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Physiology of Sound Perception

Sound perception begins with the receptor cells of the organ of Corti, which, being secondary-sensory hair mechanoreceptor cells, convert mechanical sound vibrations into electrical nerve impulses. Sound perception corresponds to the concepts of an auditory analyzer (according to I.P. Pavlov) and an auditory sensory system, combining auditory receptors, pathways and auditory centers of various levels of the central nervous system, including the cortex of the temporal lobe of the brain.

There are various theories of hearing that explain the mechanism of sound perception in a spiral organ - the receptor of the auditory system.

1. Theories of peripheral sound analysis.

- resonant theory (Helmholtz, 1863)

- hydrodynamic theories:

* traveling wave (Bekesi, 1960)

* column of fluid (Roaf-Fletcher, 1930)

- Flock theory (1977)

- Ukhtomsky theory (1945)

2. Theories of central sound analysis.

- telephone (Rutherford, 1886)

- standing waves (Ewald, 1899)

3. The dualistic theory (Rebul, 1938)

Theories of peripheral sound analysis suggest the possibility of a primary analysis of its properties in the cochlea, due to its anatomical and functional features.

The Helmholtz resonator theory is that the basilar membrane is a set of “strings” of different lengths and tensions, like a musical instrument (for example, a grand piano). “Strings” resonate and respond to their corresponding frequencies of a sound wave, such as an open grand piano to a human vote. The Helmholtz theory is confirmed by the morphological structure of the main membrane - at the base of the cochlea, the strings are shorter (0.16 mm), resonate to high sounds, and at the apex they are longer (0.52 mm) and respond to low-frequency signals. When applying complex sounds, several sections of the main membrane oscillate simultaneously, which explains the timbre. The amplitude of sound perception depends on the amplitude of membrane vibrations. Helmholtz theory for the first time allowed to explain the main naienoaa ooa - ii? Aaaeaiea auniou, neeu e oaia? A caoea, but it does not explain the phenomenon of masking high sounds by low-frequency sounds. At the same time, modern knowledge does not confirm the possibility of oscillation of individual “strings” of the main membrane, as well as the presence of their huge number on the membrane with a length of 35 mm, receiving frequencies in the range of 0.2-20 kHz.

According to the Bekesi hydrodynamic theory, a sound wave passing in the perilymph of both stairs causes oscillations of the main membrane in the form of a traveling wave. Depending on the frequency of the sound, the maximum bending of the membrane occurs in its limited area. Low sounds cause a traveling wave along the entire length of the main membrane, causing its maximum displacement near the tip of the cochlea. Mid-frequency tones displace the middle of the main membrane as much as possible, and high sounds - in the region of the main curl of the spiral organ, where the basilar membrane is more elastic and elastic.

The Roaf - Fletcher hydrodynamic theory, based on Lutz's experiments with U - shaped tubes and a liquid, confirms Bekesi's conclusions that sound waves propagate with high frequency near the main scroll of the cochlea, and with a low frequency - to the helicotreme.

Flock (1977) believes that in the formation of frequency selectivity, the main role is played by the basilar membrane with external hair cells, and not internal, as many authors think. These cells have efferent bonds. Their cilia are arranged in the form of a rigid bending W-structure; therefore, any changes in the cell length under the influence of the potential difference will lead to a displacement of the basilar membrane. Actin and myosin, the necessary components of any contractile system, were found in the structure of external hair cells. The bioelectric activity of external hair cells in the mechanical sway of the basilar membrane is confirmed by the experiments of W. Brownell, G. Bander et al. (1985). Currently, there are mathematical and physical models of snail hydrodynamics, including nonlinear and active mechanisms (Shuplyakov BC et al., 1987; Zwicker E., 1986).

The theory of "physiological resonance of cells" by Ukhtomsky consists in unequal physiological lability of hair cells, which selectively respond to different frequencies of sound waves. With great lability of hair cells, they respond to high-frequency sounds and vice versa, which resembles physiological resonance.

