The Spark of Life: Electricity in the Human Body

by Ashcroft, Frances

  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.

  In essence, each electroplaque behaves like a miniature living battery with the stimulated side (facing the tail) bearing a negative charge and the opposite side (facing the head) a positive charge. These tiny batteries are stacked up in a head-to-tail fashion in a long column. A simple analogy is with a battery-powered torch in which the cylindrical handle contains several batteries, stacked one on top of another (positive to negative). Their individual voltages add up to give the total needed to power the torch. In the same way, the tiny voltages produced by the individual electroplaques when they are excited
add up to give a very large voltage. The more cells in the stack, the bigger the jolt. Young eels, which have fewer electric cells per stack, can still produce a significant shock, but it is much less than their full-grown cousins. Each individual shock does not last long, as the electrical impulse at the innervated face of the electroplaque is all over within a couple of milliseconds. However, the eel produces a barrage of jolts by firing off rapid bursts of impulses in quick succession – as many as 400 a second.

  Top. The electric eel has three electric organs, but only the main one generates the large electric shock it uses to stun its prey. Middle and bottom. Two of the wafer-like electroplaques that make up one of the columns of the main organ. When a cell is at rest (inactive), its inside is negatively charged and both its external faces are positively charged; thus there is no voltage difference between the two outside faces. When the eel fires a shock (active), the voltage at the posterior face of the electroplaque becomes negatively charged, so that the voltage between the two outside faces of the cell now amounts to around 150 millivolts. The voltages from the individual electroplaques summate to deliver a substantial shock

  Although the voltage difference between one end of a stack and the other is considerable, the current that flows out of the end of the stack into the surrounding water is relatively small. This is advantageous, as it is not enough to fry the eels’ own cells. However, the currents through the whole collection of parallel stacks add up, so that the total current generated is much more – it amounts to about an amp. The space between each electroplaque is filled with a highly conductive jelly-like material, which is probably what von Humboldt found so distasteful to eat. This serves a very important function; it ensures that the current flows easily from one electroplaque to the next in the stack, and between the end of the column and the surrounding water. Equally important is that each column is well insulated along its length, in order to coax the current to flow along the column rather than leaking out sideways into the eel’s own surrounding tissues.

  It is clearly valuable to have the electroplaques as thin as possible, because the more cells that can be crammed into the column, the greater the voltage that can be developed, and the larger the shock produced. However, the thinner the cell, the more quickly it fills up with sodium ions, which enter during the electrical impulse. This creates problems because it reduces the concentration gradient that drives sodium ions to move into the cell, which means that during a train of impulses the size of the electrical impulse each cell produces steadily falls. Consequently, the magnitude of the shock, and the frequency at which it can be generated, gradually decreases and finally fails. The electric organ is then discharged – just like an overworked battery. It was this phenomenon the Indians exploited in their novel fishing technique. Recharging the electric organ takes some time and is achieved by molecular pumps that laboriously pump all the sodium ions that have entered the cell back out again, thereby restoring the sodium gradient that powers the electrical impulse.

  Zapped!

  The electric ray Torpedo uses a system similar to that of the electric eel to produce an electric shock, but with some modifications because it is a marine fish rather than a freshwater one. In freshwater, there are few dissolved salts to carry an electric current, so it does not travel very far and the eel must be close to its prey to order to stun it. Thus the eel generates a much greater voltage, which helps force the current through the water. Seawater is a far better conductor of electricity than freshwater because it contains more salts, so the magnitude of the current diminishes less rapidly with distance. The torpedo is perfectly adapted to its marine environment as it generates a higher current but a lower voltage than Electrophorus.

  The electric organs of Torpedo lie on either side of the head. The path of current flow when the electric organs discharge is shown in the cross-section through the fish on the right.

