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PubMed1978

The cochlear nerve in the cat: topography, cochleotopy, and fiber spectrum.

The topographical and cytological features of the cochlear nerve in normal adult cats were studied by microdissection, light microscopy, transmission and scanning electron microscopy. The 2-mm long cochlear nerve trunk is situated within the internal acoustic meatus. The nerve is wrapped like a roll with the more basal fibers situated peripheral to the more apical ones. The fibers are fasciculated according to their target in the cochlea and follow a helical course with the sharpest spiralling in the distal portion of the fiber. Apical and basal coil fibers are of approximately the same length measured from the spiral lamina to the rostral border of the anteroventral cochlear nucleus. The nerve contains about 90% thick, myelinated fibers, about 10% unmyelinated axons, and a few thin, but heavily myelinated axons. The first group shows a unimodal distribution of axonal diameters, with a gradual increase in average diameter from basal to apical coil fibers. The number of axonal microtubules and neurofilaments have a high linear correlation to axonal circumference without principal cochleotopic differences. The myelin sheaths are thicker and the internodal segments shorter than expected from similarly sized peripheral fibers in rodents. The possible nature and origin of the three fiber categories are discussed.

Animals↗
PubMed1994

Preservation of hearing after surgery for acoustic schwannomas: correlation between cochlear nerve function and operative findings.

The results of surgery in 66 patients with acoustic schwannoma in whom total tumor removal was accomplished are reviewed in terms of the relationships among tumor size, preoperative hearing level, operative findings (including the shape and location of the cochlear nerve at the tumor surface), and the extent of postoperative hearing preservation. Both tumor size and the preoperative hearing level were correlated with the shape of the cochlear nerve, which more frequently formed a solid bundle when the tumor was small or the preoperative hearing was excellent. Hearing was retained postoperatively only in cases in which the nerve formed a solid bundle and could be differentiated and separated from the tumor capsule without difficulty. These findings suggest that tumor size and preoperative hearing level, which have been reported to be the main prognostic factors of postoperative hearing preservation, may influence the results of surgery for acoustic schwannoma through the shape of the cochlear nerve.

Cochlear Nerve↗
PubMed1991

Details of the central projections of the cochlear nerve in the hamster revealed by the fluorescent tracer DiI.

A new fluorescent tracer, DiI, may be used postmortem in perfused animals making placement in small structures such as the cochlea much less difficult. We have mapped cochlear nerve terminations in the cochlear nucleus with DiI and, using three-dimensional reconstructions, have demonstrated the topography and geometry of the cochlear input. By placing DiI in two regions of the cochlea, we have demonstrated that terminations from the cochlea form stacked sheets of inputs. If enough closely situated ganglion cells were labeled in the cochlea, a well-delineated stripe of label could be found in each section of the cochlear nucleus up to the superficial granule cell region. Here a small extension of label, separate from the stripe may represent the medial olivocochlear axons. When fewer axons were labelled in the cochlear nerve, the sheets were absent and discontinuous patches of label were found. These patches apposed rostrocaudally to form bands of label that run orthogonal to the tonotopic organization.

Animals↗
PubMed2005

Microsurgical results with large vestibular schwannomas with preservation of facial and cochlear nerve function as the primary aim.

OBJECTIVE: To evaluate our microsurgical results in dealing with vestibular schwannomas (VS) greater than or equal to 30 mm when preservation of cranial nerve function was considered more important than total tumour removal. METHODS: Sixteen consecutive cases were operated on by the same neurosurgeon according to a prospective protocol using intraoperative neuro-monitoring (IONM) based on electromyographic and brain stem auditory evoked potential recordings. Facial nerve function was evaluated on the House-Brackmann Scale and cochlear nerve function on the Gardner-Robertson Scale. Someone not involved in the clinical management of our patients collected all data. RESULTS: Fifteen patients showed facial nerve (FN) function of House-Brackmann grade (HBG) I or II at one year postoperatively and one kept the HBG IV she had preoperatively. Two patients of four maintained a cochlear nerve function of Gardner-Robertson grade (GRG) II. The tumour excision rates were: total, 68.7%; near total, 6.3%; subtotal, 18.7%, and partial, 6.3%. The average follow-up was 55 months (1-106). Three patients underwent radiotherapy later with growth stabilisation and no additional morbidity. CONCLUSION: When dealing with VS greater than or equal to 30 mm, microsurgery guided by IONM, with a rate of total or near-total tumour excision of about 75%, can retain socially acceptable facial nerve function (HBG I or II) in all cases and serviceable hearing (GRG I or II) in two cases out of four. Maintaining serviceable cranial nerve function should take precedence over total tumour excision.

