Vestibular Nerve Fibers Research Articles

Screening testing and computerized vestibulometry in vestibular neuronitis

Introduction. The demonstrative example of unilateral peripheral vestibulopathy is vestibular neuronitis. This disease of infectious-toxic origin has the involvement of vestibular ganglia neurons or vestibular nerve fibers. The overwhelming moment in topic diagnostics of vestibular impairments is within-patient comparison anamnesis data and vestibulometry results. In cases of vestibular neuronitis exception of central vestibular system pathology is mandatory. Objective. To attain objectification of disequilibrium in vestibular neuronitis by means of registration and assessment of oculogyric and postural reactions. Materials and methods. Vestibular passport trials are recommended in the examination of patients with acute vertiginous complaints. Special attention has been given to the revelation of impairments of command eye movements, disturbances of ocular pursuit, spontaneous nystagmus, and catch-up saccades in Halmagyi – Curthoys test. Possible impairments of static and dynamic balance control are assessed in modified Romberg test and Unterberger test. Gaze tests with registration of saccadic and pursuit eye movements, spontaneous and optokinetic nystagmus reactions as well bithermal caloric test are carried out in computerized electrooculography and videooculography. Deviations of body gravity center are analyzed with the help of sensory organization test in computerized dynamic posturography. Results and discussion. The results of screening and computerized vestibular testing of 24 patients are presented. High diagnostic value of registration and analysis of oculogyric reactions as well as postural control signs is demonstrated in cases of acute vestibular neuronitis and acerbation of its chronic recurrent variant. The key objective symptom, which confirms peripheral level of vestibular dysfunction, is spontaneous nystagmus registration in elimination of gaze fixation. Unsteadiness in static and dynamic trials confirms the vestibular decompensation extent in acute phase of vestibular ganglia / neurons pathology in cases of vestibular neuronitis. Decompensation signs revelation of this peripheral vestibular dysfunction needs urgent medicine: possible etiotropic and obligate symptomatic therapy as well as spare vestibular rehabilitation exercises. Conclusion. Combination of oculogyric impairments and disturbances of postural control enables to improve accuracy of diagnostics in decompensated stage of vestibular neuronitis.

Molecular biology of sporadic vestibular schwannomas including genetic and epigenetic alterations

Introduction: Vestibular schwannomas (VSs) are benign tumors, developing from neoplastic Schwann cells, that produce myelin sheath covering vestibular nerve fibers. The majority of these tumors are sporadic VS and very rarely they can be associated with neurofibromatosis type 2 (NF2). VSs are usually characterized with slow growth rate, however some of them appear to have a cystic structure, grow more rapidly, can cause facial nerve palsy and brainstem compression. The molecular hallmark of both sporadic and NF-2 associated VS is the inactivation of supresor gene NF-2, also called merlin gene. Aim: In this review we summarize the current knowledge on the molecular biology of VS, including genetic and epigenetic aberrations, altered gene expression and specific microRNA expression profiles.

Efferent synaptic transmission at the vestibular type II hair cell synapse.

In the vestibular peripheral organs, type I and type II hair cells (HCs) transmit incoming signals via glutamatergic quantal transmission onto afferent nerve fibers. Additionally, type I HCs transmit via "non-quantal" transmission to calyx afferent fibers, by accumulation of glutamate and potassium in the synaptic cleft. Vestibular efferent inputs originating in the brainstem contact type II HCs and vestibular afferents. Here, synaptic inputs to type II HCs were characterized by using electrical and optogenetic stimulation of efferent fibers combined with in vitro whole cell patch-clamp recording from type II HCs in the rodent vestibular crista. Properties of efferent synaptic currents in type II HCs were similar to those found in cochlear HCs and mediated by activation of α9-containing nicotinic acetylcholine receptors (nAChRs) and small-conductance calcium-activated potassium (SK) channels. While efferents showed a low probability of release at low frequencies of stimulation, repetitive stimulation resulted in facilitation and increased probability of release. Notably, the membrane potential of type II HCs during optogenetic stimulation of efferents showed a strong hyperpolarization in response to single pulses and was further enhanced by repetitive stimulation. Such efferent-mediated inhibition of type II HCs can provide a mechanism to adjust the contribution of signals from type I and type II HCs to vestibular nerve fibers, with a shift of the response to be more like that of calyx-only afferents with faster non-quantal responses.NEW & NOTEWORTHY Type II vestibular hair cells (HCs) receive inputs from efferent neurons in the brain stem. We used in vitro optogenetic and electrical stimulation of vestibular efferent fibers to study their synaptic inputs to type II HCs. Stimulation of efferents inhibited type II HCs, similar to efferent effects on cochlear HCs. We propose that efferent inputs adjust the contribution of signals from type I and II HCs to vestibular nerve fibers.

