Neural pathways from the vestibular labyrinths to the flocculus in the cat

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.

Inferior peduncle lesion presenting with bilaterally impaired vestibular responses to horizontal and posterior head impulses

Differentiating central from peripheral origins of vestibulo-ocular reflex (VOR) lesions can be challenging. A 36-year old man presented with a 1-year history of progressive unsteadiness. The video-Head Impulse Test revealed a significantly reduced VOR gain in both horizontal and posterior canals (0.49 ± 0.05 and 0.38 ± 0.06) but normal VOR responses in both anterior canals (0.89 ± 0.08 and 1.04 ± 0.15). No plausible combination of end-organ lesion should be responsible for these observations. A brain magnetic resonance imaging disclosed a left inferior cerebellar peduncle lesion suggestive of a glioma.

Long-term depression as a model of cerebellar plasticity

Long-term depression (LTD) here concerned is persistent attenuation of transmission efficiency from a bundle of parallel fibers to a Purkinje cell. Uniquely, LTD is induced by conjunctive activation of the parallel fibers and the climbing fiber that innervates that Purkinje cell. Cellular and molecular processes underlying LTD occur postsynaptically. In the 1960s, LTD was conceived as a theoretical possibility and in the 1980s, substantiated experimentally. Through further investigations using various pharmacological or genetic manipulations of LTD, a concept was formed that LTD plays a major role in learning capability of the cerebellum (referred to as "Marr-Albus-Ito hypothesis"). In this chapter, following a historical overview, recent intensive investigations of LTD are reviewed. Complex signal transduction and receptor recycling processes underlying LTD are analyzed, and roles of LTD in reflexes and voluntary movements are defined. The significance of LTD is considered from viewpoints of neural network modeling. Finally, the controversy arising from the recent finding in a few studies that whereas LTD is blocked pharmacologically or genetically, motor learning in awake behaving animals remains seemingly unchanged is examined. We conjecture how this mismatch arises, either from a methodological problem or from a network nature, and how it might be resolved.

A novel path for rapid transverse communication of vestibular signals in turtle cerebellum

Voltage-sensitive dye activity within the thin, unfoliated turtle cerebellar cortex (Cb) was recorded in vitro during eighth cranial nerve (nVIII) stimulation. Short latency responses were localized to the middle of the lateral edges of both ipsilateral and contralateral Cb [vestibulocerebellum (vCb)]. Even with a severed contralateral Cb peduncle, stimulation of the nVIII ipsilateral to the intact peduncle evoked contralateral vCb responses with a mean latency of only 0.25 ms after the ipsilateral responses, even though the distance between them was ∼ 5 mm. We investigated whether a rapidly conducting commissure exists between each vCb by stimulating one of them directly. Responses in both vCb spread sagittally, but, surprisingly, there was no sequential activation along a transverse Cb beam between them. In contrast, stimulation medial to either vCb evoked transverse beams that required ∼ 20 ms to cross the Cb. Therefore, the rapid commissural connection between each vCb is not mediated by slowly conducting parallel fibers. Also, the vCb was not strongly activated by climbing fiber stimulation, suggesting that inputs to vCb involve distinct cerebellar circuits. Responses between the two vCb remained following knife cuts through the rostral and caudal Cb along the midline, through both peduncles, and even shallow midline cuts to the middle Cb through its white matter and granule cell layer. Commissural responses were still observed only with a narrow transverse bridge between each vCb or in thick transverse Cb slices. Horseradish peroxidase transport from one vCb labeled transverse axons traveling within the Purkinje cell layer that were larger than parallel fibers and lacked varicosities. In sagittal sections, cross-section profiles of myelinated axons were observed around Purkinje cells midway between the rostral and caudal Cb. This novel pathway for transverse communication between lateral edges of turtle Cb suggests that afferents may directly conduct vestibular information rapidly across the Cb to coordinate vestibulomotor reflex behaviors.

