Dimensions of the human vestibular and tympanic scalae

The cochlear scalae provide a practical access route for the insertion of cochlear implant electrodes. A microanatomical study was carried out on 25 human temporal bones obtained from cadavers. These bones were dissected with the aid of an operation microscope, in which their perilymphatic spaces were filled with coloured latex and further prepared in a formalin stain. Each of the rubber moulds was removed from the osseous matrix using standard otosurgical equipment, and subsequently cut into 1 mm segments. The height and width of the vestibular and tympanic scalae were measured. The results, presented in diagrams, indicate that the vestibular scala is less prominent than the tympanic scala in the basic and middle coil of the cochlea and in the upper coil, they display greater dimensions which could serve as a place for electrode insertion in cochlear implant procedures. In addition, the vestibular and tympanic scalae present alternate dominance in their width and height as corroborated by the calculated coefficients. The results obtained in this study supplement our knowledge of the anatomy of the cochlea thus far lacking a full investigation of the scalae, and could serve as a basis for other studies dealing with the physiology of the organs of hearing.

Peripherin immunoreactivity labels small diameter vestibular 'bouton' afferents in rodents

Recent morphophysiological studies have described three different subpopulations of vestibular afferents. The purpose of this study was to determine whether peripherin, a 56-kDa type III intermediate filament protein present in small sensory neurons in dorsal root ganglion and spiral ganglion cells, would also label thin vestibular afferents. Peripherin immunohistochemistry was done on vestibular sensory organs (cristae ampullares, utriculi and sacculi) of chinchillas, rats, and mice. In these sensory organs, immunoreactivity was confined to the extrastriolar region of the utriculus and the peripheral region of the crista. The labelled terminals were all boutons, except for an occasional calyx. In vestibular ganglia, immunoreactivity was restricted to small vestibular ganglion cells with thin axons. The immunoreactive central axons of vestibular ganglion cells form narrow bundles as they pass through the caudal spinal trigeminal tract. As they exit this tract, several bundles coalesce to form a single, narrow bundle passing caudally through the ventral part of the lateral vestibular nucleus. Finally, we conclude that all labelled axons and terminals were vestibular afferents rather than efferents, as no immunoreactivity in the vestibular efferent nucleus of the brainstem was observed.

Dizziness in the elderly and age-related degeneration of the vestibular system

The peripheral and central vestibular systems exhibit an age-related structural deterioration which may be responsible for vestibular reflex deficits and dizziness in the elderly. However, it seems likely that the central nervous system is capable of compensating for a certain degree of decline in function, since not all elderly people are impaired to the extent that the clinical signs of vestibular dysfunction are apparent. Dizziness and other vestibular disorders may develop only when the degree of deterioration of the vestibular system exceeds the ability of the nervous system to compensate. If dizziness does eventuate, it can have profound psychological consequences, particularly in terms of loss of confidence in independent activity, and may lead to the development of anxiety disorders. Vestibular rehabilitation programs may help to minimise the effects of age-related deterioration of the vestibular system and its psychological impact.

Morphological, morphometric, and functional differences in the vestibular organ of different breeds of the rat (Rattus norvegicus)

In the laboratory rat, differences in shape, dimension and function of the cochlea have been reported for various breeds. In contrast, no comparable investigations to date have been undertaken for the vestibular organ in different breeds of the rat. Vestibular organs of two breeds of rat (Wistar, Sprague-Dawley) were analyzed morphologically and morphometrically by means of microdissection techniques in order to determine the mechanical sensitivity of the cupula according to Oman et al; (Acta Otolaryngol., 1987;103:1-13, 1987). Differences in shape of the lateral semicircular duct exist between the two breeds and the cupular mechanical sensitivity is significantly higher in Wistar than in Sprague-Dawley rats. With respect to the other semicircular ducts, no differences in shape were found between the two strains. The cupular mechanical sensitivity of the anterior semicircular duct, however, is higher in Wistar than in Sprague-Dawley rats. The breeds also differ in the shape of their utriculus; obviously a correlation exists between the latter and the cupular mechanical sensitivity of the semicircular ducts. There are differences in the vestibular organs between the two breeds of the laboratory rat investigated. The cupular mechanical sensitivity of the semicircular duct does not seem to be correlated to body mass. The size and morphology of the utriculus influence the mechanical sensitivity of a single duct, but differences only become significant if other parameters also differ.

