The effects of stimulating the cerebellar nodulus in the cat on the responses of vestibular neurons

Downbeat nystagmus: a clinical and pathophysiological review

Downbeat nystagmus (DBN) is a neuro-otological finding frequently encountered by clinicians dealing with patients with vertigo. Since DBN is a finding that should be understood because of central vestibular dysfunction, it is necessary to know how to frame it promptly to suggest the correct diagnostic-therapeutic pathway to the patient. As knowledge of its pathophysiology has progressed, the importance of this clinical sign has been increasingly understood. At the same time, clinical diagnostic knowledge has increased, and it has been recognized that this sign may occur sporadically or in association with others within defined clinical syndromes. Thus, in many cases, different therapeutic solutions have become possible. In our work, we have attempted to systematize current knowledge about the origin of this finding, the clinical presentation and current treatment options, to provide an overview that can be used at different levels, from the general practitioner to the specialist neurologist or neurotologist.

Vestibular neurons with direct projections to the solitary nucleus in the rat

Anterograde and retrograde tract tracing were combined with neurotransmitter and modulator immunolabeling to identify the chemical anatomy of vestibular nuclear neurons with direct projections to the solitary nucleus in rats. Direct, sparsely branched but highly varicose axonal projections from neurons in the caudal vestibular nuclei to the solitary nucleus were observed. The vestibular neurons giving rise to these projections were predominantly located in ipsilateral medial vestibular nucleus. The cell bodies were intensely glutamate immunofluorescent, and their axonal processes contained vesicular glutamate transporter 2, supporting the interpretation that the cells utilize glutamate for neurotransmission. The glutamate-immunofluorescent, retrogradely filled vestibular cells also contained the neuromodulator imidazoleacetic acid ribotide, which is an endogenous CNS ligand that participates in blood pressure regulation. The vestibulo-solitary neurons were encapsulated by axo-somatic GABAergic terminals, suggesting that they are under tight inhibitory control. The results establish a chemoanatomical basis for transient vestibular activation of the output pathways from the caudal and intermediate regions of the solitary nucleus. In this way, changes in static head position and movement of the head in space may directly influence heart rate, blood pressure, respiration, as well as gastrointestinal motility. This would provide one anatomical explanation for the synchronous heart rate and blood pressure responses observed after peripheral vestibular activation, as well as disorders ranging from neurogenic orthostatic hypotension, postural orthostatic tachycardia syndrome, and vasovagal syncope to the nausea and vomiting associated with motion sickness.NEW & NOTEWORTHY Vestibular neurons with direct projections to the solitary nucleus utilize glutamate for neurotransmission, modulated by imidazoleacetic acid ribotide. This is the first direct demonstration of the chemical neuroanatomy of the vestibulo-solitary pathway.

Deciphering the Neural Control of Sympathetic Nerve Activity: Status Report and Directions for Future Research

Sympathetic nerve activity (SNA) contributes appreciably to the control of physiological function, such that pathological alterations in SNA can lead to a variety of diseases. The goal of this review is to discuss the characteristics of SNA, briefly review the methodology that has been used to assess SNA and its control, and to describe the essential role of neurophysiological studies in conscious animals to provide additional insights into the regulation of SNA. Studies in both humans and animals have shown that SNA is rhythmic or organized into bursts whose frequency varies depending on experimental conditions and the species. These rhythms are generated by brainstem neurons, and conveyed to sympathetic preganglionic neurons through several pathways, including those emanating from the rostral ventrolateral medulla. Although rhythmic SNA is present in decerebrate animals (indicating that neurons in the brainstem and spinal cord are adequate to generate this activity), there is considerable evidence that a variety of supratentorial structures including the insular and prefrontal cortices, amygdala, and hypothalamic subnuclei provide inputs to the brainstem regions that regulate SNA. It is also known that the characteristics of SNA are altered during stress and particular behaviors such as the defense response and exercise. While it is a certainty that supratentorial structures contribute to changes in SNA during these behaviors, the neural underpinnings of the responses are yet to be established. Understanding how SNA is modified during affective responses and particular behaviors will require neurophysiological studies in awake, behaving animals, including those that entail recording activity from neurons that generate SNA. Recent studies have shown that responses of neurons in the central nervous system to most sensory inputs are context-specific. Future neurophysiological studies in conscious animals should also ascertain whether this general rule also applies to sensory signals that modify SNA.

Vestibulo-sympathetic responses

Evidence accumulated over 30 years, from experiments on animals and human subjects, has conclusively demonstrated that inputs from the vestibular otolith organs contribute to the control of blood pressure during movement and changes in posture. This review considers the effects of gravity on the body axis, and the consequences of postural changes on blood distribution in the body. It then separately considers findings collected in experiments on animals and human subjects demonstrating that the vestibular system regulates blood distribution in the body during movement. Vestibulosympathetic reflexes differ from responses triggered by unloading of cardiovascular receptors such as baroreceptors and cardiopulmonary receptors, as they can be elicited before a change in blood distribution occurs in the body. Dissimilarities in the expression of vestibulosympathetic reflexes in humans and animals are also described. In particular, there is evidence from experiments in animals, but not humans, that vestibulosympathetic reflexes are patterned, and differ between body regions. Results from neurophysiological and neuroanatomical studies in animals are discussed that identify the neurons that mediate vestibulosympathetic responses, which include cells in the caudal aspect of the vestibular nucleus complex, interneurons in the lateral medullary reticular formation, and bulbospinal neurons in the rostral ventrolateral medulla. Recent findings showing that cognition can modify the gain of vestibulosympathetic responses are also presented, and neural pathways that could mediate adaptive plasticity in the responses are proposed, including connections of the posterior cerebellar vermis with the vestibular nuclei and brainstem nuclei that regulate blood pressure.

