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 violations of conditioned reflex reactions arising after this led to the conclusion that there are primary projection zones (nuclear zones) and the so-called scattered elements in the cortical section of the analyzer 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 the environment and are 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 excitation process.
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 several 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 acting 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.
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 a fiber that propagates excitation.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;
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 apparatuses 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) part 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 disks 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 there are light-sensitive pigments (complex protein substances) - chromoproteins, which become discolored 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 (, and 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 under the action of bright light, only a small part of the rhodopsin present in the sticks (about 0.006%) is cleaved.
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 on 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, that is, oscillatory movements of particles of elastic bodies that propagate in the form of waves in a wide variety of media, including air, and are 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 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 atmospheric air and air in the tympanic cavity leads to 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-receiving 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 the thinnest cilia 10 microns long protrude. 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.
The presence of a huge amount of 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 part 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 different 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 above-mentioned areas of the cerebral cortex, but also the gyrus of Ammon's horn and the lower part of the postcentral gyrus take part in the analysis and synthesis of odor stimuli. 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 can 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, the vital importance 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. In children, positive conditioned reflexes to odor stimuli can be developed 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, the distinction between different varieties of cologne) begin to develop 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 the functional state of the body.
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 drum 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 hyperplarization. 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 features. 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 skin receptor formations 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. The peripheral fibers of these cells run as part of the three branches of the trigeminal nerve that innervate the skin of the face. The central fibers of 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 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 is connected by five holes to the pistil of the vestibule The initial end of each semicircular canal has an extension, which is called the ampulla ear H and 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) that 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 poppies Kinocilia of all hair cells are immersed in the gelatinous mass located above them the so-called otolithic membrane containing numerous crystals of calcium phosphate and carbonate, called otoliths (literally translated as ear stones) The ends of the stereocilia in the hair cells of the macula freely support and hold the otolithic membrane (Fig. eighteen).
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 responds 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 about 2 mm in diameter. 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. there is an acceleration or deceleration of rotation in certain planes. The fact is that 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. and then the cupula begins to move, exciting the receptor cells. The rotation recognition threshold for the receptors of the semicircular canals is approximately 2-3 ° / this is 2-3 ° / sec.
The peripheral fibers of the bipolar neurons of the vestibular ganglion, which are located in the inner ear (the 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 pair of cranial and 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 with the cerebellum (the part of the brain that provides coordination of movements), as well as with 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 it 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 a new point of fixation of vision of this type provide the possibility of viewing the 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 musculo-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 the cardiovascular system, gastrointestinal tract and other organs. Such reactions can manifest themselves in changes in heart rate, vascular tone, blood pressure, nausea and vomiting can occur (for example, like this happens with prolonged and strong action of specific stimuli of traffic 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, phenomenal (for example, exercises of circus acrobats, gymnasts, etc.).
When a receptor is exposed to an adequate stimulus (to which it is evolutionarily adapted), which can cause confirmation changes in the 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 over 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 a 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.
Receptors are called special formations that perceive and convert the energy of external irritation into the specific energy of a nerve impulse.
All receptors are divided into exteroreceptors, receiving stimuli from the external environment (receptors of the organs of hearing, sight, smell, taste, touch), interoreceptors responsive to stimuli from internal organs, and proprioreceptors, perceiving irritations from the motor apparatus (muscles, tendons, articular bags).
Depending on the nature of the stimulus to which they are tuned, distinguish chemoreceptors(receptors of taste and smell, chemoreceptors of blood vessels and internal organs), mechanoreceptors (proprioreceptors of the motor sensory system, baroreceptors of blood vessels, receptors of the auditory, vestibular, tactile and pain sensory systems), photoreceptors (receptors of the visual sensory system) and thermoreceptors (receptors of the sensory system of the skin and internal organs).
By the nature of the connection with the stimulus, distant receptors are distinguished, which react to signals from distant sources and cause warning reactions of the body (visual and auditory), and contact, receiving direct influences (tactile, etc.).
According to structural features, primary (primary-sensing) and secondary (secondary-sensing) receptors are distinguished.
Primary receptors are the endings of sensitive bipolar cells, the body of which is outside the CNS, one process approaches the surface perceiving irritation, and the other goes to the CNS (for example, proprioreceptors, tactile and olfactory receptors).
Secondary receptors are represented by specialized receptor cells that are located between the sensitive neuron and the point of application of the stimulus. These include receptors for taste, vision, hearing, and the vestibular apparatus. In practical terms, the most important is the psychophysiological classification of receptors according to the nature of the sensations that arise when they are stimulated. According to this classification, a person distinguishes between visual, auditory, olfactory, taste, tactile receptors, thermoreceptors, receptors for the position of the body and its parts in space (proprio- and vestibular receptors) and skin receptors.
