Receptors are presented. Educational portal. Auditory sensory system

There are several classifications of receptors:

By position

• Exteroceptors (exteroceptors) - located on or near the surface of the body and perceive external stimuli (signals from the environment)

  Interoreceptors (interoceptors) - located in internal organs and perceive internal stimuli (for example, information about the state of the internal environment of the body)

  • Proprioceptors (proprioceptors) are receptors of the musculoskeletal system, allowing one to determine, for example, the tension and degree of stretching of muscles and tendons. They are a type of interoreceptors.

Ability to perceive different stimuli

• Monomodal - responding to only one type of stimulus (for example, photoreceptors to light)

  Polymodal - responsive to several types of stimuli (for example, many pain receptors, as well as some invertebrate receptors that respond simultaneously to mechanical and chemical stimuli).

According to an adequate stimulus

• Chemoreceptors- perceive the effects of dissolved or volatile chemical substances.

  Osmoreceptors- perceive changes osmotic concentration liquids (usually the internal environment).

  Mechanoreceptors- perceive mechanical stimuli (touch, pressure, stretching, vibrations of water or air, etc.)

  Photoreceptors- perceive visible and ultraviolet light

  Thermoreceptors- perceive a decrease (cold) or increase (heat) in temperature

  Pain receptors, stimulation of which leads to pain. There is no such physical stimulus as pain, so separating them into a separate group based on the nature of the stimulus is to some extent arbitrary. In fact, they are high-threshold sensors of various (chemical, thermal or mechanical) damaging factors. However, a unique feature of nociceptors, which does not allow them to be classified, for example, as “high-threshold thermoreceptors,” is that many of them are polymodal: the same nerve ending can be excited in response to several different damaging stimuli .

  Electroreceptors- perceive changes in the electric field

  Magnetic receptors- perceive changes in the magnetic field

Humans have the first six types of receptors. Taste and smell are based on chemoreception, touch, hearing and balance are based on mechanoreception, as well as sensations of body position in space, and vision is based on photoreception. Thermoreceptors are found in the skin and some internal organs. Most interoreceptors trigger involuntary, and in most cases unconscious, autonomic reflexes. Thus, osmoreceptors are included in the regulation of kidney activity, chemoreceptors that perceive pH, concentration carbon dioxide and oxygen in the blood, are included in the regulation of breathing, etc.

Sometimes it is proposed to distinguish a group of electromagnetic receptors, which includes photo-, electro- and magnetoreceptors. Magnetoreceptors have not been precisely identified in any group of animals, although they are believed to be some cells in the retina of birds, and possibly a number of other cells. .

26gmanhole (lat. oculus) - touch organ(organ visual system) humans and animals, with the ability to perceive electromagnetic radiation V light wavelength range and providing the function vision. In a person through eye About 90% of information comes from the outside world .

Eye vertebrates is the peripheral part visual analyzer, in which the photo receptor perform the function neurons- photosensory cells (“neurocytes”) retina. Internal structure

1. Rear camera 2. Serrated edge 3. Eyelash ( accommodative) muscle 4. Ciliary (ciliary) girdle 5. Schlemm's canal 6. Pupil 7. Front camera 8. Cornea 9. Iris 10. Bark lens 11. Core lens 12. Ciliary process 13. Conjunctiva 14. Inferior oblique muscle 15. Inferior rectus muscle 16. Medial rectus muscle 17. Retinal arteries and veins 18. Blind spot 19. Dura mater 20. Central artery retina 21. Central vein retina 22. Optic nerve 23. Vorticose vein 24. Vagina of the eyeball 25. Yellow spot 26. Fossa fovea 27. Sclera 28. Choroid of the eye 29. Superior rectus muscle 30. Retina

The eyeball consists of membranes that surround the inner nucleus of the eye, representing its transparent contents - vitreous, lens, aqueous humor in the anterior and posterior chambers.

The nucleus of the eyeball is surrounded by three membranes: outer, middle and inner.

Outer - very dense fibrous membrane of the eyeball ( tunica fibrosa bulbi), to which they are attached extrinsic muscles of the eyeball, performs a protective function and, thanks to turgor, determines the shape of the eye. It consists of a front transparent part - cornea, and the rear opaque part is whitish in color - sclera.

The middle, or choroid, layer of the eyeball ( tunica vasculosa bulbi), plays an important role in metabolic processes, providing nutrition to the eye and removing metabolic products. It is rich in blood vessels and pigment (pigment-rich cells choroids prevent light from penetrating through the sclera, eliminating light scattering). She is educated iris, ciliary body And the choroid itself. In the center of the iris there is a round hole - the pupil, through which light rays penetrate into the eyeball and reach the retina (the size of the pupil changes as a result of the interaction of smooth muscle fibers - sphincter and dilator, contained in the iris and innervated parasympathetic And sympathetic nerves). The iris contains varying amounts of pigment, which determines its color - “ eye color».

