The contribution of the contralateral labyrinth to second order vestibular neuronal activity in the cat

The Nature and Origin of Synaptic Inputs to Vestibulospinal Neurons in the Larval Zebrafish

Vestibulospinal neurons integrate sensed imbalance to regulate postural reflexes. As an evolutionarily conserved neural population, understanding their synaptic and circuit-level properties can offer insight into vertebrate antigravity reflexes. Motivated by recent work, we set out to verify and extend the characterization of vestibulospinal neurons in the larval zebrafish. Using current-clamp recordings together with stimulation, we observed that larval zebrafish vestibulospinal neurons are silent at rest, yet capable of sustained spiking following depolarization. Neurons responded systematically to a vestibular stimulus (translation in the dark); responses were abolished after chronic or acute loss of the utricular otolith. Voltage-clamp recordings at rest revealed strong excitatory inputs with a characteristic multimodal distribution of amplitudes, as well as strong inhibitory inputs. Excitatory inputs within a particular mode (amplitude range) routinely violated refractory period criteria and exhibited complex sensory tuning, suggesting a nonunitary origin. Next, using a unilateral loss-of-function approach, we characterized the source of vestibular inputs to vestibulospinal neurons from each ear. We observed systematic loss of high-amplitude excitatory inputs after utricular lesions ipsilateral, but not contralateral, to the recorded vestibulospinal neuron. In contrast, while some neurons had decreased inhibitory inputs after either ipsilateral or contralateral lesions, there were no systematic changes across the population of recorded neurons. We conclude that imbalance sensed by the utricular otolith shapes the responses of larval zebrafish vestibulospinal neurons through both excitatory and inhibitory inputs. Our findings expand our understanding of how a vertebrate model, the larval zebrafish, might use vestibulospinal input to stabilize posture. More broadly, when compared with recordings in other vertebrates, our data speak to conserved origins of vestibulospinal synaptic input.

The nature and origin of synaptic inputs to vestibulospinal neurons in the larval zebrafish

Vestibulospinal neurons integrate sensed imbalance to regulate postural reflexes. As an evolutionarily-conserved neural population, understanding their synaptic and circuit-level properties can offer insight into vertebrate antigravity reflexes. Motivated by recent work, we set out to verify and extend the characterization of vestibulospinal neurons in the larval zebrafish. Using current clamp recordings together with stimulation, we observed that larval zebrafish vestibulospinal neurons are silent at rest, yet capable of sustained spiking following depolarization. Neurons responded systematically to a vestibular stimulus (translation in the dark); responses were abolished after chronic or acute loss of the utricular otolith. Voltage clamp recordings at rest revealed strong excitatory inputs with a characteristic multimodal distribution of amplitudes, as well as strong inhibitory inputs. Excitatory inputs within a particular mode (amplitude range) routinely violated refractory period criteria and exhibited complex sensory tuning, suggesting a non-unitary origin. Next, using a unilateral loss-of-function approach, we characterized the source of vestibular inputs to vestibulospinal neurons from each ear. We observed systematic loss of high-amplitude excitatory inputs after utricular lesions ipsilateral, but not contralateral to the recorded vestibulospinal neuron. In contrast, while some neurons had decreased inhibitory inputs after either ipsilateral or contralateral lesions, there were no systematic changes across the population of recorded neurons. We conclude that imbalance sensed by the utricular otolith shapes the responses of larval zebrafish vestibulospinal neurons through both excitatory and inhibitory inputs. Our findings expand our understanding of how a vertebrate model, the larval zebrafish, might use vestibulospinal input to stabilize posture. More broadly, when compared to recordings in other vertebrates, our data speak to conserved origins of vestibulospinal synaptic input.

The human vestibular cortex: functional anatomy of OP2, its connectivity and the effect of vestibular disease

Area OP2 in the posterior peri-sylvian cortex has been proposed to be the core human vestibular cortex. We investigated the functional anatomy of OP2 and adjacent areas (OP2+) using spatially constrained independent component analysis (ICA) of functional magnetic resonance imaging (fMRI) data from the Human Connectome Project. Ten ICA-derived subregions were identified. OP2+ responses to vestibular and visual motion were analyzed in 17 controls and 17 right-sided vestibular neuritis patients who had previously undergone caloric and optokinetic stimulation during fMRI. In controls, a posterior part of right OP2+ showed: (i) direction-selective responses to visual motion and (ii) activation during caloric stimulation that correlated positively with perceived self-motion, and negatively with visual dependence and peak slow-phase nystagmus velocity. Patients showed abnormal OP2+ activity, with an absence of visual or caloric activation of the healthy ear and no correlations with vertigo or visual dependence-despite normal slow-phase nystagmus responses to caloric stimulation. Activity in a lateral part of right OP2+ correlated with chronic visually induced dizziness in patients. In summary, distinct functional subregions of right OP2+ show strong connectivity to other vestibular areas and a profile of caloric and visual responses, suggesting a central role for vestibular function in health and disease.

Mature Retina Compensates Functionally for Partial Loss of Rod Photoreceptors

Loss of primary neuronal inputs inevitably strikes every neural circuit. The deafferented circuit could propagate, amplify, or mitigate input loss, thus affecting the circuit’s output. How the deafferented circuit contributes to the effect on the output is poorly understood because of lack of control over loss of and access to circuit elements. Here, we control the timing and degree of rod photoreceptor ablation in mature mouse retina and uncover compensation. Following loss of half of the rods, rod bipolar cells mitigate the loss by preserving voltage output. Such mitigation allows partial recovery of ganglion cell responses. We conclude that rod death is compensated for in the circuit because ganglion cell responses to stimulation of half of the rods in an unperturbed circuit are weaker than responses after death of half of the rods. The dominant mechanism of such compensation includes homeostatic regulation of inhibition to balance the loss of excitation.

Care et al. ablate half of the rods in mature mouse retina and find that primary neuron loss is functionally compensated for by balanced inhibition and excitation at the secondary neuron. Changes in cone-mediated, but not rod-mediated, output neuron spikes are recapitulated by half stimulation, demonstrating independent regulation of pathways.

