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At least 577 records · Page 32Linked to original sources
PubMed1993

Is the endolymphatic K secretion electrogenic?

The endolymphatic potential is assumed to result from active K transport into the endolymphatic compartment and passive K diffusion in the opposite direction. However, in several in vivo experiments, changes in the endolymphatic potential differed from those in the endolymphatic K concentration. Moreover, in in vitro experiments, a negative endolymphatic potential was observed in the presence of ouabain without K gradient between the two compartments. These observations suggest that the coupling between the K transport and the genesis of the endolymphatic potential is not tight. Several factors may separately influence the endolymphatic potential and the K transport such as the acid-base equilibrium, the integrity of Reissner's membrane, the hormonal status, and the Na transport.

Animals↗
PubMed2000

A review of the effects of selective inner hair cell lesions on distortion product otoacoustic emissions, cochlear function and auditory evoked potentials.

Carboplatin, a second-generation antineoplastic drug, is much less ototoxic than cisplatin in humans and many laboratory animals. However, when a moderate dose of carboplatin is administered to chinchillas, it can selectively destroy inner hair cells (IHCs) and type-I ganglion neurons without damaging the outer hair cells (OHCs). One of the earliest signs of injury from carboplatin is damage to type I, spiral ganglion neurons. Selective destruction of IHCs has no effect on the cochlear microphonic (CM) potential and distortion product otoacoustic emissions (DPOAEs). However, very high doses of carboplatin can destroy both OHCs and IHCs resulting in a decline in CM and DPOAE amplitude. In cases where carboplatin partially destroys the IHCs, the auditory nerve fibers that contact the residual IHCs have normal thresholds and tuning, but their spontaneous and driven discharge rates are reduced. These results suggest that OHCs are responsible for the sharp tuning and exquisite sensitivity of the cochlea. IHC loss leads to a reduction in neural input (i.e., sensory deprivation) to the central auditory system. Surprisingly, although the neural input to the central auditory system is reduced, evoked response amplitudes recorded from the auditory cortex are often enhanced. These results indicate that when the neural input to the central auditory brain is reduced, the central auditory system compensates for the reduced input by increasing its gain.

Journal Article↗
PubMedDate not supplied

Instrumental perforation of the round window. Animal experiments using cochleography and ERA.

The round window membrane of the inner ear of the guinea pig was perforated under Ketanest anaesthesia. A very rapid and almost total loss of cochlear microphonics, auditory nerve action potential and brain-stem response developed. Replacement of the perilymph with Ringer's solution and surgical closure of the membrane damage had no effect on auditory nerve action potential or brain-stem response. Further decline in the amplitude of cochlear microphonics was halted, however. When perfusion of the round window membrane was carried out whilst the bulla was full with Ringer's solution, no essential decline was seen in the amplitudes of microphonics and auditory nerve action. Only brain-stem response was reduced--temporarily by 40% of the original amplitudinal level--but this showed subsequent recovery. Preservation of cochlear microphonics and auditory nerve action after perforation in Ringer's solution, i.e. excluding air, indicates that air entering the scala tympani is the cause of the sudden hearing loss after round window perforation. Using a simple physical model, an attempt is made to demonstrate this process. The anatomic connections between the round window, cochlear aqueduct and scala tympani are discussed, as are the different ways in which the round window membrane can be perforated.

Action Potentials↗
PubMed2003

In vivo characterization of piezoelectric transducers for implantable hearing AIDS.

