Insights into Inner Ear Function and Disease Through Novel Visualization of the Ductus Reuniens, a Seminal Communication Between Hearing and Balance Mechanisms

The sensory end-organs responsible for hearing and balance in the mammalian inner ear are connected via a small membranous duct known as the ductus reuniens (also known as the reuniting duct (DR)). The DR serves as a vital nexus linking the hearing and balance systems by providing the only endolymphatic connection between the cochlea and vestibular labyrinth. Recent studies have hypothesized new roles of the DR in inner ear function and disease, but a lack of knowledge regarding its 3D morphology and spatial configuration precludes testing of such hypotheses. We reconstructed the 3D morphology of the DR and surrounding anatomy using osmium tetroxide micro-computed tomography and digital visualizations of three human inner ear specimens. This provides a detailed, quantitative description of the DR's morphology, spatial relationships to surrounding structures, and an estimation of its orientation relative to head position. Univariate measurements of the DR, inner ear, and cranial planes were taken using the software packages 3D Slicer and Zbrush. The DR forms a narrow, curved, flattened tube varying in lumen size, shape, and wall thickness, with its middle third being the narrowest. The DR runs in a shallow bony sulcus superior to the osseus spiral lamina and adjacent to a ridge of bone that we term the "crista reuniens" oriented posteromedially within the cranium. The DR's morphology and structural configuration relative to surrounding anatomy has important implications for understanding aspects of inner ear function and disease, particularly after surgical alteration of the labyrinth and potential causative factors for Ménière's disease.

Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water

Major evolutionary transitions, in which animals develop new body plans and adapt to dramatically new habitats and lifestyles, have punctuated the history of life. The origin of cetaceans from land-living mammals is among the most famous of these events. Much earlier, during the Mesozoic Era, many reptile groups also moved from land to water, but these transitions are more poorly understood. We use computed tomography to study changes in the inner ear vestibular system, involved in sensing balance and equilibrium, as one of these groups, extinct crocodile relatives called thalattosuchians, transitioned from terrestrial ancestors into pelagic (open ocean) swimmers. We find that the morphology of the vestibular system corresponds to habitat, with pelagic thalattosuchians exhibiting a more compact labyrinth with wider semicircular canal diameters and an enlarged vestibule, reminiscent of modified and miniaturized labyrinths of other marine reptiles and cetaceans. Pelagic thalattosuchians with modified inner ears were the culmination of an evolutionary trend with a long semiaquatic phase, and their pelagic vestibular systems appeared after the first changes to the postcranial skeleton that enhanced their ability to swim. This is strikingly different from cetaceans, which miniaturized their labyrinths soon after entering the water, without a prolonged semiaquatic stage. Thus, thalattosuchians and cetaceans became secondarily aquatic in different ways and at different paces, showing that there are different routes for the same type of transition.

Inner ear sensory system changes as extinct crocodylomorphs transitioned from land to water

Major evolutionary transitions, in which animals develop new body plans and adapt to dramatically new habitats and lifestyles, have punctuated the history of life. The origin of cetaceans from land-living mammals is among the most famous of these events. Much earlier, during the Mesozoic Era, many reptile groups also moved from land to water, but these transitions are more poorly understood. We use computed tomography to study changes in the inner ear vestibular system, involved in sensing balance and equilibrium, as one of these groups, extinct crocodile relatives called thalattosuchians, transitioned from terrestrial ancestors into pelagic (open ocean) swimmers. We find that the morphology of the vestibular system corresponds to habitat, with pelagic thalattosuchians exhibiting a more compact labyrinth with wider semicircular canal diameters and an enlarged vestibule, reminiscent of modified and miniaturized labyrinths of other marine reptiles and cetaceans. Pelagic thalattosuchians with modified inner ears were the culmination of an evolutionary trend with a long semiaquatic phase, and their pelagic vestibular systems appeared after the first changes to the postcranial skeleton that enhanced their ability to swim. This is strikingly different from cetaceans, which miniaturized their labyrinths soon after entering the water, without a prolonged semiaquatic stage. Thus, thalattosuchians and cetaceans became secondarily aquatic in different ways and at different paces, showing that there are different routes for the same type of transition.

The evolution of the vestibular apparatus in apes and humans

Phylogenetic relationships among extinct hominoids (apes and humans) are controversial due to pervasive homoplasy and the incompleteness of the fossil record. The bony labyrinth might contribute to this debate, as it displays strong phylogenetic signal among other mammals. However, the potential of the vestibular apparatus for phylogenetic reconstruction among fossil apes remains understudied. Here we test and quantify the phylogenetic signal embedded in the vestibular morphology of extant anthropoids (monkeys, apes and humans) and two extinct apes (Oreopithecus and Australopithecus) as captured by a deformation-based 3D geometric morphometric analysis. We also reconstruct the ancestral morphology of various hominoid clades based on phylogenetically-informed maximum likelihood methods. Besides revealing strong phylogenetic signal in the vestibule and enabling the proposal of potential synapomorphies for various hominoid clades, our results confirm the relevance of vestibular morphology for addressing the controversial phylogenetic relationships of fossil apes.

