Selective mode of commissural inhibition induced by semicircular canal afferents on secondary vestibular neurones in the cat

Pathways for Naturalistic Looking Behavior in Primate I: Behavioral Characteristics and Brainstem Circuits

Organisms control their visual worlds by moving their eyes, heads, and bodies. This control of "gaze" or "looking" is key to survival and intelligence, but our investigation of the underlying neural mechanisms in natural conditions is hindered by technical limitations. Recent advances have enabled measurement of both brain and behavior in freely moving animals in complex environments, expanding on historical head-fixed laboratory investigations. We juxtapose looking behavior as traditionally measured in the laboratory against looking behavior in naturalistic conditions, finding that behavior changes when animals are free to move or when stimuli have depth or sound. We specifically focus on the brainstem circuits driving gaze shifts and gaze stabilization. The overarching goal of this review is to reconcile historical understanding of the differential neural circuits for different "classes" of gaze shift with two inconvenient truths. (1) "classes" of gaze behavior are artificial. (2) The neural circuits historically identified to control each "class" of behavior do not operate in isolation during natural behavior. Instead, multiple pathways combine adaptively and non-linearly depending on individual experience. While the neural circuits for reflexive and voluntary gaze behaviors traverse somewhat independent brainstem and spinal cord circuits, both can be modulated by feedback, meaning that most gaze behaviors are learned rather than hardcoded. Despite this flexibility, there are broadly enumerable neural pathways commonly adopted among primate gaze systems. Parallel pathways which carry simultaneous evolutionary and homeostatic drives converge in superior colliculus, a layered midbrain structure which integrates and relays these volitional signals to brainstem gaze-control circuits.

Neural Circuits of Inputs and Outputs of the Cerebellar Cortex and Nuclei

This article is dedicated to the memory of Masao Ito. Masao Ito made numerous important contributions revealing the function of the cerebellum in motor control. His pioneering contributions to cerebellar physiology began with his discovery of inhibition and disinhibition of target neurons by cerebellar Purkinje cells, and his discovery of the presence of long-term depression in parallel fiber-Purkinje cell synapses. Purkinje cells formed the nodal point of Masao Ito's landmark model of motor control by the cerebellum. These discoveries became the basis for his ideas regarding the flocculus hypothesis, the adaptive motor control system, and motor learning by the cerebellum, inspiring many new experiments to test his hypotheses. This article will trace the achievements of Ito and colleagues in analyzing the neural circuits of the input-output organization of the cerebellar cortex and nuclei, particularly with respect to motor control. The article will discuss some of the important issues that have been solved and also those that remain to be solved for our understanding of motor control by the cerebellum.

Heterogeneous vestibulocerebellar mossy fiber projections revealed by single axon reconstruction in the mouse

A significant population of neurons in the vestibular nuclei projects to the cerebellum as mossy fibers (MFs) which are involved in the control and adaptation of posture, eye-head movements, and autonomic function. However, little is known about their axonal projection patterns. We studied the morphology of single axons of medial vestibular nucleus (MVN) neurons as well as those originating from primary afferents, by labeling with biotinylated dextran amine (BDA). MVN axons (n = 35) were classified into three types based on their major predominant termination patterns. The Cbm-type terminated only in the cerebellum (15 axons), whereas others terminated in the cerebellum and contralateral vestibular nuclei (cVN/Cbm-type, 13 axons), or in the cerebellum and ipsilateral vestibular nuclei (iVN/Cbm-type, 7 axons). Cbm- and cVN/Cbm-types mostly projected to the nodulus and uvula without any clear relationship with longitudinal stripes in these lobules. They were often bilateral, and sometimes sent branches to the flocculus and to other vermal lobules. Also, the iVN/Cbm-type projected mainly to the ipsilateral nodulus. Neurons of these types of axons showed different distribution within the MVN. The number of MF terminals of some vestibulocerebellar axons, iVN/Cbm-type axons in particular, and primary afferent axons were much smaller than observed in previously studied MF axons originating from major precerebellar nuclei and the spinal cord. The results demonstrated that a heterogeneous population of MVN neurons provided divergent MF inputs to the cerebellum. The cVN/Cbm- and iVN/Cbm-types indicate that some excitatory neuronal circuits within the vestibular nuclei supply their collaterals to the vestibulocerebellum as MFs.

