P.M. Sellick

Measurement of basilar membrane motion in the guinea pig using the Mössbauer technique

Basilar membrane motion was measured at the 16-19 kHz place of the guinea pig cochlea using the Mössbauer technique. The threshold of the gross cochlear action potential (CAP) evoked by pure-tone bursts was used as an indication of neural threshold. CAP threshold deteriorated progressively after the cochlea was opened and the Mössbauer source placed on the basilar membrane. A close relationship was found between the amplitude of basilar membrane motion at the source place frequency and CAP threshold. Basilar membrane velocity at CAP threshold SPL was about 0.04 mm/s over a 60-dB range of CAP threshold. Intensity functions for basilar membrane motion were linear for frequencies more than an octave below the source place frequency but demonstrated progressive saturation for frequencies greater than an octave below the CF. This nonlinear behavior was eliminated as the CAP threshold became less sensitive and was absent post mortem. Isovelocity curves at the 0.04 mm/s criterion were remarkably similar to frequency threshold curves from primary afferent fibers innervating a similar place on the basilar membrane. The isovelocity curve was a better fit than the isoamplitude curve suggesting that inner hair cells respond to basilar membrane velocity. As the CAP threshold deteriorated, the isovelocit curves lost sensitivity around the best frequency, whereas sensitivity to frequencies below 10 kHz remained constant even after the animal was killed. We suggested that most of the frequency response and nonlinear behavior of inner hair cells and afferent fibers may be found in basilar motion.

The modulation of the sensitivity of the mammalian cochlea by low frequency tones. III. Basilar membrane motion.

The simultaneous presentation of an intense low frequency tone and a moderate intensity high frequency tone produced modulation of the high frequency motion of the cochlear partition in the first turn of the guinea pig cochlea. This modulation was in synchrony with the displacement caused by the low frequency tone. Maximum reduction in mechanical sensitivity was observed for peak displacements towards scala tympani, whereas a less pronounced reduction was observed for peak displacements towards scala vestibuli. Modulation was reduced or absent for high frequency tones more than an octave below the characteristic frequency of the mechanical tuning curve and was absent post mortem.

Modulation of responses of spiral ganglion cells in the guinea pig cochlea by low frequency sound

Period histograms were generated from single unit data obtained from the spiral ganglion of the first turn of the guinea pig cochlea in response to continuous tones between 40 and 500 Hz. With the lowest intensities used, spontaneous activity was suppressed during basilar membrane displacement (inferred from cochlear microphonic phase) towards scala vestibula and activity was enhanced during displacement towards scala tympani. At higher intensities the response changed to excitation during maximal basilar membrane velocity towards scala vestibuli. These patterns were delayed by about 0.5 ms producing large phase delays at the higher frequencies. We postulate that the displacement response is produced by cochlear microphonic originating from the outer hair cells acting on the inner hair cell membrane. In contrast, the velocity response is produced by the inner hair cell receptor potential. The effect of a 40 Hz tone on activity evoked by tones above, at, and below the characteristic frequency was investigated by generating period histograms synchronous with the 40 Hz tone. We found that activity evoked by tones around the characteristic frequency of the cell was suppressed during displacement of the basilar membrane towards scala tympani and enhanced in the opposite direction at 40 Hz intensities that had no effect on spontaneous activity. Further increase in the 40 Hz intensity produced suppression during scala vestibuli displacement with activity remaining only during the zero crossings. Still further increase produces the 40 Hz tone alone response. Activity evoked by tones in the low frequency tails of the frequency threshold curve was not similarly modulated. This phenomenon is though to be related to basilar membrane nonlinearity for frequencies close to the cut-off. Investigation of the effect of a 40 Hz tone on the threshold of the compound action potential confirmed data obtained from single units.

The influence of Mossbauer source size and position on phase and amplitude measurements of the guinea pig basilar membrane

Phase and amplitude measurements were made from the incus and basilar membrane in guinea pig using the Mossbauer technique. The basilar membrane/incus ratio had a maximum of about 60 dB and a phase accumulation of between 9 and 12 radians to CF. Two source sizes were used (60 X 85 and 20 X 60 microns) and the source was placed either on the modiolar edge of the basilar membrane or in the middle. Notches in plots of the basilar membrane/incus ratio occur at stimulus frequencies that appear to be associated with source size rather than position, suggesting that artefacts could be produced by the presence of the Mossbauer source.

Comparison between the tuning properties of inner hair cells and basilar membrane motion.

Measurements were made of inner hair cell receptor potentials and basilar membrane motion in the 17-21 kHz region of the guinea pig cochlea. The latter were made using the Mossbauer technique. Isoamplitude curves at 0.9 mV d.c. receptor potential were compared with isovelocity curves at 0.04 mm/s and the corresponding basilar membrane displacement at CF. The Mossbauer source (20 X 60 or 60 X 85 microns) was placed either in the middle of the basilar membrane or on the extreme modiolar edge. These two source positions yielded broad and narrow mechanical tuning curves, respectively. The latter approximated the receptor potential curves most closely but deviated by 10-15 dB on the low frequency side of the tuning curve tip.

