Receptors are called special formations that perceive and convert the energy of external irritation into the specific energy of a nerve impulse.

All receptors are divided into exteroreceptors, receiving stimuli from the external environment (receptors of the organs of hearing, vision, smell, taste, touch), interoreceptors responsive to stimuli from internal organs, and proprioreceptors, perceiving irritations from the motor apparatus (muscles, tendons, articular bags).

Depending on the nature of the stimulus to which they are tuned, distinguish chemoreceptors(receptors of taste and smell, chemoreceptors of blood vessels and internal organs), mechanoreceptors (proprioreceptors of the motor sensory system, baroreceptors of blood vessels, receptors of the auditory, vestibular, tactile and pain sensory systems), photoreceptors (receptors of the visual sensory system) and thermoreceptors (receptors of the sensory system of the skin and internal organs).

By the nature of the connection with the stimulus, distant receptors are distinguished that respond to signals from distant sources and cause warning reactions of the body (visual and auditory), and contact receptors that receive direct influences (tactile, etc.).

According to structural features, primary (primary-sensing) and secondary (secondary-sensing) receptors are distinguished.

Primary receptors are the endings of sensitive bipolar cells, the body of which is outside the CNS, one process approaches the surface perceiving irritation, and the other goes to the CNS (for example, proprioreceptors, tactile and olfactory receptors).

Secondary receptors are represented by specialized receptor cells that are located between the sensitive neuron and the point of application of the stimulus. These include receptors for taste, vision, hearing, and the vestibular apparatus. In practical terms, the most important is the psychophysiological classification of receptors according to the nature of the sensations that arise when they are stimulated. According to this classification, a person distinguishes between visual, auditory, olfactory, taste, tactile receptors, thermoreceptors, receptors for the position of the body and its parts in space (proprio- and vestibular receptors) and skin receptors.

The mechanism of excitation of receptors . In primary receptors, the energy of an external stimulus is directly converted into a nerve impulse in the most sensitive neuron. In the peripheral ending of sensitive neurons, under the action of a stimulus, a change in the permeability of the membrane for certain ions and its depolarization occurs, local excitation occurs - the receptor potential , which, having reached a threshold value, causes the appearance of an action potential propagating along the nerve fiber to the nerve centers.

In secondary receptors, the stimulus causes the appearance of a receptor potential in the receptor cell. Its excitation leads to the release of the mediator in the presynaptic part of the contact of the receptor cell with the fiber of the sensitive neuron. Local excitation of this fiber is reflected by the appearance of an excitatory postsynaptic potential (EPSP), or the so-called generator potential . When the threshold of excitability is reached in the fiber of the sensitive neuron, an action potential arises that carries information to the CNS. Thus, in secondary receptors, one cell converts the energy of an external stimulus into a receptor potential, and the other into a generator potential and an action potential. The postsynaptic potential of the first sensitive neuron is called the generator potential and it leads to the generation of nerve impulses.

4. Receptor Properties

1. The main property of receptors is their selective sensitivity to adequate stimuli, to the perception of which they are evolutionarily adapted (light for photoreceptors, sound for receptors of the cochlea of ​​the inner ear, etc.). Most receptors are tuned to perceive one type (modality) of stimulus - light, sound, etc. The sensitivity of the receptors to such specific stimuli is extremely high. The excitability of the receptor is measured by the minimum value of the energy of an adequate stimulus, which is necessary for the occurrence of excitation, i.e. excitation threshold .

2. Another property of receptors is the very low value of thresholds for adequate stimuli. . For example, in the visual sensory system, photoreceptors can be excited by a single quantum of light in the visible part of the spectrum, olfactory receptors can be excited by the action of single molecules of odorous substances, etc. Excitation of receptors can also occur under the action of inadequate stimuli (for example, the sensation of light in the visual sensory system during mechanical and electrical stimuli). However, in this case, the excitation thresholds are much higher.

Distinguish between absolute and differential ( differential ) rapids . Absolute thresholds are measured by the minimum perceived magnitude of the stimulus. Differential thresholds represent the minimum difference between two stimulus intensities that is still perceived by the body (differences in color shades, light brightness, degree of muscle tension, articular angles, etc.).

3. The fundamental property of all living things is adaptation , those. adaptability to environmental conditions. Adaptation processes cover not only receptors, but also all links of sensory
systems.