BACKGROUND: Piezoelectric bimorph transducers may be used at the input stage of implantable hearing aids to convert ossicle vibrations into electrical waveforms, and at the output stage to convert electrical signals into mechanical motion that drives the ossicles. This study assessed transducer performance in anesthetized, acutely implanted cats using computer-averaged, laser-Doppler vibrometer measures and cochlear potentials. METHODS: Measures of output linearity and distortion for a transducer placed on the umbo were obtained from averaged laser-Doppler vibrometer outputs. Frequency response and equivalent sound pressure level for transducers placed against the stapes were estimated by comparing compound action potentials and cochlear microphonics elicited preoperatively by acoustic signals with responses elicited postoperatively by signals presented through transducers. RESULTS: The transducer placed on the umbo exhibited an effective bandwidth that exceeded 8 kHz, linear response behavior for driving voltages up to 2 Vrms, and harmonic distortion of -40 dB or better at all frequencies greater than 250 Hz. Except for a shorter latency, transducer-elicited cochlear potentials were indistinguishable from acoustically elicited responses. Frequency response varied widely across transducers, ranging from reasonably flat to possessing a bandpass characteristic with a peak at 2 to 4 kHz; 1-Vrms signals applied to transducers with various geometries yielded equivalent intensities of 62 to 108 dB sound pressure level at 4 kHz, 51 to 98 dB sound pressure level at 2 kHz, and 55 to 80 dB sound pressure level at 1 kHz. Differences in frequency response and equivalent sound pressure level stemmed from different resonance frequencies in transducers with dissimilar lengths and, more importantly, from variation in transducer-stapes contact force. CONCLUSIONS: Appropriately designed piezoelectric transducers can provide the cochlea with high-fidelity, wide-bandwidth signals. However, using them in implantable hearing aids requires that geometry and contact force be optimized to reduce variability in output level. Recording cochlear potentials is a cost-effective means of assessing transducer performance intraoperatively, but care must be exercised to take into account any temporary, drill-induced sensitivity loss.

Action Potentials↗
PubMed1990

Distortion product emissions in humans. III. Influence of sensorineural hearing loss.

The realization that otoacoustic emissions are sensitive to cochlear disorders has resulted in the speculation that they may have considerable clinical potential as objective measures of hearing. To assess the clinical utility of one type of emission, the distortion product emission (DPE), a study was undertaken in individuals with hearing impairments representing a number of common otologic disorders. The results of this investigation provided evidence that tests of DPEs promise to satisfy a number of requirements important to clinical testing, including objectivity of measurement procedures, test-retest reliability, simple subject preparation, readily available instrumentation, and relatively brief examination periods. The fine resolution of DPEs within the stimulus frequency and level domains also permits an accurate confirmation of the pattern of hearing loss. For example, tests of DPEs detected a 20-dB hearing level impairment at a single frequency in an ear exhibiting early signs of noise-induced hearing loss, and a 10-dB improvement in sensitivity following ingestion of the hyperosmotic agent glycerol in an ear displaying a mild to moderate hearing loss due to Meniere's disease. Finally, the application of DPEs to the objective testing of otologic disorders suggests that the ability of these responses to assess the sensory component of a sensorineural disorder may contribute to the eventual understanding of the complicated pathogenesis of many cochlear diseases. When all the positive features of DPE testing are realized, the potential contribution that these measures can make in a clinical setting becomes apparent.

Adult↗
PubMed1987

DC potentials of the lateral wall of the scala media.

We recorded DC potential and resistance changes of the cochlear lateral wall in guinea pigs, using (3M KCl) electrolyte glass microelectrodes. The patterns found for slow penetration showed different potential and resistance characteristics at different segments of the lateral wall. In addition, our findings demonstrate that the monitoring of microelectrode tip resistance is a useful procedure for DC cochlear recordings.

Animals↗
PubMed1990

Early alterations of cochlear function in experimental perilymph fistulas.

Alterations of cochlear function secondary to perforation of the round window membrane in guinea pigs were expected to reveal sensitive and/or specific audiometric parameters in perilymph fistulas. A perforation of the round window membrane either did not affect acoustic compound action potential thresholds, or induced a delayed, possibly conductive, inner ear hearing loss. Also, evoked suprathreshold responses deteriorated progressively and independently of the increase of compound action potential thresholds. A sudden (conductive) hearing loss and a poor response to suprathreshold stimulation might indicate inner ear damage secondary to a perilymphatic fistula.

Animals↗
PubMed1996

The fine structure of spiral ligament cells relates to ion return to the stria and varies with place-frequency.

