Whole-nerve Action Potential Research Articles

Dynorphin release by the lateral olivocochlear efferents may inhibit auditory nerve activity: A cochlear drug delivery study

Dynorphin (dyn) is suggested to excite the auditory nerve (AN) when released by the lateral olivocochlear (LOC) efferents. However, previous studies evaluated either intravenously delivered dyn-like agents, raising the potential for systemic (central) effects, or agent concentrations unlikely to be achieved via endogenous cochlear release. This study tested the hypothesis that biologically relevant increases in dyn levels in the cochlea achieved via diffusion of the drug of (−)pentazocine across the round window membrane enhances AN firing. In general, amplitude of the cochlear whole-nerve action potential (CAP) was depressed following drug application. These results suggest that dyn released by the LOC neurons would likely act as an inhibitory transmitter substance in the LOC system; neurotransmission is one of the LOC system's vast unknowns.

Inferior colliculus stimulation causes similar efferent effects on ipsilateral and contralateral cochlear potentials in the guinea pig

The inferior colliculus (IC) is a processing center in both the ascending and descending auditory pathways. It has been demonstrated anatomically to send descending projections to the region of the medial olivocochlear (MOC) neurons in the auditory brainstem. Activation of MOC system produces reductions in cochlear neural activity. Individual MOC fibers innervate relatively restricted regions of the cochlea. Recent studies have shown that selective electrical stimulation within the IC central nucleus (ICC) produces frequency-specific reductions of neural activity in the contralateral cochlea (Ota, Y., Oliver, D.L., Dolan, D.F., 2004. Frequency-specific effects on cochlear responses during activation of the inferior colliculus in the guinea pig. J. Neurophysiol. 91, 2185–2193). This efferent effect is likely mediated through selective activation of MOC cells. In this study, we investigated the effects of selective stimulation of one ICC on cochlear output in both ears in anesthetized and paralyzed guinea pigs to explore possible differences in the effective efferent innervation of the two ears. ICC stimulation had a similar tonotopically tuned effect on the distortion product otoacoustic emission (DPOAE) and the cochlear whole-nerve action potential (CAP) in each cochlea. The bandwidth of the efferent effect in each ear was measured and compared at different stimulation levels. For a given ICC stimulation site, the efferent effects were larger for the CAP response. The effect on each response measure was greater in the contralateral than the ipsilateral ear. The effective bandwidth of the efferent effect on the CAP was current-level-dependent but less so for the DPOAE. The results of transections at various locations within the brainstem suggest that the effects were mediated by the MOC system. From the results presented here, the descending efferent system, which originates in the auditory cortex, has frequency-specific, spatially restricted, bilateral effects. The effects are greater in the contralateral ear.

Long-Term Effects of Acoustic Trauma on Electrically Evoked Otoacoustic Emission

Electrically evoked otoacoustic emissions (EEOAEs) are sounds measured in the ear canal when alternating current (AC) stimulation is passed into the cochlea. These sounds are attributed to the motile responses of outer hair cells (OHCs). The EEOAE has characteristic amplitude, phase, and fine structure. Multicomponent analysis of the EEOAE shows short (SDC) and long delay components (LDC) that are thought to originate from OHCs near the AC stimulating site and from OHCs at more remote locations, respectively. We measured the effects of various loud noise exposures on the EEOAE and the cochlear whole-nerve action potential (CAP) in animals chronically implanted with a scala tympani electrode. Noise exposures that produced permanent (PTS) or temporary threshold shifts (TTS) were associated with frequency-specific changes in CAP thresholds, EEOAE fine structure, and reductions in the amplitude of the LDC. A frequent observation in this study was an increase in the overall EEOAE amplitude after the noise exposure. The increase was correlated with increased SDC amplitude. The SDC was present in animals chemically treated with ototoxic drugs and mechanical damage to the cochlea. The SDC was eliminated after disarticulation of the ossicular chain. The presence of EEOAE fine structure in the postexposure response is an indicator of TTS in advance of CAP recovery. The results suggest that the EEOAE might be used to differentiate the mechanisms associated with TTS and PTS.

Frequency-specific effects on cochlear responses during activation of the inferior colliculus in the Guinea pig.