Nasal vestibule wall elasticity: interactions with a nasal dilator strip

We studied the effect of an adhesive external nasal dilator strip (ENDS) on external nasal geometry in 20 healthy Caucasian adults (10 men, 10 women; age 21-45 yr). The recoil force exerted by ENDS was estimated by bending the device (n = 10) with known weights. In the horizontal direction, a small/medium-sized ENDS in situ exerted a unilateral recoil force of 21.4-22.6 g. Application of ENDS resulted in a displacement of the lateral nasal vestibule walls that had both anterosuperior and horizontal components and that was maintained over an 8-h period. The resultant unilateral nasal vestibule wall displacement at the tip of the device was at 47.6 +/- 2.0 degrees to the horizontal (as related to the plane of the device when in situ) and had a magnitude of 3.5 +/- 0.1 mm. ENDS increased external nasal cross-sectional area by 23.0-65.3 mm2. Nasal vestibule wall compliance was estimated at 0.05-0.16 mm/g. Thus ENDS applies a relatively constant abducting force irrespective of nasal width. Variable responsiveness to ENDS may be related to differences in elastic properties of the nasal vestibule wall.

Hybrid rendering and virtual endoscopy of the auditory and vestibular system

A hybrid rendering method (color-coded 3D shaded-surface and volume display) with the possibility of virtual endoscopy using image data sets from HR-SCT was developed. To show the possible advantages and benefits of the improved rendering algorithm we have specifically highlighted the use in relation to the auditory and vestibular system. Postprocessing image visualization offers improved morphological analysis, and will benefit radiological diagnostics, medical education, surgical planning, surgical training and postoperative assessment.

Importance of type IV collagen, laminin, and heparan sulfate proteoglycan in the regulation of labyrinthine fluid in the rat cochlear duct

The distribution of major components of the basement membrane, such as type IV collagen, laminin, and heparan sulfate proteoglycan (HSPG), was investigated in the rat cochlear duct. Immunofluorescence demonstrated that type IV collagen, laminin and HSPG were distributed along capillaries in the cochlear duct, including the stria vascularis, spiral ligament, spiral prominence and spiral limbus. Additionally, type IV collagen, laminin and HSPG were found to be distributed from the basement membrane of Reissner's membrane to that of the spiral prominence in a linear pattern. The scala media was surrounded by these basement membrane components, demarcating endolymph from perilymph, along epithelial cells except at the stria vascularis. These findings suggest that type IV collagen, laminin and HSPG create the anatomical separation between endolymph and perilymph, thus indicating that they may be involved in the regulation of fluid transport between the endolymph and perilymph.

Surgical exposure of the fundus of the internal auditory canal: anatomic limits of the middle fossa versus the retrosigmoid transcanal approach

OBJECTIVE

To define the anatomic limitations and advantages of the middle cranial fossa and the retrosigmoid transcanal approaches in the exposure of the fundus of the internal auditory canal (IAC).

STUDY DESIGN

A series of 15 cadaver temporal bone specimens were dissected and the measurements of the lateral recess of the IAC were made with a millimeter rule and rounded to the nearest quarter millimeter.

METHODS

Retrospective case review, surgical observation, review, and measurements recorded from magnetic resonance scans. Surgical observations and measurements recorded from cadaver specimens.

RESULTS

These results were compared with historical studies of the retrosigmoid transcanal approach. The results utilizing a combination of these approaches to remove acoustic neuromas at a tertiary referral center during the preceding 11 years are also presented. Previous studies have shown that for the retrosigmoid transcanal approach, it is impossible to expose 3 to 4 mm of the lateral recess of the IAC without violating the vestibule and/or the endolymphatic duct. This has led some authors to advocate the middle cranial fossa approach to the IAC when hearing preservation is a consideration. The current study shows that the falciform crest obscures the inferior half of the fundus. This creates a pocket that cannot be visualized, which on average is 1.82 x 2.33 mm.

CONCLUSION

The fundus of the IAC cannot be completely exposed without violating the labyrinth through either the posterior fossa or middle fossa approach. The clinical implications of these studies are unknown at this time. Low recurrence rates are achieved with both approaches. The anatomic limitations of both approaches must still be considered when planning or performing these approaches, to minimize the risk of recurrence.

