Anatomy, Physiology, and Pathology of the Organs of Hearing, Vision, and Speech - Shvetsov A.G. 2006

Auditory Analyzer
Physiology of Hearing

Sound is the adequate stimulus for the Auditory analyzer. The primary characteristics of each sound tone are the frequency and amplitude of the sound wave. The higher the frequency, the higher the pitch of the sound. The intensity of sound, expressed as its loudness, is proportional to the amplitude and is measured in dB. The human ear is capable of perceiving sound in the range from 20 Hz to 20 000 Hz (children up to 32 000 Hz). The ear has the highest sensitivity to sounds with a frequency of 1000 to 4000 Hz. Below 1000 and above 4000 Hz, the sensitivity of the ear decreases significantly.

Sound with an intensity of up to 30 dB is barely audible, 30 to 50 dB corresponds to a whisper, 50 to 65 dB to normal speech, 65 to 100 dB to loud noise, 120 dB is the "pain threshold", and 140 dB causes damage to the middle (ruptured tympanic membrane) and inner (destruction of The Organ of Corti) ear.

The speech audibility threshold in children aged 6-9 years is 17-24 dBA, and in adults, it is 7-10 dBA. With a loss of The ability to perceive sounds from 30 to 70 dB, difficulties in conversation are observed, and below 30 dB, near-total deafness is diagnosed.

Different auditory capabilities are assessed by difference thresholds (DT), i.e., the detection of minimal changes in any of the sound parameters, such as its intensity or frequency. In humans, the difference threshold for intensity is 0.3-0.7 dB, and for frequency, it is 2-8 Hz.

Bone conducts sound well. In some forms of deafness where the auditory nerve is undamaged, sound travels through the bones. Deaf individuals can sometimes dance by listening to music through the floor, perceiving its rhythm with their feet. Beethoven listened to the piano being played through a cane, resting one end on the piano and holding the other end in his Teeth. With bone conduction, it is possible to hear ultrasound—sounds with a frequency above 50 000 Hz.

Under the prolonged exposure of the ear to loud sounds (2-3 minutes), auditory acuity decreases, and in silence, it recovers; 10-15 seconds is sufficient for this (auditory adaptation).

A temporary decrease in auditory sensitivity with a longer recovery period of normal Hearing acuity, which also occurs under prolonged exposure to intense sounds but recovers after a short rest, is called auditory fatigue. Auditory fatigue, which is based on temporary protective inhibition in the Cerebral Cortex, is a physiological phenomenon that serves a protective function against the pathological exhaustion of nerve centers. Auditory fatigue that does not recover after a short rest, based on persistent transmarginal inhibition in Brain structures, is called auditory overfatigue, requiring a range of special therapeutic and health measures to alleviate it.

Physiology of sound perception. Under METABOLISM/18.html">The Influence of sound waves, complex displacements occur in the membranes and fluid of the cochlea. Their study is complicated by both the small magnitude of the vibrations and the extremely small size of the cochlea, as well as its deep Location within the dense capsule of the labyrinth. It is even more difficult to identify The Nature of the physiological processes occurring during the transformation of mechanical energy into neural excitation in the receptor, as well as in Neural Pathways and centers. Consequently, there are only A number of hypotheses (assumptions) explaining the processes of sound perception.

The earliest of these is Helmholtz's theory (1863). According to this theory, mechanical Resonance phenomena occur in the cochlea, As a result of which complex sounds are decomposed into simple ones. A tone of any frequency has its own limited area on the basilar membrane and stimulates strictly defined nerve fibers: low sounds cause vibrations at the apex of the cochlea, and high sounds at its base.

According to the modern hydrodynamic theory of Békésy and Fletcher, which is currently considered the primary one, the active principle of auditory perception is not frequency, but the amplitude of sound. The amplitude maximum of each frequency in the audible range corresponds to a specific area of the basilar membrane. Under the influence of sound amplitudes, complex dynamic processes and membrane deformations occur in the Lymph of both scalae of the cochlea, with the site of maximum deformation corresponding to the spatial distribution of sounds on the basilar membrane, where vortex Movements of the lymph were observed. Sensory Cells are most strongly excited where the amplitude of vibrations is maximal; therefore, different frequencies act on different cells. In any case, the vibrating Hair cells Touch the tectorial membrane and change their shape, which leads to the generation of an excitation potential in them. The excitation arising in specific groups of receptor cells propagates as nerve impulses along the fibers of the auditory nerve to the Brainstem nuclei and subcortical centers located in the Midbrain, where the information contained in the sound stimulus is repeatedly recoded as it passes through various levels of the Auditory pathway. During this process, Neurons of one type or another extract "their own" Features of the stimulus, which ensures a fairly specific activation of higher-level neurons. Upon reaching the auditory cortex, located in the temporal lobes (Brodmann areas 41—primary auditory cortex, and 42—secondary, associative auditory cortex), this repeatedly recoded information is transformed into an auditory sensation. At the same time, as a result of the decussation of the pathways, the sound signal from the right and left ears enters both hemispheres of the brain simultaneously.

Age-related features of The Development of auditory sensitivity. The Development of the peripheral and subcortical Divisions of the auditory analyzer is largely complete by the time of birth, and the auditory analyzer begins to function from the very first hours of a child's life. The first reaction to sound in an infant is manifested by pupillary dilation, breath-holding, and certain movements. Then, the child begins to listen to the voices of adults and respond to them, which is already associated with a sufficient degree of development of the cortical divisions of the analyzer, although their development is completed at rather late stages of ontogenesis. In the second half of the first year, the child perceives certain sound combinations and associates them with specific objects or actions. At the age of 7-9 months, the infant begins to imitate the speech sounds of others, and by one year, their first words appear.

In newborns, the perception of pitch and loudness of sound is reduced, but by 6-7 months, sound perception reaches the adult norm, although the functional development of the auditory analyzer, associated with the development of fine differentiation to auditory stimuli, continues until 6-7 years. The greatest auditory acuity is characteristic of adolescents and young adults (14-19 years), and then it gradually declines.