestin, a molecular motor that is sensitive to the voltage difference across the cell membrane, and changes in this voltage difference produced by the exogenous electrical stimulus caused the cell to dance. In life, such voltage changes are produced by opening of mechanosensitive ion channels in response to the movement of the hair bundle within, as Huxley so evocatively put it, the storm in the cochlear fluid. The twitching of the outer hair cells amplifies the movement of the basilar membrane, leading to greater stimulation of the sensory inner hair cells. This intrinsic biological amplifier is the basis of our ability to hear very quiet sounds. High doses of aspirin inhibit the motor protein and induce a reversible hearing loss.

The Song of the Ear

It may come as a surprise, but your ears can also generate sounds. Technically known as otoacoustic emissions, these are produced by the outer hair cells. They arise because the vibrations the hair cells produce as they bounce up and down set up waves in the cochlear fluid, which in turn are passed on to the air in the middle ear and ultimately back to the eardrum. The sounds made by healthy ears are quieter than a whisper and those of individuals with cochlear damage are even weaker. However, they can be picked up by a special microphone placed within the ear canal, and such ‘ear songs’ provide doctors with a valuable non-invasive measure of the health of the ear and a simple way to check if a young baby has impaired hearing. This enables a child to be fitted with a hearing aid or cochlear implant before the time window for learning speech has passed.

Living Under a Deaf Sentence

Helen Keller, who was both blind and deaf, once said that whereas blindness separates people from things, deafness separates people from people. The isolation, confusion, frustration and depression often experienced by those who lose their hearing is poignantly stated by Ludwig van Beethoven in his ‘Heiligenstadt Testament’, written at the age of thirty-two, six years after he had started to go deaf. ‘Oh you men who think or say that I am malevolent, stubborn or misanthropic, how greatly do you wrong me. You do not know the secret cause which makes me seem that way to you [. . .] it was impossible for me to say to people, “Speak louder, shout, for I am deaf.” [. . .] for me there can be no relaxation with my fellow men, no refined conversations, no mutual exchange of ideas. I must live almost alone, like someone who has been banished.’ By the time he was forty-five, Beethoven was almost totally deaf. Yet although performance became impossible, he continued to compose and conduct. At the première of his Ninth Symphony (when he was fifty-four), he had to be gently turned around to witness the rapturous applause of the audience, for he could hear nothing. He wept at the sight.

Unlike a musician, a deaf painter is still able to practise his art. Indeed, in Goya’s case it seems to have led to his greatest works. After a serious illness left him stone deaf, the devastating isolation he experienced precipitated a remarkable change in his art: increasingly, he focused on nightmarish fantasies, black visions and satirical portrayals of human behaviour. Liberated from the cacophonous distractions of daily life, it is said, he saw the world for what it was – although whether Goya, like the art critics, perceived his deafness as a blessing is questionable.

Hear Today, Gone Tomorrow

It is estimated that about nine million people in the UK – as many as one in seven of the population – suffer from some degree of hearing loss. Almost inevitably, it seems, as we grow older our ability to hear high frequencies declines. Alan Bennett wrote, ‘I did not think my hearing had deteriorated at all but [. . .] R. asks me if I can hear the crickets and I cannot believe that, the night tingling with the sound, I am dead to it.’ Such high frequency hearing loss often creeps up on you gradually, almost unnoticed, until suddenly hearing is gone. The poet Philip Larkin only discovered it when a friend remarked on the beauty of a skylark’s song and he was unable to hear it. For him, as for many others, partial deafness darkened his melancholy. Age-related high frequency hearing loss has even been exploited by the manufacturers of a controversial device termed the Mosquito that emits an ear-splitting high-pitched whine, audible to teenagers but not to adults. Its painful buzz has been used to disperse loiterers and prevent antisocial behaviour on UK streets. In an ironic twist to the story, ingenious teenagers subsequently stole the frequency and used it as a mobile phone ringtone that they – but not their teachers – could hear.

The deterioration in hearing we all experience with age is caused because our hair cells naturally die off with time and once gone they are lost forever. Loud noises destroy our hearing even faster. The rock musician Pete Townshend
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instantly lost the hearing in one ear in a famous incident in which a staged explosion was far louder than expected. Thousands of troops fighting in Iraq and Afghanistan returned home with permanent hearing loss, mostly caused by roadside bombs. The thunder of warfare, the deafening sound of a pop concert, the roar of jets and loud machinery – all exact a heavy toll. This is because our outer hair cells are very vulnerable and can be irretrievably damaged by loud noises.

Left
. Normal hair cells, showing the three layers of outer hair cells and, below, the single layer of inner hair cells.
Right
. Loud noises damage the outer hair cells before the inner ones.

Chronic exposure to moderately loud sounds can also cause permanent hearing loss because there is no time for partially damaged hair cells to recover. Many people, often unwittingly, routinely subject their ears to noise levels that can ruin their hearing. Exposure to sounds louder than 85 decibels for an extended period of time can cause hearing loss: this noise level is similar to that associated with using a power drill, riding a motorcycle, going to the cinema and many other everyday pursuits. It is also lower than the maximum volume levels on many portable MP3 players. Turn up the volume too loud for too long and you may be unable hear your grandchildren in later life. Sadly, it seems inevitable that within the next few decades many people will become far more interested in how their ears work than they might wish.