Integration of vestibular and gastrointestinal inputs by cerebellar fastigial nucleus neurons: multisensory influences on motion sickness

Previous studies demonstrated that ingestion of the emetic compound copper sulfate (CuSO4) alters the responses to vestibular stimulation of a large fraction of neurons in brainstem regions that mediate nausea and vomiting, thereby affecting motion sickness susceptibility. Other studies suggested that the processing of vestibular inputs by cerebellar neurons plays a critical role in generating motion sickness and that neurons in the cerebellar fastigial nucleus receive visceral inputs. These findings raised the hypothesis that stimulation of gastrointestinal receptors by a nauseogenic compound affects the processing of labyrinthine signals by fastigial nucleus neurons. We tested this hypothesis in decerebrate cats by determining the effects of intragastric injection of CuSO4 on the responses of rostral fastigial nucleus to whole-body rotations that activate labyrinthine receptors. Responses to vestibular stimulation of fastigial nucleus neurons were more complex in decerebrate cats than reported previously in conscious felines. In particular, spatiotemporal convergence responses, which reflect the convergence of vestibular inputs with different spatial and temporal properties, were more common in decerebrate than in conscious felines. The firing rate of a small percentage of fastigial nucleus neurons (15%) was altered over 50% by the administration of CuSO4; the firing rate of the majority of these cells decreased. The responses to vestibular stimulation of a majority of these cells were attenuated after the compound was provided. Although these data support our hypothesis, the low fraction of fastigial nucleus neurons whose firing rate and responses to vestibular stimulation were affected by the administration of CuSO4 casts doubt on the notion that nauseogenic visceral inputs modulate motion sickness susceptibility principally through neural pathways that include the cerebellar fastigial nucleus. Instead, it appears that convergence of gastrointestinal and vestibular inputs occurs mainly in the brainstem.

Effects of visceral inputs on the processing of labyrinthine signals by the inferior and caudal medial vestibular nuclei: ramifications for the production of motion sickness

Neurons located in the caudal aspect of the vestibular nucleus complex have been shown to receive visceral inputs and project to brainstem regions that participate in generating emesis, such as nucleus tractus solitarius and the "vomiting region" in the lateral tegmental field (LTF). Consequently, it has been hypothesized that neurons in the caudal vestibular nuclei participate in triggering motion sickness and that visceral inputs to the vestibular nucleus complex can affect motion sickness susceptibility. To obtain supporting evidence for this hypothesis, we determined the effects of intragastric infusion of copper sulfate (CuSO4) on responses of neurons in the inferior and caudal medial vestibular nuclei to rotations in vertical planes. CuSO4 readily elicits nausea and emesis by activating gastrointestinal (GI) afferents. Infusion of CuSO4 produced a >30 % change in spontaneous firing rate of approximately one-third of neurons in the caudal aspect of the vestibular nucleus complex. These changes in firing rate developed over several minutes, presumably in tandem with the emetic response. The gains of responses to vertical vestibular stimulation of a larger fraction (approximately two-thirds) of caudal vestibular nucleus neurons were altered over 30 % by administration of CuSO4. The response gains of some units went up, and others went down, and there was no significant relationship with concurrent spontaneous firing rate change. These findings support the notion that the effects of visceral inputs on motion sickness susceptibility are mediated in part through the caudal vestibular nuclei. However, our previous studies showed that infusion of CuSO4 produced larger changes in response to vestibular stimulation of LTF neurons, as well as parabrachial nucleus neurons that are believed to participate in generating nausea. Thus, integrative effects of GI inputs on the processing of labyrinthine inputs must occur at brain sites that participate in eliciting motion sickness in addition to the caudal vestibular nuclei. It seems likely that the occurrence of motion sickness requires converging inputs to brain areas that generate nausea and vomiting from a variety of regions that process vestibular signals.

Clinical manifestations of cerebellar infarction according to specific lobular involvement

Lesions in the cerebellum produce various symptoms related to balance and motor coordination. However, the relationship between the exact topographical localization of a lesion and the resulting symptoms is not well understood in humans. In this study, we analyzed 66 consecutive patients with isolated cerebellar infarctions demonstrated on diffusion-weighted magnetic resonance imaging. We identified the involved lobules in these patients using a cross-referencing tool of the picture archiving and communication system, and we investigated the relationships between the sites of the lesions and specific symptoms using χ (2) tests and logistic regression analysis. The most common symptoms in patients with isolated cerebellar infarctions were vertigo (87%) and lateropulsion (82%). Isolated vertigo or lateropulsion without any other symptoms was present in 38% of patients. On the other hand, limb ataxia was a presenting symptom in only 40% of the patients. Lateropulsion, vertigo, and nystagmus were more common in patients with a lesion in the caudal vermis. Logistic regression analysis showed that lesions in the posterior paravermis or nodulus were independently associated with lateropulsion. Lesions in the nodulus were associated with contralateral pulsion, and involvement of the culmen was associated with ipsilateral pulsion and isolated lateropulsion without vertigo. Nystagmus was associated with lesions in the pyramis lobule, while lesions of the anterior paravermis were associated with dysarthria and limb ataxia. Our results showed that the cerebellar lobules are responsible for producing specific symptoms in cerebellar stroke patients.

