A model proposing synaptic and extra-synaptic influences on the responses of cochlear nerve fibres

Ensemble spontaneous activity in the guinea-pig cochlear nerve

Spectral analysis of electrical noise recorded from the round window (RW) of the cochlea is referred to as the ensemble spontaneous activity (ESA) of the cochlear nerve. The ESA is considered to represent the summed spontaneous activity of single fibers of the auditory nerve and changes in the spectral characteristics of the ESA have been observed in humans with tinnitus. Experiments were undertaken to determine the relationship of the ESA to auditory neurotransmission. The ESA consisted of energy centered at approximately 900 Hz, similar to the spectral peak of single auditory neuron discharges. The amplitude of the ESA was correlated with good auditory sensitivity in the 12-30 kHz region of the cochlea. Constant pure tones of 12-22 kHz suppressed the ESA reducing its amplitude in a frequency and intensity dependent manner implying that the ESA recorded at the RW is generated or dominated by neurons in the basal region of the cochlea. The ESA was significantly suppressed by round window perfusion of the P2X receptor agonist adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS) (10 mM) the glutamate receptor antagonist 6-7-dinitroquinoxaline-2,3-dione (DNQX) (1 mM), and the sodium channel antagonist tetrodotoxin (TTX) (20 microM). Following intravenous furosemide injection (40 mg/kg) reduction and recovery of the ESA correlated with similar changes in the endocochlear potential (EP). Following DNQX and ATPgammaS an additional spectral peak at 200 Hz was often observed. This peak has been postulated to be a correlate of tinnitus in humans but had not previously been observed in a guinea-pig model of tinnitus. These data confirm the spectral characteristics of the ESA in guinea-pigs and show it is dependent on the sensitivity of the auditory nerve and intact auditory neurotransmission. In addition these experiments support the view that the ESA represents summed spontaneous neural activity in the cochlea and provide a platform for studies of the influence of ototoxic compounds on the spontaneous neural outflow of the cochlea as a model of tinnitus.

Reciprocal synapses between inner hair cell spines and afferent dendrites in the organ of corti of the mouse

We provide, for the first time, ultrastructural evidence for the differentiation of reciprocal synapses between afferent dendrites of spiral ganglion neurons and inner hair cells. Cochlear synaptogenesis of inner hair cells in the mouse occurs in two phases: before and after the onset of hearing at 9-10 postnatal (PN) days. In the first phase, inner hair cells acquire afferent innervation (1-5 PN). Reciprocal synapses form around 9-10 PN on spinous processes emitted by inner hair cells into the dendritic terminals, predominantly in conjunction with ribbon afferent synapses. During the second phase, which lasts up to 14 PN, synaptogenesis is led by the olivocochlear fibers of the lateral bundle, which induce the formation of compound and spinous synapses. The afferent dendrites themselves also develop recurrent presynaptic spines or form mounds of synaptic vesicles apposed directly across inner hair cell ribbon synapses. Thus, in the adult 2-month mouse, afferent dendrites of spiral ganglion neurons are not only postsynaptic but also presynaptic to inner hair cells, providing a synaptic loop for an immediate feedback response. Reciprocal synapses, together with triadic, converging, and serial synapses, are an integral part of the afferent ribbon synapse complex. We define the neuronal circuitry of the inner hair cell and propose that these minicircuits form synaptic trains that provide the neurological basis for local cochlear encoding of the initial acoustic signals.

