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PubMed1992

Modulation of cochlear nerve spike rate by cardiac activity in the gerbil.

Among primary auditory axons with characteristic frequencies (CFs) below 2500 Hz, a substantial subpopulation was found in which spike activity was driven by cardiac events. The presence of cardiac-driven activity was inferred from cycle histograms triggered on the peak of the electrocardiogram (ECG). This driven activity was either like a simple onset response (often followed by a reduction of spike activity to below background level), or as a longer lasting series of peaks and troughs. In two axons with high CFs (7 kHz and 12.5 kHz), cardiac-driven suppression was observed. Recordings made by a probe microphone revealed the presence of heart-related sound in the external ear canal. The onset of that sound coincided with the onset of cardiac-driven spike activity (and suppression).

Action Potentials↗
PubMed1991

Enhancement of the cochlear nerve compound action potential: sharply defined frequency-intensity domains bordering the tuning curve.

Forward masking can either decrease or increase the response to a subsequent stimulus. In the gerbil, frequency-intensity domains of the maskers that decrease the amplitude of the compound action potential (CAP) can be plotted as the sharply defined CAP tuning curve (TC). Regions were also found over which masking increases (enhances) the amplitude of the CAP. Center-frequency (CF) enhancement domains were found in approximately 2/3 of the animals tested, in response to maskers having frequencies very near that of the probe stimulus, at levels ranging from below the CAP detection threshold to just below the tip threshold of the TC. Approximately 2/3 of the animals showing CF enhancement also displayed low-frequency (LF) enhancement, in response to a domain which borders the low-frequency tail of the TC.

Acoustic Stimulation↗
PubMed1985

Origin of latency shift of cochlear nerve potentials with sound intensity.

Compound action potentials were recorded from the round window in anesthetized rats in response to tonebursts and to continuous tones. The responses to tonebursts were compared to cross-correlograms of the responses to continuous tones that were amplitude modulated by pseudorandom noise. The cross-correlograms were similar in shape to the responses to tonebursts, but the latencies of the two cross-correlogram peaks decreased less when the sound intensity was increased from near threshold values than did the latencies of the peaks in the responses to tonebursts. When an unmodulated tone was added to these stimuli, the latencies of peaks in the response to tonebursts increased as the intensity of the unmodulated tone was increased. However, the effect of an unmodulated tone on the latencies of the peaks in the cross-correlograms was more complex: when the frequency of the unmodulated tone was below that of the modulated tone, there was a decrease in the latency of the peaks in cross-correlograms. This decrease in latency was a function of the intensity of the unmodulated tone, and was similar to the decrease in latency seen when the intensity of a single modulated tone is increased. These results are interpreted as supporting the findings of earlier research, which showed that the cross-correlogram of the unit response to amplitude-modulated tones has a latency that is nearly independent of the stimulus intensity. In addition, the data indicate that the decrease in the latency of the peaks in the cross-correlograms of the gross response from the round window is a result of nonlinearity of the cochlear frequency analyzer and is not directly related to neural excitation in the cochlea.

Animals↗
PubMed1990

Latency and amplitude tuning curves of the N1 and N2 components of the cochlear nerve compound action potential.

Compound action potential tuning curves (CAP TCs) generated by masking the N1 component of the CAP provide a means of assessing the ability of the cochlea to selectively tune to certain stimuli. This paper examines some of the factors which can influence this TC when a moderately intense (i.e. 40-80 dB SPL) probe stimulus is used. At these levels, each of the four corners of the trapezoidal stimulus envelope is capable of generating a CAP. Also, short stimulus rise times can merge the CAPs produced by the first two corners, but this does not appear to have a major effect on the CAP TC. It was shown that the N2 component of the CAP for the first corner of the stimulus is equally capable of producing a well-tuned TC. Another study has shown that, in addition to amplitude decrements, one can use latency increases as a criterion for CAP TCs. We have demonstrated that latency TCs are more finely tuned than amplitude TCs at high levels, especially when the stimulus rise time is short.

