The Spark of Life: Electricity in the Human Body (40 page)

Bionic Ears

Electricity has been used to power hearing aids for years, but these are no more than simple amplifiers that boost the sound. If the sensory cells in your ear are damaged you will be unable to hear, no matter how loud the stimulus. Normally, the hair cells in the cochlea of your inner ear sense sound signals and translate them into electrical impulses that are sent to the brain via the auditory nerve. In deaf people who have an auditory nerve that is at least partially intact it is possible to bypass the damaged hair cells and stimulate the auditory nerve directly. This is what cochlear implants do.

Currently, they come with both internal and external components, the former being implanted under the skin of the head and the other being worn behind the ear. The outer device, which is about the size of a small hearing aid, consists of a microphone, a speech processor and a transmitter. The microphone picks up sounds from the environment and converts them to electrical signals, the speech processor filters out background noise and the transmitter forwards the signals to a receiver mounted close to it, but inside the body. The receiver then sends the electrical signals to an array of tiny electrodes that lie alongside different regions of the auditory nerve. The array is introduced into one of the fluid-filled chambers of the cochlea during surgery, so that it lies close enough to the auditory nerve fibres to stimulate them externally.

The hair cells of the cochlea are arranged along its length according to the tones (frequencies) to which they are sensitive, with those that respond to high notes at one end and those responsive to bass notes at the other, much like the keys of a piano. The brain is able to discriminate pitch because different branches of the auditory nerve innervate hair cells that respond to different frequencies. Thus if a branch of the nerve is artificially stimulated the brain will detect it as a note of a particular pitch. The number of electrodes in a cochlear implant varies, with current devices having from sixteen to twenty-four. The more you have, the wider the range of frequencies you will be able to detect. This is why present devices cannot compare with the ear itself, which has more than 3,000 inner hair cells, enabling a far finer discrimination of pitch and the ability to hear musical composition.

Cochlear implants are currently only used in severely deaf people with damaged hair cells. They work best for adults who have lost their hearing and for very young children who are born deaf. There is a critical window for acquisition of language skills and it is important that children receive implants within that time period – two to six years is the typical age. The use of implants is still in its infancy and current devices do not provide people with entirely normal hearing: the British politician Jack Ashley once famously described it as sounding like a ‘croaking Dalek with laryngitis’. It takes practice and training to understand the sounds that are heard, and it is particularly difficult for tonal languages like Mandarin in which pitch discrimination is essential. Nevertheless, many people who were once completely deaf can now hear, and even use the telephone. Understanding speech in noisy situations, like a busy restaurant or bar, however, remains a challenge.

Cochlear implants work only if a few auditory nerve fibres remain intact, which is not the case in all deaf individuals. To get round this problem, electrode arrays have been designed that are implanted into one or other of the hearing centres of the brain. While these work even less well than cochlear implants, they hold some promise for providing otherwise totally deaf individuals with a crude sense of hearing. Not all deaf people are interested in devices to help them hear, however. They resent the implicit implication that they are disabled, and prefer to rely on sign language, which enables them to communicate easily and fluently with one another.

Gripping Stuff

Every morning Christian Kandlbauer gets up, eats breakfast, climbs into his car and drives off to work. Nothing remarkable about that you may think, except that Christian lost both his arms at the age of seventeen in an accident. He now has two prosthetic arms: a conventional one and one that is controlled by his brain. The nerve that once controlled one of his lost limbs was surgically redirected to his chest and different branches of the nerve implanted in different muscle groups. Over time, new nerve terminals innervated the chest muscles so that now when Christian wishes to move his arm, his brain sends a signal down the nerve that excites the chest muscles. The tiny electrical impulses in the muscles are then picked up by an amplifier placed on the surface of his chest and translated into movements of his prosthetic arm. Prosthetic limbs controlled by thought alone are still in the development stage and Christian was one of the first people to be fitted with one.