Bionic balance organs: progress in the development of vestibular prostheses

The vestibular system is a sensory system that is critically important in humans for gaze and image stability as well as postural control. Patients with complete bilateral vestibular loss are severely disabled and experience a poor quality of life. There are very few effective treatment options for patients with no vestibular function. Over the last 10 years, rapid progress has been made in developing artificial 'vestibular implants' or 'prostheses', based on cochlear implant technology. As of 2017, 13 patients worldwide have received vestibular implants and the results are encouraging. Vestibular implants are now becoming part of an increasing effort to develop artificial, bionic sensory systems, and this paper provides a review of the progress in this area.

Gender Differences in Perception of Self-Orientation: Software or Hardware?

We evaluated the contribution of attentional strategy to the perception of self-orientation with and without a body tilt in the median plane. Reinking et al (1974 Journal of Personality and Social Psychology30 807–811) found that the frame dependence of females on the rod-and-frame test could be mediated by instructions prompting them to focus on internal cues (ie arising from inside of the body). Here, we measured the influence of attentional instructions on the perception of the morphological horizon. Eleven females and thirteen males estimated their morphological horizon in an upright and a 45° body tilt in the median plane under three instruction conditions. All participants first performed without attentional instructions. Then, participants performed under both internal and external attentional instructions. For females, but not for males, perception of morphological horizon was more footward in the supine than in the upright orientation. Although instructions did not eliminate gender differences, internal instructions allowed females to reduce their perceptual bias in the supine orientation.

Anatomy of the para-vestibular canaliculus

A histologic study of the para-vestibular canaliculus (PVC), its contents, and its relationship to the vestibular aqueduct (VA), is presented. 20 normal human temporal bones were fixed in 10% formalin solution, embedded in celloidin, and sectioned horizontally at intervals of 20 micrometers. Every tenth section was stained with hematoxylin and eosin (HE) and studied under a light microscope. Three significant observations were made. First, in 80% of the specimens, two rather than one PVC were found in the area of the vestibular orifice of the VA. Second, in 70% of the specimens, the PVC was found to merge with the VA rather than to enter the posterior cranial fossa (PCF) separately. Third, in all the specimens examined, a vein was seen to traverse the entire length of the PVC. However, in 17 specimens, no artery could be identified within the PVC. In the 13 (65%) specimens in which arteries could be identified in the PVC, the arteries extended only half the length of the PVC, from the PCF to the VA. In no specimen examined could arteries be seen extending the full length of the PVC from the PCF to the vestibule.

Nobel Prize centenary: Robert Bárány and the vestibular system

The hundredth anniversary of Robert Bárány's Nobel Prize in Medicine offers the opportunity to highlight the importance of his discoveries on the physiology and pathophysiology of the vestibular organs. Bárány developed the method of caloric vestibular stimulation that revolutionized the investigation of the semicircular canals and that is still widely used today. Caloric vestibular stimulation launched experimental vestibular research that was relevant to comprehend the evolution of human locomotion, and Bárány's tests continue to be used in neuroscience to understand the influence of vestibular signals on bodily perceptions, cognition and emotions. Only during the last 20 years has caloric vestibular stimulation been merged with brain imaging to localize the human vestibular cortex.

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.

Vestibular disease: diseases causing vestibular signs

Having determined whether a patient has central or peripheral vestibular disease, clinicians must then determine what diseases are likely to result in such a presentation. This article describes the more common diseases causing vestibular disease in dogs and cats. Having formulated a list of potential causes of vestibular disease, clinicians should proceed through a systematic investigation to diagnose the underlying condition. A companion article describes the anatomy, physiology, and clinical signs associated with vestibular disease.

[Histological changes of peripheral vestibular organs in the inner ears of Smad4 conditional knockout mice]

OBJECTIVE

To investigate the histological changes in the vestibular endorgans of Smad4 gene conditional knockout mice and to explore the influence of the Smad4 gene on vestibular development.

METHODS

Histological changes of periphery vestibular organs in inner ear of Smad4 conditional knockout mice were investigated by frozen sections, immunofluorescence, confocal microscopy, scanning electron microscopy and transmission electron microscopy.

RESULTS

There was no Smad4 expression in the inner ear cartilage capsule of Smad4-/- mice. In Smad4+/- mice, Smad4 expression in the same cartilage capsule was positive, and it was strong positive in Smad4+/+ mice. Smad4 expression in vestibular sense epithelium, crista ampullaris and macula, was positive. And no difference was found among these three genotypes. Studying at scanning electron microscopy and transmission electron microscopy levels and anti-filament immunofluorescence showed that no pathological changes were observed in all the three genotype mice.

CONCLUSION

Although the Smad4 gene was knockout effectively in the auricular cartilage capsule of Smad4 conditional knockout mice,the histological changes of Smad4 conditional knockout mice in vestibulum auris internal were slightly.

Vestibular disease: anatomy, physiology, and clinical signs

The vestibular system is responsible for keeping an animal oriented with respect to gravity. It is a sensory system that maintains the position of the eyes, body, and limbs in reference to the position of the head. Proper interpretation of neurologic deficits and precise neuroanatomic localization are essential to diagnose and prognosticate the underlying disorder. Neurologic examination can confirm whether the vestibular dysfunction is of peripheral or central nervous system origin. Idiopathic vestibular syndrome is the most common cause of peripheral vestibular disease in dogs and, despite its dramatic clinical presentation, can improve without intervention. Central vestibular diseases generally have a poorer prognosis.