Adult↗
PubMed1997

Radiation tolerance of the cochlear nerve at the gamma-knife in rabbits.

Since the first treatment of acoustic neurinoma using the gamma-knife by Leksell, a series of cases have been reported with good control rates. However, the most frequent complication is delayed hearing loss which occurs in more than 50% of patients. The purpose of this study was to define a safe dose by analyzing the radiosurgical dose-response relationship and histological effects on the normal cochlear nerve in rabbit. The rabbits had computed tomography (CT)-guided stereotactic radiosurgery on their cochlear nerves in the internal auditory canal with a 4 mm collimator focusing of a gamma-unit. Maximum doses of 10, 20, 30, 40, 60, 80, 100, 200 and 500 Gy were administered. After the radiosurgery, auditory brain stem responses (ABR) and the behavior of the rabbits were evaluated periodically. At the conclusion, histological investigations were performed. No physiological or histological findings were observed from doses of 30 Gy or below during the 12 month period after the radiosurgery. A dose of 100 Gy caused a severe ABR threshold elevation, vestibular dysfunction and facial palsy. Necrosis and demyelination of nerves were observed pathologically. In this study, we determined that the safe dose to the normal cochlear nerve during radiosurgery was under 40 Gy in rabbits, and complications seemed to vary due to individual differences in radiation tolerance.

Animals↗
PubMed1983

[Early auditory evoked potentials (EAEP) following cochlear nerve lesions in the cerebellopontile angle. An animal experiment study].

After exposure of the cerebello-pontine angle, various lesions were applied to the cochlear nerve in 20 rabbits. Changes of the EAEP were registered at the same time. Lesions proximal to the internal auditory meatus hardly exercised any influence on wave I. Waves II-V disappeared immediately after the trauma, Partial or complete recovery was observed, depending on the extension of the nerve lesion. Dissection or crushing of the cochlear nerve resulted in permanent disappearance of the waves II-V. Wave I often showed a slight reduction of amplitude, combined with a broadening of its duration and increase in latency. Lesions of minor intensity produced a transient disappearance of waves II-V. Recovery of these waves occurred within the following 2-3 days.

Animals↗
PubMed1993

Electrophysiologic identification of the cochlear nerve fibers during cerebello-pontine angle surgery.

To facilitate identification and preservation of the auditory nerve during cerebello-pontine angle surgery, bipolar recording of cochlear nerve compound action potentials (CNAPs) was performed. Two silver wires insulated with teflon up to the exposed ends were utilized as electrodes. They were twisted together, the distance between the two tips being 1 mm or less. Rarefaction polarity clicks (31/s) ranging from the psychoacoustical threshold to 120 dB pe SPL were used as stimuli. The investigation was performed in three groups of patients. The first group consisted of 9 patients submitted to vestibular neurectomy and 4 patients operated on by microvascular decompression of the eighth nerve. The second group comprised 8 patients with acoustic tumors smaller than 2 mm and serviceable hearing. Postoperative audiometric results in the subjects in the second group were compared with those obtained in well-matched homogeneous controls consisting of patients with acoustic neuroma operated on without the aid of CNAP recording. Bipolar recording from the eighth nerve was extremely selective, a good response being obtained only when positioning the electrode on the cochlear portion of the eighth nerve. During removal of the acoustic neuroma, repeated bipolar probing of the tumor and eighth nerve facilitated the task of distinguishing the cochlear nerve from other nervous structures and from the tumor, and contributed to preserving hearing in most patients.

Action Potentials↗
PubMed1987

Computer-aided three-dimensional reconstruction of the cochlear nerve root.