Ultrastructural analysis of cells in the cochlea's lateral wall was undertaken to investigate morophologic features relevant to the route of K+ cycling from organ of Corti (OC) to stria vascularis (StV) and to the question of a transcellular versus an extracellular path. The fine structure of outer sulcus cells (OSCs) evidenced their capacity for uptake of K+ from Claudius cells and from perilymph in inferior spiral ligament. Plasmalemmal amplification and mitochondrial density together with known content of Na,K-ATPase testified to activity of type II, IV and V fibrocytes in resorbing K+. Location and fine structure afforded a basis for distinguishing subtypes among the type I, II and IV cells. The type II, IV and V fibrocytes can be viewed as drawing K+ from surrounding perilymph and from OSCs and generating an intracellular downhill diffusion gradient for K+ flow through gap junctions to subtype Ib and Ia fibrocytes and strial basal cells. Pumping action enabled by extreme structural specialization of type II fibrocytes is considered to mediate K+ translocation across the interruption between the gap junction connected epithelial and gap junction connected fibrocyte systems and to explain ion flow directed toward StV through OSCs and fibrocytes despite their lack of polarity. The OSC bodies shrank, their root bundles expanded and the gap junction contact between OSCs and Claudius cells increased toward the base of the cochlea. Expanding root bundles and type I and IIb fibrocyte populations contrasted with shrinking OHCs and Deiters and tectal cells from the apex to the base of the cochlea. These differences indicated an increased magnitude and alternate route of K+ transport toward the StV in high as compared to low-frequency regions. The augmented K+ transport through spiral ligament in basal cochlea correlates with and provides a possible basis for the larger endocochlear potential in the base. The findings appear consistent with current flow extracellularly through scalae tympani and vestibuli and transcellularly through OC, OSCs and class I, II, IV and V fibrocytes.

Acoustic Stimulation↗
PubMed1989

Electrically stimulated increases in cochlear blood flow: II. Evidence of neural mediation.

In a companion paper, we reported that electrical stimulation increased cochlear blood flow (CBF). This response was found to be an increasing function of current intensity and was frequency-selective, with the best response at approximately 500 Hz continuous sinusoidal current. The present investigation seeks to discover the mechanism of this effect. Direct measurement of cochlear temperature during electrical stimulation revealed no evidence of local heating. Autonomic neuronal activation is not likely, as neither atropine, hexamethonium, nor propranolol abolished the evoked CBF response. Strial activity could be suppressed by the use of furosemide, but the evoked CBF response persisted. Inactivation of auditory afferent neurons with kainic acid also did not change the evoked CBF response. Dimethyl sulfoxide, a potent oxygen-free radical scavenger, did suppress the evoked CBF response to a small but significant degree. This suggests that oxygen-free radicals may be produced within the cochlea during electrical stimulation. Finally, the evoked CBF response was completely suppressed by procaine and tetrodotoxin, with recovery of evoked CBF response accompanying recovery of cochlear action potentials. These data indicate that stimulation of neural fibers, distinct from autonomic and auditory afferent neurons, may modulate CBF.

Animals↗
PubMed1990

[Clinical and electrophysiological investigations of the influence of phenytoin natrium on tinnitus].

Anticonvulsant drugs like Carbamazepine and Phenytoin natrium (PHT) have been used for suppressing the tinnitus. In this paper, a clinical experience was reported, in that 100 patients with tinnitus were treated with intravenous administration of PHT, and the influence of PHT on auditory systems in the guinea pig investigated in detail electrophysiological examinations was also reported. The results are summarised as follows: 1. Clinical evaluation in the treatment of tinnitus (1) Of the 100 patients, more than 70% were well responded. (2) Pith match, loudness balance test correlated closely with the suppression of tinnitus. (3) Side effects were found in 5%, but there were all mild enough to be well tolerated. 2. Experimental evaluation in the guinea pig (1) In intravenous administration of PHT, latency and amplitude of each waveform peak in ABR were prolonged and decreased. As for the latency, waves III and IV were most prominently affected. The results may indicated that PHT works in the upper brain stem predominantly and effects also to the cochlear nucleus and cochlear nerve. (2) The amplitude of N1 in AP was depressed and its latency was slightly prolonged. These phenomena related closely with change of wave I in ABR. This may indicate that PHT can directly suppress the cochlear nerve. (3) Even at the dose that apparently influenced ABR and N1 in AP, no obvious change was observed on amplitudes in CM and EP, that may indicate PHT cannot affect on hair cells at this dose or it cannot proceed to the cochlea. (4) These changes were considered to be direct effect of PHT on auditory systems, since they were not correlated with change in the circulatory system. (5) In the perilymphatic perfusion of PHT, amplitudes in EP and CM were depressed slightly at the beginning of the perfusion. Changes in EP preceded those in CM. The results may indicate that PHT effects directly on the stria vascularis and secondarily on CM. (6) In this perfusion, no obvious change was found in AP. Nevertheless, the amplitude of N1 in AP increased over the initial level after the perfusion corresponding to the recovery of EP in some animals. The results may indicate that PHT cannot work directly on hair cells nor cochlear nerve and that the suppression of efferent inhibitory neurons can increase AP potentials. (7) PHT works on the large part of auditory pathway without cochlea, as a role of inhibitor. Considering antitinnitic effect of PHT, tinnitus bases on unusual excitement of the auditory pathway.

