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

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 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 first are stimulated by the action of stimuli external environment(electromagnetic and sound waves, pressure, the action of odorous molecules), and the second - 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.

3. Taste system: receptor, conduction and cortical sections. Taste perception mechanisms. Methods for studying the taste system.

Taste system

In the process of evolution, taste was formed as a mechanism for choosing or rejecting food. Under natural conditions, taste sensations are combined with olfactory, tactile and thermal sensations, also created by food. An important circumstance is that the preferred choice of food is partly based on innate mechanisms, but to a large extent depends on the connections developed in ontogenesis by a conditioned reflex.

Taste, like smell, is based on chemoreception. Taste buds carry information about the nature and concentration of substances entering the mouth. Their excitation triggers a complex chain of reactions from different parts of the brain, leading to different work of the digestive organs or to the removal of substances harmful to the body that have entered the mouth with food.

taste receptors. Taste buds - taste receptors - are located on the tongue, back of the pharynx, soft palate, tonsils and epiglottis. Most of them are at the tip, edges and back of the tongue. Each of the approximately 10,000 human taste buds consists of several (2-6) receptor cells and, in addition, of supporting cells. The taste bud is flask-shaped; in humans, its length and width are about 70 microns. The taste bud does not reach the surface of the mucous membrane of the tongue and is connected to the oral cavity through the taste pore.

Taste cells are the shortest-lived epithelial cells of the body: on average, every 250 hours, the old cell is replaced by a young one moving towards the center of the taste bud from its periphery. Each of the receptor taste cells 10-20 µm long and 3-4 µm wide has 30-40 finest microvilli 0.1-0.2 µm thick and 1-2 µm long at the end facing the lumen of the pore. Think they are playing important role in the excitation of the receptor cell, perceiving certain chemical substances adsorbed in the kidney canal. It is assumed that active centers are located in the area of microvilli - stereospecific sections of the receptor, which selectively perceive different adsorbed substances. The stages of the primary transformation of the chemical energy of gustatory substances into the energy of nervous excitation of taste receptors are not yet known.

Electrical potentials of the taste system. In experiments with the introduction of a microelectrode into the taste buds of animals, it was shown that the total potential of receptor cells changes when the tongue is irritated by various substances (sugar, salt, acid). This potential develops rather slowly: its maximum is reached by the 10-15th second after exposure, although the electrical activity in the fibers of the gustatory nerve begins much earlier.

Pathways and centers of taste. The conductors of all types of taste sensitivity are the tympanic string and the glossopharyngeal nerve, the nuclei of which in the medulla oblongata contain the first neurons of the taste system. Many of the fibers coming from the taste buds are distinguished by a certain specificity, since they respond with an increase in impulse discharges only to the action of salt, acid and quinine. Other fibers react to sugar. The most convincing hypothesis is considered according to which information about the 4 main taste sensations: bitter, sweet, sour and salty is encoded not by impulses in single fibers, but by a different distribution of the frequency of discharges in a large group of fibers differently excited by the taste substance.

Taste afferent signals enter the nucleus of a single bundle of the brainstem. From the nucleus of a single bundle, the axons of the second neurons ascend as part of the medial loop to the arcuate nucleus of the thalamus, where the third neurons are located, the axons of which are directed to the cortical center of taste. The results of the studies do not yet allow us to assess the nature of the transformations of gustatory afferent signals at all levels of the gustatory system.

Taste and perception. In different people, the absolute thresholds of taste sensitivity to different substances differ significantly up to "taste blindness" to individual agents (for example, to creatine). The absolute thresholds of taste sensitivity largely depend on the state of the body (they change in case of starvation, pregnancy, etc.). When measuring absolute taste sensitivity, two assessments are possible: the occurrence of an indefinite taste sensation (different from the taste of distilled water) and the conscious perception or recognition of a certain taste. The threshold of perception, as in other sensory systems, is higher than the threshold of sensation. The discrimination thresholds are minimal in the range of medium concentrations of substances, but increase sharply when moving to high concentrations. Therefore, a 20% sugar solution is perceived as the most sweet, 10% sodium chloride solution - as the most salty, 0.2% hydrochloric acid solution - as the most acidic, and 0.1% solution of quinine sulfate - as the most bitter. Threshold contrast (dI/I) for different substances fluctuates significantly.

Taste adaptation. With prolonged action of the gustatory substance, adaptation to it is observed (the intensity of the gustatory sensation decreases). The duration of adaptation is proportional to the concentration of the solution. Adaptation to sweet and salty develops faster than to bitter and sour. Cross-adaptation has also been found, i.e., a change in sensitivity to one substance under the action of another. The use of several taste stimuli simultaneously or sequentially gives the effects of taste contrast or mixing of taste. For example, adaptation to bitter. increases sensitivity to sour and salty, adaptation to sweet sharpens the perception of all other taste stimuli. When mixing several flavoring substances, a new taste sensation may arise, which differs from the taste of the components that make up the mixture.

4. Olfactory system: receptors, their localization, odor perception mechanism, conductive and cortical sections.

Olfactory system

The olfactory receptor cell is a bipolar cell, on the apical pole of which there are cilia, and an unmyelinated axon departs from its basal part. The axons of the receptors form the olfactory nerve, which penetrates the base of the skull and enters the olfactory bulb. Like taste cells and the outer segments of photoreceptors, olfactory cells are constantly renewing themselves. The lifespan of the olfactory cell is about 2 months.

Molecules of odorous substances enter the mucus produced by the olfactory glands, with direct current air or from the mouth during meals. Sniffing accelerates the flow of odorous substances to the mucus. In the mucus, the molecules of odorous substances on a short time bind to olfactory non-receptor proteins. Some molecules reach the olfactory receptor cilia and interact with the olfactory receptor protein located there. In turn, the olfactory protein activates, as in the case of photoreception, the GTP-binding protein (G-protein), which, in turn, activates the enzyme adenylate cyclase, which synthesizes cAMP. An increase in the concentration of cAMP in the cytoplasm causes opening in plasma membrane receptor cells of sodium channels and, as a result, the generation of a depolarization receptor potential. This leads to a pulsed discharge in the axon of the receptor (olfactory nerve fiber).

It was previously believed that the low selectivity of an individual receptor was due to the presence of many types of olfactory receptor proteins in it, but recently it has been found that each olfactory cell has only one type of membrane receptor protein. This protein itself is able to bind many odorous molecules of various spatial configurations. The rule "one olfactory cell - one olfactory receptor protein" greatly simplifies the transmission and processing of information about odors in the olfactory bulb - the first nerve center for switching and processing chemosensory information in the brain.

The presence of only one olfactory protein in each receptor is due not only to the fact that each olfactory cell expresses only one of the hundreds of olfactory protein genes, but also to the fact that only one of the two alleles, maternal or paternal, is expressed within a given gene. It is likely that genetically determined individual differences in the perception thresholds of certain odors are associated with functional differences in the mechanisms of expression of the olfactory receptor protein gene.

