How sound enters the ear. Hearing analyzer

Snail is a flexible tube formed from three fluid-filled chambers. The fluid is practically incompressible, so any movement of the foot plate of the stapes in the oval window must be accompanied by movement of the fluid elsewhere. At auditory frequencies, the fluid-filled cochlea, the vestibular aqueduct, and other connecting pathways between the cochlea and the cerebrospinal fluid are virtually closed, and this is reflected in the round window membrane, which provides mobility to the foot plate.

When footplate the stirrup moves inward, the round window deviates outward. (The footplate and round window have approximately the same volumetric velocity, but move in opposite directions.) It is this interaction between the round and oval windows, as well as the incompressibility of the cochlear fluids, that determines the role of the difference in sound pressure exerted on the two cochlear windows in stimulating the inner ear.

Snail It is divided into chambers by the basilar membrane, organ of Corti, cochlear duct and Reissner's membrane. The mechanical properties of the cochlear chambers largely depend on the mechanical properties of the basilar membrane; the latter is narrow, rigid, thick at the base and wider, more flexible and thin at the apex. Because the fluid is essentially incompressible, the inward movement of the stapes causes instantaneous transmission of motion through the fluids of the cochlea, resulting in protrusion of the round window.

Thus, with fluid movement, there is an almost instantaneous distribution of pressure across different parts of the cochlea. The reaction of different sections of the cochlea with their different mechanical properties in relation to pressure distribution leads to the appearance of a traveling wave and displacement of the cochlear chambers. The maximum displacement of this wave depends on the tonality and corresponds to certain areas where there is a difference in mechanical properties. High-frequency sounds produce maximum displacement near the hard, thick base, while low-frequency sounds produce maximum displacement at the pliable, thin apex.

Because the wave begins its path from the base to the apex, and also stops immediately after the place of maximum displacement; there is an asymmetry in the movement of different sections of the cochlea. All sounds produce some displacement of the basement membrane, while low-frequency sounds produce a predominant displacement at the apex. This asymmetry affects our perception of complex sounds (where low-frequency sounds may influence our ability to perceive high-frequency sounds, but not vice versa) and is thought to influence the sensitivity of the base of the cochlea, which is responsible for high-frequency sounds in sound trauma or presbycusis. The movement of the internal structures of the cochlea stimulates the hair cells of the organ of Corti, providing greater stimulus with strong movement.

Anatomy of the ear in three sections.
Outer ear: 1 - auricle; 2 - external auditory canal; 3 - eardrum.
Middle ear: 4 - tympanic cavity; 5 - auditory tube.
Inner ear: 6 and 7 - labyrinth with internal auditory canal and vestibular-cochlear nerve; 8 - internal carotid artery;
9 - cartilage of the auditory tube; 10-muscle, lifting the velum palatine;
11 - muscle that strains the velum palatine; 12 - muscle that strains the tympanic membrane (Toynbee muscle).

A) Phase difference of the sound wave of the cochlear windows. As noted earlier, the cochlea responds to the difference in sound pressure between the fenestrae, where the sound pressure exerted on the oval window is the sum of the pressure generated by the ossicular system and the acoustic pressure in the middle ear cavity. It is important to understand how this difference (a critical stimulus to the inner ear) depends on the relative amplitude and phase of the individual sound pressures in the two windows.

With significant difference sound pressure amplitudes between the oval and round windows (both in the healthy ear and in the ear after successful tympanoplasty, where the ossicular system increases the pressure acting on the oval window), the phase difference has a negligible effect in determining the pressure difference between the windows.

Decline importance of the phase with the difference in magnitude is shown in the figure below, demonstrating a hypothetical situation in which the sound pressure magnitude of an oval window is ten times (20 dB) greater than the sound pressure of a round window. The range of possible pressure differences in windows is shown by two curves, one with an amplitude of 9 representing the difference when the window pressures are in phase (0° phase difference) and the other curve (with an amplitude of 11) showing the pressure difference when the window completely out of phase (phase difference 180°). Even with the maximum effect of changing the phase difference, the two curves shown in the figure below are similar in magnitude, within 2 dB.

With significant difference At magnitudes around 100 and 1000 (40-60 dB), occurring in the normal ear and in ears that have undergone successful tympanoplasty, the phase difference has little effect.