Multicomponent stimulus interactions observed in basilar-membrane vibration in the basal region of the chinchilla cochlea

Multicomponent stimuli consisting of two to seven tones were used to study suppression of basilar-membrane vibration at the 3-4-mm region of the chinchilla cochlea with a characteristic frequency between 6.5 and 8.5 kHz. Three-component stimuli were amplitude-modulated sinusoids (AM) with modulation depth varied between 0.25 and 2 and modulation frequency varied between 100 and 2000 Hz. For five-component stimuli of equal amplitude, frequency separation between adjacent components was the same as that used for AM stimuli. An additional manipulation was to position either the first, third, or fifth component at the characteristic frequency (CF). This allowed the study of the basilar-membrane response to off-CF stimuli. CF suppression was as high as 35 dB for two-tone combinations, while for equal-amplitude stimulus components CF suppression never exceeded 20 dB. This latter case occurred for both two-tone stimuli where the suppressor was below CF and for multitone stimuli with the third component=CF. Suppression was least for the AM stimuli, including when the three AM components were equal. Maximum suppression was both level- and frequency dependent, and occurred for component frequency separations of 500 to 600 Hz. Suppression decreased for multicomponent stimuli with component frequency spacing greater than 600 Hz. Mutual suppression occurred whenever stimulus components were within the compressive region of the basilar membrane.

Basilar-membrane response to multicomponent stimuli in chinchilla

The response of chinchilla basilar membrane in the basal region of the cochlea to multicomponent (1, 3, 5, 6, or 7) stimuli was studied using a laser interferometer. Three-component stimuli were amplitude-modulated signals with modulation depths that varied from 25% to 200% and the modulation frequency varied from 100 to 2000 Hz while the carrier frequency was set to the characteristic frequency of the region under study (approximately 6.3 to 9 kHz). Results indicate that, for certain modulation frequencies and depths, there is enhancement of the response. Responses to five equal-amplitude sine wave stimuli indicated the occurrence of nonlinear phenomena such as spectral edge enhancement, present when the frequency spacing was less than 200 Hz, and mutual suppression. For five-component stimuli, the first, third, or fifth component was placed at the characteristic frequency and the component frequency separation was varied over a 2-kHz range. Responses to seven component stimuli were similar to those of five-component stimuli. Six-component stimuli were generated by leaving out the center component of the seven-component stimuli. In the latter case, the center component was restored in the basilar-membrane response as a result of distortion-product generation in the nonlinear cochlea.

Medial efferent effects on auditory-nerve responses to tail-frequency tones. I. Rate reduction

One way medial efferents are thought to inhibit responses of auditory-nerve fibers (ANFs) is by reducing the gain of the cochlear amplifier thereby reducing motion of the basilar membrane. If this is the only mechanism of medial efferent inhibition, then medial efferents would not be expected to inhibit responses where the cochlear amplifier has little effect, i.e., at sound frequencies in the tails of tuning curves. Inhibition at tail frequencies was tested for by obtaining randomized rate-level functions from cat ANFs with high characteristic frequencies (CF > or = 5 kHz), stimulated with tones two or more octaves below CF. It was found that electrical stimulation of medial efferents can indeed inhibit ANF responses to tail-frequency tones. The amplitude of efferent inhibition depended on both sound level (largest near to threshold) and frequency (largest two to three octaves below CF). On average, inhibition of high-CF ANFs responding to 1 kHz tones was around 5 dB. Although an efferent reduction of basilar-membrane motion cannot be ruled out as the mechanism producing the inhibition of ANF responses to tail frequency tones, it seems more likely that efferents produce this effect by changing the micromechanics of the cochlear partition.

Basilar membrane motion in relation to two-tone suppression

It is proposed that two-tone suppression of rate responses in auditory-nerve fibres by a low-side suppressor cannot be explained in terms of basilar membrane motion. In a model, the amplitude of the mechanical response, either to the tone at characteristic frequency (CF), or to the CF tone combined with a second, lower frequency tone (a suppressor), is taken as the effective stimulus to inner hair cells (IHC), the voltage response of which is considered responsible for excitatory drive to auditory-nerve fibres. Many empirical mechanical and physiological effects are simulated accurately by the model, particularly phenomena observed in two-tone experiments using low-side suppressor tones, that authors have described as two-tone suppression. It is argued in this paper, however, that such phenomena strictly do not constitute suppression in the cochlear response and provide no explanation for rate suppression in nerve fibres. According to the model presented here and consistent with experimental data, suppression of the spike response to a CF tone in an auditory-nerve fibre by a low-side suppressor cannot be explained in terms of the mechanics of the BM. Conclusions by others that experiments support a mechanical explanation for low-side rate suppression are shown to be questionable. It is concluded that low-side suppression of neural responses is explicable only in terms of a non-mechanical factor derived from the response to the low frequency tone, that depresses responsiveness in fibres at the CF location. Adherence to the model of low-side neural rate suppression depending on reduced net mechanical response of the BM is contrary to experimental evidence; furthermore it overlooks a profound influence additional to synaptic drive, that is implied in the shaping of responses in auditory-nerve fibres.