The central theories of Rutherford and Ewald, unlike the previous ones, deny the possibility of primary analysis of sound in the cochlea.

According to Rutherford's telephone theory, the basis of the transmission mechanism for all frequencies is a Corti's tire, like a telephone membrane with a microphone effect. When pressure is applied to the hair cells, the membrane transmits microphone potentials into signals synchronous in frequency to the centers of the brain, where they are analyzed. The role of mechanical vibrations of the main membrane is ignored.

According to Ewald’s theory, under the influence of sound, “standing” waves are established on the main membrane, like Hladni’s figures (sound images), which are analyzed in the brain centers as corresponding various auditory sensations.

The dualistic theory of Rebula consists in an attempt to combine spatial theory with telephone theory. In his opinion, low-frequency sounds are transmitted immediately to the higher auditory centers, and high sounds have their exact localization in a certain place of the main membrane. This contradicts the facts, since the pulses of the higher sections of the central nervous system do not correspond to the frequency and nature of the sound wave. For example, the frequency of tones of the round window of the cochlea is 16,000 Hz, the auditory nerve is 3,500 Hz, the medulla is 2,500 Hz, and the auditory cortex is 100 Hz.

In the oscillatory process and rocking of the main membrane, otoacoustic emission may also be of importance (Kemp D., 1978; Kemp D., Chum R., 1980). It consists in generating acoustic signals in the cochlea without sound stimulation or after it, which are recorded using a miniature and highly sensitive low-noise microphone in the external auditory canal. These signals are different in frequency and wave shape in different people. An individual emission pattern may correspond to individual deviations of the audiogram. With pathology of the inner ear, the “thresholds” of emission change. Spontaneous emission that occurs without acoustic stimulation is credited with a role in the acute frequency tuning of the cochlea and the activity of auditory receptors. It is possible that the traveling wave and resonance act simultaneously on the main membrane due to otoacoustic emission.

In a spiral organ, the mechanical process of sound conduction is transformed into an electrophysiological mechanism of sound perception.

During functional dormancy between the outer and inner sides of the cell membrane, there is a membrane potential of the receptor cell or resting potential (PP) due to the uneven distribution of sodium and potassium ions between the cytoplasm and the environment. The inner side of the membrane is negatively charged relative to the outer. The maximum resting potential of internal hair cells is –42 mV in the apical part of the cochlea (Dallos P., 1985) and –55 mV –– in its basal part (Rassel J., 1985), and the external hair cells –– –71 mV and –100 mV, respectively . Other large cells of the body do not have such great resting potential.

Vibration of the ear lymph causes displacement of the main membrane by the traveling wave and bending of stereocilia of hair cells. As a result of this, in the apical part of the cell, its membrane is depolarized. With a 10–20% decrease in PP, a gradual receptor potential (RP) arises, which with a decrement of attenuation extends to the base of the cell and leads to the release of the mediator into the synaptic cleft. On the postsynaptic membrane of the nerve ending of the auditory nerve there is a generator potential (GP). Its value is also directly proportional to the strength of the stimulus. GP electrotonically reaches the electrogenic site of the postsynaptic nerve membrane, on which periodically, when the GP reaches a critical level, an action potential (PD) arises, called excitatory postsynaptic potential (EPSP) in secondary-sensing receptors. He obeys the law “ana eee ie? Aai”. Its amplitude is higher than the amplitude of the PP nerve. The occurrence of PD is associated with the aspiration of sodium ions into the cell upon its excitation. The descending phase of polarization depends on the flow of potassium ions from the cell and the initial charge is restored. Nerve impulses along the conduction paths of the auditory system reach the cortical center of the hearing of the temporal lobe of the brain.

In addition to the electrical potentials of receptor cells and the auditory nerve, the microphone, summation and endocochlear electrical potentials of the cochlea are isolated.

