Zonal organization of the climbing fiber projection to the flocculus and nodulus of the rabbit: a combined axonal tracing and acetylcholinesterase histochemical study

An Indirect Pathway from the Rat Interstitial Nucleus of Cajal to the Vestibulocerebellum Involved in Vertical Gaze Holding

The neural network, including the interstitial nucleus of Cajal (INC), functions as an oculomotor neural integrator involved in the control of vertical gaze holding. Impairment of the vestibulocerebellum (VC), including the flocculus (FL), has been shown to affect vertical gaze holding, indicating that the INC cooperates with the VC in controlling this function. However, a network between the INC and VC has not been identified. In this study, we aimed to obtain anatomical evidence of a neural pathway from the INC to the VC (the INC-VC pathway) in rats. Injection of dextran-conjugated Alexa Fluor 488 or adeno-associated virus 2-retro (AAV2retro) expressing GFP into the FL or another VC region (uvula/nodulus) did not reveal any retrogradely labeled neurons in the INC, suggesting that INC neurons do not project directly to the VC. Rabies virus-based transsynaptic tracing experiments revealed that the INC-VC pathway is mediated via synaptic connections with the prepositus hypoglossi nucleus (PHN) and medial vestibular nucleus (MVN). The INC neurons in the INC-VC pathway were mainly localized bilaterally within the rostral region of the INC. Transsynaptic tracing experiments involving the INC-FL pathway revealed that INC neurons connected to the FL via the bilateral PHN and MVN. These results indicate that the INC-VC pathway is not a direct pathway but is mediated via the PHN and MVN. These findings can provide clues for understanding the network mechanisms responsible for vertical gaze holding.

Adaptive Balance in Posterior Cerebellum

Vestibular and optokinetic space is represented in three-dimensions in vermal lobules IX-X (uvula, nodulus) and hemisphere lobule X (flocculus) of the cerebellum. Vermal lobules IX-X encodes gravity and head movement using the utricular otolith and the two vertical semicircular canals. Hemispheric lobule X encodes self-motion using optokinetic feedback about the three axes of the semicircular canals. Vestibular and visual adaptation of this circuitry is needed to maintain balance during perturbations of self-induced motion. Vestibular and optokinetic (self-motion detection) stimulation is encoded by cerebellar climbing and mossy fibers. These two afferent pathways excite the discharge of Purkinje cells directly. Climbing fibers preferentially decrease the discharge of Purkinje cells by exciting stellate cell inhibitory interneurons. We describe instances adaptive balance at a behavioral level in which prolonged vestibular or optokinetic stimulation evokes reflexive eye movements that persist when the stimulation that initially evoked them stops. Adaptation to prolonged optokinetic stimulation also can be detected at cellular and subcellular levels. The transcription and expression of a neuropeptide, corticotropin releasing factor (CRF), is influenced by optokinetically-evoked olivary discharge and may contribute to optokinetic adaptation. The transcription and expression of microRNAs in floccular Purkinje cells evoked by long-term optokinetic stimulation may provide one of the subcellular mechanisms by which the membrane insertion of the GABAA receptors is regulated. The neurosteroids, estradiol (E2) and dihydrotestosterone (DHT), influence adaptation of vestibular nuclear neurons to electrically-induced potentiation and depression. In each section of this review, we discuss how adaptive changes in the vestibular and optokinetic subsystems of lobule X, inferior olivary nuclei and vestibular nuclei may contribute to the control of balance.

Striped Distribution Pattern of Purkinje Cells of Different Birthdates in the Mouse Cerebellar Cortex Studied with the Neurog2-CreER Transgenic Line

Heterogeneity of Purkinje cells (PCs) that are arranged into discrete longitudinally-striped compartments in the cerebellar cortex is related to the timing of PC generation. To understand the cerebellar compartmental organization, we mapped the PC birthdate (or differentiation timing) in the entire cerebellar cortex. We used the birthdate-tagging system of Neurog2-CreER (G2A) mice hybridized with the AldocV strain which visualizes the zebrin (aldolase C) longitudinal striped pattern. The birthdate-specific distribution pattern of PCs was arranged into longitudinally-oriented stripes consistently throughout almost all lobules except for the nodulus, paraflocculus, and flocculus, in which distinct stripes were observed. Boundaries of the birthdate stripes coincided with the boundary of zebrin stripes or located in the middle of a zebrin stripe. Each birthdate stripe contained PCs born in a particular period between embryonic day (E) 10.0 and E 13.5. In the vermis, PCs were chronologically distributed from lateral to medial stripes. In the paravermis, PCs of early birthdates were distributed in the long lateral zebrin-positive stripe (stripe 4+//5+) and the medially neighboring narrow zebrin-negative substripe (3d-//e2-), while PCs of late birthdates were distributed in the rest of all paravermal areas. In the hemisphere, PCs of early and late birthdates were intermingled in the majority of areas. The results indicate that the birthdate of a PC is a partial determinant for the zebrin compartment in which it is located. However, the correlation between the PC birthdate and the zebrin compartmentalization is complex and distinct among the vermis, paravermis, hemisphere, nodulus, and flocculus.

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.