The Anatomical and Physiological Basis of Clinical Tests of Otolith Function. A Tribute to Yoshio Uchino

Otolithic receptors are stimulated by gravitoinertial force (GIF) acting on the otoconia resulting in deflections of the hair bundles of otolithic receptor hair cells. The GIF is the sum of gravitational force and the inertial force due to linear acceleration. The usual clinical and experimental tests of otolith function have used GIFs (roll tilts re gravity or linear accelerations) as test stimuli. However, the opposite polarization of receptors across each otolithic macula is puzzling since a GIF directed across the otolith macula will excite receptors on one side of the line of polarity reversal (LPR at the striola) and simultaneously act to silence receptors on the opposite side of the LPR. It would seem the two neural signals from the one otolith macula should cancel. In fact, Uchino showed that instead of canceling, the simultaneous stimulation of the oppositely polarized hair cells enhances the otolithic response to GIF—both in the saccular macula and the utricular macula. For the utricular system there is also commissural inhibitory interaction between the utricular maculae in each ear. The results are that the one GIF stimulus will cause direct excitation of utricular receptors in the activated sector in one ear as well as indirect excitation resulting from the disfacilitation of utricular receptors in the corresponding sector on the opposite labyrinth. There are effectively two complementary parallel otolithic afferent systems—the sustained system concerned with signaling low frequency GIF stimuli such as roll head tilts and the transient system which is activated by sound and vibration. Clinical tests of the sustained otolith system—such as ocular counterrolling to roll-tilt or tests using linear translation—do not show unilateral otolithic loss reliably, whereas tests of transient otolith function [vestibular evoked myogenic potentials (VEMPs) to brief sound and vibration stimuli] do show unilateral otolithic loss. The opposing sectors of the maculae also explain the results of galvanic vestibular stimulation (GVS) where bilateral mastoid galvanic stimulation causes ocular torsion position similar to the otolithic response to GIF. However, GVS stimulates canal afferents as well as otolithic afferents so the eye movement response is complex.

Why the cerebellar shutdown/clampdown hypothesis of vestibular compensation is inconsistent with neurophysiological evidence

BACKGROUND

Vestibular compensation is the process by which the central nervous system (CNS) attempts to adapt to the loss of vestibular sensory inputs. As such, the compensation process is critically involved in the vestibular rehabilitation programs that are implemented by physical therapists for patients with vestibular disorders. One hypothesis regarding vestibular compensation, which has persisted in some of the published vestibular compensation literature and particularly on some vestibular and physical therapy websites, is the 'cerebellar shutdown' or 'cerebellar clampdown' hypothesis proposed by McCabe and Ryu in 1969. This hypothesis proposes that the cerebellum inhibits neuronal activity in the bilateral vestibular nuclei (VN) following unilateral vestibular loss (UVL), causing the VN contralateral to the UVL to be electrically silent during the early phases of vestibular compensation. Despite a wealth of evidence against this idea, it has gained traction amongst some physical therapists and has implications for vestibular rehabilitation early in the compensation process.

CONCLUSIONS

In this paper it is argued that the 'cerebellar shutdown' or 'clampdown' hypothesis is inconsistent with well accepted neurophysiological and imaging evidence and that it is also logically flawed.

Concepts and Physiological Aspects of the Otolith Organ in Relation to Electrical Stimulation

BACKGROUND

This paper discusses some of the concepts and major physiological issues in developing a means of electrically stimulating the otolithic system, with the final goal being the electrical stimulation of the otoliths in human patients. It contrasts the challenges of electrical stimulation of the otolith organs as compared to stimulation of the semicircular canals. Electrical stimulation may consist of trains of short-duration pulses (e.g., 0.1 ms duration at 400 Hz) by selective electrodes on otolith maculae or otolithic afferents, or unselective maintained DC stimulation by large surface electrodes on the mastoids - surface galvanic stimulation.

SUMMARY

Recent anatomical and physiological results are summarized in order to introduce some of the unique issues in electrical stimulation of the otoliths. The first challenge is that each otolithic macula contains receptors with opposite polarization (opposing preferred directions of stimulation), unlike the uniform polarization of receptors in each semicircular canal crista. The puzzle is that in response to the one linear acceleration in the one macula, some otolithic afferents have an increased activation whereas others have decreased activation. Key Messages: At the vestibular nucleus this opposite receptor hair cell polarization and consequent opposite afferent input allow enhanced response to the one linear acceleration, via a "push-pull" neural mechanism in a manner analogous to the enhancement of semicircular canal responses to angular acceleration. Within each otolithic macula there is not just one uniform otolithic neural input to the brain - there are very distinctly different channels of otolithic neural inputs transferring the neural data to the brainstem. As a simplification these channels are characterized as the sustained and transient systems. Afferents in each system have different responses to stimulus onset and maintained stimulation and likely different projections, and most importantly different thresholds for activation by electrical stimulation and different adaptation rates to maintained stimulation. The implications of these differences are considered.

Brain Stem Neural Circuits of Horizontal and Vertical Saccade Systems and their Frame of Reference

Sensory signals for eye movements (visual and vestibular) are initially coded in different frames of reference but finally translated into common coordinates, and share the same final common pathway, namely the same population of extraocular motoneurons. From clinical studies in humans and lesion studies in animals, it is generally accepted that voluntary saccadic eye movements are organized in horizontal and vertical Cartesian coordinates. However, this issue is not settled yet, because neural circuits for vertical saccades remain unidentified. We recently determined brainstem neural circuits from the superior colliculus to ocular motoneurons for horizontal and vertical saccades with combined electrophysiological and neuroanatomical techniques. Comparing well-known vestibuloocular pathways with our findings of commissural excitation and inhibition between both superior colliculi, we proposed that the saccade system uses the same frame of reference as the vestibuloocular system, common semicircular canal coordinate. This proposal is mainly based on marked similarities (1) between output neural circuitry from one superior colliculus to extraocular motoneurons and that from a respective canal to its innervating extraocular motoneurons, (2) of patterns of commissural reciprocal inhibitions between upward saccade system on one side and downward system on the other, and between anterior canal system on one side and posterior canal system on the other, and (3) between the neural circuits of saccade and quick phase of vestibular nystagmus sharing brainstem burst neurons. In support of the proposal, commissural excitation of the superior colliculi may help to maintain Listing's law in saccades in spite of using semicircular canal coordinate.

