Auditory sensory system structure of the ear hearing acuity. Mechanisms of excitation of receptors

The auditory analyzer is the second most important analyzer in providing cognitive activity person. The auditory system is used for perception sound signals what gives her special role associated with the perception of articulate speech. A child who has lost his hearing early childhood also loses the ability to speak.

Structure auditory analyzer:

The peripheral part is the receptor apparatus in the ear (inner);

The conductor part is the auditory nerve;

central part– auditory cortex cerebral hemispheres(temporal lobe).

Structure of the ear.

The ear is an organ of hearing and balance, includes:

Outer ear - Auricle, which catches sound vibrations and directs them to the outside ear canal. The auricle is formed by elastic cartilage, covered on the outside with skin. The external auditory canal looks like a curved canal 2.5 cm long. Its skin is covered with hairs. The ducts of the glands open into the auditory canal, which produce earwax. Both hairs and earwax perform protective function;

Middle ear. Consists of: eardrum, tympanic cavity(filled with air) auditory ossicles- malleus, incus, stirrups (transmit sound vibrations from the eardrum to oval window inner ear, prevent its overload), the Eustachian tube (connects the middle ear cavity with the pharynx). The eardrum is a thin elastic plate located on the border of the outer and middle ear. The malleus is connected at one end to the eardrum, and at the other end to the incus, which is connected to the stapes. The stapes is connected to the oval window, which separates the tympanic cavity from the inner ear. The auditory (Eustachian) tube connects the tympanic cavity with the nasopharynx, lined from the inside with mucous membrane. It maintains equal pressure externally and internally on eardrum.

The middle ear is separate from the inner ear bone wall, in which there are two holes (round window and oval window);

Inner ear. Located in temporal bone and is formed by the bony and membranous labyrinths. Membranous labyrinth of connective tissue located inside the bony labyrinth. Between the bony and membranous labyrinth there is a fluid - perilymph, and inside the membranous labyrinth - endolymph.

The bony labyrinth consists of the cochlea (sound-receiving apparatus), the vestibule (part of the vestibular apparatus) and three semicircular canals (the organ of hearing and balance). The membranous labyrinth is located inside the bony labyrinth. Between them there is a liquid - perilymph, and inside the membranous labyrinth - endolymph. In the membranous labyrinth of the cochlea there is the organ of Corti - the receptor part of the auditory analyzer, which converts sound vibrations into nervous excitement. The bony vestibule that forms middle part labyrinth of the inner ear, has two open windows in the wall, oval and round, which connect the bone cavity with the eardrum. The oval window is closed by the base of the stapes, and the round window is covered by a movable elastic connective tissue plate.

Sound perception: sound waves through the auricle enter the external auditory canal and cause oscillatory movements of the eardrum - vibrations of the eardrum are transmitted to the auditory ossicles, the movements of which cause vibration of the stapes, which closes the oval window - movements of the stapes of the oval window vibrate the perilymph, its vibrations are transmitted - oscillation endolymph, entails vibration of the main membrane - during the movements of the main membrane and endolymph, the covering membrane inside the cochlea with a certain force and frequency touches the microvilli of receptor cells, which are excited - excitation along the auditory nerve to the subcortical hearing centers ( midbrain) –– higher analysis and the synthesis of auditory stimuli occurs in cortical center auditory analyzer, which is located in temporal lobe. Here the character of the sound, its strength, and height are distinguished.

The auditory sensory system (auditory analyzer) is the second most important distant analyzer of a person. Rumor plays vital role specifically in humans in connection with the emergence of articulate speech. Acoustic (sound) signals are air vibrations with different frequency and strength. They stimulate the auditory receptors located in the cochlea of the inner ear. Receptors activate the first auditory neurons, after which sensory information is transmitted to the auditory cortex big brain(temporal) through a series of successive structures.

The organ of hearing (ear) is peripheral section auditory analyzer, in which auditory receptors are located. The structure and functions of the ear are presented in table. 12.2 and in Fig. 12.9 2.