Central projections of the saccular and utricular nerves in macaques

The central projections of the utricular and saccular nerve in macaques were examined using transganglionic labeling of vestibular afferent neurons. In these experiments, biotinylated dextran amine was injected directly into the saccular or utricular neuroepithelium of fascicularis (Macaca fascicularis) or rhesus (Macaca mulatta) monkeys. Two to 5 weeks later, the animals were killed and the peripheral vestibular sensory organs, brainstem, and cerebellum were collected for analysis. The principal brainstem areas of saccular nerve termination were lateral, particularly the spinal vestibular nucleus, the lateral portion of the superior vestibular nucleus, ventral nucleus y, the external cuneate nucleus, and cell group l. The principal cerebellar projection was to the uvula with a less dense projection to the nodulus. Principle brainstem areas of termination of the utricular nerve were the lateral/dorsal medial vestibular nucleus, ventral and lateral portions of the superior vestibular nucleus, and rostral portion of the spinal vestibular nucleus. In the cerebellum, a strong projection was observed to the nodulus and weak projections were present in the flocculus, ventral paraflocculus, bilateral fastigial nuclei, and uvula. Although there is extensive overlap of saccular and utricular projections, saccular inputs to the lateral portions of the vestibular nuclear complex suggest that saccular afferents contribute to the vestibulospinal system. In contrast, the utricular nerve projects more rostrally into areas of known concentration of vestibulo-ocular related cells. Although sparse, the projections of the utricle to the flocculus/ventral paraflocculus suggest a potential convergence with floccular projection inputs from the vestibular brainstem that have been implicated in vestibulo-ocular motor learning.

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.

Central projections of the vestibular nerve: a review and single fiber study in the Mongolian gerbil

The primary purpose of this article is to review the anatomy of central projections of the vestibular nerve in amniotes. We also report primary data regarding the central projections of individual horseradish peroxidase (HRP)-filled afferents innervating the saccular macula, horizontal semicircular canal ampulla, and anterior semicircular canal ampulla of the gerbil. In total, 52 characterized primary vestibular afferent axons were intraaxonally injected with HRP and traced centrally to terminations. Lateral and anterior canal afferents projected most heavily to the medial and superior vestibular nuclei. Saccular afferents projected strongly to the spinal vestibular nucleus, weakly to other vestibular nuclei, to the interstitial nucleus of the eighth nerve, the cochlear nuclei, the external cuneate nucleus, and nucleus y. The current findings reinforce the preponderance of literature. The central distribution of vestibular afferents is not homogeneous. We review the distribution of primary afferent terminations described for a variety of mammalian and avian species. The tremendous overlap of the distributions of terminals from the specific vestibular nerve branches with one another and with other sensory inputs provides a rich environment for sensory integration.

Motor dynamics encoding in cat cerebellar flocculus middle zone during optokinetic eye movements

We investigated the relationship between eye movement and simple-spike (SS) frequency of Purkinje cells in the cerebellar flocculus middle zone during the optokinetic response (OKR) in alert cats. The OKR was elicited by a sequence of a constant-speed visual pattern movement in one direction for 1 s and then in the opposite direction for 1 s. Quick-phase-free trials were selected. Sixty-six cells had direction-selective complex spike (CS) activity that was modulated during horizontal (preferring contraversive) but not vertical stimuli. The SS activity was modulated during horizontal OKR, preferring ipsiversive stimuli. Forty-one cells had well-modulated activity and were suitable for the regression model. In these cells, an inverse dynamics approach was applied, and the time course of the SS rate was reconstructed, with mean coefficient of determination 0.76, by a linear weighted superposition of the eye acceleration (mean coefficient, 0.056 spikes/s per deg/s(2)), velocity (5.10 spikes/s per deg/s), position (-2.40 spikes/s per deg), and constant (mean 34.3 spikes/s) terms, using a time delay (mean 11 ms) from the unit response to the eye response. The velocity and acceleration terms contributed to the increase in the reconstructed SS rates during ipsilateral movements, whereas the position term contributed during contralateral movements. The standard regression coefficient analyses revealed that the contribution of the velocity term (mean coefficient 0.81) was predominant over the acceleration (0.03) and position (-0.17) terms. Forward selection analysis revealed three cell types: Velocity-Position-Acceleration type (n = 27): velocity, position, and acceleration terms are significant (P < 0.05); Velocity-Position type (n = 12): velocity and position terms are significant; and Velocity-Acceleration type (n = 2): velocity and acceleration terms are significant. Using the set of coefficients obtained by regression of the response to a 5 deg/s stimulus velocity, the SS rates during higher (10, 20, and 40 deg/s) stimulus velocities were successfully reconstructed, suggesting generality of the model. The eye-position information encoded in the SS firing during the OKR was relative but not absolute in the sense that the magnitude of the position shift from the initial eye position (0 deg/s velocity) contributed to firing rate changes, but the initial eye position did not. It is concluded that 1) the SS firing frequency in the cat middle zone encodes the velocity and acceleration information for counteracting the viscosity and inertia forces respectively, during short-duration horizontal OKR and 2) the apparent position information encoded in the SS firing is not appropriate for counteracting the elastic force during the OKR.

