REMEMBER

Question 1. What is the importance of hearing for a person?

With the help of hearing, a person perceives sounds. Hearing makes it possible to perceive information at a considerable distance. Articulate speech is associated with the auditory analyzer. A person who was deaf from birth or who lost their hearing at early childhood, loses the ability to pronounce words.

Question 2: What are the main parts of any analyzer?

Any analyzer consists of three main links: receptors (peripheral receiving link), nerve pathways (conducting link) and brain centers (central processing link). The higher sections of the analyzers are located in the cerebral cortex, and each of them occupies a specific area.

QUESTIONS FOR THE PARAGRAPH

Question 1. What is the structure of the auditory analyzer?

The auditory analyzer includes the organ of hearing, the auditory nerve and the brain centers that analyze auditory information.

Question 2. What hearing disorders do you know and what are their main causes?

Sometimes too much earwax accumulates in the external auditory canal and forms a plug, reducing hearing acuity. Such a plug must be removed very carefully, as it can damage the eardrum. Various types of pathogens can penetrate from the nasopharynx into the middle ear cavity and can cause inflammation of the middle ear - otitis media. With the right and timely treatment Otitis media passes quickly and does not affect hearing sensitivity. Mechanical injuries - bruises, blows, exposure to super-strong sound stimuli - can also lead to hearing impairment.

1. Prove that the “organ of hearing” and the “auditory analyzer” are different concepts.

The organ of hearing is the ear, which consists of three sections: the outer, middle and inner ear. The auditory analyzer includes the auditory receptor (located in the inner ear), the auditory nerve and the auditory zone of the cerebral cortex, located in the temporal lobe.

2. Formulate the basic rules of hearing hygiene.

To prevent a decrease in hearing acuity and protect the hearing organs from harmful influences external environment, virus penetration and development dangerous diseases, adhering to the basic rules of hearing hygiene and monitoring the condition of your ears, the cleanliness and condition of your hearing is necessary constantly and without fail.

Hearing hygiene suggests that ears should be cleaned no more than twice a week unless they are very dirty. There is no need to get rid of the sulfur that is in the ear canal too carefully: it protects the human body from the penetration of pathogenic microorganisms into it, removes debris (skin flakes, dust, dirt), and moisturizes the skin.

THINK!

What features of the auditory analyzer allow a person to determine the distance to the sound source and the direction towards it?

An important property of the auditory analyzer is its ability to determine the direction of sound, called ototopics. Ototopics is possible only if you have two ears that can hear normally, i.e. with good binaural hearing. Determination of the direction of sound is ensured by the following conditions: 1) the difference in sound strength perceived by the ears, since the ear that is closer to the sound source perceives it as louder. What also matters here is that one ear is in the sound shadow; 2) perception of the minimum time intervals between the arrival of sound to one and the other ear. In humans, the threshold for this ability to distinguish between minimal time intervals is 0.063 ms. The ability to localize the direction of sound disappears if the sound wavelength is less than double the distance between the ears, which is on average 21 cm. Therefore, ototopy of high-pitched sounds is difficult. The greater the distance between sound receivers, the more accurate the determination of its direction; 3) the ability to perceive phase differences sound waves, entering both ears.

In the horizontal plane, a person distinguishes the direction of sound most accurately. Thus, the direction of sharp impact sounds, such as gunshots, is determined with an accuracy of 3-4°. Orientation in determining the direction of the sound source in the sagittal plane depends to a certain extent on the ears.

PHYSIOLOGY OF THE HEARING ANALYZER

(Auditory sensory system)

Lecture questions:

1. Structural and functional characteristics of the auditory analyzer:

a. Outer ear

b. Middle ear

c. Inner ear

2. Divisions of the auditory analyzer: peripheral, conductive, cortical.

3. Perception of height, sound intensity and sound source location:

a. Basic electrical phenomena in the cochlea

b. Perception of sounds of different pitches

c. Perception of sounds of varying intensities

d. Identifying the sound source (binaural hearing)

e. Auditory adaptation

1. The auditory sensory system is the second most important distant human analyzer, plays important role specifically in humans in connection with the emergence of articulate speech.

Hearing analyzer function: transformation sound waves into the energy of nervous excitation and auditory sensation.

Like any analyzer, the auditory analyzer consists of a peripheral, conductive and cortical section.

PERIPHERAL DEPARTMENT

Converts the energy of sound waves into energy nervous excitation – receptor potential (RP). This department includes:

· inner ear (sound-receiving apparatus);

· middle ear (sound-conducting apparatus);

· outer ear (sound-collecting apparatus).

The components of this department are combined into the concept organ of hearing.

Functions of the organs of hearing

Outer ear:

a) collecting sound (auricle) and directing the sound wave into the external auditory canal;

b) conducting a sound wave through the ear canal to the eardrum;

c) mechanical and temperature protection environment all other parts of the hearing organ.

Middle ear(sound-conducting section) is the tympanic cavity with 3 auditory ossicles: the malleus, the incus and the stapes.

The eardrum separates the external auditory canal from the tympanic cavity. The handle of the malleus is woven into the eardrum, its other end is articulated with the incus, which, in turn, is articulated with the stapes. The stapes is adjacent to the membrane of the oval window. The pressure in the tympanic cavity is equal to atmospheric pressure, which is very important for adequate perception of sounds. This function is performed by the Eustachian tube, which connects the middle ear cavity to the pharynx. When swallowing, the tube opens, resulting in ventilation of the tympanic cavity and equalization of the pressure in it with atmospheric pressure. If external pressure changes rapidly (rapid rise to altitude) and swallowing does not occur, then the pressure difference between atmospheric air and air in the tympanic cavity leads to tension eardrum and the occurrence of unpleasant sensations (“stuffy ears”), decreased perception of sounds.

The area of ​​the tympanic membrane (70 mm2) is significantly larger than the area of ​​the oval window (3.2 mm2), due to which gain the pressure of sound waves on the membrane of the oval window is 25 times. Lever mechanism of bones reduces the amplitude of sound waves is 2 times, so the same amplification of sound waves occurs at the oval window of the tympanic cavity. Consequently, the middle ear amplifies sound by about 60-70 times, and if we take into account the amplifying effect of the outer ear, then this value increases by 180-200 times. In this regard, during strong sound vibrations, in order to prevent the destructive effect of sound on the receptor apparatus of the inner ear, the middle ear reflexively turns on “ defense mechanism" It consists of the following: in the middle ear there are 2 muscles, one of them stretches the eardrum, the other fixes the stapes. Under strong sound impacts, these muscles, when contracting, limit the amplitude of vibration of the eardrum and fix the stapes. This “quenches” the sound wave and prevents excessive stimulation and destruction of the phonoreceptors of the organ of Corti.

Inner ear: represented by the cochlea - a spirally twisted bone canal (2.5 turns in humans). This channel is divided along its entire length into three narrow parts (ladders) with two membranes: the main membrane and the vestibular membrane (Reisner).

