The orientation of the semicircular canals in the guinea pig

Quantitative analysis and correlative evaluation of video-oculography, micro-computed tomography, and histopathology in Pendrin-null mice

Patients with SLC26A4 mutations exhibit highly variable hearing loss and vestibular dysfunction. Although Slc26a4 mutant mice similarly exhibit vestibular deficits, including circling behavior, head tilting, and torticollis, the underlying pathogenesis of the vestibular symptoms remains unclear, hindering its effective management for patients with SLC26A4 mutations. In this study, we evaluated the equilibrium function using the inspection equipment, which can record eye movements against rotational, gravitational, and thermal stimulations. Moreover, we correlated the degree of functional impairment with the morphological alterations observed in Slc26a4^Δ/Δ mice. The rotational stimulus and ice water caloric tests revealed considerable impairment of the semicircular canal, while the tilted gravitational stimulus test showed a severe functional decline of the otolithic system in Slc26a4^Δ/Δ mice. Generally, the degree of impairment was more severe in circling Slc26a4^Δ/Δ mice than in non-circling Slc26a4^Δ/Δ mice. In non-circling Slc26a4^Δ/Δ mice, the semicircular canal function was normal. Micro-computed tomography results showed enlargement of the vestibular aqueduct and bony semicircular canals but no correlative relationship between the severity of the caloric response and the size of bony labyrinths. Giant otoconia and a significant decrease in total otolith volume in the saccule and utricle were observed in Slc26a4^Δ/Δ mice. However, the giant otoconia were not overly dislocated in the bony otolithic system and ectopic otoconia were absent in the semicircular canal. The number and morphology of the utricular hair cells in Slc26a4^Δ/Δ mice were not significantly reduced compared to those in Slc26a4^Δ/+ mice. Collectively, we can conclude that vestibular impairments are mainly associated with otoconia formation and morphology rather than hair cell degeneration. In addition, severe disturbances of semicircular canals cause circling behavior in Slc26a4^Δ/Δ mice. Our comprehensive morphological and functional assessments apply to mouse models of other genetic diseases with vestibular impairment.

Functional implications of ubiquitous semicircular canal non-orthogonality in mammals

The 'canonical model' of semicircular canal orientation in mammals assumes that 1) the three ipsilateral canals of an inner ear exist in orthogonal planes (i.e., orthogonality), 2) corresponding left and right canal pairs have equivalent angles (i.e., angle symmetry), and 3) contralateral synergistic canals occupy parallel planes (i.e., coplanarity). However, descriptions of vestibular anatomy that quantify semicircular canal orientation in single species often diverge substantially from this model. Data for primates further suggest that semicircular canal orthogonality varies predictably with the angular head velocities encountered in locomotion. These observations raise the possibility that orthogonality, symmetry, and coplanarity are misleading descriptors of semicircular canal orientation in mammals, and that deviations from these norms could have significant functional consequences. Here we critically assess the canonical model of semicircular canal orientation using high-resolution X-ray computed tomography scans of 39 mammal species. We find that substantial deviations from orthogonality, angle symmetry, and coplanarity are the rule for the mammals in our comparative sample. Furthermore, the degree to which the semicircular canals of a given species deviate from orthogonality is negatively correlated with estimated vestibular sensitivity. We conclude that the available comparative morphometric data do not support the canonical model and that its overemphasis as a heuristic generalization obscures a large amount of functionally relevant variation in semicircular canal orientation between species.

Adaptation of vestibular tone studied with electrical stimulation of semicircular canal afferents

Damage to one vestibular labyrinth or nerve causes a central tone imbalance, reflected by prominent spontaneous nystagmus. Central adaptive mechanisms eliminate the nystagmus over several days, and the mechanisms underlying this process have received extensive study. The characteristics of vestibular compensation when the tone imbalance is presented gradually or repeatedly have never been studied. We used high-frequency electrical stimulation of semicircular canal afferents to generate a vestibular tone imbalance and recorded the nystagmus produced when the stimulation was started abruptly or gradually and when it was repeatedly cycled on and off. In the acute-onset protocol, brisk nystagmus occurred when stimulation started, gradually resolved within 1 day, and reversed direction when the stimulation was stopped after 1 week. Repeated stimulation cycles resulted in progressively smaller nystagmus responses. In the slow-onset protocol, minimal nystagmus occurred while the stimulation ramped-up to its maximum rate over 12 h, but a reversal still occurred when the stimulation was stopped after 1 week, and repeated stimulation cycles did not affect this pattern. The absence of nystagmus during the 12 h ramp of stimulation demonstrates that central vestibular tone can rebalance relatively quickly, and the reduction in the stimulation-off nystagmus with repeated cycles of the acute-onset but not the slow-onset stimulation suggests that dual-state adaptation may have occurred with the former paradigm but not the latter.

High morphological variation of vestibular system accompanies slow and infrequent locomotion in three-toed sloths

The semicircular canals (SCs), part of the vestibular apparatus of the inner ear, are directly involved in the detection of angular motion of the head for maintaining balance, and exhibit adaptive patterns for locomotor behaviour. Consequently, they are generally believed to show low levels of intraspecific morphological variation, but few studies have investigated this assumption. On the basis of high-resolution computed tomography, we present here, to our knowledge, the first comprehensive study of the pattern of variation of the inner ear with a focus on Xenarthra. Our study demonstrates that extant three-toed sloths show a high level of morphological variation of the bony labyrinth of the inner ear. Especially, the variation in shape, relative size and angles of their SCs greatly differ from those of other, faster-moving taxa within Xenarthra and Placentalia in general. The unique pattern of variation in three-toed sloths suggests that a release of selection and/or constraints on their organ of balance is associated with the observed wide range of phenotypes. This release is coincident with their slow and infrequent locomotion and may be related, among other possible factors, to a reduced functional demand for a precise sensitivity to movement.

The bony labyrinth of the early platyrrhine primate Chilecebus

We document the morphology of the bony labyrinth of Chilecebus carrascoensis, one of the best preserved early platyrrhines known, based on high resolution CT scanning and 3D digital reconstruction. The cochlea is low and conical in form, as in other anthropoids, but has only 2.5 spiral turns. When the allometric relationship with body mass is considered, cochlear size is similar to that in extant primates. The relative size of the semicircular canals, which is well within the range of other primates, indicates that Chilecebus carrascoensis was probably not as agile in its locomotion as other small-bodied platyrrhines such as Leontopithecus rosalia, Saguinus oedipus, and Callithrix jacchus, but it probably was not a suspensory acrobat or a slow climber. The proportion, shape, and orientation of the semicircular canals in Chilecebus carrascoensis also mirror that typically seen in extant primates. However, no single variable can be used for predicting the locomotor pattern in Chilecebus carrascoensis. Based on Principle Component Analysis (PCA) scores we calculated rescaled Euclidean distances for various taxa; primates with similar locomotor patterns tend to share shorter distances. Results for Chilecebus carrascoensis underscore its general resemblance to living quadrupedal primate taxa, but it is not positioned especially near any single living taxon.

The relationship of head movements to semicircular canal size in cetaceans

The semicircular canals measure head rotations, providing information critical for maintaining equilibrium. The canals of cetaceans (including whales, dolphins and porpoises) are extraordinarily small, making them unique exceptions to the allometric relationship shared by all other vertebrates between canal size and animal mass. Most modern cetaceans have shorter and less flexible necks than those of their ancestors, an adaptation hypothesized to have led to exaggerated head movements during locomotion. These movements are thought to have necessitated a decrease in the size and sensitivity of the canals, increasing their operating range to accommodate increased head motion. We tested whether the size of the semicircular canals in cetaceans is related to their head movements by comparing the rotational head velocities, frequencies and accelerations of the bottlenose dolphin (Tursiops truncatus) and a terrestrial relative, cattle (Bos taurus), using an array of three orthogonal head-fixed miniaturized rotational ratemeters. We collected data during typical locomotion (swimming; trotting) and during behaviors with enhanced head movements (rapid spiraling underwater; bucking). Cattle head movements always exceeded those of dolphins. Maximum head velocities were 528 deg. s(-1) in dolphins and 534 deg. s(-1) in cattle; maximum head frequencies were 2.86 Hz in dolphins and 3.45 Hz in cattle; and maximum head accelerations were 5253 deg. s(-2) in dolphins and 10,880 deg. s(-2) in cattle. These results indicate that accentuated head movements cannot explain the reduced size and sensitivity of cetacean semicircular canals. The evolutionary cause for their reduced canal size remains uncertain.