The Video Head Impulse Test

In 1988, we introduced impulsive testing of semicircular canal (SCC) function measured with scleral search coils and showed that it could accurately and reliably detect impaired function even of a single lateral canal. Later we showed that it was also possible to test individual vertical canal function in peripheral and also in central vestibular disorders and proposed a physiological mechanism for why this might be so. For the next 20 years, between 1988 and 2008, impulsive testing of individual SCC function could only be accurately done by a few aficionados with the time and money to support scleral search-coil systems—an expensive, complicated and cumbersome, semi-invasive technique that never made the transition from the research lab to the dizzy clinic. Then, in 2009 and 2013, we introduced a video method of testing function of each of the six canals individually. Since 2009, the method has been taken up by most dizzy clinics around the world, with now close to 100 refereed articles in PubMed. In many dizzy clinics around the world, video Head Impulse Testing has supplanted caloric testing as the initial and in some cases the final test of choice in patients with suspected vestibular disorders. Here, we consider seven current, interesting, and controversial aspects of video Head Impulse Testing: (1) introduction to the test; (2) the progress from the head impulse protocol (HIMPs) to the new variant—suppression head impulse protocol (SHIMPs); (3) the physiological basis for head impulse testing; (4) practical aspects and potential pitfalls of video head impulse testing; (5) problems of vestibulo-ocular reflex gain calculations; (6) head impulse testing in central vestibular disorders; and (7) to stay right up-to-date—new clinical disease patterns emerging from video head impulse testing. With thanks and appreciation we dedicate this article to our friend, colleague, and mentor, Dr Bernard Cohen of Mount Sinai Medical School, New York, who since his first article 55 years ago on compensatory eye movements induced by vertical SCC stimulation has become one of the giants of the vestibular world.

Plasticity within non-cerebellar pathways rapidly shapes motor performance in vivo

Although cerebellar mechanisms are vital to maintain accuracy during complex movements and to calibrate simple reflexes, recent in vitro studies have called into question the widely held view that synaptic changes within cerebellar pathways exclusively guide alterations in motor performance. Here we investigate the vestibulo-ocular reflex (VOR) circuitry by applying temporally precise activation of vestibular afferents in awake-behaving monkeys to link plasticity at different neural sites with changes in motor performance. Behaviourally relevant activation patterns produce rapid attenuation of direct pathway VOR neurons, but not their nerve input. Changes in the strength of this pathway are sufficient to induce a lasting decrease in the evoked VOR. In addition, indirect brainstem pathways display complementary nearly instantaneous changes, contributing to compensating for the reduced sensitivity of primary VOR neurons. Taken together, our data provide evidence that multiple sites of plasticity within VOR pathways can rapidly shape motor performance in vivo.

The extent to which non-cerebellar pathways can refine motor performance is debated. Here, the authors demonstrate behaviourally relevant patterns of activation evoke rapid plasticity within direct and indirect vestibulo-ocular reflex pathways in vivo, leading to changes in evoked eye movements.

The Video Head Impulse Test (vHIT) of Semicircular Canal Function - Age-Dependent Normative Values of VOR Gain in Healthy Subjects

BACKGROUND/HYPOTHESIS

The video Head Impulse Test (vHIT) is now widely used to test the function of each of the six semicircular canals individually by measuring the eye rotation response to an abrupt head rotation in the plane of the canal. The main measure of canal adequacy is the ratio of the eye movement response to the head movement stimulus, i.e., the gain of the vestibulo-ocular reflex (VOR). However, there is a need for normative data about how VOR gain is affected by age and also by head velocity, to allow the response of any particular patient to be compared to the responses of healthy subjects in their age range. In this study, we determined for all six semicircular canals, normative values of VOR gain, for each canal across a range of head velocities, for healthy subjects in each decade of life.

STUDY DESIGN

The VOR gain was measured for all canals across a range of head velocities for at least 10 healthy subjects in decade age bands: 10-19, 20-29, 30-39, 40-49, 50-59, 60-69, 70-79, 80-89.

METHODS

The compensatory eye movement response to a small, unpredictable, abrupt head rotation (head impulse) was measured by the ICS impulse prototype system. The same operator delivered every impulse to every subject.

RESULTS

Vestibulo-ocular reflex gain decreased at high head velocities, but was largely unaffected by age into the 80- to 89-year age group. There were some small but systematic differences between the two directions of head rotation, which appear to be largely due to the fact that in this study only the right eye was measured. The results are considered in relation to recent evidence about the effect of age on VOR performance.

CONCLUSION

These normative values allow the results of any particular patient to be compared to the values of healthy people in their age range and so allow, for example, detection of whether a patient has a bilateral vestibular loss. VOR gain, as measured directly by the eye movement response to head rotation, seems largely unaffected by aging.

Neurobiological mechanisms of acute vertigo

The vestibular system provides us with reflexive responses of eye movements and balance control, as well as with perceptual estimates of self-motion and gravity direction. Crucial to its proper functioning is a bilaterally balanced vestibular signal originating from the vestibular end organs in the inner ears and projecting via vestibular nerve afferents to the brainstem vestibular nuclei. Disturbances of the bilateral vestibular interplay become evident in cases of acute unilateral peripheral vestibular deafferentation. The resultant sudden imbalance of vestibular afferent tone at the level of the vestibular nuclei leads to pronounced ocular–motor and postural impairment, as well as to intensive vertigo and/or dizziness, accompanied by autonomic symptoms, such as nausea and vomiting. Subsequent compensatory mechanisms efficiently diminish these static symptoms (such as spontaneous nystagmus) within days and allow functional recovery of dynamic symptoms (such as blurred vision during fast head turns) to such a degree that most patients return to their normal daily activities within weeks. This article aims to provide an understanding about the pathophysiological changes after unilateral vestibular deafferentation and the current knowledge on the compensatory mechanisms.

Voxel-based morphometry depicts central compensation after vestibular neuritis

OBJECTIVE

Patients who have had vestibular neuritis (VN) show a remarkable clinical improvement especially in gait and posture >6 months after disease onset.

METHODS

Voxel-based morphometry was used to detect the VN-induced changes in gray and white matter by means of structural magnetic resonance imaging. Twenty-two patients were compared an average 2.5 years after onset of VN to a healthy sex-and age-matched control group.

RESULTS

Our analysis revealed that all patients had signal intensity increases for gray matter in the medial vestibular nuclei and the right gracile nucleus and for white matter in the area of the pontine commissural vestibular fibers. A relative atrophy was observed in the left posterior hippocampus and the right superior temporal gyrus. Patients with a residual canal paresis also showed an increase of gray matter in middle temporal (MT)/V5 bilaterally.

