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Results for “COCHLEAR NERVE”

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At least 19 recordsLinked to original sources
PubMed1982

Intensity coding in the auditory periphery of the cat: responses of cochlear nerve and cochlear nucleus neurons to signals in the presence of bandstop masking noise.

The dynamic range over which fine intensity discrimination is possible has been reported to be largely unaffected by limitation of the spread of neuronal activity to neighbouring frequency regions by bandstop noise masking. We have therefore examined the responses of cochlear nerve and nucleus neurons to tone and noise signals in the presence of a bandstop masking noise designed to be comparable to that employed in the psychophysical experiments. Under these conditions, the vast majority of cochlear nerve fibres were saturated by sound levels at which some 50% of our sample of cochlear nucleus neurons still responded to signal level differences. The extended dynamic ranges of these cochlear nucleus neurons was shown to be a result of activation, by the masking noise, of the lateral inhibitory side-bands 'biassing' the neuron's discharge. A small proportion of cochlear fibres, having low spontaneous discharge rates and showing strong two-tone suppression effects, demonstrated analogous but not so pronounced effects. It is unclear in what form information on the level of stimuli under these conditions is transmitted by the majority of apparently saturated cochlear nerve fibres, but several possible mechanisms are discussed.

Animals↗
PubMed1984

Noise masking of tone responses and critical ratios in single units of the mouse cochlear nerve and cochlear nucleus.

Responses of single units in the cochlear nerve and cochlear nucleus to tone bursts in a background of continuous white broadband noise were recorded. Tone and noise intensities ranged from threshold to saturation levels. Masking of the tone response by the noise was demonstrated by comparing peristimulus-time histograms and spike rates recorded during the tone and between tone presentations. The response of a unit to masking was found to be predictable based upon the difference in its rate of response to the tone and to the noise when the tone was masked. Several nonlinearities of the masking process are described. The most prominent one is an increase in the difference between tone and noise levels at the threshold of masking with increasing tone levels, i.e. neural critical ratios increase with increasing tone level. On the average, the frequency dependence of single unit effective bandwidths and of critical ratio bandwidths is similar to that of mean behavioral critical ratio bands.

Action Potentials↗
PubMed1998

Long-term degeneration in the cochlear nerve and cochlear nucleus of the adult chinchilla following acoustic overstimulation.

Adult chinchillas were exposed once to an octave-band noise, centered at 4 kHz, and allowed to survive for 16 days or for 1, 2, 4, and 8 months. Axonal degeneration was mapped in the cochlear nucleus, using the Nauta-Rasmussen silver method, and related to hair cell damage and to loss of myelinated nerve fibers in the osseous spiral lamina of the cochlea. Axonal degeneration in the dorsal cochlear nucleus had already reached a peak by 16 days and disappeared after 1 month. Meanwhile, myelinated nerve fiber degeneration in the cochlea extended basally, followed 2 weeks to 2 months later by spread of axonal degeneration into the corresponding high-frequency region of the ventral cochlear nucleus. Axonal degeneration occurred early in the low-frequency region of the ventral cochlear nucleus, followed 2-4 weeks later by spread of myelinated fiber degeneration into more apical regions of the cochlea. New degeneration of axons in the cochlear nerve and in the ventral cochlear nucleus continued to occur for up to 8 months after stimulation. These findings imply that plastic changes in the central auditory pathways could play a role in the long-term effects of cochlear damage and acoustic overstimulation, possibly leading to a chronic neurodegenerative condition in the ear and in the brain.

Acoustic Stimulation↗
PubMed1986

The representation of steady-state vowel sounds in the temporal discharge patterns of the guinea pig cochlear nerve and primarylike cochlear nucleus neurons.

We have recorded the responses of fibers in the cochlear nerve and cells in the cochlear nucleus of the anesthetized guinea pig to synthetic vowels [i], [a], and [u] at 60 and 80 dB SPL. Histograms synchronized to the pitch period of the vowel were constructed, and locking of the discharge to individual harmonics was estimated from these by Fourier transformation. In cochlear nerve fibers from the guinea pig, the responses were similar in all respects to those previously described for the cat. In particular, the average-localized-synchronized-rate functions (ALSR), computed from pooled data, had well-defined peaks corresponding to the formant frequencies of the three vowels at both sound levels. Analysis of the components dominating the discharge could also be used to determine the voice pitch and the frequency of the first formants. We have computed similar population measures over a sample of primarylike cochlear nucleus neurons. In these primarylike cochlear nucleus cell responses, the locking to the higher-frequency formants of the vowels is weaker than in the nerve. This results in a severe degradation of the peaks in the ALSR function at the second and third formant frequencies at least for [i] and [u]. This result is somewhat surprising in light of the reports that primarylike cochlear nucleus cells phaselock, as well as do cochlear nerve fibers.

