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According to specialization to the perception of a certain type of information, there are:

1. visual,

2. auditory,

3. olfactory,

4. taste,

5. tactile receptors,

6. thermo-, proprio- and vestibuloreceptors (receptors for the position of the body and its parts in space) and

7. pain receptors.

Depending on the localization, all receptors are divided into:

1. external (exteroreceptors) and

2. internal (interoreceptors).

Exteroreceptors include auditory, visual, olfactory, gustatory, tactile.

Interoreceptors include vestibulo- and proprioceptors (receptors of the musculoskeletal system), as well as visceroreceptors (signaling the state of internal organs).

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

Depending on the nature of the stimulus to which they are optimally tuned, receptors can be divided into:

1. photoreceptors,

2. mechanoreceptors, which include auditory, vestibular receptors, and tactile receptors of the skin, receptors of the musculoskeletal system, baroreceptors of the cardiovascular system;

3. chemoreceptors, including taste and smell receptors, vascular and tissue receptors;

4. thermoreceptors (skin and internal organs, as well as central thermosensitive neurons);

5. pain (nociceptive) receptors.

All receptors are initially divided into:

1. primary feeling and

2. secondarily sentient.

To the primary-sentient include olfactory receptors, tactile receptors, and proprioceptors. They are characterized by the fact that the conversion of the energy of irritation into the energy of a nerve impulse occurs in them in the first neuron of the sensory system.

To the second-sentient include receptors for taste, vision, hearing, vestibuloreceptors. They have a highly specialized receptor cell between the stimulus and the first neuron. In this case, the first neuron is not excited directly, but indirectly through the receptor (not nerve) cell.

General mechanisms of receptor excitation.

When a stimulus acts on a receptor cell, the energy of an external stimulus is converted into a receptor signal, or transduction of a sensory signal. This process includes three main steps:

1) the interaction of a stimulus, i.e., a molecule of an odorous or gustatory substance (smell, taste), a quantum of light (vision) or mechanical force (hearing, touch) with receptor protein molecules that are part of the cell membrane of the receptor cell;

2) the emergence of intracellular processes of amplification and transmission of sensory stimulus within the receptor cell; and

3) opening of ion channels located in the receptor membrane, through which the ion current begins to flow, which leads to depolarization of the cell membrane of the receptor cell and the emergence of the so-called receptor potential.

Receptor potential is a change in the membrane potential that occurs in the receptor under the action of an adequate stimulus due to a change in the ion permeability of the receptor membrane and depends gradually on the intensity of the stimulus.

Under the action of a stimulus, the protein molecules of the protein-lipid layer of the receptor membrane change their configuration, the ion channels open and the conductivity of the membrane for sodium increases, a local response occurs or receptor potential. When the receptor potential reaches the threshold value, a nerve impulse occurs in the form of an action potential - a spreading excitation.

The receptor potential obeys the following laws:

1. it is local, i.e. does not apply,

2. depends on the strength of the stimulus,

3. can be stacked,

4. may be depolarizing or hyperpolarizing.

Secondary receptors differ from primary receptors in the mechanism by which the stimulus is transformed into neural activity.

In the second-sentient receptors, a highly specialized receptor cell is synaptically connected to the endings of a sensory neuron. Therefore, a change in the electrical receptor potential of this cell under the influence of a stimulus leads to the release of mediator quanta from the presynaptic end of the receptor cell. This mediator (for example, acetylcholine), acting on the postsynaptic membrane of the end of the first neuron, changes its polarization and EPSP occurs on it. This EPSP is called generator potential, since it subsequently electrotonically causes the generation of an impulse binary response in the form of an action potential.

In primary receptors, the receptor and generator potentials do not differ and are virtually identical.

So, the transformation of the energy of an external stimulus - the encoding of information and the transfer of information to the sensory nuclei of the brain is provided by two functionally different processes:

1. gradual analog receptor or generator potentials, obeying the laws of force and

2. binary action potential (impulse) following the all-or-nothing law.



32. 1. Receptor: concept, function, classification of receptors, properties and their features, mechanism of excitation of receptors.

The sensor system performs the following main functions, or operations, with signals: 1) detection; 2) distinction; 3) transfer and transformation; 4) coding; 5) feature detection; 6) recognition of images.

Detection and primary discrimination of signals is provided by receptors, and detection and recognition of signals - by neurons of the cerebral cortex. Transmission, transformation and encoding of signals is carried out by neurons of all layers of sensory systems.