INTERPRETATION

These findings indicate that the processes of central compensation after VN seem to occur in 3 different sensory systems. First of all, the vestibular system itself showed a white matter increase in the commissural fibers as a direct consequence of an increased internuclei vestibular crosstalk of the medial vestibular nuclei. Second, to regain postural stability, there was a shift to the somatosensory system due to an elevated processing of proprioceptive information in the right gracile nucleus. Third, there was a bilateral increase in the area of MT/V5 in VN patients with a residual peripheral vestibular hypofunction. This seems to be the result of an increased importance of visual motion processing.

Functional organization of vestibular commissural connections in frog

Central vestibular neurons receive substantial inputs from the contralateral labyrinth through inhibitory and excitatory brainstem commissural pathways. The functional organization of these pathways was studied by a multi-methodological approach in isolated frog whole brains. Retrogradely labeled vestibular commissural neurons were primarily located in the superior vestibular nucleus in rhombomeres 2/3 and the medial and descending vestibular nucleus in rhombomeres 5-7. Restricted projections to contralateral vestibular areas, without collaterals to other classical vestibular targets, indicate that vestibular commissural neurons form a feedforward push-pull circuitry. Electrical stimulation of the contralateral coplanar semicircular canal nerve evoked in canal-related second-order vestibular neurons (2 degrees VN) commissural IPSPs (approximately 70%) and EPSPs (approximately 30%) with mainly (approximately 70%) disynaptic onset latencies. The dynamics of commissural responses to electrical pulse trains suggests mediation predominantly by tonic vestibular neurons that activate in all tonic 2 degrees VN large-amplitude IPSPs with a reversal potential of -74 mV. In contrast, phasic 2 degrees VN exhibited either nonreversible, small-amplitude IPSPs (approximately 40%) of likely dendritic origin or large-amplitude commissural EPSPs (approximately 60%). IPSPs with disynaptic onset latencies were exclusively GABAergic (mainly GABA(A) receptor-mediated) but not glycinergic, compatible with the presence of GABA-immunopositive (approximately 20%) and the absence of glycine-immunopositive vestibular commissural neurons. In contrast, IPSPs with longer, oligosynaptic onset latencies were GABAergic and glycinergic, indicating that both pharmacological types of local inhibitory neurons were activated by excitatory commissural fibers. Conservation of major morpho-physiological and pharmacological features of the vestibular commissural pathway suggests that this phylogenetically old circuitry plays an essential role for the processing of bilateral angular head acceleration signals in vertebrates.

An increase in glycinergic quantal amplitude and frequency during early vestibular compensation in mouse

The process of vestibular compensation includes both behavioral and neuronal recovery after unilateral loss of peripheral vestibular organs. The mechanisms that underlie this process are poorly understood. Previous research has shown the presence of both gamma-aminobutyric acid type A (GABA(A)) and glycine receptors in the medial vestibular nuclei (MVN). It has been suggested that inhibitory transmission mediated by these receptors may have a role in recovery during vestibular compensation. This study investigated changes in fast inhibitory synaptic transmission of GABA(A)ergic and glycinergic quantal events after unilateral labyrinthectomy (UL) at three different time points. Mice were anesthetized and peripheral vestibular organs were removed from one side of the head. After recovery, transverse brain stem sections (300 mum) were prepared from mice that had undergone UL either 4 hours, 2 days, or 7 days earlier. Our experiments do not show evidence for alterations in synaptic GABA(A) receptor properties in MVN neurons after UL at any time point investigated. In contrast, during early vestibular compensation (4 hours post UL) there is a significant increase in the glycinergic quantal current amplitude in contralesional MVN neurons compared with control. Our results also show an increase in the frequency of glycinergic quantal events of both ipsi- and contralesional MVN neurons during this early period. We suggest that changes in both pre- and postsynaptic glycine receptor mediated inhibitory synaptic transmission after sensory loss is an important mechanism by which neuronal discharge patterns can be modulated.

The effects of unilateral eighth nerve block on fictive VOR in the turtle

Multiunit activity during horizontal sinusoidal motion was recorded from pairs of oculomotor, trochlear, or abducens nerves of an in vitro turtle brainstem preparation that received inputs from intact semicircular canals. Responses of left oculomotor, right trochlear and right abducens nerves were approximately aligned with leftward head velocity, and that of the respective contralateral nerves were in-phase with rightward velocity. We examined the effect of sectioning or injecting lidocaine (1-2 microL of 0.5%) into the right vestibular nerve. Nerve block caused a striking phase shift in the evoked response of right oculomotor and left trochlear nerves, in which (rightward) control responses were replaced by a smaller-amplitude response to leftward table motion. Such "phase-reversed" responses were poorly defined in abducens nerve recordings. Frequency analysis demonstrated that this activity was advanced in phase relative to post-block responses of the respective contralateral nerves, which were in turn phase-advanced relative to pre-block controls. Phase differences were largest (approximately 10 degrees) at low frequencies (approximately 0.1 Hz) and statistically absent at 1 Hz. The phase-reversed responses were further investigated by eliminating individual canal input from the left labyrinth following right nVIII block, which indicated that the activation of the vertical canal afferents is the source of this activity.

Bilateral vestibular loss leads to active destabilization of balance during voluntary head turns in the standing cat

The purpose of this study was to determine the source of postural instability in labyrinthectomized cats during lateral head turns. Cats were trained to maintain the head in a forward orientation and then perform a rapid, large-amplitude head turn to left or right in yaw, while standing freely on a force platform. Head turns were biomechanically complex with the primary movement in the yaw plane accompanied by an ipsilateral ear-down roll and nose-down pitch. Cats used a strategy of pushing off by activating extensors of the contralateral forelimb while using all four limbs to produce a rotational moment of force about the vertical axis. After bilateral labyrinthectomy, the initial components of the head turn and accompanying postural responses were hypermetric, but otherwise similar to those produced before the lesion. However, near the time of peak yaw velocity, the lesioned cats produced an unexpected burst in extensors of the contralateral limbs that thrust the body to the ipsilateral side, leading to falls. This postural error was in the frontal (roll) plane, even though the primary movement was a rotation in the horizontal (yaw) plane. The response error decreased in amplitude with compensation but did not disappear. We conclude that lack of vestibular input results in active destabilization of balance during voluntary head movement. We postulate that the postural imbalance arises from the misperception that the trunk was rolling contralaterally, based on signals from neck proprioceptors in the absence of vestibular inputs.