Electroolfactogram. The total electrical potential recorded from the surface of the olfactory epithelium is called the electroolfactogram (Fig. 14.24). This is a monophasic negative wave with an amplitude of up to 10 mV and a duration of several seconds, which occurs in the olfactory epithelium even with a short-term exposure to an odorous substance. Often on the electroolfactogram one can see a small positive deviation of the potential preceding the main negative wave, and with a sufficient duration of exposure, a large negative wave is recorded to stop it (off-reaction). Sometimes fast oscillations are superimposed on the slow waves of the electroolfactogram, reflecting synchronous pulsed discharges of a significant number of receptors.

Olfactory information encoding. Studies using microelectrodes show that single receptors respond with an increase in the frequency of impulses, which depends on the quality and intensity of the stimulus. Each olfactory receptor responds not to one, but to many odorous substances, giving "preference" to some of them. It is believed that on these properties of receptors that differ in their tuning to different groups substances, the coding of odors and their recognition in the centers of the olfactory sensory system can be based. In electrophysiological studies of the olfactory bulb, it was revealed that the electrical response recorded in it under the action of smell depends on the odorous substance: when different smells the spatial mosaic of the excited and inhibited sections of the bulb changes. Whether this serves as a way of encoding olfactory information is still difficult to judge.

Central projections of the olfactory system. A feature of the olfactory system is, in particular, that its afferent fibers do not switch in the thalamus and do not pass to the opposite side of the cerebrum. The olfactory tract leaving the bulb consists of several bundles that go to different parts of the forebrain: the anterior olfactory nucleus, the olfactory tubercle, the prepiriform cortex, the periamygdala cortex, and part of the nuclei of the amygdala complex. The connection of the olfactory bulb with the hippocampus, piriform cortex and other parts of the olfactory brain is carried out through several switches. It has been shown that the presence of a significant number of centers of the olfactory brain (rhinencephalon) is not necessary for the recognition of odors, therefore, most of the nerve centers into which the olfactory tract is projected can be considered as associative centers that ensure the connection of the olfactory sensory system with other sensory systems and organization on this basis. a number of complex forms of behavior - food, defensive, sexual, etc.

The efferent regulation of the activity of the olfactory bulb has not yet been studied enough, although there are morphological prerequisites that indicate the possibility of such influences.

The sensitivity of the human olfactory system. This sensitivity is extremely high: one olfactory receptor can be excited by one molecule of an odorous substance, and the excitation of a small number of receptors leads to the appearance of a sensation. At the same time, the change in the intensity of the action of substances (the threshold of discrimination) is estimated by people rather roughly (the smallest perceived difference in the strength of the smell is 30-60% of its initial concentration). In dogs, these figures are 3-6 times higher. Adaptation in the olfactory system occurs relatively slowly (tens of seconds or minutes) and depends on the air flow velocity over the olfactory epithelium and on the concentration of the odorous substance.

5. The vestibular system and its role in assessing the position of the body in space and during movement. Receptor, conduction and cortical sections. The results of one- and two-sided destruction of labyrinths in a frog.

vestibular system

The vestibular system, along with the visual and somatosensory systems, plays a leading role in the spatial orientation of a person. It receives, transmits and analyzes information about accelerations or decelerations that occur in the process of rectilinear or rotational movement, as well as when changing the position of the head in space. With uniform movement or at rest, the receptors of the vestibular sensory system are not excited. Impulses from the vestibuloreceptors cause a redistribution of skeletal muscle tone, which ensures that the balance of the body is maintained. These influences are carried out in a reflex way through a number of departments of the central nervous system.

The structure and functions of the receptors of the vestibular system . The peripheral part of the vestibular system is the vestibular apparatus, located in the labyrinth of the pyramid of the temporal bone. It consists of a vestibule (vestibulum) and three semicircular canals (canales cemicircularis). In addition to the vestibular apparatus, the cochlea enters the labyrinth, in which auditory receptors are located. The semicircular canals (Fig. 14.17) are located in three mutually perpendicular planes: the upper one in the frontal, the posterior one in the sagittal and the lateral one in the horizontal. One of the ends of each channel is expanded (ampulla).

The vestibular apparatus also includes two sacs: spherical (sacculus) and elliptical, or uterus (utriculus). The first of them lies closer to the cochlea, and the second - to the semicircular canals. In the sacs of the vestibule there is an otolith apparatus: accumulations of receptor cells (secondary-sensing mechanoreceptors) on elevations, or spots (macula sacculi, macula utriculi). The part of the receptor cell protruding into the cavity of the sac terminates in one longer movable hair and 60-80 glued immobile hairs. These hairs penetrate a jelly-like membrane containing calcium carbonate crystals - otoliths. The excitation of the hair cells of the vestibule occurs due to the sliding of the otolith membrane along the hairs, i.e., their bending (Fig. 14.18).

In the membranous semicircular canals, filled, like the entire labyrinth, with dense endolymph (its viscosity is 2-3 times greater than that of water), receptor hair cells are concentrated only in ampoules in the form of cristae (cristae ampularis). They are also provided with hairs. When the endolymph moves (during angular accelerations), when the hairs bend in one direction, the hair cells are excited, and when the movement is opposite, they are inhibited. This is due to the fact that the mechanical control of the ion channels of the hair membrane with the help of microfilaments, described in the section "mechanisms of auditory reception", depends on the direction of the hair fold: deviation in one direction leads to the opening of the channels and depolarization of the hair cell, and deviation in the opposite direction causes channel closure and receptor hyperpolarization. In the hair cells of the vestibule and ampulla, when they are bent, receptor potential, which enhances the release of acetylcholine and through synapses activates the endings of the fibers of the vestibular nerve.

The fibers of the vestibular nerve (processes of bipolar neurons) are sent to the medulla oblongata. The impulses coming through these fibers activate the neurons of the bulbar vestibular complex, which includes nuclei: the superior vestibule, or Bekhterev, the lateral vestibule, or Deiters, Schwalbe, etc. From here, the signals are sent to many parts of the central nervous system: the spinal cord, cerebellum, oculomotor nuclei , cerebral cortex, reticular formation and autonomous ganglia nervous system.

Complex reflexes associated with vestibular stimulation. The neurons of the vestibular nuclei provide control and management of various motor reactions. The most important of these reactions are the following: vestibulospinal, vestibulo-vegetative and vestibulo-oculomotor. Vestibulospinal influences through the vestibulo-, reticulo- and rubrospinal tracts change the impulses of neurons at the segmental levels of the spinal cord. This is how the dynamic redistribution of skeletal muscle tone is carried out and the reflex reactions necessary to maintain balance are turned on. The cerebellum is responsible for the phasic nature of these reactions: after its removal, the vestibulospinal influences become predominantly tonic. During voluntary movements, vestibular influences on the spinal cord are weakened.

Vestibulooculomotor reflexes (ocular nystagmus) consist of a slow movement of the eyes in the opposite direction to rotation, followed by a jump of the eyes back. The very occurrence and characteristics of rotational ocular nystagmus are important indicators of the state of the vestibular system; they are widely used in marine, aviation and space medicine, as well as in experiment and clinic.

Main afferent pathways and projections of vestibular signals. There are two main pathways for the entry of vestibular signals into the cerebral cortex: a direct route through the dorsomedial part of the ventral postlateral nucleus and an indirect vestibulocerebellothalamic pathway through the medial part of the ventrolateral nucleus. In the cerebral cortex, the main afferent projections of the vestibular apparatus are localized in the posterior part of the postcentral gyrus. In the motor cortex, in front of the lower part of the central sulcus, a second vestibular zone was found.