With all the variety of stimuli and sensory systems, all systems have the same structural plan. Each sensory system consists of a peripheral part - receptors, a conductive part - nerve pathways and subcortical nerve centers, a cortical part - it contains the final analysis of information received from peripheral receptors and nerve centers of the cerebral cortex.

Receptor classification. 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, gustatory, tactile receptors, thermo-, proprio- and vestibuloreceptors (receptors for the position of the body and its parts in space) and pain receptors.

There are external (exteroreceptors) and internal (interoreceptors) receptors. Exteroreceptors include auditory, visual, olfactory, gustatory, tactile. Interoreceptors include vestibulo- and proprioceptors (receptors of the musculoskeletal system), as well as visceroreceptors (signaling the state of internal organs).

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

Depending on the nature of the stimulus to which they are optimally tuned, receptors can be divided into photoreceptors, mechanoreceptors, which include auditory, vestibular receptors, and tactile skin receptors, receptors of the musculoskeletal system, baroreceptors of the cardiovascular system; chemoreceptors, including taste and smell receptors, vascular and tissue receptors; thermoreceptors (skin and internal organs, as well as central thermosensitive neurons); pain (nociceptive) receptors.

All receptors are divided into primary-sensing and secondary-sensing. The former include olfactory, tactile and proprioceptors. They differ in that the conversion of the energy of irritation into the energy of a nerve impulse occurs in them in the first neuron of the sensory system. Secondary-sensing include receptors of taste, vision, hearing, vestibular apparatus. They have a specialized receptor cell between the stimulus and the first neuron that does not generate impulses. Thus, the first neuron is not excited directly, but through a receptor (not nerve) cell.

General mechanisms of receptor excitation. Receptors are cells that distinguish between natural stimuli and send information about them to the central nervous system. Stimulation of the receptor causes a change in the resting potential in the dendrites towards depolarization. When a stimulus acts on a receptor cell, the energy of an external stimulus is converted into a receptor signal, or transduction of a sensory signal. This process includes three main steps:

1) the interaction of a stimulus, i.e., a molecule of an odorous or gustatory substance (smell, taste), a quantum of light (vision) or a mechanical force (hearing, touch) with a receptor protein molecule, which is part of the cell membrane of the receptor cell;

2) intracellular processes of amplification and transmission of sensory stimulus within the receptor cell; and

3) 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).

In the primary sensory receptors, this potential acts on the most sensitive parts of the membrane, capable of generating action potentials - electrical nerve impulses. In secondary-sensing receptors, the receptor potential causes the release of mediator quanta from the presynaptic terminal of the receptor cell. A mediator (for example, acetylcholine), acting on the postsynaptic membrane of the first neuron, changes its polarization (a postsynaptic potential is generated). The postsynaptic potential of the first neuron of the sensory system is called the generator potential, since it causes the generation of an impulse response.

In primary sensory receptors, the receptor and generator potentials are one and the same.

Generation of excitation in receptors. The emergence of the receptor potential is due to an increase in the Na + -conductivity of the dendrites. The excitation that arises in them electrotonically spreads to the soma - a transformation or primary transformation of the stimulus into a receptor potential occurs. Therefore, the receptor is a transducer, a sensor. Excitation in the form of a receptor potential covers only the soma. In the axon of the primary receptors, starting from the axon hillock - the place where the axon departs from the soma - this excitation is transformed into a series of action potentials.

It is very important that after the first action potential, the axon membrane hyperpolarizes significantly below the level of the resting potential. Due to this circumstance, Na + channels after inactivation are restored to such an extent that the depolarization phase that occurs after the first trace hyperpolarization potential again reaches a threshold value sufficient to generate the next action potential. Consequently, the hyperpolarization trace potential serves as the basis for the formation of rhythmic excitation of the nerve fiber.

In secondary receptors, only the receptor potential arises, and a series of action potentials is formed in the terminals of the afferent nerve cell that forms contact with the receptor. In particular, visual and auditory receptors are secondary.

When the generator potential (GP) reaches a critical value, it causes a discharge of afferent impulses in the nearest intercept of Ranvier. The discharge frequency is directly proportional to the HP value (logarithmic dependence corresponding to the Weber-Fechner law). The sensation also increases in proportion to the logarithm of the strength of the stimulus. Novocaine interrupts the flow of these impulses, which is associated with its analgesic effect.