Purkinje cells in posterior cerebellar vermis encode motion in an inertial reference frame

The ability to orient and navigate through the terrestrial environment represents a computational challenge common to all vertebrates. It arises because motion sensors in the inner ear, the otolith organs, and the semicircular canals transduce self-motion in an egocentric reference frame. As a result, vestibular afferent information reaching the brain is inappropriate for coding our own motion and orientation relative to the outside world. Here we show that cerebellar cortical neuron activity in vermal lobules 9 and 10 reflects the critical computations of transforming head-centered vestibular afferent information into earth-referenced self-motion and spatial orientation signals. Unlike vestibular and deep cerebellar nuclei neurons, where a mixture of responses was observed, Purkinje cells represent a homogeneous population that encodes inertial motion. They carry the earth-horizontal component of a spatially transformed and temporally integrated rotation signal from the semicircular canals, which is critical for computing head attitude, thus isolating inertial linear accelerations during navigation.

Membrane and firing properties of glutamatergic and GABAergic neurons in the rat medial vestibular nucleus

In previous studies, neurons in the medial vestibular nucleus (MVN) were classified mainly into 2 types according to their intrinsic membrane properties in in vitro slice preparations. However, it has not been determined whether the classified neurons are excitatory or inhibitory ones. In the present study, to clarify the relationship between the chemical and electrophysiological properties of MVN neurons, we explored mRNAs of cellular markers for GABAergic (glutamic acid decarboxylase 65, 67, and neuronal GABA transporter), glutamatergic (vesicular glutamate transporter 1 and 2), glycinergic (glycine transporter 2), and cholinergic neurons (choline acetyltransferase and vesicular acetylcholine transporter) expressed in electrophysiologically characterized MVN neurons in rat brain stem slice preparations. For this purpose, we combined whole cell patch-clamp recording analysis with single-cell reverse transcription-polymerase chain reaction (RT-PCR) analysis. We examined the membrane properties such as afterhyperpolarization (AHP), firing pattern, and response to hyperpolarizing current pulse to classify MVN neurons. From the single-cell RT-PCR analysis, we found that GABAergic neurons consisted of heterogeneous populations with different membrane properties. Comparison of the membrane properties of GABAergic neurons with those of other neurons revealed that AHPs without slow components and a firing pattern with delayed spike generation (late spiking) were preferential properties of GABAergic neurons. On the other hand, most glutamatergic neurons formed a homogeneous subclass of neurons exhibiting AHPs with slow components, repetitive firings with constant interspike intervals (continuous spiking), and time-dependent inward rectification in response to hyperpolarizing current pulses. We also found a small number of cholinergic neurons with various membrane properties. These findings clarify the electrophysiological properties of excitatory and inhibitory neurons in the MVN, and the information about the preferential membrane properties may be useful for identifying GABAergic and glutamatergic MVN neurons electrophysiologically.

Gravity dependence of ocular drift in patients with cerebellar downbeat nystagmus

Downbeat nystagmus is a frequent ocular motor sign in patients with lesions of the vestibulocerebellum. The upward drift in downbeat nystagmus is a combination of a gaze-evoked drift, due to an impaired vertical neural integrator, and a velocity bias. Using a three-dimensional turntable, we analyzed the influence of gravity on these two mechanisms. Patients with cerebellar downbeat nystagmus (n = 6) and healthy subjects (n = 12) were placed in various whole-body positions along the roll, pitch, and oblique vertical planes of the head. Ocular drift was monitored with scleral search coils. Although there was no gravity dependence of the vertical gaze-evoked drift, the vertical velocity bias consisted of two components: a gravity-dependent component that sinusoidally modulated as a function of body position along the pitch plane, and a gravity-independent component that was directed upward. The combination of the two components led to an overall drift that was minimal in supine and maximal in prone position. In healthy subjects, only the gravity-dependent component was present, but in a scaled-down manner. Our results suggest that the intact vestibulocerebellum minimizes an overacting otolith-ocular reflex elicited by pitch tilt and cancels an inherent upward ocular drift that is independent of gravity-modulated otolith signals.