Effects of galvanic vestibular stimulation on resting state brain activity in patients with bilateral vestibulopathy.

We examined the effect of galvanic vestibular stimulation (GVS) on resting state brain activity using fMRI (rs‐fMRI) in patients with bilateral vestibulopathy. Based on our previous findings, we hypothesized that GVS, which excites the vestibular nerve fibers, (a) increases functional connectivity in temporoparietal regions processing vestibular signals, and (b) alleviates abnormal visual–vestibular interaction. Rs‐fMRI of 26 patients and 26 age‐matched healthy control subjects was compared before and after GVS. The stimulation elicited a motion percept in all participants. Using different analyses (degree centrality, DC; fractional amplitude of low frequency fluctuations [fALFF] and seed‐based functional connectivity, FC), group comparisons revealed smaller rs‐fMRI in the right Rolandic operculum of patients. After GVS, rs‐fMRI increased in the right Rolandic operculum in both groups and in the patients' cerebellar Crus 1 which was related to vestibular hypofunction. GVS elicited a fALFF increase in the visual cortex of patients that was inversely correlated with the patients' rating of perceived dizziness. After GVS, FC between parietoinsular cortex and higher visual areas increased in healthy controls but not in patients. In conclusion, short‐term GVS is able to modulate rs‐fMRI in healthy controls and BV patients. GVS elicits an increase of the reduced rs‐fMRI in the patients' right Rolandic operculum, which may be an important contribution to restore the disturbed visual–vestibular interaction. The GVS‐induced changes in the cerebellum and the visual cortex were associated with lower dizziness‐related handicaps in patients, possibly reflecting beneficial neural plasticity that might subserve visual–vestibular compensation of deficient self‐motion perception.

Residual vestibular function after vestibular schwannoma surgery

ObjectivesThis study aimed to assess vestibular function in 39 patients who underwent neurectomy for vestibular schwannoma. MethodSemicircular canal reactivity was measured by video head-impulse test using high-frequency passive head acceleration. Response gain was calculated as a ratio between the areas under the eye-velocity curve and the head-velocity curve. Statistical analysisStudent t-test was used for to compare quantitative variables. ANOVA was used to test inter-group differences in categoric variables. ResultsIn all cases, surgery-side gain on head impulse test was low, with increased gain asymmetry. A subgroup of 7 patients (18%) showed relatively high gain in vestibulo-ocular reflex on the surgery side. Caloric reaction was absent in all cases. These findings indicate that residual vestibular function can be conserved following vestibular schwannoma extirpation. ConclusionCases with moderate vestibulo-ocular reflex gain were a subgroup with partial conservation of vestibular nerve fibers. Whether this is a predictor of better functional prognosis remains to be elucidated. Higher gain correlated with less extensive surgery and sparing of the inferior vestibular nerve. Low gain correlated with complete vestibular neurectomy. This information may guide rehabilitation strategy following surgery.