Geometry of the semicircular canals and extraocular muscles in rodents, lagomorphs, felids and modern humans

The vestibulo-ocular reflex (VOR) exacts compensatory movements of the extraocular muscles in response to stimulation of the semicircular canals to allow gaze fixation during head movements. In this study, the spatial relationships of these muscles and canals were investigated to assess their relative alignments in mammalian species commonly used in studies of the VOR. The head region of each specimen was scanned using magnetic resonance imaging and 28 anatomical landmarks were recorded from the images to define the six extraocular muscles and the anatomical planes of the three semicircular canals. The vector rotation of a semicircular canal that does not stimulate either of its two sister canals, referred to as the prime direction, was also calculated as an estimate of the maximal response plane. Significant misalignments were found between the extraocular muscles and the canals by which they are principally stimulated in most of the species under study. The deviations from parallel orientation were most pronounced in the human and rabbit samples. There were also significant departures from orthogonality between the semicircular canals in most species. Only the guinea pig displayed no significant difference from 90 degrees in any of its three inter-canal angles, although humans and rabbits deviated from orthogonality in just one semicircular canal pair - the anterior and posterior canals. The prime directions were found to deviate considerably from the anatomical canal planes (by over 20 degrees in rats). However, these deviations were not always compensatory, i.e. prime planes were not always more closely aligned with the muscle planes. Results support the view that the vestibular frame remains relatively stable and that the spatial mismatch with the extraocular co-ordinate frame is principally driven by realignment of the muscles as a result of changes in the position of the orbits within the skull (orbital convergence and frontation).

Short-term habituation of eye-movement responses induced by galvanic vestibular stimulation (GVS) in the alert guinea pig

In a recent study, we showed that primary afferent neurons innervating all vestibular end organs were sensitive to galvanic vestibular stimulation (GVS) in guinea pigs. In order to determine the three-dimensional character of eye movements induced by GVS, changes in eye position were recorded using digital video oculography during delivery of bilateral GVS ranging in intensity between 20 and 80 microA. Pulses were delivered in repetitive trains in order to also ascertain the involvement of vestibular habituation. At low intensities of GVS (up to 40 microA), maintained changes in eye position were induced toward the anode and away from the cathode. These eye movements were predominantly vertical with some horizontal eye movement and very little or no torsion. At higher intensities of GVS (>40 microA), horizontal nystagmus was initially observed, as well as an overall deviation of the beating field toward the anode. Nystagmus was found to habituate rapidly over successive presentations of GVS, whereas the tonic deviation of the eye remained consistent without any detectable habituation. The direction of eye movements induced by GVS was similar to that observed in humans during trans-mastoidal GVS, and the threshold differences between tonic and phasic components for GVS were also similar to previous human GVS studies. The observed habituation appears to be more specific to the phasic VOR component in quadrupedal animals such as guinea pigs, and this may reflect a considerable emphasis placed on otolithic stimulation in these animals during complex locomotor activities.

A new approach to visualizing the membranous structures of the inner ear - high resolution X-ray micro-tomography

CONCLUSION

Through the application of high resolution X-ray micro-tomography and a method of contrast enhancement based on en bloc staining in osmium tetroxide (OsO4), we report an approach that facilitates accurate three-dimensional (3D) reconstruction to reveal the fine structure of the inner ear.

OBJECTIVES

To overcome the problems of artefacts, including tissue distortion and loss of 3D context that are inherent in existing methods that rely on manual dissection and/or histological sectioning.

MATERIALS AND METHODS

A staining protocol was developed that involved the en bloc application of the OsO4 solution (2% w/v) for an extended period of time. The samples were then scanned using an X-ray micro-tomography platform and subsequent 3D visualizations were constructed.

RESULTS

The digital nature of the data allowed a complete 3D contextual visualization to be constructed whereby the individual sensory structures could be seen in relation to other inner ear structures. This included a detailed anatomy of the membranous labyrinth and nerve supply including the spatial configuration of the utricular and saccular maculae. This is a new way of undertaking temporal bone histology.

The role of 3-canal biomechanics in angular motion transduction by the human vestibular labyrinth

The present work examines the role of the complex geometry of the human vestibular membranous labyrinth in the process of angular motion transduction by the semicircular canals. A morphologically descriptive mathematical model was constructed to address the biomechanical origins of temporal signal processing and directional coding in determining the inputs to the brain. The geometrical model was developed based on shrinkage-corrected temporal bone sections using a segmentation/data-fitting procedure. Endolymph fluid dynamics within the 3-canal labyrinth was modeled using an asymptotic form of the Navier-Stokes equations and solved to estimate endolymph and cupulae volume displacements. The geometrical model was manipulated to study the role of major morphological features on directional and temporal coding. Anatomical results show that the bony osseous canals provide reasonable estimates of the orientation of the delicate membranous canals--the two differed by only 3.48 +/- 1.89 degrees . Biomechanical results show that the maximal response directions are distinct from the anatomical canal planes, but can be closely approximated by fitting a flat plane to the centerline of the canal of interest and weighting each location along the centerline with the inverse of the cross-sectional area squared. Vector cross-products of these maximal response directions, in turn, determine the null planes and prime directions that transmit the direction of angular motion to the brain as three independent directional channels associated with the nerve bundles. Finally, parameter studies indicate that changes in canal cross-sectional area and shape only moderately affect canal temporal and directional coding, while three-canal orientation is critical to directional coding.

Morphology of the mammalian vestibulo-ocular reflex: The spatial arrangement of the human fetal semicircular canals and extraocular muscles

The vestibulo-ocular reflex is the system of compensatory ocular movements in response to stimulation of the kinetic labyrinth seen in all vertebrates. It allows maintenance of a stable gaze even when the head is moving. Perhaps the simplest influence on the VOR is the spatial orientation of the planes of the semicircular canals relative to the extraocular muscles. It is hypothesized that the extraocular muscles are in parallel alignment with their corresponding semicircular canals in order to reduce the amount of neural processing needed and hence keep reflex times to a minimum. However, despite its obvious importance, little is known of this spatial arrangement. Moreover, nothing is known about any ontogenetic changes in the relative orientations of the extraocular muscles and semicircular canals. The morphologies of fetal and adult specimens of Homo sapiens were examined using magnetic resonance (MR) images. Three-dimensional co-ordinate data were taken from the images and used to calculate vector equations of the extraocular muscles and planes of best fit for the semicircular canals. The relative orientations of the muscles and canals were then calculated from the vectors and planes. It was shown that there are significant correlations between both the anterior and lateral semicircular canals and their corresponding extraocular muscles during ontogeny. In the case of the lateral canal with the medial rectus, the lateral canal with the lateral rectus, and the anterior canal with the inferior oblique, the trend is towards, though never reaching, alignment, whereas the anterior canal and the superior rectus muscle move out of alignment as age increases. Furthermore, it was noted that none of the six muscle-canal pairs is in perfect alignment, either during ontogeny or in adulthood. It was also shown that the three semicircular canals are not precisely orthogonal, but that the anterior and posterior canals form an angle of about 85 degrees , while the anterior and lateral canals diverge by approximately 100 degrees . Overall, it was shown that there is significant reorientation of the extraocular muscles and semicircular canals during ontogeny, but that, in most cases, there is little realignment beyond the fetal period.