The microphone potential (MP) is an alternating electrical oscillation in the liquid environment of a cochlea, repeating the shape of a sound wave.
They were first registered in 1930 by Weaver and Bray (Wever E., Bray C), and in 1937 by G.V. Gershuni and VF.Undritz. These potentials are discharged by the electrodes from the cochlea, from a round window or from the wall of the external auditory canal, the eardrum. They reflect the function of hair cells, more external, and are similar to a technical microphone that converts pressure pulses into electrical signals.

The summation potential (SP) associated with acoustic stimulation is constant and does not reproduce the shape of the sound wave. It is small and does not depend on the oxygen supply of the cochlea and other factors affecting the microphone effect.

Endocochlear potential (ECP) or endolymphatic resting potential was discovered by Bekesy (Bekesy G., 1952). It is independent of stimulation and ranges from +60 to +80 mV. Tasaki (Tasaki J., 1960) associates it with the function of the vascular streak. Of all the labyrinth fluids, in terms of chemical composition, one perilymph is similar to other extracellular fluids, in particular, cerebrospinal fluid and blood serum, since the perilymphatic space communicates via the cochlear duct (ductus cochlearis) with the subarachnoid cerebrospinal fluid. In perilymph, a high concentration of sodium ions and a low concentration of potassium. Endolymph sharply differs from other extracellular fluids, since it has a high level of potassium ions and a very low level of sodium. Thanks to this ionic ratio, a constant resting potential is created, which is an amplifier of all microelectric processes in a spiral organ. Since the spiral organ has no vessels, the vascular strip provides its trophic, saturating the endolymph with oxygen. Cortilymph, located in the tunnel and washing the hair cells and their nerve endings, is rich in sodium. It is isolated from the endolymph by a cuticular plate. The source of cortilymph formation is probably the capillary network located under the basilar membrane. Homeostasis of intra labyrinth media depends on the state of the hematolabyrinth barrier.

According to V.O. Samoilov (1986), ECP occurs due to the active transport of hydrogen ions (H +) to the endolymph from the mitochondrial matrix of marginal cells. ECP is essential for the normal functioning of the auditory receptor. It maintains a large potential difference (up to 150 mV) between the endolymph and cytoplasm of the hair cells at the base of stereocilia. This potential difference arises from the summation of a positive EPC with a negative resting potential of the receptor cell. Its fall is observed in case of a violation of the oxygen supply of the vascular strip (Lantsov A.A., 1982).

The internal hair cells (3500–4000) have connections mainly with afferent nerve fibers, and the external (18000-20000) - with efferent fibers. The auditory nerve contacts the hair cells on the main membrane through synapses. 10-20 afferent fibers depart from one inner cell, and 1-2 - outside. A small group of nerve fibers of the auditory nerve conducts pulses of mainly the same frequency. If the sound stimulus is a complex oscillation, then all the fibers corresponding to the spectrum of the sound are activated in the auditory nerve. At the level of auditory receptors, a frequency analysis of sounds occurs, and their duration is encoded by the activation time of afferent fibers of the auditory nerve.

In the humoral regulation of the function of the organ of Corti, specific cells of the vascular strip, apudocytes, which are elements of the endocrine cell regulation system, have a certain value. Apudocytes produce biogenic amines - serotonin, melatonin and peptide hormones - adrenaline, norepinephrine.

The function of the subcortical auditory centers has been studied relatively little. Through them, an unconditioned reflex connection is made with motor reactions to sound (head, eye rotations), ankylosing spondylitis reflex, ankylosing spondylitis reflex Shurygin and others.

Basic information on the localization of cortical centers was obtained using conditioned reflexes, experiments with extirpation of the cortex and abduction of biocurrents. When the cortical centers of hearing are damaged, the higher analysis of sound signals is violated, their synthesis into a cohesive image, poor speech intelligibility with a satisfactory perception of pure tones (Undrits V.F., 1923).