Efferent Inputs Are Required for Normal Function of Vestibular Nerve Afferents

A group of vestibular afferent nerve fibers with irregular-firing resting discharges are thought to play a prominent role in responses to fast head movements and vestibular plasticity. We show that, in C57BL/6 mice (either sex, 4-5 weeks old), normal activity in the efferent vestibular pathway is required for function of these irregular afferents. Thermal inhibition of efferent fibers results in a profound inhibition of irregular afferents' resting discharges, rendering them inadequate for signaling head movements. In this way, efferent inputs adjust the contribution of the peripheral irregular afferent pathway that plays a critical role in peripheral vestibular signaling and plasticity.SIGNIFICANCE STATEMENT Vestibular end organs in the inner ear receive efferent inputs from the brainstem. Previously, electrical stimulation of efferents was linked to an increase in resting discharges of afferents and a decrease in their sensitivities. Here, we show that localized thermal inhibition of unmyelinated efferents results in a significant decrease in the activity of afferent nerve fibers, particularly those with irregular resting discharges implicated in responses to fast head movements and vestibular compensation. Thus, by upregulating and downregulating of afferent firing, particularly irregular afferents, efferents adjust neural activity sensitive to rapid head movements. These findings support the notion that peripheral vestibular end organs are not passive transducers of head movements and their sensory signal transmission is modulated by efferent inputs.

Galvanic vestibular stimulation: from basic concepts to clinical applications.

Galvanic vestibular stimulation (GVS) plays an important role in the quest to understand sensory signal processing in the vestibular system under normal and pathological conditions. It has become a highly relevant tool to probe neuronal computations and to assist in the differentiation and treatment of vestibular syndromes. Following its accidental discovery, GVS became a diagnostic tool that generates eye movements in the absence of head/body motion. With the possibility to record extracellular and intracellular spikes, GVS became an indispensable method to activate or block the discharge in vestibular nerve fibers by cathodal and anodal currents, respectively. Bernie Cohen, in his attempt to decipher vestibular signal processing, has used this method in a number of hallmark studies that have added to our present knowledge, such as the link between selective electrical stimulation of semicircular canal nerves and the generation of directionally corresponding eye movements. His achievements paved the way for other major milestones including the differential recruitment order of vestibular fibers for cathodal and anodal currents, pronounced discharge adaptation of irregularly firing afferents, potential activation of hair cells, and fiber type-specific activation of central circuits. Previous disputes about the structural substrate for GVS are resolved by integrating knowledge of ion channel-related response dynamics of afferents, fiber type-specific innervation patterns, and central convergence and integration of semicircular canal and otolith signals. On the basis of solid knowledge of the methodology, specific waveforms of GVS are currently used in clinical diagnosis and patient treatment, such as vestibular implants and noisy galvanic stimulation.

EVestG Recordings are Vestibuloacoustic Signals

A field potential can be defined as the ‘synchronous’ firing of many of vestibuloacoustic nerve fibres. Electrovestibulography has been considered as a method to record and detect vestibuloacoustic signals originating mainly from the vestibular system. A clear picture of the likely acoustic, vestibular or vestibuloacoustic origin and the physiologic basis of these field potentials is lacking. Recordings were conducted on anesthetized Guinea Pigs [ketamine (60 mg/kg) and xylazine (6 mg/kg)] to determine the origin of these minute spontaneous field potentials detected, recorded and averaged during Electrovestibulography recordings. Recordings were made with (1) normal hearing and balance (vestibular function), (2) following cisplatin deafening then (3) following both cisplatin deafening and gentamicin vestibular ablation. The detected minute spontaneous field potentials were determined to be both acoustic and vestibular in origin i.e., vestibuloacoustic. Based on the spontaneous activity of acoustic and vestibular nerve fibers arguments are presented that these field potentials are predominantly vestibular.

A Low-Power ASIC Signal Processor for a Vestibular Prosthesis.

A low-power ASIC signal processor for a vestibular prosthesis (VP) is reported. Fabricated with TI 0.35 μm CMOS technology and designed to interface with implanted inertial sensors, the digitally assisted analog signal processor operates extensively in the CMOS subthreshold region. During its operation the ASIC encodes head motion signals captured by the inertial sensors as electrical pulses ultimately targeted for in-vivo stimulation of vestibular nerve fibers. To achieve this, the ASIC implements a coordinate system transformation to correct for misalignment between natural sensors and implanted inertial sensors. It also mimics the frequency response characteristics and frequency encoding mappings of angular and linear head motions observed at the peripheral sense organs, semicircular canals and otolith. Overall the design occupies an area of 6.22 mm (2) and consumes 1.24 mW when supplied with ± 1.6 V.

