Behavioural and functional vestibular disorders after space flight: 2. Fish,amphibians and birds

The review presents data on functional changes in fish, amphibians and birds associated with otolith organ activity after exposure to weightlessness during spaceflight. These data are of importance both for solving some fundamental problems of vestibulology and for practice. In the latter case, lower vertebrates are considered as a convenient and, most importantly, adequate model to unravel the mechanisms of vestibular disorders in humans. Analysis of the experimental results shows that weightlessness exerts no substantial effect on the formation and functional state of the otolith system in embryos of fish, amphibians and birds developing during spaceflight. Moreover, they even promote faster embryonic development of fish and amphibians as shown for mammalian fetuses. The experiments show that both in lower and higher vertebrates weightlessness brings about similar functional and behavioral changes. For example, in fish hatchlings and amphibian tadpoles (without lordosis) the vestibulo-ocular reflex was more pronounced immediately after orbital spaceflight than in control. An analogous alteration in the otolith reflex was observed in most cosmonauts after short-time space missions. In adult terrestrial vertebrates, as well as in humans, immediately after landing there was found a drop in the level of activity and deterioration of the equilibrium function and motor coordination. Another interesting finding was an unusual looping behavior when fish and tadpoles swam in loops post landing. Presumably, unusual motor activity of animals, as well as illusions arising in cosmonauts and astronauts during the transition from 1 to 0 g, have the same background being associated with changes in the stimulation pattern of the otolith organs. Considering the similarity of vestibular responses, the use of animal models seems very promising as allowing different invasive techniques.     

Neurosensory Mechanisms of Space Adaptation Syndrome

The results of studies on oculomotor reactions, both spontaneous and induced by visual and vestibular stimuli, that were performed during missions of Russian orbital complexes are described. It is demonstrated that abnormal sensorimotor reactions (spontaneous nystagmus; disturbance of the tracking function of the eyes; and decrease and increase in the tonic and dynamic vestibular excitability, respectively) are appropriate reactions of sensory systems during weightlessness. Abnormal sensorimotor reactions during spaceflights are individualized with respect to the strength, pattern of expression, time of development, and duration and dynamics of adaptation processes. Periods of adaptation (the initial, disintegration, and adaptation periods and alternation of compensation and decompensation periods) of sensorimotor functions to the conditions of weightlessness are determined. The possible mechanisms of neurosensory disturbances are described. These are otolith deafferentation, channel–otolith conflict, interlabyrinth asymmetry, intersensory mismatch, sensory deprivation, and suppression of vestibular afferent signals inadequate to new conditions.     

Saccular otolith mass asymmetry in adult flatfishes

A dimensionless measure of otolith mass asymmetry, χ, was calculated as the difference between the masses of the right and left paired otoliths divided by average otolith mass. Saccular otolith mass asymmetry was studied in eight flatfish species (110 otolith pairs) and compared with data from a previously published study on roundfishes. As in the case of symmetrical fishes, the absolute value of χin flatfishes does not depend on fish length and otolith growth rate, although otolith mass and the absolute value of otolith mass difference are correlated with fish length. The values of χwere between ?0·2 and +0·2 in 96·4% of flatfishes studied. The mean ±s .e . value of χin flatfishes was significantly larger than in standard bilaterally symmetrical marine fishes (‘roundfishes’), respectively 0·070 ± 0·006 and 0·040 ± 0·006. The most prominent distinction is the existence of downside prevalence of saccular otolith mass in flatfishes, which contrasts with no right–left prevalence in roundfishes found in a previous study. In the right‐eyed flatfishes (Soleidae), the left saccular otoliths are heavier than the right otoliths. In the left‐eyed flatfishes (Bothidae and Citharidae), the right saccular otoliths are heavier than the left otoliths. Not all flatfishes, however, fit in this design: 11·8% of flatfishes studied had the heavier saccular otoliths in the upside labyrinth and 5·4% of flatfishes had no otolith mass asymmetry (within the accuracy of the analysis). At the same time, the more mobile flatfishes (bothids and citharids) have more symmetrical and, hence, more precisely organized saccular otolith organs than the bottom‐associated flatfishes (soleids). It is possible to assume that the value of the otolith asymmetry is not only correlated with flatfish placement in a particular family, or position of eyes, but also may correlate with general aspects of their ecology. Mathematical modelling indicated that for most flatfishes one‐side saccular prevalence had no substantial significance for sound processing. On the other hand, calculations showed that 49% of flatfishes (but only 14·5% of roundfishes) have |χ| which exceed the critical level and, in principle, could sense the difference between the static displacement of the large and small paired otoliths. At that, the number of the soleids that could sense this difference is greater than the number of the bothids and citharids, 84 and 27%, respectively.     

