5.1.1. THE CONCEPT OF RECEPTORS

In physiology, the term "receptor" is used in two senses.

First, this sensory receptors -

specific cells tuned to the perception of various stimuli of the external and internal environment of the body and have a high sensitivity to an adequate stimulus. Sensory receptors (lat. ge-ceptum - take) perceive irritation

inhabitants of the external and internal environment of the body by converting the energy of irritation into a receptor potential, which is converted into nerve impulses. To others - inadequate stimuli - they are insensitive. Inadequate stimuli can excite receptors: for example, mechanical pressure on the eye causes a sensation of light, but the energy of an inadequate stimulus must be millions and billions of times greater than an adequate one. Sensory receptors are the first link in the reflex pathway and the peripheral part of a more complex structure - analyzers. A set of receptors, the stimulation of which leads to a change in the activity of any nerve structures, is called the receptive field. Such a structure can be an afferent fiber, an afferent neuron, a nerve center (respectively, the receptive field of an afferent fiber, neuron, reflex). The receptive field of the reflex is often called the reflexogenic zone.

Secondly, this effector receptors (cytoreceptors), which are protein structures of cell membranes, as well as cytoplasm and nuclei, capable of binding active chemical compounds (hormones, mediators, drugs, etc.) and triggering cell responses to these compounds. All cells of the body have effector receptors; in neurons there are especially many of them on the membranes of synaptic intercellular contacts. This chapter deals only with sensory receptors that provide information about the external and internal environment of the body to the central nervous system (CNS). Their activity is a necessary condition for the implementation of all the functions of the central nervous system.

5.1.2. CLASSIFICATION OF RECEPTORS

The nervous system is distinguished by a wide variety of receptors, the various types of which are shown in Fig. 5.1.

A. The central place in the classification of receptors is occupied by their division depending on the type of perceived stimulus. There are five such types of receptors.

1. Mechanoreceptors excited by mechanical deformation. They are located in the skin, blood vessels, internal organs, musculoskeletal system, auditory and vestibular systems.

2. Chemoreceptors perceive chemical changes in the external and internal

body environment. These include taste and olfactory receptors, as well as receptors that respond to changes in the composition of blood, lymph, intercellular and cerebrospinal fluid (changes in O 2 and CO 2 voltage, osmolarity, pH, glucose levels and other substances). Such receptors are found in the mucous membrane of the tongue and nose, the carotid and aortic bodies, the hypothalamus, and the medulla oblongata.

3. thermoreceptors - perceive temperature changes. They are divided into heat and cold receptors and are located in the skin, blood vessels, internal organs, hypothalamus, middle, medulla oblongata and spinal cord.

4. Photoreceptors in the retina, the eyes perceive light (electromagnetic) energy.

5. Nociceptors - their excitation is accompanied by pain sensations (pain receptors). The irritants of these receptors are mechanical, thermal and chemical (histamine, bradykinin, K + , H +, etc.) factors. Painful stimuli are perceived by free nerve endings that are found in the skin, muscles, internal organs, dentin, and blood vessels.

B. From a psychophysiological point of view Receptors are divided according to the sense organs and the sensations formed into visual, auditory, gustatory, olfactory and tactile.

B. By location in the body Receptors are divided into extero- and interoreceptors. Exteroreceptors include receptors of the skin, visible mucous membranes and sensory organs: visual, auditory, gustatory, olfactory, tactile, skin pain and temperature. Interoreceptors include receptors of internal organs (visceroreceptors), blood vessels and the central nervous system. A variety of interoreceptors are receptors of the musculoskeletal system (proprioreceptors) and vestibular receptors. If the same kind of receptors (for example, chemoreceptors for CO 2) are localized both in the central nervous system (medulla oblongata) and in other places (vessels), then such receptors are divided into central and peripheral.

D. Depending on the degree of specificity of the receptors, those. their ability to respond to one or more types of stimuli distinguish between monomodal and polymodal receptors. In principle, each receptor can respond not only to an adequate, but also to an inadequate stimulus, however,

attitude towards them is different. Receptors whose sensitivity to an adequate stimulus is much greater than that to an inadequate stimulus are called monomodal. Monomodality is especially characteristic of exteroreceptors (visual, auditory, gustatory, etc.), but there are monomodal and interoreceptors, for example, carotid sinus chemoreceptors. Polymodal receptors are adapted to the perception of several adequate stimuli, for example, mechanical and temperature or mechanical, chemical and pain. Polymodal receptors include, in particular, irritant receptors of the lungs, which perceive both mechanical (dust particles) and chemical (odorous substances) irritants in the inhaled air. The difference in sensitivity to adequate and inadequate stimuli in polymodal receptors is less pronounced than in monomodal ones.

