How sharks use Ohm's law and probability theory

In 1951, the English scientist Lissman studied the behavior of the fish of the gymnasium. This fish lives in opaque opaque water in the lakes and swamps of Africa and therefore cannot always use sight for orientation. Lissman suggested that these fish, like bats, are used for orientation echolocation.

The amazing ability of bats to fly in complete darkness, without bumping into obstacles, was discovered a long time ago, in 1793, that is, almost simultaneously with the discovery of Galvani. Did it Lazaro Spallanzani - Professor at the University of Pavia (the one where Volta worked). However, experimental evidence that bats emit ultrasounds and are guided by their echoes was obtained only in 1938 at Harvard University in the USA, when physicists created equipment for recording ultrasounds.

Having tested the ultrasonic hypothesis of the orientation of the gymnasium experimentally, Lissman rejected it. It turned out that the gymnarch is oriented somehow differently. Studying the behavior of the gymnast, Lissman found out that this fish has an electric organ and begins to generate very weak current discharges in opaque water. Such a current is not suitable for either defense or attack. Then Lissman suggested that the gymnarch should have special organs for the perception of electric fields - sensor system.

It was a very bold hypothesis. Scientists knew that insects see ultraviolet light, and many animals hear inaudible sounds for us. But this was only a slight extension of the range in the perception of signals that people can perceive. Lissman allowed an entirely new type of receptor to exist.

The situation was complicated by the fact that the reaction of fish to weak currents at that time was already known. It was observed back in 1917 by Parker and Van Heuser on the catfish (all catfish seem to have electroreceptors). However, these authors gave their observations a completely different explanation. They decided that by passing a current through the water, the ion distribution in it changes, and this affects the taste of the water. Such a point of view seemed quite plausible: why come up with some new organs, if the results can be explained by well-known ordinary organs of taste. True, these scientists did not prove their interpretation in any way; they did not set a control experiment. If they cut the nerves coming from the organs of taste, so that the taste sensations in the fish disappear, they would find that the reaction to the current persists. Having limited themselves to a verbal explanation of their observations, they passed a great discovery.

Lissman, on the contrary, came up with a variety of experiments and, after a decade of work, proved his hypothesis. About 25 years ago, the existence of electroreceptors was recognized by science. Electroreceptors began to be studied, and soon they were found in many marine and freshwater fish (sharks, stingrays, catfish, etc.), as well as lampreys. About 5 years ago, such receptors were discovered in amphibians (salamanders and axolotl), and recently - in mammals (platypuses).

Where are the electroreceptors located and how are they arranged?

Fishes (and amphibians) have lateral line mechanoreceptors located along the body and on the head of the fish; they perceive the movement of water relative to the animal. Electroreceptors are another type of lateral line receptor. During embryonic development, all lateral line receptors develop from the same area of ​​the nervous system as the auditory and vestibular receptors. So the auditory bats and fish electroreceptors are close relatives.

In different fish, electroreceptors have different localization - they are located on the head, on the fins, along the body (sometimes in several rows), as well as a different structure. Often, electroreceptor cells form specialized organs. We consider here one of such organs found in sharks and stingrays - the Lorencini ampoule (this organ was described by the Italian scientist Lorencini in 1678).

Lorencini thought that ampoules are glands that produce fish mucus (although they did not exclude other possibilities). The Lorenzini ampoule is a subcutaneous canal, one end of which is open to the external environment (its inlet is called sometimes), and the other ends with a dull extension (ampoule); the lumen of the channel is filled with a jelly-like mass; electroreceptor cells line the “bottom” of the ampoule in one row.

It is interesting (indeed, an irony of fate) that Parker, who first noticed that fish react to weak electric currents, also studied Lorenzini's ampoules, but attributed completely different functions to them. He found that by pushing the wand on the external entrance of the channel (“pore”), a shark reaction (for example, a change in the frequency of heartbeats) can be caused.

From such experiments, he concluded that the Lorenzini ampoule is a manometer for measuring the depth of immersion of fish, especially since the structure of the organ was similar to a manometer. But this time, Parker's interpretation turned out to be erroneous. If you place a shark in a pressure chamber and create increased pressure in it (simulating an increase in immersion depth), then the Lorencini ampoule does not respond to it - and this can be done without experimenting: water presses from all sides and there is no effect). And with pressure only on the pore in the jelly that fills it, a potential difference arises, similar to how a potential difference arises in a piezoelectric crystal (although the physical mechanism of the potential difference in the channel is different).

