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Sound Physiology

Carrying sounds in the ear is a mechanical process (Fig. 1.2.5).

The propagation of a sound wave in the perilymph is possible due to the presence of a secondary membrane of the round window, and in the endolymph due to the elastic endolymphatic sac communicating with the endolymphatic space of the labyrinth through the endolymphatic duct.

The movement of the labyrinth fluids causes oscillations of the basilar membrane of the membranous labyrinth, on which the organ of Corti with sensitive hair cells is located.

Fig. 1.2.5

The auricle is important in the ototope, the concentration of sound energy and the coordination of impedances (resistance) of the sound wave of a free acoustic field and the external auditory canal. The auricle and the external auditory meatus have their own resonant frequency (3 and 5 kHz). In particular, the amplification by the auditory canal of sounds of 3 kHz by 10-12 dB occurs due to the correspondence of the length of the auditory canal to 1/4 of the wavelength of this resonant frequency, which improves the perception of speech.

Thus, the outer ear plays a role in enhancing high-frequency sounds and localizing the sound source in space.

The energy of the sound wave is lost during the transition from the air to the liquid (ear lymph) by 99.9% due to the higher impedance of the peri- and endolymph and its reflection, which is about 30 dB. However, the possible loss of sound energy is compensated by other mechanisms.

The area of ​​the human eardrum is about 85 mm2, of which only 55 mm2 vibrate under the influence of a sound wave. The area of ​​the stirrup foot plate is about 3.2 mm2. The difference in these areas provides a 17-fold increase in pressure on the stirrup foot plate, which is 24.6 dB, that is, the impedance loss of sound energy is almost completely compensated.

In addition, the auditory ossicles act according to the law of the lever system, which creates a positive amplification effect with a coefficient of 1.3. An additional increase in energy on the foot plate of the stapes is due to the conical shape of the eardrum, with vibration of which the pressure on the hammer doubles.

Thus, the sound energy applied to the eardrum is amplified on the foot plate of the stapes in 17. 1.3. 2 = 44.2 times, which corresponds to 33 dB and compensates for the impedance energy loss.

The amplification of the sound wave pressure also depends on the frequency of stimulation. So, at a frequency of 2500 Hz, the pressure increases by 30 dB, and above this frequency, the gain decreases.

Under the influence of low sounds, the membrane vibrates in the direction of the tympanic cavity up to 0.5 mm during vibration, and under the influence of high sounds - within a fraction of a micron. The greatest vibration of the membrane is observed in the posterior regions. The stirrup moves in an oval window of 3-7 mm at a sound pressure level of 80 dB, at a frequency of 1000 Hz. The sound wave exerts pressure 60 times stronger on the foot of the stirrup than on the round window, so it extends from the oval to the round window, and not vice versa. Thanks to the transformational mechanism of the middle ear, the pressure of the sound wave in the ear lymph becomes 36 times greater than in air.

In addition, the sound wave enters the labyrinth windows in unequal phases with a maximum pressure difference between them. In the thickening phase, the oval window membrane oscillates inside the vestibular staircase with a perilymph shift, and the round window membrane, being in the rarefaction phase, toward the tympanic cavity, and vice versa when changing phases. The wave propagation velocity in the vestibular canal is constant and equal to the speed of sound in water, and in the tympanal canal the wave propagates with a sharp deceleration due to large pressure gradients from the side of the basilar membrane. The volume displacements of the windows are the same, but the sound pressure in the vestibular and tympanic stairs is different, which is a necessary condition for the movement of fluid in the labyrinth and the excitation of the auditory receptor.

One of the conditions for the normal functioning of the sound-conducting apparatus is its good mobility, and especially the membranes of the oval and round windows with a maximum pressure difference on them. With a complete defect of the eardrum, when the difference in sound pressure on the labyrinth windows is minimal, hearing decreases by 45 - 50 dB, and when the chain of auditory ossicles is destroyed - by 50-60 dB.

Morphological changes and mobility disorders of the sound-conducting system are the causes of conductive hearing loss of various etiologies.