Table 12.2

Structure and functions of the ear

Ear part	Structure	Functions
Outer ear	Auricle, external auditory canal, eardrum	Protective (sulfur release). Captures and transmits sounds. Sound waves vibrate the eardrum, which vibrates the auditory ossicles
Middle ear	An air-filled cavity containing the auditory ossicles (hammer, anvil, stirrup) and Eustachian (auditory) tube	The auditory ossicles conduct and amplify sound vibrations 50 times. The Eustachian tube is connected to the nasopharynx to equalize pressure on the eardrum.
Inner ear	Hearing organ: oval and round windows, cochlea with a cavity filled with liquid, and the organ of Corti - a sound-receiving apparatus	Auditory receptors located in the organ of Corti convert sound signals into nerve impulses that are transmitted to the auditory nerve, and then to the auditory zone of the cerebral cortex
Inner ear	Balance organ (vestibular apparatus): three semicircular canals, otolithic apparatus	Perceives the position of the body in space and transmits impulses to the medulla oblongata, then to the vestibular zone of the cerebral cortex; response impulses help maintain body balance

1 See: Rezanova E.L., Antonova I.P., Rezanov A.A. Decree. Op.
2 See: Human Physiology: Textbook. In 2 vols.

Rice. 12.9.

The mechanism of sound transmission and perception. Sound vibrations are picked up by the auricle and transmitted through the external auditory canal to the eardrum, which begins to vibrate in accordance with the frequency of the sound waves. Vibrations of the eardrum are transmitted to the chain of ossicles of the middle ear and, with their participation, to the membrane of the oval window. Vibrations of the membrane of the vestibule window are transmitted to the perilymph and endolymph, which causes vibrations of the main membrane along with the organ of Corti located on it. In this case, the hair cells with their hairs touch the integumentary (tectorial) membrane, and due to mechanical irritation, excitation occurs in them, which is transmitted further to the fibers of the vestibulocochlear nerve (Fig. 12.10).

Location and structure of receptor cells of the organ of Corti. On the basilar membrane there are two types of receptor hair cells: internal and external, separated from each other by the arches of Corti.

The inner hair cells are arranged in a single row; total number them along the entire length membranous canal reaches 3500. Outer hair cells are arranged in three to four rows; their total number is 12,000-20,000. Each hair cell has an elongated

Rice. 12.10.

The cochlear canal is divided into the scala tympani and vestibular canal and the membranous canal (middle scala), which houses the organ of Corti. The membranous canal is separated from the scala tympani by the basilar membrane. It contains peripheral processes of neurons of the spiral ganglion, forming synaptic contacts with outer and inner hair cells

shape; one of its poles is fixed on the main membrane, and the second is located in the cavity of the membranous canal of the cochlea. There are hairs at the end of this pole, or stereotypies. Their number on each inner cage is 30-40, and they are very short - 4-5 microns; on each outer cell the number of hairs reaches 65-120, they are thinner and longer. The hairs of the receptor cells are washed by the endolymph and come into contact with the integumentary (tectorial) membrane, which is located above the hair cells along the entire course of the membranous canal.

The mechanism of auditory reception. When exposed to sound, the main membrane begins to vibrate, the longest hairs of the receptor cells (stereocilia) touch the integumentary membrane and tilt slightly. Deviation of the hair by several degrees leads to tension in the thinnest vertical filaments (microfilaments) connecting the tops of neighboring hairs of a given cell. This tension, purely mechanically, opens from one to five ion channels in the stereocilium membrane. Through open channel a potassium ion current begins to flow into the hair. The tension force of the thread required to open one channel is negligible - about 2-10 -13 N. What seems even more surprising is that the weakest sounds felt by a person stretch the vertical threads connecting the tops of neighboring stereocilia to a distance half as large as the diameter hydrogen atom.

The fact that the electrical response of the auditory receptor reaches a maximum after 100-500 μs means that the membrane ion channels open directly from the mechanical stimulus without the participation of intracellular second messengers. This distinguishes mechanoreceptors from much slower-acting photoreceptors.