Adaptation of receptors is a common property of all receptors, which consists in adapting to the strength of the stimulus. It manifests itself in a decrease in sensitivity to a constantly acting stimulus. A person "gets used" to the action of constant stimuli - the smell, the pressure of clothing, the sound of a clock, etc. and stop noticing them. With adaptation, the magnitude of the generator potential and the frequency of impulses passing through the afferent nerve decrease.

There are slowly adapting receptors (pain) and rapidly adapting (eye). Only the vestibulo- and proprioceptors do not adapt (or almost do not adapt). When the action of the constant stimulus ceases, the adaptation disappears and the sensitivity of the receptor increases (the effect of excitation after inhibition).

When the action of any stimulus begins, the receptor reacts to it very energetically. As stimulation continues, the receptor adapts to it and activity in the sensory fiber decreases to a lower level. With short and periodic presentations of a stimulus, the receptor each time responds to it completely, without adaptation.

Slowly adapting receptors serve to control long-lasting stimuli, for example, the degree of muscle stretch, H + concentration. Rapidly adapting receptors are characteristic of sensory systems in which stimuli are recorded with high sensitivity and high temporal resolution.

Signal discrimination. An important characteristic of the sensory system is the ability to notice differences in the properties of simultaneously or sequentially acting stimuli. Discrimination begins in the receptors, but the neurons of the entire sensory system are involved in this process. It characterizes the minimum difference between stimuli that the sensory system can detect. (differential, or difference, threshold).

The dependence of the strength of sensation on the strength of stimulation (Weber-Fechner law) is expressed by the formula: E=a∙logI +b, where E is the magnitude of sensation, I is the strength of stimulation, and and b are constants that are different for different modalities of stimuli. According to this formula, sensation increases in proportion to the logarithm of the intensity of stimulation.

The absolute sensitivity of the sensory system is measured by the response threshold. Sensitivity and threshold are opposite concepts: the higher the threshold, the lower the sensitivity, and vice versa. Usually, such a strength of stimulus is taken as the threshold, the probability of perception of which is 0.5 or 0.75 (the correct answer is the presence of a stimulus in half or 3/4 of the cases of its action). Lower intensity values ​​are considered subthreshold, and higher intensity values ​​are considered suprathreshold. It turned out that even in the subthreshold range, a reaction to superweak stimuli is possible, but it is unconscious (does not reach the threshold of sensation). So, if the intensity of a flash of light is reduced so much that a person can no longer say whether he saw it or not, an imperceptible galvanic skin reaction to this signal can be registered from his hand. The sensitivity of receptor elements to adequate stimuli, to which they are evolutionarily adapted, is extremely high. So, the olfactory receptor can be excited by the action of a single molecule of an odorous substance, the photoreceptor - by a single quantum of light. The sensitivity of auditory receptors is also marginal: if it were higher, we would hear a constant noise due to the thermal movement of molecules.

Above, we mentioned the difference in the strength of stimuli. Spatial discrimination is based on the distribution of excitation in the receptor layer and in the neural layers. So, if two stimuli excited two neighboring receptors, then it is impossible to distinguish between these stimuli and they will be perceived as a whole. There must be at least one unexcited receptor between two excited receptors. For a temporary distinction between two stimuli, it is necessary that the nervous processes caused by them do not merge in time and that the signal caused by the second stimulus does not fall into the refractory period from the previous stimulation.

The amplitude (intensity) of the stimulus is encoded as the frequency of impulses or action potentials sent from the receptor to the CNS. An increase in the amplitude of the stimulus, provided that it is above the threshold value, correspondingly increases the frequency of action potentials.

32.2. Analyzers (I.P. Pavlov): concept, classification of analyzers, three divisions of analyzers and their meaning, principles of constructing cortical divisions of analyzers.

An analyzer, according to I.P. Pavlov, is a part of the nervous system, consisting of perceiving elements - sensory receptors that receive stimuli from the external or internal environment, nerve pathways that transmit information from receptors to the brain, and those parts of the brain that process this information.

Methods for studying sensory systems. To study sensory systems, electrophysiological, neurochemical, behavioral and morphological studies on animals, psychophysiological analysis of perception in a healthy and sick person, methods of mapping his brain are used. Sensory functions are also modeled and prosthetic.

Modeling of sensory functions makes it possible to study on biophysical or computer models such functions and properties of sensory systems that are not yet available for experimental methods.

Basic principles of the structure of analyzers. The main general principles for constructing sensory systems in higher vertebrates and humans are as follows:

1) layering, i.e., the presence of several layers of nerve cells, the first of which is associated with receptors, and the last with neurons in the motor areas of the cerebral cortex.