A mathematical model of human semicircular canal geometry: a new basis for interpreting vestibular physiology

We report a precise, simple, and accessible method of mathematically measuring and modeling the three-dimensional (3D) geometry of semicircular canals (SCCs) in living humans. Knowledge of this geometry helps understand the development and physiology of SCC stimulation. We developed a framework of robust techniques that automatically and accurately reconstruct SCC geometry from computed tomography (CT) images and are directly validated using micro-CT as ground truth. This framework measures the 3D centroid paths of the bony SCCs allowing direct comparison and analysis between ears within and between subjects. An average set of SCC morphology is calculated from 34 human ears, within which other geometrical attributes such as nonplanarity, radius of curvature, and inter-SCC angle are examined, with a focus on physiological implications. These measurements have also been used to critically evaluate plane fitting techniques that reconcile many of the discrepancies in current SCC plane studies. Finally, we mathematically model SCC geometry using Fourier series equations. This work has the potential to reinterpret physiology and pathophysiology in terms of real individual 3D morphology.

Inner ear of a notoungulate placental mammal: anatomical description and examination of potentially phylogenetically informative characters

We provide the first detailed description of the inner ear of a notoungulate, an extinct group of endemic South American placental mammals, based on a three-dimensional reconstruction extracted from CT imagery of a skull of Notostylops murinus. This description provides new anatomical data that should prove to be phylogenetically informative, an especially significant aspect of this research given that both the interrelationships of notoungulates and the position of Notoungulata within Placentalia are still unresolved. We also assess the locomotor agility of Notostylops based on measurements of the semicircular canals. This is the best available data on the locomotion of a notostylopid because significant postcranial remains for this group have not been described. The cochlea of Notostylops has 2.25 turns, and the stapedial ratio is 1.6. The stapedial ratio is one of the lowest recorded for a eutherian, which typically have ratios greater than 1.8. The fenestra cochleae is located posterior to the fenestra vestibuli, a condition previously only reported for some stem primates. The separation of the saccule and utricule of the vestibule is visible on the digital endocast of the bony labyrinth. The posterior arm of the LSC and the inferior arm of the PSC are confluent, but these do not form a secondary crus commune, and the phylogenetic or functional significance of this confluence is unclear at this time. Locomotor agility scores for Notostylops suggest a medium or 'average' degree of agility of motion compared to extant mammals. In terms of its locomotion, we tentatively predict that Notostylops was a generalized terrestrial mammal, with cursorial tendencies, based on its agility scores and the range of locomotor patterns inferred from postcranial analyses of other notoungulates.

Assessment of the reuniting duct by three-dimensional CT rendering

CONCLUSION

The rendering strategy sometimes induces misunderstanding of the image. We demonstrated a more accurate image of the bony groove of the reuniting duct using three-dimensional (3D) cone beam CT image, which was less affected by artifacts created by the rendering effect.

OBJECTIVE

To obtain a suitable image of the groove of the reuniting duct for future morphological study.

MATERIALS AND METHODS

The grooves of reuniting ducts in 10 healthy human subjects were analyzed by cone beam CT in comparison with a cadaver study.

RESULTS

We could obtain more accurate 3D CT images of the bony groove in human subjects by checking the landmarks of 3D CT images.

Three-dimensional virtual model of the human temporal bone: a stand-alone, downloadable teaching tool

OBJECTIVE

To develop a three-dimensional virtual model of a human temporal bone based on serial histologic sections.

BACKGROUND

The three-dimensional anatomy of the human temporal bone is complex, and learning it is a challenge for students in basic science and in clinical medicine.

METHODS

Every fifth histologic section from a normal 14-year-old male was digitized and imported into a general purpose three-dimensional rendering and analysis software package called Amira (version 3.1). The sections were aligned, and anatomic structures of interest were segmented.

RESULTS

The three-dimensional model is a surface rendering of these structures of interest, which currently includes the bone and air spaces of the temporal bone; the perilymph and endolymph spaces; the sensory epithelia of the cochlear and vestibular labyrinths; the ossicles and tympanic membrane; the middle ear muscles; the carotid artery; and the cochlear, vestibular, and facial nerves. For each structure, the surface transparency can be individually controlled, thereby revealing the three-dimensional relations between surface landmarks and underlying structures. The three-dimensional surface model can also be "sliced open" at any section and the appropriate raw histologic image superimposed on the cleavage plane. The image stack can also be resectioned in any arbitrary plane.

CONCLUSION

This model is a powerful teaching tool for learning the complex anatomy of the human temporal bone and for relating the two-dimensional morphology seen in a histologic section to the three-dimensional anatomy. The model can be downloaded from the Eaton-Peabody Laboratory web site, packaged within a cross-platform freeware three-dimensional viewer, which allows full rotation and transparency control.

Vestibular orientation for craniofacial surgery: application to the management of unicoronal synostosis

BACKGROUND

Standardized cephalometric measurements are necessary to compare skulls of different ages and sizes, in normal and diseased subjects, and in different species. In diseases involving the skull base, classical cephalometry is often impossible because the cranial landmarks are modified. In vestibular orientation (VO), the plane of the lateral semicircular canal (LSCC) of the inner ear, which has a constant relation to gravity, defines the horizontal plane of reference. Defining a reference plane independent of external landmarks is especially important in unicoronal craniosynostosis (UCCS), because the skull base is asymmetrical.