The modular architecture and neurochemical patterns in the cerebellar cortex

The review deals with topical issues of the neuronal arrangement underlying basic cerebellar functions. The cerebellum and its auxiliary structures contain several hundreds of modules (so called "microzones"). Each module receives the corticopetal input specific for the lobule it belongs to and forms the topographic projection. The precision of the major input-output signal flow in the cerebellar cortex is provided by a pronounced stratification of its synaptic zones of a various origin and regular topography of its afferent connections, interneurons, and efferent neurons. There is a nice match between the anatomical and functional coordinates of the modules, whose spatial boundaries are determined by the spread of afferent excitation and local interneuron connections. The dynamic characteristics of the modules are analyzed by the example of the formation of the nitrergic neuron ensembles and cerebellar projections of corticopetal fibers. The authors discuss the cerebellar blood flow and its relation to the activity of NO/GABAergic Lugaro cells and other interneurons in the cerebellar cortex. A generalized scheme of intra- and intermodular communication is proposed.

Plane-specific Purkinje cell responses to vertical head rotations in the cat cerebellar nodulus and uvula

We recorded simple spike (SS) and complex spike (CS) firing of Purkinje cell in the cerebellar nodulus and uvula of awake, head-restrained cats during sinusoidal vertical rotation of the head in four stimulus planes (pitch, roll, and two vertical canal planes). Two SS response types (position- and velocity-types) with response phases close to those of head position and velocity, respectively, were recognized. Optimal response planes and directions for SS and CS of each cell were estimated from the response amplitudes in the four stimulus planes by fitting with a sinusoidal function. The principal findings are as follows: 1) two rostrocaudally oriented functional zones of Purkinje cells can be distinguished; 2) the medially located parasagittal band is active during rotation in the pitch plane; 3) the laterally located band is active during rotation in the roll plane. These two zones are the same as previously reported zones in the cerebellar flocculus active during head rotation in the canal planes in the point that both cerebellar sagittal zones are plane-specific functional zones, suggesting that the anatomical sagittal zones serve as functional plane-specific zones at least in the vestibulocerebellum.

Nonvisual complex spike signals in the rabbit cerebellar flocculus

In addition to the well-known signals of retinal image slip, floccular complex spikes (CSs) also convey nonvisual signals. We recorded eye movement and CS activity from Purkinje cells in awake rabbits sinusoidally oscillated in the dark on a vestibular turntable. The stimulus frequency ranged from 0.2 to 1.2 Hz, and the velocity amplitude ranged from 6.3 to 50°/s. The average CS modulation was evaluated at each combination of stimulus frequency and amplitude. More than 75% of the Purkinje cells carried nonvisual CS signals. The amplitude of this modulation remained relatively constant over the entire stimulus range. The phase response of the CS modulation in the dark was opposite to that during the vestibulo-ocular reflex (VOR) in the light. With increased frequency, the phase response systematically shifted from being aligned with contraversive head velocity toward peak contralateral head position. At fixed frequency, the phase response was dependent on peak head velocity, indicating a system nonlinearity. The nonvisual CS modulation apparently reflects a competition between eye movement and vestibular signals, resulting in an eye movement error signal inferred from nonvisual sources. The combination of this error signal with the retinal slip signal in the inferior olive results in a net error signal reporting the discrepancy between the actual visually measured eye movement error and the inferred eye movement error derived from measures of the internal state. The presence of two error signals requires that the role of CSs in models of the floccular control of VOR adaption be expanded beyond retinal slip.

Gating of neural error signals during motor learning

Cerebellar climbing fiber activity encodes performance errors during many motor learning tasks, but the role of these error signals in learning has been controversial. We compared two motor learning paradigms that elicited equally robust putative error signals in the same climbing fibers: learned increases and decreases in the gain of the vestibulo-ocular reflex (VOR). During VOR-increase training, climbing fiber activity on one trial predicted changes in cerebellar output on the next trial, and optogenetic activation of climbing fibers to mimic their encoding of performance errors was sufficient to implant a motor memory. In contrast, during VOR-decrease training, there was no trial-by-trial correlation between climbing fiber activity and changes in cerebellar output, and climbing fiber activation did not induce VOR-decrease learning. Our data suggest that the ability of climbing fibers to induce plasticity can be dynamically gated in vivo, even under conditions where climbing fibers are robustly activated by performance errors.

DOI:http://dx.doi.org/10.7554/eLife.02076.001

The cerebellum (or ‘little brain’) is located underneath the cerebral hemispheres. Despite comprising around 10% of the brain’s volume, the cerebellum contains roughly half of the brain’s neurons. Many of the functions of the cerebellum are related to the control and fine-tuning of movement, and people whose cerebellum has been damaged have problems with balance and coordination, and with learning new motor skills.

One of the roles of the cerebellum is to control a reflex known as the vestibulo-ocular reflex, which enables us to keep our gaze fixed on an object as we turn our heads. The cerebellum relays information about head movements to the muscles that control the eyes, instructing the eyes to move in the opposite direction to the head. This keeps the image of the object we are looking at stable on the retina.

The vestibulo-ocular reflex is controlled by a circuit that includes Purkinje cells (which are the main output cells of the cerebellum) and climbing fibres (which originate in the brainstem). Any failure of the vestibulo-ocular reflex to fully compensate for head movements generates an error signal that activates the climbing fibres. These in turn modify the output of Purkinje cells, leading ultimately to adjustments in eye movements.