Nevertheless, phase difference may be significant in conditions where the sound pressure magnitudes in the area of ​​the oval and round windows are similar (for example, when the chain of auditory ossicles is damaged). With similar amplitude and phase of window pressure, there is a tendency to neutralize each other and create only a small pressure difference. On the other hand, if the window pressures have similar amplitude but opposite phases, they will potentiate each other, resulting in a window pressure difference similar to the magnitude of the applied pressure.

If there is a reliable difference in magnitude between the pressures at the cochlea windows, then the phase difference is of little importance in determining the difference between the two sound pressures.
In the particular case presented, the sound pressure at the oval window is 10 times (20 dB) greater than at the round window.
One window pressure wave cycle (P WD) is presented for two conditions.
The dotted line shows P WD when the pressure on the oval and round windows is in phase, resulting in a peak amplitude of pressure change of 9 = 10-1.
The solid line shows P WD when there is no phase match, and the resulting amplitude of P WD is 11 = 10-(-1).
Note that both peak amplitude differences differ by less than 2 dB (20log 10 11/9 = 1.7 dB), even though the phase difference is due to the maximum difference in magnitude possible.
Thus, in the normal ear and in the ear that has undergone successful tympanoplasty, when the sound pressure at the oval window is greater due to greater sound conduction along the ossicular chain, the difference in sound pressure phases at the oval and round windows has little effect in determining hearing outcome. .

b) Ways of sound stimulation of the inner ear. The middle ear's contribution to the window pressure difference that stimulates the inner ear can be divided into several stimulating pathways. The previous section described how the ossicular system transforms sound pressure in the external auditory canal, transmitting it to the oval window. This pathway has been called ossicular transmission. There is another mechanism called acoustic transmission by which the middle ear can stimulate the inner ear.

Movement eardrum in response to sound arising in the ear, it creates sound pressure in the middle ear cavity. A few millimeters of distance between the cochlear windows means that the acoustic sound pressure at the oval and round windows is similar, but not identical. Small differences between the magnitudes and phases of the sound pressures on the outside of two windows result in a small but measurable sound pressure difference between them. In a normal ear, the magnitude of the pressure difference that is provided by acoustic transmission is small, about 60 dB, which is less than transmission through the ossicles. Consequently, ossicular transmission dominates in the healthy middle ear, and acoustic transmission can be ignored.

However, below will be shown that acoustic transmission can be of great importance in the case of a defect in the ossicular chain that occurs in certain diseases, as well as in the ear that has undergone reconstruction.

Ambient sound can also reach the inner ear, through vibration of the whole body or head, the so-called sound conduction of the body. This is a more general process than bone conduction, in which vibration only affects the mastoid process. Sound-induced vibrations throughout the body and head can stimulate the inner ear:
(1) generating pressure in the external auditory canal or middle ear by exerting pressure on their walls,
(2) producing mutual movements between the auditory ossicles and the inner ear and
(3) direct compression of the inner ear and its contents through compression of the surrounding fluid and bone.

ABOUT the role of body sound conductivity Little is known about normal hearing function. However, measurements of hearing loss due to conditions such as congenital atresia of the ear canal suggest that the entire body can provide stimulation to the inner ear that is 60 dB less than normal ossicular function.

Diagram of conduction pathways along the auditory ossicular chain and acoustic conduction.
The transmission of the auditory ossicles is created by the movement of the eardrum, the auditory ossicles and the foot plate of the stapes.
Acoustic transmission occurs due to sound pressure in the middle ear, which is created by the sound pressure of the external auditory canal and the movement of the eardrum.
Because the cochlear fenestrae are spatially distant, the sound pressures in the middle ear cavity affecting the oval and round fenestrae (RW) are similar, but not identical.
The slight difference between the amplitudes and pressure phases of the two windows results in a small but measurable difference in sound pressure between the two windows.
This difference is called acoustic transmission. In the normal ear, acoustic transmission is extremely small, and its magnitude is approximately 60 dB less than transmission through the ossicles.