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.

The frog vestibular system as a model for lesion-induced plasticity: basic neural principles and implications for posture control

Studies of behavioral consequences after unilateral labyrinthectomy have a long tradition in the quest of determining rules and limitations of the central nervous system (CNS) to exert plastic changes that assist the recuperation from the loss of sensory inputs. Frogs were among the first animal models to illustrate general principles of regenerative capacity and reorganizational neural flexibility after a vestibular lesion. The continuous successful use of the latter animals is in part based on the easy access and identifiability of nerve branches to inner ear organs for surgical intervention, the possibility to employ whole brain preparations for in vitro studies and the limited degree of freedom of postural reflexes for quantification of behavioral impairments and subsequent improvements. Major discoveries that increased the knowledge of post-lesional reactive mechanisms in the CNS include alterations in vestibular commissural signal processing and activation of cooperative changes in excitatory and inhibitory inputs to disfacilitated neurons. Moreover, the observed increase of synaptic efficacy in propriospinal circuits illustrates the importance of limb proprioceptive inputs for postural recovery. Accumulated evidence suggests that the lesion-induced neural plasticity is not a goal-directed process that aims toward a meaningful restoration of vestibular reflexes but rather attempts a survival of those neurons that have lost their excitatory inputs. Accordingly, the reaction mechanism causes an improvement of some components but also a deterioration of other aspects as seen by spatio-temporally inappropriate vestibulo-motor responses, similar to the consequences of plasticity processes in various sensory systems and species. The generality of the findings indicate that frogs continue to form a highly amenable vertebrate model system for exploring molecular and physiological events during cellular and network reorganization after a loss of vestibular function.

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.

New neurons in the vestibular nuclei complex after unilateral vestibular neurectomy in the adult cat

Recent findings revealed a reactive neurogenesis after lesions and in several models of disease. After unilateral vestibular neurectomy (UVN), we previously reported gamma-aminobutyric acid (GABA)ergic neurons are upregulated in the vestibular nuclei (VN) in the adult cat. Here, we ask whether this upregulation of GABAergic neurons resulted from a reactive neurogenesis. To determine the time course of cell proliferation in response to UVN, 5-bromo-2'-deoxyuridine (BrdU) was injected 3 h, 1, 3, 7, 15 and 30 days after UVN. We investigated the survival and differentiation in UVN cats injected with BrdU at 3 days and perfused 30 days after UVN. Results show a high number of BrdU-immunoreactive nuclei in the deafferented VN with a peak at 3 days after UVN and a decrease at 30 days. Most of the newly generated cells survived up to 1 month after UVN and gave rise to a variety of cell types. Confocal analysis revealed three cell lineages: microglial cells (OX 42/BrdU-immunoreactive cells); astrocytes [glial fibrillary acidic protein (GFAP)/BrdU-immunoreactive cells]; and neurons (NeuN/BrdU-immunoreactive cells). That UVN induced new neurons was confirmed by an additional marker (nestin) expressed by neural precursor cells. We show that most of the newly generated neurons have a GABAergic phenotype [glutamate decarboxylase (GAD)-67/BrdU-immunoreactive cells]. Morphological analysis showed two subtypes of GABAergic neurons: medium and small (30 vs. 10 microm, respectively). This is the first report of reactive neurogenesis in the deafferented VN in the adult mammalian CNS.

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.

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.

Ultrastructural features of non-commissural GABAergic neurons in the medial vestibular nucleus of the monkey