However, Kimpo et al. have now obtained evidence that Purkinje cells can modulate their response to the instructions they receive from climbing fibres. Monkeys sat in a rotating chair while a visual object they were trained to track with their eyes was moved to induce errors in the vestibulo-ocular reflex. When the object was moved so that a bigger reflexive eye movement was required to stabilize the image, the activation of the climbing fibres in response to the error led to a change in the response of the Purkinje cells, as expected. However, when a smaller reflexive eye movement was needed, the error-driven responses of the climbing fibres did not alter the responses of Purkinje cells. Similar results were obtained using pulses of light to artificially activate climbing fibres and thus simulate error signals.

The work of Kimpo et al. indicates that the cerebellum does not blindly follow the instructions it receives from the brainstem, but can instead modulate its responses to incoming information about performance errors. Further work is now required to identify factors that influence the responsiveness of the cerebellum: such information could ultimately be used to improve learning of motor skills and recovery from injury.

DOI:http://dx.doi.org/10.7554/eLife.02076.002

Detailed expression pattern of aldolase C (Aldoc) in the cerebellum, retina and other areas of the CNS studied in Aldoc-Venus knock-in mice

Aldolase C (Aldoc, also known as "zebrin II"), a brain type isozyme of a glycolysis enzyme, is expressed heterogeneously in subpopulations of cerebellar Purkinje cells (PCs) that are arranged longitudinally in a complex striped pattern in the cerebellar cortex, a pattern which is closely related to the topography of input and output axonal projections. Here, we generated knock-in Aldoc-Venus mice in which Aldoc expression is visualized by expression of a fluorescent protein, Venus. Since there was no obvious phenotypes in general brain morphology and in the striped pattern of the cerebellum in mutants, we made detailed observation of Aldoc expression pattern in the nervous system by using Venus expression in Aldoc-Venus heterozygotes. High levels of Venus expression were observed in cerebellar PCs, cartwheel cells in the dorsal cochlear nucleus, sensory epithelium of the inner ear and in all major types of retinal cells, while moderate levels of Venus expression were observed in astrocytes and satellite cells in the dorsal root ganglion. The striped arrangement of PCs that express Venus to different degrees was carefully traced with serial section alignment analysis and mapped on the unfolded scheme of the entire cerebellar cortex to re-identify all individual Aldoc stripes. A longitudinally striped boundary of Aldoc expression was first identified in the mouse flocculus, and was correlated with the climbing fiber projection pattern and expression of another compartmental marker molecule, heat shock protein 25 (HSP25). As in the rat, the cerebellar nuclei were divided into the rostrodorsal negative and the caudoventral positive portions by distinct projections of Aldoc-positive and negative PC axons in the mouse. Identification of the cerebellar Aldoc stripes in this study, as indicated in sample coronal and horizontal sections as well as in sample surface photos of whole-mount preparations, can be referred to in future experiments.

Modulated discharge of Purkinje and stellate cells persists after unilateral loss of vestibular primary afferent mossy fibers in mice

Cerebellar Purkinje cells are excited by two afferent pathways: climbing and mossy fibers. Climbing fibers evoke large "complex spikes" (CSs) that discharge at low frequencies. Mossy fibers synapse on granule cells whose parallel fibers excite Purkinje cells and may contribute to the genesis of "simple spikes" (SSs). Both afferent systems convey vestibular information to folia 9c-10. After making a unilateral labyrinthectomy (UL) in mice, we tested how the discharge of CSs and SSs was changed by the loss of primary vestibular afferent mossy fibers during sinusoidal roll tilt. We recorded from cells identified by juxtacellular neurobiotin labeling. The UL preferentially reduced vestibular modulation of CSs and SSs in folia 8-10 contralateral to the UL. The effects of a UL on Purkinje cell discharge were similar in folia 9c-10, to which vestibular primary afferents project, and in folia 8-9a, to which they do not project, suggesting that vestibular primary afferent mossy fibers were not responsible for the UL-induced alteration of SS discharge. UL also induced reduced vestibular modulation of stellate cell discharge contralateral to the UL. We attribute the decreased modulation to reduced vestibular modulation of climbing fibers. In summary, climbing fibers modulate CSs directly and SSs indirectly through activation of stellate cells. Whereas vestibular primary afferent mossy fibers cannot account for the modulated discharge of SSs or stellate cells, the nonspecific excitation of Purkinje cells by parallel fibers may set an operating point about which the discharges of SSs are sculpted by climbing fibers.

Processing of visual signals related to self-motion in the cerebellum of pigeons

In this paper I describe the key features of optic flow processing in pigeons. Optic flow is the visual motion that occurs across the entire retina as a result of self-motion and is processed by subcortical visual pathways that project to the cerebellum. These pathways originate in two retinal-recipient nuclei, the nucleus of the basal optic root (nBOR) and the nucleus lentiformis mesencephali, which project to the vestibulocerebellum (VbC) (folia IXcd and X), directly as mossy fibers, and indirectly as climbing fibers from the inferior olive. Optic flow information is integrated with vestibular input in the VbC. There is a clear separation of function in the VbC: Purkinje cells in the flocculus process optic flow resulting from self-rotation, whereas Purkinje cells in the uvula/nodulus process optic flow resulting from self-translation. Furthermore, Purkinje cells with particular optic flow preferences are organized topographically into parasagittal "zones." These zones are correlated with expression of the isoenzyme aldolase C, also known as zebrin II (ZII). ZII expression is heterogeneous such that there are parasagittal stripes of Purkinje cells that have high expression (ZII+) alternating with stripes of Purkinje cells with low expression (ZII-). A functional zone spans a ZII± stripe pair. That is, each zone that contains Purkinje cells responsive to a particular pattern of optic flow is subdivided into a strip containing ZII+ Purkinje cells and a strip containing ZII- Purkinje cells. Additionally, there is optic flow input to folia VI-VIII of the cerebellum from lentiformis mesencephali. These folia also receive visual input from the tectofugal system via pontine nuclei. As the tectofugal system is involved in the analysis of local motion, there is integration of optic flow and local motion information in VI-VIII. This part of the cerebellum may be important for moving through a cluttered environment.

