Plasma osmolality variations and their effect on the hearing threshold of the guinea pig.
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Animals↗
Biomedical subjects
B M Johnstone
Publications and source records attributed to B M Johnstone.
Basilar membrane measurements and the travelling wave.
From the original measurements of G. von Békésy (1942) until a few years ago, the basilar membrane was considered to undergo simple passive linear vibration. Recent measurements have completely altered this notion. It is now known that the BM is highly non linear and very sharply tuned. Indeed, BM can now account for most of the properties of the eighth nerve response to sound. The non linearity can be approximated by a hyperbolic function and appears to be part of an active process in the outer hair cell. At the characteristic frequency, CAP threshold (10 dB SPL) corresponds to 0.3 nm motion and the non linearity shows half saturation at 10 nm. The sigmoid shape of the full range BM input-output curve is due to the combination of a less sensitive linear passive component with the added sensitivity of the active non linear function. A hyperbolic input-output function is also present in the cochlear microphonics, and at low frequencies the half saturation value again corresponds to 10 nm BM displacement. With induced threshold loss (e.g. noise trauma) the nonlinearity disappears from the BM, but is still present in the CM. This suggests that the pathology is in the active mechanical feedback process, rather than in the receptor system. It appears that BM mechanics at low amplitudes near the resonant frequency is controlled by a nonlinear mechano-electrical transducer followed by a vulnerable, linear, active mechanism (electro-mechanical?) feeding back in positive phase onto BM vibration.
Animals↗
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↗
Kainic acid selectively alters auditory dendrites connected with cochlear inner hair cells.
Cochleas of adult guinea pigs and rats, and 6-day-old rat pups, were injected, through the round window, with 2 microliters of artificial classical Konishi perilymph containing 1 nmol kainic acid (KA). 5 min later, they were fixed, removed, and processed for electron microscopy. In all KA-treated cochleas, the injection resulted in a severe swelling of auditory dendrites below the inner hair cells (IHCs). Below the outer hair cells (OHCs), the swelling appeared only in the 6-day-old rats, not in adult animals. These results are significant in three different ways: (1) They confirm the strong difference between afferents innervating the IHCs and the OHCs in adult cochleas. (2) They shed some light on the synaptic plasticity found at the OHC level during synaptogenesis. (3) They support the hypothesis that glutamate, or a related substance, is the IHC neurotransmitter.
Age Factors↗
Very rapid adaptation in the guinea pig auditory nerve.
Guinea pig auditory ganglion cell responses to 100-ms duration tone bursts were recorded over a range of stimulus intensities. The responses, recorded in the form of peristimulus/poststimulus time histograms, were analysed by reduction into two phases. The first phase was a rapid exponential adaptation from an initial onset response; the second was a more gradual reduction in the firing rate which, over the 100 ms duration of the stimulus, appeared to be a linear function of time. The first, rapid, phase was nonlinear in its response to changes in stimulus intensity, exhibiting a change in amplitude and having a time constant which decreased with increasing intensity. Individual units were consistent in the magnitude and time course of this phase. The second phase was also nonlinear with intensity, and was far more variable from unit to unit. With the recording parameters employed it was not possible to determine whether the effect of intensity on the second phase was an effect on the magnitude or time course, or both. Stimulus termination responses were also analysed, and typically were of one of two forms. If, at any particular stimulus intensity, the unit under study showed little sign of the slower adaptation then the termination response was a simple depression of activity (perhaps to zero) which recovered with an exponential time constant of about 25 ms, independent of intensity. If, however, the peristimulus responses showed a significant amount of the slow adaptation then the termination responses also exhibited a second, slower, phase of recovery. This was modelled over the recording epoch as a linear function of time. The magnitude of the slow offset response also increased with intensity faster than did the average firing rate.
Acoustic Stimulation↗
Measurement of the combination tone 2f1-f2 using two-tone forward masking and compound action potential responses.
The presence of the 2f1-f2 combination distortion tone generated when two tones are presented to the cochlea, can be detected using a two-tone complex to forward mask a probe tone. The combination tone is the effective masking agent and the amplitude of the compound action potential response to the probe tone reflects the degree of masking. Excitation patterns, frequency relationships and intensity functions that are obtained with this procedure are consistent with previously published data.
Action Potentials↗
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.
Acoustic Stimulation↗
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.
Acoustic Stimulation↗
The alteration of the vibration of the basilar membrane produced by loud sound.
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Animals↗
Group delay measurement from spiral ganglion cells in the basal turn of the guinea pig cochlea.
Measurements of group delay were made extracellularly from spiral ganglion cells in the 3.7 to 5.0-mm region of the guinea pig cochlea, using sinusoidally amplitude modulated tones with constant modulating frequency (100 Hz) and depth of modulation (0.19). Threshold cochlear tuning was accompanied by frequency-dependent group delays. The group delay on the low-frequency tail was independent of carrier frequency; the interunit variation was 0.28-1.28 ms. The difference in group delay between CF and the low-frequency tail decreased as the CF threshold increased (-0.09 +/- 0.02 ms per 10 dB, beginning at 0.62 +/- 0.07 ms at 0 dB SPL). The group delay decreased above CF; at the units' maximum frequency it was less than the low-frequency tail value, and was sometimes negative. Following arterial injections of furosemide the CF threshold increased and the group delay peak decreased; the low-frequency tail was unaffected. The group delay decreased with increasing intensity; the reduction near and above CF was not only larger than that on the low-frequency tail, but also the change at 5-10 dB above threshold was far greater than expected from the Q10dB of the suprathreshold iso-rate tuning curves. A minimum-phase analysis suggested that the group delay response above CF, together with its nonlinear behavior, can be accounted for by a high-frequency, level-independent, amplitude plateau, in combination with the single unit, amplitude nonlinearity which is known to exist above CF.