Functions of the vestibular system. The vestibular system helps the body to navigate in space during active and passive movement.

During passive movement, the cortical sections of the system remember the direction of movement, turns and the distance traveled. It should be emphasized that under normal conditions, spatial orientation is provided by the joint activity of the visual and vestibular systems. The sensitivity of the vestibular system of a healthy person is very high: the otolith apparatus allows one to perceive the acceleration of rectilinear movement, equal to only 2 cm/s2. The threshold for distinguishing the tilt of the head to the side is only about 1 °, and forward and backward - 1.5-2 °. The receptor system of the semicircular canals allows a person to notice accelerations of rotation of 2-3°∙ s-2.

The mechanism of excitation of sensory receptors (receptor potential and action potential).

The mechanism of excitation in sensory receptors is different. In the primary sensory receptor, the transformation of the energy of the stimulus and the emergence of impulse activity takes place in the sensory neuron itself. In the secondarily sensitive receptors, between the sensory neuron and the stimulus, there is a receptor cell, in which, under the influence of the stimulus, the processes of transformation of the energy of the stimulus into the process of excitation take place. But there is no impulse activity in this cell. Receptor cells are connected by synapses to sensory neurons. Under the influence of the potential of the receptor cell, a mediator is released, which excites the nerve ending of the sensory neuron and causes a local response in it - the postsynaptic potential. It has a depolarizing effect on the outgoing nerve fiber, in which impulse activity occurs.

Consequently, in secondary sensory receptors, local depolarization occurs twice: in the receptor cell and in the sensory "neuron. Therefore, it is customary to call the gradual electrical response of the receptor cell the receptor potential, and the local depolarization of the sensory neuron the generator potential, meaning that it generates in the nerve leaving the receptor spreading excitation in the fiber.In primary-sensing receptors, the receptor potential is also generator.Thus, the receptor act can be depicted in the form of the following diagram.

For primary sensory receptors:

Stage I - specific interaction of the stimulus with the receptor membrane;

Stage II - the emergence of a receptor potential at the site of interaction of the stimulus with the receptor as a result of a change in the permeability of the membrane for sodium (or calcium) ions;

Stage III - electrotonic propagation of the receptor potential to the axon of the sensory neuron (passive propagation of the receptor potential along the nerve fiber is called electrotonic);

IV stage - generation of action potential;

Stage V - conduction of the action potential along the nerve fiber in the orthodromic direction.

For secondary sensory receptors:

Stages I-III coincide with the same stages of primary sensory receptors, but they proceed in a specialized receptor cell and end on its presynaptic membrane;

Stage IV - the release of the mediator by the presynaptic structures of the receptor cell;

Stage V - the emergence of a generator potential on the postsynaptic membrane of the nerve fiber;

Stage VI - electrotonic propagation of the generator potential along the nerve fiber;

When a receptor is exposed to an adequate stimulus (to which it is evolutionarily adapted), which can cause confirmation changes in perceiving structures (activation of the receptor protein), a receptor potential (RP) is formed.

In receptors (except for photoreceptors), the energy of the stimulus, after its transformation and amplification, leads to the opening of ion channels and the movement of ions, among which the main role is played by the movement of Na + into the cell. This leads to depolarization of the receptor membrane. It is believed that in mechanoreceptors, membrane stretching leads to channel expansion. The receptor potential is local, it can only propagate electrotonically to short distances- up to 3 mm.

The occurrence of AP in primary and secondary receptors occurs in different ways.

In the primary receptor, the receptor zone is part of the afferent neuron - the end of its dendrite. It is attached to the receptor. The resulting RP, spreading electrotonically, causes depolarization of the nerve ending and the occurrence of PD. In myelinated fibers, AP occurs in the nearest nodes of Ranvier, i.e. in areas with a sufficient concentration of potential-dependent sodium and potassium channels, with short dendrites, for example, in olfactory cells - in the axon hillock. When membrane depolarization reaches a critical level, AP is generated.

In secondary receptors, RP occurs in a receptor cell that is synaptically connected to the end of the dendrite of an afferent neuron.

The receptor potential ensures the release of the mediator by the receptor cell into the synaptic cleft. Under the influence of a mediator, a generator potential arises on the postsynaptic membrane, which ensures the occurrence of AP in the nerve ending near the postsynaptic membrane. The generator potential, like the receptor potential, is a local potential.

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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 the primary sensory receptors, which are free bare endings 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. Consequently, 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 a 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.

1. Pavlov about analyzers. Structure and functions of analyzers. The mechanism of excitation in receptors. Receptor and generator potentials.

The doctrine of analyzers was created. The analyzer considered the totality of neurons involved in the perception of stimuli, the conduction of excitation, as well as the analysis of its properties by the cells of the cerebral cortex. The analyzer was first considered as a single system, including the receptor apparatus (peripheral section of the analyzer), afferent neurons and pathways (conductor section) and areas of the cerebral cortex that perceive afferent signals (the central end of the analyzer). Experiments with the removal of sections of the cortex and the study of the resulting violations of conditioned reflex reactions led to the conclusion that in the cortical section of the analyzer there are primary projection zones (nuclear zones) and the so-called scattered elements that analyze incoming information outside the nuclear zone of the cerebral cortex. Even before the advent of modern analytical (in particular, electrophysiological) research methods, he made available for objective experimental analysis the spatio-temporal interaction of nervous processes at the higher, cortical levels of analyzer systems.

Analyzers are complex sensitive formations of the nervous system that perceive stimuli from environment and responsible for the formation of sensations. There are three parts of any analyzer:

Ø Peripheral or receptor department, which carries out the perception of the energy of the stimulus and its transformation into a specific process of excitation.

Ø The conductor department, represented by afferent nerves and subcortical centers, it transfers the resulting excitation to the cerebral cortex.

Ø The central or cortical section of the analyzer, represented by the corresponding zones of the cerebral cortex, where the highest analysis and synthesis of excitations and the formation of the corresponding sensation are carried out.

Analyzers perform a large number of functions or operations on signals. Among them the most important:

I. Signal detection.

II. Distinguishing signals.

III. Transmission and conversion of signals.

IV. Encoding of incoming information.

V. Detection of certain signs of signals.

VI. Image recognition.

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.

In primary sensory receptors, the stimulus acts on the perceptive substrate embedded in the sensory neuron itself, which in this case is excited directly (primarily) by the stimulus. Primary sensory receptors include: olfactory, tactile receptors and muscle spindles.

Secondary sensory receptors include those receptors in which additional receptor cells are located between the active stimulus and the sensory neuron, while the sensory neuron is not excited directly by the stimulus, but indirectly (secondarily) - by the potential of the receptor cell. Secondary sensory receptors include: receptors for hearing, vision, taste, vestibular receptors.

The mechanism of excitation in these receptors is different. In the primary sensory receptor, the transformation of the energy of the stimulus and the emergence of impulse activity takes place in the sensory neuron itself. In secondary sensory receptors, between the sensory neuron and the stimulus, there is a receptor cell, in which, under the influence of the stimulus, the processes of transformation of the energy of the stimulus into the process of excitation take place. But there is no impulse activity in this cell. Receptor cells are connected by synapses to sensory neurons. Under the influence of the potential of the receptor cell, a mediator is released, which excites the nerve ending of the sensory neuron and causes a local response in it - the postsynaptic potential. It has a depolarizing effect on the outgoing nerve fiber, in which impulse activity occurs.