Computer-aided three-dimensional reconstruction techniques have been used for reconstruction of the cochlear nuclei and the cochlear nerve root. The cochlear nuclei, the nerve root, and the other adjacent structures were studied in serial frontal and horizontal sections. Boundaries of these structures were digitized into a computer and eight three-dimensional models were produced. Two solid models (reconstructions) showed the spatial surface topography, while the others revealed the appearance of the nerve root within the nuclei. The reconstructions also showed the position of the tenia of the tela choroidea on the surface of the nuclei and its relation to the root. This knowledge makes it possible to use the tenia as a landmark for determination of the root position within the nuclei. As the nerve enters the nucleus and forms the root, there is an appreciable reduction in its thickness. At the same time, the distance between the nuclei surface and the root, which has a cone shape, increases. The cone is curved along its longitudinal axis toward the inferior cerebellar peduncle. The top of the root borders the dorsal cochlear nucleus. This study stems from work in this institution on placement of the central electroauditory prosthesis on the cochlear nuclei surface. The results obtained further knowledge of the anatomy of the nuclei, specifically the areas used for the prosthesis implantation and the underlying tissue.

Brain Stem↗
PubMed1989

[Activities of single cochlear nerve fibers in rats].

Activities of 325 single cochlear nerve fibers in response to pure tones at various frequencies including ultrasonic sounds were observed in rats. The stimulus sounds were measured with a probe tube inserted into an ear canal and a microphone and expressed in dB SPL. The best frequency (BF, characteristic frequency) and the threshold at BF were determined in every fiber. The measured BF ranged from 0.58 kHz to 62.6 kHz. BF and the threshold at BF of a fiber with the lowest BF-threshold were 27.49kHz and 6dB SPL respectively. The highest sensitivity was found in fibers with BF from 20 to 50 kHz. The tuning curve rose from BF and its slope was steeper for the high side than the low one as observed in other animals. The sharpness of the tuning curve was expressed by QN values, where QN is the ratio of BF to the band width at N dB above the BF threshold. Mean and the variance of Q10dB, Q20dB, Q30dB, Q40dB, Q50dB, all increased as the BF did. Almost all fibers had spontaneous discharge. The discharge rate increased with the increase in sound intensity and saturated at about 30 dB above the threshold. The auditory threshold curve obtained from distribution of the lowest BF-threshold of 325 fibers was consistent with the audiograms determined behaviorally.

Acoustic Stimulation↗
PubMed1989

Cochlear nerve action potentials during cerebellopontine angle surgery: relationship of latency, amplitude, and threshold measurements to hearing.

The cochlear nerve action potential (AP) was monitored during two types of hearing preservation surgery: (1) removal of small cerebellopontine angle tumors and (2) retrolabyrinthine vestibular nerve section. The purpose of this investigation was to study the relationship between changes in intraoperative AP latency, amplitude, and threshold, and changes in audiometric hearing subsequent to surgery. The presence or absence of a cochlear nerve AP toward the end of surgery accurately predicted the presence or absence of postoperative hearing (93%). Regression analyses were performed to explore the relationship between audiometric pre- and postoperative thresholds and intraoperative AP threshold changes. Statistically significant relationships were found for (1) pre-operative audiometric thresholds and initial AP click thresholds (p = 0.0068); (2) final AP click thresholds and postoperative audiometric thresholds (p = 0.0003); (3) AP click (p = 0.0001) and 1 kHz (p = 0.0006) threshold shifts and pre- to postoperative audiometric threshold shifts. No relationship was found between either AP latency or amplitude values and postoperative hearing. Intraoperative threshold measurements can be used as an indicator of hearing change. If amplitude or latency indices are also monitored it may be better to use stimulus levels close to the evoked potential threshold.

Action Potentials↗
PubMed1970

Cochlear nerve fiber discharge patterns: relationship to the cochlear microphonic.

Fourier analysis of discharge patterns in response to sinusoidal acoustic stimulation provides a consistent and repeatable measure of response phase and amplitude. The variation of the fundamental and harmonic components of the patterns as stimulus parameters are changed is strikingly similar to that of cochlear microphonics. The results are significantly different for single fibers with different characteristic frequencies; the variations parallel those of microphonics recorded from different cochlear turns.