2) Multichannel sensory system, i.e., the presence in each layer of a multitude (from tens of thousands to millions) of nerve cells associated with a multitude of cells of the next layer. The presence of many such parallel channels for processing and transmitting information provides the sensor system with the accuracy and detail of signal analysis and greater reliability;

3) Presence of sensory funnels. A different number of elements in adjacent layers forms "sensor funnels". So, in the human retina there are 130 million photoreceptors, and in the layer of ganglion cells of the retina there are 100 times fewer neurons (“narrowing funnel”). At the next levels of the visual system, an "expanding funnel" is formed: the number of neurons in the primary projection area of ​​the visual cortex is thousands of times greater than the number of retinal ganglion cells. In the auditory and in a number of other sensory systems, there is an "expanding funnel" from the receptors to the cerebral cortex. The physiological meaning of the “shrinking funnel” is to reduce the redundancy of information, and the “expanding” one is to provide a fractional and complex analysis of various signal features; differentiation of the sensory system vertically and horizontally.

4) Vertical and horizontal differentiation. Vertical differentiation consists in the formation of departments, each of which consists of several neural layers. Thus, a department is a larger morphofunctional formation than a layer of neurons. Each department (for example, olfactory bulbs, cochlear nuclei of the auditory system or geniculate bodies) performs a specific function. Horizontal differentiation consists in different properties of receptors, neurons and connections between them within each of the layers. So, in vision, there are two parallel neural channels running from photoreceptors to the cerebral cortex and processing information coming from the center and from the periphery of the retina in different ways.

General principles for the formation of analyzers. Common to most conductive paths of analyzers is that before entering the nuclear zones of the cortex, they give off collaterals of the reticular formation and interact with it, and also pass through the thalamus.

The cortical representation of the analyzers are the primary and secondary fields, mainly located in the occipital, postcentral and temporal sections of the second block (the block for receiving, processing and storing exteroceptive information) of the brain.

All analyzer systems operate on the basis of the following general principles:

1) information analysis with the help of special neuron-detectors;

2) parallel multi-channel processing of information, ensuring its reliability;

3) selection of information in the interval from the receptor to the projection field;

4) sequential complication of information processing from level to level;

5) holistic representation of the signal in the CNS in conjunction with other signals;

6) implementation of the principles of increasing the reliability of processing various signal features.

The basis of the cortical sections of the analyzers are the primary or projection zones of the cortex (fields), which perform a highly specialized function of reflecting only stimuli of one modality. Their task is to identify the stimulus by its quality and signal value, in contrast to the peripheral receptor, which differentiates the stimulus only by its physical or chemical characteristics. The main function of the primary fields is the subtlest reflection of the properties of the external and internal environment at the level of sensation.

All primary cortical fields are characterized by the topical (screen) principle of organization, according to which any area of ​​the receptor surface corresponds to a certain area in the primary cortex (according to the “point to point” principle), which gave reason to call the primary cortex projection. The size of the zone of representation of one or another receptor site in the primary burrow depends on the functional significance of this site, and not on its actual size.

Primary fields include: 17th (for vision). 3rd (for skin-kinesthetic sensitivity) and 41st (for hearing). Exteroceptor information enters these areas of the brain after passing through the relay nuclei of the thalamus.

Secondary fields represent cellular structures, morphologically and functionally, as if built on top of projection ones. In them, there is a consistent complication of the process of processing information, which is facilitated by the preliminary conduction of afferent impulses through the associative nuclei of the thalamus. Secondary fields ensure the transformation of somatotropic impulses into such a functional organization, which at the level of the psyche is equivalent to the process of perception.

On the surface of the brain, secondary fields border or surround projection fields. Numbers of secondary fields: 18.19 - for vision, 1.2 and partially 5 - for skin-kinesthetic sensitivity, 42 and 22 - for hearing. Primary and secondary fields refer to the nuclear zones of the analyzers located on the three spatial poles of the hindbrain - occipital, parietal and temporal, respectively.

Fig.56. fields of the cerebral cortex.

Tertiary fields (associative, overlap zone) take on the most complex functional load. They are located outside the nuclear zones and are mainly located in the gap between the secondary fields or along their perimeter. A large and important part of the tertiary fields is formed at the border of the parietal, occipital and temporal regions, being equidistant from each of these poles, and has no direct access to the periphery. Their functions are almost completely reduced to the integration of excitations coming from the secondary cortex of the entire complex of analyzers. The work of the tertiary zones has its psychological equivalent in the perception of the world in its entirety and a combination of spatial, temporal and intensity characteristics of the external environment. All this gives grounds to consider them as an apparatus of inter-analyzer syntheses.