For primary sensory receptors:

Stage I - specific interaction of the stimulus with the receptor membrane;

Stage II - the emergence of a receptor potential at the site of interaction of the stimulus with the receptor as a result of a change in the permeability of the membrane for sodium (or calcium) ions;

Stage III - electrotonic propagation of the receptor potential to the axon of the sensory neuron (passive propagation of the receptor potential along the nerve fiber is called electrotonic);

IV stage - generation of action potential;

Stage V - conduction of the action potential along the nerve fiber in the orthodromic direction.

For secondary sensory receptors:

Stages I-III coincide with the same stages of primary sensory receptors, but they proceed in a specialized receptor cell and end on its presynaptic membrane;

Stage IV - the release of the mediator by the presynaptic structures of the receptor cell;

Stage V - the emergence of a generator potential on the postsynaptic membrane of the nerve fiber;

Stage VI - electrotonic propagation of the generator potential along the nerve fiber;

Stage VII - generation of an action potential by electrogenic sections of the nerve fiber;

Stage VIII - conduction of the action potential along the nerve fiber in the orthodromic direction.

2. Physiology of the visual analyzer. receptor apparatus. Photochemical processes in the retina under the action of light.

The visual analyzer is a set of structures that perceive light energy in the form of electromagnetic radiation with a wavelength of nm and discrete particles of photons, or quanta, and form visual sensations. With the help of the eye, 80-90% of all information about the world around us is perceived.

Thanks to the activity of the visual analyzer, the illumination of objects, their color, shape, size, direction of movement, the distance at which they are removed from the eye and from each other are distinguished. All this allows you to evaluate the space, navigate the world around you, and perform various types of purposeful activities.

Along with the concept of the visual analyzer, there is the concept of the organ of vision.

The organ of vision is the eye, which includes three functionally different elements:

Ø the eyeball, in which the light-perceiving, light-refracting and light-regulating apparatus are located;

Ø protective devices, i.e., the outer shells of the eye (sclera and cornea), lacrimal apparatus, eyelids, eyelashes, eyebrows;

Ø motor apparatus, represented by three pairs of eye muscles (external and internal rectus, superior and inferior rectus, superior and inferior oblique), which are innervated by III (oculomotor nerve), IV (trochlear nerve) and VI (abducens nerve) pairs of cranial nerves.

The receptor (peripheral) section of the visual analyzer (photoreceptors) is subdivided into rod and cone neurosensory cells, the outer segments of which are, respectively, rod-shaped (“rods”) and cone-shaped (“cones”) shapes. A person has 6-7 million cones and million daddies.

The exit point of the optic nerve from the retina does not contain photoreceptors and is called the blind spot. Lateral to the blind spot in the region of the fovea lies the area of best vision - the yellow spot, containing mainly cones. Towards the periphery of the retina, the number of cones decreases, and the number of rods increases, and the periphery of the retina contains only rods.

Differences in the functions of cones and rods underlie the phenomenon of dual vision. Rods are receptors that perceive light rays in low light conditions, that is, colorless, or achromatic, vision. Cones, on the other hand, function in bright light conditions and are characterized by different sensitivity to the spectral properties of light (color or chromatic vision). Photoreceptors have a very high sensitivity, which is due to the peculiarity of the structure of the receptors and the physicochemical processes that underlie the perception of light stimulus energy. It is believed that photoreceptors are excited by the action of 1-2 light quanta on them.

Rods and cones consist of two segments - outer and inner, which are interconnected by means of a narrow cilium. The rods and cones are oriented radially in the retina, and the molecules of photosensitive proteins are located in the outer segments in such a way that about 90% of their photosensitive groups lie in the plane of the discs that make up the outer segments. Light has the greatest exciting effect if the direction of the beam coincides with the long axis of the rod or cone, while it is directed perpendicular to the discs of their outer segments.

Photochemical processes in the retina. In the receptor cells of the retina are light-sensitive pigments (complex protein substances) - chromoproteins, which discolor in the light. The rods on the membrane of the outer segments contain rhodopsin, the cones contain iodopsin and other pigments.

Rhodopsin and iodopsin consist of retinal (vitamin A1 aldehyde) and glycoprotein (opsin). Having similarities in photochemical processes, they differ in that the absorption maximum is located in different regions of the spectrum. Rods containing rhodopsin have an absorption maximum in the region of 500 nm. Among the cones, three types are distinguished, which differ in the maxima in the absorption spectra: some have a maximum in the blue part of the spectrum (nm), others in the green (, others - in the red (nm) part, due to the presence of three types of visual pigments. The red cone pigment received the name "iodopsin". Retinal can be in various spatial configurations (isomeric forms), but only one of them - the 11-CIS isomer of retinal acts as a chromophore group of all known visual pigments. Carotenoids serve as a source of retinal in the body.

Photochemical processes in the retina proceed very economically. Even with action bright light only a small part of the rhodopsin present in the sticks is cleaved (about 0.006%).

In the dark, resynthesis of pigments takes place, proceeding with the absorption of energy. The recovery of iodopsin proceeds 530 times faster than that of rhodopsin. If the content of vitamin A in the body decreases, then the processes of resynthesis of rhodopsin weaken, which leads to impaired twilight vision, the so-called night blindness. With constant and uniform illumination, a balance is established between the rate of disintegration and resynthesis of pigments. When the amount of light falling on the retina decreases, this dynamic balance is disturbed and shifted towards higher pigment concentrations. This photochemical phenomenon underlies dark adaptation.

Of particular importance in photochemical processes is the pigment layer of the retina, which is formed by an epithelium containing fuscin. This pigment absorbs light, preventing its reflection and scattering, which determines the clarity of visual perception. The processes of pigment cells surround the light-sensitive segments of rods and cones, taking part in the metabolism of photoreceptors and in the synthesis of visual pigments.

Due to photochemical processes in the photoreceptors of the eye, under the action of light, a receptor potential arises, which is a hyperpolarization of the receptor membrane. This is a distinctive feature of the visual receptors, the activation of other receptors is expressed in the form of depolarization of their membrane. The amplitude of the visual receptor potential increases with increasing intensity of the light stimulus. So, under the action of red, the wavelength of which is nm, the receptor potential is more pronounced in the photoreceptors of the central part of the retina, and blue (nm) - in the peripheral.

The synaptic endings of the photoreceptors converge to the bipolar neurons of the retina. In this case, the photoreceptors of the fovea are associated with only one bipolar. The conduction section of the visual analyzer starts from the bipolar cells, then the ganglion cells, then the optic nerve, then the visual information enters the lateral geniculate bodies of the thalamus, from where it is projected onto the primary visual fields as part of the visual radiation.

The primary visual fields of the cortex are field 16 and field 17 is the spur sulcus of the occipital lobe.

A person is characterized by binocular stereoscopic vision, that is, the ability to distinguish the volume of an object and view it with two eyes. Characterized by light adaptation, that is, adaptation to certain lighting conditions.

3. auditory analyzer. Sound-catching and sound-conducting apparatus of the organ of hearing.

