Cochlear microphonic potential

Abstract: Outer hair cell electromotility is a rapid, force generating, length change in response to electrical stimulation. DC electrical pulses either elongate or shorten the cell and sinusoidal electrical stimulation results in mechanical oscillations at acoustic frequencies. The mechanism underlying outer hair cell electromotility is thought to be the origin of spontaneous otoacoustic emissions. The ability of the cell to change its length requires that it be mechanically flexible. At the same time the structural integrity of the organ of Corti requires that the cell possess considerable compressive rigidity along its major axis. Evolution appears to have arrived at novel solutions to the mechanical requirements imposed on the outer hair cell. Segregation of cytoskeletal elements in specific intracellular domains facilitates the rapid movements. Compressive strength is provided by a unique hydraulic skeleton in which a positive hydrostatic pressure in the cytoplasm stabilizes a flexible elastic cortex with circumferential tensile strength. Cell turgor is required in order that the pressure gradients associated with the electromotile response can be communicated to the ends of the cell. A loss in turgor leads to loss of outer hair cell electromotility. Concentrations of salicylate equivalent to those that abolish spontaneous otoacoustic emissions in patients weaken the outer hair cell's hydraulic skeleton. There is a significant diminution in the electromotile response associated with the loss in cell turgor. Aspirin's effect on outer hair cell electromotility attests to the role of the outer hair cell in generating otoacoustic emissions and demonstrates how their physiology can influence the propagation of otoacoustic emissions.

Abstract: The dose and duration limiting toxic effects of cisplatin are ototoxicity and nephrotoxicity. While several studies have attempted to shed some light on the causes of nephrotoxicity, the reasons for ototoxicity induced by cisplatin are poorly understood. Therefore, this investigation was undertaken to delineate the potential mechanisms underlying cisplatin ototoxicity. The role of glutathione (GSH), oxidized glutathione (GSSG) and malondialdehyde levels, and antioxidant enzyme activities [superoxide dismutase, catalase, GSH peroxidase, and GSH reductase] were examined in cochlear toxicity following an acute dose of cisplatin. Male Wistar rats were treated with various doses of cisplatin. Pretreatment auditory brain stem evoked responses (ABR) were performed and then post-treatment ABRs and endocochlear potentials were also performed after three days. Acute cochlear toxicity (ototoxicity) was evidenced as elevated hearing thresholds and prolonged wave I latencies in response to various stimuli (clicks and tone bursts at 2, 8, 16 and 32 kHz) on ABRs. The endocochlear potentials were reduced (50% control) in cisplatin-treated rats as compared to control animals. The rats were sacrificed and cochleae isolated. The GSH, GSSG and malondialdehyde levels, and antioxidant enzyme activities were determined. Cisplatin ototoxicity correlated with a decrease in cochlear GSH [0.45 +/- 0.012 nmol/mg] after cisplatin administration compared to 0.95-012 nmol/mg in control cochleae (P < 0.05). Superoxide dismutase, catalase activities and malondialdehyde levels were significantly increased in the cochleae of cisplatin injected rats. Cochlear GSH-peroxidase and GSH reductase activity significantly decreased after cisplatin administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Abstract: An advanced cochlear echo test technique has been specially developed to tackle some specific problems associated with measurements from infants. The test procedure and data processing combination has greatly improved noise immunity and has given very reliable results on poorly co-operating children. Technical details are given and the results of a study of 105 ears from 55 children are presented. The potential of the advanced technique as a screening test is discussed.

Abstract: Intracellular receptor potentials were recorded from inner and outer hair cells in response to low-frequency tones, from the basal, high-frequency region of the guinea-pig cochlea. The receptor potentials recorded from inner hair cells are asymmetrical about the resting membrane potential with the depolarizing phase, which corresponds to rarefaction in sound pressure, exceeding the phase of hyperpolarization by a factor of about 3. It was found that the relationship between the peak-to-peak voltage responses and sound pressure level could be described by rectangular hyperbolae. When the frequency of the sound stimulus was progressively increased from 100 Hz to 4 kHz, the 'periodic' (a.c.) component of the receptor potential was attenuated with respect to the 'continuous' (d.c.) component. The characteristics of the inner hair cells could be described by two stages of low-pass filtering, with one of the filters having the same corner frequency as the electrical time constants which varied in different cells between 178 and 840 Hz. Receptor potentials recorded intracellularly from two morphologically identified outer hair cells were symmetrical about the resting membrane potential (about -65-70 mV) and had a maximal amplitude of only 5 mV at frequencies and intensities which yield 20-30 mV voltage responses from inner hair cells. No d.c. component receptor potentials were recorded in response to high-frequency tones. Phase and amplitude measurements were made from receptor potentials from inner hair cells, and from 'cochlear microphonic potentials' which were recorded from the organ of Corti and scala tympani. The phase of depolarization in both potentials was associated with displacement of the basilar membrane towards the scala vestibuli. The phase of the intracellular receptor potentials leads the cochlear microphonic by about 90 degrees and the sound pressure by about 180 degrees at frequencies below 100 Hz. Above this frequency the phase lead progressively declines and at higher frequencies becomes a phase lag. These phase relationships indicate that inner hair cells respond to the velocity of the basilar membrane at frequencies below 200-600 Hz, and to its displacement above this, and that the voltage responses of the inner hair cells are limited by their membrane time constants. It is suggested that outer hair cells respond to basilar membrane displacement throughout their frequency range. It is shown that, with respect to frequency, the different growth rates of the cochlear microphonic potentials and inner hair cell receptor potentials, and the dominance of cochlear microphonic potentials in the organ of Corti, result in an effective electrical interaction between inner hair cells and cochlear microphonic potentials.