Peripheral Vestibular System Disease in Vestibular Schwannomas: A Human Temporal Bone Study.

Dizziness is a common symptom in patients with vestibulo-cochlear schwannoma (VS), and several recent studies have identified this symptom as the single most important concerning the quality of life. Clinical and histological observations regarding hearing loss have suggested that this may be caused by both cochlear and retrocochlear mechanisms. Multiple mechanisms may also be at play in the case of dizziness, which may broaden perspectives of therapeutic approach. This study presents a systematic and detailed assessment of vestibular histopathology in temporal bones from patients with VS. Retrospective analysis of vestibular system histopathology in temporal bones from 17 patients with unilateral VS. The material was obtained from The Copenhagen Temporal Bone Collection. Vestibular schwannomas were associated with atrophy of the vestibular ganglion, loss of fiber density of the peripheral vestibular nerve branches, and atrophy of the neuroepithelium of the vestibular end organs. In cases with small tumors, peripheral disease occurred only in the tissue structures innervated by the specific nerve from which the tumor originated. Vestibular schwannomas are associated with distinctive disease of the peripheral vestibular tissue structures, suggesting anterograde degeneration and that dizziness in these patients may be caused by deficient peripheral vestibular nerve fibers, neurons, and end organs. In smaller tumors, a highly localized disease occurs, which opens perspectives of differentiated clinical assessment and subsequent, targeted therapy.

Hair Cell Ribbon Synapse Function - Differently Optimized for Hearing and Balance

In the cochlea, inner hair cells transmit the sound signal via type I auditory nerve fibers to the brain, and outer hair cells mainly operate as local amplifiers of the sound signal in the periphery. However, outer hair cells also contact a small group of afferent type II fibers, of unknown function. In the vestibular organs in the inner ear, hair cells transmit head rotation signals to the brain, via vestibular nerve fibers. All of these different hair cell types release glutamate onto their afferent contacts via ribbon synapses. Interestingly, synaptic transmission at these individual hair cell ribbon synapses operates differently in many respects, most likely optimized for their specific function.For example, at the inner hair cell synapse, known for its high reliability and precision, EPSC are fast and amplitudes are quite variable in size and unusually large, with distributions from 20 pA to 1 nA (holding potential −94 mV). In comparison, at outer and vestibular type I hair cells, EPSCs show rather ‘conventional’ sizes of 20 - 50 pA. At inner hair cell ribbon synapses, EPSC amplitude distributions are quite diverse for individual nerve fibers, and we propose that specific distributions underlie ‘high’ and ‘low spontaneous rate fibers carrying different aspects of the sound signal. Interestingly, EPSC distributions are calcium-independent, as hair cell depolarizations do not change the distributions. At the vestibular type I hair cell afferent synapse, hair cell depolarization shifts the amplitude distribution, suggesting a calcium-dependence. Postsynaptically, EPSCs are unusually slow, causing slow, summed depolarizations of the postsynaptic membrane and thereby a change in firing rate.In summary, different mechanisms of ribbon synapse transmission and their function at different hair cells will be compared and discussed.Supported by NIDCD R01DC006476 and R01DC012957 to EG.

The anatomical and physiological framework for vestibular prostheses.

This article reviews the structure function of the vestibular system and its pathology with respect to requirements for the design and construction of a functional vestibular prosthesis. The ultimate goal of a vestibular prosthesis is to restore balance and equilibrium through direct activation of vestibular nerve fibers. An overview of the peripheral and central vestibular systems that highlights their most important functional aspects re: the design of a prosthesis is provided. Namely, the peripheral labyrinth faithfully transduces head motion and gravity in both the time and frequency domains. These signals are described in hopes that they may be prosthetically replicated. The peripheral and central connections of the vestibular nerve are also discussed in detail, as are the vestibular nuclei in the brainstem that receive VIIIth nerve innervation. Lastly, the functional effector pathways of the vestibular system, including the vestibulo-ocular, vestibulo-spinal, vestibulo-colic, vestibulo-autonomic, and vestibular efferent innervation of the labyrinth are reviewed.