Differences of the primate flocculus and ventral paraflocculus in the mossy and climbing fiber input organization

Potential sources of cerebellar cortical afferent fibers were identified in the vestibular ganglion, medulla oblongata, pons, and cerebellar nucleus of seven anesthetized Macaca fuscata after local injections of wheat germ agglutinin-conjugated horseradish peroxidase or Fast Blue into the flocculus (FL) or ventral paraflocculus (VP). There were differences in the sources of mossy fibers to the FL and VP. Labeled neurons, after injections into the FL, were located mainly in the ipsilateral vestibular ganglion, bilaterally in the vestibular and prepositus hypoglossal nuclei, nucleus reticularis tegmenti pontis, and the central part of the mesencephalic reticular formation including the raphe nuclei. Labeled neurons were rarely seen in the pontine nuclei after injections into the FL. By contrast, after injections into the VP, numerous labeled neurons were located in the contralateral pontine nuclei, but relatively few in the vestibular nuclei bilaterally. Sources of climbing fibers to the FL and VP were completely contralateral to the injection side. After the injection into the FL and VP, labeled neurons were located in the dorsal cap, ventrolateral outgrowth, and ventral part of the medial accessory olivary nucleus. The projections from these three olivary areas were generally consistent with a zonal pattern of terminations in the FL and VP. The present results are consistent with a hypothesis that the FL is mainly involved in the control of vestibulo-ocular reflex and that the VP is mainly involved in the control of smooth pursuit eye movements.

Simple-spike activity of floccular Purkinje cells responding to sinusoidal vertical rotation and optokinetic stimuli in alert cats

To understand how the cerebellar flocculus is involved in the processing of semicircular canal signals in the vertical vestibulo-ocular reflex (VOR), we analyzed the simple-spike activity of floccular Purkinje (P) cells that was modulated by sinusoidal pitch rotation, and then analyzed their activity during presentation of sinusoidal vertical optokinetic stimuli in alert, head-fixed cats. The great majority of P cells also responded to optokinetic stimuli with peak discharge near peak stimulus velocity. Eighty percent of P cells that responded to both pitch and optokinetic stimuli showed increased activity when the directions of the resultant eye movements were the same. During rapid modification of the VOR induced by visual pattern movement, modulation amplitudes of the cells tested increased together with the eye velocity increase. Maximal activation directions of these cells studied during vertical rotation in many planes were near the vertical canal planes, similar to those in our previous studies. The remaining 20% of P cells showed increased discharge for the same direction of stimulus movement. These results suggest that the activity of the majority of pitch-responding P cells contains, at least partly, a vertical eye velocity component during presentation of vestibular or optokinetic stimuli in addition to canal inputs during pitch rotation.

Postsynaptic targets of Purkinje cell terminals in the cerebellar and vestibular nuclei of the rat

The cerebellar and vestibular nuclei consist of a heterogeneous group of inhibitory and excitatory neurons. A major proportion of the inhibitory neurons provides a GABAergic feedback to the inferior olive, while the excitatory neurons exert more direct effects on motor control via non-olivary structures. At present is is not clear whether Purkinje cells innervate all types of neurons in the cerebellar and vestibular nuclei or whether an individual Purkinje cell axon can innervate different types of neurons. In the present study, we studied the postsynaptic targets of Purkinje cell axons in the rat using a combination of pre-embedding immunolabelling of the Purkinje cell terminals by L7, a Purkinje cell-specific marker, and postembedding GABA and glycine immunocytochemistry. In the cerebellar nuclei, vestibular nuclei and nucleus prepositus hypoglossi Purkinje cell terminals were found apposed to GABAergic and glycinergic neurons as well as to larger non-GABAergic, non-glycinergic neurons. In the cerebellar and vestibular nuclei individual Purkinje cell terminals innervated both the inhibitory and excitatory neurons. Both types of neurons were contacted no only by non-GABAergic Purkinje cell terminals but also by GABA-containing terminals that were not labelled for L7 and by non-GABAergic, non-glycinergic terminals that formed excitatory synapses. Glycine-containing terminals were relatively scarce ( < 2% of the GABA-containing terminals) and frequently contacted the larger non-GABAergic, non-glycinergic neurons. To summarize, Purkinje cell axons evoke their effects through different types of neurons present in the cerebellar and vestibular nuclear complex. The observation that individual Purkinje cells can innervate both excitatory and inhibitory neurons suggests that the excitatory cerebellar output system and the inhibitory feedback to the inferior olive are controlled simultaneously.