Animals↗
PubMed1989

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

A unique property of sensory coding in the vertebrate auditory system is the existence of the classical form of excitatory centre-inhibitory surround in relative spike rate along the stimulus frequency dimension, in addition to a representation of temporal fine structure of high frequency periodic stimuli in the discharge pattern of primary afferent spike trains. We present a model which designates three factors that influence rate and temporal synchrony in spike responses; an excitatory factor, a suppressive factor and a synchronizing factor. The model proposes that an essential integration of bioelectric signals occurs in the primary afferent fibre. It is presumed that mean spike rate depends on mean level of membrane depolarization and synchronization depends on periodic modulation of membrane potential at the spike initiating zone. In the model, the excitatory factor is synaptically-mediated, excitatory post-synaptic potential (e.p.s.p.); the suppressive factor is negative DC polarization of the fibre membrane and the synchronizing factor is AC modulation of the fibre membrane potential. It is proposed that both the negatively-polarizing and high-frequency modulating signals are derived from extracellular current flow in the cochlea.

Action Potentials↗
PubMed1975

[Mauration of unitary responses to tonal stimulation in the cochlear nerve of the kitten].

The maturation of single auditory nerve fiber responses to long-duration (500 ms) tone-bursts was studied in kittens at various stages after birth. Spike discharges were examined as a function of three criteria. 10 Latency. Mean value of the "on" peak latency was about 25 ms at 1 to 3 days after birth. It then regressed, reaching 8 to 12 ms at the end of the first week, and 2 to 4 ms at 20 days. 20 Peristimulus histograms (PSTH). Evolution of PSTH revealed the characteristic sequence of unit reactivity to long duration stimuli (i.e. on, rhythmic, and continuous responses). Rhythmic responses is assumed up to now to be related to the immaturity of synaptic junctions below the hair cells. 30 interval histograms. The interspike interval histograms of the discharges showed a similar evolution. A bimodal distribution corresponding to the rhythmic mode of reactivity, appeared at the end of the first week.

Acoustic Stimulation↗
PubMed1992

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.

Acoustic Stimulation↗
PubMed1997

Transmitter release in inner hair cell synapses: a model analysis of spontaneous and driven rate properties of cochlear nerve fibres.

The inner hair cell (IHC) synapse is one of the stages of cochlear processing that determine the relation between sound pressure level and spike rate in auditory nerve fibres. Transmitter released in the non-stimulated condition is held responsible for the wide range of spontaneous spike rates (SR) observed in these fibres. Properties of stimulated spike activity in auditory nerve fibres, including rate threshold and operating range of a fibre, are known to systematically vary with SR. This paper presents a model analysis of the relation between IHC transmembrane potential and transmitter release rate as becoming manifest in these spontaneous and driven rate properties. A previously developed computational model is used to identify those transfer properties of its synapse section which lead to reproduction of the variation of rate thresholds, shapes of rate-intensity functions and maximal driven rate with SR known from the literature. First a simple additive release model, in which driven transmitter release depends linearly on IHC potential, is elaborated. Its results lead to the hypothesis that the true release function is non-linear and variable across synapses generating different SR. An exponential release function is then introduced, with parameters varying across SR in a physiologically dictated way. This approach leads to adequate reproduction of the variation in rate thresholds and rate-intensity functions with SR. Finally, the model is applied in an inverse way to directly estimate the release function from given rate-intensity functions. The conclusion of both forward and inverse model analyses is that transmitter release is a non-linear function of IHC potential which, by the systematic variation of its parameters across SR, effectively leads to the physiological variation in dynamic range across fibres of different SR. Possible relations of these results with ultrastructural morphology and basic physiology of IHC synapses are discussed.

Acoustic Stimulation↗
PubMed1988

Vascular decompression of the cochlear nerve in tinnitus sufferers.

An estimated 40 million Americans suffer from tinnitus, and approximately 20% of these sufferers feel that the quality of their life is significantly impaired by this symptom. Despite thorough evaluation, the underlying etiology in the majority of these patients remains obscure or conjectural. Most of these patients will, however, benefit from consultation and avoidance of caffeine, nicotine, and salt, while others require biofeedback, amplification, masking, and even psychotherapy. On rare occasions, physicians are presented with a patient complaining of unilateral tinnitus of undetermined etiology who, in spite of a thorough evaluation and all conventional therapies, continues to be severely handicapped by that symptom. Early findings suggest that these patients may be suffering from vascular loop compression of the cochlear division of the eighth cranial nerve. When patients are carefully selected, retrosigmoid decompression of that vascular loop has provided gratifying relief.