The anatomy of the vestibular nuclei

The vestibular portion of the eighth cranial nerve informs the brain about the linear and angular movements of the head in space and the position of the head with respect to gravity. The termination sites of these eighth nerve afferents define the territory of the vestibular nuclei in the brainstem. (There is also a subset of afferents that project directly to the cerebellum.) This chapter reviews the anatomical organization of the vestibular nuclei, and the anatomy of the pathways from the nuclei to various target areas in the brain. The cytoarchitectonics of the vestibular brainstem are discussed, since these features have been used to distinguish the individual nuclei. The neurochemical phenotype of vestibular neurons and pathways are also summarized because the chemical anatomy of the system contributes to its signal-processing capabilities. Similarly, the morphologic features of short-axon local circuit neurons and long-axon cells with extrinsic projections are described in detail, since these structural attributes of the neurons are critical to their functional potential. Finally, the composition and hodology of the afferent and efferent pathways of the vestibular nuclei are discussed. In sum, this chapter reviews the morphology, chemoanatomy, connectivity, and synaptology of the vestibular nuclei.

Bilateral processing of vestibular responses revealed by injecting lidocaine into the eighth cranial nerve in vitro

Extracellular unit responses were recorded from the vestibular nucleus (VN) and medial longitudinal fasciculus during horizontal head rotation of an in vitro turtle brainstem in which the temporal bones remained attached. Units were characterized as type I or type II based on the responses to ipsiversive or contraversive rotation, respectively. Lidocaine injections (0.5-2 microl of 0.5%) into the root of the eighth cranial nerve within the cranium caused rapid effects on unit responses to head rotation. Responses of type I units were reduced by ipsilateral injection but enhanced following contralateral injection. On the other hand, type II units had their responses increased by ipsilateral injections yet decreased by contralateral injections. In approximately half of the type II cells, decrease of the contraversive response was accompanied by the appearance of latent ipsiversive activity. Our findings not only confirm that each eighth nerve has afferents that drive ipsiversive excitation of both vestibular nuclei but also suggest that both nerves compete to dominate a central neuron's vestibular response. These results may be inconsistent with the push-pull vestibular model in which each nerve drives the central neuron with a complementary response that enhances the vestibular output. An alternate model is described in which vestibular neurons receive bilateral excitation, and that excitatory input is antagonized by crossed inhibition during contraversive motion.

Contribution of GABAergic inhibition to the responses of secondary vestibular neurons to head rotation in the rat

To assess the contribution of GABAA receptor-mediated inputs in control of vestibular responses of secondary vestibular neurons, we examined the effects of the GABAA receptor antagonists, bicuculline and picrotoxin, on these neurons in anesthetized rats. Horizontal canal-related secondary vestibular neurons were identified by their monosynaptic excitation from the ipsilateral vestibular nerve and by the modulation of their firing rate for head rotation. Responses to sinusoidal head rotation were recorded before and during iontophoretic application of the drugs. Application of bicuculline increased DC level of the responses (mean firing rate in each cycle) in all of the 10 neurons examined. In seven of these, the gain was increased along with the DC level, but the phase was virtually unaffected. Similarly, picrotoxin increased both the DC level (4/4) and the gain (3/4), but did not affect the phase. In the 10 neurons that increased the gain, the mean percent increase in the gain was 31% (8-54%). These results indicate that the majority of neurons received inhibitory inputs that were in phase with the excitatory inputs from primary afferents. This suggests that these neurons received GABAergic input of non-commissural origin, most likely from the flocculus.

Postnatal changes in locomotor movements after labyrinthectomy in rats

The influence of unilateral or bilateral labyrinthectomy on the postnatal development of locomotor movements was studied in newborn rats. Locomotor movements of the rats were measured on the surface of a digitizer table by attaching a miniature resonance coil to the abdomen of the rat. Labyrinthectomy was performed in rats on postnatal day (PND) 11. From PND 10 to PND 14, the total distance and mean velocity of locomotor movements were significantly lower in the labyrinthectomized rats than in the control rats. From PND 17 to PND 20, the total distance and mean velocity of the locomotor movements of the unilateral labyrinthectomized rats gradually recovered to an almost normal level. However, those of the bilateral labyrinthectomized rats remained at insufficient levels even at PND 20.

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)

Neurotransmitters in the Vestibular Commissural System of the Cat

The present study focused on the transmitters that control the vestibular neural activity and, in particular, that regulate commissural inhibition. Extracellular spikes of a single vestibular neuron were recorded in decerebrate cats. The seven barrels of the electrode, with the exception of the center barrel, were filled with transmitter candidates and their specific antagonists, while the center barrel was filled with 2 M NaCl for extracellular recording. After isolation of a type 1 neuron, chemicals were iontophoretically applied to examine their effects on its activity. The results were as follows: (1) GABA and glycine markedly decreased spontaneous firing of the neurons, while serotonin did not affect their activity. (2) Bicuculline abolished the inhibitory effects of GABA on the neurons. (3) Strychine abolished the effects of glycine. (4) Commissural inhibition induced by electrical stimulation of the contralateral labyrinth was not abolished by strychine but was abolished by bicuculline. We conclude that (1) vestibular type 1 neurons are controlled by GABAergic and glycinergic but not serotoninergic neurons, and (2) commissural inhibition is activated by the GABAA receptor, but not by the GABAB receptor.

Evidence for inhibitory amino acid receptors on guinea pig medial vestibular nucleus neurons in vitro

There is little evidence to indicate the identity of the inhibitory receptors which mediate inhibitory interaction between the two medial vestibular nuclei ('brainstem commissural inhibition'). In the present study we tested the hypothesis that medial vestibular nucleus (MVN) neurons have gamma-aminobutyric acid (GABA) or glycine receptors by recording from single MVN neurons in isolated guinea pig MVN slices maintained in vitro while superfusing with GABA (10(-8) M) and the non-competitive GABAA antagonist picrotoxin (10(-6) M or 2 x 10(-6) M), or glycine (10(-6) M) and the competitive glycine antagonist strychnine (10(-6) M). Forty-four % (16/36) of the neurons tested with GABA showed a decrease in firing; in 7 out of 8 cases in which a decrease in firing occurred, the addition of the antagonist picrotoxin completely blocked the effect of the GABA alone. Fifty % (7/14) of the neurons tested with glycine showed a decrease in firing; in 4 out of 6 cases where a decrease occurred, the addition of the antagonist strychnine completely blocked the effect of the glycine alone. In one case only did a cell respond both to GABA and glycine (8 neurons tested with both). These results are consistent with the hypothesis that some MVN neurons have GABA or glycine receptors (but in most cases not both), which may mediate brainstem commissural inhibition.