Topographical organization of inferior olive cells projecting to translation and rotation zones in the vestibulocerebellum of pigeons

Previous electrophysiological studies in pigeons have shown that the vestibulocerebellum can be divided into two parasagittal zones based on responses to optic flow stimuli. The medial zone responds best to optic flow resulting from self-translation, whereas the lateral zone responds best to optic flow resulting from self-rotation. This information arrives from the retina via a projection from the accessory optic system to the medial column of the inferior olive. In this study we investigated inferior olive projections to translational and rotational zones of the vestibulocerebellum using the retrograde tracer cholera toxin subunit B. Extracellular recordings of Purkinje cell activity (complex spikes) in response to large-field visual stimuli were used to identify the injection sites. We found a distinct segregation of inferior olive cells projecting to translational and rotational zones of the vestibulocerebellum. Translation zone injections resulted in retrogradely labeled cells in the ventrolateral area of the medial column, whereas rotation zone injections resulted in retrogradely labeled cells in the dorsomedial region of the medial column. Motion of any object through space, including self-motion of organisms, can be described with reference to translation and rotation in three-dimensional space. Our results show that, in pigeons, the brainstem visual systems responsible for detecting optic flow are segregated into channels responsible for the analysis of translational and rotational optic flow in the inferior olive, which is only two synapses from the retina.

Cerebral functional magnetic resonance imaging of vestibular, auditory, and nociceptive areas during galvanic stimulation

Cerebral activation was investigated with functional magnetic resonance imaging (fMRI) during galvanic stimulation of the mastoid in 6 normal volunteers. Cutaneous stimulation at the neck C4-5 level served as a control. During mastoid stimulation, bilateral vestibular activation occurred in the posterior insula (parietoinsular vestibular cortex, PIVC), the transverse temporal (Heschl's) gyrus, and thalamic pulvinar. The cutaneous pain elicited by galvanic stimulation caused bilateral activity of the medial part of the insula and the anterior median thalamus. Thus, galvanic stimulation at the mastoid level activates cortical areas of three different sensory systems in the insulathalamic region, the vestibular, the auditory, and the nociceptive systems.

Measurement of guinea pig basilar membrane using computer-aided three-dimensional reconstruction system

Cochleas are known to have the ability to analyze a frequency widely, and this ability seems to be owed mostly to the basilar membrane (BM) configuration. However, the relationship between the cochlear frequency-position map and the BM configuration is not clear. Therefore, in this paper, the internal structures of a guinea pig cochlea, especially the BM configuration, were reconstructed and measured using a computer-aided three-dimensional (3-D) reconstruction system. Then, an attempt was made to examine the influence of the BM configuration on the cochlear frequency-position map. The measurement results indicate that the width of the BM increased and its thickness decreased with an increase in the distance from the basal turn towards the apical turn. Theoretical consideration reveals that the wide frequency-position of the cochlea is achieved by not only the BM configuration change along the length of the cochlea but also the change of the Young's modulus of the BM along the length of the cochlea.

Physiological Principles of Vestibular Function on Earth and in Space

Physiological mechanisms underlying vestibular function have important implications for our ability to understand, predict, and modify balance processes during and after spaceflight. The microgravity environment of space provides many unique opportunities for studying the effects of changes in gravitoinertial force on structure and function of the vestibular system. Investigations of basic vestibular physiology and of changes in reflexes occurring as a consequence of exposure to microgravity have important implications for diagnosis and treatment of vestibular disorders in human beings. This report reviews physiological principles underlying control of vestibular processes on earth and in space. Information is presented from a functional perspective with emphasis on signals arising from labyrinthine receptors. Changes induced by microgravity in linear acceleration detected by the vestibulo-ocular reflexes. Alterations of the functional requirements for postural control in space are described. Areas of direct correlation between studies of vestibular reflexes in microgravity and vestibular disorders in human beings are discussed. (Otolaryngol Head Neck Surg 1998;118:S5-S15.)

Vestibular-hippocampal interactions

Since the early 1960s, researchers have speculated that the vestibular system, the sensory system concerned with the perception of balance and self-motion, contributes to spatial information processing and the development of spatial memory in the hippocampus. Anatomical studies have suggested that various parts of the thalamus are likely to transmit vestibular information to the hippocampus, perhaps via the parietal cortex; however, more direct pathways are possible. Over the last 2-3 years there have been a number of direct electrophysiological demonstrations that vestibular stimulation affects head direction cells in the anterior thalamic nuclei and place cells in the hippocampus. These studies demonstrate the importance of vestibular-hippocampal interactions for hippocampal function but also raise the possibility that the hippocampus may be important for compensation of vestibular function following peripheral or central vestibular lesions.