Comparative view of the central organization of afferent and efferent circuitry for the inner ear

In all vertebrates, eighth nerve fibres from the inner ear distribute to target nuclei situated in the dorsolateral wall of the rhombencephalon. In amniotes, primary auditory and vestibular nuclei are readily delineated in that acoustic nuclei lie dorsal and sometimes rostral to vestibular nuclei. Fishes and aquatic amphibians have, in addition to labyrinthine organs, hair cell receptors in the lateral line system. Eighth nerve and lateral line fibres from these sense organs project to the octavolateralis region of the rhombencephalon. In this region, the primary nuclei cannot be easily divided into functionally distinct units. However, modality-specific zones seem to be present for auditory as well as lateral line projections lie dorsal and sometimes rostral to those from vestibular organs. Projections from the primary auditory and vestibular nuclei to higher order centres follow pathways which are conservative in their architecture among vertebrates. Ascending auditory fibres project either directly or via relay nuclei to a large midbrain center, the torus semicircularis (inferior colliculus) and hence to the forebrain. In fishes and aquatic amphibians, the lateral line system also sends a projection to the midbrain and information from this system may be integrated with auditory input at that level. The organization of vestibulospinal and vestibulo-ocular pathways shows little variation throughout vertebrate phylogeny. The sense organs of the inner ear of all vertebrates and of the lateral line system of anamniotes receive an efferent innervation. In anamniotes and some reptiles, the efferent supply originates from a single nucleus (Octavolateralis Efferent Nucleus) while that of "higher" vertebrates arises from separate auditory and vestibular efferent nuclei. The biological significance of this innervation for all vertebrates is not yet understood. However, an important feature common to all is the association of the efferent system with the motor centres of the hindbrain.     

Behavioral responses to short periods of lowered gravitational force in blind goldfish

1. The movements of blind goldfish flown in an aircraft through vertical flight patterns show a consistent correlation with the varyingg loads as recorded by ag meter.
2. Decreasing theg load in all tested cases caused the fish to rapidly dive downwards after an approximate delay of 0.5–1.5 sec. The opposite reaction, a tilt and movement upwards, was observed in transitions from a lower to a higherg load, though the latter reaction was not as pronounced.
3. Because any potential influence of visual cues, swimming bladder reflexes, and changing barometric pressure was excluded by experimental precautions, we concluded that the response of the fish to varyingg loads were vestibular reflexes probably resulting from displacements of the otoliths during vertical accelerations.
4. It is concluded from data in the literature and from the present results, that the behavioral responses resulting from static or dynamic forces are counterdirected toward the otolith displacements caused by these forces. Consequently, the behavioral, responses are counterdirected toward the gravitational pull on the animal, but must have the same direction as mechanical forces, which accelerate the whole organism and displace the otolith by inertia. It is concluded therefore, that the otolith reflexes have adaptive value only in a gravitational or centrifugal field, but are unsuitable to subserve any stabilizing function in the case of inertial forces during weightlessness.

