How sound enters the ear. auditory analyzer

Snail is a flexible tube formed from three fluid-filled chambers. The fluid is practically incompressible, so any movement of the footplate of the stapes in the foramen ovale 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 CSF are virtually closed, and this is reflected in the round window membrane that allows the footplate to move.

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

Snail 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 broader, mobile and thin at the apex. Because the fluid is inherently incompressible, the inward movement of the stirrup causes instantaneous transmission of motion through the fluids of the cochlea, resulting in a protrusion of the circular window.

In this way, with the movement of liquids, there is an almost instantaneous distribution of pressure in various departments of the cochlea. The reaction of different sections of the cochlea with their different mechanical properties in relation to the 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 tone and corresponds to certain areas where there is a difference in mechanical properties. High frequency sounds produce maximum displacement near a hard and thick base, while low frequency sounds produce maximum displacement at a soft and thin top.

Because the wave starts its way from the base to the top, 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 basement membrane displacement, while low frequency sounds produce a predominant displacement at the apex. This asymmetry affects our perception of complex sounds (where low frequency sounds can affect our ability to perceive high frequency sounds, but not vice versa) and is thought to affect 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 in the organ of Corti, providing more stimulus with strong movement.

Anatomy of the ear in three sections.
outer ear: 1 - auricle; 2 - external auditory meatus; 3 - tympanic membrane.
Middle ear: 4 - tympanic cavity; 5 - auditory tube.
inner ear: 6 and 7 - labyrinth with internal auditory meatus and vestibulocochlear nerve; 8 - internal carotid artery;
9 - cartilage of the auditory tube; 10-muscle that raises the palatine curtain;
11 - muscle straining the palatine curtain; 12 - muscle that strains the eardrum (Toynbee muscle).

a) Phase difference of sound wave of cochlear windows. As noted earlier, the cochlea responds to the sound pressure difference between the cochlear windows, 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 (the most important stimulus for the inner ear) depends on the relative amplitude and phase of the individual sound pressures in the two windows.

With a significant difference amplitudes of sound pressure between the foramen ovale and the foramen ovale (both in the healthy ear and in the ear after successful tympanoplasty, when the ossicular system increases the pressure exerted on the foramen ovale), the phase difference has little effect in determining the pressure difference across the windows.

decline phase importance with a difference in magnitude is shown in the figure below, showing a hypothetical situation in which the magnitude of the sound pressure 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 the windows is shown by two curves, one of which, with an amplitude of 9, represents the difference when the window pressures are in phase (phase difference 0°) and the other curve (with an amplitude of 11), showing the pressure difference when the window completely out of phase (180° phase difference). 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 a significant difference in 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.

Nonetheless, phase difference may be significant in conditions where the sound pressure magnitudes in the region of the oval and round windows are similar (for example, when the ossicular chain is damaged). With a similar amplitude and phase of the pressure windows, there is a tendency to mutually neutralize and create only a small pressure difference. On the other hand, if the window pressures are of 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 significant 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 specific case presented, the sound pressure at the oval window is 10 times (20 dB) greater than that at the round window.
One cycle of the window pressure wave (P WD) is presented for two states.
The dotted line shows P WD when the pressure on the oval and round windows are in phase, resulting in a peak amplitude of pressure change of 9 = 10-1.
The solid line shows P WD in the absence of phase matching, and as a result, the 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 possible magnitude difference.
Thus, in the normal ear and in the successful tympanoplasty ear, when the sound pressure at the foramen ovale is greater due to the greater conduction of sound along the ossicular chain, the difference in sound pressure phase between the foramen ovale and the round window has little effect in determining hearing outcome. .

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

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

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

environmental sound can also reach the inner ear, through the 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 only the mastoid process is affected by vibration. Sound-induced vibrations of the whole 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 reciprocal 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.

O the role of sound conduction of the body little is known about normal auditory 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.