The inferior colliculus (IC) is a major processing center in the ascending auditory pathway. The role of the IC in the descending efferent auditory system is less clear. Although the IC central nucleus (ICC) is the major relay station for the ascending auditory pathways, the IC's cortex receives its main input from the neocortex and nonauditory sources. The goal of this study was to determine if the IC subdivisions had different functions in the descending efferent auditory system. IC subdivisions were identified by their tuning curves evoked by tone stimulation, and the effects of localized electrical stimulation on the cochlear whole-nerve action potential (CAP). Sharp tuning curves were obtained from ICC in contrast to broad tuning curves from the lateral, external cortex. Electrical stimulation within the central nucleus had a sharply tuned effect on the CAP. The frequency region affected within the cochlea closely matched the best frequency of local cells within the central nucleus. The effect of electrical stimulation within the lateral, external cortex on the CAP was smaller in comparison to central nucleus stimulation. Similar to the broad tuning of cells within the lateral cortex, electrical stimulation had a broad frequency effect on CAP thresholds.

Comparison of Electrically Evoked Whole-Nerve Action Potential and Electrically Evoked Auditory Brainstem Response Thresholds in Nucleus CI24R Cochlear Implant Recipients

In this study, differences between electrically evoked whole-nerve action potential (EAP) and electrically evoked auditory brainstem response (EABR) measurements within Nucleus CI24R cochlear implant recipients were evaluated. Precurved modiolus-hugging internal electrode arrays, such as the CI24R, are designed to provide more direct stimulation of neural elements of the modiolus. If the electrode array is closer to the modiolus, electrically evoked and behavioral levels might be lower than were previously recorded for the straight electrode array, the CI24M. EAP and EABR growth functions and behavioral levels were obtained for 10 postlingually deafened adults. Results revealed no significant differences between EAP and EABR threshold levels, and these levels were not significantly lower than those obtained using the CI24M.

Effects of repeated cocaine injections on cochlear function

The effects of repeated cocaine administration on cochlear function were evaluated by measuring amplitude-intensity and latency-intensity functions of the whole-nerve action potential of the auditory nerve. Whole-nerve action potential input/output functions obtained using tone-pips of 0.5, 1, 2, 4 and 8 kHz in a group of cocaine-treated subjects were compared with those obtained in saline-treated animals. All measurements were made 24 h after the last treatment. Amplitudes of whole-nerve action potentials were enhanced in the cocaine-treated animals compared to the control group. No statistically significant differences in latency-intensity functions were seen after cocaine treatment. The effect of chronic cocaine exposure also was examined on catecholamine innervation in the cochlea using immunohistochemical techniques. The density of adrenergic innervation was reduced in the cocaine-treated animals.

A comparison of N1 of the whole nerve action potential and wave i of the brain-stem auditory evoked response in Mongolian gerbil

The present study seeks to provide empirical support for the assumption that wave i of the gerbil brain-stem auditory evoked response (BAER) corresponds to N1 of the whole nerve action potential (WNAP) by comparing the latency and amplitude of BAER wave i and WNAP N1. Fourteen 3-month old gerbils were anesthetized with Nembutal and Urethane-Dial. Normothermia was maintained by a homeothermic blanket system. BAERs were recorded with Grass needle electrodes placed subdermally. The WNAP was recorded with a silver wire placed in the round window niche. WNAP and BAER were simultaneously recorded with a passband of 100-10,000 Hz. Responses consisted of 500 sweeps, and two responses were obtained for each condition. Clicks were 25-microseconds electrical pulses. Tonebursts were shaped with a Hanning window, with 1-ms rise and fall times. Toneburst frequencies included 1, 2, 4, 8, and 16 kHz. For each stimulus, responses were obtained at levels of 30, 50, 70, and 90 dB pSPL. SPL was measured near the entrance to the ear canal with an Etymotic ER-7C probe microphone. Dependent variables were the latency and amplitude of N1 of the WNAP and wave i of the BAER. The latencies of wave i and N1 were very similar. Mean (across animal) latencies of N1 and wave i were within 70 microseconds for all six stimuli (clicks, tonebursts) and all four levels. Latency/intensity function slopes for N1 and wave i were also very similar, with both dependent variables showing an increasing latency/intensity function slope with decreasing toneburst frequency. The N1/wave i amplitude ratio was computed.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrically evoked whole-nerve action potentials: Data from human cochlear implant users