Majority receptors excited in response to the action of stimuli of only one physical nature and therefore belong to monomodal. They can also be aroused by some inappropriate stimuli, for example photoreceptors- strong pressure on the eyeball, and taste buds - by touching the tongue to the contacts of the galvanic battery, but it is impossible to get qualitatively distinguishable sensations in such cases. Along with monomodal receptors, there are polymodal receptors, which can be adequately stimulated by stimuli of a different nature. To this type of receptors belong some pain receptors, or nociceptors (lat. nocens - harmful), which can be excited by mechanical, thermal and chemical stimuli. Polymodality is present in thermoreceptors that respond to an increase in the concentration of potassium in the extracellular space in the same way as to an increase in temperature.

Depending on the structure of the receptors, they are divided into primary, or primary sentient, which are the specialized endings of a sensory neuron, and secondary, or secondarily sentient, which are cells of epithelial origin capable of generating a receptor potential in response to an adequate stimulus. Primary sensory receptors can themselves generate action potentials in response to irritation with an adequate stimulus if the value 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.

Secondary sensing receptors respond to the action of the stimulus only by the appearance receptor potential, on the value of which depends on the amount of mediator secreted by these cells. With its help, secondary receptors act on the nerve endings of sensory neurons that generate action potentials depending on the amount of mediator released from the secondary sensory receptors. Secondary receptors 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 more often referred to as primary receptors, but their lack of the ability to generate action potentials indicates their similarity to secondary receptors.

Depending on the source of adequate incentives receptors divided into external and internal, or exteroreceptors and interoreceptors; the former are stimulated by the action of environmental stimuli (electromagnetic and sound waves, pressure, the action of odorous molecules), and the latter are internal (this type of receptor includes not only visceroreceptors of internal organs, but also proprioceptors and vestibular receptors). Depending on whether the stimulus acts at a distance or directly on the receptors, they are also divided into distant and contact.

Classification of receptors. The classification of receptors is based on several criteria.

Psychophysiological nature of sensation: heat, cold, pain, etc.

The nature of an adequate stimulus: mechano-, thermo-, chemo-, photo-, baro-, osmbreceptors, etc.

The environment in which the receptor perceives the stimulus: extero-, interoreceptors.

Relation to one or more modalities: mono- and polymodal (monomodal convert only one type of stimulus into a nerve impulse - light, temperature, etc., polymodal can convert several stimuli into a nerve impulse - mechanical and thermal, mechanical and chemical, etc.). d.).

The ability to perceive an irritant located at a distance from the receptor or in direct contact with it: contact and distant.

Level of sensitivity (irritation threshold): low-threshold (mechanoreceptors) and high-threshold (nociceptors).

Speed ​​of adaptation: fast-adapting (tactile), slow-adapting (pain) and non-adapting (vestibular receptors and proprioceptors).

Attitude to different moments of the action of the stimulus: when the stimulus is turned on, when it is turned off, throughout the entire time of the action of the stimulus.

Morphofunctional organization and mechanism of the emergence of excitation: primary-sensing and secondary-sensing.

Under the influence of irritation of the receptors, nerve impulses arise in them, that is, they, as it were, transform irritation into excitation. On this basis receptors often compared with transducers used in technology, in which, when external influences are applied, an electric current or voltage is generated or their electrical characteristics change. Such a comparison is very conditional. Unlike the processes that occur in transducer sensors, which work due to the energy acting on them, the transformation of the irritation energy into the process of excitation in the receptors occurs due to the metabolism of the receptors themselves, and not due to the external energy applied to them. The mechanism of excitation in receptors complicated enough.

An external stimulus, acting on the receptor, causes depolarization of its surface membrane. This depolarization, similar in properties local response, is called the receptor, or generator, potential. The receptor potential does not obey the all-or-nothing law, depends on the strength of the stimulus, is able to sum up or use rapidly following each other stimuli and does not spread along the nerve fiber.

One of the distinguishing features of the receptor potential is its duration: in some receptors it can remain unchanged for many minutes while the stimulus is acting; in the pressoreceptors of the carotid sinus, responsive to an increase in blood pressure, receptor potentials lasting several hours were registered. Maintaining such a long-term depolarization of the membrane is associated with the expenditure of energy released as a result of metabolic processes; therefore, it is clear that substances that disrupt intracellular oxidative processes lead to the disappearance of receptor potentials.