The ultrastructural characteristics of non-degenerating GABAergic neurons in rostrolateral medial vestibular nucleus were identified in monkeys following midline transection of vestibular commissural fibers. In the previous papers, we reported that most degenerated cells and terminals in this tissue were located in rostrolateral medial vestibular nucleus, and that many of these neurons were GABA-immunoreactive. In the present study, we examined the ultrastructural features of the remaining neuronal elements in this medial vestibular nucleus region, in order to identify and characterize the GABAergic cells that are not directly involved in the vestibular commissural pathway related to the velocity storage mechanism. Such cells are primarily small, with centrally-placed nuclei. Axosomatic synapses are concentrated on polar regions of the somata. The proximal dendrites of GABAergic cells are surrounded by boutons, although distal dendrites receive only occasional synaptic contacts. Two types of non-degenerated GABAergic boutons are distinguished. Type A terminals are large, with very densely-packed spherical synaptic vesicles and clusters of large, irregularly-shaped mitochondria with wide matrix spaces. Such boutons form symmetric synapses, primarily with small GABAergic and non-GABAergic dendrites. Type B terminals are smaller and contain a moderate density of round/pleomorphic vesicles, numerous small round or tubular mitochondria, cisterns and vacuoles. These boutons serve both pre- and postsynaptic roles in symmetric contacts with non-GABAergic axon terminals. On the basis of ultrastructural observations of immunostained tissue, we conclude that at least two types of GABAergic neurons are present in the rostrolateral portion of the monkey medial vestibular nucleus: neurons related to the velocity storage pathway, and a class of vestibular interneurons. A multiplicity of GABAergic bouton types are also observed, and categorized on the basis of subcellular morphology. We hypothesize that "Type A" boutons correspond to Purkinje cell afferents in rostrolateral medial vestibular nucleus, "Type B" terminals represent the axons of GABAergic medial vestibular nucleus interneurons, and "Type C" boutons take origin from vestibular commissural neurons of the velocity storage pathway.

The ultrastructure of GABA-immunoreactive vestibular commissural neurons related to velocity storage in the monkey

The purpose of the present study was to visualize the synaptic interactions of GABAergic neurons involved in the mediation of velocity storage. In the previous report, ultrastructural studies of degenerating neurons were conducted following midline section of rostral medullary commissural fibers with subsequent behavioral testing. The midline lesion caused functionally discrete damage to the velocity storage component, but not to the direct pathway, of the angular vestibulo-ocular reflex, and the degenerating neurons were interpreted as potential participants in the velocity storage network. We concluded that at least some of the commissural axons mediating velocity storage originate from clusters of neurons in the lateral crescents of the rostral medial vestibular nucleus. In the present report, immunocytochemical evidence is presented that many vestibular commissural neurons, putatively involved in mediating velocity storage, are GABAergic. These cells have large nuclei, small round or narrow tubular mitochondria, occasional cisterns and vacuoles, but few other organelles. Their axons are thinly-myelinated, and terminate in boutons containing mitochondria of similar ultrastructural appearance and a moderate density of round/pleomorphic synaptic vesicles. Such terminals often form axoaxonic synapses, and less frequently axodendritic contacts, with non-GABAergic elements. On the basis of the present results, we conclude that a portion of the commissural neurons of the velocity storage pathway is GABAergic. The observation of GABAergic axoaxonic synapses in this pathway is interpreted as a structural basis for presynaptic inhibition of medial vestibular nucleus circuits by velocity storage-related commissural neurons. Conversely, substantial ultrastructural evidence for postsynaptic inhibition of non-GABAergic commissural cells argues for a dual role for GABAergic terminals mediating velocity storage: presynaptic inhibition of non-GABAergic vestibular cells by GABAergic velocity storage commissural axons, and postsynaptic inhibition of non-GABAergic velocity storage cells by GABAergic axons. Both pre- and postsynaptic inhibitory arrangements could provide the morphologic basis for disinhibitory activation of the velocity storage network within local neuronal circuits.

Neurotransmitters of vestibular commissural inhibition in the cat

The present study examined transmitters that regulate commissural inhibition. Extracellular spikes of a single vestibular neuron were recorded in decerebrated cats. Multibarreled electrodes were filled with transmitter candidates (GABA and glycine), their specific antagonists (bicuculline, strychnine) and 2 M NaCl for extracellular recording. After isolation of a type I neuron, chemicals were iontophoretically applied to examine their effects on the activity of the neuron. The results were as follows. Commissural inhibition caused by electrical stimulation of the contralateral labyrinth was not abolished by the application of strychnine (a glycine antagonist), but was abolished by bicuculline (a GABA antagonist). Commissural inhibition was not abolished by phaclofen. Some bicuculline-sensitive neurons, with a short-latency commissural inhibition (presumably disynaptic inhibition), showed spacial summation when the conditioning stimulation (contralateral vestibular nerve stimulation) was applied with the test stimulation (vestibular nucleus stimulation). It was concluded that commissural inhibition was activated by the GABAA receptor, but not by the GABAB receptor, that the inhibitory type I neurons located in the contralateral vestibular nucleus were GABAergic, and that inhibitory type II neurons were also GABAergic neurons.