On the main membrane there is a spiral organ - the organ of Corti (organ of Corti) - this is the actual sound-receiving apparatus with receptor cells - this is the peripheral section of the auditory analyzer.

The helicotrema (orifice) connects the superior and inferior canals at the apex of the cochlea. The middle channel is separate.

Above the organ of Corti is a tectorial membrane, one end of which is fixed and the other remains free. The hairs of the outer and inner hair cells of the organ of Corti come into contact with the tectorial membrane, which is accompanied by their excitation, i.e. the energy of sound vibrations is transformed into the energy of the excitation process.

Structure of the organ of Corti

The transformation process begins with sound waves entering the outer ear; they move the eardrum. Vibrations of the eardrum through the system auditory ossicles middle ear are transmitted to the membrane of the oval window, which causes vibrations of the perilymph of the scala vestibule. These vibrations are transmitted through the helicotrema to the perilymph of the scala tympani and reach the round window, protruding it towards the middle ear (this prevents the sound wave from dying out when passing through the vestibular and tympanic canal of the cochlea). Vibrations of the perilymph are transmitted to the endolymph, which causes vibrations of the main membrane. The fibers of the basilar membrane begin to vibrate together with the receptor cells (outer and inner hair cells) of the organ of Corti. In this case, the phonoreceptor hairs come into contact with the tectorial membrane. The cilia of the hair cells are deformed, this causes the formation of a receptor potential, and on its basis - an action potential (nerve impulse), which is carried along the auditory nerve and transmitted to the next department auditory analyzer.

CONDUCTING DEPARTMENT OF THE HEARING ANALYZER

The conductive section of the hearing analyzer is presented auditory nerve. It is formed by the axons of neurons of the spiral ganglion (1st neuron of the pathway). The dendrites of these neurons innervate the hair cells of the organ of Corti (afferent link), the axons form fibers auditory nerve. The auditory nerve fibers end on the neurons of the nuclei of the cochlear body (VIII pair of h.m.n.) (second neuron). Then, after partial decussation, the fibers of the auditory pathway go to the medial geniculate body of the thalamus, where switching occurs again (third neuron). From here, excitation enters the cortex (temporal lobe, superior temporal gyrus, transverse gyri of Heschl) - this is the projection auditory zone of the cortex.

CORTICAL DIVISION OF THE AUDITORY ANALYZER

Presented in the temporal lobe of the cerebral cortex - superior temporal gyrus, transverse temporal gyri of Heschl. Cortical gnostic auditory zones are associated with this projection zone of the cortex - Wernicke's sensory speech area and praxial zone – Broca's speech motor center(inferior frontal gyrus). The cooperative activity of the three cortical zones ensures the development and function of speech.

The auditory sensory system has feedbacks, which provide regulation of the activity of all levels of the auditory analyzer with the participation descending paths, which start from the neurons of the “auditory” cortex and sequentially switch in the medial geniculate body of the thalamus, the inferior colliculus of the midbrain with the formation of tectospinal descending tracts and on the nuclei of the cochlear body of the medulla oblongata with the formation of vestibulospinal tracts. This ensures, in response to the action of a sound stimulus, the formation of a motor reaction: turning the head and eyes (and in animals, the ears) towards the stimulus, as well as increasing the tone of the flexor muscles (flexion of the limbs in the joints, i.e. readiness to jump or run ).

Auditory cortex

PHYSICAL CHARACTERISTICS OF SOUND WAVES THAT ARE PERCEIVED BY THE HEARING ORGAN

1. The first characteristic of sound waves is their frequency and amplitude.

The frequency of sound waves determines the pitch of the sound!

A person distinguishes sound waves with a frequency from 16 to 20,000 Hz (this corresponds to 10-11 octaves). Sounds whose frequency is below 20 Hz (infrasounds) and above 20,000 Hz (ultrasounds) by humans not felt!

Sound that consists of sinusoidal or harmonic vibrations is called tone(high frequency - high tone, low frequency - low tone). A sound consisting of unrelated frequencies is called noise.

2. The second characteristic of sound that the auditory sensory system distinguishes is its strength or intensity.

The strength of sound (its intensity) together with the frequency (tone of sound) is perceived as volume. The unit of loudness measurement is bel = lg I/I 0, but in practice it is more often used decibel (dB)(0.1 bel). A decibel is 0.1 decimal logarithm of the ratio of sound intensity to its threshold intensity: dB = 0.1 log I/I 0. Maximum volume level when sound causes painful sensations, equal to 130-140 dB.

The sensitivity of the auditory analyzer is determined by the minimum sound intensity that causes auditory sensations.

In the range of sound vibrations from 1000 to 3000 Hz, which corresponds to human speech, the ear has the greatest sensitivity. This set of frequencies is called speech zone(1000-3000 Hz). Absolute sound sensitivity in this range is 1*10 -12 W/m2. For sounds above 20,000 Hz and below 20 Hz, absolute hearing sensitivity decreases sharply - 1*10 -3 W/m2. In the speech range, sounds are perceived that have a pressure of less than 1/1000 of a bar (a bar is equal to 1/1,000,000 of normal atmospheric pressure). Based on this, in transmitting devices, in order to ensure adequate understanding of speech, information must be transmitted in the speech frequency range.

MECHANISM OF PERCEPTION OF HEIGHT (FREQUENCY), INTENSITY (STRENGTH) AND LOCALIZATION OF SOUND SOURCE (BINAURAL HEARING)

Perception of sound wave frequency

The structure of the auditory analyzer is the topic of our article. How are its structure and functions related? What is the importance of hearing for a person? Let's figure it out together.

What are sensory systems

Every second, our body perceives information from the environment and reacts to it accordingly. This is possible thanks to sensor or analyzing systems. The structure of the auditory analyzer is similar to other similar structures.

In total, there are five sensory systems in the human body. In addition to auditory, these include visual, olfactory, tactile, and gustatory. Scientists claim that humans also have a sixth sense. We are talking about intuition - the ability to foresee events. But the structure that is responsible for the formation of this feeling is still unknown.

Operating principle of analyzers

If we briefly describe the structure of the auditory analyzer, we can name its three sections. They are called peripheral, conductive and central. All sensory systems have such a structure.

The peripheral section is represented by receptors. These are sensitive formations that perceive various types of irritations and convert them into impulses. Nerve fibers, which represent the conductive section, transmit information to the brain. Here it is analyzed and a response to irritation is formed.

Structure and functions of the auditory analyzer: briefly

How is sound vibration perceived? The structure of the auditory analyzer is similar to all others. Its peripheral section is represented by the ear. The conduction nerve is the auditory nerve. Along it, nerve impulses move to the central part. This is the auditory area of ​​the cerebral cortex.

Adaptability

A common property for all sensory systems is their ability to adapt the level of their sensitivity to the intensity of the stimulus. This property is also called adaptation. And the structure of the human auditory analyzer is no exception.