     

Motor reactions and vestibular reflexes in cats and monkey in weightlessness

Close morpho-functional relationships of the cerebellum and vestibular system at all stages of phylogenesis of vertebrates suggest that cerebellum can be regarded as an important center of gravireceptive function. Direct examination of electrical activity of the labyrinth in cats during transient (1-2 sec) state of weightlessness produced by free fall has shown that there was an almost two fold increase in both the rate and amplitude of electrical activity in the vestibular ganglion. It is commonly accepted at present time that the conditions of orbital flight around Earth closely connect with weightlessness that usually manifests itself as undesirable factor of flight. It is known, that vestibular, proprioceptive, visual and other sensory modalities are converted on the cerebellum, which would indicate that this information is used for motor coordination and spatial orientation. Undoubtedly, origin of many vestibulo-motor disturbances during flight and in postflight period to a considerable degree depends on weightlessness. On the whole the visual illusions, motor discoordination, and space sickness, including vomiting are referred to the "space adaptation syndrome." But nature of these disturbances still is not well understood. This investigation was dedicated to study of vestibular and motor reactions of cats and monkey in short-term microgravity.     

Compensatory processes during unilateral loss of vestibular function]

Changes in the compensation of the sequences of the unilateral loss of the labyrinthine function were studied in rabbits. Destruction of the labyrinth was accompanied by ocular nystagmus, increase of frequency of respiration and heart contractions, and EEG-activation. Investigations carried out showed these reactions to be extinguished at different time. At the late periods of labyrinthectomy a considerable asymmetry of nystagmus reaction to the angular accelerations equal in intensity, but opposite in direction was revealed. Stimulation of an intact otolith labyrinth was accompanied by the appearance of positional nystagmus. The results obtained indicated imperfection of the compensatory mechanisms during complete unilateral loss of the vestibular function.     

Hypergravity decreases carbonic anhydrase-reactivity in inner ear maculae of fish

Previous investigations revealed that fish inner ear otolith growth depends on the amplitude and the direction of gravity. Both otolith total size, otolith bilateral size-asymmetry and the total and bilateral calcium-incorporation are also affected by gravity. Hypergravity, e.g., slows down otolith growth and diminishes bilateral otolith asymmetry as compared to 1 g control specimens raised in parallel. Since the enzyme carbonic anhydrase (CA) plays a prominent role in otolithic calcification, the reactivity of inner ear CA during otolith growth under hypergravity was investigated. CA-reactivity was demonstrated histochemically and densitometrically on sections of inner ear maculae of larval cichlid fish (Oreochromis mossambicus), that were kept for 6 hrs in a 3 g hypergravity centrifuge. The total unilateral macular CA-reactivity and the bilateral difference in CA between the left and the right maculae were significantly lower in 3 g animals than in 1g controls. The result is in complete agreement with previous studies indicating that a regulatory mechanism, which adjusts otolith size and asymmetry towards the gravity vector, acts via activation/deactivation of macular CA.     

Effect of Gravity Changes on the Cyanobacterium Synechocystis sp. PCC 6803

The impact of hypergravity and simulated weightlessness were studied to check whether cyanobacteria perceive changes of gravity as stress. Hypergravity generated by a low-speed centrifuge increased slightly the overall activity of dehydrogenases, but the increase was the same for 90 g and 180 g. The protein pattern did not show qualitative alterations during hypergravity treatment up to 180 g. Cells of Synechocystis PCC 6803 subjected to common stressors like salt, heat, and light clearly accumulated at least four general stress proteins (25, 31, 34, and 63 kDa, respectively). Three of these proteins could also be detected after hypergravity, but in such small amounts that their occurrence could only be taken as a weak indication of stress. Low-molecular-weight stress metabolites were not synthesized in response to hypergravity, indicating that this gravity change was unable to activate the osmotic signal transduction chain. Gravity-dependent alterations were observed only during simulated weightlessness (generated by a fast-rotating clinostat). The glutamate/glutamine ratio was significantly shifted toward a higher glutamine portion. Altogether, the results may indicate that moderate changes of gravity were hardly, if ever, sensed as stress by cyanobacteria. Received: 20 May 1997 / Accepted: 25 June 1997     