V) Bone conduction audiology. The acoustic energy transmitted to the skull when the bone vibrates (tuning fork or electromagnetic vibration of an audiometer) sets the basement membrane in motion and is perceived as sound. Clinical bone conduction tests are performed to diagnose cochlear function. The mechanisms by which bone vibration stimulates the inner ear have been described by Tonndorf and others and are similar to those previously described for the conduction of sound throughout the body. It is important to understand that all hypothetical mechanisms of sound conduction take into account the relative mobility between the auditory ossicles and the inner ear, and that bone conduction audibility depends on the pathological state of the external auditory canal and middle ear.

The auditory analyzer perceives air vibrations and transforms the mechanical energy of these vibrations into impulses, which are perceived in the cerebral cortex as sound sensations.

The perceptive part of the auditory analyzer includes the outer, middle and inner ear (Fig. 11.8.). The outer ear is represented by the auricle (sound collector) and the external auditory canal, the length of which is 21-27 mm and the diameter is 6-8 mm. The outer and middle ears are separated by the eardrum - a membrane that is poorly pliable and weakly stretchable.

The middle ear consists of a chain of interconnected bones: the malleus, the incus and the stapes. The handle of the malleus is attached to the tympanic membrane, the base of the stapes is attached to the oval window. This is a kind of amplifier that amplifies vibrations 20 times. The middle ear also has two small muscles that attach to the bones. Contraction of these muscles leads to a decrease in vibrations. The pressure in the middle ear is equalized by the Eustachian tube, which opens into the oral cavity.

The inner ear is connected to the middle ear by the oval window, to which the stapes is attached. In the inner ear there is a receptor apparatus of two analyzers - perceptive and auditory (Fig. 11.9.). The hearing receptor apparatus is represented by the cochlea. The cochlea, 35 mm long and having 2.5 whorls, consists of a bony and membranous part. The bone part is divided by two membranes: the main and vestibular (Reisner) into three canals (upper - vestibular, lower - tympanic, middle - tympanic). The middle part is called the cochlear passage (membranous). At the apex, the upper and lower canals are connected by a helicotrema. The upper and lower canals of the cochlea are filled with perilymph, the middle ones with endolymph. Perilymph resembles plasma in ionic composition, endolymph resembles intracellular fluid (100 times more K ions and 10 times more Na ions).

The main membrane consists of weakly stretched elastic fibers, so it can vibrate. On the main membrane - in the middle channel - there are sound-perceiving receptors - the organ of Corti (4 rows of hair cells - 1 internal (3.5 thousand cells) and 3 external - 25-30 thousand cells). Above is the tectoreal membrane.

Mechanisms of sound vibrations. Sound waves passing through the external auditory canal vibrate the tympanic membrane, the latter sets in motion the bones and the membrane of the oval window. The perilymph oscillates and the oscillations fade towards the apex. Vibrations of the perilymph are transmitted to the vestibular membrane, and the latter begins to vibrate the endolymph and the main membrane.

The following is recorded in the cochlea: 1) The total potential (between the organ of Corti and the middle channel - 150 mV). It is not associated with the conduction of sound vibrations. It is due to the level of redox processes. 2) Action potential of the auditory nerve. In physiology, the third - microphone - effect is also known, which consists in the following: if electrodes are inserted into the cochlea and connected to a microphone, after amplifying it, and pronouncing various words in the cat's ear, then the microphone reproduces the same words. The microphonic effect is generated by the surface of the hair cells, since the deformation of the hairs leads to the appearance of a potential difference. However, this effect exceeds the energy of the sound vibrations that caused it. Hence, the microphone potential is a difficult transformation of mechanical energy into electrical energy, and is associated with metabolic processes in hair cells. The place of occurrence of the microphone potential is the region of the roots of the hairs of the hair cells. Sound vibrations acting on the inner ear impose an emerging microphonic effect on the endocochlear potential.

The total potential differs from the microphone one in that it reflects not the shape of the sound wave, but its envelope and occurs when high-frequency sounds act on the ear (Fig. 11.10.).

The action potential of the auditory nerve is generated as a result of electrical excitation that occurs in the hair cells in the form of a microphone effect and a net potential.

There are synapses between hair cells and nerve endings, and both chemical and electrical transmission mechanisms take place.