  The torpedo has two large kidney-shaped electric organs positioned one on either side of the head. Each consists of 500 to 1,000 closely packed stacks of electroplaques, and there are around 1,000 cells in each column. Because there are fewer cells per stack, Torpedo cannot generate as high a voltage as Electrophorus; the maximum shock is only about fifty volts, around a tenth that of the eel. However, the current is greater because of the much larger number of columns, so that the torpedo can produce a current as high as fifty amps and a power output of more than a kilowatt at the peak of its discharge. The fact the torpedo generates more amps and fewer volts than the electric eel is dictated by the greater conductivity of the medium in which it lives. The exigencies of marine life also explain why its electric organs are short and wide whereas those of the eel are long and thin: this is because you need many short stacks to get high current with lower volts.

  The columns of electroplaques are stacked vertically between the upper and lower surfaces of the wings of the ray. When the electric organ discharges, the current spreads out into the surrounding medium, being greatest directly above or below the electric organ. The hunting behaviour of the torpedo exploits this fact. It rests on the bottom of the sea floor until a fish comes close, whereupon it swims upward, emitting a stunning series of electric shocks and orientating itself so its prey will receive the greatest jolt. It then drops down onto the immobilized prey, wraps its wings around it and manipulates it into its mouth.

  As in Electrophorus, only the lower surface of each of the torpedo’s electroplaques is innervated. This modified muscle membrane is packed with so many acetylcholine receptors that they form a semi-crystalline array. In essence, it is one giant synapse. Excitation of the nerve supplying the electric organ releases the transmitter acetylcholine (see Chapter 4), which opens acetylcholine receptors in the electroplaque’s lower membrane and produces a potential difference between one side of the cell and the other of around 100 millivolts. This is significantly less than that produced in the electroplaques of the electric eel. Nevertheless, the main reason the torpedo generates a lower voltage is because it has fewer cells per stack. It takes a lot of energy to produce an electric shock and it cannot be maintained continuously so, like the electric eel, the torpedo produces bursts of pulses (about 100 per second), with each shock lasting just a few milliseconds.

  Why Does the Torpedo Not Shock Itself?

  Why the torpedo (or indeed the electric eel) is not incapacitated by the shock it produces is a puzzle that is still not fully understood. Current flows from one end of the electroplaque stack to the other and then out through the tissue and skin into the water. Because the electric organs sit in the wings, the current does not flow directly through the torpedo’s heart or brain. Furthermore, the current flowing through any individual part of the fish is small as each column of electroplaques produces only a small amount. The prey, however, experiences a substantial shock because the weak currents through the different columns add up to create a much greater current in the water. It is also believed that fatty layers in the fish’s skin act as an insulator to protect it from its own shocks, because if the skin is scratched or damaged (which renders this insulation less effective) an electric eel twitches when it discharges, suggesting it now feels the shock. Of course, it is also important that the skin above the electric organs is not well insulated so that the current can escape into the water and, as expected, the skin over the top and bottom of the torpedo’s electric organs is of higher conductance than that covering other areas of the body.

  Shark Attack!

  In September 1985, the telecommunications company AT&T laid an undersea fibre optic cable between Gran Canaria and Tenerife in the Canary Isles. A mere month later, the cable shorted out ten kilometres (six miles) out from Tenerife at a depth of 1,000 metres, interrupting telecommunications. AT&T were faced with the laborious, time-consuming and expensive task of raising the cable and replacing the damaged section. Mysteriously, the cable developed a similar fault twice the next year and yet again in April 1987. Careful examination of the damaged cables revealed that they were stu
dded with sharks’ teeth, suggesting that the damage was caused by a shark’s bite. The main culprit was the crocodile shark, Pseudocarcharias kamoharai, which has very powerful jaws.

  To understand what was happening, AT&T went fishing. Hundreds of sharks were caught and examined. In a bizarre experiment they even tried force-feeding one shark a sample of cable. ‘He was not happy about having someone try to shove it down his mouth,’ Mr Barrett of AT&T reported.

  Fibre optic cables are supplied with undersea repeater stations along their length that boost the optical signals. The high voltage required to power these amplifiers is supplied by a copper sheath surrounding the optical fibre core, and what seems to have happened is that the shark bit through the insulation, exposing the copper sheath to seawater. This short-circuited the power system and thereby interrupted communications.