Depolarization of the presynaptic ending of the hair cell leads to the release of synaptic cleft neurotransmitter (glutamate or aspartate). By acting on the postsynaptic membrane of the afferent fiber, the mediator causes the generation of excitation of the postsynaptic potential and then the generation of impulses propagating in the nerve centers.

The opening of just a few ion channels in the membrane of one stereocilium is clearly not enough to generate a receptor potential of sufficient magnitude. An important mechanism amplification of the sensory signal at the receptor level of the auditory system is the mechanical interaction of all stereocilia (about 100) of each hair cell. It turned out that all stereocilia of one receptor are interconnected into a bundle by thin transverse filaments. Therefore, when one or more of the longer hairs bend, they pull all the other hairs with them. As a result, the ion channels of all hairs open, providing a sufficient receptor potential.

Binaural hearing. Humans and animals have spatial hearing, i.e. the ability to determine the position of a sound source in space. This property is based on the presence of two symmetrical halves of the auditory analyzer ( binaural hearing).

The acuity of binaural hearing in humans is very high: it is able to determine the location of the sound source with an accuracy of about 1 angular degree. Physiological basis This is the ability of the neural structures of the auditory analyzer to evaluate interaural (inter-aural) differences in sound stimuli by the time of their arrival in each ear and by their intensity. If the sound source is located away from the midline of the head, the sound wave arrives at one ear somewhat earlier and with greater force than at the other. Estimation of the distance of sound from the body is associated with the weakening of the sound and the change in its timbre.

See: Human Physiology: Textbook. In 2 vols.

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The auditory sensory system is a system that provides encoding of acoustic stimuli and determines the ability of animals to navigate in environment through the assessment of acoustic stimuli. Peripheral parts of the auditory system - hearing organs located in the inner ear and phonoreceptors.

Sound is the oscillatory movements of elastic bodies that propagate in different environments in the form of waves. Sound waves have two important characteristics: frequency (Hz), which determines the pitch of the sound, and amplitude (dB), which reflects the loudness of the sound. The range of frequencies of sound waves perceived by humans is from 16 Hz to 20,000 Hz. Human ear most sensitive in the range from 1000 to 4000 Hz, (the range of human speech).

The auditory sensory system is the mechanical, receptor and neural structures that perceive and analyze sound vibrations.

The human auditory system is characterized by binaural hearing - the perception of sounds by both ears simultaneously and the combination of the signals they receive, which makes it possible to determine the source of sound in space, the degree of its remoteness and its direction movement. For low frequencies The main factor in binaural hearing is the difference in the time of sound entering the right and left ear, and for high frequencies – differences in sound intensity. If the sound source is in the middle, then the sound reaches both ears at the same time, but usually the sound source is shifted, so that the sound reaches the ear that is closer to the sound source first. The slightest shift to the right or left is already perceived by a person.

Peripheral auditory system

The auditory system is characterized by a rather complexly organized pre-receptor unit, which is represented by the outer and middle ear, and the receptors themselves are located in the inner ear.

The outer ear includes:

the auricle is a speaker that helps concentrate sounds that come from different parts of space;

external auditory canal – enhances the intensity of sounds, protects the eardrum from adverse influences, ensures constant temperature and humidity in this area;

eardrum – transmits sound vibrations to the middle ear.

The middle ear consists of inner surface the eardrum and three bones (malleus, incus and stapes). It is connected to back pharynx through a narrow canal - the eustachian tube, which equalizes the pressure in the middle ear with the pressure in the environment. Vibrations of the eardrum lead to consistent movement of the bone. The base of the stapes is attached to the oval window of the cochlea (part of the inner ear). Thanks to the work of the middle ear bones, the sound is amplified approximately 20 times. At high sound volumes, the gain decreases due to the contraction of two muscles of the middle ear, which reduce vibrations of the eardrum and ossicles, reducing the gain of sound vibrations. Muscle contraction occurs when sound intensity exceeds 90 dB. In addition, muscles contract during swallowing, chewing, and speaking.