Mechanism for transmitting sound of different frequencies. For a long time, the resonator system dominated in physiology. Helmholtz theory: strings of different lengths are stretched on the main membrane, like a harp they have different vibration frequencies. Under the action of sound, that part of the membrane that is tuned to resonance with a given frequency begins to oscillate. Vibrations of the tensioned threads irritate the corresponding receptors. However, this theory is criticized because the strings are not stretched and their vibrations at any given moment include too many membrane fibers.

Deserves attention Bekes theory. There is a resonance phenomenon in the cochlea, however, the resonating substrate is not the fibers of the main membrane, but a column of liquid of a certain length. According to Bekeshe, the higher the frequency of sound, the shorter the length of the oscillating column of liquid. Under the influence of low-frequency sounds, the length of the oscillating column of liquid increases, capturing most of the main membrane, and not individual fibers vibrate, but a significant part of them. Each pitch corresponds to a certain number of receptors.

Currently, the most common theory of perception of sound of different frequencies is "place theory"”, according to which the participation of perceiving cells in the analysis of auditory signals is not excluded. It is assumed that hair cells located in different parts of the main membrane have different lability, which affects sound perception, i.e. we are talking about tuning hair cells to sounds of different frequencies.

Damage in various parts of the main membrane leads to a weakening of electrical phenomena that occur when irritated by sounds of different frequencies.

According to the resonance theory, different parts of the main plate respond by vibrating their fibers to sounds of different pitches. The strength of sound depends on the magnitude of the vibrations of sound waves that are perceived by the eardrum. The stronger the sound, the greater the vibration of the sound waves and, accordingly, the eardrum. The pitch of the sound depends on the frequency of vibration of the sound waves. The frequency of vibrations per unit time will be greater. perceived by the organ of hearing in the form of higher tones (fine, high-pitched sounds of the voice) Lower frequency vibrations of sound waves are perceived by the organ of hearing in the form of low tones (bass, rough sounds and voices).

Perception of pitch, sound intensity, and sound source location begins when sound waves enter the outer ear, where they vibrate the eardrum. Vibrations of the tympanic membrane through the system of auditory ossicles of the middle ear are transmitted to the membrane of the oval window, which causes vibrations of the perilymph of the vestibular (upper) scala. These vibrations are transmitted through the helicotrema to the perilymph of the scala tympani (lower) and reach the round window, displacing its membrane towards the cavity of the middle ear. Vibrations of the perilymph are also transmitted to the endolymph of the membranous (middle) canal, which causes the main membrane, consisting of individual fibers stretched like piano strings, to vibrate. When exposed to sound, the membrane fibers begin to vibrate along with the receptor cells of the organ of Corti located on them. In this case, the hairs of the receptor cells come into contact with the tectorial membrane, and the cilia of the hair cells are deformed. First, a receptor potential appears, and then an action potential (nerve impulse), which is then carried along the auditory nerve and transmitted to other parts of the auditory analyzer.

And morphologists call this structure organelukha and balance (organum vestibulo-cochleare). It has three departments:

• external ear (external auditory canal, auricle with muscles and ligaments);
• middle ear (tympanic cavity, mastoid appendages, auditory tube)
• (membranous labyrinth located in the bony labyrinth inside the bone pyramid).

1. The outer ear concentrates sound vibrations and directs them to the external auditory opening.

2. The auditory canal conducts sound vibrations to the eardrum

3. The eardrum is a membrane that vibrates under the influence of sound.

4. The malleus with its handle is attached to the center of the eardrum with the help of ligaments, and its head is connected to the incus (5), which, in turn, is attached to the stapes (6).

Tiny muscles help transmit sound by regulating the movement of these ossicles.

7. The Eustachian (or auditory) tube connects the middle ear to the nasopharynx. When the ambient air pressure changes, the pressure on both sides of the eardrum is equalized through the auditory tube.

The organ of Corti consists of a number of sensory, hair-bearing cells (12) that cover the basilar membrane (13). Sound waves are picked up by hair cells and converted into electrical impulses. These electrical impulses are then transmitted along the auditory nerve (11) to the brain. The auditory nerve consists of thousands of tiny nerve fibers. Each fiber starts from a specific part of the cochlea and transmits a specific sound frequency. Low-frequency sounds are transmitted through fibers emanating from the apex of the cochlea (14), and high-frequency sounds are transmitted through fibers connected to its base. Thus, the function of the inner ear is to convert mechanical vibrations into electrical ones, since the brain can only perceive electrical signals.