With the help of an auditory analyzer, a person orients himself in the sound signals of the environment, forms appropriate behavioral reactions, such as defensive or food-procuring ones. The ability of a person to perceive spoken and vocal speech, musical works makes the auditory analyzer a necessary component of the means of communication, cognition, and adaptation.

An adequate stimulus for the auditory analyzer is sounds, i.e., oscillatory movements of particles of elastic bodies propagating in the form of waves in a wide variety of media, including air, and perceived by the ear. Sound wave vibrations (sound waves) are characterized by frequency and amplitude. The frequency of sound waves determines the pitch of the sound. A person distinguishes sound waves with a frequency of 20 kHz. Sounds whose frequency is below 20 Hz - infrasounds and above Hz (20 kHz) - ultrasounds, are not felt by a person. Sound waves that have sinusoidal, or harmonic, oscillations are called tone. Sound consisting of unrelated frequencies is called noise. At a high frequency of sound waves, the tone is high, at a low frequency, it is low.

The second characteristic of sound that the auditory sensory system distinguishes is its strength, which depends on the amplitude of the sound waves. The strength of a sound or its intensity is perceived by a person as loudness. The sensation of loudness increases with amplification of the sound and also depends on the frequency of sound vibrations, i.e., the loudness of the sound is determined by the interaction of the intensity (strength) and height (frequency) of the sound. The unit of sound loudness is bel, in practice the decibel (dB) is usually used, i.e. 0.1 bela. A person also distinguishes sounds by timbre, or "color". The timbre of the sound signal depends on the spectrum, i.e., on the composition of additional frequencies (overtones) that accompany the main tone (frequency). By timbre, one can distinguish sounds of the same height and loudness, on which the recognition of people by voice is based. The sensitivity of the auditory analyzer is determined by the minimum sound intensity sufficient to produce an auditory sensation. In the region of sound vibrations from 1000 to 3000 per second, which corresponds to human speech, the ear has the greatest sensitivity. This set of frequencies is called the speech zone. In this area, sounds are perceived having a pressure of less than 0.001 bar (1 bar is approximately one millionth of normal atmospheric pressure). Based on this, in transmitting devices, in order to provide an adequate understanding of speech, speech information must be transmitted in the speech frequency range.

Structural and functional characteristics

The receptor (peripheral) section of the auditory analyzer, which converts the energy of sound waves into the energy of nervous excitation, is represented by receptor hair cells of the organ of Corti (the organ of Corti) located in the cochlea. Auditory receptors (phonoreceptors) are mechanoreceptors, are secondary and are represented by inner and outer hair cells. Humans have approximately 3,500 inner and outer hair cells, which are located on the basilar membrane inside the middle canal of the inner ear.

The inner ear (sound-receiving apparatus), as well as the middle ear (sound-transmitting apparatus) and the outer ear (sound-catching apparatus) are united in the concept of the organ of hearing.

The outer ear, due to the auricle, captures sounds, concentrates them in the direction of the external auditory canal and enhances the intensity of sounds. In addition, the structures of the outer ear perform a protective function, protecting the eardrum from the mechanical and thermal effects of the external environment.

The middle ear (the sound-conducting section) is represented by the tympanic cavity, where three auditory ossicles are located: the hammer, anvil and stirrup. The middle ear is separated from the external auditory canal by the tympanic membrane. The handle of the malleus is woven into the eardrum, its other end is articulated with the anvil, which, in turn, is articulated with the stirrup. The stirrup is adjacent to the membrane of the oval window. The area of the tympanic membrane (70 mm2) is much larger than the area of the oval window (3.2 mm2), due to which the pressure of sound waves on the membrane of the oval window increases by about 25 times. Since the lever mechanism of the ossicles reduces the amplitude of sound waves by about 2 times, then, consequently, the same amplification of sound waves occurs at the oval window. Thus, there is a general amplification of sound by the middle ear approximately at once. If we take into account the reinforcing effect of the outer ear, then this value reaches times. The middle ear has a special protective mechanism, represented by two muscles: the muscle that stretches the eardrum and the muscle that fixes the stirrup. The degree of contraction of these muscles depends on the strength of the sound vibrations. With strong sound vibrations, the muscles limit the amplitude of vibrations of the tympanic membrane and the movement of the stirrup, thereby protecting the receptor apparatus in the inner ear from excessive excitation and destruction. With instantaneous strong irritations (hitting the bell), this protective mechanism does not have time to work. The contraction of both muscles of the tympanic cavity is carried out according to the mechanism of the unconditioned reflex, which closes at the level of the brain stem. In the tympanic cavity, pressure equal to atmospheric pressure is maintained, which is very important for adequate perception of sounds. This function is performed by the Eustachian tube, which connects the middle ear cavity with the pharynx. When swallowing, the tube opens, ventilating the middle ear cavity and equalizing the pressure in it with atmospheric pressure. If the external pressure changes rapidly (rapid rise to a height), and swallowing does not occur, then the pressure difference between the atmospheric air and the air in the tympanic cavity leads to the tension of the eardrum and the appearance of unpleasant sensations, a decrease in the perception of sounds.

The inner ear is represented by the cochlea - a spirally twisted bone canal with 2.5 curls, which is divided by the main membrane and Reissner's membrane into three narrow parts (ladders). The upper canal (scala vestibularis) starts from the foramen ovale and connects to the lower canal (scala tympani) through the helicotrema (apical opening) and ends with a round window. Both channels are a single whole and are filled with perilymph, similar in composition to the cerebrospinal fluid. Between the upper and lower channels is the middle (middle staircase). It is isolated and filled with endolymph. Inside the middle canal on the main membrane is the actual sound-perceiving apparatus - the organ of Corti (organ of Corti) with receptor cells, representing the peripheral section of the auditory analyzer.

Mace" href="/text/category/bulava/" rel="bookmark"> club, from which protrudes along the thinnest cilia 10 microns long. Olfactory cilia are immersed in a liquid medium produced by the olfactory glands. The presence of cilia tenfold increases the contact area receptor with odor molecules.

With a calm breath, the air stream does not enter the narrow gap between the superior nasal concha and the nasal septum, where the olfactory region is located, and therefore the molecules of odorous substances can penetrate into it only with the help of diffusion. Forced inhalation, as well as quick, short inhalations made during sniffing, cause vortex air movements in the nasal cavity, which contributes to the penetration of air into the olfactory region. From the oral cavity (for example, during meals), odorous molecules diffuse into the nasopharynx and easily enter the nasal cavity along with the exhaled air. To act on the receptors, they must be adsorbed and dissolved on the moist surface of the olfactory epithelium.

The sensitivity of the olfactory receptors is unusually high. Some substances, such as trinitrobutyltoluene, can be detected by a person by smell even when the content of air in a liter is billionths of a milligram. In many animals, the sensitivity of the olfactory analyzer is many times higher than in humans.

Availability huge amount organic and inorganic odorous substances of the most varied structure renders untenable attempts at a purely chemical explanation of their effect on receptors. It is possible that the energy of intramolecular vibrations causes those physicochemical shifts in the olfactory vesicle that lead to the onset of the excitation process. Such a mechanism of irritation, if it really exists, would be similar to the photochemical mechanism of irritation of the photosensitive elements of the retina.