The inner ear consists of the cochlea and the membranous labyrinth, which belongs to vestibular apparatus. The cochlea contains the organ of Corti, which contains auditory receptors called hair cells. Inside the cochlea there are two membranes that divide it into three scalae - vestibular, tympanic and medial. The scalae are filled with incompressible fluids (endolymph and perilymph). Receptors are located on the basal (main) membrane, and are covered on top by a covering membrane. When sound vibrations pass through the outer and middle ear, the last bone of the middle ear - the stapes - transmits vibrations to the oval window of the cochlea, and this, in turn, transmits vibrations to the fluids of the inner ear. If liquids vibrate, then so does basement membrane, as a result of which the hairs of the receptor cells touch the integumentary membrane. This is an adequate stimulus for the auditory receptors. A receptor potential arises in them, and then a spreading PD

Inner ear

Conductive and cortical sections of the auditory system

The hair cells of the organ of Corti give off fibers that form the auditory nerve, which carries signals to the dorsal and ventral cochlear (auditory) nuclei in the brain stem. There the first switching of auditory information occurs. From the cochlear nuclei, signals arrive to the nuclei of the superior olive (medulla oblongata), where partial decussation is observed auditory tract: the minority of them remains in their hemisphere, and the majority moves to the opposite side. Next, the information goes to the midbrain, to the posterior (lower) tubercles of the quadrigeminal region. Having left there, most of the fibers cross again and go to the medial geniculate bodies of the thalamus - the last subcortical stage of auditory information processing.

The projection areas of the auditory sensory system are temporal areas bark b.p.

The auditory system is a collection of mechanical, receptor and nerve structures, perceiving and analyzing sound vibrations.

The range of frequencies of sound waves perceived by humans is very wide - from 16 Hz to 20,000 Hz.

The human auditory system is characterized by the phenomenon of binaural hearing. This feature allows a person to use spatial hearing, with which one can establish the location of the sound source, the degree of its distance and the direction of its movement, and also increases the clarity of perception.

The hearing organ consists of the outer, middle and inner ear. The auditory receptors are located in the organ of Corti of the inner ear.

Rice. 10.4. auditory asymmetry in healthy people(based on: Maryutina T.M., Ermolaev O.Yu., 2001). A – presentation of the syllable “ba” only in the left ear, B – presentation of the syllable “ga” only in right ear, B – dichotic (simultaneous) presentation of the syllable “ba” to the left ear, and the syllable “ga” to the right ear, while transmission to the ipsilateral hemisphere is suppressed, the person calls the syllable “ga”, since the syllable “ba” enters the speech left hemisphere later on commissions.

Experimental studies have shown that even an infant aged 50 days pays more attention to sounds presented through the right.

The auditory system consists of two sections - peripheral and central.

The peripheral section includes the outer, middle and inner ear (cochlea) and the auditory nerve. The functions of the peripheral department are:

reception and transmission of sound vibrations by the receptor of the inner ear (cochlea);
conversion of mechanical vibrations of sounds into electrical impulses;
transmission of electrical impulses along the auditory nerve to the auditory centers of the brain.

The central department includes the subcortical and cortical auditory centers. Functions auditory centers The brain is processing, analyzing, remembering, storing and interpreting sound and speech information.

The ear consists of 3 parts: the outer, middle and inner ear. Almost all parts of the outer ear can be seen: the pinna, the external auditory canal, and the eardrum, which separates the outer ear from the middle ear. Behind the eardrum is the middle ear - this is a small cavity (tympanic cavity), in which there are 3 small bones (hammer, incus, stirrup), connected in series to each other. The first of these ossicles (the malleus) is attached to the eardrum, the last (the stapes) is attached to the thin membrane of the oval window that separates the middle ear from the inner ear. The middle ear system also includes the auditory (Eustachian) tube, which connects the tympanic cavity to the nasopharynx, equalizing the pressure in the cavity.