AIM OF THE STUDY

To illustrate the interest of VO in clinical practice, we report on our experience with VO-based correction of UCCS.

MATERIALS AND METHODS

Since 1992, we have used VO-3D computed tomography scanner for surgical planning of all patients with UCCS, measuring the required correction as the discrepancy between the theoretical and the observed midline.

RESULTS

Thirty-eight children were operated under the age of 2 years for UCCS and evaluated after a mean follow-up of 66 months. Thirty-two (84%) were considered perfect, four (11%) had mild imperfection not requiring reoperation, and two (5%) required reoperation because of progressive craniosynostosis involving the sagittal suture. Good surgical results were obtained when the orbits were correctly oriented relative to the plane of the LSCC.

CONCLUSIONS

VO is a useful reference system for the evaluation and surgical planning of UCCS. We hypothesize that the mismatch between the visual and labyrinthic sensorial inputs plays a role in the pathophysiology of UCCS.

Imaging anatomy of the vestibular and visual systems

PURPOSE OF REVIEW

This review will outline the imaging anatomy of the vestibular and visual pathways, using computed tomography and magnetic resonance imaging, with emphasis on the more recent developments in neuroimaging.

RECENT FINDINGS

Technical advances in computed tomography and magnetic resonance imaging, such as the advent of multislice computed tomography and newer magnetic resonance imaging techniques such as T2-weighted magnetic resonance cisternography, have improved the imaging of the vestibular and visual pathways, allowing better visualization of the end organs and peripheral nerves. Higher field strength magnetic resonance imaging is a promising tool, which has been used to evaluate and resolve fine anatomic detail in vitro, as in the labyrinth. Advanced magnetic resonance imaging techniques such as functional magnetic resonance imaging and diffusion tractography have been used to identify cortical areas of activation and associated white matter pathways, and show potential for the future identification of complex neuronal relays involved in integrating these pathways.

SUMMARY

The assessment of the various components of the vestibular and the visual systems has improved with more detailed research on the imaging anatomy of these systems, the advent of high field magnetic resonance scanners and multislice computerized tomography, and the wider use of specific techniques such as tractography which displays white matter tracts not directly accessible until now.

Vestibular function testing

PURPOSE OF REVIEW

This review provides an overview of vestibular function testing and highlights the new techniques that have emerged during the past 5 years.

RECENT FINDINGS

Since the introduction of video-oculography as an alternative to electro-oculography for the assessment of vestibular-induced eye movements, the investigation of the utricle has become a part of vestibular function testing, using unilateral centrifugation. Vestibular evoked myogenic potentials have become an important test for assessing saccular function, although further standardization and methodological issues remain to be clarified. Galvanic stimulation of the labyrinth also is an evolving test that may become useful diagnostically.

SUMMARY

A basic vestibular function testing battery that includes ocular motor tests, caloric testing, positional testing, and earth-vertical axis rotational testing focuses on the horizontal semicircular canal. Newer methods to investigate the otolith organs are being developed. These new tests, when combined with standard testing, will provide a more comprehensive assessment of the complex vestibular organ.

Morphometric investigations of sensory vestibular structures in tadpoles (Xenopus laevis) after a spaceflight: implications for microgravity-induced alterations of the vestibuloocular reflex

In lower vertebrates, gravity deprivation by orbital flights modifies the vestibuloocular reflex. Using the amphibian Xenopus laevis, the experiments should clarify to which extent macular structures of the labyrinth are responsible for these modifications. In particular, the shape of otoconia and number and size of sensory macular cells expressing CalBindin were considered. CalBindin is common in mature sensory cells including vestibular hair cells and is probably involved in otoconia formation. Two developmental stages were used for this study: stage 26/27 embryos, which were unable to perform the roll-induced vestibuloocular reflex (rVOR) at onset of microgravity, and stage 45 tadpoles, which had already developed the reflex. The main observations were that the developmental progress of the animals was not affected by microgravity; that in the young tadpole group with normal body shape the rVOR was not modified by microgravity, while in the older group with microgravity experience, the rVOR was augmented; and that significant effects on the shape of otoconia and on the number and size of CalBindin-expressing cells of the labyrinthine maculae cells were absent. In addition, behavioural data were never significantly correlated with morphological features of macular structures such as size and number of CalBindin-expressing cells. It is postulated that mechanisms of vestibular adaptation to microgravity during early development are probably based on mechanisms located in central structures of the vestibular system.

Vestibular efferents contain peripherin

Vestibular efferents have a common origin with the motoneurons of the facial nerve. In adults they share a number of common features, such as the same transmitter. Here we show using retrograde transport and immunohistochemistry, that the vestibular efferents, like facial motoneurons, contain peripherin. This supports the suggestion that peripherin-positive fibers at the apex of the cristae ampullaris are efferents.

