Living Electricity Around Us

We have discussed why potential difference on the membrane of living cells appear, and have examined the process of the propagation of impulses along a nerve fiber. All the electric phenomena we are speaking about take place only on the cell membrane. But what was it that E. du Bois-Reymond registered in 1843 with the help of a simple galvanometer connected to a nerve? Since microelectrodes were not used until a hundred years later, it means that his galvanometer registered an electric field in the solution surrounding the nerve.

Examining the cable properties of a fiber, for the sake of simplicity we considered the outer solution of an electrolyte to be equipotential. Indeed, the voltage drop in the outer solution should be hundreds of times less than that inside a fiber because of much larger dimensions of the outer conductor (solution). Nevertheless, under sufficient amplification, an electric field can always be detected around an excited cell or organ, especially when all the cells of an organ are excited simultaneously. Our heart is such an organ in which all the cells are excited almost simultaneously. Like all other internal organs, it is all surrounded with electroconductive medium (the blood resistivity is ~ 100 Ohm-cm). Thus, at each excitation, the heart surrounds itself with an electric field. Therefore, a cardiologist, by measuring and analyzing the potential differences between different points of the body that appear in systole (electrocardiogram), sees how the heart works. In 1924, a Dutch physician Willem Einthoven was awarded the Nobel Prize for the introduction of the cardiograph technique into the diagnostics of cardiac diseases.

People knew for a long time that fish can be a source of electric discharges. You can see an electric catfish on ancient Egyptian tombs, and "electrotherapy" with the help of this fish was recommended by an ancient Greek physician Galen (130-200 A.D.). Another ancient doctor who treated Roman emperor Claudius (first century A.D.) prescribed electrical treatment in the following way: "A headache, even if it is chronic and unbearable, vanishes, if a live black ray is placed on a painful spot and is kept there until the pain disappears." Gout was treated in the same way: "With any type of gout, when pain starts, a live black ray should be placed under the feet. Meanwhile the patient should stand on wet sand washed by seawater and remain so until his leg below the knee goes numb." At the same time people noticed that the shock of a ray could pass through iron spears or sticks moistened with seawater and thus affect people who have no immediate contact with the ray.

Some fish are able to produce very strong discharges, immobilizing (paralyzing) other fish and even animals of a human size. Ancient Greeks, who believed that an electric ray could "enchant" both fish and fishermen, called it narke. This Greek word means a making rigid, or striking fish. The word narcotic is of the same origin.

Before the electric theory appeared, the theory that explained the shock of a ray as a mechanical effect was considered most efficient. Among the protagonists of this theory was French natural scientist R. Reaumur, whose name was given to one of the temperature scales. Reaumur assumed that a ray produces a shock by a muscle that can contract at a high rate. This is why if you touch such a muscle, a limb can go numb for some time, as it happens, for example, when you hit your elbow.

It was not until the end of the eighteenth century that the experiments which revealed the electric nature of the shock produced by a ray were carried out. A Leyden jar, the main electric capacitor of the time, played a certain part. Those who experienced the discharges of the Leyden jar and a ray claimed that the two were very similar in their effect. Like the discharge of a Ley den jar, the shock of a ray can simultaneously affect several people holding one another by hand, as long as one of the hands touches the ray.

The last doubts regarding the nature of the shock produced by a ray disappeared in 1776, when it was demonstrated that under certain circumstances, this shock could bring about an electric spark. For this purpose two metal wires with the tiniest possible air gap between them were partially submerged into a tank where the fish swam. A brief closure of the wires attracted the fish's attention, and it moved close to the wires and produced an electric shock; sometimes at this very moment sparks flashed between the wires. For the sparks to be more visible, the experiments were carried out at night. Soon after the experiments were over, some London newspapers started advertising a shake-up with a discharge of an electric fish for only two shillings and six pence. Benjamin Franklin, one of the authors of the theory of electricity, supported electric treatment. That is why the use of static electricity in medicine is still called franklinization.

By the early nineteenth century people already had known that the discharge of electric fish can pass through metals, but not through glass and air. In the eighteenth and nineteenth centuries physicists often used electric fish as a source of electric current. For example, Faraday showed while studying the discharges of an electric ray that the animal electricity did not dramatically differ from other kinds of electricity, which then were considered to be five: static (produced by rubbing), thermal, magnetic, chemical, and animal. Faraday believed that if people understood the nature of animal electricity, it would be possible "to convert the electric force into the nervous."

The strongest discharges are produced by the South American electric eel. They can be as strong as 500-600 V. The impulses of an electric ray can have voltage of up to 50 V and discharge current more than 10A so that their power often is more than 500 W. All the fish that produce electric discharges use special electric organs. In high-voltage electric fish, such as marine electric ray, freshwater electric eel, and catfish, these organs may take up a considerable part of the animal body volume. For example, in an electric eel they are located along the whole body, which is about 40% of the total volume of the fish.

The diagram of an electric organ is shown in Figure 1.11. It consists of electrocytes — much flattened cells packed into stacks. The endings of nerve fibers reach up to the membrane of one of the two flat sides of an electrocyte strength of electric field electrocytes

electrocytes

rierve innervated membrane

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