A - transverse section through the ear; B - vertical section through the cochlea; B - cross section of the cochlea

The inner ear is the smallest and most important part of the ear. The inner ear (labyrinth) is a system of canals and cavities located in the temporal bone of the skull. They consist of the vestibule, 3 semicircular canals (the organ of balance) and the cochlea (the organ of hearing). The hearing organ is called the cochlea because it is shaped like a shell. grape snail. It is into the cochlea that during cochlear implantation surgery a chain of active CI electrodes is introduced, which stimulate the fibers auditory nerve.

The cochlea has 2.5 turns and is a spiral bone canal 30 - 35 mm long, which spirals around the bone column (or spindle, modiolus). The cochlea is filled with fluid. A spiral bone plate runs along its entire length, located perpendicular to the bone column (modiolus), to which an elastic membrane is attached - the basilar membrane, reaching the opposite wall of the cochlea. The spiral bony plate and the basilar membrane divide the cochlea along its entire length into 2 parts (staircases): the lower one, facing the base of the cochlea, the tympanic scala, and the upper one, the scala vestibular. The scala tympani is connected to the middle ear cavity through the round window, and the vestibular window is connected through the oval window. Both scalae communicate with each other through a small opening (helicotrema) at the top of the cochlea.

In the vestibular scala, an elastic membrane, Reisner’s membrane, departs from the bony plate, which with the basilar membrane forms the third ladder, the median or cochlear scala. In the scala cochlea and basilar membrane there is an organ of hearing - the organ of Corti with auditory receptors (external and internal hair cells). The hairs of the hair cells are immersed in the integumentary membrane located above them. The inner hair cells are approached by most of the dendrites of the cochlear ganglion, which are the beginning of the afferent/ascending auditory pathway, which transmits information to the auditory centers of the brain. Outer hair cells have more synaptic contacts with the effective/descending pathways of the auditory system, providing feedback its higher departments with the lower ones. Outer hair cells are involved in fine-tuning the basilar membrane of the cochlea.

Hair cells are located on the basilar membrane in a certain order - in the initial part of the cochlea there are cells that respond to high-frequency sounds, in the upper (apical) part of the cochlea there are cells that respond to low-frequency sounds. This ordered arrangement of elements of the auditory system is called tonotopic organization. It is typical for all levels - auditory organ, subcortical auditory centers, auditory cortex. This important property auditory system, which is one of the principles of encoding sound information - the “principle of place”, i.e. sound of a certain frequency is transmitted and stimulates very specific areas of the auditory pathways and centers.

Hearing is the ability of the human and animal body to perceive sound stimuli. Sound, in turn, can be defined as the oscillatory movement of particles of an elastic medium (gas, liquid, solid), propagating in the form of a longitudinal wave. Sound vibrations are characterized by frequency (infrasound - up to 15-20 Hz; sound itself, i.e. sound, human audible, – from 16 Hz to 20 kHz; ultrasound - above 20 kHz), propagation speed (depending on the properties of the medium): in air - approximately 340 m/s, in sea water– 1550 m/s) and intensity (force). In practice, a comparative value is used to measure sound intensity - the sound pressure level, which is measured relative to the human hearing threshold in decibels (dB). Sounds containing vibrations of only one frequency (pure tones) are rare. Most sounds are formed by the superposition of several frequencies.

Hearing sensitivity is assessed by absolute hearing threshold– minimum detectable sound intensity. The lower the hearing threshold, the higher the sensitivity of hearing. The absolute hearing threshold, in turn, depends on the frequency of the tone. For a person the most low threshold audibility is recorded at 1-4 kHz. When exposed to sounds of very high intensity, a painful sensation occurs.

The auditory system, like other sensory systems, is capable of adaptation. Both the peripheral part and the neurons of the central nervous system are involved in this process. Adaptation manifests itself in a temporary increase in the auditory threshold.

As already mentioned, a person perceives sounds with a frequency from 16 to 20,000 Hz. This range decreases with age due to a reduction in its high-frequency part. After 40 years upper limit frequencies audible sounds every year it decreases by about 160 Hz.