Morphometry of otoliths in chicken macula lagena

The macula lagena located at the apical end of the cochlea in birds is characterized by the presence of numerous otoliths with unclear sensory functions. These otoliths are reported to be similar to those in the vestibular system but their detailed features in morphology are unknown. In the present study, we examined the number, size and shape of otoliths from the macula lagena in Chinese domestic chickens (Gallus Ling Nan) with a scanning electron microscope for morphometry. For chickens aged 10-15 post-hatch days, the otoliths in each macula lagena were counted to be 16,055 +/- 4038 (mean +/- S.D., n = 4). The average length and width were 12.98 +/- 3.70 microm and 5.10 +/- 1.48 microm (n = 526 otoliths), respectively. The ratio of length to width for the otolith was 2.58 +/- 0.39 (n = 526 otoliths) and remained relatively constant despite their variations in physical size. Almost all the otoliths were in regular shape and appeared like isolated cylinders with smooth facets at each end, but a few of them (0.025% of 64,221 otoliths screened) were found to be in odd shapes, such as T-shape and cross-shape. The results suggest that otoliths in the macula lagena and those in the vestibular system of bird's inner ear have similar physical properties and may play a similar role in sensing gravitational and acceleration signals.

Three-dimensional regular arrangement of the annular ligament of the rat stapediovestibular joint

The stapes footplate articulates with the vestibular window through the annular ligament. This articulation is known as the stapediovestibular joint (SVJ). We investigated the ultrastructure of adult rat SVJ and report here on the characteristic ultrastructure of the corresponding annular ligament. Transmission electron microscopy showed that this annular ligament comprises thick ligament fibers consisting of a peripheral mantle of microfibrils and an electron-lucent central amorphous substance that is regularly arranged in a linear fashion, forming laminated structures parallel to the horizontal plane of the SVJ. Scanning electron microscopy revealed that transverse microfibrils cross the thick ligament fibers, showing a lattice-like structure. The annular ligament was vividly stained with elastica van Gieson's stain and the Verhoeff's iron hematoxylin method. Staining of the electron-lucent central amorphous substance of the thick ligament fibers by the tannate-metal salt method revealed an intense electron density. These results indicate that the annular ligament of the SVJ is mainly composed of mature elastic fibers.

An electronic prosthesis mimicking the dynamic vestibular function

This paper presents a functional architecture, system level design, and electronic evaluation of a unilateral vestibular prosthesis. The sensing unit of the prosthesis is a custom-designed one-axis micro-electromechanical system (MEMS) gyroscope. Similar to the natural semicircular canal, the MEMS gyroscope senses angular motion of the head and generates voltages proportional to the corresponding angular acceleration. The voltage is then converted into electric current pulses according to the physiological data relating angular acceleration to the spike count in the vestibular nerve. The current pulses can be delivered to stimulate the corresponding vestibular nerve branch. Electronic properties of the vestibular prosthesis prototype have been systematically evaluated and found to meet the design specifications. A unique feature of the present vestibular implant prototype is the scalability: the sensing unit, pulse generator, and the current source can be potentially implemented on a single chip using integrated MEMS technology.

MRI and fMRI analysis of oculomotor function

This chapter reviews the anatomical correlations of the cortical oculomotor centers in humans. The modern structural methods allow a better anatomical definition of the parietal, frontal and temporal structures involved in oculomotor control. Functional imaging reveals the cortical networks involved in saccadic, pursuit, and vestibular eye movements. Finally, the interaction of the network between attention and eye movements is discussed.

Organization of dye-coupled cerebellar granule cells labeled from afferent vestibular and dorsal root fibers in the frogRana esculenta

Application of neurobiotin to the nerves of individual labyrinthine organs and dorsal root fibers of limb-innervating segments of the frog resulted in labeling of granule cells in the cerebellum showing a significant overlap with a partial segregation in the related areas of termination. In different parts of the cerebellum, various combinations of different canal and otolith organ-related granule cells have been discerned. The difference in the extension of territories of vertical canals vs. horizontal canals may reflect their different involvement in the vestibuloocular and vestibulospinal reflex. Dye-coupled cells related to the lagenar and saccular neurons were localized in more rostral parts of the cerebellum, whereas cells of the utricle were represented only in its caudal half. This separation is supportive of the dual function of the lagena and the saccule. The territories of granule cells related to the cervical and lumbar segments of the spinal cord were almost completely separated along the rostrocaudal axis of cerebellum, whereas their territories were almost entirely overlapping in the mediolateral and ventrodorsal directions. The partial overlap of labyrinthine organ-related and dorsal root fiber-related granule cells are suggestive of a convergence of sensory modalities involved in the sense of balance. We propose that the afferent input of vestibular and proprioceptive fibers mediated by gap junctions to the cerebellar granule cells subserve one of the possible morphological correlates of a very rapid modification of the motor activity in the vestibulocerebellospinal neuronal circuit.

Vertigo and motion sickness. Part I: vestibular anatomy and physiology

Control of the symptoms of vertigo and motion sickness requires consideration of the neurophysiology of areas both intrinsic and extrinsic to the vestibular system proper. We review the essential anatomy and physiology of the vestibular system and the associated vomiting reflex.