Olfactory cells, equipped with a receptor formation at the end of their peripheral process, represent the first neuron of the pathways of the olfactory analyzer. These are typical bipolar cells, homologous to the cells of the intervertebral nodes of the spinal cord. Their axons, not covered with a myelin sheath, form up to 20 thin nerve trunks. Through the holes of the ethmoid bone, they pass into the cranial cavity and penetrate into the olfactory bulb, that is, into the anterior, thickened end of the olfactory tract. Here are the bodies of the second neuron. The terminal branches of the axons of several bipolar cells approach the dendrites of each of them. The axons of the second neuron form the olfactory tract and go to the bodies of the third neuron, located in the amygdala nucleus, in the anterior, curved end of the ammonian gyrus and in the subcallosal gyrus. The axons of the third neuron are sent to the cortical section of the olfactory analyzer.

In addition to these main pathways that reach the cortical region of the olfactory analyzer, there are also pathways that connect the axons of the second neuron with the diencephalon, as well as with various accumulations of gray matter in the middle, posterior, and spinal cord. Through these pathways, motor and sensory reactions to irritation of olfactory receptors are carried out. Apparently, the bridle, which is part of the epithalamus, plays the same role in relation to reflexes to irritation of the olfactory organs as the quadrigemina in relation to reflexes to light and sound stimuli.

The nucleus of the olfactory analyzer in humans is located in the formations of the old cortex, namely, in the depths of the furrow of the Ammon's horn. The nuclei of the analyzer of both hemispheres are connected to each other by conducting paths. Some neighboring formations of the interstitial cortex should also be attributed to the olfactory analyzer. The adjacent areas of the insular region, which lie deep in the Sylvian fissure, apparently have the same significance for smell as projection-associative fields 18 and 19 have for the visual function.

There is reason to believe that the olfactory analyzer also includes a small portion of the marginal region located on the inner surface of the hemisphere in the form of a narrow strip along the corpus callosum. From the cortical section of the olfactory analyzer, efferent paths go to the underlying parts of the brain, in particular to the nipple bodies of the hypothalamic region and to the epithelium frenulum. Through these pathways, cortical reflexes to olfactory stimuli are carried out.

Some irritants, such as vanillin and guaiacol, act only on olfactory receptors. Many other volatile substances simultaneously irritate other receptors.

So, benzene, nitrobenzene, chloroform act on taste buds, as a result of which their smell has a sweetish aftertaste. Chlorine, bromine, ammonia, formalin excite pain and tactile receptors of the nasal mucosa. Menthol, phenol, camphor irritate cold receptors, and ethyl alcohol - heat and pain. Acetic acid acts on taste and pain receptors (hence the sour and pungent smell of vinegar), etc. There are pharmacological and physiological data on the existence of various types of receptors that have unequal sensitivity to individual odors. This indicates that the analysis of olfactory stimuli begins at the periphery. Higher analysis and synthesis of odor stimuli occurs in the cerebral cortex.

The complex nature of most olfactory sensations, associated with simultaneous stimulation of not only olfactory, but also other receptors, determines the close interaction of the cortical sections of the three analyzers - olfactory, gustatory, and that part of the skin where impulses from the nasal mucosa are received. Therefore, not only the areas of the cerebral cortex mentioned above, but also the gyrus of the Ammon's horn and Bottom part postcentral gyrus. A person does not distinguish between the individual components that make up a complex odor. If you mix two or more different-smelling substances, then the smell of the mixture may be either similar to the smell of one of them, or sharply different from the smell of each of its constituent parts.

Using various combinations of volatile substances in strictly defined proportions, perfumers achieve great skill in creating new scents. The ability to suppress one odor with another is used for deodorization purposes, that is, to neutralize the smell of malodorous substances.

The cortical sections of the olfactory analyzer of both hemispheres are so closely interconnected that with a purely olfactory stimulation, a person does not distinguish which half of the nasal cavity the volatile substance has entered. Stimulation of other receptors in the nasal cavity produces localized sensations. By injecting one odorous substance through the right nostril and another through the left, one can obtain the suppression of one odor by another, as well as the appearance of a completely new odor. This shows that the analysis and synthesis of olfactory stimuli mainly takes place not on the periphery, but in the cortical section of the analyzer. In some cases, it is possible to observe the following phenomenon - instead of a continuous sensation of the same smell of a mixture, alternating sensations of the smell of one or another substance appear.

In most mammals, the perfection of the analytical-synthetic function of the olfactory analyzer reaches extremely high limits. In humans, in connection with the development of speech and labor activity vitality of this analyzer has sharply decreased in comparison with the value of the visual, auditory, tactile and motor analyzers. Conditioned reflexes to the action of odor stimuli are formed in a person in an immeasurably smaller amount than in a dog or cat; this corresponds to the relatively weak development of the cortical section of the olfactory analyzer. Children have positive conditioned reflexes it is possible to develop on odor stimuli at the 5-6th week of life; the formation of gross differentiations becomes possible for the most part not earlier than the beginning of the third month. However, subtle differentiations (for example, distinguishing different varieties cologne) begin to be produced much later, and even then with great difficulty. Often, even adults, despite the absence of any disturbances in the peripheral section of the analyzer, distinguish odors very poorly.

In those cases where odor stimuli acquire significant significance for a person, the analytical-synthetic activity of the olfactory analyzer can reach great perfection, up to distinguishing between the components of an odor mixture. This is observed in some perfumers, cooks, etc.

5. Taste reception. Types of taste sensations. Features of the conduction department.

The sense of taste is associated with irritation of not only chemical, but also mechanical, temperature and even pain receptors of the oral mucosa, as well as olfactory receptors. The taste analyzer determines the formation of taste sensations, is a reflexogenic zone. With the help of a taste analyzer, various qualities of taste sensations are evaluated, the strength of sensations, which depends not only on the strength of irritation, but also on functional state organism.

Structural and functional characteristics of the taste analyzer.

Peripheral department. Taste receptors (taste cells with microvilli) are secondary receptors, they are an element of taste buds, which also include supporting and basal cells. Taste buds contain serotonin-containing cells and histamine-producing cells. These and other substances play a role in the formation of the sense of taste. Individual taste buds are polymodal formations, as they can perceive various types of taste stimuli. Taste buds in the form of separate inclusions are located on the back wall of the pharynx, soft palate, tonsils, larynx, epiglottis and are also part of the taste buds of the tongue as an organ of taste.

The peripheral part of the taste analyzer is represented by taste buds, which are located mainly in the papillae of the tongue. Taste cells are dotted at their end with microvilli, which are also called taste hairs. They reach the surface of the tongue through the taste pores.

There are a large number of synapses on the taste cell, which form the fibers of the tympanic string and the glossopharyngeal nerve. The fibers of the tympanic string (a branch of the lingual nerve) approach all fungiform papillae, and the fibers of the glossopharyngeal nerve approach the grooved and foliate ones. The cortical end of the taste analyzer is located in the hippocampus, the parahippocampal gyrus, and in the lower part of the posterocentral gyrus.