The evolution of tetrapod ears and the fossil record

In the earliest tetrapods, the fenestra vestibuli was a large hole in the braincase wall bounded by bones of different embryological origins: the otic capsule and occipital arch components, and also, in all except the Devonian Acanthostega, the dermal parasphenoid. This means that the hole lay along the line of the embryonic metotic fissure. Early tetrapod braincases were poorly ossified internally, and no specialized opening for a perilymphatic duct is evident. It is arguable that the earliest tetrapods had neither a perilympllatic duct crossing the otic capsule nor a specialized auditory receptor in a separate lagenar pouch. The primitive tetrapod condition is found in the earliest amniotes, and the separate development of (1) a fenestra vestibuli confined to the limits of the otic capsule, (2) a specialized pressure relief window also derived from components on the line of the metolic fissure, (3) a nonstructural, vibratory stapes and (4) increased internal ossification of the internal walls of the otic capsule, can be traced separately in synapsids, lepidosauromorph diapsids, archosauromorph diapsids, probably turtles, and amphibians. This suggests separate development of true tympanic ears in each of these groups. Developments indicating the existence of a true tympanic ear in amniotes are first found in animals from the Triassic period, and a correlation with the evolution of insect sound production is suggested.

Corticovestibular interactions: anatomy, electrophysiology, and functional considerations

This review summarizes anatomical and electrophysiological observations related to corticovestibular interactions as a step toward understanding their possible functions. Vestibular information is represented in at least three distinct regions of the cerebral cortex in cats and monkeys: the parietal and somatosensory cortex and the parietoinsular vestibular cortex. In addition, vestibular-related signals are found in more extensive regions, including the motor and premotor regions and frontal eye fields. Most of these regions also project directly to the vestibular nuclei. In monkeys, at least six cortical regions have been identified, including the motor, somatosensory, parietal and temporal areas. Most of these regions receive vestibular projections via the thalamus. Most neurons in those cortical areas respond to head velocity and receive converging vestibular, visual and somatosensory input. Electrical stimulation of some of these cortical areas in anesthetized cats influences the activity of many vestibular nuclear neurons including those projecting to the spinal cord. Lesions of the parietal vestibular regions impair the vestibulo-ocular reflex (VOR) and visual suppression of the VOR as well as vestibular-related cognitive functions such as spatial perception and memory in human subjects. Diffuse cortical damage also results in similar impairment of the VOR and suppression of the VOR and possibly the vestibulo-collic reflex. Such impairments after cortical lesions may well be due in part to interruption of cortico-vestibular connections. Future studies in alert animals should focus on the role of different cortical regions projecting to the vestibular nuclei, specifically on how each affects the processing of vestibular signals that mediate vestibulo-motor reflexes and that are used for vestibular related cognitive processes.

The development of vestibular connections in rat embryos in microgravity

Existing experimental embryological data suggests that the vestibular system initially develops in a very rigid and genetically controlled manner. Nevertheless, gravity appears to be a critical factor in the normal development of the vestibular system that monitors position with respect to gravity (saccule and utricle). In fact several studies have shown that prenatal exposure to microgravity causes temporary deficits in gravity-dependent righting behaviors, and prolonged exposure to hypergravity from conception to weaning causes permanent deficits in gravity-dependent righting behaviors. Data on hypergravity and microgravity exposure suggest some changes in the otolith formation during development, in particular the size although these changes may actually vary with the species involved. In adults exposed to microgravity there is a change in the synaptic density in the optic sensory epithelia suggesting that some adaptation may occur there. However, effects have also been reported in the brainstem. Several studies have shown synaptic changes in the lateral vestibular nucleus and in the nodulus of the cerebellum after neonatal exposure to hypergravity. We report here that synaptogenesis in the medial vestibular nucleus is retarded in developing rat embryos that were exposed to microgravity from gestation days 9 to 19.

Age-related changes of the globular substance in the otoconial membrane of mice

The globular substance, which occurs in the vestibular macula as a precursor of otoconia, was examined in aged mice in comparison with young adult mice. Dissected otoconial membrane from the utricular macula of C57BL/6J mice was loaded with fluo-3-AM, and directly observed under a confocal laser scanning microscope. Internal free Ca2+ concentration ([Ca2+]i) of the globular substance was determined through in situ calibration performed by superfusion with ionomycin and Mn2+. Total area of the otoconial membrane, average diameter of the globular substance, and [Ca2+]i showed no significant differences between young adult and aged groups. However, the number of globular substances in young adult mice was significantly larger than those of aged mice. These results suggest a reduced rate of otoconial formation in the aged vestibule, which would result in the sparseness of otoconia in the aged vestibule and lead to balance disorders commonly seen in elderly persons.