Adaptation consists in the adaptation of all parts of the sensory system to a long-acting stimulus, and it manifests itself in a decrease in the absolute sensitivity of the sensory system. Subjectively, adaptation manifests itself in getting used to the action of a constant stimulus: entering a smoky room, a person stops smelling smoke after a few minutes; a person does not feel the constant pressure of his clothes on the skin, does not notice the continuous ticking of the clock, etc.

According to the rate of adaptation to prolonged stimuli, receptors are divided into quickly and slowly adapting. . The former, after the development of the adaptation process, practically do not inform the next neuron about the ongoing stimulation, in the latter this information is transmitted, although in a significantly reduced form (for example, , so-called secondary endings in muscle spindles , which inform the central nervous system about static stresses).

Adaptation can be accompanied by both a decrease and an increase in the excitability of receptors. So, when moving from a bright room to a dark one, there is a gradual increase in the excitability of the photoreceptors of the eye, and a person begins to distinguish dimly lit objects - this is the so-called dark adaptation. However, such a high excitability of the receptors turns out to be excessive when moving into a brightly lit room (“the light hurts the eyes”). Under these conditions, the excitability of photoreceptors rapidly decreases - light adaptation occurs. .

For optimal perception of external signals, the nervous system finely regulates the sensitivity of receptors depending on the needs of the moment through efferent regulation of receptors. In particular, during the transition from a state of rest to muscular work, the sensitivity of the receptors of the motor apparatus increases markedly. , which facilitates the perception of information about the state of the support - locomotive system ( gamma - regulation ) . The mechanisms of adaptation to different stimulus intensities can affect not only the receptors themselves, but also other formations in the sense organs. For example, when adapting to different sound intensities, there is a change in the mobility of the auditory ossicles (hammer, anvil and stirrup) in the human middle ear.

5. Information encoding

The amplitude and duration of individual nerve impulses (action potentials) coming from the receptors to the centers remain constant under different stimuli. However, the receptors transmit adequate information to the nerve centers not only about the nature, but also about the strength of the acting stimulus. Information about changes in the intensity of the stimulus is encoded (transformed into the form of a nerve impulse code) in two ways:

■ change in pulse frequency, going along each of the nerve fibers from the receptors to the nerve centers;

■ change in the number and distribution of impulses- their number in a pack (portion), intervals between packs, the duration of individual bursts of impulses, the number of simultaneously excited receptors and the corresponding nerve fibers (a diverse space-time picture of this impulsation, rich in information, is called a pattern).

The greater the intensity of the stimulus, the greater the frequency of afferent nerve impulses and their number. This is due to the fact that an increase in the strength of the stimulus leads to an increase in the depolarization of the receptor membrane, which, in turn, causes an increase in the amplitude of the generator potential and an increase in the frequency of impulses arising in the nerve fiber. There is a directly proportional relationship between the strength of stimulation and the number of nerve impulses.

There is another possibility of encoding sensory information. The selective sensitivity of receptors to adequate stimuli already makes it possible to separate different types of energy acting on the body. However, even within the same sensory system, there may be different sensitivity of individual receptors to stimuli of the same modality with different characteristics (distinguishing taste characteristics by different taste receptors of the tongue, color discrimination by different photoreceptors of the eye, etc.).

Receptors, their classification. The mechanism of excitation in receptors. Receptor and generator potential.

Receptor - This is a specialized structure that perceives stimuli from the external or internal environment of the body and transforms their energy into a bioelectric potential. The receptor may be highly sensitive ending of a sensory neuron (for example, thermoreceptors, chemoreceptors, mechanoreceptors, etc.). The receptor may be special specialized cell , which, on the one hand, is in contact with the stimulus, and on the other hand, with a sensory neuron (for example, hair cells of the organ of Corti or photoreceptors of the retina).

functional (physiological) classifications of receptors.

In relation to stimuli coming from the external or internal environment:

a) exteroreceptors- perceive stimuli from the external environment;

b) interoreceptors- perceive stimuli from within the body. They are also called visceroreceptors. They are located in the internal organs, excretory ducts, vessels, etc.

Separately allocate proprioreceptors and vestibuloreceptors:

Proprioceptors - found in muscles, tendons and ligaments. They perceive changes in the state of the musculoskeletal system resulting from active and passive movements.

vestibuloreceptors - located in the inner ear, are an integral part of the vestibular apparatus and respond to changes in the position of the head and the whole body in space.

By the nature of an adequate stimulus:

a) mechanoreceptors- respond to mechanical stress;

b) chemoreceptors- react to chemicals of varying degrees of complexity;

c) photoreceptors- react to light quanta;

d) thermoreceptors- react to the absolute value of temperature in the internal or external environment, as well as to its change;

e) osmoreceptors- respond to osmotic pressure

(in blood, tissue fluid, cerebrospinal fluid).