The inner, or reticular, membrane of the eyeball ( tunica interna bulbi), - retina- the receptor part of the visual analyzer, here the direct perception of light, biochemical transformations of visual pigments, changes in the electrical properties of neurons and the transfer of information to central nervous system.

WITH functional From the point of view of the eye shell and its derivatives, they are divided into three apparatuses: refractive (light-refracting) and accommodative (adaptive), which form the optical system of the eye, and the sensory (receptive) apparatus.

Receptors

Two thousand years ago, Aristotle wrote that humans have five senses: sight, hearing, touch, smell and taste. Over two millennia, scientists have repeatedly discovered organs of new “sixth senses”, such as the vestibular apparatus or temperature receptors. These sense organs are often called “gates to the world”: they allow animals to navigate the external environment and perceive signals from their own kind. However, “looking inside oneself” also plays no less important role in the life of animals; scientists have discovered a variety of receptors that measure blood pressure, blood sugar and carbon dioxide levels, blood osmotic pressure, degree of muscle stretch, etc. These internal receptors, the signals of which, as a rule, do not reach consciousness, allow our nervous system to control a variety of processes inside 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 sensory organs of the various organisms that inhabit the Earth.

At the same time, as we studied this diversity, it was discovered that the operation of all sense organs is based on one principle. External influences are received by special cells - receptors and change 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.

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 looks at the panel, where instruments are collected that show temperature, pressure, turbine rotation speed, etc., and does not have to go around all the units in turn. In fact, living organisms have developed such a progressive measurement system different sizes hundreds of millions of years before the advent of technology. The role of the shield to which all signals are received is played by the brain.

It is natural to classify various receptors according to the types of external influences they perceive. 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 stretching and tendons, etc.

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

4) Thermoreceptors, cells that perceive temperature.

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

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

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

Photoreceptors

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

Look at fig. 59. It depicts a stick of a vertebrate animal. It has an inner segment and an outer segment connected by a neck. In the area of ​​the inner segment, the rod forms synapses and releases a transmitter that acts on the retinal neurons associated with it. The transmitter is released, as in other cells, during depolarization. In the outer segment there are special formations - disks, the membrane of which contains rhodopsin molecules. 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, i.e., it has the highest possible sensitivity. When one photon is absorbed, the rod's MP changes by approximately 1 mV. Calculations show that for such a potential shift it is necessary to influence approximately 1,000 ion channels. How can one photon affect so many channels? It was known that a photon, penetrating 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 unclear how this molecule manages to change the MP of the rods, especially since the disks with rhodopsin are not electrically connected to the outer membrane of the cell.

The answer to how sticks work has mostly been discovered in the last few years. It turned out that rhodopsin, having absorbed a quantum of light, acquires the properties of a catalyst for some time 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 is triggered by the absorption of just one quantum of light and leads to the appearance inside the rod of thousands of molecules of a substance that can influence 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 passing 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 think for a moment about what you just learned, and you will be very surprised. It turns out that our photoreceptors release the most transmitter 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 have encountered another type of channel - 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 rhodopsin-type pigment; the signal from the excited pigment is transmitted to the outer membrane using an intracellular mediator; the cell is not capable of generating AP. The difference is that the intracellular transmitter acts on different ion channels in different organisms: in vertebrates it causes hyperpolarization of the receptor, and in invertebrates, as a rule, it causes depolarization. For example, at sea ​​mollusk- 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 very significant; it can always be changed during further processing. It is only important that the light signal is reliably converted into an electrical signal.

Let us consider, as an example, the further fate of the emerging electrical signal in the visual system of barnacles already familiar to us. In these animals, the photoreceptors depolarize when illuminated and release more transmitter, but this does not cause any reaction in the animal. But when shading the eyes, the cancer takes action: removes the antennae, etc. How does this happen? The fact is that the transmitter of the photoreceptors of barnacles is inhibitory; it hyperpolarizes the next cell of the neural chain, and it begins to release less transmitter, so when the light becomes brighter, no reaction occurs. On the contrary, when the photoreceptor is shaded, it releases less transmitter 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 chain is called an 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 doesn’t need much; he leads an attached lifestyle and it is enough for him to know that the enemy is approaching. In other animals, the system of second- and third-order neurons is much more complex,

In photoreceptors, the receptor potential is transmitted further electrotonically and affects the amount of released mediator. 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 turns of the spiral formed by the non-myelinated part of the fiber move away from each other and a G-receptor potential arises in them - depolarization caused by the opening of Ka-channels, sensitive to membrane deformation; this potential creates a current flowing through the node of Ranvier of the same fiber, and the node generates impulses. The more the muscle is stretched, the greater the receptor potential and the higher the impulse frequency.