Projections from the subdivisions of the fastigial nucleus to the vestibular complex and the prepositus hypoglossal nucleus in the albino rat: an anterograde tracing study using biocytin

Differential projections from the subdivisions of the fastigial nucleus to the vestibular complex and the prepositus hypoglossal nucleus were investigated by an anterograde tracing method using biocytin in the albino rat. The caudomedial subdivision of the nucleus projected ipsilaterally to the dorsal and medial parts of the superior vestibular nucleus (Su Ve), the dorsomedial part of the lateral vestibular nucleus (LVe), and the dorsal parts of the medial (MVe) and spinal (Sp Ve) vestibular nuclei, and projected contralaterally to the ventrolateral corners of the Su Ve and LVe, the ventral part of the MVe, and the lateral part of the Sp Ve. The bilateral prepositus hypoglossal nuclei received sparse projections from the caudomedial subdivision. The middle subdivision of the fastigial nucleus projected ipsilaterally to the dorsal and/or ventral parts of the Su Ve, the dorsomedial pats of the LVe and Sp Ve, and the dorsolateral part of the MVe, and projected contralaterally to the dorsal margin of the Su Ve, the ventrolateral part of the LVe, and the lateral part of the Sp Ve. The dorsolateral protuberance of the fastigial nucleus projected ipsilaterally to the dorsal margin of the Su Ve, the dorsomedial part of the LVe, the dorsal or lateral parts of the Sp Ve, and the lateral part of the MVe, and projected contralaterally to the ventrolateral part of the LVe and the lateral part of the Sp Ve. The subnuclei x, y, and f, interstitial nucleus of the vestibular nerve, and the infracerebellar nucleus received bilateral or ipsilateral fastigiovestibular projections.

Secondary vestibulo-oculomotor projections in larval sea lamprey: anterior octavomotor nucleus

The ventral octavolateral area of lampreys contains three nuclei: the anterior, intermediate and posterior octavomotor nuclei, formed of large neurons that are contacted by thick primary vestibular fibres. We used horseradish peroxidase (HRP) or fluorescein-dextran-amine (FDA) labelling to study the projections of the anterior octavomotor nucleus (AON) in the larval sea lamprey, Petromyzon marinus. The tracers were injected either in the AON, the oculomotor nucleus or the rostralmost spinal cord. HRP injection in the AON labelled thick axons that coursed to the basal mesencephalic tegmentum, where most decussate and project to the oculomotor nucleus and the third Müller cell. Electron microscopy confirmed that AON axons contact with the contralateral third Müller cell and with oculomotor neurons. Some AON axons run in the mesencephalic tegmentum and the ventral diencephalon. An AON axon was observed to run close to the axon of the contralateral third Müller cell, establishing what appeared to be en passant contacts. HRP injection in the AON also revealed commissural fibres projecting to the contralateral octavolateral area. HRP or FDA injections in the oculomotor nucleus labelled both large and small neurons of the AON, mostly contralateral to the injection site, as well as of cells in the intermediate octavomotor nucleus, mainly ipsilateral. HRP injection in the AON or in the rostral spinal cord did not reveal any projections from the AON to the spinal cord. Our results indicate that the pattern of octavo-oculomotor connections in the lamprey is different from that observed in other vertebrates.

Transmission between the type I hair cell and its calyx ending

The long, uninterrupted apposition between the type I hair cell and the calyx ending has implications for the intercellular communication between these structures. Conventional synaptic transmission will be compromised unless the impedance of the ending is made relatively high. The apposition also creates the possibility of ephaptic transmission between the hair cell and the ending. Ephaptic transmission from the hair cell to the outer face of the calyx ending is too weak to make more than a minor contribution to sensory coding. Basolateral currents associated with hair-cell transduction can result in a substantial accumulation of K+ ions in the intercellular space. The accumulation can alter conventional transmission by depolarizing the hair cell and can alter afferent firing by depolarizing the ending. Reasons were presented suggesting that K+ accumulation may play an essential role in transduction involving type I hair cells, including the linearization of input-output relations and an increase in the maximal rate of discharge.