This study describes a method for recording the electrically evoked, whole-nerve action potential (EAP) in users of the Ineraid cochlear implant. The method is an adaptation of one originally used by Charlet de Sauvage et al. [J. Acoust. Soc. Am. 73, 615-627 (1983)] in guinea pigs. The response, recorded from 11 subjects, consists of a single negative peak that occurs with a latency of approximately 0.4 ms. EAP input/output functions are steeply sloping and monotonic. Response amplitudes ranging up to 160 micro V have been recorded. Slope of the EAP input/output function correlates modestly (approximately 0.6-0.69) with results of tests measuring word recognition skills. The refractory properties of the auditory nerve were also assessed. Differences across subjects were found in the rate of recovery from the refractory state. These findings imply that there may be difference across subjects in the accuracy with which rapid temporal cues can be coded at the level of the auditory nerve. Reasonably strong correlations (approximately 0.74-0.85) have been found between the magnitude of the slope of these recovery curves and performance on tests of word recognition.

Neural responses to auditory temporal patterns

Sets of regularly repeating auditory stimuli elicit unique perceptions; listeners are able to identify specific temporal patterns. Some temporal patterns are unambiguous (only one pattern can be perceived), while others are ambiguous (numerous patterns can be detected). While the psychophysical properties of such percepts have been well studied, little is known about the underlying neurological bases of temporal pattern perception. In this experiment, the role of adaptation in temporal pattern perception is examined by studying neural responses in four cats to a temporal pattern that is perceptually unambiguous and one that is perceptually ambiguous. Measurements were made of the whole-nerve action potential, the auditory brainstem response, and potentials from the surface of the primary auditory cortex. The adaptation patterns corresponded with the perceptual organization of temporal patterns in humans at all levels of the nervous system studied.

Effects of acute lead acetate exposure on adult guinea pigs: Electrophysiological study of the inner ear

The effects on humans of lead acetate exposure may involve the cranial nerves, since vertigo and sensory neuronal deafness have been reported in lead workers; however, there exist only a few reports concerning the dose effects of lead acetate both on the cochlea and the eighth cranial nerve. The effects of lead acetate on the cochlea and the eighth nerve were investigated systematically using cochlear microphonics (CM), wholenerve action potential (AP), and endocochlear potential (EP) in guinea pigs (male albino Hartley). Guinea pigs were injected with 2 ml of a 1% solution of lead acexate (20 mg) once a week for 1–5 weeks. The threshold of whole-nerve AP (N 1) was elevated by injection of lead acetate, even 40 mg, and whole-nerve AP (N 1) output voltage decreased after injection of 100 mg of lead acetate. On the other hand, no change was observed in CM after lead acetate injection (100 mg) or in EP after lead acetate exposure (40 mg). The blood concentrations of lead acetate were as follows (mean): control, 4.5 μg/dl; Expt 1, 80 μg/dl; Expt 2, 126 μg/dl; Expt 3, 142 μg/dl. We conclude that dysfunction of the eighth nerve is induced by high-dose lead exposure, but that lead exposure does not induce electrophysiological dysfunction of the organ of Corti and the stria vascularis.

Recoveries of whole-nerve AP thresholds, amplitudes and tuning curves in gerbils following noise exposure

The recoveries of whole-nerve action potential (AP) thresholds, AP amplitudes and AP tuning curves in gerbils were monitored following two weeks of exposure to band-pass noise at 85 dBA. Recordings were made by means of electrodes chronically implanted in the subjects' bullas. The noise exposure caused threshold elevations in all of our subjects, with the greatest shifts occurring an octave or more above the 2 kHz upper cutoff frequency of the noise. The magnitudes of the shifts varied greatly (up to 26 dB) across subjects. Thresholds of animals with the smallest initial loss of sensitivity returned to pre-exposure values within 16 days, while those of animals with greater initial losses remained elevated beyond 16 days. AP amplitudes and AP tuning curves (APTCs) were most affected at frequencies where the initial threshold shifts were greatest. At these frequencies AP amplitudes were reduced, and APTCs showed broadened tips, reduced tip-to-tail ratios, and in some cases a shift in the frequency of the tip. These effects did not necessarily reverse with threshold recovery, suggesting that AP amplitudes and AP tuning curves are more sensitive indices of acoustic injury than are AP thresholds.