Currently, most electrically powered artificial arms are controlled by electrical signals picked up from muscles of the residual limb, and the amputee must learn which muscles to contract to control the arm and then consciously do so. In general, such arms only allow a single movement – such as opening the fingers or rotating the wrist – at a time. They are also rather slow and not suitable for people who have lost the whole of their arm or leg. The more advanced type of limb, like the one that Christian possesses, enables far more complex movements that are controlled intuitively – as one patient said, ‘I just think about moving my hand and elbow and they move.’ But even these advanced artificial arms suffer from the drawback that there is no sensory feedback to indicate, for example, just how much force to apply to pick up an object – that needed to grasp a heavy jug might break a fragile egg. Bionic arms are also expensive and must be replaced every few years due to wear and tear. There is thus a pressing need to develop even better prostheses. As is so often the case with medical advances, war is the spur, and considerable investment in new prosthetic technologies has been stimulated by the large number of young US soldiers who lost one or more of their limbs while fighting in Iraq or Afghanistan.

A future dream is to enable the paralysed to walk by mimicking the pattern of electrical activity normally supplied to our limb muscles by our nerves. This is simple to state but extremely difficult to do, for walking is a highly complex task. It is not just that the artificial electrical signals must be supplied in the correct pattern and at the right rate to many different muscles, but that our movements are constantly adjusted by feedback from our limbs. Deep within our muscles lie sensors known as muscle spindles that detect the position of our limbs and the extent of muscle contraction. The information they supply is needed not only to enable us to walk properly, but also to cope with difficulties such as uneven ground or stairs. Thus some sort of feedback system may be essential if an artificial device is to send the correct electrical signals to the muscles.

Forward to the Future

The use of electrical devices in medicine is now routine. Deep brain stimulation has had transformative effects on the lives of people formerly incapacitated by the shakes, and its use in reducing severe depression is currently under investigation. Many people are able to lead normal lives due to cardiac pacemakers. Hearing aids have advanced into new territories. Prosthetic limbs are becoming ever more sophisticated. Devices to help the blind see and the paralysed walk are still in their infancy and there is a long way to go before commercial devices will be available, but there is no reason to think they will not exist eventually.

But it is unlikely to stop there. Functional magnetic resonance imaging (fMRI) can already be used to determine a person’s answer to a yes–no question. In the future, with more sophisticated interpretation of brain scans, it may be possible to enable patients with ‘locked-in’ syndrome to communicate more fully. Whether it will be possible to read someone’s mind, however, is a different matter. Current fMRI technology is massive, spatial and temporal resolution are limited, and how much can be interpreted from the signals produced remains controversial. Yet we should not forget that although the first ECG machine needed two rooms to house it, portable devices are now commonplace.

While pacemakers, deep brain stimulation and fMRI cause little comment, the idea of connecting your brain to a computer is far more startling. In one sense, many of us are already interfaced with our laptops and mobiles – although that connection is mediated via our eyes and fingertips rather than directly with our brain. But as I grow older, I would appreciate a more intimate connection. How wonderful to be able to access all my memories at will. To be prompted with the name of the person standing in front of me whom I taught twenty years ago and whose name now escapes me. To search the Web for information simply by thinking. Frightening though it at first appears to consider wiring your brain up to a computer, it is the nature of the connection that is the crux of the matter. Providing that it can be switched on or off at will, and that any information downloaded to a personal storage device (such as our brain) is both secure and under our own control, it seems likely that many of us will eventually succumb to its seductive lure. Thinking is, after all, faster than typing and reading.

