The mechanical stimulus leads to deformation of the receptor membrane. As a result electrical resistance membrane decreases, its permeability for Na + increases. An ion current begins to flow through the receptor membrane, leading to the generation of the receptor potential. With an increase in the receptor potential to a critical level of depolarization in the receptor, impulses are generated that propagate along the fiber in the CNS.

The set of points on the periphery from which peripheral stimuli affect a given sensory cell in the CNS is called receptive field.

In one receptive field there are receptors that send nerve impulses to other central neurons, i.e. individual receptive fields overlap. Overlapping receptive fields increases the resolution of the reception and recognition of the localization of the stimulus.

Relationship between stimulus intensity and response. There is a quantitative relationship between stimulus intensity and response in the form of the frequency of action potentials occurring. The same dependence describes the sensitivity of a sensory neuron in the CNS. The only difference is that the receptor responds to the amplitude of the stimulus, while the central sensory neuron responds to the frequency of action potentials coming to it from the receptor.

For central sensory neurons, it is not so much the absolute threshold S0 of the stimulus that is important, but differential, i.e. differential threshold. The differential threshold is understood as the minimum change in a given stimulus parameter (spatial, temporal, and others) that causes a measurable change in the firing rate of a sensory neuron. It usually depends most of all on the strength of the stimulus. In other words, the higher the intensity of the stimulus, the higher the differential threshold, i.e. the worse the differences between stimuli are recognized (Fig. 24).

For example, for pressure on the skin in a limited range of some intensities, the differential threshold is equal to a pressure increase of 3%. This means that two stimuli whose absolute magnitudes differ by 3% or more will be recognized. If their intensities differ by less than 3%, then the stimuli will be perceived as the same. Therefore, if after a load of 100 g we put a load of 110 g on our hand, then we can feel this difference. But if you put 500 g first, and then 510 g, then in this case the difference of 10 grams will not be recognized, since it is less than 3% (ie less than 15 g) of the value of the original load.

Rice. 24. Skin mechanoreceptors different type

Top row - diagrams of receptive fields, middle - receptor morphology, bottom - electrical activity of receptors.

(a) Rapidly adapting receptors: Meissner bodies (left) and Pacini bodies (right).

(b) Slowly adapting receptors: Merkel discs (left) and Ruffini bodies (right).

Majority receptors excited in response to the action of stimuli 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. 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. Photoreceptors in the retina common origin with nerve cells are often referred to as primary receptors, but their lack of 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.

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, generation occurs electric current or voltage or change in their electrical characteristics. Such a comparison is very conditional. Unlike the processes that occur in transducer sensors, which work due to the energy acting on them, the transformation of the irritation energy into the process of excitation in the receptors occurs due to the metabolism of the receptors themselves, and not due to the external energy applied to them. The mechanism of excitation in receptors complicated enough.

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

One of distinctive features receptor potential is its duration: in some receptors it can remain unchanged for many minutes while the stimulus acts; in the pressure receptors of the carotid sinus, responsive to increased blood pressure, registered receptor potentials lasting several hours. 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.

In the case of primary receptors, the action of the stimulus is perceived by the ending of the sensitive neuron. An active stimulus can cause hyperpolarization or depolarization of the surface membrane of receptors, mainly due to changes in sodium permeability. An increase in the permeability to sodium ions leads to membrane depolarization and a receptor potential appears on the receptor membrane. It exists as long as the stimulus acts.

Receptor potential does not obey the law "All or nothing", its amplitude depends on the strength of the stimulus. It has no refractory period. This allows the receptor potentials to be summed up under the action of subsequent stimuli. It spreads meleno, with extinction. When the receptor potential reaches a critical threshold, it triggers an action potential at the nearest node of Ranvier. In the interception of Ranvier, an action potential arises, which obeys the law "All or Nothing". This potential will be propagating.