Bone conducted vibration selectively activates irregular primary otolithic vestibular neurons in the guinea pig

The main objective of this study was to determine whether bone-conducted vibration (BCV) is equally effective in activating both semicircular canal and otolith afferents in the guinea pig or whether there is preferential activation of one of these classes of vestibular afferents. To answer this question a large number (346) of single primary vestibular neurons were recorded extracellularly in anesthetized guinea pigs and were identified by their location in the vestibular nerve and classed as regular or irregular on the basis of the variability of their spontaneous discharge. If a neuron responded to angular acceleration it was classed as a semicircular canal neuron, if it responded to maintained roll or pitch tilts it was classified as an otolith neuron. Each neuron was then tested by BCV stimuli-either clicks, continuous pure tones (200-1,500 Hz) or short tone bursts (500 Hz lasting 7 ms)-delivered by a B-71 clinical bone-conduction oscillator cemented to the guinea pig's skull. All stimulus intensities were referred to that animal's own auditory brainstem response (ABR) threshold to BCV clicks, and the maximum intensity used was within the animal's physiological range and was usually around 70 dB above BCV threshold. In addition two sensitive single axis linear accelerometers cemented to the skull gave absolute values of the stimulus acceleration in the rostro-caudal direction. The criterion for a neuron being classed as activated was an audible, stimulus-locked increase in firing rate (a 10% change was easily detectable) in response to the BCV stimulus. At the stimulus levels used in this study, semicircular canal neurons, both regular and irregular, were insensitive to BCV stimuli and very few responded: only nine of 189 semicircular canal neurons tested (4.7%) showed a detectable increase in firing in response to BCV stimuli up to the maximum 2 V peak-to-peak level we delivered to the B-71 oscillator (which produced a peak-to-peak skull acceleration of around 6-8 g and was usually around 60-70 dB above the animal's own ABR threshold for BCV clicks). Regular otolithic afferents likewise had a poor response; only 14 of 99 tested (14.1%) showed any increase in firing rate up to the maximum BCV stimulus level. However, most irregular otolithic afferents (82.8%) showed a clear increase in firing rate in response to BCV stimuli: of the 58 irregular otolith neurons tested, 48 were activated, with some being activated at very low intensities (only about 10 dB above the animal's ABR threshold to BCV clicks). Most of the activated otolith afferents were in the superior division of the vestibular nerve and were probably utricular afferents. That was confirmed by evidence using juxtacellular injection of neurobiotin near BCV activated neurons to trace their site of origin to the utricular macula. We conclude there is a very clear preference for irregular otolith afferents to be activated selectively by BCV stimuli at low stimulus levels and that BCV stimuli activate some utricular irregular afferent neurons. The BCV generates compressional and shear waves, which travel through the skull and constitute head accelerations, which are sufficient to stimulate the most sensitive otolithic receptor cells.

Planar relationships of the semicircular canals in two strains of mice

The mouse is increasingly important as a subject of vestibular research. Although many studies have focused on the vestibular responses of mice to angular rotation, the geometry of their semicircular canals has not been described. High-voltage X-ray computed tomography was used to measure the anatomy of the semicircular canals of two strains of mice, C57Bl/6J and CBA/CaJ. The horizontal plane of a stereotaxic coordinate system was defined by the midpoints of the external auditory meati and the point where the incisors emerge from the maxilla. The centroids of the lumens of the bony canals were calculated, and planes that describe the canals were fit using a least-squares regression analysis to the resulting points. Vectors normal to each regressed plane were used to represent the corresponding canal's axis of rotation, and angles of these vectors relative to skull landmarks as well as to each other were calculated. The horizontal canal of the mouse was found to be angled anteriorly upward 17.8 degrees for CBA/CaJ and 32.6 degrees for C57Bl/6J from the reference horizontal plane. Angles between ipsilateral canals deviated up to 12.3 degrees from orthogonal, and angles between contralateral synergistic canals (left anterior-right posterior, right anterior-left posterior, and horizontal-horizontal) deviated from parallel by up to 14.8 degrees. The orientations of the canals within the head as well as the orientations of the canals relative to each other were significantly different between the two strains, suggesting that care must be taken in the design and interpretation of developmental and physiologic studies involving different mouse strains.

Geometry of the semicircular canals of the chinchilla (Chinchilla laniger)

The orientations of the semicircular canals determines the response of the canals to head rotations and, in turn, the brain's ability to interpret those motions. The geometry of chinchillas' semicircular canals has never been reported. Volumetric representations of three chinchilla skulls were generated using a microCT scanner. The centroids of each semicircular canal lumen were identified as they passed through the image slices and were regressed to a plane. Unit vectors normal to the plane representing canal orientations were used to calculate angles between canal pairs. Pitch and roll maneuvers required to bring any canal into the horizontal plane for physiologic investigation were calculated. The semicircular canals of the chinchilla were found to be relatively planar. The horizontal canal was found to be oriented 55.0 degrees anteriorly upward. Pairs of ipsilateral chinchilla canals were not orthogonal and contralateral synergistic pairs were not parallel. Despite this arrangement, the canal plane unit normal vectors were organized to respond with approximately equal overall sensitivity to rotations in any direction. The non-orthogonal chinchilla labyrinth may provide an opportunity to determine whether the frame of reference used by the central vestibular and oculomotor system is based on directions of afferent maximum sensitivity or prime directions.

Responses of primary vestibular neurons to galvanic vestibular stimulation (GVS) in the anaesthetised guinea pig

Previous studies in humans and animals which have shown that DC galvanic vestibular stimulation (GVS) induces horizontal and torsional eye movements have been interpreted as being due to a preferential activation of primary vestibular afferents innervating the horizontal semicircular canals and otoliths by GVS. The present study sought to determine in guinea pigs whether GVS does indeed selectively activate primary horizontal canal and otolith afferents. Constant-current GVS was passed between electrodes implanted in the tensor-tympani muscle of each middle ear or between electrodes on the skin over the mastoid. During this stimulation, responses from single primary vestibular neurons were recorded extracellularly by glass microelectrodes in Scarpa's ganglion. Afferents from all vestibular sensory regions were activated by both surface and tensor-tympani galvanic stimulation. Tensor tympani GVS was approximately 10 times more effective than surface GVS. At larger current intensities irregularly discharging afferents showed an asymmetrical response: cathodal stimulation resulted in a larger change in firing (increase) than anodal stimulation (decrease), whereas regularly discharging afferents responded symmetrically to the two polarities of GVS. Across all afferents tuned for different types of natural vestibular stimulation, neuronal sensitivity for GVS was found to increase with discharge variability (as indexed by CV*). Anterior canal afferents showed a slightly higher sensitivity than afferents from other vestibular sensory regions. Hence, the present study concluded that GVS activates primary vestibular afferents innervating all sensory regions in a uniform fashion. Therefore, the specific pattern of GVS-induced eye movements reported in previous studies are not due to differential sensitivity between different vestibular sensory regions, but are likely to reflect an involvement of central processing.

Spatial properties of central vestibular neurons of monkeys after bilateral lateral canal nerve section

Thirty-seven neurons were recorded in the superior vestibular nucleus (SVN) of two cynomolgus monkeys 1-2 yr after bilateral lateral canal nerve section to test whether the central neurons had spatially adapted for the loss of lateral canal input. The absence of lateral canal function was verified with eye movement recordings. The relation of unit activity to the vertical canals was determined by oscillating the animals about a horizontal axis with the head in various orientations relative to the axis of rotation. Animals were also oscillated about a vertical axis while upright or tilted in pitch. In the second test, the vertical canals are maximally activated when the animals are tilted back about -50 degrees from the spatial upright and the lateral canals when the animals are tilted forward about 30 degrees . We reasoned that if central compensation occurred, the head orientation at which the response of the vertical canal-related neurons was maximal should be shifted toward the plane of the lateral canals. No lateral canal-related units were found after nerve section, and vertical canal-related units were found only in SVN not in the rostral medial vestibular nucleus. SVN canal-related units were maximally activated when the head was tilted back at -47 +/- 17 and -50 +/- 12 degrees (means +/- SD) in the two animals, close to the predicted orientation of the vertical canals. This indicated that spatial adaptation of vertical canal-related vestibular neurons had not occurred. There were substantial neck and/or otolith-related inputs activating the vertical canal-related neurons in the nerve-sectioned animals, which could have contributed to oculomotor compensation after nerve section.