There is evidence that the receptor potential arises as a result of release in the receptor under the influence of acetylcholine irritation, which changes the permeability of the membrane, which leads to its depolarization. Such an effect was observed with the introduction of acetylcholine into the area of ​​the receptors.

In photoreceptors, the appearance of a generator potential is associated with the decomposition reaction of visual purple. The receptor potential can arise in a number of receptors as a result of a direct change in the properties of the surface membrane under the influence of stimuli acting on it, without an intermediate chemical link.

When the receptor potential reaches a certain critical value, it causes a discharge of afferent impulses in the nerve fiber associated with the receptor. This discharge occurs in the first node of Ranvier closest to the receptor. Novocaine, which destroys the sensitivity of receptors, does not affect the receptor potential, but stops the discharge of afferent impulses in nerve fibers.

As shown by direct measurements made on some experimental objects, for example, on frog muscle spindles, the frequency of afferent impulses in nerve fibers is directly proportional to the magnitude of the depolarization of the receptor membrane, i.e., to the magnitude of the receptor potential ( rice. 189, A). At the same time, the frequency of afferent discharges in the nerve fiber is proportional to the logarithm of the stimulus strength ( rice. 189, B).

From a comparison of these facts, it follows that between the strength of irritation and the magnitude of the receptor potential there is not a direct, but a logarithmic relationship. These electrophysiological observations correspond to the mathematical expression proposed by G. Fechner .

Rice. 189. The ratio between the frequency of impulses and the depolarization of the frog muscle spindle receptor membrane (according to B. Katz) (A) and the ratio between the frequency of impulses in the muscle spindle and the logarithm of the load acting on the muscle (according to B. Matthews) (B). The circles show the results of individual experiments.

Physiology of sensory systems

1. General principles of the structure of sensory systems. The main functions of the sensory system are: detection, discrimination, transmission and transformation, coding, feature detection, pattern recognition. Adaptation of the sensory system.

The main general principles for constructing sensory systems in higher vertebrates and humans are as follows:

1) layering, i.e. the presence of several layers of nerve

cells, the first of which is associated with receptors, and the latter with neurons in the motor areas of the cerebral cortex. This property makes it possible to specialize neural layers in the processing of different types of sensory information, which allows the body to quickly respond to simple signals analyzed already at the first levels of the sensory system.

2) the multichannel nature of the sensory system, i.e., the presence in each layer of a multitude (from tens of thousands to millions) of nerve cells associated with a multitude of cells of the next layer. The presence of many such parallel channels for processing and transmitting information provides the sensor system with the accuracy and detail of signal analysis and greater reliability;

3) a different number of elements in adjacent layers, which forms "sensor funnels". So, in the human retina there are 130 million photoreceptors, and in the layer of ganglion cells of the retina there are 100 times fewer neurons (“narrowing funnel”).

At the following levels of the visual system, an "expanding funnel" is formed: In the auditory and in a number of other sensory systems, there is an "expanding funnel" from the receptors to the cerebral cortex. The physiological meaning of the “shrinking funnel” is to reduce the redundancy of information, and the “expanding” one is to provide a fractional and complex analysis of various signal features;

4) differentiation of the sensory system vertically and horizontally. Vertical differentiation consists in the formation of departments, each of which consists of several neural layers. Thus, a department is a larger morphofunctional formation than a layer of neurons. Each department (for example, olfactory bulbs, cochlear nuclei of the auditory system or geniculate bodies) performs a specific function. Horizontal differentiation consists in different properties of receptors, neurons and connections between them within each of the layers. So, in vision, there are two parallel neural channels running from photoreceptors to the cerebral cortex and processing information coming from the center and from the periphery of the retina in different ways.

The sensor system performs the following main functions, or operations, with signals: 1) detection; 2) distinction; 3) transfer and transformation; 4) coding; 5) feature detection; 6) recognition of images. Detection and primary discrimination of signals is provided by receptors, and detection and recognition of signals - by neurons of the cerebral cortex. Transmission, transformation and encoding of signals is carried out by neurons of all layers of sensory systems.

Signal detection. It begins in the receptor - a specialized cell, evolutionarily adapted to the perception of a stimulus of a certain modality from the external or internal environment and its transformation from a physical or chemical form into a form of nervous excitation.