D. By structural and functional organization distinguish between primary and secondary receptors. Primary are sensitive endings of the dendrite of the afferent neuron. The body of a neuron is usually located in the spinal ganglion or in the ganglion of the cranial nerves, in addition, for the autonomic nervous system - in the extra- and intra-organ ganglia. In the primary prescription

re stimulus acts directly on the endings of the sensory neuron (see Fig. 5.1). A characteristic feature of such a receptor is that the receptor potential generates an action potential within one cell - a sensory neuron. Primary receptors are phylogenetically more ancient structures, they include olfactory, tactile, temperature, pain receptors, proprioceptors, receptors of internal organs.

In secondary receptors there is a special cell synaptically connected to the end of the dendrite of the sensory neuron (see Fig. 5.1). This is a cell of epithelial nature or neuroectodermal (for example, photoreceptor) origin. For secondary receptors, it is characteristic that the receptor potential and the action potential arise in different cells, while the receptor potential is formed in a specialized receptor cell, and the action potential is formed at the end of the sensory neuron. Secondary receptors include auditory, vestibular, taste receptors, retinal photoreceptors.

E. According to the speed of adaptation Receptors are divided into three groups: adaptable(phase), slowly adapting(tonic) and mixed(phase-tonic), adapt-

running at medium speed. Examples of rapidly adapting receptors are the receptors for vibration (Pacini corpuscles) and touch (Meissner corpuscles) of the skin. Slowly adapting receptors include proprioceptors, lung stretch receptors, and part of pain receptors. Retinal photoreceptors and skin thermoreceptors adapt at an average speed.

5.1.3. RECEPTORS AS SENSOR TRANSDUCERS

Despite the great variety of receptors, in each of them three main stages can be distinguished in the conversion of stimulus energy into a nerve impulse.

1. Primary transformation of the energy of irritation. The specific molecular mechanisms of this process are not well understood. At this stage, the selection of stimuli occurs: the perceiving structures of the receptor interact with the stimulus to which they are evolutionarily adapted. For example, with the simultaneous action of light, sound waves, molecules of an odorous substance on the body, receptors are excited only under the action of one of the listed stimuli - an adequate stimulus that can cause conformational changes in perceiving structures (activation of the receptor protein). At this stage, in many receptors, the signal is enhanced, so the energy of the emerging receptor potential can be many times (for example, in the photoreceptor 10 5 times) greater than the threshold energy of stimulation. A possible mechanism of the receptor enhancer is a cascade of enzymatic reactions in some receptors, similar to the action of a hormone through second mediators. Repeatedly enhanced reactions of this cascade change the state of ion channels and ion currents, which forms the receptor potential.

2. Formation of receptor potential (RP). In receptors (except for photoreceptors), the energy of the stimulus, after its transformation and amplification, leads to the opening of sodium channels and the appearance of ion currents, among which the incoming sodium current plays the main role. It leads to depolarization of the receptor membrane. It is believed that in chemoreceptors, the opening of channels is associated with a change in the shape (conformation) of the gate protein molecules, and in mechanoreceptors, with membrane stretching and channel expansion. In photoreceptors, sodium

the current flows in the dark, and under the action of light, sodium channels are closed, which reduces the incoming sodium current, so the receptor potential is represented not by depolarization, but by hyperpolarization.

3. Turning RP into an action potential. The receptor potential, unlike the action potential, does not have regenerative depolarization and can only propagate electrotonically over small (up to 3 mm) distances, since in this case its amplitude decreases (attenuation). In order for information from sensory stimuli to reach the CNS, the RP must be converted into an action potential (AP). In primary and secondary receptors, this happens in different ways.

in primary receptors. the receptor zone is part of the afferent neuron - the end of its dendrite. The resulting RP, propagating electrotonically, causes depolarization in the areas of the neuron, in which the occurrence of AP is possible. In myelinated fibers, PD occurs in the nearest intercepts of Ranvier, in non-myelinated ones - in the nearest areas with a sufficient concentration of voltage-dependent sodium and potassium channels, and in short dendrites (for example, in olfactory cells) - in the axon hillock. If the membrane depolarization reaches a critical level (threshold potential), then AP is generated (Fig. 5.2).