According to the nature of subjective sensations:

a) visual(sensation of light);

b) auditory(sensation of sound);

c) taste(sense of taste);

d) olfactory(sensation of smell);

e) tactile(sensation of touch);

f) temperature(sensation of heat and cold);

g) vestibular(sensation of the position and movement of the body in space);

h) proprioceptors(sensation of movement, vibration, position of the body in space)

i) nocireceptors(feeling of pain).

According to the place of occurrence of excitation:

a) primary sentient(primary) - they have a receptor potential and an action potential (see questions 3,4) arise on the same sensory neuron, only in its different parts. For example, in the Pacinian corpuscle, which responds to pressure or vibration, the receptor potential occurs on the receptor membrane, which does not have fast sodium channels. (see question 5), and the action potential - on the electro-excitable membrane, which is a continuation of the receptor

b) secondarily sentient(secondary) - in them, the receptor potential and the action potential arise in different cells: the receptor potential - in a special receptor cell, and the action potential - in the sensory neuron. For example, in the visual analyzer, the receptor potential occurs in rods or cones, and the action potential in ganglion cells, the processes of which form the optic nerve (Fig. 2B). Moreover, between the receptor cell and the ganglion neuron there are bipolar neurons, in which the generator potential arises (see question 7);

According to the degree of excitability:

a) low threshold(have a higher excitability);

b) high threshold(have lower excitability).

By the number of perceived modalities:

(see classification of neurons)

a) monomodal;

b) polymodal.

By the number of perceived valences:

(see classification of neurons)

a) monovalent;

b) polyvalent.

According to the ability to adapt:

a) quickly adaptable(Fig. 3A);

b) slowly adapting(Fig. 3B);

c) non-adaptable(Fig. 3B).

The mechanism of the occurrence of excitation in the primary sensory receptors resembles the mechanism of the occurrence of excitation on the postsynaptic membrane of a chemical synapse ) and consists of the following. First, under the action of an irritant, a receptor potential (RP) arises on the receptor membrane. Since RP is always a decrease in the degree of polarization of the membrane (hyperpolarization RP does not give excitation), then local currents arise between the partially depolarized receptor membrane and the neighboring section of the electrically excitable membrane, which depolarize the electrically excitable membrane to a critical level, and therefore lead to the occurrence of AP.

The surface cell membrane does not have "fast" (electrically excitable) sodium channels. Therefore, a recharge of the surface membrane cannot occur here, but a change in the resting membrane potential under the action of stimuli is possible. This change in resting membrane potential is called receptor potential (RP).

In most receptor formations, the origin of RP is due to the fact that under the action of an adequate stimulus on the receptor membrane, the permeability for sodium ions increases, which through the opening “slow” (chemo-excitable, mechano-excitable, etc.) channels penetrate the concentration gradient into the cell and depolarize the surface cell membrane . The degree of this depolarization (RP amplitude) depends on the strength of the stimulus, that is, the higher the strength of the stimulus, the greater the depolarization of the membrane. This depolarization is local and does not extend to neighboring areas (since there are no electrically excitable channels here). Thus, RP is essentially a local or gradual response and manifests itself in local membrane depolarization.

In rods and cones (visual analyzer), in response to the action of a light quantum, hyperpolarization of the surface cell membrane occurs. Hyperpolarization RP can also occur in the vestibuloreceptors of the vestibule of the cochlea and ampullae of the semicircular canals.

generator called the potential, which is the cause of the excitation in the receptor. Therefore, the receptor potential is sometimes called the generator potential. But more often, the generator is called the potential arising in the secondary-sensing receptors on the cell, which is located after the receptor. This cell receives information from the receptor cell (in the form of a portion of the mediator) and, as a result, changes its membrane potential (Fig. 2B). This change in the MPP is called the generator potential (GP). In turn, HP is the cause of the occurrence of AP on the next in this chain - the nerve cell (that is, it generates AP). For example, in the visual analyzer, HP occurs on a bipolar neuron, which is depolarized due to a mediator released from a rod or cone. In turn, the bipolar neuron, when HP occurs, also releases a mediator, due to which excitation occurs on the ganglionic neuron. Further, the excitation along the axon of the ganglion cell, as part of the optic nerve, spreads through the conductive section of the visual analyzer.