Effects of Intratympanic Gentamicin on Vestibular Afferents and Hair Cells in the Chinchilla

Gentamicin is toxic to vestibular hair cells, but its effects on vestibular afferents have not been defined. We treated anesthetized chinchillas with one injection of gentamicin (26.7 mg/ml) into the middle ear and made extracellular recordings from afferents after 5–25 (early) or 90–115 days (late). The relative proportions of regular, intermediate, and irregular afferents did not change after treatment. The spontaneous firing rate of regular afferents was lower ( P < 0.001) on the treated side (early: 44.3 ± 16.3; late: 33.9 ± 13.2 spikes·s−1) than on the untreated side (54.9 ± 16.8 spikes·s−1). Spontaneous rates of irregular and intermediate afferents did not change. The majority of treated afferents did not measurably respond to tilt or rotation (82% in the early group, 76% in the late group). Those that did respond had abnormally low sensitivities ( P < 0.001). Treated canal units that responded to rotation had mean sensitivities only 5–7% of the values for untreated canal afferents. Treated otolith afferents had mean sensitivities 23–28% of the values for untreated otolith units. Sensitivity to externally applied galvanic currents was unaffected for all afferents. Intratympanic gentamicin treatment reduced the histological density of all hair cells by 57% ( P = 0.04). The density of hair cells with calyx endings was reduced by 99% ( P = 0.03), although some remaining hair cells had other features suggestive of type I morphology. Type II hair cell density was not significantly reduced. These findings suggest that a single intratympanic gentamicin injection causes partial damage and loss of vestibular hair cells, particularly type I hair cells or their calyceal afferent endings, does not damage the afferent spike initiation zones, and preserves enough hair cell synaptic activity to drive the spontaneous activity of vestibular afferents.

Disappearance of click-evoked potentials on the neck of the guinea pig by pharmacological and surgical destruction of the peripheral vestibular afferent system

In order to establish an animal model of acoustically evoked vestibulo-collic reflex, the so-called vestibular evoked myogenic potential in humans, potentials evoked by loud clicks on the neck of the guinea pig were recorded using subjects whose peripheral vestibular endorgans or vestibular afferents had been damaged. Four normal control guinea pigs, four guinea pigs that received an intramuscular injection of gentamicin for 20 days (90 mg/kg/day) and five guinea pigs whose vestibular nerves were surgically sectioned were used in this study. Under general anesthesia with an intraperitoneal injection of pentobarbital sodium (40 mg/kg), auditory brainstem responses (ABRs) were recorded. Then, potentials evoked by loud clicks on the pre-vertebral muscle at the level of the third cervical vertebral bone were recorded using silver ball electrodes. As a result, a distinctive negative peak (NP) with a latency of 6-8 ms was recorded in all animals in the control group. NP was not observed in the gentamicin-administered group while ABR was preserved. After sectioning the vestibular nerve, NP was abolished while ABR was preserved. From these results, NP could be of vestibular origin. These results are in agreement with a previous report of NP using subjects whose cochlea had been damaged pharmacologically.

Changes in electrovestibular brainstem responses after aminoglycoside intoxication in guinea pigs

OBJECTIVE

To verify the usefulness of short-latency vestibular responses evoked by a combination of round window electrical stimulation and sinusoidal rotation (electrovestibular brainstem responses; EVBRs) as a new monitoring tool of the vestibular function in animal experiments.

METHODS

EVBRs were obtained before, during, and after treatment with aminoglycosides, along with compound action potential (CAP) audiograms. The changes in EVBRs were compared with morphological changes observed by scanning electron microscopy.

RESULTS

EVBR amplitudes did not change in the group of guinea pigs treated with amikacin, but markedly decreased in the streptomycin and gentamicin- treated groups. CAP audiograms indicated a significant threshold elevation in the amikacin group, a moderate elevation in the gentamicin group, and no change in the streptomycin group. Under scanning electron microscopy, the loss of the sensory hair cells observed in the cristae ampullares was slight to moderate in the amikacin group, moderate to severe in the streptomycin group, and severe in the gentamicin group.

CONCLUSION

EVBRs reflect overall pathological changes undergone by vestibular hair cells, and support the vestibular specificity of EVBRs.

[Skeletal configurations of cervical column during rest to motion transition: generalization that underlie the motor repertoire in rodents]

During forward locomotion, guinea pigs adopted a posture which, although different, was as stereotyped as at rest. The whole extent of the vertebral column was extended, parallel to the earth-horizontal plane. This result supports the hypothesis that, in mammals, gaze and postural control by the central nervous system would be simplified by the adoption of a limited number of skeletal configurations. They would be optimized for energy saving, biomechanical efficacy and limitation of the number of degrees of freedom of the skeletal apparatus. Transition between rest and forward locomotion displayed two prominent characteristics: the head was stabilized versus space throughout the whole transition and the modifications of the skeletal geometry were confined to the median plane and to two major articular regions. Such an organization revealed a fourth characteristic of the guinea pig postural control: the skeletal configurations retained by evolution would ensure that smooth transitions take place between rest and the various activities of its motor repertoire. Finally, our purely descriptive data shed new light on the architecture of the neuronal network which underlies the vestibular control of gaze and posture.

The planes of the utricular and saccular maculae of the guinea pig

To establish a link between otolith anatomy and function it is necessary to know the regions of the utricular and saccular maculae, which are stimulated by any arbitrary linear acceleration stimulus. That requires accurate information about the location and orientation of the spatially extended maculae in head-fixed coordinates and referred to head-fixed landmarks (such as Reid's line). New data showing the location of the otolithic maculae in the guinea pig with respect to head-fixed stereotaxic coordinates are presented. Guinea pigs were perfused with Karnovsky's fixative and the maculae were exposed while the head was held in a guinea pig stereotaxic device. An electrolytically sharpened fine wire held in a calibrated micromanipulator was touched to points all over the surface of each macula under visual observation with the aid of a high-power operating microscope. The x, y, z coordinates of these points were plotted using a three-dimensional plotting program. Both maculae have pronounced curvature so that dorsoventral shear forces will stimulate regions of both the utricular and saccular maculae.

Neuronal correlates of vestibulo-ocular reflex adaptation in the alert guinea-pig

The spiking behaviour of 66 second-order vestibular neurons was studied in alert, chronically prepared guinea-pigs during the horizontal vestibulo-ocular reflex (VOR). Among the 66 studied neurons, seven were held long enough (> 1 h) to compare their spiking behaviour before and after a training procedure inducing a decrease in the gain of the VOR. When tested in darkness following adaptation, five of them showed a significant decrease of their sensitivity to head rotation. However, the resting discharge of these five neurons remained unchanged. This suggests that VOR adaptation is mediated not only by changes in synaptic efficacities but also by modifications in the spike generator which transforms synaptic inputs into a pattern of action potentials.