In 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 is 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 can be very exotic; however, each such story would ultimately come down to the same thing: how an external signal is converted into a receptor potential that controls the release of a transmitter or causes the generation of impulses.

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

Electroreceptors. How sharks use Ohm's law and probability theory

In 1951 The English scientist Lissman studied the behavior of the gymnarch fish. This fish lives in muddy, opaque water in the lakes and swamps of Africa and therefore cannot always use its eyesight 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 long time ago, in 1793, i.e. almost simultaneously with Galvani’s discovery. This was done by Lazaro Spallanzani, a professor at the University of Pavia. However, experimental evidence that the 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.

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

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

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 catfish. However, these authors gave a completely different explanation for their observations. They decided that when current is passed through water, the distribution of ions in it changes, and this affects the taste of the 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 perform a control experiment. If they were to cut the nerves coming from the taste organs so that the fish's taste sensations disappeared, they would find that the response to the current remained. Limiting themselves to verbal explanations of their observations, they passed by the big discovery.

Lissman, on the contrary, invented and performed many different 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 they were soon discovered in many marine and freshwater fish, as well as in lampreys. About 5 years ago, such receptors were discovered in amphibians, and more recently in mammals.

Where are the electroreceptors located and how are they structured?

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

In different fish, electroreceptors have different localizations - they are located on the head, on the fins, along the body, and also have different structures. Electroreceptor cells often form specialized organs. We will look here at one of these organs found in sharks and rays - the ampulla of Lorenzini. Lorenzini thought that the ampoules 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 blind expansion; the canal 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, studied the ampoules of Lorenzini, but attributed completely different functions to them. He discovered that by pressing a stick on the external entrance of the canal, he could provoke a reaction from the shark. From such experiments, he concluded that the ampulla of Lorenzini was a pressure gauge for measuring the diving depth of a fish, especially since the structure of the organ was similar to a pressure gauge. But this time, Parker’s interpretation turned out to be wrong. If a shark is placed in a pressure chamber and increased pressure is created in it, then the ampulla of Lorenzini does not react to it - and this can not be seen without doing an experiment: the 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, 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 connected to each other by special “tight junctions”, 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 the electric organ is a piece of good cable with a 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 channel at one end, and at the other end it forms a synapse, where it releases an excitatory transmitter that acts on the ending of the nerve fiber that approaches it. 10-20 afferent fibers approach each ampulla and each gives many terminals going to the receptors, so that as a result, approximately 2,000 receptor cells act on each fiber.

Let's now see what happens to the electro-receptor 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 sign of the GS will coincide with the sign of the field strength, and in the other it will 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 clear: in this case, the problem of converting an external signal into a natural signal for the cell - electrical - disappears. Thus, electroreceptor cells work very simply: 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 transmitter.

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

Yes, qualitatively, any neuron can be considered an electroreceptor, but if we move on 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 transmitter 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 device of the ampulla of Lorenzini contributes to ensuring 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 fall 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 = 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 resistance: the synaptic membrane, where the transmitter is released, has a high resistance, and the opposite section of the membrane has a small one, so that here the potential difference is distributed as advantageously as possible,

As for the sensitivity of the synaptic membrane to MP shifts, it can be explained by various reasons: the Ca channels of this membrane or the transmitter release mechanism itself may be highly sensitive to potential shifts. A very interesting explanation for the high sensitivity of transmitter 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 transmitter; as a result, a positive feedback occurs: the release of the transmitter causes PSP, while current flows through the synapse, and this enhances the release of the transmitter. In principle, such a mechanism must operate. But even in this case, the question is quantitative: how effective is such a mechanism to play some kind of functional role? Recently A.L. Challenge and his collaborators were able to obtain convincing experimental data confirming that such a mechanism actually operates in photoreceptors.

Fighting noise

So, due to various tricks using Ohm’s law, a potential shift of about 1 μV is created on the electroreceptor membrane. 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 causes new problem- the problem of dealing with noise. We called the sensitivity of an electroreceptor that 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, that is, they move chaotically 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 arise. In relation to metals, this problem was considered back in 1913 by de Haas and Lorentz. Thermal noise in conductors was discovered experimentally in 1927 by Johnson. In the same year, G. Nyquist gave a detailed and general theory this phenomenon. Theory and experiment agreed well: 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 difference potentials that appears on it due to randomly occurring currents, and the higher the temperature, the greater the speed of movement of charge carriers. Thus, the greater the resistance of the conductor, the greater the potential fluctuations that arise in it under the influence of thermal movement of charges.