Compartmentation of the reeler cerebellum: segregation and overlap of spinocerebellar and secondary vestibulocerebellar fibers and their target cells

The cerebellum of the reeler mutant mouse has an abnormal organization; its single lobule is composed of a severely hypogranular cortex and a central cerebellar mass (CCM) consisting of Purkinje cell clusters intermixing with the cerebellar nuclei. As such the reeler represents an excellent model in which to examine the effect of the abnormal distribution of cerebellar cells on afferent-target relationships. To this effect we studied the organization of the spinocerebellar and secondary vestibulocerebellar afferent projections in homozygous reeler mice (rl/rl) using anterograde tracing techniques. Spinal cord injections resulted in labeled spinocerebellar mossy fiber rosettes in specific anterior and posterior regions of the cerebellar cortex. Some vestiges of parasagittal organization may be present in the anterior projection area. Within the CCM, labeled fibers appeared to terminate on distinct groups of Purkinje cells. Thus, the spinocerebellar mossy fibers seem to form both normal and heterologous synapses in the reeler cerebellum. Secondary vestibular injections resulted in both retrograde and anterograde labeling. Retrograde labeling was seen in clusters of Purkinje cells and cerebellar nuclear cells; anterograde labeling was distributed in the white matter and in specific regions of the anterior and posterior cortex of the cerebellum. The labeled spinocerebellar and secondary vestibulocerebellar afferents overlapped in the anterior region but in the posterior region the vestibulocerebellar termination area was ventral to the spinocerebellar area. An area devoid of labeled terminals was also observed ventral to the posterior secondary vestibulocerebellar termination field. Using calretinin immunostaining it was determined that this area contains unipolar brush cells, a cell type found primarily in the vestibulocerebellum of normal mice. Our data indicate that despite of the lack of known landmarks (fissures, lobules) the spinocerebellar and vestibulocerebellar afferent projections in the reeler cerebellum do not distribute randomly but have specific target regions, and the position of these regions, relative to each other, appears to be conserved. Two caveats to this were the finding of overlapping terminal fields of these afferents in the anterior region, and a posteroventral region that contains unipolar brush cells yet is devoid of secondary vestibulocerebellar afferents. The distribution of Purkinje cells and cerebellar nuclear cells is not random either; those that give rise to cerebellovestibular efferents form distinct groups within the central cerebellar mass.

Optimizing the vertebrate vestibular semicircular canal: could we balance any better?

The fluid-filled semicircular canals (SCCs) of the vestibular system are used by all vertebrates to sense angular rotation. Despite masses spanning seven decades, all mammalian SCCs are nearly the same size. We propose that the SCC represents a sensory organ that evolution has "optimally designed." Four geometric parameters characterize the SCC, and "building materials" of given physical properties are assumed. Identifying physical and physiological constraints on SCC operation, we find the most sensitive SCC has dimensions consistent with available data. Since natural selection involves optimization, this approach may find broader use in understanding biological structures.

Functional and anatomical correlates of impaired velocity storage

Velocity storage (VS), a brainstem function, extends the low-frequency response of the vestibular system. To better understand VS mechanisms and characteristics in humans, we analyzed retrospectively functional measures of gait, electrophysiological measures of vestibular function, and imaging studies in an attempt to determine clinical, electrophysiological, and anatomical correlates of abnormalities in VS. Two cohorts of patients referred to our Risk of Falls Assessment Clinic participated in this investigation. Group 1 (control) patients demonstrated normal caloric and rotary chair tests. Group 2 patients with impaired velocity storage (experimentals) differed clinically from Group 1 only by demonstrating abnormal multifrequency vestibulocular reflex phase measures on rotational testing. Results showed that Group 2 patients had greater impairments in postural stability and gait than Group 1 patients. Additionally, 80% of patients in Group 2 and none in Group 1 showed pontine hyperintense lesions on MRI.

[Anatomy and physiology of the vestibular system: review of the literature]

The vestibular system is a complex system involving not only posterior labyrinth but also central structures such as cerebellum, striatum, thalamus, frontal and prefrontal cortex to assure balance, movements and walking. Information reaching the vestibular complex are not purely vestibular but also from visual, somatosensory and cerebellar origins. The equilibrium is also a complex physiological function needing concordance of vestibular, visual and somatosensory information or either central compensation after an injury but also an integrity of the central nervous system.

Etude anatomo-histologique de l’anastomose vestibulo-cochléaire de Von Oort

OBJECTIVES

The vestibulocochlear anastomosis was first described in 1918 by von Oort. It is situated deeply at the bottom of the internal acoustic meatus, and spreads from the saccular nerve before its terminal ramifications, to the cochlear nerve before its penetration into the cochlea. Nerve fibers of the cochlear efferent system are thought to pass through it. The aim of our study was to investigate the anatomy of the vestibulocochlear anastomosis and characterize its histological features.

METHOD

[corrected] Ten human temporal bones were dissected. Serial sections were obtained for histological evaluation.

RESULTS

The vestibulocochlear anastomosis was found in seven of the specimens, perfectly visualized in six. Average diameter was 0.5 mm with lengths varying from 0.5 to 1 mm. Serial histological sections demonstrated the nervous nature of the anastomosis and its relations with the saccular and cochlear nerves. The epinevrium of the saccular nerve was continuous with the supposed anastomosis in five of the specimens, demonstrating the distinct nature of the anastomosis from the saccular and cochlear nerves. We did not find any evidence linking these fibers to the cochlear efferent system.