What is the essence of the adaptation process? The fact is that the sensitivity of auditory receptors can be adjusted depending on the degree of exposure to the stimulus. If the signal is strong, the level of perception decreases, and vice versa. For example, remember how we gradually begin to distinguish quiet sounds after a certain time.

For the human body, adaptation has a protective significance. It also improves the functionality of the analyzers through long repetitions. This is how professional musicians train their ears. People who work in conditions of intense noise for a long time or live next to a railway stop noticing it after a certain period. This is also a manifestation of adaptation.

Like all sensory systems, the auditory system is compensated by the functioning of the others. A striking example This is the greatest composer Ludwig Beethoven. He was a recognized master already at a young age, and by the age of thirty his deafness began to progress rapidly. But even when Beethoven completely lost his hearing, he continued to compose musical masterpieces. He placed a small wooden stick in his mouth and pressed it against the musical instrument. In this way, the tactile sensory system compensated for the auditory analyzer. And the lack of vision is partially replaced by developed hearing and smell.

The meaning of hearing

Is it possible to live deaf? Naturally, there are a huge number of people with hearing impairments. Despite the fact that a person perceives most information through vision, the perception of sounds also has great importance.

The basic principles of the structure of the auditory analyzer make its operation continuous. We hear even during sleep. Hearing allows you to perceive information at a distance, transfer experience across generations, and is a means of communication.

What is sound pressure

Are we able to perceive all sounds? Far from it. In the process of evolution, sensory systems have adapted to analyze information only in a certain range. This protects the brain from overload.

Sounds are formed from air vibrations. The structure of the auditory analyzer ensures their transformation into nerve impulses, which are analyzed in the brain. The amplitude of such vibrations is called sound pressure. Its unit of measurement is decibel. During a normal conversation, this value is 60 dB.

The frequency of sound vibrations is measured in hertz. We perceive a very narrow range - from 16 to 20 kHz. We are unable to hear other vibrations. If the vibration frequency is below 16 Hz, it is called infrasound. In nature, it is used for communication by whales and elephants.

Ultrasound occurs at vibration frequencies greater than 20 kHz. Bats use it for orientation at night. They make sounds that are reflected from objects. This method is called echolocation.

Hearing organ

The auditory analyzer, the structure and functions of which we discuss in our article, consists of three sections. The peripheral is represented by the ear. Or more correctly, the organ of hearing. Next comes the wiring department. This is the auditory nerve. It transmits information to central department, represented by the auditory cortex of the telencephalon.

Outer ear

What are the features of the anatomical structure peripheral part auditory analyzer? First of all, it also consists of three parts. These are the outer, middle and inner ear.

The elements of the first part are the auricle and the external auditory canal. They capture and direct sound vibrations to internal departments. The auricle is formed by elastic cartilage tissue, which forms characteristic curls.

The external auditory canal is about 2.5 cm long, ending at the eardrum. His skin is rich in modified sweat glands. They secrete a special substance - earwax. Together with hairs, it traps dust and microorganisms.

Auditory ossicles

The structure of the hearing organ and auditory analyzer continues with the middle ear. Sound vibrations are transmitted to the eardrum, causing it to vibrate. The higher the sound, the more intense the vibrations.

The location of the middle ear is the skull. Its boundaries are two membranes - the tympanic membrane and the oval window. Here the vibrations are transmitted to the auditory ossicles. They have a characteristic shape, which determines their names: malleus, stapes and anvil. The auditory ossicles are anatomically connected to each other. The narrow part of the hammer is attached to the anvil. The latter is movably connected to the stirrup. Vibrations from the eardrum travel through the auditory ossicles to the membrane of the oval window.

In this section, the middle ear is anatomically connected to the nasopharynx using the eustachian, or auditory tube. This structure allows air from the environment to penetrate here. Therefore, the pressure on the eardrum is equal on both sides.

Inner ear

Much has already been said about the structure and functions of the auditory analyzer, but not a word about the receptors themselves. It's not a mistake. They are contained by the inner ear. Its location is the temporal bone. This a complex system convoluted tubules and cavities. They are filled with a special liquid.

From the oval window, the structure of the auditory analyzer continues with a canal consisting of 2.5 turns. This is the cochlea, which contains the auditory receptors, or hair cells. In the cochlea, there are main and integumentary membranes. The first is formed from transverse fibers having different lengths. There are a lot of them - up to 24 thousand. The integumentary membrane overhangs the hair cells. As a result, a sound-receiving apparatus is formed, which is called the organ of Corti. It consists of membranes and auditory receptors.

Mechanism of action

When the membrane of the oval window begins to vibrate, this irritation is transmitted to the cochlear fluid. As a result, the phenomenon of resonance occurs. Fibers begin to vibrate different lengths and auditory receptors.

This process has its own laws. A strong sound causes a large range of oscillatory movements of the fibers. At high pitches, short fibers begin to resonate.

Next, the mechanical energy of the oscillatory movements is converted into electrical energy. This is how nerve impulses arise. Their further movement occurs with the help of neurons and their processes. They enter the auditory cortex of the telencephalon, which is located in the temporal lobe.

Sound analysis - also important function auditory analyzer. The brain determines the strength of sound, its character, height, direction in space. The intonation of words is also perceived. As a result, a sound image is formed.

Even with eyes closed we can determine from which direction the signal is heard. What makes this possible? If sound enters both ears, we perceive the sound in the middle. Or rather, front and back. If sound enters one ear earlier than the other, then the sound is perceived from the right or left.

Have you ever noticed that people perceive the same sound differently? For one, the TV is too quiet, while the other cannot hear anything. It turns out that each person has his own threshold of auditory sensitivity. What does this indicator depend on? It is determined not only by the structure, functions and age characteristics of the auditory analyzer. People aged 15 to 20 years have the most acute perception of sounds. Further, hearing acuity gradually decreases.

There is also such a thing as the threshold of hearing. This is the smallest sound strength at which it begins to be perceived. This indicator is also determined by individual characteristics.

The process of forming an auditory analyzer

When does a person begin to perceive sounds? Immediately after birth. The response to sounds during this period is the manifestation of conditioned reflexes. This continues for about two months. Now the body already reacts conditionally. For example, a mother's voice becomes a sign of feeding.

By the third month, the baby can already distinguish the tone, timbre, pitch and direction of sounds. By the age of one year, as a rule, the child already understands the semantic meaning of words.

Hearing hygiene

The structure of the auditory analyzer, although completely natural, requires constant attention. The most basic rules of hygiene will allow you to maintain the ability to perceive sounds for a long time.

The simplest reason for sound deterioration is the accumulation of wax in the external auditory canal. If this substance is not removed, so-called plugs may form. To prevent this, sulfur must be removed periodically.

We also need to take the consequences of viral diseases seriously. The most basic rhinitis, sore throat or flu can lead to inflammation in the middle ear. This disease is called otitis media. Dangerous microorganisms enter the middle ear from the nasopharynx through the auditory tube.