Animals↗
PubMed1997

Translabyrinthine vestibular schwannoma surgery: postoperative tinnitus and cochlear nerve integrity.

Cochlear neurectomy as a surgical treatment for tinnitus is known to be only partially successful in many conditions. Translabyrinthine surgery for acoustic neuromas may be performed using a dissection technique which preserves an 'anterior curtain' which helps to protect the facial nerve. The cochlear nerve may be retained within this tissue though it can be cut during the dissection. To assess whether its integrity affects tinnitus the video recordings of 117 patients undergoing this operation were reviewed to grade the likelihood of the cochlear nerve having been retained intact. A postal questionnaire, with a response rate of 83%, was used to assess tinnitus. Patients who had a probable or definite nerve section had significantly lower postoperative tinnitus severity. The same relationship was found when patients were matched for tumour size and preoperative tinnitus. This procedure could act as a model for the effect of cochlear neurectomy on tinnitus associated with acoustic neuromas in a prospective trial.

Adult↗
PubMed1992

Morphology of HRP-labelled cochlear nerve axons in the dorsal cochlear nucleus of the developing hamster.

To study the development of the central terminal arbors of the cochlear nerve fibers in the dorsal cochlear nucleus, horseradish peroxidase-labelled axons in young and adult hamsters were analyzed morphometrically. Brainstem slices with whole cochlear nuclei were maintained in a slice chamber and the cochlear nerve root was injected with a mixture of wheat germ agglutinin-horseradish peroxidase, horseradish peroxidase and poly-L-ornithine. The poly-L-ornithine was added to keep the injection site small; small injections resulted in only a few axons being labelled and permitted reconstruction of individual fibers. Axons underwent an initial period of ingrowth that was completed prior to the onset of hearing (postnatal day 16). After this time the morphology and area of influence of the axons remained unchanged but the nucleus continued to increase in size. Since no additional cochlear nerve axons grow into the nucleus during this period of nuclear growth, the existing axons necessarily become more widely spaced as development proceeds. These anatomical changes may contribute to the progressive narrowing of auditory cell tuning curves.

Acoustic Stimulation↗
PubMed1986

The unrecognized rotation of the vestibular and cochlear nerves from the labyrinth to the brain stem: its implications to surgery of the eighth cranial nerve.

The cochlear and vestibular nerves rotate 90 degrees from the inner ear to the brain stem. Most of the rotation occurs within the internal auditory canal (IAC); only minimal rotation occurs in the cerebellopontine (CP) angle. At the labyrinthine end of the IAC, the cochlear nerve--which at first lies anterior to the inferior vestibular nerve (saccular nerve)--rapidly fuses with the inferior vestibular nerve. It then rotates to become inferior as the nerves leave the porus acousticus. The cochleovestibular (C-V) cleavage plane lies in a superior-inferior direction in the lateral IAC and rotates to become anterior-posterior in the CP angle. In 25% of patients in whom no C-V cleavage plane can be seen, it is not possible to completely transect all vestibular fibers. The surgical implications are that the most complete vestibular neurectomy can be done only in the lateral IAC, the cochlear and inferior vestibular nerves, because of their intimate association, should not be separated in the mid-IAC, in order to prevent damage to the cochlear nerve, and to create a complete denervation of the vestibular labyrinth, only the posterior ampullary nerve along with the superior vestibular nerve should be transected.

Brain Stem↗
PubMed1997

Advantages of a new, atraumatic, self-retaining electrode for direct cochlear nerve monitoring.