DISCUSSION

The vestibulocochlear anastomosis was found in seven of our ten dissections. The anastomosis is probably an anatomic reality composed of nerve fibers. The efferent function of these fibers remains to be demonstrated.

Polysialic acid and HNK-1 are expressed in the adult rat vestibular endorgans

Polysialic acid (PSA) and human natural killer (HNK)-1 carbohydrate epitopes are expressed mainly in developing neurons but also in restricted areas, even in adulthood. In the present study, we demonstrated the expression of PSA and HNK-1 epitopes in adult primary vestibular afferent neurons. In addition, we confirmed the presence of two distinct polysialyltransferases, PST and STX, that form PSA, as well as two types of glucuronyltransferases, GlcAT-P and GlcAT-S involved in the biosynthesis of HNK-1 epitopes in the vestibular endorgans. These results combined suggest that both PSA and HNK-1 carbohydrate epitopes are synthesized and may have an important role in the adult peripheral vestibular endorgans.

3-D reconstruction of the vestibular endorgans in pediatric temporal bones

We investigated the vestibular endorgans in three children using 3-D reconstructions from histological sections. The right temporal bone of a newborn child without peripheral vestibular pathology was used as reference model and the temporal bones from a child with Goldenhar syndrome and a child with Pierre Robin sequence with known peripheral vestibular pathology were studied. All five temporal bones were prepared by the celloidin technique and sectioned at 20 microm. Each available section was digitized with a slide scanner. The imaging data were layered anatomically correctly and rendered in a 3-D software. With this technique all vestibular endorgans were reconstructed and measured. The standard deviations in distances ranged between 0.5 and 1.2% and in angles between 0.1 and 2.9 degrees. Both maculae were curved in the longitudinal and transverse axes which described a curve of approximately 35 degrees. The angles between the semicircular ducts varied between 97 and 110 degrees. The pathological models demonstrated a distorted configuration of the semicircular canals and differed substantially from the reference model in most of the measured distances and angles. The method presented is capable of generating 3-D models of the vestibular system from histological sections with an acceptable precision without previously inserted reference marks. Archival celloidin sections are widely available and will be an important resource in understanding the detailed 3-D geometry of the vestibular system which has not yet been accomplished.

Vestibulo-ocular physiology underlying vestibular hypofunction

The vestibular system detects motion of the head and maintains stability of images on the fovea of the retina as well as postural control during head motion. Signals representing angular and translational motion of the head as well as the tilt of the head relative to gravity are transduced by the vestibular end organs in the inner ear. This sensory information is then used to control reflexes responsible for maintaining the stability of images on the fovea (the central area of the retina where visual acuity is best) during head movements. Information from the vestibular receptors also is important for posture and gait. When vestibular function is normal, these reflexes operate with exquisite accuracy and, in the case of eye movements, at very short latencies. Knowledge of vestibular anatomy and physiology is important for physical therapists to effectively diagnose and manage people with vestibular dysfunction. The purposes of this article are to review the anatomy and physiology of the vestibular system and to describe the neurophysiological mechanisms responsible for the vestibulo-ocular abnormalities in patients with vestibular hypofunction.

Vestibular defects in head-tilt mice result from mutations in Nox3, encoding an NADPH oxidase

The vestibular system of the inner ear is responsible for the perception of motion and gravity. Key elements of this organ are otoconia, tiny biomineral particles in the utricle and the saccule. In response to gravity or linear acceleration, otoconia deflect the stereocilia of the hair cells, thus transducing kinetic movements into sensorineural action potentials. Here, we present an allelic series of mutations at the otoconia-deficient head tilt (het) locus, affecting the gene for NADPH oxidase 3 (Nox3). This series of mutations identifies for the first time a protein with a clear enzymatic function as indispensable for otoconia morphogenesis.

Perspective on the vestibular cortex throughout history

The human vestibular organ transmits sensory information to various components of the central nervous system related to head movement and, obviously, among these components, to its terminal region(s) in the vestibular parts of the cerebral cortex. Study of vestibular structures dates back to historical epochs when primitive considerations on cerebral global function were made without knowledge of a cerebral cortical region related to vestibular function. At the time of Menière in the 19th century, patients with vertigo were defined as having cerebral congestion. Cerebral mapping and computational anatomy in the 20th century significantly expanded our knowledge of cerebral structure and its function and the concept of cerebral processing of a variety of types of information, including that generated by the vestibular system. These modern techniques include nuclear magnetic resonance imaging, functional magnetic resonance imaging, and positron emission tomography. These techniques have allowed researchers to define the cortical representation of the vestibular system in human beings and in other species, a representation generally assumed to be located in various cerebral temporal and parietal regions. Although vestibular activation has been recorded in frontal lobe regions, the main vestibular cortical zone has been defined as being located in the parietal lobe; others have recognized a vestibular cortical function in the insula.