The range of frequencies perceived by various animals differs from that of humans. Thus, in reptiles it extends from 50 to 10,000 Hz, and in birds from 30 to 30,000 Hz. A number of animals (dolphins, the bats) are able to determine the position of an object in space thanks to a special type of hearing echolocation– perception of sound signals that are emitted by the animal itself and reflected from the object.

Hearing organ

The organ of hearing is the ear, which has three sections - the outer ear, the middle ear and the inner ear, which actually contains the auditory receptors.

Outer and middle ear

Outer ear(Fig. 13) consists of the auricle and the external auditory canal.

The auricle is elastic cartilage covered with skin. The function of the auricle is sound location; it directs sound vibrations into the external auditory canal, providing improved perception of sounds coming from a specific direction. In humans, the auricle is rudimentary and lacks mobility.

The external auditory canal is a tube-shaped cavity covered with skin that leads to the middle ear. The average length of the human external auditory canal is 26 mm, the average area is 0.4 cm 2. The skin of the ear canal contains a large number of sebaceous glands, as well as glands that produce earwax, which plays protective role, trapping dust and microorganisms and protecting the eardrum from drying out.

The external auditory canal ends at the eardrum, which separates it from the middle ear. It is a stretched, funnel-shaped membrane between the outer and middle ear that transmits sound vibrations to the auditory ossicles of the middle ear. The membrane consists of connective tissue fibers and has an area of about 0.6 cm 2.

Middle ear- a cavity in the petrous part of the temporal bone, filled with air and containing the auditory ossicles (Fig. 13). The volume of the middle ear cavity, or tympanic cavity, is about 1 cm3.

The main part of the middle ear is auditory ossicles- small bones (hammer, incus and stapes), sequentially connected to each other and transmitting sound vibrations from the eardrum to the membrane of the oval window of the inner ear. The malleus is connected to the tympanic membrane, and the stapes is connected to the oval window. The auditory ossicles are connected to each other movably, using joints. Associated with them are two small muscles that regulate the movements of the ossicular chain. The degree of contraction of these muscles varies depending on the volume of the sound, protecting the inner ear from too much vibration.

The tympanic cavity is connected to the nasopharynx eustachian tube. Thanks to it, a balance is maintained between the pressure in the tympanic cavity and the external atmospheric pressure. In the absence of such balance, a feeling of “fullness” of the ears occurs (for example, on an airplane), which can be relieved by swallowing. When swallowing, the lumen eustachian tubes expands, which facilitates the flow of air into the middle ear cavity. Unfortunately, microorganisms can penetrate through this same channel, causing inflammation - otitis middle ear.

Inner ear

Inner ear or labyrinth(Fig. 13) - a system of cavities and convoluted canals lying in the petrous part of the temporal bone. A distinction is made between the bony labyrinth and the membranous labyrinth lying inside it.

Bone labyrinth limited by bone. It has three parts - the vestibule ( vestibulum), semicircular canals ( canales semicirculares) and snail ( cochlea). The vestibule and semicircular canals belong to the vestibular analyzer, the cochlea – to the auditory analyzer. Membranous labyrinth is located inside the bone and more or less repeats the shape of the latter. The walls of the membranous labyrinth are formed by a thin connective tissue membrane. Between the bony and membranous labyrinths there is a fluid - perilymph; the membranous labyrinth itself is filled with endolymph. All cavities of the membranous labyrinth are connected to each other by a system of ducts.

Snail- part of the inner ear in the form of a spirally twisted canal. The cochlea makes approximately 2.5 turns around the bony shaft. At the base of this rod there is a cavity in which the spiral ganglion lies.

In longitudinal and transverse sections through the cochlea, it can be seen (Fig. 13, 14) that it is divided into three sections by two membranes - basilar or main (lower) and vestibular or Reisner (upper). Middle section- This is the membranous labyrinth of the cochlea, it is called the middle scala or cochlear duct. Above it is the scala vestibularis, and below it is the scala tympani. The cochlear duct ends blindly; the vestibular and tympanic scala at the top of the cochlea are connected by a small opening - the helicotrema, essentially forming a single canal filled with perilymph. The cavity of the middle scala is filled with endolymph.