Hearing loss can also be caused by purely mechanical reasons. One of them is damage to the eardrum. It can be caused by the action of a sharp object, or excessive loud sound. For example, an explosion. If you expect this to happen, you need to open your mouth. This action makes the pressure equal on both sides of the eardrum.

But let's get back to everyday life. We do not think that the systematic use of headphones, constant household and traffic noise gradually reduce the elasticity of the ear drum. As a result, hearing acuity decreases significantly. But this process is irreversible. Just imagine that a pneumatic drill operates with a sound intensity of up to 100 decibels, and a disco - 110!

So, the human auditory sensory system consists of three sections, such as:

• Peripheral. Represented by the organ of hearing: the outer, middle and inner ear. The curls of the auricle direct air vibrations into the external auditory canal, from there to specialized bones (the malleus, stem and incus), the membrane of the oval window and the cochlea. The last structure contains hair cells. These are auditory receptors that convert mechanical vibrations into nerve impulses.
• Conductive. This is the auditory nerve through which impulses are transmitted.
• Central. Located in the cerebral cortex. Here the information is analyzed, resulting in the formation of sound sensations.

Hearing organ of a person (Fig. 7) captures (outer ear), amplifies (middle ear) and perceives (inner ear) sound vibrations, representing, in fact, a distant analyzer, the peripheral (sensory) section of which is located in the pyramid temporal bone(to the snail).

Outer ear includes the auricle and the external auditory canal, which ends in a dense fibrous membrane - the tympanic membrane, which is the boundary between the outer and middle ear. The auricle serves as a collector of sound waves and determines the direction of the sound source when listening with two ears ( binaural hearing). Both ears perform the same job, but do not communicate, which contributes to a more complete reception of information. The auditory canal is not only a conductor of sounds, but also a resonator in the range of speech frequencies from 2,000 to 2,500 Hz. The sound is amplified at these frequencies by 5 to 10 dB. Longitudinal air vibrations carrying sound cause mechanical vibrations of the eardrum, but in order to be transmitted to the membrane of the cochlear window separating the middle ear from the inner ear, and then to the endolymph of the inner ear, these vibrations must be significantly amplified.

Rice. 7. Ear structure

Outer ear: 1 – auricle; 2 – auditory canal; 3 – eardrum.

Middle ear: 4 – middle ear cavity; 5 – auditory tube; middle ear bones: malleus (a), incus (b), stapes (c);

Inner ear: 6 – snail; 7 – auditory nerve.

Vestibular apparatus: 8 – vestibule with sacs; 9 – semicircular canals.

Middle ear– amplifier of sound vibrations caught by the ear. The human sound-conducting apparatus is a very advanced mechanical system. It is capable of responding to minimal air vibrations and conducting them to the sound-receiving system, where the primary analysis of the sound wave is carried out. Vibrations of the eardrum, which converts air sound waves into mechanical vibrations, are transmitted to the auditory ossicles located in the cavity of the middle ear, articulated with each other - hammer, anvil and stapes(Fig. 7). This system of auditory ossicles provides, according to the latest data, amplification of sound coming from the eardrum by 20–25 times, which makes it possible to overcome the resistance of the membrane of the oval window, which separates the cavity of the middle ear from the cavity of the inner ear, and transmit vibrations to the endolymph of the inner ear. The role of the eardrum and auditory ossicles is reduced to the transformation of air vibrations of large amplitude and relatively low force into vibrations of the ear endolymph with relatively small amplitude but high pressure.

With sounds of high intensity, the articulation system of the auditory ossicles acquires a protective, shock-absorbing value. The main route of sound delivery to the cochlea is air, the second route is bone. In this case, the sound wave directly acts on the bones of the skull.

One of important conditions normal air transmission of sounds - the absence of a difference in pressure on both sides of the eardrum, which is ensured by the ventilation capacity of the auditory (“Eustachian”) tube. The latter has a length of 3.5 cm and a width of only 2 mm, and connects the tympanic cavity with the nasopharynx in the form of a canal. When swallowing, this passage opens, ventilating the middle ear and equalizing the pressure in it with atmospheric pressure.

Most complex structure It has inner ear . Located in the petrous part of the temporal bone, it is a bony labyrinth, inside of which there is a membranous labyrinth of connective tissue. The membranous labyrinth is, as it were, inserted into the bony labyrinth and, in general, repeats its shape. Between the bony and membranous labyrinths there is perilymph, inside the membranous – endolymph. In the inner ear there are three sections: the cochlea, the vestibule of the cochlea and the semicircular canals, but only the cochlea is the sensory hearing apparatus. The other two formations belong to the vestibular analyzer system.

The hearing organ is located in snail , which is a spiral bone canal that spirals around a cone-shaped bone shaft for 2.5–2.75 turns, and ends blindly at the apex of the pyramid.

Rice. 8. Spiral organ in the cochlea

A – opened cochlea: 1 – position of the spiral organ in the cochlea;

2 – main membrane; 3 – auditory nerve.

B – spiral organ: 1 - cover membrane; 2 - reticulate membrane;

3 – outer and inner hair cells; 4 - supporting cells;

5 – fibers of the cochlear nerve (in cross section);

6 - external and internal pillars; 7 – cochlear nerve

The spiral canal of the cochlea is 28–30 mm long. The diameter of the spiral canal in the initial section is wide (6 mm), and as it approaches the apex of the cochlea it gradually narrows, reaching 2 mm. From the rod around which this canal passes, a bony spiral basilar (main) plate extends into the lumen of the latter, and, heading towards the peripheral wall of the spiral canal, ends, without reaching it, in the middle of the diameter of the canal. From the free edge of the bony spiral plate to the opposite wall of the cochlea, the basilar plate, which is part of the membranous cochlea, is stretched along its entire length. Thus, the spiral canal of the cochlea is divided by longitudinal partitions into upper (scalena vestibule), middle (spiral organ) and lower (scalena tympani) parts filled with endolymph. Hearing receptors are located in the basilar plate of the spiral organ, located in the middle part of the canal (Fig. 8A).

The basilar plate consists of approximately 20 thousand thin elastic fibers, stretched in the form of strings of varying lengths between the bony spiral ridge and the outer wall of the cochlea (like musical instrument– harps). In the initial curl of the cochlea, the fibers are shorter and thinner, and in the last curl, the fibers are longer and thicker. The tension of the fibers gradually weakens from the base to the apex of the cochlea. The connection between the fibers is very weak, and therefore isolated vibrations of individual sections of the membrane are possible. Only those hairs that are similar to the frequency of the received signal are involved in the oscillation (similar to the phenomenon of resonance). The fewer oscillating hairs, and the closer they are located to the window of the vestibule, the lower the frequency of the sound.