In the secondary receptor, the action of the stimulus is perceived by the receptor cell. In this cell, a receptor potential arises, which will result in the release of a mediator from the cell into the synapse, which acts on the postsynaptic membrane of the sensitive fiber and the interaction of the mediator with receptors leads to the formation of another, local potential, which is called generator. It is identical in its properties to the receptor. Its amplitude is determined by the amount of mediator released. Mediators - acetylcholine, glutamate.

Action potentials occur periodically, tk. they are characterized by a period of refractoriness, when the membrane loses the property of excitability. Action potentials arise discretely and the receptor in the sensory system works as an analog-to-discrete converter. In the receptors, an adaptation is observed - adaptation to the action of stimuli. Some are fast adapting and some are slow adapting. With adaptation, the amplitude of the receptor potential and the number of nerve impulses that go along the sensitive fiber decrease. Receptors encode information. It is possible by the frequency of potentials, by the grouping of impulses into separate volleys and by the intervals between volleys. Coding is possible according to the number of activated receptors in the receptive field.

Threshold of irritation and threshold of entertainment.

Irritation threshold- the minimum strength of the stimulus that causes a sensation.

Threshold entertainment- the minimum force of change in the stimulus, at which a new sensation arises.

Hair cells are excited when the hairs are displaced by 10 to -11 meters - 0.1 amstrem.

In 1934, Weber formulated a law that establishes a relationship between the initial strength of irritation and the intensity of sensation. He showed that the change in the strength of the stimulus is a constant value

∆I / Io = K Io=50 ∆I=52.11 Io=100 ∆I=104.2

Fechner determined that sensation is directly proportional to the logarithm of irritation.

S=a*logR+b S-sensation R- irritation

S=KI in A degree I is the strength of irritation, K and A are constants

For tactile receptors S=9.4*I d 0.52

Sensory systems have receptors for self-regulation of receptor sensitivity.

Influence of the sympathetic system - the sympathetic system increases the sensitivity of receptors to the action of stimuli. This is useful in a situation of danger. Increases the excitability of receptors - the reticular formation. Efferent fibers were found in the composition of sensory nerves, which can change the sensitivity of receptors. There are such nerve fibers in the auditory organ.

Sensory hearing system

For most people living in a modern stop, hearing progressively declines. This happens with age. This is facilitated by sound pollution. environment- transport, discotheque, etc. Changes in the hearing aid become irreversible. Human ears contain 2 sensitive organs. Hearing and balance. Sound waves propagate in the form of compressions and rarefaction in elastic media, and the propagation of sounds in dense media is better than in gases. Sound has 3 important properties - pitch or frequency, power or intensity and timbre. The pitch of the sound depends on the frequency of vibrations and the human ear perceives with a frequency of 16 to 20,000 Hz. With maximum sensitivity from 1000 to 4000 Hz.

The main frequency of the sound of the larynx of a man is 100 Hz. Women - 150 Hz. When talking, additional high-frequency sounds appear in the form of hissing, whistling, which disappear when talking on the phone and this makes speech clearer.

The sound power is determined by the amplitude of the vibrations. Sound power is expressed in dB. Power is a logarithmic relationship. Whispered speech - 30 dB, normal speech - 60-70 dB. The sound of transport - 80, the noise of the engine of the aircraft - 160. The sound power of 120 dB causes discomfort, and 140 leads to pain.

The timbre is determined by secondary vibrations on sound waves. Ordered vibrations - create musical sounds. Random vibrations just cause noise. The same note sounds differently on different instruments due to different additional vibrations.