Adult↗
PubMed2001

Is there a functioning vestibulocochlear nerve? Cochlear implantation in a child with symmetrical auditory findings but asymmetric imaging.

The finding of an abnormally narrow internal auditory meatus during the assessment of a child for cochlear implantation raises the possibility that the meatus may not contain the normal number of nerves. Even with currently available MRI techniques it may be extremely difficult to decide whether or not to offer cochlear implant in such cases. We present a child of 4 1/2 years, assessed for cochlear implantation. MRI and CT imaging suggested aplasia of one vestibulocochlear nerve and hypoplasia of the other. However, audiological tests showed clear responses to sound in both ears and functional use of sound in her daily life. This child was eventually implanted with encouraging postoperative results.

Audiometry↗
PubMed1995

Patterns of degeneration in the human cochlear nerve.

The patterns of neural degeneration of the spiral ganglion were studied in 12 human pathologic specimens and 2 normal neonatal specimens. Morphometric analysis of spiral ganglion cells included the maximum cross-sectional areas of both large (type 1) and small (type II) spiral ganglion cells. The organ of Corti in segments corresponding to the spiral ganglion, was evaluated for the presence or absence of inner (IHC) and outer (OHC) hair cells and supporting cells. The relationship between degeneration of spiral ganglion cells and degeneration in the organ of Corti, the age, sex, duration of deafness, cochlear location and delay between death and fixation was evaluated statistically. Both primary and secondary degeneration of the spiral ganglion were more severe in the basal than apical half of the cochlea. Degeneration of the spiral ganglion was most severe when both IHCs and OHCs were absent in the organ of Corti. No survival advantage was identified for type II ganglion cells as has been previously reported. That is, there was no correlation between the degree of degeneration of the spiral ganglion and the prevalence of type II ganglion cells. In fact, there was more severe degeneration of type II cells when the corresponding organ of Corti was severely degenerated. These findings in the human were compared with animal models of degeneration of the spiral ganglion, and the implications for cochlear implantation were discussed.

Aged↗
PubMedDate not supplied

Factors inducing retrograde degeneration of the cochlear nerve.

The process of retrograde secondary degeneration is described and its mechanism, discussed. The extent of degeneration following transection of the central or peripheral axon and following various types of damage to the organ of Corti, including the time course of degeneration, is presented in animal experimentation and human temporal bones. Of greatest practical importance is secondary neuronal degeneration induced by alteration in the organ of Corti. The effect of damage to the outer hair cells, inner hair cells, supporting structures in the Corti, and nerve endings or peripheral dendrites is analyzed and related to different types of inner ear disease.

Acoustic Stimulation↗
PubMed2005

[Disabling positional vertigo (DPV): syndrome of vestibulo-cochlear organ impairment during vascular compression of the vestibulo-cochlear nerve (VCS)].

In 1984 Jannetta et al. introduced a new term-disabling positional vertigo (DPV). DPV is a term used to describe syndrome of cochleo-vestibular organ impairment during vascular compression syndrome of eight cranial nerve. They introduced this term to distinct this syndrome from other established vertigo syndromes on the basis of recognized in angio - MRI exam compression of VIII nerve by vessel and clinical and electrophysiological criteria. Existence of DPV syndrome is still not universally accepted, because it is difficult to diagnose vertigo as vascular compression syndrome of the eight cranial nerve when there are no specific finding to detect this syndrome. Only Moller proposed specific ABR abnormalities as a criterion in DPV diagnosis (prolongation of I - III interval). The authors performed retrospective analysis of 28 patients (16 female, 12 men, average age 43) with recognized on basis angio - MRI vascular compression syndrome of eight cranial nerve. Contrasted magnetic resonance imaging identified a vascular loop near to cochleo - vestibular nerve in all 28 cases. All patients were performed pure tone audiometry, DPOAE, ABR and ENG exam. The most common symptoms were unilateral tinnitus (89% cases), unilateral hearing loss (86%) and dizziness (61%). The most frequent abnormalities in above mentioned exams were sensorineural hearing loss in pure tone audiometry (92%). ABR data were interpreted with respect to Mollerís criteria and asymmetry of the I - III, III - V and I - V interval and prolongation of V waves in the auditory brainstem response was found in 36% cases. Abnormal changes in electronystagmography were found: absence (10%) or weakness (36%) of caloric response. The differential diagnosis of DPV syndrome are discussed. We could not find any specific clinical findings valuable for DPV diagnosis. There is no significantly more weakness or absence of caloric response of vestibular organ in patients with DPV. Disabling positional vertigo is the syndrome which should be considered in differential diagnosis in every case of vertigo.