Neuronal activity in the vestibular nuclei after contralateral or bilateral labyrinthectomy in the alert guinea pig

In the guinea pig, a unilateral labyrinthectomy is followed by an initial depression and a subsequent restoration of the spontaneous activity in the neurons of the ipsilateral vestibular nuclei. In two previous works, we have established the time course of these changes in the alert guinea pig using electrical stimulation as a search stimulus to select the analyzed neurons. The latter criterion was important to capture the many ipsilateral neurons that are silent at rest during the immediate postlabyrinthectomy stage. Because it is known that a pathway originating from the vestibular nuclei on one side crosses the midline and functionally inhibits the activity of the vestibular nuclei on the other side, we investigated in the first part of this study the spiking behavior of the neurons in the vestibular nuclei contralateral to the labyrinthectomy using the same procedure as that used for the ipsilateral neurons. The spiking behavior of 976 neurons was studied during 4-h recording sessions in intact animals and 1 h, 1 day, 2 days, or 1 wk postlabyrinthectomy. Neurons selected according to the electrical activation criterion were classified further as type I (their firing rate increased during ipsilateral rotation), type II (their firing rate increased during contralateral rotation), or unresponsive. The resting activity of type I neurons, which was 38.1 +/- 20.9 spikes/s (mean +/- SD) in the control state, increased statistically significantly 1 h after the lesion (53.3 +/- 29.1 spikes/s) and remained at this level 1 wk later (56.0 +/- 20.3 spikes/s). The sensitivity of type I units, which was 0.80 +/- 0.46 spikes/s per deg/s in the control population, decreased to 0.49 +/- 0.26 spikes/s per deg/s 1 h after the lesion and remained at this level 1 wk later (0.50 +/- 0.39 spikes/s per deg/s). When all monosynaptically activated neurons (type I, type II, unresponsive) were pooled, the sensitivity to horizontal rotation fell from 0.58 +/- 0.51 spikes/s per deg/s in the control state to 0. 15 +/- 0.25 spikes/s per deg/s 1 h after the lesion and to 0.20 +/- 0.32 spikes/s per deg/s 1 wk later. The major findings of the first part of this study in the alert guinea pig are thus in accord with those of Curthoys et al. and Smith and Curthoys in anesthetized guinea pigs. In the second part of this work, we studied the spiking behavior of the neurons in the vestibular nuclei after bilateral labyrinthectomy. After unilateral labyrinthectomy, the resting discharge of the ipsilateral monosynaptically activated vestibular neurons fell from 36.9 +/- 21 spikes/s (basal activity) to 6.7 +/- 17.0 spikes/s 1 h after the lesion and then recovered, reaching 17.4 +/- 18.9 and 40.8 +/- 23.7 spikes/s 1 day and 1 wk after the lesion, respectively. These observations raise the two following questions. What are the relative contributions of the loss of the excitatory influence from the ipsilateral labyrinth (destroyed) and of the persistence of the inhibitory influence from the contralateral labyrinth (intact) in the labyrinthectomy-induced depression of activity? And are the left-right asymmetries caused by a unilateral labyrinthectomy the driving force for restoration of activity? Here, we addressed these two questions by studying the spiking behavior of 473 second-order vestibular neurons in the alert guinea pig after a bilateral labyrinthectomy. In the acute stage, 1 h after bilateral labyrinthectomy, the resting discharge of the second-order vestibular neurons was 16.2 +/- 22.4 spikes/s. From comparison with the results obtained in the acute stage after a unilateral labyrinthectomy, we inferred that the ipsilateral excitatory influence was between two and three times more powerful than the contralateral inhibitory influence. (ABSTRACT TRUNCATED)

Reappearance of activity in the vestibular neurones of labyrinthectomized guinea-pigs is not delayed by cycloheximide

1. In mammals, unilateral labyrinthectomy induces an immediate depression of the resting discharges in the neurones of the ipsilateral vestibular nuclei. Later on, a spontaneous restoration of this activity occurs. The aim of the present study was to test the possibility that protein synthesis could be involved in the start of this process in the guinea-pig. 2. Cycloheximide (CHX), a protein synthesis inhibitor, was injected intramuscularly 1 h before (30 mg kg-1) and 5 h after (15 mg kg-1) labyrinthectomy. 3. In a first group of animals, CHX was found to induce an inhibition of protein synthesis at levels ranging from 71 to 93% for 9 h after labyrinthectomy. 4. In a second group of alert animals, we studied single unit activity of second-order vestibular neurones. It was found that, in the 12-16 h post-labyrinthectomy period, at a time when restoration began in guinea-pigs not treated with CHX, the discharges in the labyrinthectomized group treated with CHX were not different from those observed in a previous study in labyrinthectomized animals not treated with CHX. 5. We conclude that protein synthesis is not required for the start of restoration of activity in the vestibular neurones deprived of their ipsilateral labyrinthine input.

Three-dimensional reconstruction of the membranous vestibular labyrinth in the toadfish, Opsanus tau

Membranous vestibular labyrinths from the oyster toadfish, Opsanus tau, were fixed, dissected from the animal, stained, and embedded in rectangular blocks of clear histological resin. Photomicrographs of complete embedded labyrinths were taken from six orthogonal directions and used to construct three-dimensional (3D) geometrical models of the semicircular canals, ampullae, utricular vestibule and common crus. Membraneous ducts and ampullae were modeled using a set of cross-sectional elliptical curves laced together to generate curved tubular models of each structure. The ensemble of these curved tubes was used to generate a complete 3D reconstruction of the outside surface of the membranous labyrinth. When viewed from six orthogonal directions, reconstructions closely matched the embedded tissue. Dimensions of the reconstruction and histological sections were compared to measurements of fresh tissue taken from the same animals prior to fixation and used to correct the reconstructions for tissue shrinkage. Results provide estimates of the endolymphatic volumes, local cross-sectional areas and elliptical eccentricities as well as 3D orientations of the geometric canal planes relative to the skull. Ten micrometer histological sections of the material were also prepared to measure wall thickness in various regions of the labyrinth.

Vestibular and optokinetic function in the normal guinea pig

Vestibular and optokinetic function was quantitatively studied in the normal guinea pig through investigation of the vestibulo-ocular reflex (VOR), optokinetic nystagmus (OKN), and the visual vestibulo-ocular reflex (VVOR) by means of sinusoidal stimulation with a computer-controlled rate table (VOR and VVOR) or an optokinetic drum. The VOR exhibited high-pass filter characteristics with steady state gain achieved at 0.125 Hz. The maximum gain was 0.55 at a velocity of 60 degrees/s. The VOR was modeled by a transfer function with best fits obtained with an adaptation time constant of 12.5 seconds. The OKN showed low-pass filter characteristics with a decrease in gain for increase in stimulus amplitude. The maximum gain measured was 0.64. A fractional pole model provided a fit of these data. The VVOR exhibited a mean gain of between 0.6 and 0.7 across the stimulus bandwidth and peak velocities. A model based on a linear combination of the actual OKN and VOR gains provided an estimate of the VVOR gain.

Dissociations between behavioural recovery and restoration of vestibular activity in the unilabyrinthectomized guinea-pig

1. In the guinea-pig, a unilateral labyrinthectomy induces postural disturbances and an ocular nystagmus which abate or disappear over time. These behavioural changes are accompanied by an initial collapse and a subsequent restoration of the spontaneous activity in the neurones of the ipsilateral vestibular nuclei. Recently, it has been shown that the vestibular neuronal activity remained collapsed over at least 10 h whereas its restoration was complete 1 week after the lesion. The aims of this study were to determine when restoration of spontaneous activity in the partially deafferented vestibular neurones started and to compare the time courses of the behavioural and neuronal recoveries in guinea-pigs that had undergone a unilateral labyrinthectomy. 2. Neuronal discharge measurements were made using chronic extracellular recording of single unit activity. After a left labyrinthectomy, electrodes, were placed on the site of the destroyed labyrinth to enable stimulation of the left vestibular nerve. Behavioural measurements included chronic recording of eye movements by the scleral search coli technique. After a left labyrinthectomy, lateral deviation of the head, twisting of the head, and eye velocity of the slow phases of the nystagmus were measured. 3. The neuronal activity of the rostral part of the vestibular nuclear complex on the lesioned side was recorded in alert guinea-pigs over 4 h recording sessions between 12 and 72 h after the lesion. 4. The criterion used to select vestibular neurones for analysis was their recruitment by an electric shock on the vestibular nerve. In addition, in order to explore a uniform population, we focused on neurones recruited at monosynaptic latencies (0.85-1.15 ms). 5. For each recording period, the mean resting rate was calculated animal by animal and the grand mean of these individual resting rate means was calculated. Previously, a decline in the grand mean resting rate from 35.8 +/- 6.0 spikes s-1 (control state) to 7.1 +/- 4.2 spikes s-1 during the first 4 h after labyrinthectomy has been shown. In the present study, the first sign of recovery was observed during the 12-16 h recording period when the resting rate grand mean increased to 16.3 +/- 3.9 spikes s-1. This grand mean activity did not change significantly during the following 12 h. Thereafter, restoration of neuronal activity improved and was complete 1 week after the lesion. 6. Although the abatement of the vestibular symptoms roughly paralleled the restoration of neuronal activity in the vestibular nuclei, some discrepancies between the time courses of both phenomena emerged. An important step in postural recovery (the animals managed to stand up) and a major part of the abatement of the nystagmus occurred before the recovery of vestibular neuronal activity. In addition, lateral deviation of the head disappeared while restoration of the neuronal activity was incomplete, but significant head twisting was still evident when vestibular resting rates had recovered completely. 7. We conclude that restoration of neuronal activity in the ipsilateral vestibular nuclei starts 12 h after the lesion and that restoration of neuronal activity in the ipsilateral vestibular nuclei is not the only mechanism underlying behavioural vestibular compensation.

