The Spark of Life: Electricity in the Human Body (28 page)

One of the first signs of damage is a chronic ringing in the ears known as tinnitus.
The Times
’ music critic Richard Morrisson, who tested a device that simulates tinnitus, described it as a horrible, high-pitched whistling that made listening to music a nightmare. ‘It was like listening to the drifting signal of some Algerian radio station through the crackling static of an old wireless. Only much more distressing.’ Morrisson found immediate relief by ripping the simulator off his ears but for those unfortunates whose hair cells are ruined tinnitus can be a lifelong ordeal. For them, silence is never silent. Beethoven, who suffered from severe tinnitus from his late twenties, complained that his ears whistled and buzzed continually, day and night, and described his condition as truly frightful. It is extraordinary that despite this handicap he was able to compose some of the world’s greatest music.

Although tinnitus is often associated with hearing loss, this is not always the case and many tinnitus sufferers hear perfectly well. What causes these internally generated sounds to be perceived is still far from clear, but we do know that they originate from changes that take place within the brain.

A Matter of Taste

I first tasted the miracle fruit one hot summer’s afternoon in Puerto Rico. This smooth oval-shaped red fruit, about the size of a coffee bean, comes from the shrub
Synsepalum dulcificum
, a native of West Africa, and has the extraordinary property of making sour things taste sweet. It felt cold and hard as I rolled it over my tongue and I bit into it with a mixture of anticipation and trepidation. It had a thin, bitter skin surrounding yellow, slightly astringent flesh and a quite unremarkable taste. Ten minutes later I was able to eat a lemon without wincing and, somewhat tentatively, sip vinegar. With my eyes closed, many foods were barely recognizable; beer, in particular, tasted most peculiar. Happily, the effect wore off within a couple of hours.

The miracle fruit contains a protein called miraculin that interacts with sweet taste receptors and enables them to be activated by sour chemicals. Other natural modifiers of taste are also known. If you have ever eaten a fresh globe artichoke you will be aware that everything, including water, tastes sweet afterwards.
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This is because artichokes contain cynarin, which appears to work by suppressing the activity of bitter taste receptors while enhancing that of sweet ones. Whatever the mechanism, it makes it notoriously difficult to choose a wine to drink with globe artichokes. In contrast, gymnemic acid, from the south Asian herb
Gymnema sylvestre
, suppresses the intensity of sweet perception, but not that of bitter, so that many foods taste unusually bitter and sugar tastes of ashes.

Taste cells are not nerve cells but a specialized kind of epithelial cell (the cells that line the gut, mouth and nasal passages). They are very short-lived, being continually replaced every couple of weeks, and they are packed together in barrel-shaped taste buds. Humans have about 10,000 taste buds distributed over the surface of the tongue, each containing 50 to 100 taste cells.
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Each taste cell sends a long finger-like process, tipped with fine hairs that bear the taste receptors, up to the opening of the taste bud on the surface of the tongue where stimuli are received. The other end of the taste cell contacts the sensory nerve.

We can discriminate five basic tastes – sweet, salt, sour, bitter and savoury (umami). All the many different flavours we taste, however, are really smelt, for these two senses work in combination. This explains why your sense of taste seems impaired when you have a head cold and your nose is blocked. Anthelme Brillat-Savarin, the seventeenth-century gastronome, tells of meeting a man whose tongue had been cut out, yet who retained a full appreciation of tastes and flavours. He therefore concluded that, ‘smell and taste are in fact but a single composite sense, whose laboratory is the mouth and its chimney the nose’.

When you eat something, chemicals contained in the food dissolve in your saliva. This enables them to bind to the receptors at the tip of the taste cells, and so trigger a cascade of events that ultimately releases a chemical transmitter from the base of the taste cell. In turn, this excites the sensory nerve, and nerve impulses are then transmitted to the brain where the information is decoded, processed and tastes are identified.

