Sound perception mechanism
The sound vibrations of the air, passing through the external auditory canal, cause vibrations of the tympanic membrane and, through the auditory ossicles, are transmitted in an enhanced form to the membrane of the oval window leading to the vestibule of the cochlea.
The resulting vibration sets the perilymph and endolymph of the inner ear in motion and is sensed by the fibers of the main membrane, which carries the cells of the organ of Corti. The vibration of the hair cells of the organ of Corti causes the hairs to come into contact with the integumentary membrane. The hairs bend, causing a change in the membrane potential of these cells and the appearance of excitation in the nerve fibers that braid the hair cells. Through the nerve fibers of the auditory nerve, the excitation is transmitted to the auditory analyzer of the cerebral cortex.
The human ear can perceive sounds with a frequency of 20 to 20,000 Hz. Physically, sounds are characterized by frequency (number of periodic vibrations per second) and force (amplitude of vibrations). Physiologically, this corresponds to tone and volume. The third important characteristic is the sound spectrum, that is, a composition of additional periodic oscillations (harmonics) that arise together with the fundamental frequency and exceed it. The sound spectrum is expressed by the timbre of the sound. This is how the sounds of different musical instruments and the human voice are distinguished.
The distinction between sounds is based on the phenomenon of resonance that occurs in the fibers of the main membrane.
The width of the main membrane, that is, the length of its fibers is not the same: the fibers are longer at the apex of the cochlea and shorter at its base, although the cochlear canal is wider here. Its natural vibration frequency depends on the length of the fibers: the shorter the fiber, the more sound it resonates. When a high-frequency sound enters the ear, the short fibers of the main membrane located at the base of the cochlea resonate in it and the sensitive cells located in them are excited. In this case, not all cells are excited, but only those that are in fibers of a certain length. Low sounds are heard by the sensitive cells of the organ of Corti, located in the long fibers of the main membrane at the apex of the cochlea.
+ At the same time, the speed of the signal’s passage through the structures of the auditory organ and its entry into the cortex requires some reserves. Therefore, it is known that initially the hearing organ simply assesses the arrival of the signal and then adjusts to the level of best audibility. This means that the first stage takes between 35 and 175 milliseconds and the second between 180 and 500. At the same time, the maximum number of distinguishable sounds depends on the frequency of vibration and the functional state of the organ, and is set at 3 – 4 thousand shades.
Therefore, the primary analysis of sound signals begins already in the organ of Corti, from where the excitation is transmitted along the fibers of the auditory nerve to the auditory center of the cerebral cortex in the temporal lobe, where they are evaluated. qualitatively.