Adult↗
PubMedDate not supplied

[A model for the study of the ototoxic effects of drugs in an experiment].

The authors compared sensitivity of the acoustic analyzer elements to ototoxic drugs in cat experiments. The model proposed assesses primary responses of the brain cortical acoustic zone (BCAZ) to sound stimulation. The drugs effect on the analyzer's periphery was determined from changes in bioelectric responses of the cochlea to sound stimulation (microphone potential, potential of the acoustic nerve action). The arguments are presented for feasibility of using primary BCAZ responses evoked by electric stimulation of thalamocortical radiation fibers coming from the medial geniculate body for examination of the drug effects on the analyzer's cortex. Parallel registration of the above bioelectrical reactions in the same animal for a long time makes it possible to compare sensitivity of different elements of the acoustic analyzer to ototoxic medication.

Acoustic Stimulation↗
PubMedDate not supplied

Effects of topical sodium potassium ATPase inhibitors upon endocochlear potential in chinchilla.

Sodium orthovanadate (vanadate) is a recently discovered inhibitor of the enzyme sodium potassium adenosine triphosphatase, which has properties similar to ouabain in peripheral tissues. The effects of topical application of vanadate and ouabain to the round window membrane of the chinchilla were studied using endocochlear potential (EP) and action potential as measures of cochlear function. The EP demonstrated an initial elevation followed by a secondary decline after topical application of 2 mmol vanadate to the round window membrane. During the EP elevation, the resistance was unchanged, but the action potential response declined, with a greater effect on responses to 8-kHz tone bursts than to 2-kHz tone bursts. Thus, it does not appear that the initial increase of endocochlear potential that was observed was due to a change in the resistance of the cochlear duct; rather, some other mechanism must be involved. The round window topical application of potentially ototoxic drugs accompanied by cochlear function monitoring appears to be a useful method for testing drugs with marked systemic effects such as ouabain or vanadate and is simpler to perform than perilymphatic or vascular perfusion.

Administration, Topical↗
PubMed2004

Identification of the impedance model of an implanted cochlear prosthesis from intracochlear potential measurements.

Those suffering from a severe to profound sensorineural hearing loss can obtain substantial benefit from a cochlear implant prosthesis. An electrode array implanted in the inner ear stimulates auditory nerve fibers by direct injection of electrical current. A major limitation of today's technology is the imprecise control of intracochlear current flow, particularly the relatively wide spread of neural excitation. A better understanding of the intracochlear electrical fields is, therefore, required. This paper analyzes the structure of intracochlear potential measurements in relation to both the subject's anatomy and to the properties of the electrode array. An electrically equivalent network is proposed, composed of small lumped circuits for the interface impedance and for the cochlear tissues. The numerical methods required to estimate the model parameters from high-quality electrical potential recordings are developed. Finally, some models are presented for subjects wearing a Clarion CII device with a HiFocus electrode and discussed in terms of model reliability.

Cochlea↗
PubMed1996

Effects of aging on potassium homeostasis and the endocochlear potential in the gerbil cochlea.