Direct cochlear nerve monitoring during posterior fossa surgery offers the surgeon real-time information concerning auditory stams. However, routine utilization of this monitoring technique has been hampered by electrode designs that have not allowed the maintenance of a consistent contact between the nerve and electrode. We report on our experience with a new electrode designed to maintain consistent, atraumatic contact with the cochlear nerve and discuss the advantages of this electrode over existing wick and ball type electrodes.The utilization of this electrode during 18 posterior fossa surgeries, performed at Kaiser Permanenie Hospital, San Diego, including 8 vestibular schwannoma resections, allowed for consistent recording of high amplitude cochlear compound action potentials. Long-term exposure to pulsating cerebrospinal fluid (CSF) did not displace the electrode. Minimal cochlear nerve action potential amplitude change was noted with the electrode imrnersed in CSF. The electrode caused no trauma to the nerve, even in cases where it was accidentally dislodged from the nerve. It is hoped that by overcoming the problems previously associated with direct cochlear nerve monitoring, this electrode will allow for increased use of this advantageous monitoring technique. As a by product of the real-time data provided to the surgeon, we anticipate increased rates of hearing preservation during cerebellopontine angle surgery.

Journal Article↗
PubMed1982

Ototoxic effects of salicylates on the responses of single cochlear nerve fibres and on cochlear potentials.

Anaesthetized cats were given 400 mg/kg sodium salicylate i.v. producing blood levels in excess of 300-400 mg/kg. Within 10 min of injection, thresholds of fibers had risen by values ranging from 13 to 21 dB. The elevation in thresholds progressed rapidly over the subsequent 5 or so hours, reaching a plateau in about 10 h. The Q10 dB values for tuning of the cochlear fibres decreased by a factor of 3-4 on average over the same period. Likewise, the dynamic range of response was significantly reduced. These effects on the cochlear fibres were reflected in the elevation of the gross cochlear action potential thresholds. In contrast to the findings with other types of cochlear pathology, the mean discharge rate of the subpopulation of cochlear fibres having discharge rates above 20 sp/s was significantly increased by an average of 10-20 sp/s. There was a tendency for this increase to be more marked for fibres with higher characteristic frequencies and to be inversely related to threshold. In addition, 63% of fibres had anomalous temporal patterns of spontaneous activity. In view of the relevance of these data on the ototoxic effects of salicylates for our understanding of tinnitus, the effects of direct electrical current stimulation via the round window have been studied. Positive currents up to at least 600 microA suppressed the spontaneous and evoked activity of all cochlear fibres studied in the normal cochlea. These findings are consistent with the effects of such current stimulation in patients with tinnitus of peripheral origin and support the hypothesis that the neural correlate of such tinnitus is hyperactivity at the cochlear nerve level.

Animals↗
PubMed1988

Degeneration in the cochlear nerve of the rat following cochlear lesions.

Left unilateral cochlear lesions were performed on 26 albino rats at 1.5 months of age. After survival times ranging from 1 h to 6 months, the animals were perfused via the aorta with mixed aldehydes. Blocks including the cochlear nerves were removed, embedded in Araldite, sectioned in a plane transverse to the longitudinal axis of the nerve, and analyzed in the light microscope. Degenerating fiber profiles were grouped into 4 categories, and their relative frequencies were counted, as were numbers of normal fibers and glial cell nuclei. The cross-sectional areas of the nerves were measured. Lesion extent was evaluated by means of sections through operated cochleas from short and long survival times, and right cochlear nerves from 11 of the animals were used as controls. In the left nerves, segmental swelling of fibers occurred as early as 16 h survival, followed by collapse of fibers and breakdown of myelin sheaths. Starting at 36 h survival, increased numbers of glial cells were seen in the nerve. At longer survival times there were decreases in the cross-sectional area of the nerve and in the packing density of degenerating fiber profiles. At the longest survival times, a substantial amount of debris remained which resembled that seen in early stages. Finally, there was evidence of continued loss of nerve fibers occurring over a period of weeks to months.

Animals↗
PubMed1989

Effects of stimulation by cochlear implant on the cochlear nerve.

Degeneration of the cochlear nerve before and after placement of the cochlear implant might influence the efficacy of the device. We examined histological characteristics, including the caliber of the cochlear nerve fibers of the central segment proximal to the porus acusticus, in three profoundly deaf patients. Two of them used a cochlear implant for many years longer in one ear than in the other, and one used an implant in one ear only. No qualitative or quantitative differences between the two sides were found. However, in all three cases we found that the cochlear nerves on both sides were substantially degenerated. These results indicated no noticeable effects of stimulation by the cochlear implant on the central portion of the cochlear nerve.