Two-tone suppression of basilar membrane vibrations in the base of the guinea pig cochlea using "low-side" suppressors

The responses of the basilar membrane (BM) in the basal section of the guinea pig cochlea were measured by laser interferometry. The stimuli were pairs of harmonically related tones, presented simultaneously. One tone, at the BM's characteristic frequency (CF) of about 17 kHz, was presented at a low intensity. The other tone, presented at various intensities, was a "low-side" suppressor, with a frequency of 0.2-8 kHz. As observed by many others, the suppressor tone, when presented at high enough intensity, reduced the magnitude of the CF component of BM displacement, sometimes dramatically. However, regardless of whether the CF component was suppressed or not, the sum of the displacement amplitudes of the CF and suppressor components was always greater than the displacement amplitude of the unsuppressed CF component. For suppressor frequencies up to 4 kHz, the suppression was both tonic and phasic, and synchronized to the suppressor period. For higher suppressor frequencies, principally tonic suppression was seen.

Low-frequency modulation of inner hair cell and organ of Corti responses in the guinea pig cochlea

Low-frequency tones are used to study changes in responsiveness as a function of phase in inner hair cell (IHC) and organ of Corti (OC) responses recorded from second turn of the guinea pig cochlea. In these experiments a 40 Hz stimulus is combined with a variable frequency probe to determine the degree to which tones at and below best frequency (BF) are modulated. Changes in responsiveness produced by the low-frequency input are quantified and related to position of the basilar membrane which is estimated using the phase of the cochlear microphonic measured in the OC fluid space. Results obtained when 40 Hz is presented at its lowest effective level demonstrate that ac responses to low-level BF probes are reduced for basilar membrane displacements to scala tympani while probe tones well below BF are modulated in the opposite direction. The transition between these two response patterns occurs when the overall DC produced in the OC by the two-tone input changes from positive to negative. Because of this association, the frequency dependence exhibited in the bias results may be linked to mechanisms responsible for generating the two polarities of the summating potential and the DC receptor potentials that it reflects. An attempt is also made to relate bias-induced changes in hair cell receptor potentials to modulations in single-unit rate responses. In other words, to address variations in the temporal relationships between excitation and suppression measured in the auditory nerve.

Suppression in auditory-nerve fibers of cats using low-side suppressors. II. Effect of spontaneous rates

The responses of auditory nerve fibers with different spontaneous rates were studied in anesthetized cats, using harmonically related characteristic frequency (CF) tone and suppressor (SUP) tone (50-2000 Hz) as stimuli. The relative-response index, defined as the ratio of the maximum response level in the two-tone segment to the response level in the CF-alone segment, at or near the intensity of maximum suppression (i.e., where the two-tone rate was lowest), was dependent on fiber's spontaneous rate (SR). For all the SUP frequencies used, lower-SR fibers almost always showed values less than unity, while high-SR fibers almost always gave values near or greater than unity. The phase of maximum suppression was not dependent upon fiber SR. In one experiment, a pair of low- and high-SR fibers with the same CF (12 kHz) were recorded consecutively in the same electrode penetration, and were studied with the same stimulus parameters. Their temporal responses showed dramatic temporal resemblances, with very similar phases of suppression and response. But the relative-response indexes were different. The similarities in the lower- and high-SR fibers support the idea that the basic response and suppression patterns in all fibers are formed at or before the inner hair cell (IHC) stage, while the differences suggest that processes more central than the IHC receptor potential are important in determining the magnitudes of suppression, particularly in the lower-SR fibers.