Threshold sensitivity and frequency selectivity measured with round window whole nerve action potentials in the awake, restrained chinchilla.

Chronic electrodes, placed on the round windows of four adult chinchillas, were used to measure whole-nerve action potential (AP) thresholds and tuning curves. The AP quiet thresholds were within 17 dB of thresholds measured by behavioral methods and within 3 dB of those measured by other evoked response procedures. The AP tuning curves, obtained by a simultaneous masking procedure, were also similar to previously measured tuning curves in the chinchilla. These results indicate that long-term indwelling electrodes can be successfully placed on the chinchilla round window and used to measure threshold sensitivity and frequency selectivity. Recordings from these electrodes could be used in a variety of situations and may be particularly useful in studying the effects of noise on the cochlea.

Effect of Signal Temporal Shaping on the Frequency Specificity of the Action Potential in Gerbils: Effet de la forme temporelle du signal sur la spécificité fréquentielle du potentiel global chez la gerboise

We examined the effects of temporal shaping of a tone burst on the frequency specificity of evoked responses in gerbils. Frequency specificity was assessed by means of whole-nerve action potential (AP) tuning curves (APTCs). The envelopes of the onset and offset portions of the AP-eliciting tone burst (or probe) were either a linear or nonlinear (1/4 sine) function of time. At probe frequencies of 1, 4 and 8 kHz, APTCs in the two envelope conditions were similar in shape and had equally narrow tips. This was true at both low (40-50 dB SPL) and high (70-75 dB SPL) probe levels. It suggests that tone bursts in the two envelope conditions evoke AP responses that are equally frequency specific. Despite these similarities, a notable difference in the two conditions was found at high probe levels. Specifically, raising the level of an 8-kHz probe from 40 to 70 dB SPL resulted in a downward shift of the APTC tip frequency, and this shift was greater for probes with linear on-off envelopes than for those with non...

A comparison of brainstem, whole-nerve AP and single-fiber tuning curves in the gerbil: Normative data

Tuning curves were obtained from brainstem responses (BSRs) and whole-nerve action potential (AP) responses of gerbils using a forward masking procedure. These are compared with single-fiber tuning curves of the gerbil. The broadness of the tips, tip-to-tail ratios and the slopes of the high frequency sides of the BSR tuning curves are similar to those of the AP curves for probe frequencies ranging from 1 to 8 kHz. Also, the BSR and AP curves share a number of characteristics with single-fiber tuning curves. These results suggest that, like the AP, components of the BSR can be used to measure frequency selectivity in the periphery of an intact auditory system.

Relationship of gross cochlear potentials to hair cell pathology in the waltzing guinea pig.

Electrophysiological recordings were made from the hearing organs of young waltzing guinea pigs of different ages. Age-related morphological changes in the cochleas of the same animals were studied with scanning and transmission electron microscopy. Results were compared with the results of studying the cochleas of normal guinea pigs of the same age. Waltzing guinea pigs are born with a hearing loss, as can be concluded from the magnitude of the whole-nerve action potential and their lower than normal summating potentials. However, their cochlear microphonics are nearly normal until the time in the degeneration process when hair cells start to disappear. This degeneration process begins at the tops of the hair cells and is evident upon electron microscopical examination of these structures.

Onset responses in single auditory-nerve fibers

The maximum response at the onset of a CF tone burst was obtained as a function of sound intensity level and prior adaptation. A stimulus rise time of 1 ms was used to maximize the onset response as well as to produce a simultaneously recorded whole-nerve action potential (AP). As intensity increased, the onset response increased and in some units reached a maximum value of 1 spike per stimulus presentation within a 1-ms interval. The absolute refractory period of the fiber prevented a further increase in the number of spikes. Nevertheless, further increases in intensity affected the onset response by reducing the latency to the onset spike and the variability in its time of occurrence. Consequently, the maximum response in spikes per second continued to increase when it was measured using a sufficiently small time interval, e.g., 55 μs. Prior adaptation reduced the magnitude and increased the latency and variability of the onset response. These effects could account for some of the concurrent changes in the AP.