Laterality in the vestibulo-cerebellar mossy fiber projection to flocculus and caudal vermis in the rabbit: a retrograde fluorescent double-labeling study

The vestibular nuclear input into the flocculus and the uvula and nodulus of the caudal vermis was studied in rabbits by means of retrograde transport of the fluorescent tracers Fast Blue and Diamidino Yellow. Through simultaneous injection of the tracers in the homonymous lobules at either side of the midline, the distribution, the preference in laterality and the degree of collateralization of vestibulo-cerebellar neurons could be studied. The nucleus prepositus hypoglossi was included in the analysis. In the nucleus prepositus hypoglossi and in the medial vestibular nucleus 6% of the number of labeled neurons contained both tracers, against 4% in the descending vestibular nucleus. In the superior vestibular nucleus a statistically significant difference in the proportion of double-labeled neurons was found between cases with injections in the flocculus (1%) and the caudal vermis (9%). The relative distribution of single-labeled neurons projecting to either the flocculus or the caudal vermis was similar in most of the vestibular nuclei. A statistically significant preference for a projection to the flocculus in favor of one to the caudal vermis, was found for neurons in the medial vestibular nucleus and the prepositus hypoglossal nucleus. Statistically significant laterality preferences were found in the superior vestibular nucleus for the contralateral flocculus.

Identification of the Purkinje cell/climbing fiber zone and its target neurons responsible for eye-movement control by the cerebellar flocculus

We identified 3 Purkinje cell/climbing fiber zones in the cat cerebellar flocculus. The zones were perpendicular to the long axes of the crooked floccular folia, forming the crooked zones. Each zone was different in axonal projection areas of its target neurons. From the neuronal networks it is theoretically expected that activity changes of a particular zone control eye movement in a particular plane: (1) the rostral and caudal zones on one side control movement in the anterior canal plane on the side of the activity changes and those on both sides control movement in all vertical planes from sagittal to transverse planes; and (2) the middle zone controls movement in the horizontal plane by reciprocal activity changes on both sides. The zone-specific climbing fiber input to a particular zone may contribute to activity changes of the zone in response to mossy fiber input spreading across several zones. Electrical stimulation of each zone evoked the same pattern of eye movement as that theoretically expected from the neuronal networks. This is the first indication that there are indeed functional differences between the Purkinje cell zones in the cerebellum. Our findings support Oscarsson's proposal that each Purkinje cell/climbing fiber zone plus its target neurons may be an operational unit for control of a given motor function.

The primary vestibulocerebellar projection in the rabbit: absence of primary afferents in the flocculus

The central projection of vestibular nerve fibers was investigated with anterograde axonal transport of wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) and tritiated leucine following injection in the vestibular ganglion. Labeled fibers and terminal ramifications were observed throughout the vestibular complex, but absent from the lateral vestibular nucleus. Termination in the cortex was restricted to the vermis. Small numbers of mossy fiber terminals were present bilaterally, close to the midline in lobules I and II, and in the depth of the main fissures separating lobules II-VI. In the posterior vermis labeled mossy fiber terminals were found in lobule X and the ventral aspect of lobule IXd. Here, the entire ipsilateral hemivermis contained a large number of terminals, while contralaterally the medial one-third hemivermis contained fewer terminals. Labeled mossy fibers and terminals were absent in the flocculus and adjacent ventral paraflocculus.