Two-tone suppression, excitation and the after effect in rate responses in auditory nerve fibres in the cat

Responses were recorded from single, auditory nerve fibres in the anaesthetized cat. Acoustic stimuli consisted of two tones, one of which was at characteristic frequency (CF), the other (the suppressor) was at considerably lower frequency. Tones were presented in simultaneous and sequential configurations. For simultaneous presentations, well-known response properties were observed. The rising limb of the two-tone rate-intensity function closely matched that of the appropriately adapted response to the suppressor tone presented alone. Also, whether strongly suppressed relative to CF-driven rate, or equal to CF-driven rate, rate responses to the two-tone stimuli persisted unchanged when the CF tone was terminated and the suppressor tone continued alone. These results support the hypothesis that the suppressor tone has dual influences, suppressive and excitatory, that are distinct and additive. Peristimulus response histograms confirm in the cat that depression and slow recovery of sensitivity to CF may follow termination of the suppressor tone, as reported for the guinea pig [Hill, K.G. and Palmer, A.R. (1991) Hear. Res. 55, 167-176]. This delay in recovery of normal sensitivity to CF appeared to be directly related to the amount of excitation of the fibre that is attributable to the suppressor tone. A similar, delayed re-establishment of sensitivity also occurred in the response to a tone at CF, presented immediately following excitation by a suppressor tone. However, no delay occurred in the onset of response to the suppressor when preceded by the CF tone.(ABSTRACT TRUNCATED AT 250 WORDS)

One-tone suppression in the cochlear nerve of the gerbil

One-tone rate suppression has been reported several times for auditory nerve fibers of mammalian and non-mammalian vertebrates. Because its properties are very similar to those of two-tone rate suppression, the possibility exists that one-tone rate suppression is the result of an interaction within the inner ear of the suppressing tonal stimulus and some ongoing extraneous acoustic stimulus. For this reason, reports of one-tone rate suppression often elicit suspicions that the investigators were not sufficiently careful in controlling leaks in their acoustic barriers or in the electrical pathways to their acoustic drivers. Recent reports of one-tone rate suppression in pigeon basilar-papillar fibers and goldfish saccular fibers were accompanied by descriptions of measures taken to avoid such leaks. In this paper, we describe one-tone rate suppression in a mammal, the Mongolian gerbil; and we demonstrate that the background spike activity being suppressed is not driven by either external sounds coming from outside the acoustic isolation test chamber or by non-stimulus electrical inputs to the acoustic driver. The suppressed background spike activity evidently arises from sources within the animal. These sources may be non-acoustic, associated with spontaneous pre- or post-synaptic ion-channel activity; or they may be acoustic sources--internal sound or vibration generators.

Auditory-nerve spontaneous rates vary predictably with threshold

The variation of spontaneous rate with auditory nerve thresholds is compared with predictions from a simple assumption: that spontaneous and driven activity are basically similar, both being evoked by inner hair cell transmembrane potential. Under this view, spontaneous activity is seen as a response to a standing current within the hair cell and should therefore vary with threshold in a manner predictable from measured rate-intensity functions. A method for comparing spontaneous rates of fibres with differing thresholds is developed and applied to previously-collected data. The results show that spontaneous rates are quite consistent with the hypothesis, indicating no need for more complicated theories of spontaneous activity.

