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A Journal on Otorhinolaryngology, Head and Neck Surgery,
Plastic Reconstructive Surgery, Otoneurosurgery
Indexed/Abstracted in: EMBASE, Scopus
Otorinolaringologia 2000 December;50(4):203-7
Cochlear microphonic potentials. Hypotheses and experimental tests on a possible electrochemical origin. Preliminary note
Azzi A. A.
Libero Professionista Audiologo
According to the current opinions, the hair cells represent transducers with piezoelectric properties, responsible in giving rise to the cochlear potentials (microphonics ) and aimed to transpond the sound into signals forwarded by the VIII nerve to the brain.
The author anyway observes that microphonics are still present when the hair cells are totally destroyed by pathological events. On the basis of these facts, the author takes into consideration the hypothesis that other cochlear structures could contribute to originate the microphonic potentials as a first step of a more complex sound analysis. In order to give an experimental confirmation of this hypothesis, the author has performed some tests with electrochemical sensors, consisting in small plastic tubes 35 mm long (the approximate length of the cochlear duct) and of 1 or 2 mm internal diameter, totally filled with electrolytic solutions at various concentrations, in the absolute absence of air bubbles in order to avoid any possible interference with potentials originated from electrocapillarity phenomena. Two electrodes, each one at the two opposite poles of the tube, are immersed in the liquid and connected to a low frequency amplifier or to an RC oscillograph. Both the electrodes are made of enamelled copper wire of 0.1 mm diameter. The tubes are fastened at the center of a small loudspeaker excited with pure tones, speech and music. These tests confirm that electric potentials perfectly identical to the acoustic spectrum of the sounds emitted by the loudspeaker can be easily elicited from the sensor, which behaves like a piezoelectric element. The same happens with the piezoresistive sensors, similar to the sensors here described but polarized with an external source of 0.12 V (the same tension found as a fixed d-c potential in the cochlea by Békésy). These potentials, electrochemically generated, could contribute to the origin of the choclear microphonics. These should be transferred, through the hair cells acting like connectors, to the dendrites and then to the VIII nerve. The contribution of the microphonics to the auditory sensation could therefore be considered very important. The author hypothesizes how this new way of conceiving microphonics could not only explain some cochlear electrophysiological mechanisms but also influence the design of the cochlear implants.