Direct and relayed projection of periodontal receptor afferents to the cerebellum in the ferret

Field potentials in the cerebellar cortex of the ferret have been studied in response to stimulation of alveolar, muscular and cutaneous branches of the trigeminal nerve. Responses from the alveolar nerves are unusual in their very short latency. Evidence based on latency analysis, frequency following and comparison with other well-known inputs supports the view that the earliest field potentials are due to direct, unrelayed afferents, which terminate as mossy fibres. There is, in addition, a monosynaptically relayed afferent path via mossy fibres. The alveolar nerve afferents concerned with the direct projection are shown to come from periodontal mechanoreceptors and not from cutaneous receptors. No such connections are found from jaw-muscle spindle afferents. The direct and relayed periodontal pathways are both ipsilateral and crossed. They terminate in the cerebellar cortex in the parvermal region of lobules IV, V and VI. The functional significance of the direct periodontal afferent projection is considered particularly in the light of parallels with the vestibular system, which also has direct and relayed cerebellar projections.

Floccular influence on excitatory relay neurones of vestibular reflexes of anterior semicircular canal origin in the cat

Floccular influence on excitatory vestibular reflex arcs of anterior semicircular canal origin was examined in the anaesthetized cat. Stimulation of the anterior semicircular canal nerve (ACN) evoked disynaptic excitatory postsynaptic potentials (EPSPs) in all sampled inferior oblique (IO), superior rectus (SR), and biventor cervicis (BIV) muscle motoneurones of the contralateral side. Conditioning stimulus to the flocculus depressed the amplitude of the EPSPs in both IO and SR motoneurones by 50% on the average but not in any BIV motoneurones. The excitatory vestibulo-ocular neurones identified by orthodromic and antidromic responses to stimulation of the ACN and the contralateral IO motoneurone pool, respectively, were classified as VOC (vestibulo-ocular neurones with axons descending to the cervical segment) or VO (vestibulo-ocular proper) neurones on the basis of whether or not they responded antidromically to stimulation of the spinal cord in the C1 segment. All of the VO neurones in the superior vestibular nucleus (n = 19) were inhibited from the flocculus while the activities of three-fourths of the VO neurones (36/48) in the other vestibular nuclei were not suppressed by floccular stimulation. In contrast, none of VOC neurones (n = 49) received floccular inhibition. Besides inhibition, floccular stimulation induced the antidromic or orthodromic responses in some VO and VOC neurones.

Analysis of signal content of Purkinje cell responses to optokinetic stimuli in the rabbit cerebellar flocculus by selective lesions of brainstem pathways

The heterogeneous signal content of floccular Purkinje cell responses to optokinetic stimuli was analyzed in alert rabbits by means of selective lesions to brainstem pathways. Extracellular spike activities of Purkinje cells were recorded from rostral areas of the flocculus where local electrical stimulation elicited abduction of the ipsilateral eye. Chronic unilateral destruction of the nucleus reticularis tegmenti pontis, interrupting the visual mossy fiber afferent pathway to the flocculus, reduced the gain of the optokinetic eye movement (OKR) to one-third of the control. Concomitantly, simple spike responses of Purkinje cells to optokinetic stimuli were reduced to less than one-third of the control values. Severance of the visual climbing fiber afferent pathway by rostral inferior olivary lesions reduced the OKR gain little, and decreased the simple spike responses of the Purkinje cells only slightly. Bilateral lesions of the rostral half of the medial vestibular nucleus and rostro-ventral part of the lateral vestibular nucleus, which reduced the eye velocity in the OKR to less than one-third of the control value, did not induce any appreciable change in the simple spike responses of the Purkinje cells. It is concluded that visual mossy fiber signals are the most dominant factor which determines Purkinje cell responses to optokinetic stimuli, while visual climbing fiber signals and eye velocity mossy fiber signals make only subsidiary contributions.