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.

Neurotransmitters regulating vestibular commissural inhibition in the cat

The present study focuses on the transmitters which control the vestibular neural activity and, in particular, those which regulate commissural inhibition. Extracellular spike recordings were taken from single vestibular neurons of decerebrate cats. Seven-barreled electrodes were used. The barrels, except for the central one, were filled with several transmitter candidates and their specific inhibitors. After isolation of a type I neuron, the chemicals were iontophoretically applied to examine their effects on the activity of the neuron. The results were as follows: i) GABA and glycine markedly decreased spontaneous firing of the neurons, while serotonin did not change the activity. ii) Bicuculline abolished the inhibitory effects of GABA on the neurons. iii) Strychnine also abolished the effects of glycine, iv) Commissural inhibition caused by electrical stimulation of the contralateral labyrinth was not abolished by the application of strychnine but was abolished by bicuculline. We concluded that 1) vestibular type 1 neurons are controlled by GABAergic and glycinergic neurons, and that 2) the commissural inhibition is activated by the GABAA receptor, but not by the GABAB receptor.

Commissural and intrinsic connections of the vestibular nuclei in the rabbit: a retrograde labeling study

The intrinsic and commissural projection of the vestibular nuclei were investigated by means of retrograde transport of normal (HRP) and wheat-germ-agglutinated horseradish peroxidase (WGA-HRP). It was found that within each vestibular complex, the superior (SV), medial (MV) and descending (DV) vestibular nuclei are reciprocally connected. A rostrocaudally oriented column of medium-sized and large neurons, comprising the central SV and the magnocellular MV (MVmc) receives input from the surrounding neurons and does not reciprocate this projection. Efferents from group y terminate in the SV, MV and DV. The infracerebellar nucleus (INF) as well as the interstitial nucleus of the VIIIthe nerve (IN) supply fibers to the MV and DV. The neurons that participate in the commissural projection are distributed throughout the vestibular complex with the exception of the lateral vestibular nucleus (LV) and the group x. The largest number of cells was found in the MV. The HRP labeled cells show a tendency to cluster into rostrocaudally oriented groups. Each nucleus projects to more than one contralateral nucleus. Group y shows a more extensive contralateral projection than the bordering INF. It was concluded that quantitative differences in connectivity were present between a core region in the vestibular complex and peripheral parts. This core region comprises the central SV, the LV, the MVmc and extends into the rostral DV. It receives predominantly intrinsic input from the surrounding vestibular neurons and is in contrast to these latter neurons only minimally involved in the commissural projection.

Internuclear fiber connections of vestibular nuclear complex. A horseradish peroxidase study

Internuclear fiber connections among the superior (SVN), lateral (LVN), medial (MVN) and descending (DVN) vestibular nuclei were examined in cats using retrograde transport of horseradish peroxidase (HRP). HRP was microiontophoretically applied in the respective vestibular nucleus at doses of 300-500 nA for 5-10 min, and with the treatment the HRP injection site was limited to 0.2-0.5 mm in diameter within the nucleus. Major commissural connections were found between the bilateral SVN and between the bilateral DVN. Minor commissural connections were observed from MVN to SVN, LVN and MVN, from DVN to LVN, and from LVN to the contralateral LVN. In the ipsilateral vestibular nuclei, fiber connections were found from LVN to SVN, from MVN to SVN, LVN and DVN, and from DVN to SVN and LVN.

The fine structure of the feline superior vestibular nucleus: identification and synaptology of the primary vestibular afferents