The human ear has 3 parts - outer, middle and inner ear. The outer ear is represented by the auricle, which acts as a sound-catching funnel. The human ear picks up sounds less perfectly than that of a rabbit, a horse that can control its ears. The auricle is based on cartilage, with the exception of the earlobe. Cartilage gives elasticity and shape to the ear. If the cartilage is damaged, then it is restored by growing. External auditory canal S figurative form- inside, forward and down, length 2.5 cm. The ear canal is covered with skin with low sensitivity of the outer part and high sensitivity of the inner. There are hairs on the outside of the ear canal that prevent particles from entering the ear canal. The ear canal glands produce a yellow lubricant that also protects the ear canal. At the end of the passage is the tympanic membrane, which consists of fibrous fibers covered on the outside with skin and inside with mucous. The eardrum separates the middle ear from the outer ear. It fluctuates with the frequency of the perceived sound.

The middle ear is represented by the tympanic cavity, the volume of which is approximately 5-6 drops of water and the tympanic cavity is filled with air, lined with a mucous membrane and contains 3 auditory ossicles: the hammer, anvil and stirrup. The middle ear communicates with the nasopharynx using the Eustachian tube. At rest, the lumen of the Eustachian tube is closed, which equalizes the pressure. Inflammatory processes leading to inflammation of this tube cause a feeling of congestion. The middle ear is separated from the inner ear by an oval and round opening. fluctuations eardrum through a system of levers they are transmitted by a stirrup to the oval window, and the outer ear transmits sounds by air.

There is a difference in the area of ​​the tympanic membrane and the oval window (the area of ​​the tympanic membrane is 70 mm square, and that of the oval window is 3.2 mm square). When vibrations are transmitted from the membrane to the oval window, the amplitude decreases and the strength of the vibrations increases by 20-22 times. At frequencies up to 3000 Hz, 60% of E is transmitted to the inner ear. In the middle ear there are 2 muscles that change vibrations: the tensor tympanic membrane muscle (attached to the central part of the tympanic membrane and to the handle of the malleus) - with an increase in contraction force, the amplitude decreases; stirrup muscle - its contractions limit the movement of the stirrup. These muscles prevent injury to the eardrum. In addition to air transmission of sounds, there is also bone transmission, but this sound power is not able to cause vibrations of the bones of the skull.