Different tastes arise because different types of receptor are stimulated. Two tastes – salt and sour – are directly detected by ion channels sensitive to the ions involved, which are respectively sodium ions and hydrogen ions (protons). Salty tastes are mediated by the epithelial sodium channel (ENaC) we met in the previous chapter. Several kinds of ion channel that are sensitive to protons detect sour tastes. The carbon dioxide in fizzy drinks and champagne is also detected by sour taste receptors because it yields protons when dissolved in water. Interestingly, some soda-water manufacturers recognized this long before science showed it to be true – sauerwasser
¹⁰
and similar seltzers are named for their sour, slightly acidic taste. Umami, from the Japanese word umai, meaning ‘delicious’, describes the savoury taste of food containing monosodium glutamate. Some of the receptors that detect glutamate are also ion channels. Somewhat surprisingly, the giant panda lacks functional umami receptors, but whether this is the cause or the consequence of the fact that, unlike other bears, it prefers a strict vegetarian diet is unclear.

Sweet and bitter substances do not activate ion channels directly. Instead they bind to specific receptors, so setting in train a cascade of biochemical events that eventually leads to the opening of a specialized ion channel (known as TRPM5) that is common to both pathways. The ability to discriminate between sweet and bitter substances arises because the two types of receptors are found in different populations of taste cells, which signal separately to the brain. Thus whether something tastes sweet or bitter is decided by the brain. We have over twenty different receptors for bitter taste, but only one for sweet taste, reflecting the evolutionary drive to identify bitter-tasting substances, which are often poisonous. The sweet taste receptor is composed of two different proteins and variants in either of the genes that encode these proteins give rise to different sensitivities to sweet substances; it seems that some people really do have more of a ‘sweet tooth’ than others. Reduced sensitivity to sugar is most common in sub-Saharan African populations, suggesting that that the ability to sense sugar is more important in cold climates, where sugar sources are rare. But in today’s society the beguiling pleasure of sweet taste brings in its wake terrible public health problems – obesity and tooth decay are the handmaidens of Sachertorte, raspberry ice cream and sugary drinks.

Many patients taking anti-cancer drugs complain that food tastes terrible – less sweet and more bitter. This is because, like all epithelial cells, taste cells have a very rapid turnover and thus are especially sensitive to chemotherapeutic drugs, which destroy rapidly dividing cells. Taste is also influenced by context (although this is largely the province of the brain). I love the smell of coffee but gave up drinking it over twenty years ago and now take only tea. On the odd occasion when I am accidentally handed the wrong drink and take a sip of coffee it tastes very strange. The ability to identify the correct flavour is also reduced if the food is the wrong colour; raspberry juice does not taste quite right if it is coloured orange or green. Try it, and see if you agree.

Making Sense of Scents

Scents, as Marcel Proust famously observed, can evoke remembrance of things past. The spicy, peppery smell of lupins reminds me of my great-aunt’s garden, crammed with colourful flowers and butterflies, and humming with bees. That of mown hay evokes other childhood memories – of lying in the grass watching the village cricket match, hearing the distant cuckoo and the strangely comforting thwack of leather on willow.

The cells that detect smells lie high up in the nose, almost seven centimetres away from the nostril. These are the olfactory neurones, which send processes to the olfactory epithelium in the nose. Each nerve process terminates in a small bunch of olfactory cilia, fine hair-like processes that project up into the viscous mucous layer that covers the moist surface of the inner nose and greatly increase the membrane surface area available for odorant detection. Odorant receptors lie embedded in the surface of the cilia, ready to capture smells borne on the air you breathe.

Humans have around 350 distinct types of olfactory receptor proteins,
¹¹
although each olfactory neurone carries only a single kind. But we can detect far more than 350 aromas: most people can distinguish many thousands of substances, often in tiny amounts. A good ‘nose’, such as an expert perfumier or sommelier, has even finer discrimination. Thus it is clear that there is not a specific receptor dedicated to a given odour. Rather, it is believed that each receptor recognizes a class of odour molecule (or a specific molecular feature), that a single odorant may bind to more than one receptor, and that it is the specific combination of receptors that are stimulated that enables us to discriminate smells. In the same way that the letters of the alphabet can be used to construct a vast vocabulary, so the different combinations of odorant receptors produce a cornucopia of pure odours. Scents are even more complex and varied as they are composed of many different odours.