Compensation of horizontal canal related activity in the medial vestibular nucleus following unilateral labyrinth ablation in the decerebrate gerbil. I. Type I neurons

The spontaneous activity and dynamic responses to two frequencies (1.3 and 0.13 Hz) of sinusoidal angular horizontal head acceleration of type I neurons in the medial vestibular nucleus were recorded bilaterally in decerebrate Mongolian gerbils (Meriones unguiculatus) under three experimental conditions; normal labyrinth intact, acutely following unilateral labyrinthine lesion, and four to seven weeks following labyrinthine lesion. The mean spontaneous activity and number of detected type I neurons decreased immediately ipsilateral to the lesion but recovered significantly with time. In contrast, spontaneous activity on the contralateral side increased during compensation following hemilabyrinthectomy. The mean response gains at both frequencies of head oscillation were depressed bilaterally and asymmetrically acutely following the lesion such that the response gain of cells on the intact side exceeded that of the neurons recorded on the injured side. After compensation the number of detected type I neurons on the side ipsilateral to the injury increased but remained below normal levels. The mean gains remained depressed but became symmetric with compensation as a result of improvement in the response of ipsilateral neurons. The phase of responses were significantly advanced in the compensated animals. Although response gain is not fully restored, the linearity of the dynamic modulation in compensated animals is improved as evidenced by a continuous modulation of the increased spontaneous activity of neurons contralateral to the hemilabyrinthectomy. It is proposed that this effect is related to the concurrent improvement in the linearity of the horizontal vestibulo-ocular response. Electrical cathodal polarization of the vestibular nerve ipsilateral to the ablated labyrinth was utilized to investigate the relationship between recovery of spontaneous activity and dynamic function. Acutely following hemilabyrinthectomy, cathodal polarization restored activity in second-order type I neurons to near normal levels but their response gain to head rotation remained depressed. Similar galvanic stimulation in compensated animals also elevated ipsilateral spontaneous activity. As in the acute preparation, such stimulation did not modify the response gain or phase. Thus, the improvement in response of type I neurons in the compensated gerbil was not a direct consequence of restoration of spontaneous activity on the side of the injury.

Compensation of horizontal canal related activity in the medial vestibular nucleus following unilateral labyrinth ablation in the decerebrate gerbil. II. Type II neurons

The spontaneous activity and dynamic responses to sinusoidal horizontal head angular acceleration of type II horizontal semicircular canal related neurons in the medial vestibular nucleus (MVN) were recorded bilaterally in decerebrate Mongolian gerbils (Meriones unguiculatus) under three experimental conditions: normal labyrinths intact, acutely following unilateral labyrinthine lesion, and four to seven weeks following labyrinthine lesion. The number of type II neurons detected contralateral to the lesion was greatly reduced both in the acutely hemilabyrinthectomized animals and following compensation. The gain of the responses was depressed bilaterally acutely following the lesion. A greater reduction in response gain was noted in cells contralateral to the lesion. The gain of the contralateral type II responses increased with time such that in the compensated animal bilaterally symmetric gains were recorded. While the significant changes which occur in the gain of type II neurons with recovery from peripheral vestibular lesions can largely be attributed to type I neurons on the other side of the midline, changes in type I neurons were not entirely reflected in the type II population. The spontaneous activity of type II neurons did not undergo any significant changes following the labyrinthine lesion. We present a model utilizing the dynamic responses to estimate the functional recovery of commissural connections in compensated animals. The overall gain of the contralateral type I to ipsilateral type I commissural polysynaptic pathway appears to improve, while the efficacy in the reverse direction remains depressed, suggesting that modifications in commissural connections, particularly involving the type II to type I connections within the MVN on the injured side, mediate aspects of behavioral recovery.

Medullary electrosensory processing in the little skate. II. Suppression of self-generated electrosensory interference during respiration

1. Previous studies have demonstrated that the resting activity of electrosensory ALLN fibers is modulated by the animal's own respiratory activity and that all fibers innervating a single ampullary cluster are modulated with the same amplitude and phase relationship to ventilation. We demonstrate that ALLN fibers in the skate are modulated in this common-mode manner bilaterally, regardless of receptor group, orientation, or position of the receptor pore on the body surface (Fig. 2). 2. Ascending efferent neurons (AENs), which project to the electrosensory midbrain from the DON, are modulated through a much smaller portion of their dynamic range. AENs give larger responses to an extrinsic local electric field than to the respiratory driving, indicating that a mechanism exists for suppressing ventilatory electrosensory reafference. 3. In paralyzed animals no modulation of resting activity or of responses of extrinsic electric fields could be observed with respect to the animal's respiratory motor commands in the absence of electrosensory reafference. 4. Cells of the dorsal granular ridge (DGR) project to medullary AENs via the DON molecular layer. A majority of proprioceptive DGR neurons are modulated by ventilatory activity, however, in a given fish the modulation is not in the same phase relationship to ventilation among DGR units. 5. The modulation of AENs during respiration was increased following transection of the contralateral ALLN (Fig. 9). Resting activity and responses to excitatory stimuli were inhibited by simultaneous stimulation of the transected contralateral ALLN indicating that a common-mode rejection mechanism is mediated via the commissural interconnections of the DONs.

Mechanisms of recovery following unilateral labyrinthectomy: a review

This paper reviews the literature on the mechanisms responsible for the behavioural recovery which occurs following unilateral labyrinthectomy (UL), UL causes a syndrome of ocular motor and postural disorders, which diminish over time in a process of behavioural recovery known as vestibular compensation. Electrophysiological studies show that the VIIIth nerve does not undergo a functional recovery, therefore vestibular compensation has been attributed to CNS plasticity. However, the nature of the plasticity responsible for vestibular compensation is not understood. Single-neuron studies have demonstrated that a significant recovery of resting activity has occurred in the vestibular nuclei (VN) ipsilateral to the UL by the time symptoms such as spontaneous nystagmus and roll head tilt (static symptoms) have largely disappeared. However, many of the deficits in the response of VN neurons to head acceleration persist and may be permanent. This lack of recovery in the response of neurons to head acceleration correlates with the incomplete and sometimes poor recovery of the vestibulo-ocular and vestibulo-spinal reflex responses to head movement (dynamic symptoms). The major neuronal change in the VN during vestibular compensation appears to be the recovery of resting activity in the VN ipsilateral to the UL, although this recovery is more pronounced in the medial VN than in the lateral VN. The mechanism responsible for the regeneration of resting activity in VN neurons is unknown. In frogs, there is evidence to suggest that transcommissural synaptic input to the VN, from the contralateral (intact) labyrinth, increases in efficacy.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Vestibular control of muscular tone and posture

The vestibulospinal system helps to maintain upright posture and head stability. The semicircular canals and their short latency connections to the neck motoneurons, largely via the medial vestibulospinal tract, respond to angular accelerations so as to stabilize the head in space. The paired otolith organs, the utricles placed approximately horizontally, and the saccules vertically, respond to linear acceleration including gravity. Their influence leads, via the lateral vestibulospinal tract, to excitation of ipsilateral extensor motoneurons of the limbs and trunk, and to inhibition of reciprocal flexor motoneurons. Linear displacement of the otoliths leads to bracing of the limbs and body so as to maintain upright posture, and to extend the limbs so as to help in landing after sudden falls.