Acoustics↗
PubMed1994

Apical cochlear nerve exposed to perilymph in the gerbil and rat.

There is considerable controversy regarding the origin and composition of perilymph in the scala vestibuli and scala tympani and the barriers and transport mechanisms between them. To elucidate the anatomical separation between perilymph and the extracellular fluid which surrounds the cochlear nerve in Mongolian gerbils (Meriones unguiculatus) and Sprague Dawley rats (Rattus rattus) we examined the modiolus, osseous spiral lamina and the bone of the perilymphatic scalae using light and electron microscopy. Although the cochlear nerve, spiral ganglion and their extracellular fluid are separated from perilymph by the bone of the modiolus in the middle and basal turns, no bone separates these neural structures from perilymph in the apical turn in the two species examined. Instead, the spiral ganglion and axonal elements of the apical turn were covered by a continuation of the bone lining cell layer of the scala tympani. Gaps between lining cells appeared to provide direct communication between the perilymphatic fluid and the extracellular fluid investing the cochlear nerve. Various authors have described openings in modiolar bone in both the scala vestibuli and the scala tympani. While the bone and cellular covering of the modiolus in the basal middle turns of the cochlea is not presumed to be a complete barrier to fluid exchange between the two scalae and the modiolar canal it can be expected to impose some limitation on the rate of passive diffusion. Therefore our data indicates that in the apical turn of the Mongolian gerbil and Sprague-Dawley rat there may be a more significant communication between perilymph and the extracellular fluid and neural elements of the apical modiolar canal than previously reported.

Animals↗
PubMed1997

Characteristics of the myelin sheath membrane of the spiral ganglion cell and the cochlear nerve of guinea pig.

The membrane structures of the myelin sheath of guinea pig spiral ganglion cell and cochlear nerve were examined using freeze-fracturing, rapid freezing and freeze substitution, and deep-etching techniques. The intermediate dense lines of the myelin sheath were observed only in the outermost several layers using thin and thick (1 micron) section techniques. The spaces between the two adjacent major dense lines at any layer of myelin sheath were occupied by flocculent materials in the freeze-substituted specimen. Periods between the two adjacent major dense lines were 13.3 nm in conventional fixation and 11.7 nm in rapid freezing followed by freeze substitution, respectively. A freeze-fracturing study showed no differences in membrane structures at any layer of myelin sheath. Developed tight-junction strands were observed parallel to the long axis of the myelin sheath in the deep-etched specimen. These findings may show the characteristics of myelin sheath membrane in the spiral ganglion cell and cochlear nerve of the guinea pig.

Animals↗
PubMed1992

A statistical study of cochlear nerve discharge patterns in response to complex speech stimuli.

Cochlear nerve discharge patterns in response to the synthesized consonant-vowel stimulus /da/ were collected from a population of 223 auditory-nerve fibers from a single cat. For each nerve fiber, discharges were measured from multiple, independent stimulus presentations, with the means and variances of the post-stimulus time histograms and Fourier transforms of response generated from the ensemble of stimulus presentations. The statistics were not consistent with those predicted via an inhomogeneous Poisson counting process model. Specifically, the synchronized components as measured by the Fourier transforms of post-stimulus time histogram responses have variances that are as much as a factor of 3 times lower than the predicted by the Poisson model. To account for the non-Poisson nature of the statistics, the Markov process model of Siebert/Gaumond was adopted. Using the maximum-likelihood and minimum description length algorithms, introduced by Miller [J. Acoust. Soc. Am. 77, 1452-1464 (1985)] and Mark and Miller [J. Acoust. Soc. Am. 91, 989-1002 (1992)], estimates of the stimulus and recovery functions were computed for each nerve fiber. Then, Markov point processes were simulated with the stimulus and recovery functions generated from these nerve fibers. The statistics of the simulated Markov processes are shown to have almost identical first- and second-order statistics as those measured for the population of auditory-nerve fibers, and demonstrates the effectiveness of the Markov point process model in accounting for the correlation effects associated with the discharge history-dependent refractory properties of auditory nerve response.