Neuronal response sensitivity to bidirectional off-vertical axis rotations: a dimension of imbalance in the bilateral vestibular nuclei of cats after unilateral labyrinthectomy

In decerebrate cats after acute hemilabyrinthectomy, the response sensitivity of extracellularly recorded vestibular nuclear neurons on the lesioned and labyrinth-intact sides were examined quantitatively during constant velocity off-vertical axis rotations with an aim to elucidate the functional contribution of otolithic inputs to the ipsilateral and contralateral vestibular nuclei. The bidirectional response sensitivity, delta, was determined as the ratio of the gain during clockwise to that during counterclockwise rotations. A continuum of response sensitivity was identified: one-dimensional neurons showed symmetrically bidirectional response patterns, while two-dimensional neurons showed asymmetrically bidirectional patterns that in some cases approached unidirectional patterns with change in velocity. The proportion of two-dimensional neurons was significantly increased after acute hemilabyrinthectomy. Two-dimensional neurons that responded only to one direction of rotation in at least one of the velocities tested were described as unidirectional neurons. This unidirectional response pattern was observed in one-third of the entire neuronal population studied, but not in cats with both labyrinths intact, thus suggesting that such prominent broadly tuned responses are normally masked by converging otolithic inputs from the contralateral side. These neurons were found in higher proportion on the lesioned side than on the labyrinth-intact side. Among the 70% of unidirectional neurons that exhibited bidirectional response at some velocities and unidirectional response at others, prominent shifts in delta values (i.e. between 0/infinity and finite values) with velocity can be computed for each neuron. The shifts in delta values correlated with large shifts in the response dynamics and spatial orientation as the response pattern changed with velocity. The response orientations of the unidirectional neurons pointed in all directions on the horizontal plane. When all the two-dimensional neurons (i.e. both the unidirectionally and bidirectionally responsive ones) were pooled, imbalances in the distribution of the response orientations and in response gain were found between the ipsilateral-side-down/head-down half-circle and the contralateral-side-down/head-up half-circle on the labyrinth-intact side, but not on the lesioned side. These results, derived from spatiotemporal processing of gravitational signals, reveal a novel dimension of imbalance between neuronal populations in the two vestibular nuclear complexes after acute lesion of one labyrinth. This feature would provide, on the one hand, deranged cues of spatial orientation and direction during slow head excursions and, on the other, a framework for the dynamic behavioral deficits associated with hemilabyrinthectomy.

Using realistic models to study synaptic integration in cerebellar Purkinje cells

This review presents an approach to modeling which we call "experiments in computo". The use of realistic models makes it possible to generate new predictions that can be confirmed experimentally. Several examples are given of how this approach has improved our understanding of synaptic integration by the Purkinje cell active dendrite. The computer model was constructed to replicate neuronal behavior which has no direct relevance to synaptic integration: it was tuned to reproduce the response of Purkinje cells to current injection in vitro, which consists of a high frequency, regular rhythm of somatic Na+ spikes, interrupted by spontaneous dendritic Ca2+ spikes. The in vivo firing behavior of Purkinje cells is quite different as it consists of highly irregular simple spike firing only, without spontaneous dendritic Ca2+ spikes. The computer model predicted-that the Purkinje cell needs to receive a continuous background inhibitory synaptic drive in addition to the excitation by parallel fibers to obtain this typical in vivo firing. This prediction was confirmed by blocking inhibition during in vivo intracellular recordings. More recently, we demonstrated that the net inhibitory drive to the Purkinje cell dendrite has to be larger than the excitatory synaptic drive. Inhibition hyperpolarizes the dendrite compared to the soma, making it act as a current sink during most of the spiking cycle. These predictions have been confirmed with the dynamic clamp method in the cerebellar slice preparation. Synchronous focal excitatory input by parallel fiber leads in the model to activation of voltage-gated Ca2+ channels which amplify the somatic response by a variable amount. The variability of this graded amplification is due both to position of the input, effectively canceling the cable attenuation, and to the effect of preceding background input. Differences between the model and experimental results in this aspect can be explained by the relative hyperpolarized state of Purkinje cells in the in vitro experimental preparation. These studies led to a new theory about the function of long-term depression in the cerebellum which can explain recent experimental results. In conclusion, our modeling approach generated predictions which contradicted prevalent ideas on how the cerebellum, or neurons in general, works and led to experiments which otherwise would not have been carried out.

Analysis and neural network modeling of the nonlinear correlates of habituation in the vestibulo-ocular reflex

Through the process of habituation, continued exposure to low-frequency (0.01 Hz) rotation in the dark produced suppression of the low-frequency response of the vestibulo-ocular reflex (VOR) in goldfish. The response did not decay gradually, as might be expected from an error-driven learning process, but displayed several nonlinear and nonstationary features. They included asymmetrical response suppression, magnitude-dependent suppression for lower- but not higher-magnitude head rotations, and abrupt-onset suppressions suggestive of a switching mechanism. Microinjection of lidocaine into the vestibulocerebellum of habituated goldfish resulted in a temporary dishabituation. This suggests that the vestibulocerebellum mediates habituation, presumably through Purkinje cell inhibition of vestibular nuclei neurons. The habituated VOR data were simulated with a feed-forward, nonlinear neural network model of the VOR in which only Purkinje cell inhibition of vestibular nuclei neurons was varied. The model suggests that Purkinje cell inhibition may switch in to introduce nonstationarities, and cause asymmetry and magnitude-dependency in the VOR to emerge from the essential nonlinearity of vestibular nuclei neurons.