The mechanism of excitation of receptors . In primary receptors, the energy of an external stimulus is directly converted into a nerve impulse in the most sensitive neuron. In the peripheral ending of sensitive neurons, under the action of a stimulus, a change in the permeability of the membrane for certain ions and its depolarization occurs, local excitation occurs - the receptor potential , which, having reached a threshold value, causes the appearance of an action potential propagating along the nerve fiber to the nerve centers.
In secondary receptors, the stimulus causes the appearance of a receptor potential in the receptor cell. Its excitation leads to the release of the mediator in the presynaptic part of the contact of the receptor cell with the fiber of the sensitive neuron. Local excitation of this fiber is reflected by the appearance of an excitatory postsynaptic potential (EPSP), or the so-called generator potential . When the threshold of excitability is reached in the fiber of the sensitive neuron, an action potential arises that carries information to the CNS. Thus, in secondary receptors, one cell converts the energy of an external stimulus into a receptor potential, and the other into a generator potential and an action potential. The postsynaptic potential of the first sensitive neuron is called the generator potential and it leads to the generation of nerve impulses.
4. Receptor Properties
1. The main property of receptors is their selective sensitivity to adequate stimuli, to the perception of which they are evolutionarily adapted (light for photoreceptors, sound for receptors of the cochlea of the inner ear, etc.). Most receptors are tuned to perceive one type (modality) of stimulus - light, sound, etc. The sensitivity of the receptors to such specific stimuli is extremely high. The excitability of the receptor is measured by the minimum value of the energy of an adequate stimulus, which is necessary for the occurrence of excitation, i.e. excitation threshold .
2. Another property of receptors is the very low value of thresholds for adequate stimuli. . For example, in the visual sensory system, photoreceptors can be excited by a single quantum of light in the visible part of the spectrum, olfactory receptors can be excited by the action of single molecules of odorous substances, etc. Excitation of receptors can also occur under the action of inadequate stimuli (for example, the sensation of light in the visual sensory system during mechanical and electrical stimuli). However, in this case, the excitation thresholds are much higher.
Distinguish between absolute and differential ( differential ) rapids . Absolute thresholds are measured by the minimum perceived magnitude of the stimulus. Differential thresholds represent the minimum difference between two stimulus intensities that is still perceived by the body (differences in color shades, light brightness, degree of muscle tension, articular angles, etc.).
3. The fundamental property of all living things is adaptation ,
those. adaptability to environmental conditions. Adaptation processes cover not only receptors, but also all links of sensory
systems.
Adaptation consists in the adaptation of all parts of the sensory system to a long-acting stimulus, and it manifests itself in a decrease in the absolute sensitivity of the sensory system. Subjectively, adaptation manifests itself in getting used to the action of a constant stimulus: entering a smoky room, a person stops smelling smoke after a few minutes; a person does not feel the constant pressure of his clothes on the skin, does not notice the continuous ticking of the clock, etc.
According to the rate of adaptation to prolonged stimuli, receptors are divided into quickly and slowly adapting. . The former, after the development of the adaptation process, practically do not inform the next neuron about the ongoing stimulation, in the latter this information is transmitted, although in a significantly reduced form (for example, , so-called secondary endings in muscle spindles , which inform the central nervous system about static stresses).
Adaptation can be accompanied by both a decrease and an increase in the excitability of receptors. So, when moving from a bright room to a dark one, there is a gradual increase in the excitability of the photoreceptors of the eye, and a person begins to distinguish dimly lit objects - this is the so-called dark adaptation. However, such a high excitability of the receptors turns out to be excessive when moving into a brightly lit room (“the light hurts the eyes”). Under these conditions, the excitability of photoreceptors rapidly decreases - light adaptation occurs. .
For optimal perception of external signals, the nervous system finely regulates the sensitivity of receptors depending on the needs of the moment through efferent regulation of receptors. In particular, during the transition from a state of rest to muscular work, the sensitivity of the receptors of the motor apparatus increases markedly. , which facilitates the perception of information about the state of the support - locomotive apparatus ( gamma - regulation ) . The mechanisms of adaptation to different stimulus intensities can affect not only the receptors themselves, but also other formations in the sense organs. For example, when adapting to different sound intensities, there is a change in the mobility of the auditory ossicles (hammer, anvil and stirrup) in the human middle ear.
