Silent signals in the deep
(appeared in May 2018)

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Not sight, sound or smell – there is yet another way animals pass messages, says S.Ananthanarayanan.

Living things communicate. They use colours of feathers, fur and petals, calls and there are scents – even plants use scents to signal a predator’s enemies when under attack. But all three involve something material, like dyes, vibrating molecules of air or chemicals. Communicating through electric fields would be different.

After the signals of light, sound or chemicals reach the body, of course, it is through electrical effects that the information is conveyed to the brain. But could the cells of the body directly receive electric fields, without an active role of the intervening media?

It turns out that there is a large group of animals that do just that. Apart from an image, sound and smell, living things also radiate electric fields. And there are animals that are able to detect the fields, a faculty called electroreception, for communication, navigation and detecting prey. As salt water is a good conductor, it is predominantly fish or water dwelling animals that have evolved the ability. Nicholas W. Bellono, Duncan B. Leitch and David Julius from the University of California, San Francisco, write in the journal, Nature, about the molecular and biophysical modifications seen in the sensory cells of sharks and skates. They report that the shark is less selective of frequencies of electric fields than the skate, reflecting its use of the signal for predation, unlike the skate, which uses electric fields for communication.

The neuron, or the nerve cell, works by moving electric charges about. The cell consists of a cell body, which is normally negatively charged, and a mechanism to hold a stock of positive potassium ions, in good numbers, and sodium ions in lesser numbers. The membrane that covers the cell is normally impervious, but has openings, some that can allow either potassium ions and some that can allow sodium ions to pass through, from time to time. The concentration of sodium and potassium ions in the medium surrounding the cell is the reverse of what it is inside, that is to say, it is rich in sodium ions and depleted in potassium ions. And the pores in the cell wall are normally closed, on account of the negative charge inside the cell. There is hence tension, of sodium ions wanting to get in and potassium ions wanting to get out.

The nerve cell also has a number of protrusions, called dendrites, that branch out and have endings that are sensitive to stimuli. These stimuli could be touch, heat, light, or electrical. When the stimulus strikes, the effect on the nerve cell is that sodium channels get opened. As there are more sodium ions outside the cell than inside, sodium ions rush in through the open channel. The entry of positive sodium ions leads to reduction of the net negative charge inside the cell, an effect called depolarisation. If the reduction goes far enough, it is like tripping a switch and many more channels open and there is a surge of sodium ions. The charge, which was about minus 75 millVolts at the starts, goes to +40 milliVolts, all in 1 millisecond.

When the charge rises like, this, the sodium channels close and potassium channels open. The rich store of potassium ions in the cell then rush out, to reduce the positive charge and bring back the starting charge condition. As there is now a change in the sodium and potassium content, there are mechanisms, which consume energy and kick in to pump out the sodium and take in the potassium, to restore the original state.

The sudden rise in charge, by over a tenth of a Volt, is called an action potential, and this passes down another protrusion of the cell, the axon. The axon forms the bulk of the length of the cell and has endings, the axon terminals, which pass signals on to muscles, glands or other neurons. Typically, the axon ends in a synapse, or the junction of two neurons. The action potential, in the presynaptic, or first, neuron, leads to the axon terminal to release an electrical or chemical signal, to the dendrites of the signal-receiving or postsynaptic, the second neuron, which is there at the synapse. The signal to the second neuron then starts of the sequence of channels opening and closing, firing of action potential, and so on, while the first neuron takes time to get restored.

While the start of the cascade, which sends signals to the brain, is a stimulus that comes to the first, peripheral and stimulus-sensitive neuron, the stimulus itself could be an electrical signal. The neuron would then need to be sensitive to electric fields. For this sensitivity to be of any use, it needs to respond to extremely weak electric fields, which are generated by the feeble electric activity in nerve cells or other processes in animals. The shark is known to be the most electrically sensitive of all animals and it can detect a field as low as 5 nano Volts per centimeter, which helps it to locate prey for the final strike, at close range. The picture shows where such sensitive electroreceptors are found in the head of the shark. It is reported that approaching a shark with any object that radiates an electric field could induce the shark to attack!

The use of electric fields for detection, like the shark does, is called passive electroreception. Another form of electroreception, which the skate, another sea animal, uses, is for communication, mainly to inform other skates that it is there. Skates then emit and receive electric signals. But to be able to be made out in the general electric noise, the skate has to modulate its electric signal, and to be sensitive to identify the modulated signal from other skates.

The work of the authors of the paper in Nature traces the differences in the way the signal receptors in the shark and the skate function and they find the differences correspond to the function that electroreception discharges in the two categories. “In the shark, electroreception may act as a threshold detector for broad frequencies, potentially reflecting its role in predation. By contrast, skate sensation appears more specifically tuned to enable the detection of signals from prey as well as frequencies in the range of conspecific electric-organ discharges (i.e. the signals from animals of the same species)”, the authors say.

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