inside ear

the inner ear is a maze of interconnected tubes and extensions. The organ of balance is located in the inner ear. The labyrinth has a bone base, and inside there is a membranous labyrinth and there is an endolymph. The cochlea belongs to the auditory part, it forms 2.5 turns around the central axis and is divided into 3 ladders: vestibular, tympanic and membranous. The vestibular canal begins with the membrane of the oval window and ends with a round window. At the apex of the cochlea, these 2 canals communicate with a helicocream. And both of these canals are filled with perilymph. The organ of Corti is located in the middle membranous canal. The main membrane is built from elastic fibers that start at the base (0.04mm) and reach the top (0.5mm). To the top, the density of the fibers decreases by 500 times. The organ of Corti is located on the main membrane. It is built from 20-25 thousand special hair cells located on supporting cells. Hair cells lie in 3-4 rows (outer row) and in one row (inner). At the top of the hair cells are stereociles or kinocilies, the largest stereociles. Sensory fibers of the 8th pair of cranial nerves from the spiral ganglion approach the hair cells. At the same time, 90% of the isolated sensitive fibers end up on the inner hair cells. Up to 10 fibers converge per inner hair cell. And in the composition of nerve fibers there are also efferent ones (olive-cochlear bundle). They form inhibitory synapses on sensory fibers from the spiral ganglion and innervates the outer hair cells. Irritation of the organ of Corti is associated with the transmission of vibrations of the bones to the oval window. Low-frequency vibrations propagate from the oval window to the top of the cochlea (the entire main membrane is involved). At low frequencies, excitation of the hair cells lying on the top of the cochlea is observed. Bekashi studied the propagation of waves in a cochlea. He found that as the frequency increased, a smaller column of liquid was drawn in. High-frequency sounds cannot involve the entire fluid column, so the higher the frequency, the less the perilymph fluctuates. Oscillations of the main membrane can occur during the transmission of sounds through the membranous canal. When the main membrane oscillates, the hair cells move upward, which causes depolarization, and if downward, the hairs deviate inward, which leads to hyperpolarization of the cells. When hair cells depolarize, Ca channels open and Ca promotes an action potential that carries information about the sound. The outer auditory cells have efferent innervation and the transmission of excitation occurs with the help of Ash on the outer hair cells. These cells can change their length: they shorten during hyperpolarization and elongate during polarization. Changing the length of the outer hair cells affects the oscillatory process, which improves the perception of sound by the inner hair cells. The change in the potential of hair cells is associated with the ionic composition of the endo- and perilymph. Perilymph resembles CSF, and endolymph has a high concentration of K (150 mmol). Therefore, the endolymph acquires a positive charge to the perilymph (+80mV). Hair cells contain a lot of K; they have a membrane potential and are negatively charged inside and positive outside (MP = -70mV), and the potential difference makes it possible for K to penetrate from the endolymph into the hair cells. Changing the position of one hair opens 200-300 K-channels and depolarization occurs. Closure is accompanied by hyperpolarization. In the organ of Corti, frequency coding occurs due to the excitation of different parts of the main membrane. At the same time, it was shown that low-frequency sounds can be encoded by the same number of nerve impulses as the sound. Such coding is possible with the perception of sound up to 500 Hz. Encoding of sound information is achieved by increasing the number of volleys of fibers for a more intense sound and due to the number of activated nerve fibers. The sensory fibers of the spiral ganglion terminate in the dorsal and ventral nuclei of the cochlea of ​​the medulla oblongata. From these nuclei, the signal enters the kernels of the olive, both its own and opposite side. From its neurons there are ascending paths as part of the lateral loop that approach the inferior colliculus of the quadrigemina and the medial geniculate body of the thalamus opticus. From the latter, the signal goes to the superior temporal gyrus (Geshl gyrus). This corresponds to fields 41 and 42 (primary zone) and field 22 (secondary zone). In the CNS, there is a topotonic organization of neurons, that is, sounds are perceived with different frequencies and different intensities. The cortical center is important for perception, sound sequence and spatial localization. With the defeat of the 22nd field, the definition of words is violated (receptive opposition).

The nuclei of the superior olive are divided into medial and lateral parts. And the lateral nuclei determine the unequal intensity of sounds coming to both ears. The medial nucleus of the superior olive picks up temporal differences in the arrival of sound signals. It was found that signals from both ears enter different dendritic systems of the same perceiving neuron. Hearing impairment can be manifested by ringing in the ears when the inner ear or auditory nerve is irritated, and two types of deafness: conductive and nervous. The first is associated with lesions of the outer and middle ear (wax plug). The second is associated with defects in the inner ear and lesions of the auditory nerve. Elderly people lose the ability to perceive high-pitched voices. Due to the two ears, it is possible to determine the spatial localization of sound. This is possible if the sound deviates from the middle position by 3 degrees. When perceiving sounds, it is possible to develop adaptation due to the reticular formation and efferent fibers (by acting on the outer hair cells.

visual system.

Vision is a multi-link process that begins with the projection of an image onto the retina of the eye, then there is excitation of photoreceptors, transmission and transformation in the neural layers visual system and ends with the adoption by the higher cortical departments of a decision about a visual image.

The structure and functions of the optical apparatus of the eye. The eye has a spherical shape, which is important for turning the eye. Light passes through several transparent media - the cornea, lens and vitreous body, which have certain refractive powers, expressed in diopters. The diopter is equal to the refractive power of a lens with a focal length of 100 cm. The refractive power of the eye when viewing distant objects is 59D, close objects is 70.5D. An inverted image is formed on the retina.

Accommodation- the adaptation of the eye to a clear vision of objects at different distances. The lens is playing leading role in accommodation. When considering close objects, the ciliary muscles contract, the ligament of zinn relaxes, the lens becomes more convex due to its elasticity. When considering distant ones, the muscles are relaxed, the ligaments are stretched and stretch the lens, making it more flattened. The ciliary muscles are innervated by parasympathetic fibers of the oculomotor nerve. Normally, the farthest point of clear vision is at infinity, the nearest one is 10 cm from the eye. The lens loses elasticity with age, so the nearest point of clear vision moves away and senile farsightedness develops.