5. Information encoding
The amplitude and duration of individual nerve impulses (action potentials) coming from the receptors to the centers remain constant under different stimuli. However, the receptors transmit adequate information to the nerve centers not only about the nature, but also about the strength of the acting stimulus. Information about changes in the intensity of the stimulus is encoded (transformed into the form of a nerve impulse code) in two ways:
■ change in pulse frequency, going along each of the nerve fibers from the receptors to the nerve centers;
■ change in the number and distribution of impulses- their number in a pack (portion), intervals between packs, the duration of individual bursts of impulses, the number of simultaneously excited receptors and the corresponding nerve fibers (a diverse space-time picture of this impulsation, rich in information, is called a pattern).
The greater the intensity of the stimulus, the greater the frequency of afferent nerve impulses and their number. This is due to the fact that an increase in the strength of the stimulus leads to an increase in the depolarization of the receptor membrane, which, in turn, causes an increase in the amplitude of the generator potential and an increase in the frequency of impulses arising in the nerve fiber. Between the strength of irritation and the number of nerve impulses there is a directly proportional relationship.
There is another possibility of encoding sensory information. The selective sensitivity of receptors to adequate stimuli already makes it possible to separate different types of energy acting on the body. However, even within the same sensory system, there may be different sensitivity of individual receptors to stimuli of the same modality with different characteristics (distinguishing taste characteristics by different taste receptors of the tongue, color discrimination by different photoreceptors of the eye, etc.).
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 excitation process 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 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, which respond 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.
When a stimulus acts on the receptor membrane, the permeability for sodium or calcium ions increases, the ions enter the nerve ending, the membrane depolarizes and a receptor potential is formed.
Receptor potential has all the properties of local excitation (depends on the strength of the stimulus, is capable of summation, spreads with attenuation). Then the receptor potential with the help of local currents causes the generation of AP in the afferent fiber. The frequency of AP depends on the amplitude of the receptor potential.
Ca/Na channels open → enter the cell along a passive concentration gradient → membrane depolarization.
Formation of local currents of the outgoing direction → depolarization up to FCD → AP generation.
Mechanisms of excitation of secondary sensory receptors
In the receptor cell, under the action of an irritant, sodium or calcium channels open, which leads to the appearance of a receptor potential. Excitation of the cell causes the secretion of a mediator (acetylcholine, glutamic acid), and a generator potential is formed on the membrane of the sensitive neuron.
Generator potential with the help of local currents, it acts on the membrane of the afferent fiber, where AP occurs
Receptor potential occurs when the receptor is irritated as a result of depolarization and an increase in the conductivity of a section of its membrane, which is called receptive. The receptor potential arising in the receptive sections of the membrane electrotonically propagates to the axon hillock of the receptor neuron, where generator potential. The emergence of a generator potential in the region of the axon hillock is explained by the fact that this part of the neuron has lower excitation thresholds and the action potential develops in it earlier than in other parts of the neuron membrane. The higher the generator potential, the more intense the frequency of discharges of the propagating action potential from the axon to other parts of the nervous system. Consequently, the frequency of discharges of the receptor neuron depends on the amplitude of the generator potential.
The afferent link of the reflex arc is represented by sensory neurons, the bodies of which are located in the sensory ganglia of the spinal nerves or the corresponding cranial nerves.
The central link of the reflex is represented by interneurons, which form small excitatory and inhibitory neural networks.
The efferent link is represented by one (for somatic reflexes) or two (for autonomic reflexes) neurons.
The effector organ in somatic reflexes is skeletal muscles, in vegetative reflexes - smooth muscles, glands, cardiomyocytes.
Morphofunctional characteristics of the sympathetic division of the autonomic nervous system
Systems.
The SNS centers are located in the thoracolumbar region of the spinal cord.
Preganglionic neurons lie in the lateral horns from the last cervical to the 4th lumbar segment of the spinal cord (C8, Th1-Th12, L1-L4).
There are 2 types of ganglia in the SNS:
1. paravertebral ganglia are paired and form a sympathetic nerve chain on both sides of the spinal cord (sympathetic trunk)
2. prevertebral ganglia - unpaired, there are three of them (solar plexus, superior and inferior mesenteric nodes).
Picks and reactive systems
ñ Preganglionic neurons - cholinergic (Ax).
ñ Postganglionic neurons - adrenergic (Nad),
ñ The transmission of excitation to the organ is carried out with the help of alpha- and beta-adrenergic receptors, which are metabotropic receptors.
When excited alpha-adrenergic receptors the membrane enzyme phospholipase C is activated and two second messengers are formed: inositol triphosphate (ITP) and diacylglycerol (DAG).
When excited beta-adrenergic receptors the membrane enzyme adenylate cyclase is activated, which leads to the formation of a second messenger c-AMP. A consequence of the activation of adrenoreceptors may be a change in both sodium and potassium conductivity of the membrane of effector cells.