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At least 289 records · Page 16Linked to original sources
PubMed1992

Auditory brain-stem response in zitter rats with genetic spongiform encephalopathy.

Zitter rats with genetic spongiform encephalopathy and hypomyelination developed an abnormal auditory brain-stem response (ABR) before the appearance of spongy lesions in the central nervous system (CNS). The ABR abnormalities were characterized by a dual peak of wave I, with a longer latency than in normal rats, and decreased or absent waves III and IV. Hypomyelination in both peripheral and central nerves may have been responsible for these abnormalities. The slow negative wave became wide and obscure with aging. These changes accompanied age-dependent progression of spongy changes in the CNS. These findings suggest that at least two mechanisms, one involving hypomyelination and the other causing spongy lesions, are responsible for the brain-stem auditory pathway dysfunction in zitter rats.

Action Potentials↗
PubMed1983

On the origin of the compound action potentials (N1, N2) of the cochlea of the rat.

The origin of N2 component of the compound action potential was studied by comparing the potential recorded at the round window with the response recorded from the cochlear nucleus, by comparing the potential recorded at the round window before and after the cochlear nucleus was removed by suction, and by comparing latencies of unit responses from auditory nerve fibers and cochlear nucleus units with the responses from the round window. The results indicated that the N2 potential, as well as the positive potential between the N1 and N2, was generated by secondary auditory neurons situated in the cochlear nucleus. The results were based on the response to click sounds and cross correlograms of the response to continuous sounds that were amplitude-modulated with pseudorandom noise.

Acoustic Stimulation↗
PubMedDate not supplied

Cerebellar actions on cochlear microphonics and on auditory nerve action potential.

The influence exerted by cerebellar stimulation upon cochlear microphonics (CM) and auditory nerve action potential (AP) has been analyzed in curarized guinea pigs. Round window recordings demonstrated that conditioning electrical stimulus trains delivered to the cerebellar cortex diminished the CM and AP amplitude at the same time and in a parallel fashion. On the other hand, cooling of the cerebellar cortex showed the opposite results of increased amplitudes. All pre-receptorial mechanisms were avoided. A PDP-12 computer performed parametric and non-parametric statistical analysis showing the differences to be significant for the shifts. Evidence of simultaneous inhibitory cerebellar action on both potentials has been demonstrated and a cerebello-olivo-cochlear pathway is proposed for such action upon the receptor cells and/or incoming fibers.

Acoustic Stimulation↗
PubMed1983

Efferent effects elicited by electrical stimulation at the round window of the guinea pig.

We report a technique for activating the efferent nerve fibres to the cochlea by electrical stimulation at the round window. Such electrical stimulation caused a reduction in the amplitude of the gross nerve response (N1) to a click presented after the electrical stimulus but did not alter the latency of the response. The reduction increased with increasing current strength above 200 microA and increasing rate of electrical pulses above 50 Hz. The effect was also dependent on the duration of the shock train and the pulse width. The reduction in N1 was most pronounced at low click intensities. Recovery of the N1 was almost complete about 80 ms after the end of the electrical stimulus. The effect of electrical stimulation in reducing the N1 amplitude could almost always be blocked by intraperitoneal injections of strychnine. Recovery from the strychnine block was observed when animals were maintained for periods of more than 60 min after the administration of strychnine. The ease of this technique allows it to be used to examine the effects of efferent stimulation on various aspects of cochlear function in the guinea pig.

Animals↗
PubMed1983

Cochlear inner hair cells: effects of transient asphyxia on intracellular potentials.