Differential effects of ouabain and ethacrynic acid on the labyrinthine potentials

SummaryIntravenous injection of 20–45 mg/kg Ethacrynic acid reversibly reduced the endocochlear potential and increased endolymph sodium concentration from less than 1 mM to 5–8 mM 20 min after the injection.Intravenous injection of 42–66 mg/kg Ethacrynic acid drastically reduced the endocochlear potential to negative values while the utricular potential remained unaffected. However perfusion of 1×10−3 M Ethacrynic acid into the perilymphatic space reduced both potentials at approximately the same rate, indicating that the differential effect of Ethacrynic acid on the endocochlear and utricular potentials, was due to differences in vascularity and not to differences in the action of Ethacrynic acid.The maximum negative potential produced by intravenous Ethacrynic acid in the cochlea and by perilymphatic application in both the cochlea and utricle was rapidly reduced by anoxia without changes in the rate of influx of sodium.A similar anoxia sensitive negative potential was not produced by the application of ouabain in either the utricle or cochlea even though both potentials were reduced to negative values. Inclusion of 1×10−3 M ouabain into the 1×10−3 M Ethacrynic acid perilymph perfusate reduced the anoxia sensitive negative potential by 50 to 80% in the utricle and cochlea respectively.

Evidence for an Electrogenic Potassium Pump as the Origin of the Positive Component of the Endocochlear Potential

SummaryThe endocochlear potential (EP) and endolymph potassium concentration [K+]e were measured in the first turn of the guinea pig cochlea. Liquid ion exchanger K+ specific microelectrodes were used for [K+] measurement. The resting value of the cochlea endolymph [K+]e was 140±8.6 mM. The [K+]e was then reduced by perfusing scale media with K+ substituted Ringers containing 5, 20 or 50 mM K+. EP fell rapidly after perfusion to steady values of 9–35 mV and the [K+]e returned to stable values of 62–130 mM. The K+ conductance [GK] was calculated assuming that EP was produced by K+ current to be 0.16×10−7 mMoles/min/mV/mm of scala media. Active K+ flux was calculated from the measured flux after perfusion and the passive flux was derived fromGK and the driving force for K+. It was concluded that the assumption that EP is generated by K+ current is justified and that the electrogenic K+ flux after perfusion was accompanied by an apparently electroneutral flux.

The electrophysiology of the utricle.

SummaryThe Na+ concentration in utricular endolymph of guinea pigs was measured with Na+ specific electrodes and found to be 12.6±5.8 mEqu/l.The utricular potential was 1.8±4.4 mV. Permanent anoxia resulting in the death of the animal caused the potential to fall by 15 mV and the Na+ concentration to rise at a rate of 0.4 mEqu/l per minute. The utricular potential could be maintained during anoxia by bubbling the surrounding perilymph with air or oxygen. Maintenance of the Na+ concentration and the utricular potential did not depend on the integrity of the first turn of the cochlea. It was concluded that the utricle produced its own endolymph and potential and was effectively isolated from the cochlea. The access resistance of the utricle was measured to be 2±1.1 kohms.

The modulation of the sensitivity of the mammalian cochlea by low frequency tones. I. Primary afferent activity

The neural activity of single cells within the spiral ganglion of the first turn of the guinea pig cochlea has been recorded. Period histograms of the spike activity have been collected. The histograms were synchronised with a loud low frequency tone (40 Hz) presented at a constant intensity below the threshold of the cells to phase locking. The activity evoked by a continuous high frequency tone within the tip of the frequency threshold curve was modulated by the presence of a subthreshold low frequency tone. Suppression of this evoked activity was observed for low frequency displacements of the cochlear partition towards scala tympani, and potentiation was observed for moderate displacements towards scala vestibuli. At higher intensities of the low frequency tone suppression is also observed for peak displacements of the partition towards scala vestibuli. The changes to these histograms with increases in the intensity of the high frequency tone are described. Hysteresis, flattening of the histograms at high intensities and nonmonotonic growth of firing rate at certain phases of the low frequency tone were observed. These changes are believed to be due to fast adaptation of the neural response rather than similar changes to the d.c. receptor potential within the inner hair cells that the neurons innervate.

The modulation of the sensitivity of the mammalian cochlea by low frequency tones. II. Inner hair cell receptor potentials

Abstract Intracellular receptor potentials from inner hair cells and extracellular potentials within the tunnel of Corti immediately outside inner hair cells have been recorded in the first turn of the guinea pig cochlea. The a.c. and d.c. components of these receptor potentials elicited by high frequency tones within the tip of the isoamplitude tuning curve were modulated in synchrony with a simultaneously applied low frequency suppressor tone. The observed suppression was largest for peak low frequency displacements of the cochlear partition towards scala tympani and less pronounced for displacements towards scala vestibuli. Little suppression was observed at intermediate phases of the low frequency suppressor tone corresponding to the zero crossings of the low frequency motion of the cochlear partition. Receptor potentials elicited by high frequency tones in the tail region of the isoamplitude curve were also modulated, but less strongly and in the opposite phase (scala vestibuli) to the response to frequencies close to the characteristic frequency of the inner hair cell. The modulation for frequencies much below the characteristic frequency also occurred only over a very limited range of high frequency tone intensity. Suppression of the microphonic response to the low frequency tone by the high frequency stimulus was also observed.