Responses of units in the rat cerebellar flocculus during optokinetic and vestibular stimulation

The simple (SS) and complex spike (CS) responses of Purkinje (P-cells) and non-Purkinje (non P-cells) in the cerebellar flocculus were studied in alert pigmented rats (DA-HAN) during binocular and monocular optokinetic stimulation (OKS), vestibular stimulation and a combination of the two. Of a total of 98 P-cells whose SS discharges were activated by rotary stimulation of the horizontal canal in the dark (type I and type II P-cells), the vast majority (72%) responded to constant velocity binocular OKS that was produced by means of a horizontal shadow projector system. The remaining P-cells responded only to vestibular stimulation (19%), to OKS or to the presumed fast components of optokinetic and vestibular nystagmus (9%). The optokinetic responses of P-cells were generally bidirectional but asymmetrical, i.e., the increases in rate in one direction were larger in magnitude than decreases on opposite OKS and were synergistic with the semicircular canal input. During constant velocity OKS, the discharge of a few P-cells rose approximately exponentially, outlasted the stimulus by as much as 10-13.5s and, thus, resembled OKS responses of vestibular nucleus neurons. However, the majority exhibited a phasic-tonic response governed by a short "time constant" of from 0.5-3s. The velocity tuning curves of vestibular/OKS responding P-cells showed peak sensitivities with retinal slip velocities of 1.5-2 degrees/s. This is higher than the ca. 1 degree/s determined for other relay nuclei of the horizontal optokinetic pathway. The responses of non P-cells suggest that they originate from mossy fiber projections from vestibular, visual (optokinetic) and saccadic eye movement-related areas of the brainstem. Most of the units carried a combined vestibular and optokinetic signal. The majority showed a bidirection-selective response to OKS, and a small percentage showed unidirectional responses only. Monocular testing of P-cells revealed that most received a bidirection-selective, but asymmetrical, OKS input. Slightly more than half of these had a strongest OKS drive from the contralateral eye; the remaining units were driven most strongly by the ipsilateral eye. Unidirection-selective P-cells, driven by OKS to the ipsi- or contralateral eye, were uncommon; yet this class is common among other portions of the horizontal optokinetic system (e.g., vestibular nuclei, praepositus hypoglossi nucleus, nucleus reticularis tegmenti pontis).(ABSTRACT TRUNCATED AT 400 WORDS)

Vestibular afferent inputs to lobules I and II of the cerebellar anterior lobe vermis in the cat

Vestibular nerve stimulation evoked mossy fiber responses in lobule I and a part of lobule II of the cerebellar cortex in the anesthetized cat. The latencies of the N2 and N3 potentials were in the range of 1.7-2.0 and 3.2-5.0 ms, respectively. The contribution of both primary and secondary vestibular neurons in producing these responses were indicated by electrophysiological methods.

Effects of unilateral flocculus lesions on vestibulo-ocular responses in the cat

Unilateral lesions of the cerebellar flocculus were performed in three chronically-implanted adult cats. Following the lesion a spontaneous nystagmus was observed in the dark, with the fast phase directed to the lesioned side. Vestibulo-ocular responses in the dark became asymmetrical. Responses to velocity steps exciting the labyrinth ipsilateral to the lesion were strongly increased. A decrease, although less marked, was observed in the opposite direction. Responses to sinusoidal oscillations in the dark were also asymmetric with respect to both the cumulative eye displacement during rotation in the two directions and the interval between two consecutive reversals of eye movement. These differences were greater at the lower tested frequencies (0.01 HZ) than at the higher (0.1 HZ). Spontaneous nystagmus disappeared in about 10 days and a complete symmetry of the vestibulo-ocular responses was restored in about 3 weeks. It is concluded that a unilateral lesion of the flocculus leads to two separate, but interacting, effects upon vestibulo-ocular responses.

The superior vestibular nucleus: an intracellular HRP study in the cat. II. Non-vestibulo-ocular neurons

Superior vestibular neurons were penetrated with horseradish peroxidase (HRP)-loaded glass microelectrodes in anesthetized cats and identified electrophysiologically following electrical stimulation of the vestibular nerves and oculomotor complex. Neurons that were not antidromically activated from the oculomotor complex were stained by intracellular injection of horseradish peroxidase. Three types of neurons are identified according to their initial axonal trajectories into the cerebellum, the dorsal pontine reticular formation, or the brachium conjunctivum. Ipsilateral vestibular nerve input to all neurons is primarily monosynaptic and excitatory, whereas the contralateral is inhibitory. The neurons are located in the periphery of the superior vestibular nucleus. Soma diameters range from 20.5 micrometers to 44 micrometers. Most neurons exhibit globular and ovoid cell bodies. The dendritic arbors are intermediate between iso- and allodendritic branching patterns. The few spines and dendritic appendages present are distributed mainly distally on the dendrites. Soma size does not correlate with axon diameter, number of dendrites, or dendritic territories.