Intracellular potentials were recorded from inner hair cells in the guinea pig cochlea. Transient asphyxia was induced by interrupting respiration for brief periods. Asphyxia caused a hyperpolarization of the resting membrane potential (resting Em). The hyperpolarization averaged 2.9 mV for 30 s asphyxias and 5.7 mV for 45 s asphyxias. The membrane potential recovered quickly after normal ventilation was resumed. Asphyxia also induced a rapid and profound decrease of the d.c. receptor potential in response to moderate intensity tone bursts at the characteristic frequency of the inner hair cell. At maximal depression, the receptor potential was reduced about 60% for a 30 s asphyxia and 100% for a 45 s asphyxia. The receptor potential recovered slowly after normal ventilation was resumed. A similar percent reduction and time course of recovery were observed for the a.c. receptor potential. In recordings from the same animals, the round window compound action potential (CAP) was as severely depressed by asphyxia as the hair cell receptor potentials. The time course of recovery for the CAP was similar to the slow recovery of the d.c. receptor potential. In contrast, the round window cochlear microphonics (CM) and the endolymphatic potential (EP) were affected less by asphyxia and recovered quickly after ventilation was resumed. Frequency tuning curves (FTCs) for the d.c. receptor potential were measured during the period of maximal receptor potential depression. These FTCs showed decreased tip sensitivity and a decrease in sharpness of tuning, as measured by the Q10. These changes were fully reversible. Low frequency (tail) segments of the FTCs were much less affected by asphyxia. The inner hair cell FTC changes during asphyxia were compared with neural FTC changes reported by other investigators. The similarities lead us to the conclusion that the inner hair cell and the auditory neural response to sound are equally sensitive to asphyxia.

Animals↗
PubMed1984

Distribution of cochlear damage caused by the removal of the round window membrane.

The distribution of damage that occurs in the cochlea after removal of the round window membrane was examined in the apical, middle and basal regions with light and electron microscopy. The damage resembles that seen after acoustic trauma in many respects. The outer hair cells are often disrupted in damaged zones, and the radial afferent fibers to the inner hair cells swell enormously to form large vacuoles. 16 h after opening of round window, there is conspicuous swelling of myelinated axons in the osseous spiral lamina of the apical region. This swelling is associated with large vacuoles underneath the inner hair cells. 10 h after opening the round window, much smaller vacuoles are seen in the apical region. The distribution of the damage is not uniform throughout the cochlea. Damage is usually less severe and is not uniform in the middle region but is pronounced in the base. The nature of the damage is also variable in different animals. For example, sharply delimited, discontinuous damage to the inner hair cells was occasionally observed in the apical region. The most likely cause for the damage to the cochlea is a pressure differential across the organ of Corti that appears after removing the round window membrane. The damage apparently causes low frequency random movements of the basilar membrane that are observed in the experimental cochleas using a reflected laser beam.

Animals↗
PubMed1985

Origins of eighth nerve unit response pattern in round window cap recordings.

An experimental technique based on a combined electrical and acoustical stimulation of the cochlea (R. Charlet de Sauvage et al., 1983, J. Acoust. Soc. Am. 73, 616-627) allowing to record a close estimation of single unit contribution to the VIIIth nerve CAP has been recently proposed. D.C. Teas et al.'s (1962, J. Acoust. Soc. Am. 24, 1431-1459) theory about this pattern is that, due to conduction time in the internal auditory meatus, the propagated depolarisation partly differentiates in far-field recordings, giving rise to a specifically diphasic pattern. In order to evaluate Teas's hypothesis, 83 unit waveforms collected in 10 guinea pigs are analysed. Several reproducible peaks are identified. Latency and slope measurements are performed on these peaks. This data is processed, after identifying two homologous components which could combine in accordance with Teas's theory. The schematic pattern of a primary waveform is actually determined. Its relative amplitude on the two electrodes and the delay in the meatus are inferred. Results are in good agreement with published data. This is taken as an indirect validation of Teas's hypothesis.

Acoustic Stimulation↗
PubMed1985

Modulation at the guinea pig round window of summating potentials and compound action potentials by low-frequency sound.