Mossy fiber projections from the brain stem to the nodulus in the cat. An experimental study comparing the nodulus, the uvula and the flocculus

Mossy fiber projections from the brainstem to the nodulus were studied by means of the retrograde transport of horseradish peroxidase (HRP) in the cat. The findings indicate exclusive secondary vestibular projections to the nodulus (96.5% of the total number of labeled cells in cat 1). Major sources of such projections include the caudal half of the medial and inferior vestibular nuclei, and the dorsal half of the superior vestibular nucleus. Groups-x and -z of the vestibular nuclei give rise to minor projections. The projections from groups-f and -y, and the interstitial nucleus of the eighth nerve are very few, if any. Minor extravestibular projections originate from the prepositus hypoglossal nucleus and the infratrigeminal nucleus. All these projections are bilateral. No other nuclei in the brainstem were labeled following HRP injection in the nodulus. No indications of mediolateral and dorsoventral differences in mossy fiber projections were found. There are quantitative differences in the sources of projections to the nodulus, the ventral uvula and the flocculus, all of which belong to the vestibulocerebellum. The largest sources for projections to the nodulus and the ventral uvula are from the vestibular nuclei. Among the vestibular nuclei, group-x provides the major projections to the ventral uvula. For the flocculus, in contrast, the major sources of input are the reticular formation and raphe nuclei. These quantitative differences may play an important role for differential functions of the nodulus, the ventral uvula and the flocculus.

The effect of uvulonodular lesions on horizontal optokinetic nystagmus and optokinetic after-nystagmus in cats

The effect of uvulonodular lesions on horizontal optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN) was investigated in alert cats. A lesion in each of 6 cats was made by suction-ablation under anesthesia. In the 3 cats with only a uvular lesion, both the initial slow phase velocity and the time constant of OKAN were unchanged. In the remaining 3 cats whose lesions included the nodulus as well as the uvula, the time constant of OKAN was markedly prolonged, but the initial slow phase velocity of OKAN was not affected. The postoperative average time constant of OKAN increased from the normal value of 9 s to about 40 s. In contrast, OKN parameters (the steady-state velocity and the rising time constant) were not affected by uvular or uvulonodular lesions. These results strongly suggest that the nodulus is involved in the velocity storage mechanism and might control the discharge characteristics of the velocity storage integrator.

Vestibular primary afferent projection to the cerebellum of the rabbit

The vestibular primary afferent projection to the cerebellum of the rabbit was studied with retrograde and orthograde tracers. We injected individual lobules of the cerebellum with horseradish peroxidase (HRP) or wheat germ agglutinin-HRP (WGA-HRP). Following these injections the numbers of labeled and unlabeled cells in Scarpa's ganglion were counted. Approximately 64-89% of the cells in Scarpa's ganglion were labeled retrogradely following uvula-nodular injections. About 2% of the cells in the ipsilateral Scarpa's ganglion were labeled after injections of the flocculus. Virtually no cells were labeled following injections of the ventral paraflocculus. The vestibular primary afferent projection to the uvula-nodulus is so extensive that it must be part of a collateral system that also innervates the vestibular nuclei. This collateral projection pattern was confirmed by using fluorescent tracers injected into the uvula-nodulus and vestibular complex. Fluorogold was injected into the uvula-nodulus and peroxidase-rhodamine isothiocyanate was injected into the vestibular complex. More than 50% of the neurons in Scarpa's ganglion were double labeled by these subtotal injections. The dense vestibular primary afferent projection to the uvula-nodulus was confirmed by using the C fragment of tetanus toxin (TTC) injected into the labyrinth as an orthograde tracer. With the TTC technique, the vestibular primary afferent projection to the uvula-nodulus terminated exclusively in the ipsilateral granule cell layer of lobules 9d and 10. Much sparser vestibular primary afferent projections were found in the banks of major cerebellar sulci. A barely detectable projection was found to the flocculus and ventral paraflocculus.

Role of visuo-vestibular interaction in pathological ocular oscillation: new model and control system analysis

Control system engineering is a relatively newer field of engineering which is directly applicable to performance analysis of neuronal networks. Visuo-vestibular interaction (VVI) represents a neuronal control system which has most benefited from such analysis. In this paper, we present a new model of VVI derived from our previously reported model by adding cortical pursuit feedback. Mathematical analysis of abnormal eye oscillations and stability analysis of the VVI system provided us with a plausible hypothesis that the pathological ocular oscillations seen in oculopalatal myoclonus and see-saw nystagmus are indeed due to an unstable VVI control system. The former is due to degeneration of the inferior olivary nucleus resulting in loss of cerebellar learning effects, while the latter is due to disruption of the pathway conveying retinal error signals.