Rice. 9. Hearing analyzer

Dendrites connect to auditory hairs hair (bipolar) sensory cells, which are part of the spiral assembly located right there in the central part of the cochlea. The axons of the bipolar (hair) cells of the spiral (cochlear) ganglion form the auditory branch of the vestibulocochlear nerve (VIII pair of cranial nerves), going to the nuclei of the auditory analyzer located in the bridge (second auditory neuron), subcortical auditory centers in the quadrigeminal region (third auditory neuron) and the cortical hearing center in the temporal lobe of each hemisphere (Fig. 9), where auditory sensations are formed. There are approximately 30,000–40,000 afferent fibers in the auditory nerve. Vibrating hair cells cause excitation only in strictly defined fibers of the auditory nerve, and therefore in strictly defined nerve cells of the cerebral cortex.

Each hemisphere receives information from both ears ( binaural hearing), making it possible to determine the source of sound and its direction. If the sounding object is on the left, then impulses from the left ear arrive in the brain earlier than from the right. This small difference in time allows not only to determine the direction, but also to perceive sound sources from different parts of space. This sound is called volumetric or stereophonic.

Physiology of hearing

For the auditory analyzer, sound is an adequate stimulus. The main characteristics of each sound tone are the frequency and amplitude of the sound wave. The higher the frequency, the higher the pitch of the sound. The strength of a sound, expressed by its volume, is proportional to the amplitude and is measured in decibels (dB). Human ear capable of perceiving sound in the range from 20 Hz to 20,000 Hz (children - up to 32,000 Hz). The ear is most excitable to sounds with a frequency from 1000 to 4000 Hz. Below 1000 and above 4000 Hz, the excitability of the ear is greatly reduced.

Sound up to 30 dB is very weakly audible, from 30 to 50 dB corresponds to a human whisper, from 50 to 65 dB is normal speech, from 65 to 100 dB is strong noise, 120 dB is the “pain threshold”, and 140 dB causes damage. middle (rupture of the eardrum) and inner (destruction of the organ of Corti) ear.

The speech hearing threshold for children 6-9 years old is 17-24 dBA, for adults - 7-10 dBA. With the loss of the ability to perceive sounds from 30 to 70 dB, difficulties are observed when speaking; below 30 dB, almost complete deafness is stated.

Various hearing capabilities are assessed by differential thresholds (DT), i.e., by capturing the minimally changeable parameters of sound, for example, its intensity or frequency. In humans, the differential threshold for intensity is 0.3-0.7 dB, for frequency 2-8 Hz.

Bone conducts sound well. In some forms of deafness, when the auditory nerve is intact, sound travels through the bones. Deaf people can sometimes dance by listening to music through the floor, perceiving its rhythm with their feet. Beethoven listened to the piano playing through a cane, with which he leaned on the piano and held the other end in his teeth. When conducting bone tissue, you can hear ultrasounds - sounds with a frequency of over 50,000 Hz.

At long-term action in the ear of strong sounds (2-3 minutes), hearing acuity decreases, and in silence it is restored; 10-15 seconds are enough for this ( auditory adaptation ).

Temporary decrease in hearing sensitivity from more than long period restoration of normal hearing acuity, which also occurs with prolonged exposure to intense sounds, but recovers after a short rest, is called auditory fatigue . Hearing fatigue, which is based on temporary protective braking in the cerebral cortex, this is physiological phenomenon, which is protective against pathological exhaustion nerve centers. Auditory fatigue that does not recover after a short rest, which is based on persistent extreme braking in the structures of the brain, is called auditory fatigue , which requires a number of special therapeutic and recreational measures to remove it.

Physiology of sound perception. Under the influence of sound waves, complex movements occur in the membranes and fluid of the cochlea. Their study is complicated both by the small magnitude of the vibrations and by the too small size of the cochlea and the depth of its location in the dense capsule of the labyrinth. It is even more difficult to identify the nature of the physiological processes that occur during the transformation of mechanical energy into nervous excitation in the receptor, as well as in nerve conductors and centers. In this regard, there are only a number of hypotheses (assumptions) explaining the processes of sound perception.

The earliest of them is Helmholtz's theory (1863). According to this theory, mechanical resonance phenomena occur in the cochlea, as a result of which complex sounds are decomposed into simple ones. Any tone frequencies has its own limited area on the main membrane and irritates strictly defined nerve fibers: low sounds cause vibration at the top of the cochlea, and high sounds - at its base.

According to the latest hydrodynamic theory of Bekesy and Fletcher, which is currently considered the main one, the active principle of auditory perception is not the frequency, but the amplitude of sound. The amplitude maximum of each frequency in the audibility range corresponds to a specific section of the basilar membrane. Under the influence of sound amplitudes, complex dynamic processes and membrane deformations occur in the lymph of both scalae cochlea, with the place of maximum deformation corresponding to the spatial location of sounds on the main membrane, where vortex movements of the lymph were observed. Sensory cells are most excited where the amplitude of the vibrations is maximum, so different frequencies affect different cells.

In any case, the hair cells vibrated touch the covering membrane and change their shape, which leads to the emergence of an excitation potential in them. Excitation arising in certain groups of receptor cells, in the form nerve impulses spreads along the fibers of the auditory nerve to the nuclei of the brain stem, subcortical centers located in the midbrain, where the information contained in the sound stimulus is repeatedly recoded as it passes through different levels auditory tract. During this process, neurons of one type or another release “their” properties of the stimulus, which ensures a fairly specific activation of neurons higher levels. Upon reaching the auditory zone of the cortex, localized in the temporal lobes (fields 41 - primary auditory cortex and 42 - secondary, associative auditory cortex according to Brodmann), this repeatedly recoded information is converted into an auditory sensation. In this case, as a result of the crossing of the conductive paths, the sound signal from the right and left ears reaches both hemispheres of the brain simultaneously.

Age characteristics development of auditory sensitivity. The development of the peripheral and subcortical sections of the auditory analyzer generally ends at the time of birth, and the auditory analyzer begins to function from the first hours of the child’s life. The child's first reaction to sound is dilation of the pupils, holding his breath, and some movements. Then the child begins to listen to the voice of adults and respond to it, which is already associated with a sufficient degree of development of the cortical sections of the analyzer, although the completion of their development occurs at a fairly later stages ontogeny. In the second half of the year, the child perceives certain sound combinations and associates them with certain objects or actions. At the age of 7–9 months, the baby begins to imitate the sounds of speech of those around him, and by the age of one year he begins to speak his first words.

In newborns, the perception of pitch and volume of sound is reduced, but already by 6–7 months. sound perception reaches adult norms, although the functional development of the auditory analyzer, associated with the development of subtle differentiations to auditory stimuli, continues until 6–7 years. The greatest hearing acuity is characteristic of adolescents and young men (14–19 years old), then gradually decreases.

2.3. Pathology of the auditory analyzer

Hearing impairment is a subtle obstacle that can have far-reaching psychological and social consequences. Patients with hearing loss or complete deafness face significant difficulties. Cut off from verbal communication, they largely lose touch with loved ones and other people around them and significantly change their behavior. Other sensory channels cope extremely unsatisfactorily with tasks for which hearing is responsible, so hearing is the most important human feelings, and his loss cannot be underestimated. It is required not only for understanding the speech of others, but also for the ability to speak oneself. Children who are deaf from birth do not learn to speak because they are deprived of auditory stimuli, so deafness that occurs before the acquisition of speech is especially serious problems. The inability to speak leads to widespread developmental delays, reducing the ability to learn. Therefore, children who are hard of hearing from birth should start using hearing aids up to 18 months of age.