The superior vestibular nucleus of the cat and its primary vestibular efferents were examined by light and electron microscopy. The primary vestibular afferents branch within the nucleus in a sheet-like pattern, in the transverse plane. The dendritic fields of many secondary neurons are shaped like discs and are also oriented in the transverse plane. This relation between the primary afferents and dendritic fields may be relevant to the convergence of primary afferents innervating particular endorgans onto secondary neurons. Synaptic boutons in the SV were divided into 3 putative types on the basis of the size and shape of their synaptic vesicles. The primary afferent bouton was identified by comparing the SV of the two sides after unilateral lesions of the vestibular ganglion. Its boutons contain round vesicles of 40 nm average diameter and are associated with prominent postsynaptic densities; the two other putative bouton types contain smaller, round vesicles, and pleomorphic vesicles. The primary afferent boutons largely contact proximal dendrites, their appendages, and cell somata of the secondary neurons. In animals receiving unilateral lesions of the vestibular ganglion and allowed to survive long enough for the primary afferent boutons to disappear (5--6 days), there occurs in the denervated as compared to normal SV: (1) a decrease in the fraction of the somal surface of the secondary neurons covered by boutons with small round vesicles; and (2) a decrease in the ratio: number of boutons with small round vesicles to number of boutons with pleomorphic vesicles. In addition, there appears on the lesioned side a new group of boutons with pleomorphic vesicles smaller than those in boutons from the control side. These observations suggest plastic changes in response to deafferentation, and may be related to the marked behavioral recovery which occurs within a few days after lesion of the vestibular ganglion.

A mechanism for type III vestibular responses of frog cerebellar Purkinje cells

Type III Purkinje cells (P-cells), which are excited with both directions of horizontal rotation, are found in high numbers in the frog auricular lobe and adjacent cerebellar areas. To examine the mechanisms underlying these responses, recordings were made from P-cells in curarized animals during rotational stimulation of the horizontal canals. The horizontal canal input to these cells was then modified unilaterally by VIIth nerve section, intraperilymphatic injection of local anesthetic, or by caloric stimulation. Control recordings were also obtained from peripheral canal neurons. Type III responses were abolished by unilateral lesions or reversible blockage of the VIIIth nerve with local anesthetic. The remaining responses were attributable only to the unaffected horizontal canal, ie. only type II or type I responses were observed upon interruption of the ipsi-or contralateral nerve, respectively. The level of spontaneous activity of cerebellar input fibers was low and during rotation produced 'cell silencing' response waveform asymmetries (facilitation greater than disfacilitation). When the level of peripheral resting activity was increased (warm water irrigation), thereby increasing horizontal canal response symmetry, type III responses were reduced in magnitude or abolished. Conversely, cold water irrigation, which decreases the resting rate and response symmetry of input fibers, enhanced type III response magnitudes. On the basis of these results, it is suggested that type III responses result from the fact that single P-cells receive a facilitatory input from both horizontal canals. Since these inputs are 180 degrees phase-reversed and their response waveforms asymmetrical, their resulting postsynaptic effect is a net excitation during both portions of the stimulus cycle.

Labyrinthine input to the vestibular nuclei of the awake cat

The labyrinthine input to the vestibular nuclei was investigated in 24 awake cats. Stimulus consisted of electrical shocks given through bipolar silver wire electrodes, implanted in the utricular and lateral ampullar nerves. Throughout the vestibular nuclei, single units were recorded extracellularly with glass micropipettes filled with Fast Green. The tracts of the penetrating electrodes were identified histologically. In all four nuclei units responding to both labyrinths outnumbered unilaterally responding neurones with certain differences between the individual nuclei. Excitatory as well as inhibitory responses were observed, polysynaptic being more common than mono- or disynaptic ones. No monosynaptic contralateral responses were seen. The latency distribution of contralateral responses closely mirrored that of ipsilateral responses within each nucleus. Both excitatory and inhibitory responses fell into relatively segregated populations, based upon latency distribution. This implies separate pathways for labyrinthine input to the vestibular nuclei.

Convergence of ampullar and macular inputs on vestibular nuclei unit of the rat

181 vestibular nucleus neurons were examined for their responsiveness to rotation about the vertical axis and static tilts in roll and pitch planes in the rat. 68 of these units were sensitive to rotation and tilts (canal-otolith cells). In other words, 41.0% of the neurons responded to rotation (68/166). There was no significant difference in percentage of canal-otolith cells in type I and II neurons, which were 48.6% and 37.0% respectively. Vertical axis rotation when the head was tilted produced a simultaneous stimulation of the canal and otoliths. Using this stimulus method, the bias effect was observed in 72.5% of the canal-otolith cells (29/40). Furthermore, since vertical axis rotation with the head tilted elicited vertical canal responses, the rate of ampullary convergence was estimated by analysing response profiles obtained such rotations. The results obtained in the rat were compared with those in other species.