But Mary Shelley’s story has a long reach and first we will need to overcome our fear of the unknown, of monsters such as that created by Frankenstein. We will also need to find ways to legislate and regulate the use of such technology so that the poor are not disadvantaged. Furthermore, any such radical modification of our brains will need to be invisible (for humans prefer not to stand out in a crowd), and preferably it should be possible to remove it easily when desired. Today, we routinely enhance our senses with microscopes, telescopes and night-vision goggles, to name but a few examples, but we can take them off at the end of the day. Likewise, we have calculators and computers that immeasurably enhance our mental abilities and the Internet serves as a vast, external collective memory, with far greater capacity and speed of recall than our brains and libraries. Indeed, many of us are rarely offline and our immediate response to an unknown question is usually to ‘Google’ it. It may be that some individuals will prefer to continue to access such electronic aids via their senses – their fingers, eyes and ears – rather than by a direct connection to the brain. But I for one would like a device that effortlessly stores and retrieves my personal memories, and it would obviously be invaluable for people suffering from memory loss caused by disorders such as Alzheimer’s disease.

Artificial memory aids that plug directly into our brains are, of course, currently only science fiction. But science fiction often has a way of becoming science fact, and 100 years ago few would have imagined it would be possible to control a mechanical arm simply by thinking, or stop a charging bull with a signal to its brain. Perhaps in another hundred years such memory devices may exist. It is impossible to tell, but what I do know is that understanding how the body uses electricity, and how memories are laid down, stored and retrieved by the electrical circuits in our brain will be the key to their success.

Notes

Introduction: I Sing the Body Electric

  
1
 For those who would like a more detailed explanation, it works like this. When the K
_ATP
channel is open, potassium (K
⁺
) ions move through it, flowing out of the cell down their concentration gradient. Because K
⁺
is positively charged, its efflux makes the inside of the cell more negative. This negative membrane potential holds calcium channels closed, so preventing insulin secretion. When plasma glucose levels increase, more glucose is metabolized by the beta-cell. This generates a chemical called ATP, which binds to the K
_ATP
channel and causes it to shut. As a consequence, the membrane potential becomes less negative (as fewer K
⁺
ions leave the cell), which in turn triggers opening of the calcium channels. Calcium rushes into the cell and causes the insulin-containing secretory vesicles to fuse with the surface membrane and release their contents into the bloodstream.

1: The Age of Wonder

  
1
 This quotation is often attributed to Galvani (see, for example, W. W. Atkinson,
Dynamic Thought
(Los Angeles: The Segnogram Publishing Company, 1906, p. 179) but this is not correct. It is not Galvani’s style and he was not mocked in this way during his lifetime. The quotation was probably invented by the French astronomer Camille Flammarion as it appears in his book
L’inconnu et les problèmes psychiques
(Paris: Ernest Flammarion Editeur, 1862). I am indebted to Professor Marco Piccolino for this information.

  
2
 Prometheus was sentenced by Zeus to have his liver torn out by an eagle for eternity. His liver regenerated every night so his punishment was unceasing: it is fascinating that Zeus should have chosen the liver, as it is one of the organs most capable of regeneration.

 
3
 Anne-Robert-Jacques Turgot’s famous epigram on Franklin: ‘He snatched lightning from the sky and the sceptre from the tyrant.’

2: Molecular Pores

 
1
 Ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). DNA is the molecular blueprint of our cells and RNA the messenger molecule that carries the information stored in DNA to the protein factories in the cell.

 
2
 Rod MacKinnon won the Nobel prize in 2003, together with Peter Agre (whose story is told in Chapter 8).

3: Acting on Impulse

 
1
 Hodgkin always attributed his success to chance and good fortune.

 
2
 Huxley came from a distinguished family. He was the grandson of Thomas Huxley, Darwin’s famous bulldog and a great promoter of the theory of evolution; and his half-brothers were the novelist Aldous Huxley and the biologist Julian Huxley. Hodgkin also came from an eminent academic family, many of whom were historians.

4: Mind the Gap

 
1
 Despite its extreme virulence, botulinum toxin is easily destroyed by heating; however, the bacterial spores can survive a temperature of 100
^°
C for two hours.

 
2
 It is ironic that ‘gift’ in German translates as ‘poison’, as what was indeed a gift to the West was poison for Hitler; the scientists he expelled helped the Allies win the war.