Previous work has shown that the endocochlear potential (EP) decreases with age in the gerbil. Concomitant with the EP decrease is an age-related loss of activity of Na,K-ATPase in the lateral wall and stria vascularis. We hypothesized that the EP decrease is associated with a similar decrease in the endolymphatic potassium concentration [Ke+]. This hypothesis was tested using double-barrelled, K(+)-selective electrodes introduced into scala media through the round window in young and quiet-aged gerbils. Results show that the means (+/- S.D.) of the [Ke+] in young and aged gerbils were not significantly different (178.2 +/- 14.2 mM and 171.2 +/- 34.4 mM, respectively), although the intersubject variability was much greater in the aged animals than in the young. These values of [Ke+] are slightly higher than those found for other mammals and may reflect the higher plasma osmolarity found in the gerbil. The concentration of perilymphatic potassium [Kp+] in scala tympani at the round window was also similar for the young and aged groups (3.57 +/- 1.17 mM and 4.18 +/- 2.03 mM, respectively). On the other hand, mean EP values in the young and aged gerbils were 92.0 +/- 5.7 mV and 64.8 +/- 15.8 mV, respectively and were statistically different (P < 0.001). Overall, EP and [Ke+] showed little correlation (R2 = 0.23), except that when [Ke+] fell below 150 mM, the EP was always less than 60 mV. An analysis of the chemical potential for Ke+ with respect to Kp+ shows that it was similar for young and aged gerbils (overall mean of 103.1 +/- 13.7 mV) and remained constant with respect to the EP, in spite of an overall electrochemical potential of Ke+ that varied from 120 to 210 mV. Thus, the system maintains Ke+ homeostasis at the expense of the EP, even when the EP is on the verge of collapse.

Acoustic Stimulation↗
PubMed1986

Use of the neodymium/yttrium aluminum garnet laser in middle ear surgery: a preliminary report.

Clinical usefulness of the neodymium/yttrium aluminum garnet (Nd:YAG) laser in surgery of the middle ear is unproven. An animal study was designed to examine the influence of Nd:YAG laser irradiation on cochlear potentials following vaporization of the stapes. Electrocochleagraphic changes were observed in two of the five animals studied, and caution is advised before using this instrument in humans.

Animals↗
PubMed1995

Comparison of ion transport mechanisms between vestibular dark cells and strial marginal cells.

Morphologic similarities between strial marginal cells and vestibular dark cells have long been recognized and it has long been accepted that both of these cell types are involved in the secretion of K+ into endolymph. Functional similarities of these two epithelia however, were considered unlikely as long as strial marginal cells were assumed to generate the endocochlear potential which has no equivalent in the vestibular labyrinth. The recently introduced concept that strial marginal cells transport K+ but that the mechanism for the generation of the endocochlear potential is located in another cell type provided the basis to hypothesize that ion transport mechanisms and their regulation are similar in vestibular dark and strial marginal cells. The present review compiles evidence in support of this hypothesis.

Adenosine Triphosphate↗
PubMedDate not supplied

Effect of glycerol on furosemide-caused EP decrease in guinea pig neonates.

Furosemide, a loop diuretic, reduces the endocochlear potential in guinea pigs. In our previous experiment using adult guinea pigs, glycerol was thought to open the blood-cochlea barrier and to help furosemide to reach its site of action in the stria vascularis. The present study was designed to determine whether or not the same potentiation by glycerol occurs in neonate guinea pigs. In one group, furosemide (10-30 mg/kg) was administered, while in another group, 50 v/v% glycerol (1 ml/kg) was administered 15 minutes prior to the injection of furosemide. Furosemide reduced the EP significantly more in neonates than in adults. This effect was not potentiated in the glycerol-pretreated group. From these findings, it seemed conceivable that the neonate blood-cochlea barrier still remains immature in some functions.

Animals↗
PubMed1981

[The registration of brain stem evoked responses and middle latency with air and bone conduction stimuli in the evaluation of hearing disorders (author's transl)].

The theory and practice to record the brain stem response and middle latency have been described. Normative data and sources of error have been presented. The early evoked potentials are a powerful tool, to determine the degree of hearing loss in infants and to detect possible brain stem lesions. This exact diagnosis is an assumption for a successful rehabilitation.

Acoustic Impedance Tests↗