Secondary vestibular cholinergic projection to the cerebellum of rabbit and rat as revealed by choline acetyltransferase immunohistochemistry, retrograde and orthograde tracers

Previously we have shown that four regions of the cerebellum, the uvula-nodulus, flocculus, ventral paraflocculus, and anterior lobe 1, receive extensive, but not exclusive, cholinergic mossy fiber projections. In the present experiment we have studied the origin of three of these projections in the rat and rabbit (uvula-nodulus, flocculus, ventral paraflocculus), using choline acetyltransferase (ChAT) immunohistochemistry in combination with a double label, retrogradely transported horseradish peroxidase (HRP). We have demonstrated that in both the rat and rabbit the caudal medial vestibular nucleus (MVN) and to a lesser extent the nucleus prepositus hypoglossus (NPH) contain ChAT-positive neurons. Neurons of the caudal MVN are double-labeled following HRP injections into the uvula-nodulus. HRP injections into the uvula-nodulus also labeled less than 5% of the neurons in the cholinergic vestibular efferent complex. Fewer ChAT-positive neurons in the MVN and some ChAT-positive neurons in the NPH are double-labeled following HRP injections into the flocculus. Almost no ChAT-positive neurons in the MVN and some ChAT-positive neurons in the NPH are double-labeled following HRP injections into the ventral paraflocculus. Injections of Phaseolus leucoagglutinin (PHA-L) into the caudal MVN of both the rat and rabbit demonstrated projection patterns to the uvula-nodulus and flocculus that were qualitatively similar to those observed using ChAT immunohistochemistry. We conclude that the cholinergic mossy fiber pathway to the cerebellum in general and the uvula-nodulus in particular is likely to mediate secondary vestibular information related to postural adjustments.

Optokinetic response of simple spikes of Purkinje cells in the cerebellar flocculus and nodulus of the pigmented rabbit

Under anesthesia with N2O (70%) and halothane (2-4%), Purkinje cell activities were extracellularly recorded in the flocculus and nodulus of immobilized pigmented rabbits. Large field (60 degrees x 60 degrees) optokinetic stimulation (OKS) was delivered to the central visual field of one eye with a constant velocity (0.1-4.0 degrees/s) at 0 degrees, 45 degrees, 90 degrees or 135 degrees to the horizontal plane of the eye. Most of the Purkinje cells in the flocculus and the nodulus showed significant simple spike modulations to OKS delivered to either eye. As a whole, the preferred directions of simple spike responses in the flocculus had the same orientation as those of complex spike responses. However, the preferred directions and amplitudes of modulation of simple spike responses did not necessarily correlate with those of complex spike responses in individual flocculus Purkinje cells. On the other hand, the preferred directions of simple and complex spike responses were not necessarily in the same orientation in the nodulus. The optimum velocity for simple spike responses was in the range 0.1-2.0 degrees/s for Purkinje cells in both the flocculus and the nodulus. The amplitude and time to peak of the simple spike responses of nodulus Purkinje cells were significantly smaller and longer, respectively, than those of flocculus Purkinje cells. In both the flocculus and the nodulus, Purkinje cells whose simple spikes preferred the horizontal orientation (H cells) and the vertical orientation (V cells) showed clustering. In particular, zonal organization was noted in the flocculus. H cells were localized in a dorso-ventral zone in the rostral one third of the flocculus, and V cells were in two distinct zones rostral and caudal to the H cell zone. The locations of H and V cells in the flocculus correspond to the H zone and V zones, respectively, determined on the basis of the preferred directions of complex spike responses to OKS. This indicates that the same subdivisions of the flocculus are supplied with optokinetic signals with the same orientation selectively through both mossy and climbing fibers, and suggest that such subdivisions of the flocculus are functional units which control horizontal and vertical components of optokinetic eye movements. The present results indicate that the flocculus and the nodulus are supplied with distinct optokinetic signals through mossy fibers and play different roles in controlling optokinetic eye movements.

Effect of uvulonodular lesions on optokinetic nystagmus and optokinetic after-nystagmus in cats

To investigate the effect of uvulonodular lesions on horizontal optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN), 6 cats were tested before and after lesions made by ablation under anesthesia. In the 3 cats with only uvular lesion, the initial slow phase velocity and the time constant of OKAN were unchanged. In the remaining three cats whose lesions included not only the uvula but also the nodulus, the time constant of OKAN was markedly prolonged, and the initial OKAN velocity was not affected. The postoperative average time constant of OKAN increased from normal value of 9 s to about 40 s. In contrast, OKN was unchanged in all of the 6 animals. These findings indicate that the nodulus may contribute to the discharge characteristics of the velocity storage integrator.

Differential mossy fiber projections to the dorsal and ventral uvula in the cat

The brainstem afferents to the uvula were studied by using retrograde axonal transport of horseradish peroxidase in the cat. Findings indicate differential afferent projections to the ventral and dorsal uvula. Major sources projecting to the ventral uvula include the caudal parts of the medial and inferior vestibular nuclei, the x- and f-groups of the vestibular nuclei, the dorsal and central parts of the superior vestibular nucleus, the rostral dorsomedial part of the paramedian nucleus of the pontine nuclei, the caudal part of the prepositus hypoglossal nucleus, and the infratrigeminal nucleus. Labeled cells in the vestibular nuclei were 74.7% of the total number of labeled cells in cat 40. On the other hand, the major sources projecting to the dorsal uvula are the peduncular, paramedian, and lateral nuclei of the pontine nuclei at the rostral and intermediate levels. Labeled cells in the pontine nuclei comprised 82.1% of the total number of labeled cells in cat 1. Findings also indicate that the lateral part of the ventral uvula receives input mainly from the pontine nuclei, whereas the medial part of the ventral uvula receives input mainly from the vestibular nuclei. Mediolateral differences were not found for the dorsal uvula. These mossy fiber zones are mediolaterally wide, with a dorsoventral partition in the uvula, in contrast to the climbing fiber zones, which are narrow (about 0.4 mm) and extend longitudinally throughout the uvula. There are quantitative differences in afferent sources to the ventral uvula and flocculus, both of which belong to the vestibulocerebellum. The largest afferent sources for the ventral uvula are the vestibular nerve and nuclei, whereas the largest sources for the flocculus are the reticular formation and raphe nuclei. These quantitative differences may have an important role for differential functions between the ventral uvula and flocculus. It has been suggested that the ventral uvula controls the velocity storage integrator of the vestibuloocular and optokinetic reflexes, whereas the flocculus is responsible for rapid changes of eye velocity in these reflexes.