A quantitative study of electroencephalography, eye movements and neck electromyography characterizing the sleep-wake cycle of the guinea-pig

The qualitative and quantitative characteristics of cerebral cortex electrical activity, ocular motility and muscular activity were studied in six head-restrained guinea-pigs during wakefulness, slow-wave and paradoxical sleep. Animals were chronically implanted with bipolar electrodes in the obliquus capitis muscle for electromyographic recordings and epidurally through the parietal bones for electroencephalographic (EEG) recordings. Eye movements were recorded using the scleral search-coil technique. After postoperative recovery and a short period of habituation to immobilization, head-restrained animals exhibited a polyphasic sleep-wake cycle similar to what has already been described in the unrestrained guinea-pig. Paradoxical sleep periods of mean duration 110 +/- 42 s occurred at a mean interval of 32.2 +/- 7.2 min. Amplitude and frequency components of EEG activity were different for each state of vigilance. EEG amplitude was highest and frequency range lowest-with two well-defined peaks at 4 and 10 Hz-during slow-wave sleep. During paradoxical sleep, frequencies were higher and amplitudes lower than during wakefulness. Three types of eye movement intermingled with periods of ocular fixation were recorded: saccadic movements during wakefulness and paradoxical sleep, slow drifts during slow-wave sleep and paradoxical sleep, and a new type of eye movement-bursts of high-velocity eye oscillations during paradoxical sleep. Saccadic eye movements during paradoxical sleep were more frequent and showed higher velocities and amplitudes than during wakefulness. During paradoxical sleep the episodes of eye oscillation (8-14 Hz) occurred quite regularly every 1.6 s and had a mean duration of 1.4 s. During wakefulness, the obliquus muscle activity displayed a burst-tonic pattern. Bursting components were closely related to saccadic eye movements directed to the side of the recorded muscle. The muscle activity was predominantly tonic during slow-wave sleep and was completely absent during paradoxical sleep except for small bursts or twitches. These twitches were tightly synchronized with the occurrence of the rapid eye movements oriented towards the side of the recorded obliquus muscle, as during wakefulness. These results strongly suggest that paradoxical sleep is characterized by the oscillatory discharge of at least two neuronal populations: the brainstem saccadic generators and the tecto-reticular spinal network which underlies gaze-orienting behaviour during wakefulness. The occurrence of rhythmic discharges at approximately 11 Hz may explain the spinal motoneurons' inhibition during paradoxical sleep in order to avoid anarchic motor behavior. Whether these neuronal oscillations are simply an epiphenomenon or have functional implications remains to be determined.

Projections of the individual vestibular end-organs in the brain stem of the squirrel monkey

The central nervous system (CNS) projections of primary afferent neurons from individual vestibular receptors were studied using horseradish peroxidase (HRP) or biocytin labeling in 14 ears from 7 adult squirrel monkeys using the technique developed in the chinchilla (Lee et al., 1989, 1992). The specificity of labeling was verified by examining the location of the labeled fibers and cell bodies in the vestibular nerve and Scarpa's ganglion. Labeled fibers and cells were restricted to nerves and areas belonging to groups of cells in either the superior or the inferior ganglion of the vestibular nerve. In the vestibular nerve root, labeled primary afferent fibers also exhibited a receptor-dependent segregation at the entrance to the medulla. Fibers from the HSC and the SSC were found rostrally and those from the PSC and the SAC were found in the caudal area. The UTR fibers were situated intermediate between these two groups of fibers. (A bundle of fibers, probably vestibular efferents, was identified immediately rostrally and ventromedially to the UTR fibers.) The primary afferent fibers bifurcated into secondary ascending and descending fibers at the lateral border of the vestibular nuclei, forming a longitudinal rostrocaudal vestibular tract. The secondary fibers from individual end-organs occupied specific locations in the tract: the UTR fibers were dorsal to the SSC and the HSC fibers, PSC fibers were found most medially, and the SAC fibers occupied the lateralmost area. The secondary UTR fibers overlapped considerably with those of the SSC and the HSC. The orderly receptor-dependent segregation of fibers was more prominent in the descending tracts than in the ascending tracts. In the vestibular nuclei complex the location of the tertiary branches of various end-organs exhibited considerable overlap within the major vestibular nuclei (SN, superior nucleus; LN, lateral nucleus; MN, medial nucleus; DN, descending nucleus). There were still differences, however, in the projection pattern. Fibers from the SAC ran primarily in the lateral area, fibers from the SSC and the UTR were found ventromedially to the SAC fibers, and the HSC projected slightly medially to the fibers from the SSC. The PSC fibers projected most medially. The UTR and SAC sent numerous fibers to the cerebellum. Fibers from the semicircular canals projected through the rostrodorsal region of the SN and presumably also projected to the cerebellum. The precise termination of fibers was evaluated by studying the location of labeled boutons, which were identified in all major vestibular nuclei. Labeled boutons from all the receptors were in the rostral and central areas of the SN, and in the MN mainly in the rostral two-thirds. In the LN, boutons from all the receptors were in the rostroventral part, most of which were from the UTR and SAC. No labeled boutons were in the caudodorsal part of this nucleus. Labeled boutons in the DN primarily surrounded the descending tract fibers and were particularly prominent medially. In specimens in which superior vestibular nerve receptor organs were scratched vestibular efferent fibers were also labeled. These fibers traveled in the most ventral part of the vestibular nerve root and projected in the ventral aspect of the LN to labeled soma in the ipsilateral and contralateral brain stem. Specificity the in projection patterns of efferent fibers from different end-organs could not be ascertained.

Differential sensitivity to rotation measured on potentials evoked by electrical stimulation of the guinea-pig ear

Responses to electrical stimulation of the ear applied between round-window and vertex electrodes were recorded in awake guinea-pigs from the same electrodes or from separate vertex/mastoid subdermal needle electrodes. They were averaged during opposite phases of sinusoidal rotation or before and after constant velocity rotation. In both cases the responses were subtracted from each other and yielded differential per- or post-rotatory "electrovestibular" responses. For comparison, responses were also recorded in the same animals and conditions of electrical stimulation during silence and during silence and during presentation of a broad-band noise. The difference yielded "electroacoustic" responses. In round-window records, electrovestibular and electroacoustic responses presented typical compound nerve action potential patterns. Electrovestibular responses could be recorded for head angular velocities as low as 3 degrees sec-1 at 0.1 Hz. Response amplitude showed a logarithmic relation to head velocity. Changes in amplitude, as a function of time after rotation, were comparable to those reported for vestibular nerve fibre responses. In vertex/mastoid records, electroacoustic responses presented a sequence of peaks similar to the click-evoked auditory brain-stem responses, and electrovestibular responses presented two peaks, presumably representing contributions of central vestibular structures. Such "electrovestibulography" permits the study of an individual ear and makes available to the investigator a large range of vestibular stimulation conditions.