The scala vestibular originates from oval window – thin membrane, connected to the stapes and located between the middle ear and the vestibule of the inner ear. The drum ladder starts from round window– membrane located between the middle ear and the cochlea.

Sound waves entering the outer ear shake the eardrum, and then along the chain of auditory ossicles reach the oval window and cause its vibrations. The latter spread through the perilymph, causing vibrations of the basilar membrane. Because the fluid is incompressible, vibrations are damped at the round window, i.e. when the oval window protrudes into the cavity of the scala vestibularis, the round window arches into the cavity of the middle ear.

Basilar membrane It is an elastic plate penetrated by protein fibers weakly stretched across (up to 24,000 fibers of different lengths). The density and width of the basilar membrane is different areas different. The membrane is most rigid at the base of the cochlea, and towards its apex the plasticity increases. In humans, at the base of the cochlea the width of the membrane is 0.04 mm, then, gradually increasing, it reaches 0.5 mm at the apex of the cochlea. Those. the membrane expands where the cochlea itself narrows. The membrane length is about 35 mm.

Located on the basilar membrane organ of corti, containing more than 20 thousand auditory receptors located between supporting cells. Auditory receptors are hair cells (Fig. 15); Due to their activity, vibrations of the fluid inside the cochlea are converted into electrical signals. On the surface of each receptor cell there are several rows of hairs (stereocilia) decreasing in length, filled with cytoplasm, there are about a hundred of them. The hairs extend into the cavity of the cochlear duct, and the tips of the longest of them are immersed in a covering jelly-like membrane lying above the organ of Corti along its entire length. The tops of the hairs are connected by thin protein filaments, apparently connected to ion channels . If the hairs bend, the protein threads stretch, opening the channels. As a result, an incoming cation current occurs, depolarization and receptor potential develop. Thus, an adequate stimulus for auditory receptors is hair bending, i.e. these receptors are mechanoreceptors.

Sound wave, passing through the perilymph, causes vibrations of the basilar membrane, which are a so-called traveling wave (Fig. 16), which propagates from the base of the cochlea to its apex. Depending on the frequency of sound, the amplitude of these vibrations varies in different parts membranes. The higher the sound, the narrower part of the membrane swings with maximum amplitude. In addition, the amplitude of vibrations naturally depends on the strength of the sound. When the basilar membrane vibrates, the hairs of the receptors sitting on it, in contact with the integumentary membrane, shift. This causes ion channels to open, resulting in a receptor potential. The magnitude of the receptor potential is proportional to the degree of displacement of the hairs. The minimum displacement of the hairs that causes a response is only 0.04 nm - less than the diameter of a hydrogen atom.

Auditory hair receptors are secondary sensory receptors. To transmit a signal to the central nervous system, bipolar dendrites are suitable for each of them. nerve cells, whose bodies lie in the spiral ganglion (Fig. 14, 19). Dendrites form a synapse with hair receptors (mediator - glutamic acid). The greater the deformation of the hairs, the greater the receptor potential and the amount of released mediator, and, therefore, the greater the frequency nerve impulses, spreading along the fibers of the auditory nerve. In addition, some auditory receptors are suitable efferent fibers, coming from the central nervous system from the nuclei of the superior olivary (see below). Thanks to them, it is possible to regulate the sensitivity of receptors to some extent.

The axons of the neurons of the spiral ganglion form cochlear (cochlear) nerve(auditory part VIII pair cranial nerves). In humans, the cochlear nerve has approximately 30 thousand fibers. It goes to the auditory nuclei located on the border medulla oblongata and a bridge.

Thus, peripheral analysis of the properties of a sound stimulus consists of determining its height and volume. Moreover, each section of the basilar membrane is characterized by “tuning” to a certain frequency of sound - frequency dispersion. As a result, hair cells, depending on their location, selectively respond to sounds of different tones. Therefore, we can talk about tonotopic (Greek. tonos– tone) location of hair cells.