Now let's return to electroreceptors. We said that in order to increase sensitivity in this receptor, it is advantageous to have the membrane resistance as high as possible, so that most of the voltage drops across it. Indeed, the resistance of the membrane that releases the transmitter is very high in the electroreceptor cell, about 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 perceptible MF shift that occurs under the influence of an external field! It turns out that the situation is as if you are sitting in a room where three dozen people are talking about their own things, and you are trying to carry on a conversation with one of them. If the volume of all noise is 30 times higher than the volume of your voice, then 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 synapses of approximately 2,000 electroreceptors act on one perceptive fiber. Under the influence of thermal noise in the membrane, a transmitter is released from one or another synapse and the afferent fiber, even in the absence of electric fields outside the fish, constantly pulses. When an external signal appears, all 2,000 cells release a transmitter. As a result, the external signal is enhanced.

Wait, the thinking reader will say, after all, 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 can more easily shout over a crowd of three thousand than one over thirty? But it turns out that in reality, oddly enough, this is the case. Probably, each of us has more than once heard rhythmic, ever-increasing claps breaking through the storm of applause. Or, through the roar of the stadium stands, exclamations of “Well done!” Well done!”, chanted even by a small group of fans. The fact is that in all these cases we are faced with a confrontation between an organized, synchronous signal and noise, i.e. a chaotic signal. Roughly speaking, returning to electroreceptors, their reactions to an external signal are synchronous and add up, and only some of the random thermal noise coincides in time. Therefore, the signal amplitude grows in direct proportion to the number of receptor cells, and the noise amplitude grows much more slowly. But excuse me, the reader can 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 - probability theory, in which random phenomena and processes are studied. of different nature. Unfortunately, this section of mathematics is not taught at all in secondary school.

Now let's do a simple calculation. Let external field shifted the MP of all receptors by 1 μV. Then the total useful signal of all receptors will be equal to 2,000 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 from large number receptors, the useful signal is 1.5 times greater than the noise. It is clear that it is impossible to get by with a hundred receptor cells. 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 the retinal rods react to the stimulation of just one molecule of rhodopsin. But such excitation can occur not only under the influence of light, but also under the influence of thermal noise. As a result of the high sensitivity of the rods in the retina, “false alarm” signals should constantly arise. 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 averaging of the shifts in their potential, so that everything happens in the same way as in electroreceptors. And remember the unification of spontaneously active cells of the sinus node of the heart through highly permeable contacts, 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 combat noise in different situations.

How do animals use their electroreceptors? About the method of fish orientation muddy water we'll talk more later. But sharks and rays use their electroreceptors when searching for prey. These predators are able to detect flounder hidden under a layer of sand only by the electric fields generated by its muscles during breathing movements. This ability of sharks was shown in a series of beautiful experiments performed by Kelmin in 1971. An animal can hide and not move, can camouflage itself with the background color, but it cannot stop metabolism, stop the heart, stop breathing, so it is always unmasked by odors, and in water there are also electric fields that arise during the work of the heart and other muscles. So many predatory fish can be called “electric bloodhounds”.

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1. Central nervous system

The central nervous system is part of the nervous system of vertebrates, represented by a collection of nerve cells that form the head and spinal cord.

The central nervous system regulates the processes occurring in the body and serves as the control center for all systems. The mechanisms of central nervous system activity are based on the interaction of excitation and inhibition.

Higher nervous activity (HNA)

Higher nervous activity - according to I.P. Pavlov - is a complex form of life activity that ensures individual behavioral adaptation of humans and higher animals to changing environmental conditions.

The basis of higher nervous activity is the interaction of innate unconditioned and conditioned reflexes acquired in the process of ontogenesis, to which a second signaling system is added in humans.

The structural basis of the VND is the cerebral cortex with the subcortical nuclei of the forebrain and some structures of the diencephalon.

2. Higher nervous activity

Higher nervous activity (HNA) is the activity of the higher parts of the central nervous system, ensuring the most perfect adaptation of animals and humans to the environment (behavior). The structural basis of the VNI is the cerebral cortex with the subcortical nuclei of the forebrain and the formations of the diencephalon, however, there is no strict connection of the VNI with brain structures. Lowest nervous activity represent a function of the central nervous system aimed at regulating physiological processes in the body itself. Key Feature GNI is a signaling character that allows one to prepare in advance for one or another form of activity (food, defensive, sexual, etc.)

Characteristics of VND: variability, signaling, adaptability - provide flexibility and adaptability of reactions. The probabilistic nature of the external environment gives relativity to any behavioral reaction and encourages the body to make probabilistic forecasts. The ability to learn highly depends not only on the processes of excitation, but also inhibition. Conditioned inhibition promotes a rapid change in forms of behavior in accordance with conditions and motivations.

The term GNI was introduced by I. P. Pavlov, who considered it equivalent to the concept of “mental activity.” According to I.P. Pavlov, this is a combined reflex (conditioned and unconditioned reflex) function of the cerebral cortex and the nearest subcortex of the brain. He also introduced the concept of “signal systems” as systems of conditioned reflex connections, highlighting the first signaling system common to animals and humans and the second, specific only to humans.