With conductive hearing loss (purulent otitis media, tympanic membrane defect, adhesive otitis media, otosclerosis, and others), various types of auditory repair operations are performed to improve the transformation mechanism of the middle ear.

Muscles of the tympanic cavity (tensioning the tympanic membrane and stirrups) perform accommodation and protective functions.
They regulate the transmission of sounds of different frequencies and intensities due to changes in the voltage of the auditory ossicles chain. When exposed to strong sounds (80 dB) on the ear, both muscles come into a tetanic contraction state and protect the inner ear from sound injury. Due to muscle contraction and elasticity of the ligaments of the auditory ossicles, an accommodation function is performed to limit distortions (non-linearities) in the middle ear.

Bone-tissue conduction. In addition to airborne sound conduction, wave oscillations are transmitted to the Corti organ via the tissue pathway - through the bones of the skull (G. Kulikovsky, 1935). There are inertial and compression types of bone-tissue conductivity. At low sounds, the skull oscillates as a whole and due to the inertia of the auditory ossicle chain, relative movement of the stapes relative to the labyrinth capsule (inertial type) is obtained. At high sounds, a periodic compression of the labyrinth’s capsule by the wave and excitement of the Corti’s organ due to the difference in the pressure of the fluids from the inside to the oval and round windows (compression type) are obtained. Therefore, both air conductivity and inertial type of bone-tissue conductivity require normal mobility of the membranes of both windows. With the compression type of bone-tissue conductivity, the mobility of one membrane (for example, a round window membrane) is sufficient.

At the heart of some diseases (Meniere's disease, sensorineural hearing loss, etc.) is a violation of the circulation of the labyrinth fluids. It is believed that the endolymph is produced by the vascular strip, and is absorbed in the endolymphatic sac into which it flows through the endolymphatic duct. Excessive production of endolymph by the vascular stripe, deterioration of its resorption in the endolymphatic sac, as well as excess perilymph with an increase in cerebrospinal fluid pressure may cause increased intralabyrinth pressure and rocky conductive hearing loss. The latter is possible due to the presence of a connection between the perilymphatic space of the tympanic ladder by means of the cochlear duct and the subarachnoid space.

The auditory tube performs ventilation, drainage, protective functions and barofunction.

According to the theory of V.I.Voyachek, the mechanism of ventilation of the tympanic cavity consists in the fact that with a threshold decrease in air pressure (1-5 mm Hg) in the tympanic cavity, the membrane is slightly retracted, the tympanic string is compressed between the handle of the malleus and the long process of the anvil, as a result, salivary function is stimulated and the act of swallowing saliva is carried out. At the time of swallowing, the nasopharyngeal mouth of the auditory tube opens, and the necessary amount of air from the nasopharynx enters the tympanic cavity. Thus, the same air pressure is restored in the tympanic cavity with the atmosphere, which is a necessary condition for the normal function of the sound-conducting apparatus.

Ventilation function can also occur with blowing, sneezing, or coughing due to increased air pressure in the nasopharynx.

If the ventilation function of the auditory tube is impaired, negative pressure arises in the tympanic cavity, the tympanic membrane is retracted, the intralabyrinth pressure of the fluid rises due to the pressure of the stapes in the oval window, which leads to impaired sound conduction and hearing loss in the low frequency range to 20-30 dB.

Baroaccommodation of the ear is the ability to equalize air pressure in the tympanic cavity with aperiodic fluctuations in environmental pressure, especially significant and sharp. With atmospheric pressure drops (30-60 mmHg), barotrauma symptoms appear in the form of hyperemia of the membrane, hemorrhage, and a pressure drop of 0.3 atmosphere threatens to rupture it. Ear barotrauma is also the cause of conductive hearing loss.

The drainage function of the auditory tube consists in the outflow of secretion from the tympanic cavity into the nasopharynx. The protective function of the auditory tube is performed by the ciliated epithelium, the movements of the cilia of which are directed towards the nasopharynx.
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Sound Physiology

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