The Spark of Life: Electricity in the Human Body (15 page)

All affected pigs carry the same mutation and derive from a single founder animal in which the mutation arose spontaneously. The high incidence of porcine stress syndrome in the UK arose because pigs were selectively bred for lean meat and reduced back fat, attributes that unfortunately turned out to be associated with the gene for malignant hyperthermia. Lean, heavily muscled animals are much more likely to carry the gene. The porcine stress syndrome gene has now been almost completely bred out of the UK pig population by the simple expedient of giving each pig a whiff of a general anaesthetic (e.g. halothane) to breathe. Pigs that developed muscle rigidity and a rise in body temperature of 2°C within five minutes were removed from the breeding pool.

The pigs were the key to understanding the molecular basis of the human disease. Once the cause of porcine stress syndrome had been identified, corresponding mutations were soon found in the ryanodine receptors of about a third of families who suffer from malignant hyperthermia. It is thought that in affected people, anaesthesia causes the ryanodine receptor to become unusually leaky to calcium ions. These flood out of the intracellular stores, triggering sustained muscle contractions and muscle rigidity. In turn this stimulates muscle metabolism so much that the body temperature can soar to dangerous levels.

As the disease runs in families, it is now possible to test in advance if family members are likely to develop the problem when they are anaesthetized, and means can be taken to prevent it. In addition, the drug dantrolene sodium, which blocks calcium release from intracellular stores, is kept in all operating theatres for malignant hyperthermia emergencies. It was Shirley Bryant and Keith Ellis who first established how dantrolene acts. In doing so they saved many lives as the fatality rate due to an attack fell from 80 per cent in the 1970s to less than 10 per cent today.

Bryant had a longstanding interest in electricity, having started his career with quite a jolt. While training as an engineer he helped design an artificial lightning bolt for General Electric’s exhibit at the World’s Fair, and got a 30,000 volt shock for his pains. Fortunately for those with myotonia congenita and malignant hyperthermia, he survived.

As this chapter has shown, the electrical activity of our muscle fibres provides the trigger for muscle contraction, ensuring all parts of the fibre contract simultaneously; without it, our muscles simply would not work. In some animals, however, the muscle action potential has been commandeered for a very different purpose. Modified muscle fibres that have lost the ability to contract have evolved into specialized electric organs, in which the action potentials from many cells summate to produce a substantial electric shock. The next chapter, a slight but hopefully interesting digression, illustrates how animals use the currents produced by electric organs for such diverse purposes as offence, defence, navigation and communication.

6

Les Poissons Trembleurs

Who has not heard of the invincible skill of the dread torpedo and of the powers that win it its name?

Claudian,
Carmina minora
, XLIX (XLVI)

T
he electrifying discharge of the torpedo ray has been known since antiquity. It even features in the dialogues of Plato,
¹
where Meno, perplexed by Socrates’s arguments, compares the philosopher to the fish, saying, ‘And if I may venture to make a joke, you seem to me both in your appearance and other respects to be very like that flat sea-fish, the torpedo. For this numbs those who come near it and touch it, as now you have numbed me, I think. For my mind and my tongue are paralysed, and I do not know how to answer you.’ Other classical texts report that fishermen found that their hands were paralysed when they speared a torpedo or caught one in their nets. It is from this attribute that the fish derives its scientific name – the Latin word torpere means ‘to be paralysed’ – and it is from its Greek name – narke – that we get our word ‘narcotic’. The puzzle for classical writers was that the numbing power of the fish could be experienced at a distance; it was not necessary to touch it.

Scientific investigation of electric fish began in the 1700s, when explorers returned from Africa with tales of a ‘poisson trembleur’ that made their muscles tremble intolerably when they touched it. This was the African catfish,
Malapterurus electricus
. The French naturalist Michel Adanson, who came across it when travelling in Senegal, was the first to compare the painful sensation with the jolt from a Leyden jar, and surmise that the fish could similarly deliver an electric shock.

Bas-relief from the tomb of Ti at Sakkura (
c.
2750
BC
). The fourth fish from the left just below the boat, that lies beneath the pole, is the catfish
Malapterurus electricus
. The man sitting in the boat seems to be touching another fish with whiskers, probably also a catfish – if so, he is likely to be getting quite a shock.