Scheme of conduction pathways along the ossicular chain and acoustic conduction.
The transmission of the auditory ossicles is created by the movement of the tympanic membrane, the auditory ossicles, and the foot plate of the stirrup.
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 tympanic membrane.
Because the cochlear windows are spatially distant, the middle ear sound pressures at the oval and round windows (RW) are similar, but not identical.
A small difference between the pressure phase amplitudes at 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 low, and its magnitude is approximately 60 dB less than transmission through the auditory ossicles.

in) Bone conduction audiology. The acoustic energy transmitted to the skull during bone vibration (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 et al. and are similar to those previously described for whole body sound transmission. 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, as well as the fact that audibility during bone conduction depends on the pathological condition 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 receptive part of the auditory analyzer includes - the outer, middle and inner ear (Fig. 11.8.). The outer ear is represented by the auricle (sound catcher) and the external auditory meatus, the length of which is 21-27 mm and the diameter is 6-8 mm. The outer and middle ear are separated by the tympanic membrane - a slightly pliable and slightly stretchable membrane.

The middle ear consists of a chain of interconnected bones: the hammer, anvil, and stirrup. The handle of the malleus is attached to the tympanic membrane, the base of the stirrup is attached to the oval window. This is a kind of amplifier that amplifies vibrations 20 times. In the middle ear, in addition, there are two small muscles attached to the bones. The contraction of these muscles leads to a decrease in oscillations. Pressure in the middle ear is equalized by the Eustachian tube, which opens into the mouth.

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

The main membrane consists of loosely stretched elastic fibers, so it can fluctuate. 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). Top - tectorial membrane.

Mechanisms for conducting 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 to the top the oscillations fade. 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 related to the conduction of sound vibrations. It is due to the equation of redox processes. 2) The 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.

The mechanism for transmitting sound of different frequencies. For a long time, physiology was dominated by the resonator 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 stretched threads irritate the corresponding receptors. However, this theory is criticized because the strings are not stretched and their vibrations at any given moment involve too many membrane fibers.

Deserves attention Bekeshe theory. There is a phenomenon of resonance in the cochlea, however, the resonating substrate is not the fibers of the main membrane, but a liquid column of a certain length. According to Bekesche, the greater the frequency of sound, the shorter the length of the oscillating liquid column. Under the action of low-frequency sounds, the length of the oscillating liquid column 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 for the 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 on 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 different parts of the main membrane leads to a weakening of the electrical phenomena that occur when irritated by sounds of different frequencies.

According to the resonance theory, different sections of the main plate react 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 sound will be the stronger, the greater the magnitude of the vibrations of sound waves and, accordingly, the eardrum. The pitch of the sound depends on the frequency of vibrations of sound waves. The greater the frequency of vibrations per unit time will be. perceived by the organ of hearing in the form of higher tones (thin, high sounds of the voice) A lower frequency of vibrations of sound waves is 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 with sound waves entering the outer ear, where they set the eardrum in motion. Vibrations of the tympanic membrane are transmitted through the system of auditory ossicles of the middle ear to the membrane of the oval window, which causes oscillations of the perilymph of the vestibular (upper) scala. These vibrations are transmitted through the helicotrema to the perilymph of the tympanic (lower) scala and reach the round window, displacing its membrane towards the middle ear cavity. Vibrations of the perilymph are also transmitted to the endolymph of the membranous (middle) canal, which leads to oscillatory movements of the main membrane, consisting of individual fibers stretched like piano strings. Under the action of sound, the fibers of the membrane come into oscillatory motion along with the receptor cells of the organ of Corti located on them. In this case, the hairs of the receptor cells are in contact with the tectorial membrane, the cilia of the hair cells are deformed. A receptor potential appears first, 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 organelle and balance (organum vestibulo-cochleare). It has three departments:

• outer 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. In the auditory canal conducts sound vibrations to the eardrum

3. The eardrum is a membrane that vibrates when exposed to sound.

4. The hammer with its handle is attached to the center of the tympanic membrane with the help of ligaments, and its head is connected to the anvil (5), which, in turn, is attached to the stirrup (6).