Bilateral otolith contribution to spatial coding in the vestibular system

Recent work on the coding of spatial information in central otolith neurons has significantly advanced our knowledge of signal transformation from head-fixed otolith coordinates to space-centered coordinates during motion. In this review, emphasis is placed on the neural mechanisms by which signals generated at the bilateral labyrinths are recognized as gravity-dependent spatial information and in turn as substrate for otolithic reflexes. We first focus on the spatiotemporal neuronal response patterns (i.e. one- and two-dimensional neurons) to pure otolith stimulation, as assessed by single unit recording from the vestibular nucleus in labyrinth-intact animals. These spatiotemporal features are also analyzed in association with other electrophysiological properties to evaluate their role in the central construction of a spatial frame of reference in the otolith system. Data derived from animals with elimination of inputs from one labyrinth then provide evidence that during vestibular stimulation signals arising from a single utricle are operative at the level of both the ipsilateral and contralateral vestibular nuclei. Hemilabyrinthectomy also revealed neural asymmetries in spontaneous activity, response dynamics and spatial coding behavior between neuronal subpopulations on the two sides and as a result suggested a segregation of otolith signals reaching the ipsilateral and contralateral vestibular nuclei. Recent studies have confirmed and extended previous observations that the recovery of resting activity within the vestibular nuclear complex during vestibular compensation is related to changes in both intrinsic membrane properties and capacities to respond to extracellular factors. The bilateral imbalance provides the basis for deranged spatial coding and motor deficits accompanying hemilabyrinthectomy. Taken together, these experimental findings indicate that in the normal state converging inputs from bilateral vestibular labyrinths are essential to spatiotemporal signal transformation at the central otolith neurons during low-frequency head movements.     

Ontogeny of mouse vestibulo-ocular reflex following genetic or environmental alteration of gravity sensing

The vestibular organs consist of complementary sensors: the semicircular canals detect rotations while the otoliths detect linear accelerations, including the constant pull of gravity. Several fundamental questions remain on how the vestibular system would develop and/or adapt to prolonged changes in gravity such as during long-term space journey. How do vestibular reflexes develop if the appropriate assembly of otoliths and semi-circular canals is perturbed? The aim of present work was to evaluate the role of gravity sensing during ontogeny of the vestibular system. In otoconia-deficient mice (ied), gravity cannot be sensed and therefore maculo-ocular reflexes (MOR) were absent. While canals-related reflexes were present, the ied deficit also led to the abnormal spatial tuning of the horizontal angular canal-related VOR. To identify putative otolith-related critical periods, normal C57Bl/6J mice were subjected to 2G hypergravity by chronic centrifugation during different periods of development or adulthood (Adult-HG) and compared to non-centrifuged (control) C57Bl/6J mice. Mice exposed to hypergravity during development had completely normal vestibulo-ocular reflexes 6 months after end of centrifugation. Adult-HG mice all displayed major abnormalities in maculo-ocular reflexe one month after return to normal gravity. During the next 5 months, adaptation to normal gravity occurred in half of the individuals. In summary, genetic suppression of gravity sensing indicated that otolith-related signals might be necessary to ensure proper functioning of canal-related vestibular reflexes. On the other hand, exposure to hypergravity during development was not sufficient to modify durably motor behaviour. Hence, 2G centrifugation during development revealed no otolith-specific critical period.     

Spatial and temporal segregation of auditory and vestibular neurons in the otic placode

The otic placode generates the auditory and vestibular sense organs and their afferent neurons; however, how auditory and vestibular fates are specified is unknown. We have generated a fate map of the otic placode and show that precursors for vestibular and auditory cells are regionally segregated in the otic epithelium. The anterior-lateral portion of the otic placode generates vestibular neurons, whereas the posterior-medial region gives rise to auditory neurons. Precursors for vestibular and auditory sense organs show the same distribution. Thus, different regions of the otic placode correspond to particular sense organs and their innervating neurons. Neurons from contiguous domains rarely intermingle suggesting that the regional organisation of the otic placode dictates positional cues to otic neurons. But, in addition, vestibular and cochlear neurogenesis also follows a stereotyped temporal pattern. Precursors from the anterior-lateral otic placode delaminate earlier than those from its medial-posterior portion. The expression of the proneural genes NeuroM and NeuroD reflects the sequence of neuroblast formation and differentiation. Both genes are transiently expressed in vestibular and then in cochlear neuroblasts, while differentiated neurons express Islet1, Tuj1 and TrkC, but not NeuroM or NeuroD. Together, our results indicate that the position of precursors within the otic placode confers identity to sensory organs and to the corresponding otic neurons. In addition, positional information is integrated with temporal cues that coordinate neurogenesis and sensory differentiation.     