[Comparative neurobiology of the organization of gaze-stabilizing reflex systems in vertebrates]

During locomotion gaze is stabilized against passive head movements by compensatory eye movements. The efficacy and the neuronal organization of optokinetic and vestibular reflexes of different vertebrate species is compared. Besides many similarities between species a number of differences can be found as well. Increase in the efficacy of compensatory reflexes is not correlated with an increase in the efficacy of basic neuronal circuits but with the appearance of functionally new connections and of new network properties. This increasingly higher complexity allows to maintain gaze stability at increasingly higher speeds of locomotion or to suppress these reflexes during visual pursuit of a moving object.

Vestibular control of neck muscles in acute and chronic hemilabyrinthectomized cats

Reflex activity evoked in neck extensor muscles by head movements in the sagittal plane (the sagittal vestibulocollic reflex (v.c.r.), Dutia & Hunter, 1985), was studied in decerebrate cats with acute or chronic loss of one vestibular labyrinth. After acute hemilabyrinthectomy, tonic electromyographic (e.m.g.) activity in the biventer cervicis muscle ipsilateral to the lesion was normal, while that in the contralateral muscle was abolished. Sinusoidal head movements in the sagittal plane (0.1-5 Hz, 1-10 deg peak to peak) caused reflex modulation of e.m.g. activity in the ipsilateral muscle, but did not evoke any response in the contralateral muscle. The phase (re head position) of the reflex response in the ipsilateral muscle was similar to that in a normal cat with intact labyrinths, while reflex gain was lowered by 2-8 dB below its value before hemilabyrinthectomy. Removal of the remaining labyrinth in acutely hemilabyrinthectomized animals restored bilaterally symmetrical tonic e.m.g. activity in the neck extensors. There was no e.m.g. modulation during head movements after bilateral labyrinthectomy. In chronic hemilabyrinthectomized cats (four to seven weeks), tonic e.m.g. activity in the neck muscles on both lesioned and intact sides was similar to normal. The gain and phase of the sagittal v.c.r. were also normal over a wide range of frequencies of head movement on both lesioned and intact sides. Interruption of the medial longitudinal bundle approximately 1 mm rostral to the obex did not abolish the bilaterally symmetrical compensated reflex response in either muscle, indicating that the descending axons in the medial vestibulospinal tract are not essential in mediating the normal v.c.r. response in compensated animals.

Effect of spinal cord transection on spontaneous activity recorded from type I neurons of the medial vestibular nucleus in compensated hemilabyrinthectomized gerbils

Spontaneous activity was recorded from type I neurons of the medial vestibular nuclei (MVN) in unanesthetized, decerebrate gerbils to determine the effect of spinal cord transection on compensation following labyrinthectomy. Immediately after labyrinthectomy there was an increase in activity of type I neurons on the intact side and an absence of activity on the injured side. Following compensation from labyrinthectomy, the distribution and activity rates approximated those of nonlabyrinthectomized animals. Spinal cord transection resulted in an increase in activity in type I MVN neurons contralateral to the labyrinthectomy in compensated animals and bilaterally in nonlabyrinthectomized animals. These results illustrate that type I neurons apparently are under an indirect inhibitory control from both the contralateral labyrinth and the spinal cord. In compensated animals spinal cord inhibition exists only on the intact side. This suggests that the symmetry in type I activity bilaterally in the compensated animal is in part the result of asymmetric spinal cord input.

Intervestibular integration and its clinical consequences

Research during the last twenty years has led to the conclusion that it is time for a more integrated and dynamic view on the physiological and pathophysiological process during clinical vestibular stimulation and perhaps also to reevaluate the interpretation of the findings. Ten patients with absence of function in one labyrinth were exposed to caloric and rotatory stimulation several times during the compensating period of 8 weeks. In the acute state, caloric irrigation of the normal ear with 30 degrees or rotatory stimulation toward the damaged ear resulted in total inhibition of the spontaneous nystagmus. In the chronic state the same investigation reversed the spontaneous nystagmus in the opposite direction. The investigation has shown that it is possible clinically to study the reactions of every single neuron in the vestibular ocular reflex pathway and in this way establish a more topographic diagnosis.

Effects of lesion of the interstitial nucleus of Cajal on vestibular horizontal canal neurons in the cat

Effects of procaine infusion into the interstitial nucleus of Cajal (INC) on vestibular nuclear neurons related to the horizontal canal were studied in cats anesthetized with nitrous oxide and paralyzed with gallamine. Neurons that responded to sinusoidal horizontal rotation (at 0.18 Hz) were recorded extracellularly in the medial and descending vestibular nuclei. Spontaneous activity of type I neurons increased, whereas that of type II neurons decreased following procaine infusion into the ipsilateral INC. Gain of the neuronal response to horizontal rotation decreased after the ipsilateral INC infusion, but there was no consistent effect on phase. Infusion into the contralateral INC seemed less effective. Similar effects were obtained with electrolytic lesions that were confined to the ipsilateral INC area. These results suggest that the INC influences type I neurons through inhibitory action of type II neurons and that it eventually controls the gain, but not the phase, of the horizontal vestibular reflexes.