Visuomotor cerebellum in human and nonhuman primates

In this paper, we will review the anatomical components of the visuomotor cerebellum in human and, where possible, in non-human primates and discuss their function in relation to those of extracerebellar visuomotor regions with which they are connected. The floccular lobe, the dorsal paraflocculus, the oculomotor vermis, the uvula-nodulus, and the ansiform lobule are more or less independent components of the visuomotor cerebellum that are involved in different corticocerebellar and/or brain stem olivocerebellar loops. The floccular lobe and the oculomotor vermis share different mossy fiber inputs from the brain stem; the dorsal paraflocculus and the ansiform lobule receive corticopontine mossy fibers from postrolandic visual areas and the frontal eye fields, respectively. Of the visuomotor functions of the cerebellum, the vestibulo-ocular reflex is controlled by the floccular lobe; saccadic eye movements are controlled by the oculomotor vermis and ansiform lobule, while control of smooth pursuit involves all these cerebellar visuomotor regions. Functional imaging studies in humans further emphasize cerebellar involvement in visual reflexive eye movements and are discussed.

Crossing zones in the vestibulocerebellum: a commentary

The contention of this commentary, focused on the vestibulocerebellum (particularly the flocculus), is that the great importance for our understanding of cerebellar organization in terms of climbing fiber zones, begun years ago by Voogd [1969, 2011] and Oscarsson [1969], needs to be matched by coming more to grips with the other fundamental geometrical organization of the cerebellum, the parallel fibers. The central issue is the selection of those parallel fiber signals to be transformed into Purkinje cell activity in the different zones. At present, in comparison to our knowledge of vestibulocerebellar climbing fiber inputs, the deficiencies in our knowledge of the zonal anatomy and physiology of vestibulocerebellar mossy fibers and granule cells are glaring. The recent emphasis on molecularly oriented investigations points to the need to reinvigorate pursuit of unanswered questions about cerebellar anatomy, the handmaiden of physiology.

Cerebellar zones: a personal history

Cerebellar zones were there, of course, before anyone noticed them. Their history is that of young people, unhindered by preconceived ideas, who followed up their observations with available or new techniques. In the 1960s of the last century, the circumstances were fortunate because three groups, in Leiden, Lund, and Bristol, using different approaches, stumbled on the same zonal pattern in the cerebellum of the cat. In Leiden, the Häggqvist myelin stain divulged the compartments in the cerebellar white matter that channel the afferent and efferent connections of the zones. In Lund, the spino-olivocerebellar pathways activated from individual spinal funiculi revealed the zonal pattern. In Bristol, charting the axon reflex of olivocerebellar climbing fibers on the surface of the cerebellum resulted in a very similar zonal map. The history of the zones is one of accidents and purposeful pursuit. The technicians, librarians, animal caretakers, students, secretaries, and medical illustrators who made it possible remain unnamed, but their contributions certainly should be acknowledged.

Morphometric analysis of the neuronal numbers and densities of the inferior olivary complex in the donkey (Equus asinus)

The morphometric interrelations between the compartments of the inferior olivary complex (IOC) in the donkey (Equus asinus) were ascertained by examining serial sections throughout the entire length of the IOC for both sides. Nissl-stained celloidin sections of four brainstems of donkeys were used. The IOC consisted of three major nuclei and four small cell groups. The total neuronal count in both sides of the IOC was 202,040±8480 cells. The medial accessory olivary nucleus (MAO) had the largest relative area (46%) and the highest number of neurons (90,800±7600). The dorsal accessory olivary nucleus (DAO) had the second largest relative area (33%), while the principal olivary nucleus (PO) had the lowest relative area (21%). However, the total neuron count in the PO was larger (60,840±1840) than DAO (50,360±4040). The average neuronal density was 2700±400 cells/mm(3). The numerical values of the current study of the IOC in the donkey were similar to those of other mammals.

Organization of the marmoset cerebellum in three-dimensional space: lobulation, aldolase C compartmentalization and axonal projection

The cerebellar cortex is organized by transverse foliation and longitudinal compartmentalization. Although the latter, which is recognized through the molecular expression in subsets of Purkinje cells (PCs), is closely related to topographic axonal projection and represents functional divisions, the details have not been fully clarified in mammals other than rodents. Therefore, we examined folial and compartmental organization of the marmoset cerebellum, which resembles the macaque cerebellum, and compared it with that of the rodent cerebellum by aldolase C immunostaining, three-dimensional reconstruction of the PC layer, and labeling of olivocerebellar and corticonuclear projections. Longitudinal stripes of different aldolase C expression intensities separated the entire cerebellar cortex into multiple compartments. Lobule VIIAb-d was equivalent to rodent lobule VIc in that it contained a transverse gap in the cortical layers and served as the rostrocaudal boundary for compartments and axonal branching. Olivocortical and corticonuclear projection patterns in major compartments indicated that the compartmental organization in the marmoset cerebellum was generally equivalent to that in the rodent cerebellum, although two compartments were missing in the pars intermedia and several compartments that have not been seen in rodents were recognized in the flocculus, nodulus, and the most lateral hemisphere. Reconstruction showed that the paraflocculus and flocculus were formed by a single longitudinal sheet, the axis of which was parallel to the aldolase C compartments, PC dendrites, and olivocerebellar climbing fiber distribution. The results indicate that molecular compartmentalization in the marmoset cerebellum reflected both the common fundamental organization of the mammalian cerebellum and species-dependent differentiation.

