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In Tomales Bay we have an electric ray, Torpedo californica, whose specialty is what early scientists called animal electricity. If you find a Torpedo on the beach, don’t pick it up. Some of Torpedo’s muscles are electric organs that, like car batteries, hold an electrical charge. They’re primed to shock you. Electric fishes and eels use electricity to find prey in muddy water, to stun it before they eat it, and for defense
Small to large Torpedo rays look like this: (fig 1)
Most of the body looks like a flat, soft, flabby, rounded bluish, gray or black disc with both eyes on the topside and two wings. Torpedo’s snout is short. Its body has two dorsal fins, a short stout tail and a large tail fin. Torpedo lives on sandy bottoms, near rocky reefs, and in kelp beds to depths of 3 to 30 meters. Torpedo moves slowly as he’s the toughest guy around. He can discharge more than fifty volts at a pop. The wings of Torpedo contain layers of modified muscle cells stacked like batteries in a flashlight. Each cell, called an electrocyte or an electroplax, works like a very small battery. Stacks of these cells in Torpedo discharge together to make the electricity you can feel when you grasp him.
If an object like your hand is in the water and grasps the top and bottom of Torpedo your fingers complete the circuit, and current flows through your hand. The current is large enough to penetrate your skin and excite nerves and muscles in your hand. If the object is a hand or a small fish, and is large enough, it distorts the electric voltage around the body of Torpedo, so now Torpedo can sense your presence. He thinks you are prey, and ZAP. Electric organs are useful for fishes living in murky water where vision is impaired. Eectric organs have evolved in several groups of fishes independently of each other.
In the mid-1780s, the Italian physician Luigi Galvani connected the nerves of a recently killed frog to a long metal wire and pointed the wire at the sky in a thunderstorm. With each lightning flash the frog’s legs twitched as if it were still alive. Galvani showed that intact muscle tissue responds to electricity. He reasoned the muscle twitch resembled the living frog’s movements, and that muscles and nerves use electricity to move the frog.
After touching exposed nerves to muscles or nerves to nerves, Galvani showed that the electricity came from the muscles and nerves themselves, because he got the same muscle contractions he got from lightening. He saw that no metals or external sources of electricity were needed for contractions.
Galvani inspired fellow Italian scientist Alessandro Volta, but he could not convince him. Volta in 1800 invented the first electrical battery—the voltaic pile. He soaked pieces of cardboard in brine then piled them up between disks of various metals. Volta was not convinced that the animal electricity came from the muscle tissue or nerve fibers themselves, but that animals reacted to electricity produced by two different metals used somewhere inside the frog that connected their nerves and muscles. Did Volta’s piles make him a biased observer? To see who was right and how Torpedo creates a battery and can shock you, we need to understand some basic electricity: charge separation, voltage, current and simple series circuits.
[Fig 2 ]
Charge separation and voltage
In nature, free electric charges of opposite sign, + and –, try to keep as close together as they can. Like an old couple, + and — move around holding on tightly to each other. Like an old couple, the closer the charges are to each other the more they hold on. Like an old couple, when you pull a pair apart they strain to stick together, but the further away one partner gets from the other, the less attraction each partner feels for the old charge left behind. How hard they pull on each other weakens with increased distance between them. Voltage measures the electrical force pulling separated charges together.
Voltage is strong if the charges are closely separated, but the voltage is weaker if the separated charges are held farther apart. Torpedo uses cellular energy from metabolism to separate ions. Potassium chloride (used as a salt substitute) breaks down to K+ and Cl—and table salt (what makes seawater salty) is Na+ and Cl—or sodium chloride. Each electrocyte cell keeps these ions separated by the thickness of the cell membrane. Torpedo uses both these salts in forming electrical potentials.
Stacked Electrocytes in series circuits form Torpedo’s batteries.
Remember those old Christmas tree lights: A string of bulbs with a plug for the wall socket? Do you also recall that when a single bulb went out, the whole line of bulbs went out? If you know how this string works, you know about series circuits. As an example, consider a very simple circuit consisting of four light bulbs and one battery. (The battery and the wall socket are just sources of voltage or separated charges). If the wire joins the positive end of the battery to one bulb, to the next bulb, to the next bulb, to the next bulb, then back to the negative end of the battery, in one continuous loop, the bulbs are joined in a series circuit. Electricity passes along the string as current. Electrical current is like a river. It flows from a high point to a low point. With the four light bulbs connected in series, the same current runs through all of them, but the voltage drops across each bulb. When the current re-enters the battery, the battery increases the voltage in the circuit from low to high. When one bulb goes out, the stream of current breaks, no current flows and all the bulbs go dark together.
From Bulbs to Batteries
Now do a thought experiment. We will make each light bulb in our series circuit a battery. Unlike a bulb that consumes electricity, so that each bulb next in the line receives less, if each bulb is a battery, each battery adds an additional voltage in the series. When bulbs are replaced by batteries, the voltage in the string can increase. Think of a flashlight where the batteries are in series. As you stack the batteries in the flashlight, one on top of another, the positive end of one battery contacts the negative end of the next in line. Each time you add a battery you get a brighter flashlight, because brightness is greater as the battery voltages add. Think of voltage as pressure that drives current. Now Torpedo uses batteries also to drive current through your hand.
Stacks of electrocytes are shocking.
Electrocytes are modified muscle cells that have lost their ability to contract. Each electrocyte is a little battery, and each cell by itself produces a very small voltage. Electrocytes are stacked in piles, perhaps a thousand cells in each pile. The piles of cell batteries are oriented from top to bottom in the wing. Each Torpedo has five hundred to a thousand stacks of batteries lined up close together in each wing. Five hundred to a thousand columns each, with about a thousand electrocyte cells stacked in each column, means that now when you grab TORPEDO….
Published June 27, 2014