Taste cells are constantly dividing and constantly dying. Particularly fast is the replacement of cells located in the anterior part of the tongue, where they lie more superficially. Replacement of taste bud cells is accompanied by the formation of new synaptic structures

Conductor department. Inside the taste bud are nerve fibers that form receptor-afferent synapses. The taste buds of different areas of the oral cavity receive nerve fibers from different nerves: the taste buds of the anterior two-thirds of the tongue - from the tympanic string, which is part of the facial nerve; kidneys of the posterior third of the tongue, as well as the soft and hard palate, tonsils - from the glossopharyngeal nerve; taste buds located in the pharynx, epiglottis and larynx - from the upper laryngeal nerve, which is part of the vagus nerve.

These nerve fibers are peripheral processes of bipolar neurons located in the corresponding sensory ganglia, representing the first neuron of the conductive section of the taste analyzer. The central processes of these cells are part of a single bundle of the medulla oblongata, the nuclei of which represent the second neuron. From here, the nerve fibers in the medial loop approach the thalamus opticus (the third neuron).

Central department. The processes of the thalamus neurons go to the cerebral cortex (fourth neuron). The central, or cortical, section of the taste analyzer is localized in the lower part of the somatosensory cortex in the region of the language representation. Most of the neurons in this area are multimodal, that is, they respond not only to taste, but also to temperature, mechanical, and nociceptive stimuli. The taste sensory system is characterized by the fact that each taste bud has not only afferent, but also efferent nerve fibers that are suitable for taste cells from the central nervous system, which ensures the inclusion of the taste analyzer in the integral activity of the body.

Mechanism of taste perception. For a taste sensation to occur, the irritating substance must be in a dissolved state. A sweet or bitter taste substance, which dissolves in saliva to molecules, penetrates into the pores of the taste buds, interacts with the glycocalyx, and is adsorbed on the microvillus cell membrane, into which “sweet-sensing” or “bitter-sensing” receptor proteins are embedded. When exposed to salty or sour taste substances, the concentration of electrolytes around the taste cell changes. In all cases, the permeability of the cell membrane of the microvilli increases, sodium ions move inside the cell, the membrane depolarizes and the receptor potential is formed, which propagates both along the membrane and along the microtubular system of the taste cell to its base. At this time, a mediator (acetylcholine, serotonin, and, possibly, hormone-like substances of a protein nature) is formed in the taste cell, which in the receptor-afferent synapse leads to the emergence of a generator potential, and then an action potential in the extrasynaptic sections of the afferent nerve fiber.

Perception of taste stimuli. Microvilli of taste cells are formations that directly perceive the taste stimulus. The membrane potential of taste cells ranges from -30 to -50 mV. Under the action of taste stimuli, a receptor potential of 15 to 40 mV arises. It is a depolarization of the surface of the taste cell, which is the cause of the appearance of a generator potential in the fibers of the tympanic string and the glossopharyngeal nerve, which, upon reaching a critical level, turns into propagating impulses. From the receptor cell, excitation is transmitted through the synapse to the nerve fiber with the help of acetylcholine. Some substances, such as CaCl2, quinine, salts of heavy metals, do not cause primary depolarization, but primary hyperplyarization. Its occurrence is associated with the implementation of negative rejected reactions. In this case, no propagating pulses arise.

Unlike olfactory taste sensations can easily be combined into groups according to similar characteristics. There are four basic taste sensations - sweet, bitter, sour and salty, which in their combinations can give diverse shades of taste.

The sensation of sweet is caused by carbohydrates contained in food substances (dihydric and polyhydric alcohols, monosaccharides, etc.); bitter sensation - by affecting the taste buds of various alkaloids; the sensation of sour arises from the action of various acids dissolved in water; the feeling of salty is caused by table salt (sodium chloride) and other chlorine compounds.

6. Skin analyzer: types of reception, conduction department, representation in the cerebral cortex.

The skin analyzer includes a set of anatomical formations, the coordinated activity of which determines such types of skin sensitivity as a feeling of pressure, stretching, touch, vibration, heat, cold and pain. All receptor formations of the skin, depending on their structure, are divided into two groups: free and non-free. Non-free, in turn, are divided into encapsulated and non-encapsulated.

Free nerve endings are represented by the terminal branches of the dendrites of sensory neurons. They lose myelin, penetrate between epithelial cells and are located in the epidermis and dermis. In some cases, the terminal branches of the axial cylinder envelop the altered epithelial cells, forming tactile menisci.

Non-free nerve endings consist not only of branching fibers that have lost myelin, but also of glial cells. Non-free encapsulated receptor formations of the skin include plastic bodies, or bodies of Vater-Pacini, visible to the naked eye (for example, on a cut of the skin of the fingers), in fatty tissue. Touch is perceived by tactile bodies (Meissner bodies, Krause flasks, etc.) of the papillary layer of the skin proper, tactile discs of the germ layer of the epidermis. Hair roots are braided with nerve cuffs.

The density of the location of receptors in the skin of different parts of the body is not the same and is functionally determined. The receptors embedded in the skin serve as peripheral parts of the skin analyzer, which, due to its length, is essential for the body.

Excitation from the receptors of the skin analyzer is sent to the central nervous system through thin and wedge-shaped bundles. In addition, impulses from skin receptors travel along the dorsal-tuberous tract and the ternary loop, and from proprioreceptors - along the spinocerebellar tracts.

A thin bundle carries impulses from the body below the 5th thoracic segment, and a wedge-shaped bundle carries impulses from the upper body and arms. These paths are formed by neurites of sensitive neurons, the bodies of which lie in the spinal ganglions, and the dendrites terminate in skin receptors. Having passed the entire spinal cord and the posterior part of the medulla oblongata, the fibers of the thin and wedge-shaped bundles end on the neurons of the ton and wedge-shaped nuclei. The fibers of the thin and wedge-shaped nuclei go in two directions. Some - called external arcuate fibers - go to the opposite side, where, as part of the lower legs of the cerebellum, they end on the cells of the cortex of its worm. The neurites of the latter connect the cortex of the vermis with the nuclei of the cerebellum. The fibers of the cells of these nuclei as part of the lower cerebellar peduncles are sent to the vestibular nuclei of the bridge.

Another, most of the fibers of the cells of the thin and sphenoid nuclei in front of the central canal of the medulla oblongata crosses and forms a medial loop. The latter goes through the medulla oblongata, tires of the bridge and midbrain and ends in the ventral nucleus of the thalamus. The fibers of the neurons of the thalamic thalamic radiance go to the cortex of the central regions of the cerebral hemispheres.

The dorsal tuberculous path conducts excitation from receptors, the irritation of which causes pain and temperature sensations. The cell bodies of the sensory neurons of this pathway lie in the spinal ganglia. The central fibers of neurons are part of the posterior roots in the spinal cord, where they terminate on the bodies of intercalary neurons of the posterior horns. The processes of the cells of the posterior horns pass to the opposite side and, in the depths of the lateral funiculus, are connected to the dorsal tuberous path. The latter passes through the spinal cord, tegmentum oblongata, pons and legs of the brain and ends on the cells of the ventral nucleus of the thalamus. The fibers of these neurons go as part of the thalamic radiance to the cortex, where they end, mainly in the posterior central region.