The first signaling system (PSS) - direct sensations and perceptions, forms the basis of the GNI and is reduced to a set of diverse conditioned and unconditioned reflexes to direct stimuli. The human PSS is characterized by a greater speed of propagation and concentration of the nervous process, its mobility, which ensures rapid switching and formation of conditioned reflexes. Animals are better at distinguishing between individual stimuli, and humans are better at distinguishing between their combinations.

The second signaling system was formed in humans on the basis of the first as a system of speech signals (pronounced, audible, visible). The words contain a generalization of the signals of the first signaling system. The process of generalization by word is developed during the formation of conditioned reflexes. Generalized reflection and abstraction are formed only in the process of communication, i.e. determined by biological and social factors.

Receptor - (from Latin recipere - to receive), nerve formations that convert chemical and physical influences from the external or internal environment of the body into nerve impulses; a peripheral specialized part of the analyzer, through which only a certain type of energy is transformed into the process of nervous excitation. Receptors vary widely in the degree of structural complexity and in the level of adaptation to their function. Depending on the energy of the corresponding stimulation, the 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 taste sensitivity: many of them are located in the brain, responding to changes chemical composition fluid environment of the body. Visual receptors are also essentially chemoreceptors. Depending on their position in the body and the function they perform, receptors are divided into exteroceptors, interoreceptors and proprioceptors. Exteroceptors include distant receptors that receive information at some distance from the source of stimulation (olfactory, auditory, visual, gustatory); interoceptors signal about stimuli from the internal environment, and proprioceptors signal about the state of the body’s motor system. Individual receptors are anatomically connected to each other and form receptive fields that can overlap.

3. Receptor

From Latin Receptum - to take

A receptor is a sensitive nerve ending or specialized cell that converts perceived stimulation into nerve impulses.

All receptors are characterized by the presence of a specific membrane region containing a receptor protein that determines reception processes. Depending on the chosen classification, receptors are divided:

For primary and secondary;

In photo-, phono-, thermo-, electro- and baro-;

On extero- and intero-;

On mechano-, photo- and chemo-;

On nociceptors, heat, cold, tactile, etc.;

For mono- and polyvalent;

For auditory, visual, olfactory, tactile and gustatory;

For contact and remote;

Into phasic, tonic and phase-tonic.

Types of receptors. Adaptation of receptor mechanisms

Adaptation of receptor mechanisms is the process of reducing (reducing) the activity of receptors as a stimulus with constant physical characteristics acts.

The nature of adaptation of receptor mechanisms depends on:

From the properties of the auxiliary apparatus;

From the characteristics of the perceiving structures of the receptor;

From the properties of the regenerative elements of the nerve ending;

For secondary sensory receptors: from the properties of the synaptic contact between the receptor cell and the ending of the sensory neuron.

Pain receptor

Nocireceptor; Nociceptor

A pain receptor is a receptor whose irritation causes pain.

Vestibuloreceptors

Acceleroceptors

Vestibuloreceptors are receptors that perceive changes in the speed and direction of body movement in space. In humans, vestibuloreceptors are represented by hair cells in the membranous labyrinth of the inner ear.

Taste buds

Taste buds are chemoreceptors, the irritation of which causes taste sensations.

Taste buds:

Localized in the oral mucosa;

They react to four types of substances: sour, salty, bitter and sweet.

Secondary sensory receptor

Non-free receptor

A secondary sensory receptor is a receptor that is a specialized cell, the excitation of which is transmitted to the endings of the corresponding afferent neuron.

Glucoreceptors

Glucoreceptors are receptors that are sensitive to changes in the concentration of glucose in the blood.

Distant receptor

Telereceptor

Distant receptor - a receptor that perceives irritations, the source of which is located at some distance from the body.

Visual tuberosities

The visual thalamus is part of the diencephalon; main subcortical centers of sensitivity. Impulses from all receptors of the body enter the visual thalamus along the ascending pathways, and from here to the cerebral cortex.

Interoreceptor

Interoceptor; Visceroreceptor; Internal receptor

From Latin Interior - internally + Capio - to take

Interoreceptor - receptor:

Located in internal organs, tissues or vessels; And

Perceiving mechanical, chemical and other changes in the internal environment of the body.

Skin receptor

Cutaneous receptor - a receptor located in the skin and providing perception of mechanical, temperature and pain stimulation.

Mechanoreceptor

A mechanoreceptor is a sensitive nerve ending that perceives mechanical influences: pressure, acceleration, etc.

Monomodal receptor

Monovalent receptor

Monomodal receptor - a receptor that perceives only one type of stimulation.

Olfactory receptors

Olfactory receptors are chemoreceptors of the mucous membrane of the upper parts of the nasal cavity, the irritation of which causes the sensation of smell.

Primary sensory receptor

Primary sensory receptor - a receptor that is a sensitive nerve ending.