How are Lorenzini ampoules arranged? It turned out that all the cells of the epithelium lining the channel are firmly connected to each other by special "tight contacts", which provides a high specific resistance of the epithelium (about 6 MOhm-cm2). A channel coated with such good insulation extends under the skin and can be several tens of centimeters long. On the contrary, the jelly filling the channel of the Lorenzini ampoule has a very low resistivity (of the order of 30 Ohm-cm); this is ensured by the fact that ion pumps pump a lot of K + ions into the lumen of the channel (the concentration of K + in the channel is much higher than in sea water or in the blood of fish). Thus, the channel of an electric organ is a piece of a good cable with high insulation resistance and a well-conductive core.

The "bottom" of the ampoule is laid in one layer by several tens of thousands of electroreceptor cells, which are also tightly glued together. It turns out that the receptor cell at one end looks inside the channel, and at the other end forms a synapse, where it excites an exciting mediator acting on a suitable end of the nerve fiber. Each ampoule fits between 10 and 20 afferent fibers and each gives many terminals that go to the receptors, so that as a result about 2,000 receptor cells act on each fiber (pay attention to this - this is important!).

Let us now see what happens to the electroreceptor cells themselves under the influence of an electric field.

If any cell is placed in an electric field, then in one part of the membrane the PP sign coincides with the sign of the field strength, and in the other it turns out to be the opposite. This means that on one half of the cell, MP will increase (the membrane is hyperpolarized), and on the other hand, on the contrary, it will decrease (the membrane will be depolarized).

The action of the electric field on the cell

It turns out that every cell "feels" electric fields, that is, it is an electroreceptor. And it is clear: in this case, the problem of converting an external signal to a natural one for the cell - the electric one - disappears.Thus, electroreceptor cells work very simply: with the appropriate sign of the external field, the synaptic membrane of these cells is depolarized and this shift in potential controls the release of the mediator.

But then the question arises: what are the features of electro-receptor cells? Can any neuron perform their functions? What is the special arrangement of Lorenzini ampoules?

Yes, qualitatively, any neuron can be considered an electroreceptor, but if we turn to quantitative estimates, the situation changes. Natural electric fields are very weak, and all the tricks that nature uses in electrosensitive organs are aimed, firstly, to catch the largest potential difference on the synaptic membrane, and, secondly, to ensure high sensitivity of the mediator release mechanism to change MP.

The electric organs of sharks and stingrays have extremely high (we can say, fantastically high!) Sensitivity: fish react to electric fields with an intensity of 0.1 μV / cm! So the problem of sensitivity is brilliantly solved in nature. How are such results achieved?

Firstly, the device of the Lorenzini ampoule contributes to this sensitivity. If the field strength is 0.1 μV / cm and the channel length of the ampoule is 10 cm, then a potential difference of 1 μV will be necessary for the entire ampoule. Almost all of this voltage will fall on the receptor layer, since its resistance is much higher than the resistance of the medium in the channel.

The shark is directly using Ohm's law: V = IR, since the current flowing in the circuit is the same, the voltage drop is greater where the resistance is higher. Thus, the longer the ampoule channel and the lower its resistance, the greater the potential difference is supplied to the electroreceptor.

Secondly, Ohm’s law is “applied” by the electroreceptors themselves. Different sections of their membrane also have different resistance: the synaptic membrane, where the mediator stands out, has a high resistance, and the opposite part of the membrane is small, so here the potential difference is distributed more profitably.

As for the sensitivity of the synaptic membrane to MP shifts, it can be explained by various reasons: the channels of this membrane or the mediator ejection mechanism itself may have high sensitivity to potential shifts.

A very interesting version of the explanation of the high sensitivity of mediator release to MP shifts was proposed by A. L. Call. His idea is that at such synapses, the current generated by the postsynaptic membrane flows into the receptor cells and promotes the release of the mediator; as a result, a positive feedback arises: the release of the mediator causes a PSP, while the current flows through the synapse, and this enhances the release of the mediator.

In principle, such a mechanism must necessarily operate. But in this case, the question is quantitative: how effective is such a mechanism to play some kind of functional role? Recently, A. L. Vyzov and his co-workers have been able to obtain convincing experimental data confirming that such a mechanism really works in photoreceptors.