Differential projections from the vestibular nuclei to the flocculus and uvula-nodulus in pigeons (Columba livia)

The pigeon vestibulocerebellum is divided into two regions based on the responses of Purkinje cells to optic flow stimuli: the uvula-nodulus responds best to self-translation, and the flocculus responds best to self-rotation. We used retrograde tracing to determine whether the flocculus and uvula-nodulus receive differential mossy fiber input from the vestibular and cerebellar nuclei. From retrograde injections into the both the flocculus and uvula-nodulus, numerous cells were found in the superior vestibular nucleus (VeS), the cerebellovestibular process (pcv), the descending vestibular nucleus (VeD), and the medial vestibular nucleus (VeM). Less labeling was found in the prepositus hypoglossi, the cerebellar nuclei, the dorsolateral vestibular nucleus, and the lateral vestibular nucleus, pars ventralis. In the VeS, the differential input to the flocculus and uvula-nodulus was distinct: cells were localized to the medial and lateral regions, respectively. The same pattern was observed in the VeD, although there was considerable overlap. In the VeM, the majority of cells labeled from the flocculus were in rostral margins on the ipsilateral side, whereas labeling from uvula-nodulus injections was distributed bilaterally throughout the VeM. Finally, from injections in the flocculus but not the uvula-nodulus, moderate labeling was observed in a paramedian area, adjacent to the medial longitudinal fasciculus. In summary, there were clear differences with respect to the projections from the vestibular nuclei to functionally distinct parts of the vestibulocerebellum. Generally speaking, the mossy fibers to the flocculus and uvula-nodulus arise from regions of the vestibular nuclei that receive input from the semicircular canals and otolith organs, respectively.

Qualitative and quantitative studies of the inferior olivary complex in the water buffalo (Buballus bubalis)

The shape and neuronal number of the inferior olivary complex (IOC) were determined in the water buffalo (Buballus bubalis). The configuration and interrelations of the IOC compartments were ascertained by investigating serial sections through the whole rostro-caudal extent of the IOC. Nissl-stained celloidin sections of six water buffalo's brainstems were used. The IOC in the water buffalo consisted of three major nuclei and four small cell groups. The medial accessory olivary nucleus (MAO) had the longest rostro-caudal extent as well as the highest number of neurons (98,000 +/- 3,000). Although the total area of the principal olivary nucleus (PO) was smaller than the area of the dorsal accessory olivary nucleus (DAO), the PO had the second largest neuronal number. The total number of neurons on both sides of the IOC was 210,000 +/- 7,000 cells. The average neuronal density was 3,000 cells/mm3. Although the size of the PO relatively increases while the size of MAO decreases with the development of the cerebellar hemispheres, the IOC in most mammals maintains a similar structure except for the higher primates and marsupials. The water buffalo IOC showed morphological similarities to the almost all mammalian IOC including rats as follow; the main part of the MAO consists of three subgroups (a, b and c), the DAO is Boomerang-shaped while the PO is a simple U-shaped structure.

Morphological study on the inferior olivary nuclear complex of the donkey (Equus asinus)

This study provides basic data on the normal structure of the inferior olivary complex (IOC) of the donkey, Equus asinus, at the light microscopic level. In common with that of other mammals, the donkey IOC consisted of three major nuclei and four minor groups of cells. The former was comprised of the medial and dorsal accessory olives (MAO and DAO, respectively) and the principal olive (PO), and the latter was comprised of the dorsal cap, nucleus beta, ventrolateral outgrowth and dorsomedial cell column. The MAO had the longest rostral to caudal representation and formed the caudal pole of IOC. The DAO was located dorsally to the MAO in the caudal half of the IOC. In the rostral half, the DAO bended ventrally and merged with the dorsal lamella of PO. More rostrally, the DAO lost its connection with the dorsal lamella and then conversely connected with the ventral lamella of PO. The DAO formed the rostral pole of the IOC. The PO extended through the rostral half of the IOC. The dorsal cap was a small group of cells. Overall, the donkey IOC is similar to that of other mammals.

Zonal organization of the mouse flocculus: physiology, input, and output

The zones of the flocculus have been mapped in many species with a noticeable exception, the mouse. Here, the functional map of the mouse was constructed via extracellular recordings followed by tracer injections of biotinylated-dextran-amine and immunohistochemistry for heat-shock protein-25. Zones were identified based on the Purkinje cell complex spike modulation occurring in response to optokinetic stimulation. In zones 1 and 3 Purkinje cells responded best to rotation about a horizontal axis oriented at 135 degrees ipsilateral azimuth, whereas in zones 2 and 4 they responded best to rotation about the vertical axis. The tracing experiments showed that Purkinje cells of zone 1 projected to the parvicellular part of lateral cerebellar nucleus and superior vestibular nucleus, while Purkinje cells of zone 3 projected to group Y and the superior vestibular nucleus. Purkinje cells of zones 2 and 4 projected to the magnocellular and parvicellular parts of the medial vestibular nucleus, while some also innervated the lateral vestibular nucleus or nucleus prepositus hypoglossi. The climbing fiber inputs to Purkinje cells in zones 1 and 3 were derived from neurons in the ventrolateral outgrowth of the contralateral inferior olive, whereas those in zones 2 and 4 were derived from the contralateral caudal dorsal cap. Purkinje cells in zones 1 and 2, but not in zones 3 and 4, were positively labeled for heat-shock protein-25. The present study illustrates that Purkinje cells in the murine flocculus are organized in discrete zones with specific functions, specific input - output relations, and a specific histochemical signature.