in secondary receptors RP occurs in the epithelial receptor cell, synaptically connected with the end of the dendrite of the afferent neuron (see Fig. 5.1). The receptor potential causes the mediator to be released into the synaptic cleft. Under the influence of a mediator on the postsynaptic membrane, there is generator potential(exciting postsynaptic potential), which ensures the occurrence of AP in the nerve fiber near the postsynaptic membrane. Receptor and generator potentials are local potentials.

nerve formations that serve to convert light, mechanical, chemical, thermal energy of external and internal environmental agents into nerve impulses. Peripheral specialized parts of the analyzers, through which only a certain type of energy is transformed into the process of excitation of the nervous. Receptors vary widely in complexity of structure and level of adaptation to their function. Individual receptors are anatomically linked to each other and form receptive fields capable of overlapping.

Depending on the energy of the corresponding stimulation, receptors are divided into mechanoreceptors and chemoreceptors. Mechanoreceptors are found in the ear, vestibular apparatus, muscles, joints, skin and internal organs. Chemoreceptors serve olfactory and gustatory sensitivity; many of them are in the brain, responding to changes in the chemical composition of the body's fluid environment. Visual receptors, in essence, are also chemoreceptors. Sometimes thermoreceptors, photoreceptors, and electroreceptors are also secreted.

Depending on the position in the body and the function performed, the following are distinguished:

1) exteroceptors - this includes distant receptors that received information at a certain distance from the source of irritation - olfactory, auditory, visual, gustatory;

2) interoceptors - signal stimuli of the internal environment;

3) proprioceptors - signal the state of the body's motor system.

Receptors

Word formation. Comes from lat. receptor - receiving.

Kinds. Exteroreceptors, interoreceptors and proprioceptors are distinguished according to their location and functions.

In accordance with the nature of the perceived impact, mechano-, thermo-, photo-, chemo- and electroreceptors are distinguished.

RECEPTOR

In most general terms, a specialized nerve cell or part of it that converts physical stimuli into receptor potentials. That is, a cell that is sensitive to a certain form of stimulation and reliably undergoes a certain pattern of change. Such a definition is broad enough for everything that is discussed below and that should be attributed to receptors, (a) Peripheral cells in various sensory systems that respond to certain forms of physical energy, for example, rods and cones in the retina, hair cells in the Corti organ of the internal ears, pressure-sensitive cells in the skin, taste buds on the tongue, etc. (b) Proprioreceptors that respond to external stimulation, such as hair cells in the semicircular canals of the morning ear, stretch receptors in internal organs, kinesthetic receptors in joints and tendons, and so on. (c) Post-synaptic neurons that respond to the release of neurotransmitters in the nerve system; see receptor site here. Several receptor classification systems have been used in recent years. Some of them are based on the localization of receptors in the body, such as exteroceptors, interoceptors, and proprioceptors. Some are based on a particular modality served, such as visual receptors, auditory receptors, and so on. Some depend on determining the form of physical stimuli to which the receptors are sensitive, for example, chemical receptors such as those that serve taste and smell, mechanical receptors for pressure and hearing, light receptors in vision, temperature receptors for heat and cold, and so on. Other systems focus on neurotransmitter substances that connect nerve pathways serving a specific receptor system, for example, cholinergic receptors, paminergic receptors, etc. Note that this last topic of classification is based on an analysis of the central nervous system rather than specific sensory systems initiating neural change. It is usually clear in the context in which certain receptors are discussed which system of classification is being used.

Receptors

a peripheral specialized part of the afferent nerves that provide the perception and transformation of a certain type of energy into the process of nervous excitation. Allocate: visual receptors, auditory, olfactory, etc.

Receptor

A specialized nervous structure with a particularly high degree of irritability, capable of perceiving irritation and transforming it into a bioelectric potential - a nerve impulse. It has specificity to certain stimuli, which determines the structure of the receptor and its location (exteroreceptor, proprioreceptor, interoreceptor).