Children with hearing loss are divided into three categories (classification):

Ø deaf – These are children with total hearing loss, among whom are deaf without speech (early deafened) and deaf who have retained speech. Early deafened children also include children with bilateral persistent hearing loss. In children with congenital or acquired pre-existing speech development hearing impairment, subsequently deafness is compensated by other analyzers (visual images, instead of verbal-logical ones). The main form of communication is facial expressions and gestures.

In children who have retained speech, due to the lack of auditory control, it is unclear and blurry. Children often experience voice disorders (inadequate voice pitch, falsetto, nasality, harshness, unnatural timbre), and speech breathing disorders also occur. Mentally, children are unstable, inhibited, and have large complexes.

Ø late-deafened – children with hearing loss but relatively intact speech. They study in special schools for special programs with appropriate TSO for normalization of residual hearing (vibration device, mechanical speech protection device). Oral speech is perceived by ear with distortions, which is why difficulties arise in learning, in selecting speech perception, in expressing and pronouncing speech. These children are withdrawn, irritable, and speak with impaired lexical and grammatical structure of speech.

Ø hearing impaired – these children with partial hearing impairment, which impedes auditory development, but who retain the ability to independently accumulate a speech reserve.

Based on the depth of hearing impairment, there are 4 degrees:

– light – perception of whisper at a distance of 3-6 m, spoken speech 6-8 m;

– moderate – perception of whisper – 1-3 m, conversational speech – 4-6 m;

– significant – perception of whisper – 1 m, conversational speech – 2-4 m;

– heavy – perception of whisper is not painful. 5-10 cm from the ear, colloquially - no more than 2 meters.

Decreased hearing acuity due to any pathological processes in any part of the auditory analyzer ( hypoacusis) or hearing loss is the most common consequence of pathology of the auditory analyzer. Rarer forms of hearing loss are hyperacusis when even ordinary speech causes painful or unpleasant sound sensations (can be observed with damage to facial nerve); double sound ( diplacusia), which occurs when the left and right ears reproduce the pitch of the sound signal differently; paracusia– improvement of hearing acuity in noisy environments, characteristic of otosclerosis.

Hypoacusis can be conditionally associated with three categories of causes:

1. Sound conduction disorders. Hearing impairment due to mechanical obstruction to the passage of sound waves can be caused by accumulation in the external auditory canal earwax . It is secreted by the glands of the external ear canal and performs a protective function, but, accumulating in the external auditory canal, forms a cerumen plug, the removal of which completely restores hearing. A similar effect is produced by presence of foreign bodies in the ear canal, which is especially common in children. It should be noted that the main danger is not so much the presence of a foreign body in the ear, but rather unsuccessful attempts to remove it.

Hearing loss may be caused by ruptured eardrum when exposed to very loud noises or sounds such as a blast wave. In such cases, it is recommended to open your mouth by the time the explosion occurs. A common cause of eardrum perforation is picking at the ear with hairpins, matches and other objects, as well as inept attempts to remove foreign bodies from the ear. Violation of the integrity of the eardrum, while the remaining parts of the auditory organ are intact, has a relatively small effect on auditory function (only the perception of low sounds suffers). The main danger is subsequent infection and the development of purulent inflammation in the tympanic cavity.

Loss of elasticity of the eardrum when exposed to industrial noise, it leads to a gradual loss of hearing acuity (occupational hearing loss).

Inflammation of the tympanal-ossicular apparatus reduces its ability to amplify sound, and even with a healthy inner ear, hearing deteriorates.

Inflammation of the middle ear pose a danger to auditory perception due to their consequences (complications), which are most often noted with chronic inflammation (chronic otitis media). For example, due to the formation of adhesions between the walls of the tympanic cavity and the membrane, the mobility of the latter decreases, resulting in hearing impairment and tinnitus. A very common complication of both chronic and acute purulent otitis, is a perforation of the eardrum. But the main danger lies in possible transition inflammation of the inner ear (labyrinthitis), of the meninges (meningitis, brain abscess), or in the occurrence general infection blood (sepsis).

In many cases, even with proper and timely treatment, especially chronic otitis media, restoration of auditory function in full is not achieved due to the resulting cicatricial changes in the eardrum and the joints of the auditory ossicles. With lesions of the middle ear, as a rule, persistent hearing loss occurs, but complete deafness does not occur because it persists bone conduction. Complete deafness after inflammation of the middle ear can develop only as a result of the transition of the purulent process from the middle ear to the inner ear.

Secondary (secretory) otitis media is a consequence of blockage of the auditory tube due to inflammatory processes in the nasopharynx or proliferation of adenoids. The air in the middle ear is partially absorbed by its mucous membrane and negative air pressure is created, on the one hand, limiting the mobility of the eardrum (resulting in hearing impairment), and on the other hand, promoting the sweating of blood plasma from the vessels into the tympanic cavity. Subsequent organization of the plasma clot can lead to the development adhesive process in the tympanic cavity.

Occupies a special place otosclerosis, consisting in the growth of spongy tissue, most often in the niche of the oval window, as a result of which the stapes becomes walled up in the oval window and loses its mobility. Sometimes this growth can spread to the labyrinth of the inner ear, which leads to disruption of not only the function of sound transmission, but also sound perception. It manifests itself, as a rule, at a young age (15-16 years) with a progressive decline in hearing and tinnitus, leading to severe hearing loss or even complete deafness.

Since lesions of the middle ear affect only sound-conducting formations and do not affect sound-receiving neuroepithelial structures, the hearing loss they cause is called conductive. Conductive hearing loss (except for occupational hearing loss) in most patients is quite successfully corrected by microsurgical and hardware methods.

2. Impaired sound perception. In this case, the hair cells of the organ of Corti are damaged, so that either signal transduction or neurotransmitter release is impaired. As a result, the transmission of information from the cochlea to the central nervous system suffers and develops sensory hearing loss.

The reason is the impact of external or internal unfavorable factors: infectious diseases childhood(measles, scarlet fever, epidemic cerebrospinal meningitis, mumps), general infections (influenza, rash and relapsing fever, syphilis); medicinal (quinine, some antibiotics), household (carbon monoxide, lamp gas) and industrial (lead, mercury, manganese) intoxication; injuries; intense exposure to industrial noise and vibration; disruption of the blood supply to the inner ear; atherosclerosis, age-related changes.

Due to its deep location in the bony labyrinth, inflammation of the inner ear (labyrinthitis), as a rule, are complications of inflammatory processes of the middle ear or meninges, some childhood infections (measles, scarlet fever, mumps). Purulent diffuse labyrinthitis in the vast majority of cases ends in complete deafness, due to purulent melting of the organ of Corti. The result of limited purulent labyrinthitis is partial hearing loss for certain tones, depending on the location of the lesion in the cochlea.