Masking and suppression in auditory nerve fibers of the goldfish, Carassius auratus

The responses of single fibers of the auditory nerve of the goldfish (Carassius auratus) were recorded in response to two tones of different duration (20 ms 'signals' and 200 ms 'maskers') presented simultaneously or non-simultaneously. A single tone may produce excitation, adaptation, and suppression in auditory nerve fibers. For fibers with characteristic frequencies (CF) in the 200 to 400 Hz range, frequencies well above CF tend to produce suppression. If the net response to the masker tone is excitation, an added excitatory signal tone tends to increment the response in a way predictable from the rate-level function for the masker. A masker can attenuate the response to a signal as a result of a compressive and saturating response to the masker, and as a result of a low signal-to-masker ratio. If the net response to a masker tone is suppression, it effectively subtracts from signal excitation, causing 'suppressive masking.' In non-spontaneous fibers, suppression, additive excitatory effects, and adaptation can be revealed by responses to the signal in the absence of spike responses to the masker. In general, the ability of one tone (the masker) to reduce the response to a second tone (the signal) is greater in non-spontaneous fibers than in spontaneous fibers. These results also show that estimates of the frequency selectivity of many goldfish auditory nerve fibers will depend on whether the response of the fiber is defined by excitation, suppression, or both. The response of many fibers with CF in the 200-400 Hz region, as defined by excitation, can be masked or suppressed by a broad range of frequencies covering the effective hearing range of the goldfish.

First order temporal properties of spontaneous and tone-evoked activity of auditory afferent neurones in the cochlear ganglion of the pigeon

Spontaneous and tone-evoked single-unit activity was recorded from afferent neurones in the cochlear ganglion of the anaesthetized pigeon, and the data analysed in a way that allowed the physics of underlying mechanisms to be described. The periodicity of neural activity was quantified by Fourier analysis of the histogram of successive spike intervals. Spontaneous activity was quasiperiodic for 57% of neurones (average rate: 74 s-1); it was irregular for the remainder of neurones (average rate: 55 s-1). The preferred frequency (PF) of the quasiperiodic spontaneous activity was, on average, equal to the characteristic frequency (CF) of the neurone (70% of cases) or CF/2 (30%). This observation can be explained by supposing that preferred intervals of spontaneous activity are generated by noise passing through a filter tuned to the CF of the neurone; in most cases (70%) discharge was synchronized to CF, but in the others the neurone fired to every second cycle of the filtered signal. Consistent with this interpretation, for 79% of neurones, the modal interval of spontaneous activity was, on average, directly proportional to the CF-period, irrespective of whether preferred intervals were detected. The synchronization index at the PF was inversely related to the PF, and was quantified by the amplitude response of a first-order low-pass filter with cutoff frequency of 48 +/- 18 Hz. The spontaneous activity of 9% of neurones exhibited a second-harmonic component of the PF. For both tone-evoked and spontaneous activity, the observed synchronization indices of harmonics of the stimulus frequency or of the PF were consistent with an underlying exponential spike-generator function. If such a function does indeed govern spike generation, then it implies that the Shannon entropy of the probability density function of the instantaneous firing rate is near its maximum value and suggests that the system is close to statistical equilibrium. Single-tone rate-suppression was detected for 53% of those neurones that exhibited multiple preferred intervals of spontaneous activity. It is conjectured that the phenomena of quasiperiodic spontaneous activity and single-tone rate-suppression are different aspects of a single presynaptic process. According to this model, we would expect to find these two phenomena in animals that have auditory fibres innervating electrically tuned hair cells, and that have stereocilia firmly coupled to a tectorial membrane.