Cerebellar circuitry as a neuronal machine

Shortly after John Eccles completed his studies of synaptic inhibition in the spinal cord, for which he was awarded the 1963 Nobel Prize in physiology/medicine, he opened another chapter of neuroscience with his work on the cerebellum. From 1963 to 1967, Eccles and his colleagues in Canberra successfully dissected the complex neuronal circuitry in the cerebellar cortex. In the 1967 monograph, "The Cerebellum as a Neuronal Machine", he, in collaboration with Masao Ito and Janos Szentágothai, presented blue-print-like wiring diagrams of the cerebellar neuronal circuitry. These stimulated worldwide discussions and experimentation on the potential operational mechanisms of the circuitry and spurred theoreticians to develop relevant network models of the machinelike function of the cerebellum. In following decades, the neuronal machine concept of the cerebellum was strengthened by additional knowledge of the modular organization of its structure and memory mechanism, the latter in the form of synaptic plasticity, in particular, long-term depression. Moreover, several types of motor control were established as model systems representing learning mechanisms of the cerebellum. More recently, both the quantitative preciseness of cerebellar analyses and overall knowledge about the cerebellum have advanced considerably at the cellular and molecular levels of analysis. Cerebellar circuitry now includes Lugaro cells and unipolar brush cells as additional unique elements. Other new revelations include the operation of the complex glomerulus structure, intricate signal transduction for synaptic plasticity, silent synapses, irregularity of spike discharges, temporal fidelity of synaptic activation, rhythm generators, a Golgi cell clock circuit, and sensory or motor representation by mossy fibers and climbing fibers. Furthermore, it has become evident that the cerebellum has cognitive functions, and probably also emotion, as well as better-known motor and autonomic functions. Further cerebellar research is required for full understanding of the cerebellum as a broad learning machine for neural control of these functions.

Inferior olive and oculomotor system

Three subnuclei within the inferior olive are implicated in the control of eye movement; the dorsal cap (DC), the beta-nucleus and the dorsomedial cell column (DMCC). Each of these subnuclei can be further divided into clusters of cells that encode specific parameters of optokinetic and vestibular stimulation. DC neurons respond to optokinetic stimulation in one of three planes, corresponding to the anatomical planes of the semicircular canals. Neurons in the beta-nucleus and DMCC respond to vestibular stimulation in the planes of the vertical semicircular canals and otoliths. Each these olivary nuclei receives excitatory and inhibitory signals from pre-olivary structures. The DC receives excitatory signals from the ipsilateral nucleus of the optic tract (NOT) and inhibitory signals from the contralateral nucleus prepositus hypoglossi (NPH). The beta-nucleus and DMCC receive inhibitory signals from the ipsilateral nucleus parasolitarius (Psol) and excitatory signals from the contralateral dorsal Y group. Consequently, the olivary projection to the cerebellum, although totally crossed, still represents bilateral sensory stimulation. Inputs to the inferior olive from the NOT, NPH, Psol or Y-group discharge at frequencies of 10-100 imp/s. CFRs discharge at 1-5 imp/s; a frequency reduction of an order of magnitude. Inferior olivary projections to the contralateral cerebellum are sagittally arrayed onto multiple cerebellar folia. These arrays establish coordinate systems in the flocculus and nodulus, representing head-body movement. These climbing fiber-defined spatial coordinate systems align Purkinje cell discharge onto subjacent cerebellar and vestibular nuclei. In the oculomotor system, olivo-cerebellar circuitry enhances and modifies eye movements based on movement of the head-body in space.

Oculomotor cerebellum

The anatomical, physiological, and behavioral evidence for the involvement of three regions of the cerebellum in oculomotor behavior is reviewed here: (1) the oculomotor vermis and paravermis of lobules V, IV, and VII; (2) the uvula and nodulus; (3) flocculus and ventral paraflocculus. No region of the cerebellum controls eye movements exclusively, but each receives sensory information relevant for the control of multiple systems. An analysis of the microcircuitry suggests how sagittal climbing fiber zones bring visual information to the oculomotor vermis; convey vestibular information to the uvula and nodulus, while optokinetic space is represented in the flocculus. The mossy fiber projections are more heterogeneous. The importance of the inferior olive in modulating Purkinje cell responses is discussed.