RECEPTORS

from lat. recipere - receive] - a specialized peripheral part of each analyzer: terminal formations of afferent nerve fibers that perceive irritations from the external (exteroceptors) or from the internal (interoceptors) environment of the body and convert the physical (mechanical, thermal, etc.) or chemical energy of stimuli into excitation (nerve impulses) transmitted through sensory nerve fibers to the central nervous system (see Interoceptors, Proprioceptors, Exteroceptors)

RECEPTOR

from lat. receptor - receiving) - a peripheral specialized part of the analyzer, through which only a certain type of energy is transformed into a process of nervous excitation. According to their location, R. are classified into exteroreceptors, interoreceptors, and proprioreceptors. Exteroreceptors include distant receptors that receive information at some distance from the source of irritation (olfactory, auditory, visual, and gustatory), interoreceptors signal about stimuli in the body's internal environment, and proprioceptors signal the state of the body's motor system. Separate R. are anatomically connected with each other and form the receptive fields capable to be blocked. Depending on the nature of the stimulus, mechano-, thermo-, photo-, chemo- and electroreceptors are distinguished. R. perceiving mechanical irritations make up the most extensive group. These include skin mechanoreceptors that respond to touch and pressure; R. of the inner ear, perceiving sound irritations; R. vestibular apparatus, responding to a change in the acceleration of the movement of our body, and, finally, the mechanoreceptors of blood vessels and internal organs. Thermoreceptors respond to changes in the temperature of the external and internal environment of the body; they are divided into warm and cold. Light stimuli are perceived by photoreceptors located in the retina of the eye. Chemoreceptors include R. of taste and smell, as well as interoreceptors of internal organs. All R. are distinguished by high sensitivity to adequate stimuli, characterized by the magnitude of the absolute threshold of irritation or the minimum strength of the stimulus that can bring R. into a state of excitation. However sensitivity of different R. is not identical. Thus, rods are more sensitive than cones; phase mechanoreceptors that respond to active deformation are more sensitive than static ones that respond to permanent deformation, etc. The transformation of the energy of the external world into a nervous process of spreading excitation, which carries information to the nerve centers about the action of the stimulus, is called reception. Processes of reception are subordinated to the main psychophysical law, and R.'s functions are under the regulating control from c. n. With.

Receptors

Two thousand years ago, Aristotle wrote that humans have five senses: sight, hearing, touch, smell and taste. For two millennia, scientists have repeatedly discovered the organs of new "sixth senses", for example, the vestibular apparatus or temperature receptors. These sense organs are often referred to as the "gates to the world": they allow animals to navigate in the external environment and perceive signals from their own kind. However, no less important in the life of animals is played by the "look inside oneself"; scientists have discovered a variety of receptors that measure blood pressure, blood sugar and carbon dioxide, blood osmotic pressure, muscle tension, etc. These internal receptors, signals from which, as a rule, do not reach consciousness, allow our nervous system to control a variety of processes within the body.

From what has been said, it is clear that Aristotle's classification is clearly outdated and today the number of different "senses" would be very large, especially if we consider the sense organs of various organisms that inhabit the Earth.

At the same time, as this diversity was studied, it was found that the work of all sense organs is based on one principle. External influence is received by special cells - receptors and changes the MP of these cells. This electrical signal is called the receptor potential. And then the receptor potential controls the release of the mediator from the receptor cell, or the frequency of its impulses. Thus, the receptor is a converter of external influences into electrical signals, as Volt brilliantly guessed about it.

Receptors transmit signals to the nervous system, where they are further processed.

In the old days, in production, instruments were located directly at the measurement points. For example, each steam boiler was equipped with its own thermometer and pressure gauge. However, in the future, such devices, as a rule, were replaced by sensors that convert temperature or pressure into electrical signals; these signals could be easily transmitted over a distance. Now the operator is looking at the panel, where the instruments are assembled showing temperature, pressure, turbine speed, etc., and should not go around all the units in turn. In fact, living organisms developed such a progressive system for measuring various quantities hundreds of millions of years before the advent of technology. The role of the shield, which receives all the signals, is played by the brain.

It is natural to classify various receptors according to the types of external influences perceived by them. For example, such different receptors as receptors of the organ of hearing, receptors of the organ of balance, receptors that provide touch, respond to external influences of the same type - mechanical. From this point of view, the following types of receptors can be distinguished.

1) Photoreceptors, cells that respond to electromagnetic waves, the frequency of which lies in a certain range.

2) Mechanoreceptors, cells that respond to the displacement of their parts relative to each other; mechanoreceptors, as already mentioned, include cells that perceive sounds, i.e., vibrations of water and air of a certain frequency, and tactile mechanoreceptors, and cells of the lateral line organs of fish that perceive the movement of water relative to the body of the fish, and cells that respond to muscle stretch and tendons, etc.

3) Chemoreceptors, cells that respond to certain chemicals; their activity underlies the work of the organs of smell and taste.

4) Thermoreceptors, cells that perceive temperature.

5) Electroreceptors, cells that respond to electric fields in the environment.

Perhaps we would put these five types of receptors today in the place of the five senses described by Aristotle.