Animals↗
Recovery of eighth nerve action potential thresholds after exposure to short, intense pure tones: similarities with temporary threshold shift.
Exposure of guinea pig cochleas to short (25, 250 and 1000 ms), intense (90 or 100 dB SPL) 10 kHz pure tones reduced cochlear sensitivity to 14 kHz test tones presented at intervals varying from 5 ms to 70 s after the exposure tone. The recovery of cochlear sensitivity, determined as the SPL required to evoke a 20 microV compound action potential (CAP), depended on both the intensity and the duration of the exposure tone and appeared to take place in two or more phases. After a 25 ms, 100 dB SPL exposure, CAP threshold increased by up to 13.5 dB and generally recovered very rapidly (25 ms), although some loss persisted for as long as 400 ms. A similar, but greater and longer, elevation of threshold was seen after long exposure tones. Lower exposure tone intensities (90 dB SPL) produced threshold elevations which generally lasted for only short durations (25-35 ms). The rapid recovery is consistent with the time course of recovery from rapid adaptation, while the slow recovery component is similar to that seen in the cochlea after much longer exposures and may be related to the phenomenon of temporary threshold shift. Long and/or loud exposures also frequently resulted in a third form of threshold elevation, identified in only a few animals, which recovered with a time constant in excess of 25 s. A fourth component occasionally persisted for the duration of the experiment.
Acoustic Stimulation↗
State of stress within the basilar membrane: a re-evaluation of the membrane misnomer.
By deforming and making incisions in the basilar membrane (BM), von Békésy showed that the BM seems to behave as a thin elastic plate, rather than as a membrane. However, it has never been shown whether a traveling wave could be sustained by a prestretched material in which the tension is insufficient to cause a visible retraction of the cut edges of an incision when viewed with a light microscope. We have shown that the necessary radial tension would decrease exponentially along the cochlea, from a value of 39 +/- 9 N/m at the base, with a space constant of 4.3 +/- 1.1 mm, for the guinea pig. This variable tension would be produced by a constant prestretching surface force of 2.4 +/- 0.1 X 10(6) N/m2, acting on the BM edges. Using values of Young's modulus in the radial direction, ranging from that of elastin to collagen, it is shown that this force would most likely cause a visible retraction of the cut edges of an incision. Therefore one must either conclude, once again, that the BM is effectively an unstretched material or question the original interpretation of the incision experiments.
Animals↗
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↗
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.
Action Potentials↗
Crossed cochlear influences on monaural temporary threshold shifts.
Temporary threshold shifts (TTS) of the cochlear action potential in one (ipsilateral) ear, caused by a brief intense pure tone, were reduced either by (1) contralateral acoustic stimulation at the same frequency, or (2) destruction of the contralateral cochlea. The effect of contralateral cochlear destruction persisted, though slightly reduced in effect, after a delay of one hour between the destruction and the ipsilateral exposure. Contralateral acoustic stimulation had no effect after a delay of 10 min. The effect of contralateral cochlear destruction could be blocked by strychnine, a known antagonist of auditory efferent activity. However, contralateral cochlear destruction resulted in none of the effects upon normal ipsilateral thresholds or input-output curves for the action potential classically seen when the efferent pathways are stimulated. The results suggest that the crossed effects reported here are due to a complex interaction between the activity in both cochleas, possibly resulting in a reduction in a central inhibitory influence on an efferent feedback pathway that is then expressed during the ipsilateral exposure.
Acoustic Stimulation↗
Residual effects in monaural temporary threshold shifts to pure tones.
Temporary threshold shifts (TTS), of the cochlear action potential in the guinea pig, caused by a high frequency exposure tone could be reduced by a prior exposure at the same frequency but lower intensity. The TTS caused by the initial, low intensity exposure tone was allowed to recover to levels where the thresholds were not significantly different from normal thresholds. Re-exposure at the high intensity now resulted in less TTS being recorded, both at the half-octave point and across the range of frequencies affected by the pure tone, when compared to exposure to the high intensity tone without such priming. This protective effect of prior exposure was not affected by intraperitoneal injections of strychnine, distinguishing it from recent reports of protective mechanisms operative during TTS.
Acoustic Stimulation↗
Group delay measurements from spiral ganglion cells in the guinea pig cochlea.
Measurements of group delay were made extracellularly from spiral ganglion cells in the basal turn of the guinea pig cochlea, using sinusoidally amplitude modulated tones, with constant modulating frequency and modulation depth at the microphone input. Threshold cochlear tuning was accompanied by a frequency dependent group delay, with relative peak proportional to Q10. For sensitive units with steep intensity functions, the group delay decreased with increasing sound pressure level above threshold, without a significant change of Q10.
Animals↗
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.
Animals↗