Arachnoid Cyst Confined to the Internal Auditory Canal—Endoscope-Assisted Resection: Case Report and Review of the Literature

An arachnoid cyst confined to the internal auditory canal is a rare condition. Different pathogeneses are discussed, and a progressive enlargement of the cysts has been reported. This case illustrates the beneficial aspect of endoscopic assistance in microsurgical resection of this lesion. A slowly progressive hearing loss developed in a 35-year-old woman over 2 years; she reported experiencing tinnitus for 7 years. Magnetic resonance imaging revealed a cystic lesion in the internal auditory canal appearing hypointense on T1-weighted images and hyperintense on T2-weighted images, suggesting an arachnoid cyst. The cyst wall was fenestrated and partially resected in an endoscope-assisted microsurgical technique. Adherent vestibular nerve fibers in the cyst wall prevented total removal of the cyst. The histological examination confirmed the diagnosis of an arachnoid cyst. The endoscope-assisted microsurgical technique enables a safe cyst resection with good visualization of important neurovascular structures within the internal auditory canal. Small remnants of the capsule that are firmly attached to important neurovascular structures should be left in place rather than risk neurological deficits.

Histological considerations of the cleavage plane for preservation of facial and cochlear nerve functions in vestibular schwannoma surgery

The authors analyzed the tumor capsule and the tumor-nerve interface in vestibular schwannomas (VSs) to define the ideal cleavage plane for maximal tumor removal with preservation of facial and cochlear nerve functions. Surgical specimens from 21 unilateral VSs were studied using classical H & E, Masson trichrome, and immunohistochemical staining against myelin basic protein. The authors observed a continuous thin connective tissue layer enveloping the surfaces of the tumors. Some nerve fibers, which were immunopositive to myelin basic protein and considered to be remnants of vestibular nerve fibers, were also identified widely beneath the connective tissue layer. These findings indicated that the socalled "tumor capsule" in VSs is the residual vestibular nerve tissue itself, consisting of the perineurium and underlying nerve fibers. There was no structure bordering the tumor parenchyma and the vestibular nerve fibers. In specimens of tumors removed en bloc with the cochlear nerves, the authors found that the connective tissue layer, corresponding to the perineurium of the cochlear nerve, clearly bordered the nerve fibers and tumor tissue. Based on these histological observations, complete tumor resection can be achieved by removal of both tumor parenchyma and tumor capsule when a clear border between the tumor capsule and facial or cochlear nerve fibers can be identified intraoperatively. Conversely, when a severe adhesion between the tumor and facial or cochlear nerve fibers is observed, dissection of the vestibular nerve-tumor interface (the subcapsular or subperineurial dissection) is recommended for preservation of the functions of these cranial nerves.

Efferent-Mediated Responses in Vestibular Nerve Afferents of the Alert Macaque

The peripheral vestibular organs have long been known to receive a bilateral efferent innervation from the brain stem. However, the functional role of the efferent vestibular system has remained elusive. In this study, we investigated efferent-mediated responses in vestibular afferents of alert behaving primates (macaque monkey). We found that efferent-mediated rotational responses could be obtained from vestibular nerve fibers innervating the semicircular canals after conventional afferent responses were nulled by placing the corresponding canal plane orthogonal to the plane of motion. Responses were type III, i.e., excitatory for rotational velocity trapezoids (peak velocity, 320 degrees/s) in both directions of rotation, consistent with those previously reported in the decerebrate chinchilla. Responses consisted of both fast and slow components and were larger in irregular (approximately 10 spikes/s) than in regular afferents (approximately 2 spikes/s). Following unilateral labyrinthectomy (UL) on the side opposite the recording site, similar responses were obtained. To confirm the vestibular source of the efferent-mediated responses, the ipsilateral horizontal and posterior canals were plugged following the UL. Responses to high-velocity rotations were drastically reduced when the superior canal (SC), the only intact canal, was in its null position, compared with when the SC was pitched 50 degrees upward from the null position. Our findings show that vestibular afferents in alert primates show efferent-mediated responses that are related to the discharge regularity of the afferent, are of vestibular origin, and can be the result of both afferent excitation and inhibition.