In some cases, during infectious diseases, it is not the microbes themselves that penetrate the labyrinth, but their toxins. Dry labyrinthitis that develops in these cases occurs without purulent inflammation and usually does not lead to the death of the nerve elements of the inner ear. Therefore, complete deafness does not occur, but a significant decrease in hearing is often observed due to the formation of scars and adhesions in the inner ear.

Hearing impairment occurs due to increased endolymph pressure on the sensitive cells of the inner ear, which is observed when Meniere's disease. Despite the fact that the increase in pressure is transient, hearing loss progresses not only during exacerbations of the disease, but also in the interictal period.

3. Retrocochlear disorders – the inner and middle ear are healthy, but either the transmission of nerve impulses along the auditory nerve to the auditory zone of the cerebral cortex, or the activity of the cortical centers itself is impaired (for example, with a brain tumor).

Lesions to the conductive section of the auditory analyzer can occur on any segment of it. The most common are acoustic neuritis , by which we mean inflammatory damage not only to the trunk of the auditory nerve, but also to damage to the nerve cells that make up the spiral nerve ganglion located in the cochlea.

Nervous tissue is very sensitive to any toxic effects. Therefore, a very common consequence of exposure to certain drugs (quinine, arsenic, streptomycin, salicylic drugs, antibiotics of the aminoglycoside group and diuretics) and toxic (lead, mercury, nicotine, alcohol, carbon monoxide, etc.) substances, bacterial toxins is the death of the nerve ganglia of the spiral ganglion, which leads to secondary descending degeneration of the hair cells of the organ of Corti and ascending degeneration of the nerve fibers of the auditory nerve, with the formation of complete or partial loss of auditory function. Moreover, quinine and arsenic have the same affinity for the nerve elements of the auditory organ as methyl (wood) alcohol for nerve endings in the eye. The decrease in hearing acuity in such cases can reach a significant severity, up to deafness, and treatment, as a rule, is not effective. In these cases, rehabilitation of patients occurs through training and the use of hearing aids.

Diseases of the auditory nerve trunk occur as a result of the transition of inflammatory processes from the meninges to the nerve sheath during meningitis.

The auditory pathways in the brain can suffer from congenital anomalies and from various diseases and damage to the brain. These are, first of all, hemorrhages, tumors, inflammatory processes of the brain (encephalitis) with meningitis, syphilis, etc. In all cases, such lesions are usually not isolated, but are accompanied by other brain disorders.

If the process develops in one half of the brain and involves the auditory pathways before they cross, hearing in the corresponding ear is completely or partially impaired; above the chiasm - bilateral hearing loss occurs, more pronounced on the side opposite to the lesion, but complete hearing loss does not occur, since some of the impulses arrive along the preserved pathways of the opposite side.

Damage to the temporal lobes of the brain, where the auditory cortex is located, can occur due to cerebral hemorrhages, tumors, and encephalitis. It becomes difficult to understand speech, spatial localization of the sound source and identification of its temporal characteristics. However, such lesions do not affect the ability to distinguish between the frequency and intensity of sound. Unilateral lesions of the cortex lead to decreased hearing in both ears, more so on the opposite side. There are practically no bilateral lesions of the pathways and the central end of the auditory analyzer.

Hearing defects:

1.Allosia – congenital complete absence or underdevelopment (for example, absence of the organ of Corti) of the inner ear.

2. Atresia – fusion of the external auditory canal; at innate character usually combined with underdevelopment of the auricle or its complete absence. Acquired atresia can be a consequence of prolonged inflammation of the skin of the ear canal (with chronic suppuration from the ear), or scar changes after injuries. In all cases, only complete closure of the ear canal leads to significant and persistent hearing loss. With incomplete fusions, when there is at least a minimal gap in the ear canal, hearing usually does not suffer.

3. Protruding ears, combined with an increase in their size - macrotia, or small ear size – microtia. Due to the fact that functional value the auricle is small, all its diseases, injuries and developmental anomalies, up to complete absence, do not entail significant hearing impairment and are mainly of only cosmetic importance.

4. Congenital fistulas – cleft of the gill cleft, open on the anterior surface of the auricle, slightly above the tragus. The hole is hardly noticeable and a viscous, transparent yellow liquid is released from it.

5. Congenital anomalies of the middle ear – are accompanied by developmental disorders of the outer and inner ear (filling of the tympanic cavity with bone tissue, absence of auditory ossicles, fusion of them).

The cause of congenital ear defects most often lies in disturbances in the development of the embryo. These factors include pathological effects on the fetus from the mother’s body (intoxication, infection, injury to the fetus). Hereditary predisposition also plays a certain role.

Damage to the hearing organ that occurs during childbirth should be distinguished from congenital developmental defects. For example, even injuries to the inner ear can be the result of compression of the fetal head by the narrow birth canal or the consequences of the application of obstetric forceps during pathological childbirth.

Congenital deafness or hearing loss – this is either a hereditary disorder of the embryological development of the peripheral part of the auditory analyzer or its individual elements (outer, middle ear, bone capsule of the labyrinth, organ of Corti); or hearing impairment associated with viral infections suffered by the pregnant woman during early dates(up to 3 months) pregnancy (measles, flu, mumps); or the consequences of toxic substances entering the body of pregnant women (quinine, salicylic drugs, alcohol). Congenital hearing loss is detected already in the first year of a child’s life: he does not move from “humming” to pronouncing syllables or simple words, but, on the contrary, gradually becomes completely silent. In addition, by the middle of the second year at the latest normal child learns to turn towards a sound stimulus.

The role of the hereditary (genetic) factor as a cause of congenital hearing impairment was somewhat exaggerated in previous years. However, this factor undoubtedly has some significance, since it is known that deaf parents have children with birth defect Hearing people are born more often than hearing people.

Subjective reactions to noise. In addition to sound trauma, i.e. objectively observable hearing damage, long stay in an environment “polluted” with excessive sounds (“sound noise”), leads to increased irritability, poor sleep, headaches, increased blood pressure. The discomfort caused by noise largely depends on the subject's psychological attitude towards the source of the sound. For example, a resident of a building may be annoyed by the piano being played two floors above, although the volume level is objectively low and other residents have no complaints.

The peripheral section of the auditory analyzer is represented by the ear, with the help of which a person perceives the influence of the external environment, expressed in the form of sound vibrations that exert physical pressure on the eardrum. Most people receive less information through the organ of hearing than through the organ of vision. However, hearing is of great importance for the overall development and formation of personality, in particular for the development of speech in a child, which has a decisive influence on his mental development.

The organ of hearing and balance contains several types of sensory cells: receptors that perceive sound vibrations; receptors that determine the position of the body in space; receptors that perceive changes in direction and speed of movement. There are three parts of the organ: the outer, middle and inner ear (Fig. 12.6).

Rice. 12.6.

Outer ear perceives sounds and directs them to the eardrum. It includes conducting sections - the auricle and the external auditory canal.

