The Spark of Life: Electricity in the Human Body (16 page)

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 studded 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.

Remotely operated vehicles have filmed sharks biting electric cables, and one shark was even observed to come back for a second bite when the cable slipped from its mouth. The problem with fibre optic cables is that they are much thinner than the old-fashioned copper wire variety – often only the diameter of a garden hose (around one inch) – and thus much more vulnerable to a shark bite. Furthermore, the shark does not need to sever the cable to cause significant damage – a sharp kink is sufficient. Eventually AT&T solved the ‘Jaws problem’ by encasing the cable in two layers of steel tape and a thick polyurethane coating. They also discovered that sharks normally do not feed below about 2,000 metres, so extra protection from shark attacks is not required in deeper waters.

Electrosensory Perception

But why did the shark attack the cable? The high voltage the cable carries generates electrical and magnetic fields around and along the cable. It is presumed that the sharks were attracted by the surrounding electric field, as a shark can sense the tiny electric field caused by normal muscle activity in other organisms and so detect its prey even if it is well camouflaged. Even when access to olfactory clues is prevented, a hungry shark will find a flatfish buried in the sand. It will also become excited and ‘attack’ an artificial electric field of a similar magnitude to that generated by the breathing movements of the flatfish. A mere four microamps of current is sufficient, so it is not surprising that stray signals from underwater cables can be detected.

All organisms generate tiny electric currents when their nerves fire impulses or their muscles contract. It is not enough to stay still – breathing movements or a beating heart will give you away. As you read this, the muscles in your body are producing a background electrical crackle. Fish that live in seawater are highly sensitive to such stray electric currents because the resistance of the water is low (due to the salt it contains), so the current travels further: some fish can detect fields as small as 0.01 microvolt per centimetre (one ten-thousandth of an AA battery). A human standing still and immersed in seawater up to their neck will produce an electric field of about 0.02 microvolts per centimetre for about one metre around their body, which is easily large enough to be sensed by a shark.