All receptors are divided into primary-sensing and secondary-sensing. The former include olfactory, tactile and proprioceptors. They differ in that the conversion of the energy of irritation into the energy of a nerve impulse occurs in them in the first neuron of the sensory system. Secondary-sensing include receptors of taste, vision, hearing, vestibular apparatus. They have a specialized receptor cell between the stimulus and the first neuron that does not generate impulses. Thus, the first neuron is not excited directly, but through a receptor (not nerve) cell.

General mechanisms of excitation of receptors. When a stimulus acts on a receptor cell, the energy of an external stimulus is converted into a receptor signal, or transduction of a sensory signal. This process includes three main stages: 1) the interaction of a stimulus, i.e., a molecule of an odorous or gustatory substance (smell, taste), a quantum of light (vision) or mechanical force (hearing, touch) with a receptor protein molecule, which is located in the composition of the cell membrane of the receptor cell; 2) intracellular processes of amplification and transmission of sensory stimulus within the receptor cell; and 3) opening of ion channels located in the receptor membrane, 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). In the primary sensory receptors, this potential acts on the most sensitive parts of the membrane, capable of generating action potentials - electrical nerve impulses. In secondary-sensing receptors, the receptor potential causes the release of mediator quanta from the presynaptic terminal of the receptor cell. A mediator (for example, acetylcholine), acting on the postsynaptic membrane of the first neuron, changes its polarization (a postsynaptic potential is generated). The postsynaptic potential of the first neuron of the sensory system is called the generator potential, since it causes the generation of an impulse response. In primary sensory receptors, the receptor and generator potentials are one and the same.

The absolute sensitivity of the sensory system is measured by the response threshold. Sensitivity and threshold are opposite concepts: the higher the threshold, the lower the sensitivity, and vice versa. Usually, such a strength of stimulus is taken as the threshold, the probability of perception of which is 0.5 or 0.75 (the correct answer is the presence of a stimulus in half or 3/4 of the cases of its action). Lower intensity values ​​are considered subthreshold, and higher intensity values ​​are considered suprathreshold. It turned out that even in the subthreshold range, a reaction to superweak stimuli is possible, but it is unconscious (does not reach the threshold of sensation). So, if the intensity of a flash of light is reduced so much that a person can no longer say whether he saw it or not, an imperceptible galvanic skin reaction to this signal can be registered from his hand.

Distinguishing signals. An important characteristic of the sensory system is the ability to notice differences in the properties of simultaneously or sequentially acting stimuli. Discrimination begins in the receptors, but the neurons of the entire sensory system are involved in this process. It characterizes the minimum difference between stimuli that the sensory system can notice (differential, or difference, threshold).

The threshold for distinguishing the intensity of the stimulus is almost always higher than the previously acting stimulus by a certain fraction (Weber's law). So, an increase in pressure on the skin of the hand is felt if the load is increased by 3% (3 g must be added to a 100-gram weight, and 6 g to a 200-gram weight).

Transmission and conversion of signals. The processes of transformation and transmission of signals in the sensory system convey to the higher centers of the brain the most important (essential) information about the stimulus in a form convenient for its reliable and fast analysis.

Signal transformations can be conditionally divided into spatial and temporal. Among the spatial transformations, changes in the ratio of different parts of the signal are distinguished. So, in the visual and somatosensory systems at the cortical level, the geometric proportions of the representation of individual parts of the body or parts of the visual field are significantly distorted. In the visual area of ​​the cortex, the representation of the informationally most important central fovea of ​​the retina is sharply expanded with a relative compression of the projection of the periphery of the visual field (“cyclopean eye”). In the somatosensory area of ​​the cortex, the most important zones for fine discrimination and organization of behavior are also predominantly represented - the skin of the fingers and face ("sensory homunculus").

For temporal transformations of information in all sensory systems, compression, temporal compression of signals is typical: the transition from long-term (tonic) impulses of neurons at lower levels to short (phasic) discharges of neurons at high levels.

Information encoding. Encoding is the transformation of information into a conditional form, a code, performed according to certain rules. In a sensory system, signals are encoded by a binary code, that is, by the presence or absence of an electrical impulse at one time or another. This encoding method is extremely simple and resistant to interference. Information about the stimulation and its parameters is transmitted in the form of individual impulses, as well as groups or "packages" of impulses ("volleys" of impulses). The amplitude, duration, and shape of each pulse are the same, but the number of pulses in a burst, their frequency, the duration of bursts and the intervals between them, as well as the temporal "pattern" of the burst, are different and depend on the characteristics of the stimulus. Sensory information is also encoded by the number of simultaneously excited neurons, as well as by the place of excitation in the neuronal layer.