The auricle consists of elastic cartilage covered with a thin layer of skin. The external auditory canal is a curved rope 2.5–3 cm long. The canal has two sections: the external cartilaginous auditory canal and the internal bone one, located in the temporal bone. The external auditory canal is lined with skin with fine hairs and special sweat glands that secrete earwax. Its end is closed from the inside by a thin translucent plate - the eardrum, separating the outer ear from the middle ear.

Middle ear includes several formations enclosed in the tympanic cavity: the eardrum, the auditory ossicles, the auditory (Eustachian) tube. There are two holes on the wall facing the inner ear - oval window(window of the vestibule) and round window (window of the cochlea). On the wall of the tympanic cavity, facing the external auditory canal, there is an eardrum that perceives sound vibrations in the air and transmits them to the sound conducting system of the middle ear - the auditory ossicle complex. The barely noticeable vibrations of the eardrum are amplified and transformed here, transmitted to the inner ear similar to the action of a microphone.

The complex consists of three bones: the malleus, the anvil and the stapes. The malleus (8-9 mm long) is tightly fused with inner surface the eardrum with its handle, and the head is articulated with the anvil, which, due to the presence of two legs, resembles a molar with two roots. One leg (long) serves as a lever for the stirrup. The stirrup has a size of 5 mm, with its wide base inserted into the oval window of the vestibule, tightly adjacent to its membrane. The movements of the auditory ossicles are provided by the tensor tympani muscle and the stapedius muscle.

The auditory (Eustachian) tube, 3.5–4 cm long, connects the tympanic cavity with the upper part of the pharynx. Through it, air enters the middle ear cavity from the nasopharynx, thereby equalizing the pressure on the eardrum from the external auditory canal and the tympanic cavity. When the passage of air through the auditory tube is difficult (for example, during an inflammatory process), pressure from the external auditory canal prevails and the eardrum is pressed into the cavity of the middle ear. This leads to a decrease in the ability of the eardrum to vibrate in accordance with the frequency of sound signals.

Inner ear - very difficult organized organ, externally resembling a labyrinth or cochlea, having 2.5 circles, and located in the pyramid of the temporal bone (Fig. 12.7). Inside the bony labyrinth of the cochlea there is a closed connecting membranous labyrinth, repeating the shape of the external one. The space between the walls of the bony and membranous labyrinths is filled with fluid - perilymph, and the cavity of the membranous labyrinth is filled with endolymph.

Rice. 12.7.

The vestibule is a small oval cavity in the middle part of the labyrinth. On the wall of the vestibule, a ridge separates two pits from each other. The posterior fossa - an elliptical depression - lies closer to the semicircular canals, which open into the vestibule with five openings, and the anterior one - a spherical depression - is connected to the cochlea.

In the membranous labyrinth, elliptical and spherical sacs are distinguished. The walls of the sacs are covered flat epithelium, with the exception of a small area - spots. The spot is lined with columnar epithelium containing supporting and hair sensory cells, which have thin processes on their surface facing the cavity of the sac. The nerve fibers of the auditory nerve (its vestibular part) begin from the hair cells. The surface of the epithelium is covered with a special fine-fibrous and gelatinous membrane, called otolithic, since it contains otolith crystals consisting of calcium carbonate.

Posteriorly, three mutually perpendicular semicircular canals adjoin the vestibule - one in the horizontal and two in vertical planes. All of them are narrow tubes filled with liquid - endolymph. Each channel ends with an extension - an ampoule; in its auditory crest the cells of the sensitive epithelium are concentrated, from which the branches of the vestibular nerve begin.

In front of the vestibule is the cochlea. The cochlear canal bends in a spiral and forms 2.5 turns around the rod. The cochlear shaft consists of spongy bone tissue, between the beams of which there are nerve cells, forming a spiral ganglion. A thin bone sheet, consisting of two plates, extends from the rod in the form of a spiral; myelinated dendrites of the neurons of the spiral ganglion pass between them. The upper plate of the bony leaf passes into the spiral lip, or limbus, the lower one into the spiral main, or basilar, membrane, which extends to the outer wall of the cochlear canal. The dense and elastic spiral membrane is a connective tissue plate that consists of the main substance and collagen fibers - strings stretched between the spiral bone plate and the outer wall of the cochlear canal. At the base of the cochlea the fibers are shorter. Their length is 104 microns. Towards the apex, the length of the fibers increases to 504 µm. Their total number is about 24 thousand.

From the bone spiral plate to the outer wall of the bone canal, at an angle to the spiral membrane, another membrane extends, less dense - vestibular, or Reisner's.

The cavity of the cochlea is divided by membranes into three sections: upper channel the cochlea, or scala vestibule, starts from the window of the vestibule; the middle canal of the cochlea is located between the vestibular and spiral membranes and the lower cable, or scala tympani, starting from the window of the cochlea. At the apex of the cochlea, the scala vestibular and scala tympani communicate through a small opening, the helicotrema. The upper and lower canals are filled with perilymph. The middle canal is the cochlear duct, which is also a spirally convoluted canal with 2.5 turns. On the outer wall of the cochlear duct there is a vascular strip, the epithelial cells of which have secretory function, producing endolymph. The vestibular and tympanic scalae are filled with perilymph, and the middle canal is filled with endolymph. Inside the cochlear duct, on the spiral membrane, there is a complex device (in the form of a protrusion of the neuroepithelium), which is the actual perceptive apparatus of auditory perception - the spiral (Corti) organ.

Organ of Corti formed by sensitive hair cells (Fig. 12.8). There are inner and outer hair cells. The internal ones bear on their surface from 30 to 60 short hairs arranged in 3-5 rows. The number of inner hair cells in humans is about 3500. The outer hair cells are arranged in three rows, each of them has about 100 hairs. The total number of outer hair cells in humans is 12–20 thousand. Outer hair cells are more sensitive to the action of sound stimuli than internal ones. Above the hair cells is the tectorial membrane, which has a ribbon-like shape and a jelly-like consistency. Its width and thickness increase from the base of the cochlea to the apex.

Rice. 12.8. :

1 – cover plate; 2,3 – outer (3-4 rows) and inner (1st row) hair cells; 4 – supporting cells; 5 – fibers of the cochlear nerve (in cross section); 6 – external and internal pillars; 7 – cochlear nerve; 8 – main plate

Information from hair cells is transmitted along the dendrites of the cells forming a spiral knot. The second process of these cells - the axon - as part of the vestibular-cochlear nerve is directed to the brain stem and to the diencephalon, where a switch occurs to the next neurons, the processes of which go to the hearing center, located in the temporal part of the cerebral cortex.

The spiral organ is an apparatus that receives sound stimulation. The vestibule and semicircular canals provide balance. A person can perceive up to 300 thousand different shades of sounds and noise in the range from 16 to 20 thousand Hz. The outer and middle ear are capable of amplifying sound almost 200 times, but only weak sounds are amplified, strong sounds are attenuated.