Polymodal receptor

Polyvalent receptor

A polymodal receptor is a receptor that perceives several types of stimuli.

Tissue receptors

Tissue receptors are receptors located in organs and tissues outside specialized reflexogenic zones.

Tonic receptor

Tonic receptor - a thermoreceptor, retinal rod, or other slowly adapting receptor that responds in a more or less constant manner to the absolute magnitude of the stimulus.

Chemoreceptors

Chemoceptors; Chemoreceptors

Chemoreceptors are specialized sensitive cells or cellular structures through which the body of animals and humans perceives chemical stimuli, including changes in metabolism. The effect of chemicals on chemoreceptors leads to the appearance of bioelectric potentials in the chemoreceptors.

Exteroceptor

Exteroceptor; External receptor

From lat.Exter - lat + Recipere - take

Exteroceptor - a receptor localized on the surface of the body and perceiving irritations coming from the external environment. Typically, exteroceptors are specialized nerve epithelial formations.

The receptor is the working organ of the peripheral part of the sensory neuron. The body of the neuron is located in the intervertebral ganglion. The peripheral process of the pseudounipolar ganglion ends in the tissue with a receptor, while the central one enters the spinal cord and is involved in the formation of various sensory pathways.

Sensory nerve fibers are divided into branches, which are directed to different parts of the same tissue or to several different tissues. Nerve endings - receptors - can be located directly on the working structures of surrounding tissues, in such cases they are called free. Others adhere to the surface of special auxiliary cells and form non-free endings. Non-free endings can be enclosed in a more or less complex capsule consisting of auxiliary cells (encapsulated receptors). According to histologists, auxiliary cells perform the functions of supporting tissue and participate in the excitatory process.

From the point of view of functional specialization, it is customary to distinguish extero-, proprio- and interoreceptors. Exteroceptors, as the name suggests, are located on human integumentary tissues and are mostly represented by free endings. Some nerve fibers are strongly branched and form bushes, the branches of which end in fibrillar networks or thickenings among epithelial cells, while others are directed to the free surface of the epithelium without branching and even extend to its surface. The terminal sections of such receptors, together with the desquamating epithelial cells, die and are torn off, which is expressed by the increased regenerative activity of receptors of this structure. Among the specialized receptors of integumentary tissues, one should name non-free endings found in the taste organs (taste buds, bulbs, etc.), tactile Merkel corpuscles, olfactory bulbs, etc. From the point of view of acupuncture, it is important that in practical activities the receptors of the skin and mucous membranes of some parts of the body (nasal septum).

Deeper receptors lie in muscles, fascia, ligaments, periosteum, blood vessels and nerves.

The receptor for striated muscle tissue is a specialized formation of the neuromuscular spindle. It is a part of one or two to three muscle fibers up to several millimeters long, braided with branches of sensitive nerve fiber, which forms a kind of coupling around the muscle fibers. These receptors are free receptors that respond to stretching of muscle tissue.

Myocardial receptors are represented by the aforementioned muscle spindles and “climbing” nerve endings ending in wide fibrillar plates.

In the smooth muscles of various internal organs, only bush-like receptors of various shapes were found.

Receptors of connective tissue and blood vessels are the most diverse. Among them, free, non-free and encapsulated endings are distinguished. More often than others, a variety of bush-like or tree-like receptors of varying degrees of complexity are detected in the connective tissue. The characteristic form of connective tissue receptors are nerve endings in the form of “glomeruli”. The most loose “glomeruli” are penetrated by connective tissue fibers and are stretch receptors, others are relatively isolated from the surrounding tissues, acting as pressure receptors. There are also more complexly arranged nerve endings in the form of Vater-Paccini corpuscles, Krause flasks, Golgi-Mazzoni corpuscles, and Meissner corpuscles. It has been established that Vater - Paccini corpuscles are receptors for mechanical pressure, Krause flasks for temperature, Golgi - Mazzoni pressure and stretching, and Meissner tactile stimuli.

Vascular receptors are no less diverse. The vessels have abundant sensory innervation all the way from the heart to the intraorgan capillaries. The main form of receptors are bush-like endings, which can be free or non-free. They record the state of stretching of the vascular wall, the amount of blood pressure in the vessels, and the chemical composition of the blood. A characteristic feature of the receptors of intraorgan vessels is that they cover with their branches the area of ​​​​the surrounding tissue (vascular tissue receptors). Receptors of lymphatic vessels have been studied to a lesser extent; they are represented by ordinary connective tissue receptors.

Receptors of the peripheral nervous system and autonomic ganglia are varied in shape and perform the functions of general reception.