Refractive anomalies of the eye.

Nearsightedness (myopia). If the longitudinal axis of the eye is too long or the refractive power of the lens increases, then the image is focused in front of the retina. The person can't see well. Spectacles with concave lenses are prescribed.

Farsightedness (hypermetropia). It develops with a decrease in the refractive media of the eye or with a shortening of the longitudinal axis of the eye. As a result, the image is focused behind the retina and the person has trouble seeing nearby objects. Spectacles with convex lenses are prescribed.

Astigmatism is the uneven refraction of rays in different directions due to not strictly spherical surface of the cornea. They are compensated by glasses with a surface approaching a cylindrical one.

Pupil and pupillary reflex. The pupil is the hole in the center of the iris through which light rays pass into the eye. The pupil improves the clarity of the image on the retina by increasing the depth of field of the eye and by eliminating spherical aberration. If you cover your eye from light, and then open it, the pupil quickly narrows - the pupillary reflex. In bright light, the size is 1.8 mm, with an average of 2.4, in the dark - 7.5. Zooming in results in poorer image quality, but increases sensitivity. The reflex has an adaptive value. The sympathetic pupil dilates, the parasympathetic pupil narrows. In healthy people, the size of both pupils is the same.

Structure and functions of the retina. The retina is the inner light-sensitive membrane of the eye. Layers:

Pigmentary - a series of process epithelial cells of black color. Functions: shielding (prevents scattering and reflection of light, increasing clarity), regeneration of visual pigment, phagocytosis of fragments of rods and cones, nutrition of photoreceptors. The contact between the receptors and the pigment layer is weak, so it is here that retinal detachment occurs.

Photoreceptors. Flasks are responsible for color vision, there are 6-7 million of them. Sticks for twilight, there are 110-123 million of them. They are unevenly located. In the fovea there are only flasks, here is the greatest visual acuity. Sticks are more sensitive than flasks.

The structure of the photoreceptor. It consists of an outer receptive part - the outer segment, with a visual pigment; connecting leg; nuclear part with a presynaptic ending. The outer part consists of discs - a two-membrane structure. The outdoor segments are constantly updated. The presynaptic terminal contains glutamate.

visual pigments. In sticks - rhodopsin with absorption in the region of 500 nm. Flasks contain iodopsin with absorptions of 420 nm (blue), 531 nm (green), 558 (red). The molecule consists of the protein opsin and the chromophore part - retinal. Only the cis-isomer perceives light.

Physiology of photoreception. When a quantum of light is absorbed, cis-retinal turns into trans-retinal. This causes spatial changes in the protein part of the pigment. The pigment becomes colorless and transforms into metarhodopsin II, which is able to interact with the membrane-bound protein transducin. Transducin is activated and binds to GTP, activating phosphodiesterase. PDE destroys cGMP. As a result, the concentration of cGMP falls, which leads to the closure of ion channels, while the concentration of sodium decreases, leading to hyperpolarization and the appearance of a receptor potential that spreads throughout the cell to the presynaptic terminal and causes a decrease in glutamate release.

Restoration of the initial dark state of the receptor. When metarhodopsin loses its ability to interact with tranducine, guanylate cyclase, which synthesizes cGMP, is activated. Guanylate cyclase is activated by a drop in the concentration of calcium ejected from the cell by the exchange protein. As a result, the concentration of cGMP rises and it again binds to the ion channel, opening it. When opening, sodium and calcium enter the cell, depolarizing the receptor membrane, turning it into a dark state, which again accelerates the release of the mediator.

retinal neurons.