Parasagittal zonal pattern of olivo-nodular projections in rabbit cerebellum

Injection of horseradish peroxidase (HRP) into the nodulus of the rabbit retrogradely labeled cells in 5 subdivisions of the contralateral inferior olive. Localized HRP injections at each of the medial, intermediate and lateral parts of the nodulus revealed 6 longitudinal zones, two zones in each part, projected differentially from the 5 olivary subdivisions: (i) the medial-most zone projected from the beta nucleus, (ii) the lateral zone of the medial part and (iii) the lateral zone of the intermediate part both from the dorsal cap, (iv) the medial zone of the intermediate part from the ventrolateral outgrowth, (v) the medial zone of the lateral part from the dorsomedial cell column, and (vi) the lateral-most zone from the rostrolateral part of the medial accessory olive. A complication is that the zones (v) and (vi) seemed to cover the dorsal lamina of the nodulus, but not the ventral lamina. The lateral part of the ventral nodulus seemed to be projected from the dorsal cap and may therefore be a lateral extension of zone (iv). There was an indication that the rostral-most area of the medial accessory olive also projects to the nodulus, but a specific receptive zone for this projection was unclear. The present results suggest that the small lateral area of the nodulus previously found to be involved in cardiovascular control is projected from the rostrolateral part of the medial accessory olive and/or the dorsomedial cell column.

Tilt suppression of vestibulo-ocular reflex in patients with cerebellar lesions

The effect of tilt on the time constant of post-rotatory nystagmus was determined in a group of normal subjects and patients with cerebellar lesions. The normal subjects showed a vestibular response that decayed with a time constant of 19.6 s when upright and 7.2 s after tilt prone. Patients with midline cerebellar lesions near the uvula and nodulus had time constants that were normal in the upright position but were unaffected by head tilt. Patients with cerebellar lesions due to the Arnold-Chiari malformation showed behavior that was intermediate between that of normal subjects and the patients with midline lesions. These results provide evidence that in humans, the midline cerebellum regulates a neural network in the brainstem that perseverates peripheral vestibular input.

Dynamic modification of the vestibulo-ocular reflex by the nodulus and uvula

The time constant of the decay of slow-phase eye velocity of postrotatory nystagmus or optokinetic after-nystagmus is reduced during exposure to a stationary visual surround (visual suppression). It is also reduced after tilting the head (tilt suppression). A "dump" mechanism in the vestibulo-ocular reflex has been proposed to rapidly discharge activity from the central vestibular system during both types of suppression. Monkeys lost this mechanism after lesions of the nodulus and uvula. They also lost the ability to habituate the time constant of nystagmus on repeated exposure to optokinetic and vestibular stimuli. Periodic alternating nystagmus, which is believed to represent an instability in the vestibulo-ocular reflex, was observed in two of three monkeys. These data indicate that the nodulus and uvula play an important role in suppressing, habituating, and stabilizing the vestibulo-ocular reflex.

Post-labyrinthectomy balance compensation with preplacement of cerebellar vermis lesion

Post-unilateral labyrinthectomy compensation was compared between squirrel monkeys; one group with the pre-placement of posterior vermectomy next group with uvulonodulectomy, and the other group without any vermic lesions. The comparison was made by characterizing behavioral locomotor imbalance (by squirrel monkey platform runway test) and oculomotor imbalance (spontaneous nystagmus). Post-posterior vermectomy + unilateral labyrinthectomy group showed a much slower regainment of locomotor balance function when compared to that after uvulonodulectomy + unilateral labyrinthectomy group (which results was almost identical to that after unilateral labyrinthectomy alone). In contrast, the former group exhibited faster decay in spontaneous nystagmus, whereas the latter group showed delayed reduction of slow phase eye velocity. Thus, posterior vermis could have more functional linkage to the vestibulo-spinal system, whereas, uvula and nodulus, to the vestibulo-oculomotor system.

Collateralized projection of visual climbing fibers to the flocculus and nodulus of the rabbit

Electrical stimulation of the rabbit cerebellar flocculus evoked responses in Purkinje cells of the nodulus by an axon reflex through collateralized climbing fibers (latency 4.2 ms). These responses interfered with those evoked by stimulation of the optic nerve in the manner of occlusion. Stimulation of the nodulus induced similar axon reflex responses in the flocculus. To identify the cells of origin of this collateralized climbing fiber projection, different fluorescent retrograde tracers, propidium iodide and 4',6-diamidino-2-phenylindole, were injected into the nodulus and flocculus. In the contralateral inferior olive, the majority (85%) of double-labeled cells were found in the dorsal cap and the rest (15%) in the ventrolateral outgrowth. These results lead to a conclusion that some inferior olive neurons convey common visual signals with collateralized axons to both the flocculus and the nodulus.