Electrical responses from the chicken basilar papilla

A surgical approach to the basilar papilla of the chicken cochlea has been developed which allows recordings from within the hair cells and supporting cells in vivo. The frequency tuning curves for the AC receptor potentials measured in extracellular space immediately outside hair cells, within the hair cells and in adjacent supporting cells were similar, with best frequencies ranging from 600 Hz to 2000 Hz, and Q10 dB values between 0.6 and 2.5. In cells classified as hair cells there was no evidence of spontaneous oscillations in the membrane potentials, nor of ringing of the membrane potential in response to injected current pulses. Moreover, displacement of the papilla with the microelectrodes could modulate the hair cell membrane potential over the range 0 to -90 mV, suggesting that the current-voltage relationship in these cells was essentially linear. This view was supported by preliminary investigations of cell properties with current injection. We interpret these observations as evidence that the hair cells impaled were not electrically tuned under our experimental conditions, unlike the hair cells of the turtle cochlea. These observations, taken together with the electrode angles used, fluorescent dye-marking and previous measurements of chicken hair cells in vitro [Fuchs, P.A., Nagai, T. and Evans, M.G. (1988) J. Neuro. Sci. 8, 2460-2467], suggest that the hair cells we have impaled were the short hair cells of the papilla, and that the tuning of the AC receptor potentials we have observed was due solely to a tuned mechanical drive to their hair bundles.

Suppression and excitation in auditory nerve fibers of the goldfish, Carassius auratus

The suppression of background spike activity in the absence of deliberate acoustic stimulation occurs in fibers of the goldfish saccular nerve tuned in the region of 250 Hz. Suppression is most robust in the frequency range between 450 and 1050 Hz, the range of CF for the mid- and high-frequency saccular fibers. Suppression of background activity tends to occur following the suppressor tone offset ('off-suppression'), even though the spike response during the suppressor is below the background rate. This suggests that the suppressor tone is excitatory at the level of the hair cells and their synapses onto saccular afferents. Tones at the low- frequency edge of the suppression region may show net excitation at low intensity levels, and net suppression at higher levels. This suggests that the spike response observed is the result of the relative strengths of excitatory and suppressive effects which operate simultaneously. The magnitude and frequency of best suppression tends to increase with stimulus intensity. A suppressing tone produces transient excitation at onset. In fibers with high levels of spontaneous activity, a spike response 'rebound' often occurs 20 to 50 ms following the suppressing tone offset. These 'on' and 'off' effects are not due to energy 'splatter' in the stimulus domain. Suppression by tones can also be observed in non-spontaneous fibers when the background spike activity is evoked by noise. In these cases, however, off-suppression following a suppressed response and the 'rebound' seldom occurs. Possible sites of suppression are the hair cells and their synapses, the spike-initiation zones of the saccular afferents, and efferent inhibition. The most likely site seems to be the spike-initiation zones of saccular afferents. An important consequence of suppression for hearing is the sharpening of frequency response areas for low frequency fibers, and the partial preservation of frequency analysis in saccular fibers stimulated well above threshold.

Saturation of outer hair cell receptor currents causes two-tone suppression

Zwicker [Biol. Cybern. 35, 243-250, (1979); J. Acoust. Soc. Am. 80, 163-176 (1986)] has previously proposed that many nonlinear phenomena in the mammalian cochlea can be explained by saturation of a positive feedback process which enhances mechanical sensitivity, although the site of the nonlinearity producing this saturation has so far remained obscure. In this paper we present evidence suggesting that the nonlinearity of mechano-electrical transduction in the outer hair cells is the dominant nonlinearity producing two-tone suppression in the mammalian cochlea. In particular, we show that: (i) suppression of the extracellular summating potential (SP), recorded from a particular place within the organ of Corti, has characteristics similar to the suppression of activity in the auditory-nerve; (ii) that SP suppression occurs at approximately constant basilar membrane displacement, inferred from the SP iso-response contours; and that (iii) the onset of SP suppression with suppressor tones on the tail of the frequency tuning curve closely parallels the onset of nonlinearity in the local cochlear microphonic. Since previous studies (Patuzzi et al., 1989) have demonstrated that the vibration of the basilar membrane at its characteristic frequency is very sensitive to changes in outer hair cell receptor current, we consider that interference in outer hair cell currents caused by nonlinearity in mechano-electrical transduction is an adequate explanation of two-tone suppression. This requires that outer hair cell receptor currents deviate from linearity at a suppressor tone level below that required to produce a significant DC receptor potential within the inner hair cells, and that the active process within the cochlea is distributed along a local region of the cochlea, basal of the vibration peak.