Let's now consider, for example, one of the types of receptor cells - photoreceptors.

Photoreceptors

Photoreceptors in the retina of vertebrates are rods and cones. Back in 1866, the German anatomist M. Schultz discovered that in daytime birds, the retinas mainly contain cones, while nocturnal birds have rods. He concluded that rods serve to perceive weak light, and cones - strong. This conclusion was confirmed by subsequent studies. Comparison of different animals added many arguments in favor of this hypothesis: for example, deep-sea fish with their huge eyes have only rods in the retina.

Look at fig. 59. It depicts a vertebrate stick. It has an inner segment and an outer segment connected by a neck. In the region of the inner segment, the rod forms synapses and releases a mediator that acts on the retinal neurons associated with it. The mediator is released, as in other cells, during depolarization. In the outer segment there are special formations - disks, in the membrane of which rhodopsin molecules are embedded. This protein is the direct "receiver" of light.

When studying rods, it turned out that a rod can be excited by just one photon of light, that is, it has the highest possible sensitivity. When one photon is absorbed, the magnetic field of the rod changes by about 1 mV. Calculations show that for such a potential shift it is necessary to affect approximately 1000 ion channels. How can one photon affect so many channels? It was known that a photon, penetrating into a rod, is captured by a rhodopsin molecule and changes the state of this molecule.

But a single molecule is no better than a single photon. It remained completely incomprehensible how this molecule manages to change the MT of the rod, especially since the disks with rhodopsin are not electrically connected to the outer membrane of the cell.

The key to how wands work has mostly been found in the last few years. It turned out that rhodopsin, having absorbed a quantum of light, acquires for some time the properties of a catalyst and manages to change several molecules of a special protein, which, in turn, cause other biochemical reactions. Thus, the work of the rod is explained by the occurrence of a chain reaction, which starts when only one quantum of light is absorbed and leads to the appearance inside the rod of thousands of molecules of a substance that can influence the ion channels from inside the cell.

What does this intracellular mediator do? It turns out that the membrane of the inner segment of the rod is quite common - standard in its properties: it contains K-channels that create PP. But the membrane of the outer segment is unusual: it contains only Ka-channels. At rest, they are open, and although there are not very many of them, this is enough for the current flowing through them to reduce the MP, depolarizing the rod. So, the intracellular mediator is able to close part of the Ka-channels, while the load resistance increases and the MP also increases, approaching the potassium equilibrium potential. As a result, the rod becomes hyperpolarized when exposed to light.

Now take a moment to think about what you just learned and you will be surprised. It turns out that our photoreceptors release the most mediator in the dark, but when illuminated, they release it less, and the less, the brighter the light. This amazing discovery was made in 1968. Yu.A. Trifonov from the laboratory of A.L. Call, when little was known about the mechanism of the sticks.

So, here we met with another type of channels - channels controlled from inside the cell.

If we compare the photoreceptor of a vertebrate and an invertebrate animal, we will see that their work has a lot in common: there is a pigment like rhodopsin; the signal from the excited pigment is transmitted to the outer membrane with the help of an intracellular mediator; the cell is not capable of generating AP. The difference is that the intracellular mediator acts in different organisms on different ion channels: in vertebrates it causes hyperpolarization of the receptor, while in invertebrates, as a rule, it causes depolarization. For example, in a marine mollusk - a scallop - when the receptors of the distal retina are illuminated, their hyperpolarization occurs, as in vertebrates, but its mechanism is completely different. In the scallop, light increases the permeability of the membrane to potassium ions and the MP shifts closer to the equilibrium potassium potential.

However, the sign of the change in the photoreceptor potential is not too significant; it can always be changed in the course of further processing. It is only important that the light signal is reliably converted into an electrical signal.

Let's consider, for example, the further fate of the electrical signal that has arisen in the visual system of barnacles already familiar to us. In these animals, photoreceptors depolarize when illuminated and release more transmitter, but this does not cause any reaction of the animal. But when the eyes are shaded, cancer takes action: it removes the antennae, etc. How does this happen? The fact is that the mediator of photoreceptors in barnacles is inhibitory, it hyperpolarizes the next cell of the neuronal chain, and it begins to release less mediator, so when the light becomes brighter, no reaction occurs. On the contrary, when the photoreceptor is shaded, it releases less mediator and ceases to inhibit the second-order cell. Then this cell depolarizes and excites its target cell, in which impulses arise. Cell 2 in this circuit is called the I cell, from the word "inverting", since its main role is to change the sign of the photoreceptor signal. The barnacle has rather primitive eyes, and it needs a little; he leads an attached way of life and it is enough for him to know that the enemy is approaching. In other animals, the system of neurons of the second and third orders is much more complicated,