 
3
 At the end of World War II, Feldberg was awarded a considerable sum of ‘restitution money’ from the German government. He generously used it to set up a fund for the furtherance of good relations between German and British scientists: each year it awards a handsome prize and financial support for one British scientist to visit Germany, and one German scientist to come to the UK.

6: Les Poissons Trembleurs

 
1
 Socrates dryly replies that he resembles a torpedo only if the fish produces torpidity in itself as well as others, because the reason he baffles Meno is because he is confused himself.

 
2
 This was the price in North Carolina, USA. No doubt they would have been more costly in the UK. A guinea is 21 shillings – one pound and 10 pence in current money.

 
3
 Watts equals volts times amps.

 
4
 Each ampulla consists of a small capsule that is connected to an opening on the surface of the skin by a jelly-filled electrically conductive canal. The receptor cells sit in the wall of the ampulla. They respond to a difference in potential between the canal lumen (which is continuous with the seawater) and the body’s interior, which, in turn, stimulates electrical impulses in the nerve fibres innervating the ampulla. Cutting the nerves to the ampullae abolishes the shark’s ability to sense weak electric fields, conclusively demonstrating that the ampullae serve as electroreceptor organs.

7: The Heart of the Matter

 
1
 This was Herbert Gladstone (son of the more famous William), who was Home Secretary from 1905–10.

 
2
 The mechanism is unclear.

 
3
 He called his machine after Thanatos, the Greek god of death.

8: Life and Death

 
1
 Most of the 150 to 200 litres filtered each day is absorbed in the upper part of the kidney tubule, by other kinds of water channel.

 
2
 Raindrops do not (uselessly) trigger the trap because two hairs must be touched within 20 seconds of one another to produce a response.

9: The Doors of Perception

 
1
 Aldous Huxley took the title of his famous book
The Doors of Perception
,
about his experience of taking mescaline, from Blake’s poem. The name of the 1960s pop group,
The Doors
, refers to Huxley’s book, and through that back to Blake’s poem. Blake in turn takes the idea from Plato, who famously remarked that we are like prisoners in a cave who see the outside world only as shadows on the wall, so that what we perceive is but an illusion of reality.

 
2
 Too much vitamin A is very bad for you. The livers of some Antarctic mammals, such as polar bears and seals, contain toxic levels of vitamin A. The 1911–14 Australasian Antarctic expedition lost all their supplies and one of their party down a crevasse. The other two expedition members had to eat their huskies to try to stay alive – but Xavier Mertz died anyway, probably because he ate too much of the liver and developed fatal vitamin A poisoning.

 
3
 People can see even with the lens removed, as only about 30 per cent of the focusing power of the eye comes from the lens. The cornea does the rest. Glasses can also help focus the light in those without a lens.

 
4
 Later it was decided that colour blindness had nothing to do with it and that the driver had simply ignored the signal.

 
5
 Because sound is produced by molecules colliding with one another to produce pressure waves, sound cannot occur in a vacuum. In space, no one can hear you scream and you cannot hear an explosion.

 
6
 A similar effect is observed if you shout underwater – the sound travels less far than in air.

  
7
 Of the band The Who.

 
8
 Pickled artichokes don’t work.

  
9
 Contrary to what it says in the textbooks, the different types of taste buds are evenly distributed over the surface of the tongue.

 
10
 The name derives from ‘sauerstoff’, the German name for ‘acid’, which is a solution containing a high concentration of hydrogen ions.

 
11
 There are many more genes, but not all produce functional proteins.

 
12
 Interestingly, in humans the same receptor detects the pungent oils in wasabi.

12: Shocking Treatment

  
1
 For a video, see the Wikipedia entry on Topsy (elephant).

 
2
 One of his machines can be seen at his house in London.

  
3
 Practising as a physician without qualifications was not uncommon at that time.