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

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 equalizes through the auditory tube.

The organ of Corti consists of a number of sensitive, hairy cells (12) that cover the basilar membrane (13). Sound waves are picked up by hair cells and converted into electrical impulses. Further, these electrical impulses are transmitted along the auditory nerve (11) to the brain. The auditory nerve consists of thousands of the finest nerve fibers. Each fiber starts from a specific section of the cochlea and transmits a specific sound frequency. Low-frequency sounds are transmitted along the fibers emanating from the top of the cochlea (14), and high-frequency sounds are transmitted along the fibers associated with 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 absorber. The external auditory canal conducts sound vibrations to the eardrum. The tympanic membrane, which separates the outer ear from the tympanic cavity, or middle ear, is a thin (0.1 mm) septum shaped like an inward funnel. The membrane vibrates under the action of sound vibrations that come to it through the external auditory canal.

Sound vibrations are picked up by the auricles (in animals they can turn towards the sound source) and transmitted through the external auditory canal to the tympanic membrane, which separates the outer ear from the middle ear. Picking up the sound and the whole process of listening with two ears - the so-called binaural hearing - is important for determining the direction of the 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 negligible difference in the time the sound arrives at both ears is enough to determine its direction.

Middle ear is a sound-conducting device. It is an air cavity, which through the auditory (Eustachian) tube is connected to the nasopharyngeal cavity. Vibrations from the tympanic membrane through the middle ear are transmitted by 3 auditory ossicles connected to each other - the hammer, anvil and stirrup, and the latter through the membrane of the oval window transmits these vibrations of the fluid in the inner ear - the perilymph.

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

With 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.

Due to the connection through the auditory tube of the air cavity of the middle ear with the cavity of the nasopharynx, it becomes possible to equalize the pressure on both sides of the tympanic membrane, 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 tympanic membrane and the stirrup. The first of them, contracting, increases the tension of the tympanic membrane and thereby limits the amplitude of its oscillations during strong sounds, and the second fixes the stirrup and thereby limits its movement. The reflex contraction of these muscles occurs 10 ms after the onset of a strong sound and depends on its amplitude. In this way, the inner ear is automatically protected from overload. With instantaneous strong irritations (shocks, explosions, etc.), this protective mechanism does not have time to work, which can lead to hearing impairments (for example, among explosives and gunners).

inner ear is a sound-receiving apparatus. It is located in the pyramid of the temporal bone and contains the cochlea, which in humans forms 2.5 spiral coils. The cochlear canal is divided by two partitions by the main membrane and the vestibular membrane into 3 narrow passages: the upper one (scala vestibularis), the middle one (the membranous canal) and the lower one (the scala tympani). At the top of the cochlea there is a hole connecting the upper and lower channels into a single one, going from the oval window to the top of the cochlea and further to the round window. Its cavity is filled with a liquid - perilymph, and the cavity of the middle membranous canal is filled with a liquid 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 sound delivery to the ear is air. Approaching sound vibrates the tympanic membrane, and then vibrations are transmitted through the chain of auditory ossicles to the oval window. At the same time, air vibrations of the tympanic cavity arise, 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 ossicular chain is disturbed. In addition to the air path, the conduction of sound waves, there is a tissue, or bone, path.

Under the influence of air 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 oscillate (the bone labyrinth also begins to oscillate). Based on the latest data (Bekesy - Bekesy and others), it can be assumed that sounds propagating through the bones of the skull only excite the organ of Corti if, like air waves, they cause a certain section of the main membrane to bulge.