Photoreceptors are synaptically connected to bipolar neurons. Under the action of light on the neurotransmitter, the release of the mediator decreases, which leads to hyperpolarization of the bipolar neuron. From the bipolar signal is transmitted to the ganglion. Impulses from many photoreceptors converge to a single ganglion neuron. The interaction of neighboring retinal neurons is provided by horizontal and amacrine cells, the signals of which change the synaptic transmission between receptors and bipolar (horizontal) and between bipolar and ganglionic (amacrine). Amacrine cells carry out lateral inhibition between adjacent ganglion cells. The system also contains efferent fibers that act on synapses between bipolar and ganglion cells, regulating the excitation between them.

Nerve pathways.

The 1st neuron is bipolar.

2nd - ganglionic. Their processes go as part of the optic nerve, make a partial decussation (necessary to provide each hemisphere with information from each eye) and go to the brain as part of the optic tract, entering the lateral geniculate body of the thalamus (3rd neuron). From the thalamus - to the projection zone of the cortex, the 17th field. Here is the 4th neuron.

visual functions.

Absolute sensitivity. For the appearance of a visual sensation, it is necessary that the light stimulus has a minimum (threshold) energy. The stick can be excited by one quantum of light. Sticks and flasks differ little in excitability, but the number of receptors that send signals to one ganglion cell is different in the center and on the periphery.

Visual adaptation.

Visual adaptation sensory system to conditions of bright illumination - light adaptation. The reverse phenomenon is dark adaptation. The increase in sensitivity in the dark is gradual, due to the dark restoration of visual pigments. First, iodopsin flasks are reconstituted. It has little effect on sensitivity. Then the rhodopsin of the sticks is restored, which greatly increases the sensitivity. For adaptation, the processes of changing connections between retinal elements are also important: weakening of horizontal inhibition, leading to an increase in the number of cells, sending signals to the ganglion neuron. The influence of the CNS also plays a role. When illuminating one eye, it lowers the sensitivity of the other.

Differential visual sensitivity. According to Weber's law, a person will distinguish a difference in lighting if it is stronger by 1-1.5%.

Brightness Contrast occurs due to mutual lateral inhibition of optic neurons. A gray stripe on a light background appears darker than a gray one on a dark one, since the cells excited by the light background inhibit the cells excited by the gray stripe.

Blinding brightness of light. Too much bright light produces an unpleasant sensation of blindness. Upper bound blinding brightness depends on the adaptation of the eye. The longer the dark adaptation was, the less brightness causes glare.

Vision inertia. Visual sensation appears and disappears immediately. From irritation to perception, 0.03-0.1 s passes. The stimuli quickly following one another merge into one sensation. The minimum frequency of repetition of light stimuli, at which the fusion of individual sensations occurs, is called the critical frequency of flicker fusion. This is what cinema is based on. Sensations that continue after the cessation of stimulation are sequential images (the image of a lamp in the dark after it is turned off).

Color vision.

The entire visible spectrum from violet (400nm) to red (700nm).

Theories. Three-component theory of Helmholtz. Color sensation provided by three types of bulbs sensitive to one part of the spectrum (red, green or blue).

Goering's theory. The flasks contain substances sensitive to white-black, red-green and yellow-blue radiation.

Consistent color images. If you look at a painted object, and then at White background, then the background will acquire an additional color. The reason is color adaptation.

Color blindness. Color blindness is a disorder in which colors cannot be distinguished. With protanopia, red color is not distinguished. With deuteranopia - green. For tritanopia, it is blue. Diagnosed by polychromatic tables.

A complete loss of color perception is achromasia, in which everything is seen in shades of gray.

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

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

Wired Sensor Systems Department

The AP that has arisen spreads along the nerve fibers along the sensory pathways to the areas lying above. Distinguish the following types ways.

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

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

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

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

Cortical division of sensory systems

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

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

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

IN cortical department For each sensory system, the processes of analysis and synthesis, pattern recognition, representation formation, feature detection (selection) and organization of important information memorization processes take place.