The influence of middle ear pressure changes on the primary vestibular neurons in guinea pigs

The responses of primary vestibular neurons and perilymphatic pressure changes to middle ear pressure stimuli in guinea pigs were investigated in order to clarify the direct effects of pressure stimulus on the vestibular apparatus. The vestibular response was related to the amount of middle ear pressure change applied at a rate of +/- 100 mmH2O/s. The neural response rates of vestibular units to positive pressure in the middle ear were significantly larger than those to negative pressure. The time course pattern of the perilymphatic pressure change resembled that of the response of the vestibular units, indicating that the vestibular response is elicited by middle ear pressure via the pressure transmitted in the inner ear.

Medial vestibular nucleus in the guinea-pig: histaminergic receptors. II. An in vivo study

In a companion paper (Serafin et al. 1992) we have demonstrated in vitro that histamine depolarizes three previously described medial vestibular nucleus neuron (MVNn) types (Serafin et al. 1991a, b). It has also been shown that this effect was exclusively mediated through postsynaptic H2 receptors. All the same, the eventual contribution of presynaptic H3 receptors to the physiological response of the MVNn to histamine remained an open question since, during the slicing procedure, any histaminergic axons projecting to the vestibular nuclei would have been interrupted. This rendered our study of H3-mediated effects of histamine difficult. Hence, in the present in vivo study our aim was three-fold: (1) to investigate the presence of H3 receptors at the vestibular nuclei level; (2) to evaluate the functional importance of MVNn H2 receptors; and (3) to explore whether H3 ligands, when injected intraperitoneally (i.p.), could modulate dynamic vestibular functions. In order to address the first two questions, we investigated postural changes induced by perfusion of the guinea-pig's vestibular nuclear complex with specific ligands of the H2 and H3 receptors. Our data extend the conclusions of our in vitro study and suggest that lateral vestibular nuclei neurons and the MVNn are endowed with both H2 and H3 receptors. Our results indicate furthermore that histamine can modulate, quite effectively, static vestibular reflexes. Finally, the present study demonstrates that i.p. injection of thioperamide, an H3 antagonist, induces a significant decrease in the horizontal vestibular-ocular reflex gain and, by contrast to most of the clinically used antihistaminics, has no detrimental effect on the alertness level. Our results may thus lead to clinical testing and use of H3 antagonists as antivertigo or anti motion-sickness drugs.

Geometrical approach to neural net control of movements and posture

In one approach to modeling brain function, sensorimotor integration is described as geometrical mapping among coordinates of non-orthogonal frames that are intrinsic to the system; in such a case sensors represent (covariant) afferents and motor effectors represent (contravariant) motor efferents. The neuronal networks that perform such a function are viewed as general tensor transformations among different expressions and metric tensors determining the geometry of neural functional spaces. Although the non-orthogonality of a coordinate system does not impose a specific geometry on the space, this "Tensor Network Theory of brain function" allows for the possibility that the geometry is non-Euclidean. It is suggested that investigation of the non-Euclidean nature of the geometry is the key to understanding brain function and to interpreting neuronal network function. This paper outlines three contemporary applications of such a theoretical modeling approach. The first is the analysis and interpretation of multi-electrode recordings. The internal geometries of neural networks controlling external behavior of the skeletomuscle system is experimentally determinable using such multi-unit recordings. The second application of this geometrical approach to brain theory is modeling the control of posture and movement. A preliminary simulation study has been conducted with the aim of understanding the control of balance in a standing human. The model appears to unify postural control strategies that have previously been considered to be independent of each other. Third, this paper emphasizes the importance of the geometrical approach for the design and fabrication of neurocomputers that could be used in functional neuromuscular stimulation (FNS) for replacing lost motor control.

Effect of post-training unilateral labyrinthectomy in a spatial orientation task by guinea pigs

The effects of unilateral labyrinthectomy in guinea pigs have been studied on an angular orientation task consisting, in an open field, of running to a hidden goal oriented at 45 degrees with respect to the cephalocaudal axis of the animal placed in a starting-box. The task was conducted in light but in an homogeneous environment, i.e. without visual, auditory or olfactory cues indicating the location of the goal. A second group of animals was submitted to a similar task running to a hidden goal but the place of the goal was indicated by a colored card. All the animals were trained before the lesion and tested in their respective task for 1 month after the lesion. In the task conducted without conspicuous cues, animals were dramatically disturbed. In contrast, animals pretrained in the visually guided task were not impaired after the lesion. These results point out the important role of vestibular information in performing spatial tasks based on angular estimation, since, even if proprioceptive and visuokinesthetic information remain available, subjects seemed not able to maintain a correct angular trajectory. The trajectories being not disturbed in the visually guided task, one can exclude the hypothesis that such deficit was due to a purely motor disturbance.

Effects of Ginkgo biloba extract (EGb 761) on the guinea pig vestibular system

Previous studies have demonstrated that the administration of Ginkgo biloba extract (EGb 761) improves the compensation of the vestibular syndrome induced by transection of the VIIIth nerve. To investigate the mechanisms at play, the vestibular nuclei of alert guinea pigs were perfused with EGb 761. This perfusion always induced a stereotyped reversible postural syndrome that was the mirror image of the syndrome provoked by the unilateral lesion of the otolithical receptors. This result supports the hypothesis that EGb 761 has a direct excitatory effect on the lateral vestibular nuclei (LVN) neurons. In a second step, we quantified the horizontal vestibuloocular reflex (HVOR) of the normal guinea pig following IP injection of EGb 761. In normal guinea pig, IP administration of EGb 761 led to a reversible, dose-dependent decrease of the HVOR gain without affecting the phase of the reflex. These data help to explain the therapeutic effects of EGb 761 during vestibular syndromes and strongly suggest an impact at the neuronal level.

Orientation of the semicircular canals in rat

The orientation of the rat semicircular canals was determined using one of two techniques. Null point analysis was used to define physiologically the planar equations of the anterior (n = 15) and posterior canals (n = 15); equations for the horizontal canal (n = 19) were determined using an anatomical dissection technique. Canal orientation was defined with respect to stereotaxic coordinate system and, for comparison, relative to head position during freeze (startle) behavior. Results show that ipsilateral canal planes are orthogonal within 4-8 degrees, and pairs of right-left synergistic pairs are essentially co-planar. The horizontal canals are inclined upwards 35 degrees with respect to the horizontal plane, but a head position of 43 degrees nose-down was determined to produce near optimal horizontal canal and minimal vertical canal activation with horizontal rotation. Finally, a loud or unexpected auditory stimulus initiates a freeze (startle) response in rat characterized by an transient followed by a sustained head position lasting several seconds. Transients are complete within 300-400 ms. Thereafter, the head becomes momentarily stabilized in the startle position which averaged 14 +/- 8 degrees (nose-down with respect to horizontal stereotaxic zero) across the population (n = 14). The response habituated only slightly, but the final position was sufficiently variable so as to limit the usefulness of the freeze (startle) position as a reference of semicircular canal position in the rat.

Electrovestibulogram: first results in the guinea pig

This paper describes a method of investigation of the peripheral vestibular system. Electrical stimulations (ES) are applied on the round window of chronically implanted guinea pigs, with and without vestibular stimulation (either per or post rotational accelerations). The whole nerve action potential recorded at the output of the internal auditory meatus, the difference between the two conditions, reveals the change in electrical excitability and thus presumably in discharge rate of vestibular fibres determined by rotational stimuli. This electrical vestibular action potential (EVAP) presents as a mainly monophasic potential with a peak latency to ES of about 0.3 ms. Right and left accelerations versus rest are shown to give responses with opposite polarity, reflecting the inhibitory and excitatory influences of these opposite accelerations, whereas the transfer function of EVAP versus acceleration amplitude appears roughly linear. These observations appear altogether coherent with published mechano-physiological data about individual discharge rates of canal fibres. Also relative amplitudes of acoustical and vestibular responses are in agreement with related number of fibres in the two systems. This EVAP could be the basis of a quantitative evaluation method for the vestibular system, the electrovestibulogram (EVG).