The catfish was well known in ancient Egypt. It graces many tomb paintings and friezes, features in a bas-relief of a fishing scene from the tomb of Ti at Sakkura that dates back to as early as 2750
BC
, and mummified catfish have even been found in the tombs of the Pharaohs. It also plays an important role in the myth of Osiris. Plutarch relates how Osiris was treacherously murdered by his brother Set and his corpse torn into fourteen pieces. His distraught wife Isis managed to retrieve all of her husband’s dismembered body except for his penis, which had been thrown into the Nile and eaten by a catfish and two other fishes. As a consequence, perhaps, ancient Egyptians scrupulously avoided eating catfish.

Strangely,
Malapterurus
was considered as a love charm by Islamic authors and as an aphrodisiac by North Africans, despite an early missionary describing it as being ‘of such a nature that no man can take it in his hand while it is alive, for it filleth the arm with paine as if every joint would go asunder’. It is no wonder it hurts, as the shock it delivers can be as much as 350 volts.

The strongest shocks of all are inflicted by the South American electric eel,
Electrophorus electricus
. Notwithstanding its common name,
Electrophorus
is not an eel but a member of the knifefish family: it simply resembles an eel. Jesuit missionaries were the first to describe it, in the sixteenth century, terming it the torpedo of the Indies. But it was not until the eighteenth century that people began to investigate its electrical nature and it was recognized that the paralysing effect of touching it was due to the fact it emitted an electric shock. Although some eels were eventually brought to the United States and to London, not everyone could afford to experiment on them as the price was as much as 50 guineas an eel, a considerable sum in those days.
²
Nor were the eels always in the best shape after their long journey. An alternative, and far more attractive, proposition for an intrepid young man was to go to the eel himself. One such was the scientist-explorer Alexander von Humboldt.

What a Stunner!

Spurred on by the desire for adventure and the wish ‘to be transported from a boring daily life to a marvellous world’, the twenty-nine-year-old von Humboldt sailed for South America on a journey of scientific discovery in 1799. His ‘Personal Narrative’ of the expedition, written on his return five years later, rapidly became a bestseller. Among others, it inspired the young Charles Darwin, who wrote that it ‘stirred up in me a burning zeal to add even the most humble contribution to the noble structure of Natural Science’.

Von Humboldt was an accomplished experimenter with an avid interest in Galvani’s work on frogs (which had been published a few years earlier). He was especially eager to obtain some electric eels, which were extremely common in the tributaries of the Orinoco River. But he found this far from easy because the fear of the eel’s shock was so extreme that he was unable to persuade the local Indians to bring him any. Promises were forthcoming, but eels were not. Nor was money sufficient inducement, for it meant little to the local tribes. Frustrated by waiting, von Humboldt set out to catch them himself, so spurring his local Indian guides to offer to help by ‘fishing with horses’. Von Humboldt wrote that ‘it was hard to imagine this way of fishing; but soon we saw our guides returning from the savannah with a troop of wild horses and mules. There were about thirty of them, and they forced them into the water.’

He paints a vivid picture of the ensuing mêlée. ‘The extraordinary noise made by the stamping of the horses made the fish jump out of the mud and attack. These livid, yellow eels, like great water snakes, swim on the water’s surface and squeeze under the bellies of the horses and mules.’ The horses, of course, endeavoured to escape, but they were prevented from doing so by the Indians, who screamed and yelled and prodded them back into the river with sharp-pointed sticks. The battle was intense. ‘The eels, dazed by the noise, defended themselves with their electrical charges. For a while it seemed they might win. Several horses collapsed from the shocks received on their most vital organs, and drowned under the water. Others, panting, their manes erect, their eyes anguished, stood up and tried to escape the storm surprising them in the water.’ Some finally made it to the bank, where they collapsed onto the sand, stunned and exhausted by the electric shocks.

Within just a few minutes the violence of the combat subsided and the battle was over. The exhausted eels drifted towards the bank and were easily caught with harpoons tied to long strings. Most of the horses survived. As von Humboldt acknowledged, those that died were unlikely to have been killed by the shock itself: they were simply stunned and then trampled underfoot by the other horses and drowned. This unique method of fishing was successful because, like an electric battery, the eel has a limited store of charge and its ability to produce electric shocks is rapidly exhausted. In the interval before it has recharged itself it can be captured without danger of electrocution.