The role of the flocculus in vestibular compensation after hemilabyrinthectomy

Unilateral lesions of the cerebellar flocculus were made in two groups of cats chronically implanted for eye-movement recording. In the first group (3 cats), the floccular lesion preceded by 40-70 days a unilateral labyrinthectomy on the contralateral side. In the second group (5 cats), the serial order of the two lesions was reverted, the unilateral flocculectomy following unilateral labyrinthectomy by about 60 days in 2 animals and by 16-27 months in the other 3. The effects of the unilateral labyrinthectomy on the vestibulo-ocular reflex (spontaneous nystagmus and asymmetrical responses) were extensively tested by using natural vestibular stimulations. It was found that recovery from these effects was severely delayed in animals from the first group (flocculectomy prior to labyrinthectomy), although in animals from the second group the flocculectomy secondary to the labyrinthectomy only produced a transient asymmetry of vestibulo-ocular responses. It is concluded that the flocculus is required for initiating (not for maintaining), the compensatory process following peripheral lesions of the vestibular system.

Climbing fibre induced depression of both mossy fibre responsiveness and glutamate sensitivity of cerebellar Purkinje cells

1. In high decerebrate rabbits, cells were sampled extracellularly from the rostral flocculus. Purkinje cells were identified by their characteristic responses to stimulation of the contralateral inferior olive. Identification of basket cells was based on the absence of olivary responses and also on their location in the molecular layer adjacent to identified Purkinje cells. Mass field potentials in the flocculus were also studied.2. Single pulse stimulation of a vestibular nerve, either ipsilateral or contralateral, at a rate of 2/sec excited Purkinje cells with a latency of 3-6 msec. This early excitation represents activation through vestibular mossy fibres, granule cells and their axons (parallel fibres). Similar early excitation also occurred in putative basket cells.3. Conjunctive stimulation of a vestibular nerve at 20/sec and the inferior olive at 4/sec, for 25 sec per trial, effectively depressed the early excitation of Purkinje cells by that nerve, without an associated change in spontaneous discharge. The depression recovered in about ten minutes. This recovery was followed by the onset of a slow depression lasting for an hour.4. Conjunctive vestibular-olivary stimulation produced no such depression in the following responses: early excitation in Purkinje cells induced from the vestibular nerve not involved in the conjunctive stimulation; early excitation in putative basket cells from either vestibular nerve; inhibition or rebound facilitation in Purkinje cells following the early excitation; vestibular-evoked field potentials in the granular layer and white matter of the flocculus. These observations lead to the conclusion that the depression occurs specifically at parallel fibre-Purkinje cell synapses involved in conjunctive stimulation.5. Ionophoretic application of glutamate to Purkinje cells in conjunction with 4/sec olivary stimulation depressed the glutamate sensitivity of Purkinje cells; aspartate sensitivity was depressed to a much lesser degree. The depression diminished in about 10 min, but this recovery was succeeded by a slow depression lasting for an hour. The depression was seen only when glutamate sensitivity was relatively high, suggesting that the micro-electrode was impinging onto Purkinje cell dendrites. These observations suggest that subsynaptic chemosensitivity of Purkinje cells to the putative neurotransmitter of parallel fibres is involved in the depression observed after conjunctive stimulation of a vestibular nerve and the inferior olive.6. The present results are consistent with the Marr-Albus assumption concerning plasticity of cerebellar neuronal networks.

Properties of secondary vestibular neurons fired by stimulation of ampullary nerve of the vertical, anterior or posterior, semicircular canals in the cat

Experiments on cats were performed to study the pathway and location of the secondary vestibulo-ocular neurons in response to stimulation of the ampullary nerves of the vertical, anterior or posterior, semicircular canals. Experiments on the medial longitudinal fasciculus transection disclosed that vertical canal-evoked, disynaptic excitation and inhibition were transmitted to the extraocular motoneurons through the contra- and ipsilateral medial longitudinal fasciculus respectively. Secondary vestibular neurons, which receive input from the ampullary nerve of the vertical semicircular canals and send their axons to contralateral medial longitudinal fasciculus, were intermingled in the rostral half of the descending and lateral part of the medial vestibular nuclei. A direct excitatory connection of some of these neurons to the target extraocular motoneurons was confirmed by means of a spike-triggered signal averaging technique. It was also found that neurons activated by antidromic stimulation of ipsilateral medial longitudinal fasciculus were located in the superior vestibular nucleus, some of which made direct inhibitory connections to the target extraocular motoneurons. Both excitatory and inhibitory vestibuloocular neurons made synaptic contact in about half of the impaled target motoneurons.