In photoreceptors, the receptor potential is transmitted further electrotonically and affects the amount of mediator released. In vertebrates or barnacles, the next cell is impulseless, and only the third neuron in the chain is capable of generating impulses. But in the stretch receptor of our muscles, the situation is completely different. This mechanoreceptor is the end of a nerve fiber that coils around a muscle fiber. When stretched, the coils of the helix formed by the non-myelinated part of the fiber move away from each other and a G-ceptor potential arises in them - depolarization due to the opening of Ka-channels that are sensitive to membrane deformation; this potential creates a current through the intercept of Ranvier of the same fiber, and the intercept generates pulses. The more the muscle is stretched, the greater the receptor potential and the higher the impulse frequency.

For this mechanoreceptor, both the transformation of external influence into an electrical signal, i.e., into a receptor potential, and the transformation of the receptor potential into impulses are realized by a section of one axon.

Of course, it would be interesting for us to talk about the structure of different receptors in different animals, because in their design and application they are very exotic; however, each such story would eventually come down to the same thing: how an external signal is converted into a receptor potential that controls the release of a neurotransmitter or causes the generation of impulses.

But we will still talk about one type of receptors. This is an electroreceptor. Its peculiarity lies in the fact that the signal to which it is necessary to respond already has an electrical nature. What does this receptor do? Converts an electrical signal to electrical?

Electroreceptors. How Sharks Use Ohm's Law and Probability

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

The amazing ability of bats to fly in complete darkness without bumping into obstacles was discovered a very long time ago, in 1793, that is, almost simultaneously with the discovery of Galvani. This was done by Lazaro Spallanzani, a professor at the University of Pavia. However, experimental proof that bats emit ultrasounds and navigate by their echo was obtained only in 1938 at Harvard University in the USA, when physicists created equipment for recording ultrasound.

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

It was a very bold hypothesis. Scientists knew that insects see ultraviolet light, and many animals hear sounds that are inaudible to us. But this was only a certain expansion of the range in the perception of signals that people can also perceive. Lissman allowed the existence of a completely new type of receptor.

The situation was complicated by the fact that the reaction of the fish to weak currents at that time was already known. She was observed back in 1917 by Parker and Van Heuser on a catfish. However, these authors gave their observations a very different explanation. They decided that when current is passed through water, the distribution of ions in it changes, and this affects the taste of water. This point of view seemed quite plausible: why invent some new organs, if the results can be explained by the known ordinary organs of taste. True, these scientists did not prove their interpretation in any way; they did not set up a control experiment. If they cut the nerves from the organs of taste, so that the taste sensations in the fish disappeared, they would find that the reaction to the current was preserved. By limiting themselves to a verbal explanation of their observations, they missed the big discovery.

Lissman, on the other hand, invented and set up a wide variety of experiments and, after ten years 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, as well as in lampreys. Approximately 5 years ago, such receptors were discovered in amphibians, and recently in mammals.

Where are electroreceptors located and how are they arranged?

Fish 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 region of the nervous system as the auditory and vestibular receptors. So the auditory receptors of bats and the electroreceptors of fish are close relatives.

In different fish, electroreceptors have different localization - they are located on the head, on the fins, along the body, as well as a different structure. Often electroreceptor cells form specialized organs. We will consider here one of these organs found in sharks and rays, the ampulla of Lorenzini. Lorenzini thought that the ampullae were glands that produced fish mucus. The ampulla of Lorenzini is a subcutaneous canal, one end of which is open to the external environment, and the other ends in a deaf extension; the channel lumen is filled with a jelly-like mass; electroreceptor cells line the “bottom” of the ampoule in one row.

It is interesting that Parker, who first noticed that fish react to weak electric currents, also studied the ampullae of Lorenzini, but attributed completely different functions to them. He found that by pressing a stick against the outer entrance of the canal, the shark could react. From such experiments, he concluded that the ampulla of Lorenzini was a manometer for measuring the depth of immersion of fish, especially since the organ was similar in structure to a manometer. But this time too, Parker's interpretation was wrong. If a shark is placed in a pressure chamber and an increased pressure is created in it, then the Lorenzini ampulla does not react to it - and this can not be seen without experimenting: water presses from all sides and there is no effect *). And with pressure only on a pore in the jelly that fills it, a potential difference arises, just as a potential difference arises in a piezoelectric crystal.