Adult↗
PubMed2005

Frequency specific hearing improvement in microvascular decompression of the cochlear nerve.

BACKGROUND: Microvascular compressions of the cochlear nerve can lead to hearing loss. Due to the tonotopic organization of the cochlear nerve any focal compression of the cochlear nerve will result in a frequency specific hearing loss. Decompressing the cochlear nerve could result in a frequency specific hearing improvement, without improving overall hearing. METHOD: Thirty one patients underwent microvascular decompression operations of the vestibulocochlear nerve for vertigo or tinnitus. Preoperative audiograms were substracted from postoperative audiograms obtained 2 years after microvascular decompression. The frequencies of maximal hearing improvement postoperatively were determined. FINDINGS: Of the 31 patients studied, 19 had improvements of 5 dB or more at one or more frequencies postoperatively, and 15 patients had improvements of 10 dB or more. Three patients had improvements of 25 dB or more postoperatively. The postoperative hearing improvement was frequency-specific and related to the anatomical location of the vascular contact on the auditory nerve. The improvement of hearing becomes diluted when the difference between pre- and postoperative hearing thresholds are averaged over all audiometric frequencies. We therefore present results for each frequency that was tested. CONCLUSIONS: Microvascular decompression of the cochlear nerve can improve hearing in selected patients. The improvement seems too small to justify decompressive surgery for the sole purpose of hearing improvement, but it could be considered if associated short vertigo spells, ipsilateral tinnitus, otalgia and cryptogenic hemifacial spasm are present. Decompression should be performed early, before BAEP changes become noticeable. 3D-MRI could become a valuable tool for selecting good surgical candidates.

Adult↗
PubMed1980

Relations between frequency selectivity and two-tone rate suppression in lizard cochlear-nerve fibers.

Cochlear-nerve fibers innervating the apicial region of the alligator lizard basilar papilla show sharp frequency selectivity in response to single tones (measured with the frequency threshold contour, or FTC), and the phenomenon of two-tone rate suppression (TTRS) in response to two simultaneously presented tones (measured with the iso-TTRS contour, or ITC). The gross shapes of the FTCs, as characterized by the slopes of the sides and Q10dB, vary systematically with the fiber's characteristic frequency (CF). 'Fine-structural' features are also found: below CF, notches (frequency regions of relatively high threshold) occur in the FTC at frequencies related to CF. Above CF, a break frequency, which varies with CF, divides the FTC into segments of different slope. Features of the ITC also vary with CF. The detailed shapes of the FTCs and ITCs are related: lobes of the ITC interdigitate with notches in the FTC; the side of the FTC with steepest slope is closely associated with the side of the ITC with steepest slope. The close relation that is observed between sharp frequency selectivity and TTRS suggests that both phenomena arise from a common cochlear mechanism.

Acoustic Stimulation↗
PubMed1995

Central projection of the peripheral cochlear nerve from each turn to the cochlear nuclei in the Mongolian gerbil.

The central projections of the peripheral cochlear nerve fiber from each turn to the cochlear nuclei (CN) in the mongolian gerbil were investigated using retrograde transportation of horseradish peroxidase (HRP). The organ of Corti and the osseous spiral lamina were scratched with an electrolytically-sharpened fine needle via a small hole at each turn of the cochlea. The cochlea was filled with a 30% horseradish peroxidase (HRP) solution. After 48 hours, 50 microns transverse serial sections of the brainstem were made with a vibratome. The tissue was processed with the diaminobenzidine procedure of the cobalt-glucose method. Our experiment revealed that the fibers from the basal turn terminated at the dorsomedial portion of anteroventral cochlear nuclei (AVCN), but those from the apical turn were distributed among the ventrolateral portion of the AVCN. In the posteroventral cochlear nuclei (PVCN) and dorsal cochlear nuclei (DCN), the fibers from basal to apical turns extend from the dorsal to the ventral portion of each nuclei. A distinct tonotopic arrangement could be found between the origin of cochlear fibers of each turn and their termination in the regions of the cochlear nuclei (CN). Also, the results suggested that the scratch method combined with retrograde transportation of horseradish peroxidase was useful in investigating the tonotopic arrangement of the peripheral auditory nerve in the CN.

Animals↗