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.     

Perspectives for the comprehensive examination of semicircular canal and otolith function.

A review is presented on the three-dimensional aspects of the vestibulo-oculomotor system and the current functional tests for unilateral examination of the individual receptors in the vestibular labyrinth. In the presentation, attention is directed towards the recently developed vestibular tests, which promise a more comprehensive examination of labyrinth function. More explicitly, unilateral tests for the utricle, saccule and the individual semicircular canals are discussed. Caloric irrigation and rotatory testing are widely used as tests for the integrity of the (horizontal) semicircular canals. Little useful diagnosis is made however on the vertical canals, not to mention the otolith organs. A promising approach to the examination of individual semicircular canal function has been described. This involves the perception of self-rotation in each of the planes of the semicircular canals. The patient/subject is rotated by an arbitrary amount on a standard Barany chair and then required to return the chair to its original position, by joystick control of the chair velocity. In order to test the vertical canals, the head of the subject/patient is positioned so that the plane of each canal lies in the plane of rotation. A promising unilateral test of saccular function involves the use of vestibular evoked myogenic potentials. Here it has been demonstrated that the saccules can be activated using brief, high-intensity acoustic clicks. The myogenic potential is measured using surface electrodes over the sternocleidomastoid muscles. Initial data from patients has indicated that the test is specific for unilateral saccule disorders. The unilateral test of utricle function is based on the eccentric displacement profile. Thus, eccentric displacement of the head to 3.5 cm during constant velocity rotation about the earth-vertical axis generates an adequate unilateral stimulation of the otolith organ, without involving the semicircular canals. This paradigm has also proved efficient in localizing peripheral otolith dysfunction by means of SVV estimation. This represents a novel test of otolith function that can be easily integrated into routine clinical testing. In contrast to the otolith-ocular response, the subjective visual vertical also reflects the processing of otolithic information in the higher brain centres (thalamus, vestibular cortex). Exploitation of the two complementary approaches therefore provides useful information for both experimental and clinical scientists. Of direct interest is the finding that testing with the subject rotating on-centre is sufficient to localize peripheral otolith dysfunction by means of SVV estimation. This represents a novel test of otolith function that can be easily integrated into routine clinical testing. In addition to caloric testing, which has remained the classical unilateral test of vestibular function, the newly developed tests should improve the differential diagnosis of vestibular disorders.     

Neuronal regulation of otolith growth and kinetotic behaviour

Inner ear stones (otoliths) of larval cichlid fish were labelled with the calcium-tracer alizarin-complexone (AC) before animals were subjected to hypergravity (hg; 3 g). After the experiment, the otoliths' area between the two AC-labellings was measured. Growth of hg-otoliths was significantly slowed down as compared to 1 g-control specimens. In the course of a second experiment, the vestibular nerve was unilaterally transacted in neonate swordtail fish which were subsequently incubated in AC. Incorporation of AC was considerably lower in the otoliths of the transacted side. The results strongly suggest that otolith growth is continuously regulated in dependence of the environmental gravity vector. Since the otolithic calcium incorporation ceased on the transected head sides, it is concluded that the regulation of otolith growth is based on the central nervous efferent vestibular system.     

The vestibulo-ocular reflex of hypergravity rats

The vertebrate vestibular system detects linear (otolith organs) and angular (semicircular canals) acceleration. The function of the otolith system is twofold, 1: perception of linear acceleration of the head, and 2: assessment of the spatial orientation of the head relative to the vector of gravity. Because of the latter function, a change of gravity will affect the vestibular input which, in turn, may have a wide range of serious physiological effects, for instance on ocular reflexes. The function of the vestibulo-ocular reflex (VOR) is to stabilize the visual image on the retina. Measurement of this VOR provides a method to investigate the (processing within the) vestibular system. Discrimination between gravity and linear acceleration, caused by movement of the head, is not possible. Therefore, information from the otolith system must be constantly compared with additional information from other sensory systems in order to solve the inherent ambiguity between tilt and translation. In this processing, cues from the semicircular canals also play a role. During parabolic flight, experiments can be performed at altered gravity levels for brief periods of time. On earth, the only effective possibility to manipulate gravity for longer periods of time is a centrifuge. Together with experiments in weightlessness during orbital flight, these methods form useful tools to investigate the influence of gravity on physiology. In our laboratory, rats have been kept inside a centrifuge at 2.5 g during their entire life-span (i.e. including gestation).     