Development of postrotatory nystagmus time constants

Models of the slow phase portion of postrotatory nystagmus in the adult include time constants which describe primary and secondary nystagmus. The cupular time constant (T1) is believed to reflect the activity of the cupula of the semicircular canal, while the adaptation time constant (Ta) defines the rate of change of the baseline firing rate. Values for T1 and Ta have been published for the adult. In this study 37 normal infants under one year of age and 12 children, 3 to 13 years of age were examined for postrotatory nystagmus, and values for the two time constants were determined. The cupular time constant does not change with age and is concluded to be at an adult level at birth. This is correlated with the relative maturity of ear morphology at birth. The adaptation time constant increases from 79.5 sec at one month of age to 260.7 sec at 12-13 years of age, implying that maturation of adaption is a relatively slow process.

Vestibular nuclear neuron activity in chronically hemilabyrinthectomized cats

The activity of central vestibular neurons (Vn) of the horizontal canal system was recorded in chronically hemilabyrinthectomized cats and compared with that of labyrinth intact animals. In both groups the cerebellar vermis was removed in order to assess the efficacy of the vestibular brainstem commissure alone by means of polarizing currents applied to the labyrinths. Experiments were carried out under Ketamine anaesthesia. In control animals the mean resting rates of type I and type II Vn measured 22.4 +/- 14.0 and 27.5 +/- 14.6 imp/s respectively, and the type I responses occurred ca. 3 X more frequently than type II. In the lesioned animals a drastic reduction of the number of type I responses was found on the deafferented side, while that on the intact side remained normal. The resting rates of type I Vn on the two sides did not differ significantly from each other but were significantly lower than those of control animals. In contrast, type II responses were present on the deafferented side, but almost completely missing on the intact side. Applying polarizing stimuli in control animals, it was found that both labyrinths have similar weight in driving Vn. In lesioned animals, no major changes in the efficacy of the commissural path were found when polarizing stimuli were applied to the intact side. It is concluded that vestibular nerve section causes a severe loss of type I responses in the vestibular nuclei on the side of the lesion which apparently is not compensated by an adaptive change in the commissural path and, therefore, may be mainly responsible for the VOR asymmetry observed concomitantly.

A quantitative analysis of the spatial organization of the vestibulo-ocular reflexes in lateral- and frontal-eyed animals--II. Neuronal networks underlying vestibulo-oculomotor coordination

The neuronal connectivity underlying the vestibulo-ocular reflexes in cat and rabbit was evaluated in the light of quantitative data of the spatial orientation on semicircular canals and extraocular muscles. Neuronal connectivity was calculated using a matrix-analysis of the sensory and motor periphery, and of the brain stem pathways connecting semicircular canals and extraocular muscles. Two cases of vestibulo-ocular reflex compensation were considered. In the first case, vestibulo-oculor reflex compensation was assumed to be isotropic, i.e. the vestibulo-ocular reflex gain is the same for all directions of rotation. In the second case, the vestibulo-oculor reflex gain was assumed to be anisotropic with the "torsional" gain smaller than the "horizontal" and "vertical" gains. The theoretical calculation predicts that besides the principal vestibulo-ocular reflex pathways (classical three-neuron-arc connectivity), several accessory connections (other than principal connections, regardless of the synapses involved) exist which are characteristic for each species. These accessory connections were compared to physiological and anatomical data. In the cat theoretical connections for an isotropic vestibulo-ocular reflex gain agree with pathways observed experimentally, of which the most characteristic are excitatory connections to the superior rectus and inhibitory connections to the inferior rectus muscle from both of the anterior canals, and a mirror image pattern of connections from the posterior canals. In the rabbit experimentally obtained data and calculated connections rarely agree. However, for an anisotropic gain we find a higher rate of coincidence between experimental and theoretical connections. Our evaluation indicates, that accessory vestibulo-ocular reflex pathways serve to compensate for the incongruence between semicircular canal and extraocular muscle planes, at least in the cat. Available experimental data suggest an important role of a special subclass of accessory pathways via axon collaterals of principal projections (three-neuron-arc nature). With certain restrictions, the presented method of calculation promises to be a useful tool for a quantitative analysis of the vestibulo-ocular reflex.

Neural correlates of compensation after hemilabyrinthectomy

Vestibular brain stem neurons responsive to angular acceleration in the plane of the horizontal canal were examined in cats 30 to 52 days after contralateral labyrinthectomy and compared with similar units in cats with intact labyrinths and in other cats immediately after transection of the contralateral eighth nerve. In the compensated state, the mean spontaneous firing rate of type I neurons was 24 spikes/s, in contrast to the mean of 45/s observed immediately after contralateral labyrinthectomy. In intact cats, mean firing rate was 19 spikes/s. Sensitivity, as measured in spikes/s/deg/s2, was significantly lower immediately after labyrinthectomy than in intact controls and remained so in compensated cats. On the other hand, time constants and the ratio of adapting:nonadapting units was unchanged. Ablation of the midline cerebellum including vermis and fastigial nuclei did not materially affect these results. We concluded that (i) the main defects in static posture and nystagmus in the uncompensated state were due to the striking difference in resting firing rates between the ipsi- and contralateral vestibular nuclei; (ii) compensation in the static posture was the result of a tendency to equalize the resting firing rates in the two vestibular nuclei; and (iii) recovery in the dynamic, head-turning situation was due to partial recovery of sensitivity on the ipsilateral side, bringing it to the relatively constant, unchanging depressed sensitivity on the contralateral side.

Second-order neural responses after contralateral vestibular nerve sectioning

The characteristics of the response to an 8 degrees/sec2 acceleration of 41 neurons in the medial vestibular nucleus of the cat following vestibular nerve sectioning were compared with those of 73 control neurons at the same site. In 14 of the vestibular nerve-sectioned animals the same neuron was recorded before and after sectioning. No difference was found between the resting discharge rates, maximal response rates, and rates of adaptation of the two groups. Following contralateral vestibular nerve sectioning Type II neurons were still plentiful, suggesting many receive ipsilateral labyrinthine innervation.

Semicircular canal structure during postnatal development in cat and guinea pig

The gain of the vestibulo-ocular reflex in the cat continues to increase for some time after birth. The reason for this increase is not presently known and one possibility if that it occurs because the cat semicircular canals increase in size. The present study examined this possibility by measuring the radii of curvature (R) of individual semicircular canals and the angular relationships of the semicircular canal planes within a labyrinth in cats and guinea pigs during postnatal growth. It was found that the labyrinths do move apart substantially during postnatal development in both species, but neither the planar relations nor the radii of curvature change significantly during postnatal development. The stability of semicircular canal structure during postnatal skull growth indicates that postnatal developmental changes in canal-related function, such as increased gain in the vestibulo-ocular reflex, in these species are probably due to receptor cell or neural maturational factors.