Outer ear is a sound-collecting device. The external auditory canal conducts sound vibrations to the eardrum. The eardrum, which separates the outer ear from the tympanic cavity, or middle ear, is a thin (0.1 mm) partition shaped like an inward funnel. The membrane vibrates under the action of sound vibrations coming to it through the external auditory canal.

Sound vibrations are picked up by the ears (in animals they can turn towards the sound source) and transmitted through the external auditory canal to the eardrum, which separates the outer ear from the middle ear. Catching sound and the entire process of listening with two ears - so-called binaural hearing - is important for determining the direction of sound. Sound vibrations coming from the side reach the nearest ear a few ten-thousandths of a second (0.0006 s) earlier than the other. This insignificant difference in the time of arrival of sound to both ears is enough to determine its direction.

Middle ear is a sound-conducting device. It is an air cavity that connects through the auditory (Eustachian) tube to the cavity of the nasopharynx. Vibrations from the eardrum through the middle ear are transmitted by 3 auditory ossicles connected to each other - the hammer, incus and stapes, and the latter, through the membrane of the oval window, transmits these vibrations to the fluid located in the inner ear - perilymph.

Due to the peculiarities of the geometry of the auditory ossicles, vibrations of the eardrum of reduced amplitude but increased strength are transmitted to the stapes. In addition, the surface of the stapes is 22 times smaller than the eardrum, which increases its pressure on the oval window membrane by the same amount. As a result of this, even weak sound waves acting on the eardrum can overcome the resistance of the membrane of the oval window of the vestibule and lead to vibrations of the fluid in the cochlea.

During strong sounds, special muscles reduce the mobility of the eardrum and auditory ossicles, adapting the hearing aid to such changes in the stimulus and protecting the inner ear from destruction.

Thanks to the connection of the air cavity of the middle ear with the cavity of the nasopharynx through the auditory tube, it becomes possible to equalize the pressure on both sides of the eardrum, which prevents its rupture during significant changes in pressure in the external environment - when diving under water, climbing to a height, shooting, etc. This is the barofunction of the ear .

There are two muscles in the middle ear: the tensor tympani and the stapedius. The first of them, contracting, increases the tension of the eardrum and thereby limits the amplitude of its vibrations during strong sounds, and the second fixes the stapes and thereby limits its movements. The reflex contraction of these muscles occurs 10 ms after the onset of a strong sound and depends on its amplitude. This automatically protects the inner ear from overload. In case of instantaneous strong irritations (impacts, explosions, etc.), this protective mechanism does not have time to work, which can lead to hearing impairment (for example, among bombers and artillerymen).

Inner ear is a sound-perceiving apparatus. It is located in the pyramid of the temporal bone and contains the cochlea, which in humans forms 2.5 spiral turns. The cochlear canal is divided by two partitions, the main membrane and the vestibular membrane into 3 narrow passages: upper (scala vestibular), middle (membranous canal) and lower (scala tympani). At the top of the cochlea there is an opening that connects the upper and lower canals into a single one, going from the oval window to the top of the cochlea and then to the round window. Its cavity is filled with fluid - peri-lymph, and the cavity of the middle membranous canal is filled with a fluid of a different composition - endolymph. In the middle channel there is a sound-perceiving apparatus - the organ of Corti, in which there are mechanoreceptors of sound vibrations - hair cells.

The main route of delivery of sounds to the ear is airborne. The approaching sound vibrates the eardrum, and then through the chain of auditory ossicles the vibrations are transmitted to the oval window. At the same time, vibrations of the air in the tympanic cavity also occur, which are transmitted to the membrane of the round window.

Another way of delivering sounds to the cochlea is tissue or bone conduction . In this case, the sound directly acts on the surface of the skull, causing it to vibrate. Bone pathway for sound transmission becomes of great importance if a vibrating object (for example, the stem of a tuning fork) comes into contact with the skull, as well as in diseases of the middle ear system, when the transmission of sounds through the chain of auditory ossicles is disrupted. In addition to the air path for conducting sound waves, there is a tissue, or bone, path.