Synapses, their structure, classification and functional properties. Features of the transfer of excitation in them. Mechanism of EPSP formation. The concept of electrical synapses and the characteristics of the transmission of excitation in them.

The concept of synapse was introduced into physiology by the famous English physiologist Charles Sherrington (1897) to denote the functional contact between neurons. A synapse is a specialized intercellular contact designed to transfer information from a neuron to any other excitable cell (nerve, muscle or glandular).

There are several principles according to which the same synapses can be classified in different ways.

By type of connected cells:

a) interneuronal- provide communication between neurons located both in the CNS itself and outside it;

b) neuroeffector- provide a connection between the neuron and the effector cell (muscle or secretory);

c) neuroreceptor- provide a connection between the neuron and the receptor of the sensory neuron (thus, control over the work of the receptors is ensured, that is, their excitability is modulated).

By location:

a) central-located in the CNS

b) peripheral- located outside the central nervous system (myoneural, ganglionic, etc.).

By functional effect:

a) exciting- transmit excitation to the postsynaptic structure;

b) brake- prevent the transmission of excitation to the postsynaptic structure.

According to the mechanism of excitation transfer:

A) chemical

Classification of receptors and mechanisms of their excitation

Receptors are called special formations that transform (convert) the energy of external irritation into the specific energy of a nerve impulse.

All receptors according to the nature of the perceived environment are divided into exteroreceptors, interoreceptors and proprioreceptors. Exteroreceptors receive stimuli from the external environment (receptors of the organs of hearing, sight, smell, taste, touch). Interoreceptors respond to stimuli from internal organs. Proprioceptors perceive irritations from the motor apparatus (muscles, tendons, articular bags).

According to the type of perceived stimuli, chemoreceptors are distinguished (receptors of the taste and olfactory sensory systems, chemoreceptors of blood vessels and internal organs); mechanoreceptors (proprioreceptors of the motor sensory system, baroreceptors of blood vessels, receptors of the auditory, vestibular, tactile and pain sensory systems); photoreceptors (receptors of the visual sensory system) and thermoreceptors (receptors of the temperature sensory system of the skin and internal organs).

By the nature of the connection with the stimulus, there are distant receptors that respond to signals from distant sources and cause warning reactions of the body (visual and auditory) and contact receptors that receive direct effects (tactile, etc.)

According to structural features, primary and secondary receptors are distinguished. Primary receptors are the endings of sensitive bipolar cells, the body of which is outside the CNS, one process approaches the surface that perceives irritation, and the other goes to the CNS (for example, proprioreceptors, thermoreceptors, olfactory cells). Secondary receptors are specialized receptor cells that are located between the sensitive neuron and the point of application of the stimulus (for example, eye photoreceptors).

In primary receptors, the energy of an external stimulus is directly converted into a nerve impulse in the same cell. In the peripheral ending of sensitive cells, under the action of a stimulus, an increase in membrane permeability and its depolarization occurs, local excitation occurs - a receptor potential, which, having reached a threshold value, causes the appearance of an action potential propagating along the nerve fiber to the nerve centers.

In secondary receptors, the stimulus causes the appearance of a receptor potential in the receptor cell. Its excitation leads to the release of the mediator in the presynaptic part of the contact of the receptor cell with the fiber of the sensitive neuron. Local excitation of this fiber is reflected by the appearance of an excitatory postsynaptic potential or the so-called generator potential. When the threshold of excitability is reached in the fiber of the sensitive neuron, an action potential arises that carries information to the CNS. Thus, in secondary receptors, one cell converts the energy of an external stimulus into a receptor potential, and the other into a generator potential and an action potential.

In the primary receptors, under the action of an stimulus, it interacts with the receptor protein of the membrane of the endings of the nerve sensory cell. As a result, a receptor potential (RP) arises in the cell, which has all the properties of a local potential. It is simultaneously a generator potential (GP), since PD arises on its basis.

In secondary receptors, this process is somewhat more complicated. The stimulus interacts with the membrane of a specialized (non-nervous) receptor cell. In response to this, RP occurs, which leads to the release of the mediator from the presynaptic membrane of the receptor cell. The mediator affects the ending of the nerve cell, depolarizing it. This leads to the appearance of GP in the nerve cell, which, when a critical level of depolarization is reached, turns into AP. It should be noted that a person does not have receptors for some types of energy, for example, for X-ray and ultraviolet radiation.

Wired Sensor Systems Department

The AP that has arisen spreads along the nerve fibers along the sensory pathways to the areas lying above. There are the following types of paths.