Reappraisal of the human vestibular cortex by cortical electrical stimulation study

The cortical areas with vestibular input in humans were assessed by electrical stimulation in 260 patients with partial epilepsy who had undergone stereotactic intracerebral electroencephalogram recordings before surgery. Vestibular symptoms were electrically induced on 44 anatomical sites in 28 patients. The patients experienced illusions of rotation (yaw plane: 18, pitch plane: 6, roll plane: 6), translations (n = 6), or indefinable feelings of body motion (n = 8). Almost all vestibular sites were located in the cortex (41/44): in the temporal (n = 19), parietal (n = 14), frontal (n = 5), occipital (n = 2), and insular (n = 1) lobes. Among these sites, we identified a lateral cortical temporoparietal area we called the temporo-peri-Sylvian vestibular cortex (TPSVC), from which vestibular symptoms, and above all rotatory sensations, were particularly easily elicited (24/41 cortical sites, 58.5%). This area extended above and below the Sylvian fissure, mainly inside Brodmann areas 40, 21, and 22. It included the parietal operculum (9/24 TPSVC sites) which was particularly sensitive for eliciting pitch plane illusions, and the mid and posterior part of the first and second temporal gyri (15/24 TPSVC sites) which preferentially caused yaw plane illusions. We suggest that the TPSVC could be homologous with the monkey's parietoinsular vestibular cortex.

MuSC is involved in regulating axonal fasciculation of mouse primary vestibular afferents

Regulation of axonal fasciculation plays an important role in the precise patterning of neural circuits. Selective fasciculation contributes to the sorting of different types of axons and prevents the misrouting of axons. However, axons must defasciculate once they reach the target area. To study the regulation of fasciculation, we focused on the primary vestibulo-cerebellar afferents (PVAs), which show a dramatic change from fasciculated axon bundles to defasciculated individual axons at their target region, the cerebellar primordium. To understand how fasciculation and defasciculation are regulated in this system, we investigated the roles of murine SC1-related protein (MuSC), a molecule belonging to the immunoglobulin superfamily. We show: (i) by comparing 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) labelling and anti-MuSC immunohistochemistry, that downregulation of MuSC in PVAs during development is concomitant with the defasciculation of PVA axons; (ii) in a binding assay with cells expressing MuSC, that MuSC has cell-adhesive activity via a homophilic binding mechanism, and this activity is increased by multimerization; and (iii) that MuSC also displays neurite outgrowth-promoting activity in vestibular ganglion cultures. These findings suggest that MuSC is involved in axonal fasciculation and its downregulation may help to initiate the defasciculation of PVAs.

The evolution of concepts of vestibular peripheral information processing: toward the dynamic, adaptive, parallel processing macular model

In a letter to Robert Hooke, written on 5 February, 1675, Isaac Newton wrote "If I have seen further than certain other men it is by standing upon the shoulders of giants." In his context, Newton was referring to the work of Galileo and Kepler, who preceded him. However, every field has its own giants, those men and women who went before us and, often with few tools at their disposal, uncovered the facts that enabled later researchers to advance knowledge in a particular area. This review traces the history of the evolution of views from early giants in the field of vestibular research to modern concepts of vestibular organ organization and function. Emphasis will be placed on the mammalian maculae as peripheral processors of linear accelerations acting on the head. This review shows that early, correct findings were sometimes unfortunately disregarded, impeding later investigations into the structure and function of the vestibular organs. The central themes are that the macular organs are highly complex, dynamic, adaptive, distributed parallel processors of information, and that historical references can help us to understand our own place in advancing knowledge about their complicated structure and functions.

Transduction and adaptation in sensory hair cells of the mammalian vestibular system

The human vestibular apparatus detects head movements and gravitational stimuli which impinge upon the mechanosensory hair cells of the inner ear. The hair cells, in turn, transduce these stimuli into electrical signals which are transmitted to the brain. These sensory cells are exquisitely responsive, signaling deflections of their mechanosensitive organelles as small as 1-2 nanometers. Remarkably, they are able to preserve this level of sensitivity even when confronted with large tonic stimuli, such as gravity. To accomplish this feat hair cells have devised a novel adaptation process that repositions the mechanotransduction apparatus on a millisecond time scale to allow high sensitivity over a broad operating range. Mechanotransduction in hair cells occurs via a direct gating mechanism in which hair bundle deflection focuses tension onto membrane-bound, cation-selective ion channels located near the tips of the hair bundle. Increased tension favors an open conformation of the channel and allows calcium to enter the cell. Elevated intracellular calcium promotes adaptation which has been hypothesized to result from the activity of a cluster of molecular motors that continually adjust the tension in the transduction apparatus. Although the transduction channel itself remains elusive, myosin Ic has recently been identified as a molecular component of the "adaptation" motor.

Vestibular morphology in the mutant mix-mouse

Mix-mice, a new strain of mice with inner ear dysfunction, and their littermates were used in the present study in order to investigate vestibular morphology, visualised by light microscopy (LM) and transmission electron microscopy (TEM). Contrary to the mix-mice, their littermates showed less stained nerve chalices and it was possible to detect that hair cells showed surface herniations and so-called 'blebs' in the apical portion of the epithelium. Sensory hairs showed a disarrayed pattern. The mix-mice had severe microscopical abnormalities: collapse of the membranous cell layer, cavities inside the neuroepithelium, severe loss of hair cells, herniations of the few remaining hair cells and increased supporting cells were evident. In the utricle and saccule, hair cells were missing and the epithelial surface was covered by a single layer of smooth, flattened epithelial cells of hitherto unknown origin.