Von Humboldt’s interest in
Electrophorus
extended beyond the scientific. He also noted that its flesh did not taste too bad, although most of the body was filled with the electrical apparatus, ‘which is slimy and disagreeable’ to eat.

A Shocking Use of Muscle Power

The electric eel can produce a powerful shock of over 500 volts and a current of one ampere, which amounts to a power output of 500 watts.
³
This is sufficient to run several lightbulbs, as one Japanese aquarium demonstrated when it wired up an electric eel to power its Christmas tree lights. It is also enough to stun, or even kill, a human or large animal. In von Humboldt’s time so many mules were slain at the ford across one river that the road had to be redirected, and even in the mid-twentieth century ranchers were losing (or thought they were losing) cattle to eels in such large numbers that they instituted ‘electric eel drives’ in which the fish were encouraged to shock themselves into exhaustion and then were killed using machetes with insulated handles.

The physiological effect of a shock from an electric eel is no different from that produced by an artificial electric current of similar magnitude. It can cause involuntary muscle contraction, paralysis of the respiratory muscles, heart failure and even death, either by electrocution or more often by drowning as a result of the victim being stunned. It can also be very painful. Von Humboldt once inadvertently stepped on a large excited eel that had just been taken from the water almost fully charged. He described the pain and numbness as extreme, complaining that ‘All day I felt strong pain in my knees and in all my joints’, accompanied by twitching of the tendons and muscles (hence the Spanish name of the fish, tembladores). It is perhaps not surprising that the Indians of the llanos feared them.

Electric eels have no teeth and must swallow their prey in one gulp, which is obviously harder if it is wriggling and may be why they generate electric shocks to stun their prey. Much of the time they lurk in the mud on the river bottom, but as they get most of their oxygen by gulping air they must surface every few minutes or so to breathe. Because they breathe air they do not die if they are removed from the water and so can be easily studied. I vividly remember visiting a lab that worked on electric eels many years ago and being taken to see the fish. Before entering the room I was required to don rubber gloves that came up to my armpits, in case a fish leapt out of its tank and I inadvertently came into contact with it. It made quite an impression.

Left
. Volta’s electric battery, consisting of stacks of silver (A) and zinc (Z) discs.
Right
. Cross-section through the body of the torpedo ray showing the columns of electroplaques (H indicates one stack). The resemblance is remarkable.

Electrophorus
has a long, cylindrical, eel-like body, with a dark-grey back and yellowish belly, and it can reach an enormous size. Larger specimens weigh over twenty kilograms, exceed two and a half metres in length and are as thick as a man’s thigh. The vital organs are crammed into the front one-fifth of the body: the rest of the fish houses the backbone and swimming muscles, but most is pure power pack. The main electric organs lie on either side of the eel’s body. Each contains thousands of modified muscle cells, known as electroplaques, which have lost the capacity to contract and are specialized for producing an electric discharge. These wafer-thin, plate-like cells are stacked up in long columns, like a giant pile of coins, with as many as 5,000 to 10,000 cells per column. There are around seventy such columns on each side of the eel’s body. Each stack of electroplaques bears a strong similarity to a voltaic pile – the primitive battery discussed in Chapter 1 – a fact which Volta himself noted.

Throwing the Switch

The two faces of the electroplaque cell are markedly different. One side is smooth and criss-crossed by many nerve endings: the other is deeply invaginated and is not innervated. At rest there is no difference in voltage between the two outer faces of the cell and thus no shock is produced. When the fish decides to zap its prey, it fires off an impulse down the nerve supplying the electric organ. This triggers an electrical impulse in the electroplaque – in effect a muscle action potential – that is confined to the innervated side. As a consequence, a voltage difference develops across the two sides of the cell of as much as 150 millivolts. Because this happens simultaneously in all electroplaques, and because they are arranged in series, the voltages add up to produce a considerable jolt of 500 volts or more (about four times as much as a household electrical socket in the USA and twice as much as one in Europe). Thousands of muscle action potentials, all firing in synchrony, thus underlie the shock.