The ability of the bones of the skull to conduct sound explains why a person himself, his voice recorded on a tape, when playing back the recording, seems alien, while others easily recognize him. The fact is that the tape recording does not reproduce your voice completely. Usually, when talking, you hear not only those sounds that your interlocutors hear (i.e., 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 you listen to a tape recording of your own voice, you hear only what could be recorded - sounds that are carried by 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 the ear canal, cause the tympanic membrane to vibrate, which transmits sound vibrations through the ossicular chain to the oval window and further to the perilymph of the inner ear.

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

The tympanic membrane and the auditory ossicles not only transmit sound vibrations entering the external auditory canal, but transform them, i.e., they convert air vibrations with large amplitude and low pressure into fluctuations of the labyrinth liquid 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 anvil form an unequal lever, so that the excursions made by the foot plate of the stirrup are approximately one and a half times less than the excursions of the malleus handle.

The overall effect of the transforming action of the tympanic membrane and the lever system of the auditory ossicles is expressed in an increase in sound strength by 25-30 dB.

Violation of this mechanism in case of damage to the tympanic membrane 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 tympanic membrane and the ossicular chain, it is necessary that the air pressure on both sides of the tympanic membrane, i.e. in the external auditory canal and in the tympanic cavity, be the same.

This pressure equalization is due to the ventilatory 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 constantly maintained at atmospheric level, that is, 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 tympanic membrane and the ossicular chain; 2) protection of the inner ear from excessive sound stimulation; 3) accommodation, i.e., the adaptation of the sound-conducting apparatus to sounds of various strengths and heights.

With the contraction of the muscle stretching the eardrum, auditory sensitivity increases, which gives reason to consider this muscle "alarming". The stapedius muscle plays the opposite role - during its contraction, it limits the movement of the stirrup and thereby, as it were, muffles too strong sounds.

The outer ear includes the auricle, ear canal, and the tympanic membrane, which covers the inner end of the ear canal. The ear canal has an irregular curved shape. In an adult, it is about 2.5 cm long and about 8 mm in diameter. The surface of the ear canal is covered with hairs and contains glands that secrete earwax, which is necessary to maintain skin moisture. The auditory meatus also provides a constant temperature and humidity of the tympanic membrane.

• 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 opens the Eustachian tube, which allows air to enter the cavity and equalizes pressure on both sides of the eardrum for optimal mobility. The middle ear contains three miniature auditory ossicles: the malleus, anvil, and stirrup. One end of the malleus is connected to the tympanic membrane, its other end is connected to the anvil, which, in turn, is connected to the stirrup, and the stirrup to the cochlea of ​​the inner ear. The tympanic membrane constantly oscillates under the influence of sounds caught 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 from its spiral shape, is relevant to hearing. The cochlea is divided into three channels filled with lymphatic fluids. The fluid in the middle channel differs in composition from the fluid 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, which pick up fluctuations in the fluid in the canal caused by the movement of the stirrup and generate electrical impulses that are transmitted along the auditory nerve to the auditory cortex of the brain. Each hair cell responds to a specific sound frequency, with high frequencies being picked up by cells in the lower cochlea, and cells tuned to low frequencies are located in the upper cochlea. If the hair cells die for any reason, the person ceases to perceive the sounds of the corresponding frequencies.

• auditory pathways

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 an auditory sensation. The auditory centers are located in the temporal lobes of the brain. The time taken 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 tympanic membrane 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 intermediate nodes of the auditory pathways extract not only information about the pitch and loudness of the sound, but also other characteristics of the sound, for example, the time interval between the moments when the sound is picked up by the right and left ears - this is the basis for the ability of a person to determine the direction in which the sound comes. At the same time, the brain evaluates both the information received from each ear separately and combines all the information received into a single sensation.

Our brains store patterns for the sounds around us—familiar voices, music, dangerous sounds, and so on. This helps the brain in the process of processing information about sound to 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 damage due to aging, head trauma, or neurological diseases and disorders may be accompanied by symptoms similar to those of hearing loss, such as inattention, detachment from the environment, and inadequate response. In order to correctly hear and understand sounds, the 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!