How are the ampoules of Lorenzini arranged? It turned out that all the cells of the epithelium lining the channel are firmly interconnected by special "tight contacts", which ensures a high specific resistance of the epithelium. The channel, covered with such good insulation, passes under the skin and can be several tens of centimeters long. In contrast, the jelly filling the channel of the ampulla of Lorenzini has a very low resistivity; this is ensured by the fact that ion pumps pump many K + ions into the channel lumen. Thus, the channel of an electric organ is a piece of good cable with high insulation resistance and a well-conducting core.

The “bottom” of the ampoule is covered in one layer by several tens of thousands of electroreceptor cells, which are also tightly glued together. It turns out that the receptor cell looks inside the canal at one end, and forms a synapse at the other end, where it releases an excitatory mediator that acts on the ending of the nerve fiber that approaches it. Each ampulla has 10-20 afferent fibers, and each gives many terminals that go to the receptors, so that approximately 2,000 receptor cells act on each fiber.

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

If any cell is placed in an electric field, then in one part of the membrane the sign of the GSH will coincide with the sign of the field strength, and in the other part it will turn out to be opposite. This means that on one half of the cell, the MP will increase, and on the other, on the contrary, it will decrease. It turns out that every cell "feels" electric fields, that is, it is an electroreceptor.

And it is understandable: after all, in this case, the problem of converting an external signal into a natural signal for the cell - an electrical one - disappears. Thus, electroreceptor cells work in a very simple way: with the proper sign of the external field, the synaptic membrane of these cells is depolarized, and this potential shift controls the release of the mediator.

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

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 at, firstly, catching the largest possible potential difference on the synaptic membrane, and, secondly, ensuring high sensitivity of the mediator release mechanism to changes MP.

The electrical organs of sharks and rays are extremely sensitive: fish respond to electric fields of 0.1 μV/cm. So the problem of sensitivity is solved brilliantly in nature. How are such results achieved?

Firstly, the design of the ampulla of Lorenzini contributes to providing such sensitivity. If the field strength is 0.1 μV/cm and the length of the ampoule channel is 10 cm, then the entire ampoule will have a potential difference of 1 μV. Almost all of this voltage will drop on the receptor layer, since its resistance is much higher than the resistance of the medium in the channel. The shark here directly uses Ohm's law: V \u003d $ 11, 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 applied to the electroreceptor.

Secondly, Ohm's law is "applied" by the electroreceptors themselves; different sections of their membrane also have different resistances: the synaptic membrane, where the mediator is released, has a large resistance, and the opposite section of the membrane has a small one, so that here, too, the potential difference is distributed as advantageously as possible,

As for the sensitivity of the synaptic membrane to shifts in the MF, it can be explained by various reasons: the Ca-channels of this membrane or the mechanism of mediator release itself may be highly sensitive to potential shifts. A very interesting explanation for the high sensitivity of neurotransmitter release to MP shifts was proposed by A.L. Call. His idea is that in 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 occurs: the release of the neurotransmitter causes PSP, while current flows through the synapse, and this enhances the release of the neurotransmitter. In principle, such a mechanism must necessarily operate. But even in this case, the question is quantitative: how effective is such a mechanism to play any functional role? Recently, A.L. Call and his collaborators were able to obtain convincing experimental data confirming that such a mechanism does indeed work in photoreceptors.

Noise control

So, due to various tricks using Ohm's law, a potential shift of the order of 1 μV is created on the membrane of electroreceptors. It would seem that if the sensitivity of the presynaptic membrane is high enough - and this, as we have seen, is indeed the case - then everything is in order. But we did not take into account that increasing the sensitivity of any device raises a new problem - the problem of noise control. We called the sensitivity of the electroreceptor, which perceives 1 μV, fantastic, and now we will explain why. The fact is that this value is much lower than the noise level.

In any conductor, charge carriers participate in thermal motion, i.e., they move randomly in different directions. Sometimes more charges move in one direction than in the other, which means that in any conductor without any source of e. d.s. currents occur. As applied to metals, this problem was considered as early as 1913 by de Haas and Lorentz. Experimentally, thermal noise in conductors was discovered in 1927 by Johnson. In the same year, G. Nyquist gave a detailed and general theory of this phenomenon. Theory and experiment were in good agreement: it was shown that the noise intensity depends linearly on the resistance value and on the temperature of the conductor. This is natural: the greater the resistance of the conductor, the greater the potential difference that appears on it due to random currents, and the higher the temperature, the greater the speed of charge carriers. Thus, the greater the resistance of the conductor, the greater the potential fluctuations occur in it under the action of the thermal motion of charges.