Development of the Vestibular Apparatus and Central Vestibular Connections in a Wallaby (Macropus eugenii)

We have studied the early development of the vestibular apparatus and its central connections in the tammar wallaby (Macropus eugenii) in order to determine whether the vestibular system anatomy is sufficiently mature at birth to assist in climbing to the pouch. Structural development was studied with the aid of hematoxylin and eosin stained sections and immunoreactivity for GAP-43, whereas the development of vestibular system connections was examined by carbocyanine dye tracing. At the time of birth, the otocyst has distinct utricle, saccule and semicircular canals with immature sensory regions receiving innervation by GAP-43 immunoreactive fibers. Vestibular nerve fibers can be traced into the brainstem to the developing vestibular nuclei, which are not yet cytoarchitectonically distinct. The vestibular nuclei do not contribute direct projections to the lower cervical spinal cord at birth; most bulbospinal projections in the newborn appear to be derived bilaterally from the gigantocellular, lateral paragigantocellular reticular and ventral medullary nuclei. A substantial bilateral projection to the vestibular ganglion and apparatus from the region of the gigantocellular and lateral paragigantocellular nuclei was seen at birth, but not in subsequent ages. This is similar to a projection seen in newborn Ameridelphians. By postnatal day (P) 5, the vestibular apparatus had extensive projections to all vestibular nuclei and neurons projecting in the lateral vestibulospinal tract could be identified in the lateral vestibular nucleus. Cytoarchitectonic differentiation of the vestibular nuclei proceeded over the next 3 to 4 weeks with the emergence of discrete parvicellular and magnocellular components of the medial vestibular nucleus by P19. GAP-43 immunoreactivity stayed high in the lateral vestibulospinal tract for several months after birth, suggesting that the development of this tract followed a prolonged timecourse. Our findings indicate that central and peripheral connections of the vestibular ganglion are present at birth, but that there is no direct projection from the vestibular nuclei to the cervical spinal cord until P5. Nevertheless, the possibility remains that an indirect projection between the vestibular nuclei and the medial reticular formation is present at birth and mediates control of the climb.

Muscarinic acetylcholine receptor subtype expression in avian vestibular hair cells, nerve terminals and ganglion cells

Muscarinic acetylcholine receptors (mAChRs) are widely expressed in the CNS and peripheral nervous system and play an important role in modulating the cell activity and function. We have shown that the cholinergic agonist carbachol reduces the pigeon’s inwardly rectifying potassium channel (pKir2.1) ionic currents in native vestibular hair cells. We have cloned and sequenced pigeon mAChR subtypes M2–M5 and we have studied the expression of all five mAChR subtypes (M1–M5) in the pigeon vestibular end organs (semicircular canal ampullary cristae and utricular maculae), vestibular nerve fibers and the vestibular (Scarpa’s) ganglion using tissue immunohistochemistry (IH), dissociated single cell immunocytochemistry (IC) and Western blotting (WB). We found that vestibular hair cells, nerve fibers and ganglion cells each expressed all five (M1–M5) mAChR subtypes. Two of the three odd-numbered mAChRs (M1, M5) were present on the hair cell cilia, supporting cells and nerve terminals. And all three odd numbered mAChRs (M1, M3 and M5) were expressed on cuticular plates, myelin sheaths and Schwann cells. Even-numbered mAChRs were seen on the nerve terminals. M2 was also shown on the cuticular plates and supporting cells. Vestibular efferent fibers and terminals were not identified in our studies. Results from WB of the dissociated vestibular epithelia, nerve fibers and vestibular ganglia were consistent with the results from IH and IC. Our findings suggest that there is considerable co-expression of the subtypes on the neural elements of the labyrinth. Further electrophysiological and pharmacological studies should delineate the mechanisms of action of muscarinic acetylcholine receptors on structures in the labyrinth.