Quantitative changes in mRNA expression of glutamate receptors in the rat peripheral and central vestibular systems following hypergravity

In order to investigate the mechanisms responsible for adaptation to altered gravity, we assessed the changes in mRNA expression of glutamate receptors in vestibular ganglion cells, medial vestibular nucleus, spinal vestibular nucleus/lateral vestibular nucleus, cerebellar flocculus, and uvula/nodulus from rats exposed to hypergravity for 2 h to 1 week using real-time quantitative RT-PCR methods. The mRNA expression of GluR2 and NR1 receptors in the uvula/nodulus and NR1 receptors in the medial vestibular nucleus increased in animals exposed to 2 h of hypergravity, and it decreased gradually to the control level. The mRNA expression of GluR2 receptors in vestibular ganglion cells decreased in animals exposed to 1 week of hypergravity. Neither the metabotropic glutamate receptor 1 nor delta2 glutamate receptor in flocculus and uvula/nodulus was affected by a hypergravity load for 2 h to 1 week. It is suggested that the animals adapted to the hypergravity by enhancing the cerebellar inhibition of the vestibular nucleus neurons through activation of the NR1 and GluR2 receptors on the Purkinje cells in uvula/nodulus especially at the early phase following hypergravity. In the later phase following hypergravity, the animals adapted to the hypergravity by reducing the neurotransmission between the vestibular hair cells and the primary vestibular neurons via down-regulation of the postsynaptic GluR2 receptors in the vestibular periphery.     

Increased Molecular Mass of Hemicellulosic Polysaccharides is Involved in Growth Inhibition of Maize Coleoptiles and Mesocotyls under Hypergravity Conditions

Zea mays L. cv. Cross Bantam T51) coleoptiles and mesocotyls was suppressed by hypergravity at 30 g and above. Acceleration at 300 g significantly decreased the mechanical extensibility of cell walls of both organs. Hypergravity increased the amounts of hemicellulose and cellulose per unit length in mesocotyl walls, but not in coleoptile walls. The weight-average molecular masses of hemicellulosic polysaccharides were also increased by hypergravity in both organs. On the other hand, the activities of β-glucanases extracted from coleoptile and mesocotyl cell walls were decreased by hypergravity. These results suggest that the decreased activities of β-glucanases by hypergravity cause an increase in the molecular mass of hemicellulosic polysaccharides of both organs. The upshift of molecular mass of hemicellulosic polysaccharides as well as the thickening of cell walls under hypergravity conditions seems to be involved in making the cell wall mechanically rigid, thereby inhibiting elongation growth of maize coleoptiles and mesocotyls. Received 22 February 1999/ Accepted in revised form 20 April 1999     

Distinct contributions from the hindbrain and mesenchyme to inner ear morphogenesis

A mature inner ear is a complex structure consisting of vestibular and auditory components. Microsurgical ablations, rotations, and translocations were performed in ovo to identify the tissues that control inner ear morphogenesis. We show that mesenchyme/ectoderm adjacent to the developing ear specifically governs the shape of vestibular components - the semicircular canals and ampullae - by conferring anteroposterior axial information to these structures. In contrast, removal of individual hindbrain rhombomeres adjacent to the developing ear preferentially affects the growth and morphogenesis of the auditory subdivision, the cochlear duct, or basilar papilla. Removal of rhombomere 5 affects cochlear duct growth, while rhombomere 6 removal affects cochlear growth and morphogenesis. Rotating rhombomeres 5 and 6 along the anteroposterior axis also impacts cochlear duct morphogenesis but has little effect on the vestibular components. Our studies indicate that discrete tissues, acting at a distance, control the morphogenesis of distinct elements of the inner ear. These results provide a basis for identifying factors that are essential to vestibular and auditory development in vertebrates.