A new quantitative model of total endolymph flow in the system of semicircular ducts

1. A new concept of endolymph flow in the vertebrate vestibular system is presented. This approach describes quantitatively the flow in the entire system of three semicircular ducts interconnected by the utriculus and the crus commune. This approach is quite distinct from the classical theory in which the labyrinth is generally conceived to consist of three separate duct circuits. 2. The present approach shows the following set of distinct differences to the classical view: (a) In a labyrinth composed of three ducts perpendicular to each other the flow is non-zero in the other ducts when the labyrinth is rotated in the plane of a particular duct. (b) In a labyrinth with two equal ducts and with the duct planes under approximately 73 degrees the flow in one duct is zero when the rotation takes place in the plane of the other duct. Previous measurements of duct angles reflect this value surprisingly well. An obtuse or sharp angle between duct planes can lead to better performance of a particular labyrinth because the "external impulses" in the different ducts may amplify or compensate each other. (c) The behaviour of the flow in the entire labyrinth is a non-linear function of direction or rotation (cf. points (d), (e]. (d) Six time constants for the entire labyrinth can be distinguished (three long, three short); the flow in a particular duct is composed of six terms with these time constants. The composition of this flow and thus the relative importance of the terms depends on the positioning of the labyrinth with respect to the rotation vector. (e) The time constants also depend, for different labyrinths, on a shared influence of the dimensions of the ducts and the elastic properties of all three cupulae. (f) The forces in a particular duct depend also on the amount of motion the fluid will acquire in the other ducts. (g) The sensitivity of a particular duct depends also on the dimensions of the other parts in the vestibular system. 3. Equations for a system consisting of two ducts and for the classical single duct system are also given. Both systems are special cases of the three-duct system. The single duct equations are equivalent with equations given by Oman (1980) and Oman et al. (1987) which include the contribution of a wide utriculus. 4. The present theory of endolymph flow is mainly supported by the outcome of previously performed experiments concerning time constants and rotation of human subjects in different planes.(ABSTRACT TRUNCATED AT 400 WORDS)

Behavior of primary horizontal canal neurons in alert and anesthetized guinea pigs

The activities of the primary horizontal canal neurons were investigated in alert and anesthetized guinea pigs. The average resting rate in the alert guinea pigs was 47.3 +/- 21.2 spikes/s. This value was significantly higher than that in anesthetized animals which had an average rate of 39.1 +/- 20.5 spikes/s. The average gain from the total of 66 units tested at 0.3-Hz sinusoidal rotation was 0.42 +/- 0.37 spikes/s/degree/s, and this value was the same as that of anesthetized guinea pigs. The average phase lag to the angular acceleration was 57.3 +/- 20.7 degrees in the alert guinea pigs, which was significantly smaller than that in the anesthetized animals. The number of cutoff neurons at 0.3 Hz was 1 out of 67 neurons in the alert guinea pigs and 7 out of 64 neurons in the anesthetized guinea pigs. The functional meaning of these differences in primary semicircular canal neurons is discussed.

Neuronal activity in the contralateral medial vestibular nucleus of the guinea pig following unilateral labyrinthectomy

The recovery of normal ocular motor and postural behavior following unilateral labyrinthectomy is termed vestibular compensation and it has been attributed to the return of normal resting activity to neurons in the bilateral vestibular nuclei. Previous studies in the cat have reported that approximately normal resting activity returns to type I neurons in the medial vestibular nucleus (MVN) contralateral to the deafferentiation by 4-8 weeks post-operation (post-op.), while the gain of the response of these neurons to horizontal angular acceleration remains lower than normal. The present data demonstrate that, in the guinea pig, normal resting activity returns to the contralateral MVN type I neurons by only 52-60 h post-op., while the gain and phase of the response of these neurons to horizontal sinusoidal acceleration remains abnormal up to 8-12 months post-op. By contrast with previous studies, the present data show that type II neurons in the contralateral MVN exhibit some increase in their frequency of occurrence and gain toward normal values during vestibular compensation (at 52-60 h and 8-12 months post-op.). The rapid recovery of normal type I resting activity correlates with the disappearance of spontaneous nystagmus and postural asymmetries in the guinea pig by 52 h post-op.

Behavioural recovery from unilateral vestibular lesion is facilitated by GM1 ganglioside treatment

Exogenously administered gangliosides function in vivo to facilitate survival and repair of damaged neurones in both central and peripheral nervous systems. These effects have been attributed to their neuritogenic and neuronotrophic properties. In this investigation the effects of ganglioside treatment have been studied on the compensation that follows a unilateral labyrinthectomy, considering that the vestibular recovery is supposed to be achieved through a mechanism of sprouting acting on the deafferented neurones. Hemilabyrinthectomized guinea pigs were given a daily injection of either GM1 ganglioside (30 mg/kg, i.p.) or saline for 21 days. As regards postural symptoms, the GM1 group more quickly restored a posture of the head non-deviated in the horizontal plane, while the remaining postural symptomatology was not significantly affected by the treatment. Furthermore, the GM1-treated group attained more rapidly an ocular position characterized by very slow drifts (or by no drift at all), directed towards the side of the intact labyrinth. Ganglioside treatment did not reduce the severity of the initial hemilabyrinthectomy impairments and the effect of the treatment became evident with time. This result is consistent with the hypothesis that also in this model of neuronal plasticity, gangliosides act by enhancing reactive synaptogenesis on the deafferented vestibular neurones.

The orientation of the cervical vertebral column in unrestrained awake animals. I. Resting position

The orientation of the cervical vertebral column was studied by X-ray photography of the region containing the head and the neck in nine unrestrained species of vertebrates (man, monkey, cat, rabbit, guinea pig, rat, chicken, frog, lizard). In addition, the orientation of the horizontal semicircular canals was measured in four species using landmarks on the skull. In all vertebrates studied, with the exception of frog and lizard, the general orientation of the cervical vertebral column was vertical when animals were at rest, and not horizontal or oblique as suggested by the macroscopic appearance of the neck. The posture of the animal, whether lying, sitting or standing, had little effect on this general vertical orientation, although some variability was noticed depending on the species. This finding prompted the definition of a resting zone, where the cervical column can take any orientation within a narrow range around a mean position. The cervical vertebral column composes part of the S-shaped structure of the entire vertebral column, with one inflection around the cervico-thoracic (C7/Th1) junction. This feature is already noticable in the lizard. The vertical orientation of the cervical vertebral column is interpreted to provide a stable and energy saving balance of the head. Furthermore, when the head is lowered or raised, the atlanto-occipital and cervico-thoracic junctions are predominantly involved, while the entire cervical column largely preserves its intrinsic configuration. The curved configuration of the cervico-thoracic vertebral column embedded in long spring-like muscles is interpreted to function as a shock absorber. At rest, animals did not hold their heads with the horizontal canals oriented earth horizontally all the time, but often maintained them pitched up by ca. 5 deg, as has been reported for man. At other times, presumably when the vigilance level increased, the horizontal canals were brought into the earth horizontal plane. The vertical orientation of the cervical column results in a vertical positioning of the odontoid process of the axis (second cervical vertebra, C2), which thus provides the axis of rotation for yaw movements of the head. This axis corresponds to that of the horizontal semicircular canals. The vertical organization of the cervical vertebral column in birds and mammals, whether the animal is quadrupedal or bipedal, points to a common organizational principle for eye and head movement systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Dimensions of the horizontal semicircular duct, ampulla and utricle in rat and guinea pig

The dimensions of the membranous labyrinth partly determine the mechanical operation of the semicircular canal system. This study provides, for the first time, extensive measures in individual specimens of the sizes, cross-sectional shapes and areas of the horizontal semicircular duct, ampulla and utricle in the rat and the guinea pig. The membranous labyrinths were fixed in Karnovsky's fixative, exposed, photographed, sectioned, oriented perpendicular to the line of sight and then measured using a calibrated graticule in the eye piece of an operating microscope. As well as the expected size differences between these species, there are major differences in the shape of the utricle.