The role of the higher centers of hearing is confirmed in the clinic, when after tympanoplasty hearing acuity improves not only on the operated ear, but also on the other due to the removal of inhibition of the auditory zone of the cortex (Khilov KL; Belov IM, 1963 /. The physiological properties of the auditory system, such as binaural spatial hearing, adaptation, masking, etc., are functions of the auditory centers of the temporal cortex.

Spatial hearing, binaural hearing and ototope are interconnected (Rudenko V.P., 1967; Altman Ya.A., 1981). Spatial hearing is the ability to localize and determine the direction of sound in space, which is associated with binaural hearing based on a two-way partial intersection of auditory ways. Spatial hearing is determined by the time or intensity of sound in each ear. From the side of which ear the sound will be more intense or faster into the auditory system, from that side the signal source will be localized. Here, the distance from the sound source to each ear, as well as the phase and angle of the sound beam, matters.

Masking is the phenomenon of increasing the threshold of a test signal in the presence of another sound (masker). Masking is widely used in audiometry in order to muffle a better hearing ear. Due to the connections of each cochlea with both higher centers of hearing, a masking effect of some sounds on others in the surrounding environment is often noted. The action of the masker depends on the frequency and intensity of the signal. Lower and stronger sounds have greater disguise.

By auditory adaptation is meant a temporary increase in auditory thresholds as a result of sound stimulation. This is an adaptive and protective reaction. In conditions of silence or under the influence of very weak sounds, the sensitivity of the organ of hearing can increase, which V.F. Undrits (1962) called sensitization of sensitivity. The adaptive ability of the ear depends on the state of the central and peripheral parts of the auditory system, the balance of the processes of excitation and inhibition in the cerebral cortex (Vartanyan I.A., 1981). Fatigue of auditory function is a pathological reaction that loses the ability to reverse development of npoceca with prolonged (several months) exposure to the stimulus. Irreversible changes appear in the organ of Corti leading to hearing loss.

Hearing for a person is a biological and social factor in the development of speech and verbal communication. The central link of the entire apparatus of speech is the cerebral cortex, mainly of the left hemisphere, where the right-handed person has the auditory and kinesthetic centers. The physiological perception of speech is carried out by the auditory and visual systems. The auditory system controls the intensity, frequency, timbre and other qualities of speech. Speech signals are a collection of elements of acoustic energy with rapidly changing amplitudes and frequencies. The average frequency of the main tone of speech in men is 136 Hz, in women - 248 Hz, that is, an octave higher (Ermolaev V.G. et al., 1970). Normal speech in people has a frequency range within one octave, and for singers, artists - up to two octaves. There are singers who have a sound range of up to three and even four octaves (Ima Sumak, Maro Robin). Speech signals are limited by a frequency range of 1000 - 10000 Hz and an intensity of 50 to 80 dB.

The sound units of speech are phonemes, with the help of which a word is formed, and from words - a message. The auditory system translates simple physical or acoustic features of a speech signal into a discrete series of phonemes. At the second stage, the phoneme is directly translated into a language unit.

To hear speech, its level must be higher than the threshold of hearing or the threshold of detection. Only in this case does a person begin to distinguish individual words. With increasing intensity of speech signals, their intelligibility increases. The intelligibility thresholds depend on the phonemic composition of the speech material, the number of syllables in a word, their frequency of use, as well as the presence and nature of noise interference. Примером тесной взаимосвязи слуховой и речевой функций является опыт Ломбарда, когда при чтении текста вставляют в уши трещотки Барани и интенсивность речи резко возрастает, так как человек должен слышать и постоянно контролировать свою речь. С этой же целью в шумной обстановке люди говорят громче.

Следовательно, слуховой орган позволяет человеку воспринимать и адекватно реагировать на звуковые изменения окружаюжей среды. Каждому участку слуховой системы свойственна определённая функция, нарушение которой ведёт к частичной или полной потере слуха.
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