The nerve impulse generated in the receptors by the sensory fiber action potential reaches the first relay station for processing (perception) of the afferent flow in the central nervous system. The spinal cord (medulla spinalis) in adults is a cord 41–45 cm long, somewhat flattened from front to back. It has two thickenings corresponding to the nerve roots of the upper and lower extremities. Of these thickenings, the lumbar one is larger, but the cervical one is more differentiated, which is associated with the complexly organized motor skills of the hand. In functional terms, it should be emphasized that the organization of sensory complexes at the level of the cervical segments is subordinated to this basic function.

Receptors (Latin receptor - receiving, from recipio - accepting, receiving), special sensitive formations that perceive and transform irritations from the external or internal environment of the body and transmit information about the active agent to the nervous system, receptor. characterized by diversity in structural and functional terms. They can be represented by free endings of nerve fibers, endings covered with a special capsule, as well as specialized cells in complexly organized formations, such as the retina, organ of Corti, etc., consisting of many receptors.



The human body is endowed with the ability to perceive both the external and internal world, the impact of which can receive various signals. Such signals in the human body are capable of being perceived by receptors - special nerve endings.

What is a receptor and what is its purpose in the body?

Receptors are a set of nerve fiber endings that are highly sensitive and capable of perceiving many internal factors and external stimuli and converting them into a ready-made impulse for transmission to the brain. In other words, any information received by a person from the outside has the ability to be captured and correctly perceived by the human body precisely thanks to the receptors, of which there are a huge number.

Types of receptors and their classification

For each sensation, scientifically called a stimulus, there is its own type of analyzer that is capable of converting it into an impulse accessible to the nervous system. To better understand what receptors are, you first need to understand their classification.

Receptors can differ in location and type of signals received:

• exteroceptors are taste, visual, auditory and tactile receptors;
• interoreceptors - responsible for the musculoskeletal system and control internal organs.

Human receptors are also classified depending on the form of manifestation of the stimulus:

• chemoreceptors - receptors of smell, tongue and blood vessels;
• mechanoreceptors - vestibular, tactile, auditory;
• thermoreceptors - skin and internal organs receptors;
• photoreceptors - visual;
• nociceptive (pain) receptors.

Receptors are also distinguished by their ability to transmit quantitative impulses:

• monomodal - capable of transmitting only one type of stimulus (auditory, visual);
• polymodal - can perceive several types (pain receptors).

Principles of receptor functioning

Having considered the above classification, we can conclude that perception is distributed depending on the types of sensations for which there are certain sensory systems in the body that differ from each other functional features, namely:

• taste system (tongue receptors);
• olfactory system;
• visual system;
• vestibular apparatus (motor skills, movement);
• auditory sensory system (auditory receptors).

Let's look at each of these systems in more detail. This is the only way to fully understand what receptors are.

Taste sensory system

The main organ in this system is the tongue, thanks to the receptors of which the human brain is able to evaluate the quality and taste of food and drinks consumed.

The tongue contains mechanoreceptors that can evaluate the consistency of foods, thermoreceptors that determine the temperature level of food, and chemoreceptors that are directly involved in determining taste. The receptors of the tongue are located in taste buds (buds), which contain a set of proteins that change their properties upon contact with an irritant. Chemical properties, thereby forming a nerve impulse for transmission to the brain. They are able to distinguish four types of tastes:

• salty - the front part of the tongue (except for the tip);
• bitter - the back of the organ;
• sour - lateral receptors;
• sweet - receptors on the tip of the tongue.

But only in conjunction with the olfactory system is the human brain able to assess the completeness of the sensations transmitted by receptors and, if something happens, protect against unsuitable products for consumption.

Olfactory sensory system

The main organ in this system is the nose. The system got its name due to the content of the olfactory glands in which cells of the same name are formed. When reacting with a stimulus, they form olfactory filaments for transmission to the cavity of the cranium, and then to the brain. The olfactory system consists of:

• perceiver (olfactory organs);
• conduction (olfactory nerve);
• central sections (olfactory bulb).

In other words, the stimulus is captured by olfactory receptors and transmitted along the olfactory nerve to the bulb, which is connected by branches to the subcortex of the forebrain.

Visual sensory system

One of the most significant systems in a person's life and having complex structure. The main organs in the visual system are the eyes. Let's look at what eye receptors are. The retina of the eye is a center of nerve endings in which incoming signals are processed and converted into impulses ready for transmission to the brain. Signals are transmitted thanks to special cells with different functions:

• photoreceptors (cones and rods);
• ganglion cells;
• bipolar cells.

Thanks to photosensitive cells, the visual analyzer perceives color images in the daytime and at dusk at a speed of 720 m/s.

Vestibular apparatus

The receptors of this system are secondary sensory cells that do not have their own nerve endings. The transmission of impulses occurs when the position of the head or body changes in relation to the surrounding space. Thanks to the received impulses, human body able to maintain the desired body position. An important part of this system is the cerebellum, which senses vestibular afferents.

Auditory sensory system

A system that makes it possible to capture any sound vibrations. The hearing organ contains the following receptors:

• organ of Corti - perceives sound stimuli;
• receptors necessary to maintain body balance.