Adult↗
PubMed1993

Restoration of useful hearing after microvascular decompression of the cochlear nerve.

This report describes a patient with sudden sensorineural hearing loss who was found to have a megadolichoectasia vertebrobasilar system that appeared to be causing compression of the ipsilateral facial and vestibulocochlear nerves. The patient was treated conservatively for 4 months, during which time no hearing returned. He then underwent microvascular decompression of the affected nerves. At surgery, marked compression of the cranial nerves VII-VIII complex and the pons was observed. Postoperatively, the patient experienced a gradual return of useful hearing. We suggest that vascular compression may be a rare, but treatable, cause of sensorineural hearing loss.

Audiometry, Pure-Tone↗
PubMedDate not supplied

Tinnitus: diagnosis and treatment.

Few conditions are seen as commonly by the otologist and are more poorly understood than subjective tinnitus. Tinnitus has been reported in as high as 80% of patients seen in an otolaryngology practice. This symptom is especially marked in patients with a hearing problem and can be so severe that it becomes incapacitating. Careful diagnosis and classification of tinnitus is important for understanding of the problem. Identification of the frequency and intensity of masking, using a tinnitus analyzer, is useful in selecting the form of treatment. Analysis of the history, physical findings and the use of special electrocochleography and brain stem evoked response audiometry help to identify the site of lesion, which may be within the cochlea, cochlear nerve, cochlear nucleus, brain stem, midbrain or auditory cortex. Specific disease entities should be identified and treated. Lesions of the end-organ or cochlear nerve can be treated when necessary by translabyrinthine or middle cranial fossa section of the cochlear nerve. Tinnitus from cervical nerve lesions can be treated by rhizotomy. The use of a hearing aid or introduction of a sound with a tinnitus masker has been found to be 82% effective in suppressing tinnitus. Maskers can be combined with a hearing aid in some cases. The pathogenesis of tinnitus is discussed, but the method of action of tinnitus relief by auditory stimulation is still unclear. A thoughtful and complete examination with our new diagnostic tools and the judicious selection of therapy now makes it possible to give relief to the majority of patients suffering with disturbing tinnitus.

Adult↗
PubMed1977

[Damage of the cochlea and of the cochlear nucleus after the application of aminoglycosid antibiotics--a comparative light-, transmission- and scanning electronmicroscopic study in the guinea pig (author's transl)].

Ototoxic alterations were studied in the guinea pig organ of Corti and in the cochlear nucleus after high administrations of aminoglycosid antibiotics (Gentamicin/Tobramycin, 150 mg/kg body weight/day or Amikacin 300 mg/kg body weight for 10 days). After survival times up to 22 days the animals were examined. By means of morphology degeneration was found in the cochlear nucleus before we could state it in the organ of Corti. After longer survival times, when we observed heavy degeneration in the organ of Corti we found only a few myelin figures of axons in the cochlear nerve. Cochlear nucleus: As an early sign of degeneration alterations in the mitochondria were found. Compared with the organ of Corti the damage was less widespread in the second order neurons of the afferent auditory pathways, their dendrites or nerve endings. Quantitative calculations however, were not performed. In axosomatic synapses we could observe a way of degeneration which was not reported before in damage caused by aminoglycosid antibiotics. Free postsynaptic densities were observed to be invaginated into the neuron as a probable way of sequestration. As well we could observe reoccupation of the synaptic sites by a probable sliding of neighbouring nerve endings into the free intracellular space. (Similar observations were reported first by Gentschev and Sotelo [1973] after ablation of the cochlear nerve in rats.

Aminoglycosides↗