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.

Olivo-vestibular and cerebello-vestibular connections in albino rabbits

This study analyzes the organization of olivo-vestibular and cerebello-vestibular projections in rabbits. Iontophoretic injections of horseradish peroxidase, placed under physiological guidance into the superior, medial and lateral vestibular nuclei, produced retrogradely labeled neurons in the dorsal cap, ventrolateral outgrowth and lateral flexure of the principal olivary nucleus, the caudal half of the medial accessory olive and the caudal three-fourths of the dorsal accessory olive. This inferior olivary labeling was strictly contralateral. The same injections labeled groups of Purkinje cells in the ipsilateral cerebellar cortex, oriented perpendicular to the long axes of the folia of lobules I-V, VId-e and VIII-X of the vermis and the flocculus. The patterns of olivo-vestibular and cerebello-vestibular connections were consistent with the general hypothesis that inferior olivary axon collaterals project to both Purkinje cells and subcortical neurons inhibited by those Purkinje cells. In addition, the analysis of flocculo-nodular and dorsal cap-ventrolateral outgrowth projections to the medial and superior vestibular nuclei suggests that these connections are discrete at the level of a pool of olivary neurons which projects to functional pools of neurons in both cerebellar cortex, and in the vestibular nuclei. Thus, it is likely that inferior olivary projections define functional networks spanning cerebellar cortex and the vestibular nuclei.

Effects of vestibulocerebellar lesions upon dynamic characteristics and adaptation of vestibulo-ocular and optokinetic responses in pigmented rabbits

The vestibulo-ocular reflex (VOR) and optokinetic response (OKR) in the horizontal plane were examined in pigmented rabbits, using sinusoidal whole-body rotation and sinusoidal rotation of a striped screen. Sustained rotation of the animal (5 degrees peak-to-peak, 0.1 Hz) for 4 h, under different optokinetic stimulus conditions, induced the following adaptive changes in the VOR: (1) outphase rotation of the screen (5 degrees) increased the VOR gain by 0.3 (on average); (2) with the screen fixed in space, VOR gain increased by 0.2. (3) in-phase rotation of the screen (5 degrees) decreased the VOR gain by 0.16. However, (4) in-phase rotation of the screen at twice (10 degrees) the amplitude of whole-body rotation did not affect the gain. Instead, it induced a significant phase lead (23 degrees) in the VOR, which did not occur in other stimulus conditions. Adaptive increases of the OKR gain occurred under sustained rotation of the screen alone (2.5 degrees, 0.33 Hz). After bilateral destruction of floccular Purkinje cells with kainic acid the VOR gain and phase were affected only very slightly, but adaptive changes in the VOR were abolished. By contrast, the OKR gain was reduced and the OKR phase delayed. OKR adaptation was also affected in such a way that a gain increase initially produced could not be maintained during sustained screen rotation. Ablation of nodulus-uvala caused a gain increase and phase lead in both VOR and OKR, and its only effect on adaptability of the VOR or the OKR was observed for the VOR under stimulus condition (4).

Specific effects of unilateral lesions in the flocculus upon eye movements in albino rabbits

The horizontal vestibulo-ocular reflex (HVOR) and optokinetic response (OKR) were examined in alert albino rabbits following unilateral flocculectomy. Chemical flocculectomy with local application of kainic acid was used to avoid the retrograde degeneration of inferior olive neurons that accompanies surgical flocculectomy. Effects of chemical flocculectomy, however, were identical to those of surgical flocculectomy. The following functional deficiencies were observed in the movements of the ipsilateral eye: (1) reduction of the HVOR gain; (2) increased lag of the HVOR phase; (3) increased non-linearity of the relationship between the HVOR gain and the amplitude of turntable rotation; (4) decreased OKR gain; (5) delay with increased variation in the OKR phase; (6) impairment of rapid visual-vestibular interaction; (7) loss of the adaptation of the HVOR. Only a transient depression of the HVOR gain was seen in the contralateral eye. Control experiments with lesions in the paraflocculus, nodulus, and uvula, or lobules VI and VII, revealed no such deficiencies, except that lesions in the nodulus and uvula produced marked advancement of the HVOR phase. The effects of flocculectomy are consistent with present knowledge of both neuronal circuitry and activity of the rabbit flocculus.

Projection from the vestibular nuclei to the inferior olive in the cat: an autoradiographic and horseradish peroxidase study

Injections of horseradish peroxidase (HRP) in the inferior olive of 13 cats resulted in the labeling of neurons in the medial (MVN) and descending vestibular nuclei (DVN) as well as in subgroups g, p.h., s.v., x and z. The contribution of groups p.h. and z. could not be definitively established. Injections of triated leucine in the vestibular nuclei and adjacent reticular formation in 4 cats indicated that the previously identified vestibular nuclei and subgroups project bilaterally to the dorsomedial cell column and ipsilaterally to subnucleus beta of the inferior olive. In view of the known topography of olivocerebellar projections, the present results suggest that climbing fiber-mediated vestibular information may influence the uvula and the fastigial nucleus.

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.