Visual-vestibular interaction in the flocculus of the alert monkey. I. Input activity

Neuronal activity in the flocculus of alert Rhesus monkeys was recorded during vestibular stimulation (rotation of the monkey about a vertical axis in complete darkness), optokinetic stimulation (rotation of the visual surround around the stationary monkey), combined visual-vestibular stimulation (rotation of the monkey inside the stationary surround in light), and conflicting visual-vestibular stimulation (rotation of the monkey together with the visual surround in the same direction). The input to the flocculus was recorded as non-Purkinje cell (non-P-cell) activity. Ninety per cent of the non-P-cells which were modulated during our stimulation paradigms carry information similar to that in the neurons of vestibular nuclei. This suggests that the main mossy fiber input to the flocculus originates in the vestibular nuclei. A second input of unknown origin conveys visual information about retinal slip. Thus, part of the flocculus -- as further discussed elsewhere (Waespe and Henn 1981) -- may be specialized to subserve visual-vestibular interaction to improve the nystagmus response.

The cerebellar projection of the vestibular nerve in the cat

The projection of the vestibular nerve to the cerebellum of the cat was examined with silver degeneration methods after complete lesions of the vestibular ganglion. The majority of the primary vestibular afferents were traced to the cortex of the ipsilateral nodulus and uvula, relatively fewer entering the ipsilateral flocculus. Fibers were not traced to the paraflocculus, lingula or lateral cerebellar nucleus. A sparse projection to the ipsilateral fastigial nucleus may exist, but it remains equivocal until confirmed with additional methods. Light microscopic examination of plastic sections confirmed these observations and showed further details of the organization of the primary vestibular projection to the nodulus and uvula. These results show that the region of the cerebellum densely innervated by primary vestibular afferents is smaller than previously believed.

Response of single Purkinje neurons in the flocculus of albino rabbits to caloric stimulation

The response of Purkinje neurons of the flocculus to caloric stimulation was investigated in the Urethane-chlorarose anesthetized rabbit. Twenty-five of 37 flocculus neurons which responded to ipsilateral caloric stimulation showed an increase in firing response, while 12 neurons showed a decrease. Fifteen of 28 flocculus neurons which responded to contralateral caloric stimulation showed an increase, while the firing of 13 neurons was decreased. Forty-one percent of flocculus neurons responded to ipsilateral caloric stimulation, and 41% responded to caloric stimulation of both sides. Eighteen percent of flocculus neurons responded only to contralateral stimulation. The ipsilateral flocculus may thus be responsible for the major control of the primary vestibular signal flow in the cerebellum.

Responses of Purkinje cells in rabbit nodulus and uvula to natural vestibular and visual stimuli

1. The responses of Purkinje cells and presumed mossy fibers to natural stimulation of the horizontal semicircular canals were recorded in the nodulus and uvula of rabbit vestibulocerebellum. Units responding to vestibular stimulation were also studied with visual stimulation. 2. The responses of presumed mossy fibers were of the Type I and Type II varieties and were characterized by a low threshold for angular acceleration and high sensitivity. 3. Purkinje cell responses were divided into two groups: The first group showed only modulation of simple spike activity during rotation. According to the directionality of their responses to rotation, Purkinje cells of the first group could be further subdivided into Types I, II or III; Type II was the most frequently encountered. The second group showed modulation of both simple spike and climbing fiber activity. The simple spike response most frequently encountered was of Type II while the climbing fiber activity in the same Purkinje cells responded in the Type I mode. In another population of Purkinje cells of this group, simple spike activity was modulated by rotation in one direction only. All Purkinje cell responses had relatively high thresholds and low sensitivities. 4. Some Purkinje cells responding to rotation showed direction-selective modulation of climbing fiber discharge in response to slowly moving visual patterns.