1. Specific paths - which carry information from receptors through various levels of the central nervous system to specific nuclei of the thalamus, and from them to specific centers of the cortex - projection areas. An exception is the olfactory pathway, the fibers of which pass through the thalamus. These pathways provide information about the physical parameters of stimuli.

2. Associative thalamo-cortical pathways - do not have direct connections with receptors, receive information from the associative nuclei of the thalamus. These pathways provide awareness of the biological significance of stimuli.

3. Non-specific ways - formed by the reticular formation (RF) affect the excitability of the working nerve centers.

4. It is important to emphasize that in sensory systems there are also efferent pathways that affect the excitation of different levels of sensory systems. When impulses pass through the sensory pathways, not only excitation occurs, but also inhibition of various levels of the central nervous system. The wire section provides not only the conduction of impulses, but also their processing with the release of useful information and inhibition of less important information. This is possible because the wire section has not only nerve fibers, but also nerve cells of various levels of the central nervous system.

Cortical division of sensory systems

In the modern view, the cortical section of sensory systems is represented by projection (primary or specific) and associative (secondary, tertiary) areas.

The projection area of ​​each sensory system is the center of a certain type of sensitivity, where sensation is formed. It consists mainly of monosensory cells that receive information from specific nuclei of the thalamic type through a specific pathway. The projection area provides the perception of the physical parameters of the stimulus. Topical organization (topos - place) was found in the projection areas, that is, an ordered arrangement of projections from receptors.

Associative areas consist mainly of polysensory cells that receive information not from receptors, but from the associative nuclei of the thalamus. Due to this, associative areas provide estimates of the biological significance of the stimulus, an assessment of the sources of the stimulus.

In the cortical section of each sensory system, the processes of analysis and synthesis, pattern recognition, the formation of representations, the detection (selection) of features, and the organization of the processes of memorizing important information take place.

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When a stimulus is applied to the receptor, conversion of external stimulus energy into a receptor signal(signal transduction). This process includes three main steps:

1. interaction of a stimulus with a receptor protein molecule located in the receptor membrane;

2. amplification and transmission of the stimulus within the receptor cell

opening of ion channels located in the membrane of the receptor, through which the ion current begins to flow, which, as a rule, leads to depolarization of the cell membrane of the receptor cell (the appearance of the so-called receptor potential).
Mechanismarousalreceptors associated with a change in the permeability of the cell membrane for potassium and sodium ions. When the stimulation reaches a threshold value, a sensory neuron is excited, sending an impulse to the central nervous system. We can say that the receptors encode the incoming information in the form of electrical signals. The sensory cell sends information according to the “all or nothing” principle (there is a signal / no signal). When a stimulus acts on a receptor cell in the protein-lipid layer of the membrane, the spatial configuration of protein receptor molecules changes. This leads to a change in the permeability of the membrane for certain ions, most often for sodium ions, but in recent years the role of potassium in this process has also been discovered. Ion currents arise, the charge of the membrane changes and generation occurs receptor potential(RP). And then the excitation process proceeds in different receptors in different ways.

In primary sensory receptors, which are free naked ends of a sensitive neuron (olfactory, tactile, proprioceptive), RP acts on the neighboring, most sensitive sections of the membrane, where action potential (PD), which further in the form of impulses propagates along the nerve fiber. Thus, when the receptor potential reaches a certain value, a propagating AP arises against its background. The conversion of external stimulus energy into AP in primary receptors can occur either directly on the membrane or with the participation of some auxiliary structures.

Receptor and propagating potentials arise in primary receptors in the same elements. So, in the endings of the process of a sensory neuron located in the skin, under the action of an irritant, a receptor potential is first formed, under the influence of which a propagating potential arises in the nearest intercept of Ranvier. Therefore, in primary receptors, the receptor potential is the cause of the occurrence - generation - of a propagating AP, therefore it is also called generator potential.

In secondary sensory receptors, which are represented by specialized cells (visual, auditory, gustatory, vestibular), RP leads to the formation and release of the mediator from the presynaptic section of the receptor cell into the synaptic cleft of the receptor-afferent synapse. This mediator acts on the postsynaptic membrane of the sensitive neuron, causes its depolarization and the formation of a postsynaptic potential, which is called generator potential(GP). GP, acting on the extrasynaptic regions of the membrane of the sensitive neuron, causes the generation of AP. GP can be both de- and hyperpolarization and, accordingly, cause excitation or inhibit the impulse response of the afferent fiber.