The ternary loop transmits impulses from scalp receptors. Cells of the ternary node serve as sensitive neurons. Peripheral fibers of these cells pass through three branches trigeminal nerve that innervates the skin of the face. The central fibers of the sensory neurons emerge from the node as part of the sensory root of the trigeminal nerve and enter the brain at the point where it passes into the middle cerebellar peduncles. In the pons, these fibers divide in a T-shape into ascending and descending branches (spinal tract), which terminate in neurons that form the main sensory nucleus of the trigeminal nerve in the operculum of the pons, and the nucleus of its spinal tract in the medulla oblongata and spinal cord. The central fibers of these nuclei cross over in the upper part of the pons and, as a ternary loop, pass along the tegmentum of the midbrain to the thalamus, where they terminate independently or together with the fibers of the medial loop on the cells of its ventral nucleus. The processes of the neurons of this nucleus are sent as part of the thalamic radiation to the cortex of the lower part of the posterior central region, where the skin analyzer of the head is mainly localized.

7. Balance organ: importance in human and animal life. Features of the receptor apparatus.

The vestibular analyzer or balance organ provides a sense of the position and movement of the human body or its parts in space, and also determines the orientation and maintenance of posture in all possible types of human activity.

Fig 17 The structure and location of the labyrinth and receptors of the otolith apparatus:

1, 2, 3 - respectively horizontal, frontal and sagittal channels in a semicircle; 4,5 - otolith apparatus: oval (4) and round (5) sacs; 6,7 - nerve ganglia, 8 - vestibulo-cochlear nerve (Shra cranial nerves), 9 - otoliths; 10-jelly-like mass, 11 - hairs, 12 - receptor hair cells, 13 - supporting cells, 14 - nerve fibers

The peripheral (receptor) section of the vestibular analyzer is located, like the inner ear, in the labyrinths of the pyramid of the temporal bone. It lies in the so-called vestibular apparatus (Fig. 17) and consists of the vestibule (otolith organ) and three semicircular canals, arranged in three mutually perpendicular planes: horizontal, frontal (left to right), and agital (anterior-posterior) vestibule or vestibule consists, as indicated, of two membranous sacs: round, located closer to the curl of the inner ear and oval (pistil), located closer to the semicircular canals the membranous part of the semicircular canals are connected by five openings to the pistil of the vestibule The initial end of each semicircular canal has an extension, which is called the ampulla ear On the inner surface of the sacs there are small elevations (spots) where exactly the balance receptors are located, or the otolith apparatus, which is placed semi-vertically in the oval Bear and horizontally in the round sac. In the otolith apparatus there are receptor hair cells (mechanoreceptors), which have hairs on their top (cilia) of two types, many thin and short stereocilia and one thicker and longer hair that grows on the periphery and is called kinocilium Receptor hair cells of spots on the surface of vestibular sacs are collected in groups called poppy them of the so-called otolithic membrane containing numerous crystals of phosphate and calcium carbonate, called otoliths (literally translated as ear stones) The ends of the stereocilia of the hair cells of the macula freely support and hold the otolithic membrane (Fig. 18).

Due to the otoliths (solid inclusion), the density of the otolithic membrane is higher than the density of the environment that surrounds it. Under the influence of gravity or acceleration, the otolithic membrane is displaced relative to the receptors of the receptor cells, the hairs (kinocilia) of these cells are bent and excitation occurs in them. Thus, the otolithic apparatus every moment controls the position of the body relative to gravity, determines in which position in space (in horizontal or vertical) the body is located, and also reacts to rectilinear accelerations during vertical and horizontal movements of the body. The sensitivity threshold of the otolith apparatus to rectilinear accelerations is 2-20 cm / sec, and the threshold for recognizing the tilt of the head to the side is 1 °; forward and backward - about 2 ° With concomitant stimuli (when exposed to vibration, oscillation, shaking), the sensitivity of the vestibular analyzer decreases (for example, transport vibrations can increase the threshold for recognizing head tilt forward and backward up to 5 °, and to the side, up to 10 ° 10 ° ).

The second part of the vestibular apparatus has three semicircular canals, each with a diameter of about 2 mm. On the inner surface of the ampullae of the semicircular canals (Fig. 18) there are scallops, on top of which the hair cells are grouped into cristae, over which there is a gelatinous mass from the otoliths, which here is called a leaf-shaped membrane or Kinocilia of the hair cells of the cristae, as it was also described for the otolithic apparatus of the vestibule sacs, are immersed in the cupula and are excited by endolymph movements that occur when the body moves in space. , the mediator acetylcholine is released, which stimulates the synaptic endings of the vestibular nerve If the displacement of stereocilia is directed to the side away from the kinocilium, then the activity of the vestibular nerve, on the contrary, decreases. The endolymph of the semicircular canals has the same density as the cupula of the ampullae, and therefore rectilinear accelerations do not affect the position of the hairs of the hair cells and the cupula. When the head or body is rotated, angular accelerations occur and then the cupula begins to move, exciting the receptor cells Rotation recognition threshold for receptors semicircular canals is approximately 2-3 ° / is 2-3 ° / sec.

The peripheral fibers of the bipolar neurons of the vestibular ganglion, which are located in the inner ear (first neurons), are suitable for the receptors of the vestibular apparatus. The axons of these neurons are woven together with the nerve fibers from the receptors of the inner ear and form a single vestibulo-cochlear or synovial-cochlear nerve (VIII cranio-cochlear nerve). of the cerebral nerves) Excitatory impulses about the position in space by this nerve enter the medulla oblongata (second neuron), in particular, to the vestibular center, where nerve impulses also come from muscle and joint receptors. The third neuron is also located in the nuclei of the optic tubercles of the midbrain, which in turn, they are connected by nerve pathways to the cerebellum (a part of the brain that provides coordination of movements), as well as to subcortical formations and the cerebral cortex (centers of movement, writing, speech, swallowing, etc.) The central section of the vestibular analyzer is localized in the temporal lobe the brain.

When the vestibular analyzer is excited, somatic reactions occur (based on the vestibulo-spinal nerve connections) that contribute to the redistribution of muscle tone and the constant maintenance of body balance in space. Reflexes that ensure body balance are divided into static (out of standing, sitting, etc.) and statokinetic. An example of a statokinetic reflex may be vestibular nystagmus ocularity AGM occurs in conditions of rapid movement of the body or its rotation and consists in the fact that the eyes first slowly move in the direction opposite to the direction of movement or rotation, and then, with a quick movement in the opposite direction of the mouth, jump to new point fixation of vision Reactions of this type provide the possibility of viewing space in the conditions of movement of bodies of the body.

Thanks to the connections of the vestibular nuclei with the cerebellum, all mobile reactions and reactions for coordinating movements are provided, including during labor operations or sports exercises. Vision and muscular-articular reception also contribute to maintaining weight balance.

The connection of the vestibular nuclei with the autonomic nervous system determines the vestibulo-vegetative reactions of cardio-vascular system, gastrointestinal tract and other organs. Such reactions can manifest themselves in changes in heart rate, vascular tone, blood pressure, nausea and vomiting may occur (for example, as happens with prolonged and strong action of specific traffic stimuli on the vestibule of the ravine apparatus, which leads to motion sickness).

The formation of the vestibular apparatus in children ends earlier than other analyzers. In a newborn child, this organ functions almost in the same way as in an adult. Training of motor qualities in children from early childhood helps to optimize the development of the vestibular analyzer and, as a result, diversifies their motor capabilities, up to phenomenal (for example, exercises of circus acrobats, gymnasts, etc.).