Low-frequency sound was used to modulate responses to short single-frequency tone bursts at the guinea pig round window. Summating potentials (SP) increase (reach higher positive values) during the negative half-cycle of the low-frequency cochlear microphonic (LFCM) and decrease during the positive half-cycle of the LFCM. The compound action potential (AP) amplitude decreases during the negative half-cycle of the LFCM. The negative half-cycle of the LFCM can be identified with scala tympani displacement. SP modulation depth is defined as the difference between the highest and the lowest SP value found for tone burst stimulation at different phases of the low-frequency sound while the sound levels of the tone burst and the low-frequency bias are kept constant. When normalized with respect to the SP amplitude found without bias, the SP modulation depth is independent of the sound level of the tone burst in the range from 48 to 68 dB SPL. The normalized AP suppression tends to increase with decreasing tone burst sound level. A dynamic nonlinear mechanism which might explain these results is discussed. This mechanism is based on voltage-sensitive changes.

Acoustic Stimulation↗
PubMed1986

Direct measurement of longitudinal endolymph flow rate in the guinea pig cochlea.

The rate of longitudinal endolymph flow in the guinea pig cochlea has been measured with a novel tracer technique. The tracer we utilized was the tetramethylammonium (TMA) ion, the movement of which was monitored by ion-sensitive microelectrodes. Extremely small amounts of tracer were required as the electrodes could readily detect TMA concentrations in endolymph as low as 10 microM. TMA was introduced into scala media in the form of a small bolus, varying from 2-20 nl in volume. To examine whether longitudinal flow affects the dispersion of TMA in endolymph, we compared the characteristics of TMA spread to turn I following injection into turn II, with those of TMA spread to turn II following injection into turn I. The comparison of these data with an analytical model combining the processes of diffusion and volume flow demonstrates that the spread of tracer is dominated by passive diffusion processes with very little contribution from longitudinal endolymph flow. The rate of longitudinal endolymph flow between turn I and turn II was estimated to be less than 0.01 mm/min directed towards the basal turn. This value is considerably lower than recently published estimates using other techniques.

Cochlea↗
PubMed1986

Characterization of the electrically evoked auditory brainstem response (ABR) in cats and humans.

Electrically evoked auditory brainstem response (EABR) recordings were made from 38 humans implanted with one of three cochlear prostheses, and from 25 cats. Recognizable auditory potentials were identified in 27 of the profoundly deaf implanted subjects. In both cats and humans EABR waveform morphology and magnitude were independent of electrode configuration and paralleled those of the normal acoustic ABR, but with reduced absolute latencies. EABR recordings are highly susceptible to contamination by stimulus artifact and by elicited non-auditory potentials. Latency, morphology, and magnitude criteria are proposed for identification and analysis of EABR components.

Animals↗
PubMed1986

Single-unit response at the round window of the guinea pig.

In guinea pigs the unit contribution (unit action potential, ap) to the response of the round window was computed following the method used by Kiang and co-workers (1976), i.e., fibre discharges registered by a microelectrode in the nerve were used as trigger pulses for the averaging process of the corresponding ap registered with a gross electrode at the round window. Normally the ap was independent of the fibre-CF, had a diphasic waveform, and its amplitude was about 0.1 microV. Small inter-animal differences were found in waveform and amplitude of the ap. In a pilot experiment exploring pathological influences on ap, the auditory nerve was stretched to mimic the effect of some acoustic nerve tumors. We found that the waveforms of both ap and compound action potential (CAP) changed. The results indicate that in normal guinea pig cochleas the existence of an elementary unit waveform can be used in the convolution of the CAP [Goldstein, M.H. Jr. and Kiang, N.Y.S. (1958) J. Acoust. Soc. Am. 30, 107-114]. In abnormal cochleas, however, deteriorated aps may disturb the simple convolution concept of the CAP.

Action Potentials↗
PubMed1986

Effects of extracochlear direct current stimulation on the ensemble auditory nerve activity of cats.