Cardiovascular modules in the cerebellum

Mapping with local lesions, electrical or chemical stimulation, or recording evoked field potentials or unit spikes revealed localized representations of cardiovascular functions in the cerebellum. In this review, which is based on literatures in the field (including our own publications), I propose that the cerebellum contains five distinct modules (cerebellar corticonuclear microcomplexes) dedicated to cardiovascular control. First, a discrete rostral portion of the fastigial nucleus and the overlying medial portion of the anterior vermis (lobules I, II and III) conjointly form a module that controls the baroreflex. Second, anterior vermis also forms a microcomplex with the parabrachial nucleus. Third, a discrete caudal portion of the fastigial nucleus and the overlying medial portion of the posterior vermis (lobules VII and VIII) form another module controlling the vestibulosympathetic reflex. Fourth, the medial portion of the uvula may form a module with the nucleus tractus solitarius and parabrachial nucleus. Fifth, the lateral edge of the nodulus and the uvula, together with the parabrachial nucleus and vestibular nuclei, forms a cardiovascular microcomplex that controls the magnitude and/or timing of sympathetic nerve responses and stability of the mean arterial blood pressure during changes of head position and body posture. The lateral nodulus-uvula appears to be an integrative cardiovascular control center involving both the baroreflex and the vestibulosympathetic reflex.

Immunoreactivity for calcium-binding proteins defines subregions of the vestibular nuclear complex of the cat

The vestibular nuclear complex (VNC) is classically divided into four nuclei on the basis of cytoarchitectonics. However, anatomical data on the distribution of afferents to the VNC and the distribution of cells of origin of different efferent pathways suggest a more complex internal organization. Immunoreactivity for calcium-binding proteins has proven useful in many areas of the brain for revealing structure not visible with cell, fiber or Golgi stains. We have looked at the VNC of the cat using immunoreactivity for the calcium-binding proteins calbindin, calretinin and parvalbumin. Immunoreactivity for calretinin revealed a small, intensely stained region of cell bodies and processes just beneath the fourth ventricle in the medial vestibular nucleus. A presumably homologous region has been described in rodents. The calretinin-immunoreactive cells in this region were also immunoreactive for choline acetyltransferase. Evidence from other studies suggests that the calretinin region contributes to pathways involved in eye movement modulation but not generation. There were focal dense regions of fibers immunoreactive to calbindin in the medial and inferior nuclei, with an especially dense region of label at the border of the medial nucleus and the nucleus prepositus hypoglossi. There is anatomical evidence that suggests that the likely source of these calbindin-immunoreactive fibers is the flocculus of the cerebellum. The distribution of calbindin-immunoreactive fibers in the lateral and superior nuclei was much more uniform. Immunoreactivity to parvalbumin was widespread in fibers distributed throughout the VNC. The results suggest that neurochemical techniques may help to reveal the internal complexity in VNC organization.

Disturbance of Motor Imagery After Cerebellar Stroke

The authors studied the possible involvement of the cerebellum in nonexecutive motor functions needed for a normal performance of complex motor patterns by analyzing (using chronometric evaluation) finger movement sequences and their respective motor imagery (a mental simulation of motor patterns). Patients suffering from a cerebellar stroke (n=11) were compared with aged-matched control volunteers (n=11). Patients that had apparently recovered from a unilateral cerebellar stroke showed a marked slowing of motor performance in both hands (ipsi- and contralateral to lesion). This effect was accompanied by a similar slowing of motor imagery, suggesting that the cerebellum, traditionally implicated in the control of motor execution, is also involved in nonexecutive motor functions such as the planning and internal simulation of movements.

Inferior olivary neurons innervate multiple zones of the flocculus in pigeons (Columba livia)

Complex spike activity of floccular Purkinje cells responds to patterns of rotational optic flow about the vertical axis (rVA neurons) or a horizontal axis 45 degrees to the midline (rH45 neurons). The pigeon flocculus is organized into four parasagittal zones: two rVA zones (zones 0 and 2) interdigitated with two rH45 zones (zones 1 and 3). Climbing fiber input to the rVA and rH45 zones arises in the caudal and rostral regions of the medial column of the inferior olive (mcIO), respectively. To determine whether the two rVA zones and the two rH45 zones receive input from different areas of the caudal and rostral mcIO and whether individual neurons project to both zones of the same rotational preference, different colors of fluorescent retrograde tracer were injected into the two rVA or two rH45 zones. For the rVA injections, retrogradely labeled cells from the two zones were intermingled in the caudal mcIO, but the distribution of cells labeled from zone 0 was slightly caudal to that from zone 2. On average, 18% of neurons were double labeled. For the rH45 injections, cells retrogradely labeled from the two zones were intermingled in the rostral mcIO, but the distribution of cells labeled from zone 1 was slightly rostral to that from zone 3. On average, 22% of neurons were double labeled. In sum, each of the two rVA zones and the two rH45 zones receives input from slightly different regions of the mcIO, and about 20% of the neurons project to both zones.

Functional compartmentalization in the flocculus and the ventral dentate and dorsal group y nuclei: an analysis of single olivocerebellar axonal morphology

The cerebellar cortex consists of multiple longitudinal bands defined by their olivocerebellar projection. Single olivocerebellar axons project to a narrow longitudinal band in the cerebellar cortex and to the cerebellar nucleus with their axon collaterals. This olivocortical and olivonuclear organization is related to the functional compartmentalization of the cerebellar system. To reveal the detailed morphologic organization in the flocculus and the cerebellar and vestibular nuclei, we examined olivocerebellar projection by reconstructing the entire trajectories of 19 single olivofloccular axons and by anterograde mapping with biotinylated dextran in the rat. The flocculus was composed of 12 longitudinal band-shaped compartments that subdivided 5 previously described zones. These longitudinal bands were innervated differentially by the caudal and rostral portions of the dorsal cap (DC) and the ventrolateral outgrowth (VLO) and the rostral pole of the medial accessory olive. Single olivofloccular axons with an average of 5.1 climbing fibers usually projected to a single longitudinal band in the flocculus and to the ventral dentate or dorsal group y nucleus with their collaterals. DC neurons projected moderately to the rostrolateral portion of the ventral dentate nucleus, whereas VLO neurons projected densely to the medial portion of the ventral dentate nucleus and the dorsal group y nucleus with rostrocaudal topography. DC and VLO neurons did not project to the vestibular nuclei, although floccular Purkinje cells projected to the vestibular, ventral dentate, and dorsal group y nuclei. The fine morphologically identified longitudinal bands and topographic olivonuclear projections were correlated with previous electrophysiologically defined functional zones in the flocculus and inferior olive.