Relation between discharges in auditory nerve fibers and the whole-nerve response shown by forward masking: an empirical model for the AP.

Whole-nerve action potentials evoked by a standard click were recorded from a gross electrode on the RW and the discharges of auditory nerve fibers to the same standard click were recorded from micropipette electrodes in the auditory nerve. The effect of a preceding tone burst (2, 4, or 8 kHz) upon the responses was measured for forward-masker intensities from 20 to 80 dB SPL. All forward maskers reduced the discharges of auditory nerve fibers to the standard click with the greatest reduction occurring for fibers with characteristic frequencies (CFs) near the masker frequency. The 4- and 8-kHz forward maskers produced similar effects on N1, P1, and N2 of the RW response. However, the 2-kHz forward masker produced enhancement at P1 and N2 in the RW response to the standard click. An empirical summation model of estimated elemental waveforms at latencies dependent on CF produces synthesized AP response waveforms which are similar to the observed RW response. The model permits the exclusion of discharges from segments along the cochlear partition in a manner similar to the effect of a forward masker. Alterations in the synthesized waveforms due to the exclusion of segments are similar in direction to the alterations observed in the RW responses for forward masking.

ABR measurements in the cat using a forward-masking paradigm.

Probe-elicited wave V amplitudes of the auditory brainstem response (ABR) were measured using a forward-masking paradigm. Subjects were anesthetized cats. For individual experiments, probe frequency and intensity were fixed and masker frequencies and intensities were varied. For each masker frequency, the extent to which the probe-elicited wave V amplitude was reduced by the preceding masker was plotted as a function of masker intensity. The rising segments of the masking functions were fitted with straight lines, using a least-squares procedure, to obtain estimates of their slopes. Masking grew most rapidly for masker frequencies below probe frequency, becoming progressively less steep as masker frequency increased. ABR tuning curves were constructed by using the linear fits to define the masker intensity that caused a 50% reduction in probe-elicited wave V amplitude. The shapes of these tuning curves were comparable to whole-nerve action potential (AP) tuning curves obtained under similar stimulus conditions. These results indicate that ABR amplitude measurements in a forward-masking paradigm can be used to estimate the growth of response to masking stimuli and frequency selectivity in a manner similar to AP amplitude measurements.

An empirical evaluation of the effects of high-pass noise on the whole-nerve action potential.

A number of methods are presented for evaluating the effect of high-pass noise on the whole-nerve action potential (AP). These methods include measurements of AP thresholds, amplitude-versus-level functions, decrement in AP amplitude-versus-masker level functions, and AP tuning curves. Examinations of threshold shifts as a function of tone-burst frequency and AP amplitude-versus-level with and without the presentation of high-pass noise indicate that basal portions of the cochlear partition can be masked effectively. Decrement in AP amplitude-versus-masker level functions and subsequently constructed AP tuning curves were used to verify that the presentation of high-pass noise did not alter the frequency response of that region of the basilar membrane responding to a 4000-Hz tone-burst probe. As a result, we conclude that high-pass noise may be used to mask the response from remote regions of the cochlea without altering response characteristics from lower frequency regions.

Some ECMR properties in relation to other signals from the auditory periphery

Some basic characteristics of monkey and human acoustic emissions are reviewed and some new data presented. These characteristics are discussed in relation to the sensorineural output of the cochlea. Input-output functions and the frequency dispersa in the waveform of acoustic emissions (AE) from monkeys are described. New findings include changes in the human AE during trains of clicks and sustained suppression of the AE after short bursts of noise of moderate intensity. The similarity of click-evoked AE latencies and whole-nerve action potential (AP) latencies for low stimulus level toneburst stimuli, and the apparent discrepancy between these latencies and those of single cochlear nerve-fibre action potentials and derived impulse responses is discussed. It is argued that atlow stimulus levels AE are generated either coincident with the primary cochlear impulse or very soon afterwards. It is proposed that the AE generator is peripheral to the afferent synapse of the inner hair cells because of the lack of adaptation effects in AE. However, attention is drawn to changes in the waveform of the click-evoked AE with increasing stimulus intensity and with diurectic intoxication, that qualitatively parallel known changes in single nerve-fibre firing properties. These observations are consistent with the concept that AE are a product of a sharply tuned and vulnerable cochlear filter mechanism.