Auditory receptors are located in the cochlea of ​​the inner ear and perceive sound vibrations with the help of auxiliary structures.

Receptor called a specialized cell, evolutionarily adapted to the external or internal environment of a certain stimulus and to convert its energy from a physical or chemical form into a nervous form.

CLASSIFICATION OF RECEPTORS

The classification of receptors is based primarily on on the nature of sensations that arise in humans when they are irritated. Distinguish visual, auditory, olfactory, tactile receptors, thermoreceptors, proprioceptors and vestibuloreceptors (receptors for the position of the body and its parts in space). The question of the existence of special receptors .

Receptors by location divided into external , or exteroceptors, And internal , or interoreceptors. Exteroceptors include auditory, visual, olfactory, taste and tactile receptors. Interoreceptors include vestibuloreceptors and proprioceptors (receptors of the musculoskeletal system), as well as interoreceptors that signal the state of internal organs.

By the nature of contact with external environment receptors are divided into distant receiving information at a distance from the source of stimulation (visual, auditory and olfactory), and contact – excited by direct contact with a stimulus (gustatory and tactile).

Depending on the nature of the type of perceived stimulus , to which they are optimally tuned, there are five types of receptors.

• Mechanoreceptors are excited by their mechanical deformation; located in the skin, blood vessels, internal organs, musculoskeletal system, auditory and vestibular systems.
• Chemoreceptors perceive chemical changes in the external and internal environment of the body. 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 tension, osmolarity and pH, glucose levels and other substances). Such receptors are found in the mucous membrane of the tongue and nose, carotid and aortic bodies, and medulla oblongata.
• Thermoreceptors react to temperature changes. They are divided into heat and cold receptors and are found in the skin, mucous membranes, blood vessels, internal organs, hypothalamus, middle, oblongata and.
• Photoreceptors The retina of the eye perceives light (electromagnetic) energy.
• Nociceptors , the excitation of which is accompanied by painful 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, which are found in the skin, muscles, internal organs, dentin, and blood vessels. From a psychophysiological point of view, receptors are divided in accordance with the sensations formed into visual, auditory, gustatory, olfactory And tactile.

Depending on the structure of the receptors they are divided into primary , or primary sensory, which are specialized endings of the sensory, and secondary , or secondary sensory cells, which are cells of epithelial origin capable of forming a receptor potential in response to the action of adequate.

Primary sensory receptors can themselves generate action potentials in response to stimulation by an adequate stimulus if the magnitude of their receptor potential reaches a threshold value. These include olfactory receptors, most skin mechanoreceptors, thermoreceptors, pain receptors or nociceptors, proprioceptors and most interoreceptors of internal organs. The neuron body is located in the spinal cord or ganglion. In the primary receptor, the stimulus acts directly on the endings of the sensory neuron. Primary receptors are phylogenetically more ancient structures; they include olfactory, tactile, temperature, pain receptors and proprioceptors.

Secondary sensory receptors respond to the action of a stimulus only by the appearance of a receptor potential, the magnitude of which determines the amount of mediator released by these cells. With its help, secondary receptors act on the nerve endings of sensitive neurons, generating action potentials depending on the amount of mediator released from the secondary receptors. In secondary receptors there is a special cell synaptically connected to the end of the dendrite of the sensory neuron. This is a cell, such as a photoreceptor, of epithelial nature or neuroectodermal origin. Secondary receptors are represented by taste, auditory and vestibular receptors, as well as chemosensitive cells of the carotid glomerulus. Retinal photoreceptors, which have a common origin with nerve cells, are often classified as primary receptors, but their lack of ability to generate action potentials indicates their similarity to secondary receptors.

By speed of adaptation receptors are divided into three groups: quickly adaptable (phase), slow to adapt (tonic) and mixed (phasotonic), adapting at an average speed. An example of rapidly adapting receptors are the vibration (Pacini corpuscles) and touch (Meissner corpuscles) receptors on the skin. Slowly adapting receptors include proprioceptors, lung stretch receptors, and pain receptors. Retinal photoreceptors and skin thermoreceptors adapt at an average speed.

Most receptors are excited in response to stimuli of only one physical nature and therefore belong to monomodal . They can also be excited by some inappropriate stimuli, for example, photoreceptors - by strong pressure on the eyeball, and taste buds - by touching the tongue to the contacts of a galvanic battery, but it is impossible to obtain qualitatively distinguishable sensations in such cases.

Along with monomodal there are multimodal receptors, the adequate stimuli of which can be irritants of different nature. This type of receptor includes some pain receptors, or nociceptors (Latin nocens - harmful), which can be excited by mechanical, thermal and chemical stimuli. Thermoreceptors have polymodality, reacting to an increase in potassium concentration in the extracellular space in the same way as to an increase in temperature.