Under the influence of airborne sound vibrations, as well as when vibrators (for example, a bone telephone or a bone tuning fork) come into contact with the integument of the head, the bones of the skull begin to vibrate (the bone labyrinth also begins to vibrate). Based on the latest data (Bekesy and others), it can be assumed that sounds propagating along the bones of the skull only excite the organ of Corti if, similar to air waves, they cause arching of a certain section of the main membrane.

The ability of the skull bones to conduct sound explains why to the person himself his voice, recorded on tape, seems foreign when the recording is played back, while others easily recognize it. The fact is that the tape recording does not reproduce your entire voice. Usually, when talking, you hear not only those sounds that your interlocutors also hear (that is, those sounds that are perceived due to air-liquid conduction), but also those low-frequency sounds, the conductor of which is the bones of your skull. However, when listening to a tape recording of your own voice, you hear only what could be recorded - sounds whose conductor is air.

Binaural hearing . Man and animals have spatial hearing, that is, the ability to determine the position of a sound source in space. This property is based on the presence of binaural hearing, or hearing with two ears. For him, the presence of two symmetrical halves at all levels is also important. The acuity of binaural hearing in humans is very high: the position of the sound source is determined with an accuracy of 1 angular degree. The basis for this is the ability of neurons in the auditory system to evaluate interaural (interaural) differences in the time of sound arrival at the right and left ears and the sound intensity in each ear. If the sound source is located away from the midline of the head, the sound wave arrives at one ear somewhat earlier and has greater strength than at the other ear. Estimation of the distance of the sound source from the body is associated with the weakening of the sound and the change in its timbre.

With separate stimulation of the right and left ears through headphones, a delay between sounds as early as 11 μs or a difference in the intensity of two sounds by 1 dB leads to an apparent shift in the localization of the sound source from the midline towards an earlier or stronger sound. In the auditory centers there is with a sharp adjustment to a certain range of interaural differences in time and intensity. Cells have also been found that respond only to a certain direction of movement of the sound source in space.

The auricle, external auditory canal, tympanic membrane, auditory ossicles, annular ligament of the oval window, round window membrane (secondary tympanic membrane), labyrinth fluid (perilymph), main membrane take part in the conduction of sound vibrations.

In humans, the role of the auricle is relatively small. In animals that have the ability to move their ears, the auricles help determine the direction of the sound source. In humans, the auricle, like a mouthpiece, only collects sound waves. However, in this respect, its role is insignificant. Therefore, when a person listens to quiet sounds, he puts his hand to his ear, due to which the surface of the auricle increases significantly.

Sound waves, having penetrated into the ear canal, lead to a friendly oscillation of the tympanic membrane, which transmits sound vibrations through the ossicular chain to the oval window and further to the perilymph of the inner ear.

The eardrum responds not only to those sounds whose number of vibrations coincides with its own tone (800-1000 Hz), but also to any sound. This resonance is called universal, in contrast to acute resonance, when a secondary sounding body (for example, a piano string) responds to only one specific tone.

The eardrum and auditory ossicles do not simply transmit sound vibrations entering the external auditory canal, but transform them, that is, they transform air vibrations with large amplitude and low pressure into vibrations of the labyrinth fluid with low amplitude and high pressure.

This transformation is achieved due to the following conditions: 1) the surface of the tympanic membrane is 15-20 times larger than the area of ​​the oval window; 2) the malleus and incus form an unequal lever, so that the excursions made by the foot plate of the stapes are approximately one and a half times less than the excursions of the malleus handle.

The overall effect of the transformative effect of the eardrum and the lever system of the auditory ossicles is expressed in an increase in sound intensity by 25-30 dB.

Violation of this mechanism in case of damage to the eardrum and diseases of the middle ear leads to a corresponding decrease in hearing, i.e., by 25-30 dB.

For the normal functioning of the eardrum and the chain of auditory ossicles, it is necessary that the air pressure on both sides of the eardrum, i.e. in the external auditory canal and in the tympanic cavity, be the same.

This pressure equalization occurs due to the ventilation function of the auditory tube, which connects the tympanic cavity to the nasopharynx. With each swallowing movement, air from the nasopharynx enters the tympanic cavity, and thus the air pressure in the tympanic cavity is always maintained at atmospheric level, i.e. at the same level as in the external auditory canal.