Immunohistochemical localization of aquaporins in the human inner ear

We report the immunolocalization of aquaporins (AQPs) 1, 4, and 6 in the human auditory and vestibular endorgans. A rapid protocol was applied to audiovestibular endorgans microdissected from postmortem human temporal bones from six subjects (ages ranging from 75 to 97 years) with no history of audiovestibular disease. Temporal bones were fixed in formalin, and the endorgans were immediately microdissected. Cryostat sections were obtained from audiovestibular endorgans and were subjected to double-immunohistochemical staining with antibodies against AQPs and several cellular markers. In the human cochlea, AQP1 immunoreactivity was localized to the fibrocytes of the spiral ligament and the sub-basilar tympanic cells; AQP4 immunoreactivity was localized to the outer sulcus cells, Hensen's cells, and Claudius' cells; AQP6 immunoreactivity was localized to the apical portion of interdental cells in the spiral limbus. In the vestibular endorgans (macula utriculi and cristae), AQP1 was localized to fibrocytes and blood vessels of the underlying stroma and trabecular perilymphatic tissue; AQP4 immunoreactivity was localized to the basal pole of vestibular supporting cells; AQP6 was localized to the apical portion of vestibular supporting cells. Cochlear and vestibular hair cells and nerve fibers were not immunoreactive for any AQP. Supporting cells were identified with antibodies against glial fibrilar acidic protein. Nerve fibers and terminals were identified with antibodies against neurofilaments and Na(+)K(+)ATPase. The high degree of conservation of AQP expression in the human inner ear suggests that AQPs play a critical role in inner ear water homeostasis.

Neuroendocrine cells in the vestibular glands of the genital tract of cows and pigs

The presence of neuroendocrine (NE) cells producing biogenic amines and hormonal peptides has been investigated in the vestibular glands of the genital tracts of cows and pigs using immunohistochemistry. NE cells containing chromogranin A-, serotonin-, cholecystokinin- and somatostatin-immunoreactive material were found in both major and minor vestibular glands. Such cells were numerous, scattered in the acini and excretory duct epithelium, small in size and rounded, triangular or bipolar in shape. The function of the NE vestibular cells has been related to the secretory activity of the glands and to a sexual climax induction mechanism involving the stimulation of 5HT(3) receptors of vestibular nociceptor nerve fibers. The role of NE cells in small cell carcinomas of the vestibular glands is a topic for further investigation owing to possible parallelism between this type of tumor and the small cell carcinoma of the human prostate.

Acoustically responsive fibers in the mammalian vestibular nerve

There are afferent fibers in the vestibular nerve of cats and guinea pigs that respond to sound at levels within the normal range of hearing. Most acoustically responsive fibers are in the inferior vestibular nerve and have irregular spontaneous activity, although not as irregular as cochlear afferents. Single-fiber labeling shows that acoustically responsive, irregularly discharging (ARID) fibers originate in the saccule. ARID fibers traced centrally arborized extensively in vestibular nuclei and ventromedial to the cochlear nucleus. ARID fibers had broad, V-shaped tuning curves with best frequencies between 500 and 1000 Hz, thresholds of 90 dB SPL or more, and shapes comparable with tuning-curve tails of cochlear afferents. ARID fibers synchronized to a preferred phase of the tone cycle at levels approximately 10 dB lower than their firing-rate threshold. ARID spike rates increased monotonically with sound level without saturating at 115 dB SPL. Acoustic clicks evoked spikes in ARID fibers with minimum latencies less than 1.0 ms. Contraction of the middle-ear muscles decreased responses to sound, consistent with the sound transmission path being through the middle ear. In summary, there is strong evidence that the mammalian saccule responds to sound and sends acoustic information to the central nervous system. [Work supported by NIDCD RO1DC00235.]