And now back to electroreceptors. We said that in order to increase the sensitivity in this receptor, it is advantageous to have the highest possible resistance of the membrane, so that most of the voltage drops across it. Indeed, the resistance of the membrane that releases the mediator is very high in an electroreceptor cell, on the order of 10 10 ohms. However, everything comes at a price: the high resistance of this membrane leads to increased noise. The potential fluctuation on the electroreceptor membrane due to thermal noise is approximately 30 μV, i.e., 30 times greater than the minimum perceived MF shift that occurs under the action of an external field! It turns out that the situation is as if you are sitting in a room where each of three dozen people are talking about their own, and you are trying to have a conversation with one of them. If the volume of all the noises is 30 times louder than the volume of your voice, then the conversation will, of course, be impossible.

How does a shark "hear" such a conversation through thermal noise? Are we dealing with a miracle? Of course not. We asked you to pay attention to the fact that approximately 2,000 electroreceptors synapse on one sensory fiber. Under the influence of thermal noises in the membrane, a neurotransmitter is released from one or another synapse, and the afferent fiber, even in the absence of electric fields outside the fish, constantly impulses. When an external signal appears, all 2,000 cells secrete a mediator, and as a result, the external signal is amplified.

Wait, the thinking reader will say, because 2,000 cells should make more noise! It turns out, if we continue the analogy with a conversation in a noisy room, that 100 people will more easily outshout a crowd of three thousand than one - thirty? But, it turns out, in reality, oddly enough, the way it is. Probably, each of us has heard more than once how rhythmic, ever-increasing claps make their way through a storm of applause. Or through the roar of the stands of the stadium, exclamations are clearly audible: “Well done! Well done!”, chanted even by a small group of fans. The fact is that in all these cases we encounter a confrontation between an organized, synchronous signal and noise, i.e., a chaotic signal. Roughly speaking, returning to the electroreceptors, their reactions to an external signal are synchronous and add up, and only some part of random thermal noise coincides in time. Therefore, the signal amplitude grows in direct proportion to the number of receptor cells, while the noise amplitude grows much more slowly. But let the reader intervene again, if the noise in the receptor is only 30 times stronger than the signal, isn't nature too wasteful? Why 2,000 receptors? Maybe a hundred would be enough?

When it comes to quantitative problems, you need to count, which means you need mathematics. In mathematics there is a special section - the theory of probability, in which random phenomena and processes of a very different nature are studied. Unfortunately, this section of mathematics is not introduced at all in a comprehensive school.

Now let's do a simple calculation. Let the external field shift the MP of all receptors by 1 μV. Then the total useful signal of all receptors will be equal to 2,000 of certain units. The average value of the noise signal of one receptor is approximately 30 μV, but the total noise signal is proportional to 2000, i.e., equal to only 1350 units. We see that due to the summation of the effect of a large number of receptors, the useful signal is 1.5 times higher than the noise. It can be seen that a hundred receptor cells cannot be dispensed with. And with a signal-to-noise ratio of 1.5, the shark's nervous system is already able to detect this signal, so no miracle happens.

We said that retinal rods respond to excitation of just one molecule of rhodopsin. But such excitation can arise not only under the action of light, but also under the action of thermal noise. As a result of the high sensitivity of the rods in the retina, "false alarm" signals should always occur. However, in reality, the retina also has a noise control system based on the same principle. The rods are interconnected by ES, which leads to the averaging of their potential shifts, so that everything happens in the same way as in electroreceptors. Also remember the association through highly permeable contacts of spontaneously active cells of the sinus node of the heart, which gives a regular heart rhythm and eliminates the fluctuations inherent in a single cell. We see that nature makes extensive use of averaging to deal with noise in different situations.

How do animals use their electroreceptors? We will talk in more detail about the method of orientation of fish in muddy water in the future. But sharks and rays use their electroreceptors when searching for prey. These predators are able to detect a flounder hidden under a layer of sand only by the electric fields generated by its muscles during respiratory movements. This ability of sharks was shown in a series of beautiful experiments performed by Kelmin in 1971. The animal can lie low and not move, it can disguise itself as a background color, but it cannot stop metabolism, stop the heartbeat, stop breathing, so smells always unmask it, and in water - and electric fields arising from the work of the heart and other muscles. So many predatory fish can be called "electric sleuths".

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