It is widely believed that humans have a poor sense of smell. But tests show that we can detect some odours almost as well as dogs and much better than rats, and that we easily outperform highly sensitive measuring instruments. One reason for our supposed poor sense of smell is that we walk around with our noses high in the air, while scents are at their strongest close to the ground and quickly dissipated by air currents at higher levels, as can easily be seen by watching how a tracker dog follows a trail. Moreover, despite being able to recognize many different aromas, most of us are not very good at describing this difference in words. Yet the ability to identify a wine as distinct from all others is a highly complex and demanding task, and even those of us who are untrained find no difficulty in distinguishing the scents of oranges and lemons, which are simply mirror images of the same molecule, limonene.

When you smell a rose, the scent is wafted up to your olfactory epithelium, where the many different chemicals that make up the smell bind to their receptors on different sets of olfactory neurones. Precisely how odorants stimulate their receptors is still unclear, but it appears to be due to the different sizes and shapes of the odorant molecules. One idea is that they bind to the receptor in a lock-and-key fashion. Just as your right glove will only fit your right hand, so right-handed molecules will only bind to right-handed receptors; this explains why orange and lemon (which are left- and right-handed versions of limonene) smell different. Binding of an odorant to its receptor triggers a cascade of events in the neurone that leads to opening of a specific kind of ion channel – related to, but different from, those in the rods and cones – so giving rise to a current that in turn sets up a stream of action potentials in the olfactory neurone itself. These impulses pass along the olfactory nerve to a region of the brain known as the olfactory bulb, where they hand their signals on to other nerve cells in deeper regions of the brain. One of these is the limbic system, which is involved in emotion, which explains why smells can trigger such powerful emotions and memories.

As the olfactory nerve fibres run from the nose to the brain they pass through holes in the skull that form part of the cribiform plate. Consequently, a severe jar to the head may shear the nerves against the skull, severing or damaging the nerve processes, which usually results in a permanent loss of smell and, because smell and taste are intimately linked, it can also lead to a loss of taste.

Olfactory neurones that possess different kinds of receptors are randomly distributed across the olfactory epithelium. In the brain, however, they sort themselves out, with cells that express the same type of receptor all converging on the same place. The olfactory neurones are unique among nerve cells in that they turn over very rapidly. Each lives only about sixty days and is then replaced by a new neurone that differentiates from an olfactory stem cell. To preserve the map in the brain, replacement nerve cells that bear the same kind of receptors must always find their way to the same place in the olfactory epithelium. How this complex rewiring is achieved is still a mystery.

The King of Fruits

Like most sensations, if you are exposed to a constant smell you gradually become accustomed to it and eventually no longer perceive it. Most people fail to notice their own body odour, or even the perfume they are wearing after a while. But some smells linger longer than others. The durian is revered in South-East Asia as the most delicious of fruits. It is also one of the smelliest – so pungent, in fact, that it is banned from airplanes and hotels. I once came across a durian in a Chinese market in London and having heard of its reputation as the King of Fruits, I bought it and took it back with me on the train to Oxford. During the hour-long journey, the busy rush-hour carriage slowly emptied as the distinctive smell of the durian escaped from my bag. By the time I arrived, I was sitting in solitary state and the smell was overpowering – an indescribable mixture of smelly socks and rotting food that was so disgusting I could not tolerate the idea of having it in my house and instead left it at the lab. Next morning, when I entered the room I reeled back in shock, hit by an overpowering stench. The plan had been to taste the fruit at lunchtime, but long before that the smell had crept down the corridor and penetrated to the front lobby, and people were asking ‘What’s that funny smell?’ Rapid action was needed. So, you may well ask, was it really so delicious? Alas, to me the taste was far less memorable than the stench and not particularly pleasant. I am not alone; the French naturalist Henri Mouhot remarked, ‘On first tasting it I thought it like the flesh of some animal in a state of putrefaction.’ Clearly, it is a taste that has to be acquired.