Signal detection. This is a selective selection by a sensory neuron of one or another sign of a stimulus that has behavioral significance. Such an analysis is carried out by detector neurons that selectively respond only to certain parameters of the stimulus.

Image recognition. This is the final and most complex operation of the sensory system. It consists in assigning the image to one or another class of objects that the organism encountered earlier, i.e., in the classification of images. By synthesizing signals from neurons-detectors, the higher part of the sensory system forms an "image" of the stimulus and compares it with a multitude of images stored in memory. Recognition ends with a decision about which object or situation the organism encountered. As a result of this, perception occurs, that is, we are aware of whose face we see in front of us, whom we hear, what smell we smell.

Recognition often occurs regardless of signal variability. We reliably identify, for example, objects with different illumination, color, size, angle, orientation and position in the field of view. This means that the sensory system forms an (invariant) sensory image independent of changes in a number of signal features.

Adaptation of the sensory system

The sensory system has the ability to adapt its properties to environmental conditions and the needs of the body. Sensory adaptation is a common property of sensory systems, which consists in adaptation to a long-acting (background) stimulus. Adaptation is manifested in a decrease in the absolute and an increase in the differential sensitivity of the sensory system. Subjectively, adaptation is manifested in getting used to the action of a constant stimulus (for example, we do not notice the continuous pressure on the skin of habitual clothing).

Adaptation processes begin at the level of receptors, covering all neural levels of the sensory system. Adaptation is weak only in the vestibulo- and proprioreceptors. According to the speed of this process, all receptors are divided into fast- and slowly adapting. The first after the development of adaptations practically do not send information to the brain about the ongoing irritation. The latter transmit this information in a significantly weakened form. When the action of the constant stimulus ceases, the absolute sensitivity of the sensory system is restored. So, in the dark, the absolute sensitivity of vision increases sharply.

Efferent regulation of the properties of the sensory system plays an important role in sensory adaptation. It is carried out due to the descending influences of the higher on its lower departments. There is a kind of reconfiguration of the properties of neurons for the optimal perception of external signals in the changed conditions. The state of different levels of the sensory system is also controlled by the reticular formation, which includes them in a single system integrated with other parts of the brain and the body as a whole. Efferent influences in sensory systems most often have an inhibitory character, i.e., they lead to a decrease in their sensitivity and limit the flow of afferent signals.

The total number of efferent nerve fibers coming to receptors or elements of any neuronal layer of the sensory system is, as a rule, many times less than the number of afferent neurons coming to the same layer. This determines an important feature of efferent control in sensory systems: its broad and diffuse character. We are talking about a general decrease in the sensitivity of a significant part of the underlying neuronal layer.

2. Classification and properties of receptors. Excitation mechanisms of primary and secondary sensory receptors.

Depending on the structure of the receptors, they are divided into primary, or primary sentient, which are the specialized endings of a sensory neuron, and secondary, or secondarily sentient, which are cells of epithelial origin capable of generating a receptor potential in response to an adequate stimulus. Primary sensory receptors can themselves generate action potentials in response to irritation with an adequate stimulus if the value 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.

Secondary sensing receptors respond to the action of the stimulus only by the appearance receptor potential, on the value of which depends on the amount of mediator secreted by these cells. With its help, secondary receptors act on the nerve endings of sensory neurons that generate action potentials depending on the amount of mediator released from the secondary sensory receptors. Secondary receptors 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 more often referred to as primary receptors, but their lack of the ability to generate action potentials indicates their similarity to secondary receptors.

Classification of receptors. 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, gustatory, tactile receptors, thermo-, proprio- and vestibuloreceptors (receptors for the position of the body and its parts in space) and pain receptors.

There are external (exteroreceptors) and internal (interoreceptors) receptors. Exteroreceptors include auditory, visual, olfactory, gustatory, tactile. Interoreceptors include vestibulo- and proprioceptors (receptors of the musculoskeletal system), as well as visceroreceptors (signaling the state of internal organs).

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

Depending on the nature of the stimulus to which they are optimally tuned, receptors can be divided into photoreceptors, mechanoreceptors, which include auditory, vestibular receptors, and tactile skin receptors, receptors of the musculoskeletal system, baroreceptors of the cardiovascular system; chemoreceptors, including taste and smell receptors, vascular and tissue receptors; thermoreceptors (skin and internal organs, as well as central thermosensitive neurons); pain (nociceptive) receptors.

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