Acoustic Stimulation↗
PubMed1982

The neuronal architecture of the anteroventral cochlear nucleus of the cat in the region of the cochlear nerve root: Golgi and Nissl methods.

This report characterizes the cells and fibers in one part of the cochlear nucleus, the posterior division of the anteroventral cochlear nucleus. This includes the region where the cochlear nerve root enters the brain and begins to form endings. Nissl stains reveal the somata of globular cells with dispersed Nissl substance and those of multipolar cells with coarse, clumped Nissl bodies. Both parts of the posterior division contain cells with each Nissl pattern, but in different relative numbers and locations. Golgi impregnations demonstrate two types of neurons: bushy cells, with short bush-like dendrites, and stellate and elongate cells, with long tapered dendrites. Several varieties of bushy cells, differing in the morphology of the cell body and in the size and extent of the dendritic field, can be distinguished. Comparison of the distributions of these cell types, as well as cellular morphology, suggest that the globular cells recognized in Nissl stains correspond to bushy neurons, while the multipolar cells correspond to stellate and elongate neurons. Golgi impregnations reveal large end-bulbs and smaller boutons from cochlear nerve fibers, as well as boutons from other, unidentified sources, ending in this region. The particular arrangements of the dendritic fields of the different cell types and the axonal endings associated with them indicate that these neurons must have different physiological properties, since they define different domains with respect to the cochlear and non-cochlear inputs.

Animals↗
PubMed2005

Facial and cochlear nerve function after surgery of cerebellopontine angle meningiomas.

OBJECTIVE: Meningiomas of the cerebellopontine angle (CPA) share a common location, but their site of dural origin and their relationship to surrounding neurovascular structures of the CPA are variable. The clinical presentation and outcome after surgical resection are different because of the diversity of this tumor entity. We report on a series of 421 patients with CPA meningiomas, with special emphasis on the analysis of the preoperative and postoperative facial and cochlear nerve function in relation to the site of dural attachment and main tumor location in the CPA cistern. METHODS: Among 421 patients, the charts of 347 patients with complete clinical data, including the history and audiograms, imaging studies, surgical records, discharge letters, histological records, and follow-up records, were reviewed retrospectively. Data about preoperative and postoperative facial nerve function were available in 334 patients, and audiometric analysis was conducted in 333 patients. Patients with neurofibromatosis Type 2 were excluded from the study. RESULTS: There were 270 women and 77 men, with a mean age of 53.4 years (range, 17.6-84 yr). Among these patients, 32.9% of the tumors originated at the petrous ridge anterior to the inner auditory canal (IAC) (Group 1), 22.2% showed involvement of the IAC (Group 2), 20.2% were located superior to the IAC (Group 3), 11.8% were inferior to the IAC (Group 4), and 12.9% were posterior to the IAC, originating between the IAC and the sigmoid sinus (Group 5). Patients presented with disturbance of Cranial Nerves V-VIII, the lower cranial nerves, and ataxia, depending on the main tumor location. Tumor resection was performed through a suboccipital-retrosigmoidal approach in the semisitting position in 95% of the patients. A combined supratentorial-infratentorial presigmoidal approach was performed in 5%. Total tumor removal (Simpson Grade 1 and 2) was achieved in 85.9% and subtotal removal in 14.1%. The best initial postoperative facial and auditory nerve function was observed in tumors belonging to Groups 3 and 5. Recovery from preoperative deafness was observed in 1.8% of patients. On long-term follow-up, good facial nerve function (House-Brackmann Grade 1 or 2) was observed in 88.9% of patients. Hearing preservation among patients with preoperative functional hearing was documented in 90.8% on long-term follow-up. CONCLUSION: Although the outcome of facial and cochlear nerve function is different in CPA meningiomas, depending on the topographic classification of these tumors, preservation of the cochlear nerve is possible in every tumor group and should be attempted in every patient with CPA meningioma. It has to be kept in mind that recovery of hearing was also observed in patients with preoperative profound hearing deficits.

Adolescent↗