The influence of direct current applied by round window stimulation on the whole nerve response of the auditory nerve of the cat has been studied. Effects on acoustically driven activity (CAP) and on the ensemble spontaneous activity of the nerve were observed. Stimulation with positive current suppressed driven and spontaneous activity. The strength and spread of suppressive effects was a function of the applied current level. After a period of positive electrical stimulation, driven and spontaneous activity rapidly returned to normal values. A rebound effect was sometimes observed, marked by a brief increase in spontaneous activity above the normal level. Negative current initially produced an increase in the amplitude of driven and spontaneous responses. Prolonged stimulation with negative current (greater than 30 s) resulted in a subsequent, graded reduction of neural activity, until a profound suppression of spontaneous and evoked neural activity was attained. The amplitude/latency relationship of CAPs was altered during passing of negative currents but not during passing of positive currents. Recovery from the suppression generated by negative currents was commonly prolonged for anything from a few seconds to many minutes; prolongation was dependent on stimulus amplitude, duration and duty cycles.

Acoustic Stimulation↗
PubMed1987

Rapidly fluctuating thresholds at the onset of experimentally-induced hydrops in the guinea pig.

Pigmented guinea pigs were chronically implanted bilaterally with a platinum electrode on each round window. After recovery the endolymphatic sac was destroyed and the duct blocked on one side only; the other side was employed as a control. The round window response thresholds on both sides were recorded several times per week over a three month period. There were three main results. A sensitivity loss of up to 20 dB was observed for frequencies between 250 Hz and 6.4 kHz within two weeks post-op. At the end of three months the threshold elevation for these frequencies was as much as 50 dB. On the other hand the thresholds for frequencies between 8 and 16 kHz remained within 10 dB of their pre-operative value for at least two months. The thresholds fluctuated with a shift of as much as 25 dB within 24 h. The threshold elevation was associated with a decrease in the latency, at threshold, of the round window AP response which at frequencies between 250 Hz and 6.4 kHz was as short as that for 8 kHz. This observation suggested that it was the base, only, of the cochlea which responded. The present study has indicated that experimentally induced endolymphatic hydrops in the guinea pig mimics well the progressive and fluctuating hearing loss characteristic of Ménière's disease.

Animals↗
PubMed1988

Electrical stimulation of cochlear efferents at the round window reduces auditory desensitization in guinea pigs. I. Dependence on electrical stimulation parameters.

Electrical stimulation at the round window with pulsed short trains has been shown to elicit classical efferent effects on N1 amplitudes at the cochlea. This report demonstrates that round window stimulation as a continuous burst can reduce temporary threshold shifts (TTS) caused by a simultaneous monaural loud sound exposure. This result is similar to recent reports that stimulation of the crossed olivocochlear bundle (COCB) at the floor of the fourth ventricle can reduce TTS. Like COCB stimulation at the brainstem, the effect of round window stimulation could be abolished by strychnine, with a time course paralleling the blocking action of strychnine on the traditional COCB effects of pulsed short trains on N1 amplitudes. This report also established parameters for optimal effects of the round window stimulus and found them to be similar to the optimal parameters for the effects of brainstem stimulation on TTS. Tonic effects on TTS were also observed, with reductions in TTS being obtained as much as 7 min after a 1 min-long round window stimulus. Such tonic effects did not appear to be due to persistent effects at the cochlea but were suggested to be due to a long term resetting of some central site activated by antidromic stimulation from the round window.

Animals↗
PubMed1988

Electrical stimulation of cochlear efferents at the round window reduces auditory desensitization in guinea pigs. II. Dependence on level of temporary threshold shifts.

This report demonstrates that electrical stimulation of the efferents at the round window reduces temporary threshold shifts in a protective manner. For a standard set of stimulating parameters greatest reductions in TTS were found to exposures that caused the greatest amounts of TTS to occur. Low level exposures that caused low levels of TTS from which the cochlea could recover relatively quickly were not affected by the standard electrical stimulus. Intermediate reductions were obtained to intermediate levels of exposure, resulting in intermediate levels of TTS. Increasing current levels or duration of stimulation did not produce reductions in the low level TTS; a higher rate of stimulation was, however, able to reduce the low level TTS. Even with the higher rate of stimulation, greatest reductions in TTS occurred at the higher levels of exposure. These results are identical to the effects of COCB stimulation at the level of the brainstem and argue for viewing the COCB as a protective pathway.

Acoustic Stimulation↗