The sound-conducting apparatus also includes the muscles of the middle ear, which perform the following functions: 1) maintaining the normal tone of the eardrum and the chain of auditory ossicles; 2) protection of the inner ear from excessive sound stimulation; 3) accommodation, i.e. adaptation of the sound-conducting apparatus to sounds of varying strength and height.

When the muscle that stretches the tympanic membrane contracts, auditory sensitivity increases, which gives reason to consider this muscle “alert.” The stapedius muscle plays the opposite role - when it contracts, it limits the movements of the stirrup and thereby, as it were, muffles sounds that are too strong.

The outer ear includes the pinna, ear canal, and eardrum, which covers the inner end of the ear canal. The ear canal has an irregularly curved shape. In an adult, its length is about 2.5 cm and its diameter is about 8 mm. The surface of the ear canal is covered with hairs and contains glands that secrete earwax, which is necessary to maintain moisture in the skin. The ear canal also provides constant temperature and humidity to the eardrum.

• Middle ear

The middle ear is an air-filled cavity behind the eardrum. This cavity connects to the nasopharynx through the Eustachian tube, a narrow cartilaginous canal that is usually closed. Swallowing movements open the Eustachian tube, which allows air to enter the cavity and equalize pressure on both sides of the eardrum for optimal mobility. In the middle ear cavity there are three miniature auditory ossicles: the malleus, the incus and the stapes. One end of the malleus is connected to the eardrum, the other end is connected to the incus, which in turn is connected to the stirrup, and the stirrup to the cochlea of ​​the inner ear. The eardrum constantly vibrates under the influence of sounds picked up by the ear, and the auditory ossicles transmit its vibrations to the inner ear.

• Inner ear

The inner ear contains several structures, but only the cochlea, which gets its name because of its spiral shape, is related to hearing. The cochlea is divided into three channels filled with lymphatic fluids. The liquid in the middle channel has a different composition from the liquid in the other two channels. The organ directly responsible for hearing (the organ of Corti) is located in the middle canal. The organ of Corti contains about 30,000 hair cells that detect fluid vibrations in the canal caused by the movement of the stapes and generate electrical impulses that are transmitted along the auditory nerve to the auditory cortex. Each hair cell responds to a specific sound frequency, with high frequencies tuned to cells in the lower part of the cochlea and cells tuned to low frequencies located in the upper part of the cochlea. If hair cells die for any reason, a person ceases to perceive sounds of the corresponding frequencies.

• Auditory pathways

The auditory pathways are a collection of nerve fibers that conduct nerve impulses from the cochlea to the auditory centers of the cerebral cortex, resulting in auditory sensation. The auditory centers are located in the temporal lobes of the brain. The time it takes for the auditory signal to travel from the outer ear to the auditory centers of the brain is about 10 milliseconds.

How the human ear works (drawing courtesy of Siemens)

Sound perception

The ear sequentially converts sounds into mechanical vibrations of the eardrum and auditory ossicles, then into vibrations of the fluid in the cochlea, and finally into electrical impulses, which are transmitted along the pathways of the central auditory system to the temporal lobes of the brain for recognition and processing.
The brain and the intermediate nodes of the auditory pathways extract not only information about the pitch and volume of the sound, but also other characteristics of the sound, for example, the time interval between the moments when the right and left ear picks up the sound - this is the basis of a person’s ability to determine the direction in which the sound is coming. In this case, the brain evaluates both the information received from each ear separately and combines all the information received into a single sensation.

Our brain stores “patterns” of the sounds around us - familiar voices, music, dangerous sounds, etc. This helps the brain, when processing information about sound, quickly distinguish familiar sounds from unfamiliar ones. With hearing loss, the brain begins to receive distorted information (sounds become quieter), which leads to errors in the interpretation of sounds. On the other hand, brain problems due to aging, head injury, or neurological diseases and disorders may be accompanied by symptoms similar to those of hearing loss, such as inattention, withdrawal from the environment, and inappropriate reactions. In order to correctly hear and understand sounds, coordinated work of the auditory analyzer and the brain is necessary. Thus, without exaggeration, we can say that a person hears not with his ears, but with his brain!