Collateralization of climbing and mossy fibers projecting to the nodulus and flocculus of the rat cerebellum

Collateralization of mossy and climbing fibers was investigated using cortical injections of cholera toxin b-subunit in the rat vestibulocerebellum. Injections were characterized by their retrograde labeling within the inferior olive. Collateral labeling was plotted using color-coded density profiles of the whole cerebellar cortex. Injections in the medial part of the nodulus resulted in olivary labeling that was restricted to the rostral part of the dorsal cap. Climbing fiber collaterals were found in medial and lateral nodular zones as well as in the ventral paraflocculus and adjacent flocculus. Injections in the intermediate part of the nodulus resulted in olivary labeling of the beta-subnucleus but could also involve the ventrolateral outgrowth. In the latter case, climbing fiber collaterals were found in the two floccular zones and in a small region in the lateral-most part of crus I. All nodular injections showed a bilaterally symmetric distribution of collateral mossy fiber rosettes that was mostly confined to the vestibulocerebellum and originated predominantly from the vestibular nuclei. Injections in the flocculus labeled the caudal part of the dorsal cap and/or the ventrolateral outgrowth. Mossy fiber rosettes were observed throughout the vestibulocerebellum but also included other regions of the cerebellar cortex in a bilaterally symmetric pattern corresponding with a more widespread precerebellar origin. Climbing fibers originating in the rostral dorsal cap, labeled from an injection in the ventral paraflocculus, collateralize to a medial and lateral zone in the nodulus. Climbing fiber collaterals were usually accompanied by subjacent labeling of mossy fiber rosettes. These results demonstrate that some nodular and floccular zones are related and, at least partially, share a common input.

Zonal organization of the vestibulocerebellum in pigeons (Columba livia): III. Projections of the translation zones of the ventral uvula and nodulus

Previous electrophysiological studies in pigeons have shown that the complex spike activity of Purkinje cells in the medial vestibulocerebellum (nodulus and ventral uvula) is modulated by patterns of optic flow that result from self-translation along a particular axis in three-dimensional space. There are four response types based on the axis of preferred translational optic flow. By using a three axis system, where +X, +Y, and +Z represent rightward, upward, and forward self-motion, respectively, the four cell types are t(+Y), t(-Y), t(-X-Z), and t(-X+Z), with the assumption of recording from the left side of the head. These response types are organized into parasagittal zones. In this study, we injected the anterograde tracer biotinylated dextran amine into physiologically identified zones. The t(-X-Z) zone projected dorsally within the vestibulocerebellar process (pcv) on the border with the medial cerebellar nucleus (CbM), and labeling was found in the CbM itself. The t(-X+Z) zone also projected to the pcv and CbM, but to areas ventral to the projection sites of the t(-X-Z) zone. The t(-Y) zone also projected to the pcv, but more ventrally on the border with the superior vestibular nucleus (VeS). Some labeling was also found in the dorsal VeS and the dorsolateral margin of the caudal descending vestibular nucleus, and a small amount of labeling was found laterally in the caudal margin of the medial vestibular nucleus. The data set was insufficient to draw conclusions about the projection of the t(+Y) zone. These results are contrasted with the projections of the flocculus, compared with the primary vestibular projection, and implications for collimotor function are discussed.

Central vestibular system: vestibular nuclei and posterior cerebellum

The vestibular nuclei and posterior cerebellum are the destination of vestibular primary afferents and the subject of this review. The vestibular nuclei include four major nuclei (medial, descending, superior and lateral). In addition, smaller vestibular nuclei include: Y-group, parasolitary nucleus, and nucleus intercalatus. Each of the major nuclei can be subdivided further based primarily on cytological and immunohistochemical histological criteria or differences in afferent and/or efferent projections. The primary afferent projections of vestibular end organs are distributed to several ipsilateral vestibular nuclei. Vestibular nuclei communicate bilaterally through a commissural system that is predominantly inhibitory. Secondary vestibular neurons also receive convergent sensory information from optokinetic circuitry, central visual system and neck proprioceptive systems. Secondary vestibular neurons cannot distinguish between sources of afferent activity. However, the discharge of secondary vestibular neurons can distinguish between "active" and "passive" movements. The posterior cerebellum has extensive afferent and efferent connections with vestibular nuclei. Vestibular primary afferents are distributed to the ipsilateral uvula-nodulus as mossy fibers. Vestibular secondary afferents are distributed bilaterally. Climbing fibers to the cerebellum originate from two subnuclei of the contralateral inferior olive; the dorsomedial cell column and beta-nucleus. Vestibular climbing fibers carry information only from the vertical semicircular canals and otoliths. They establish a coordinate map, arrayed in sagittal zones on the surface of the uvula-nodulus. Purkinje cells respond to vestibular stimulation with antiphasic modulation of climbing fiber responses (CFRs) and simple spikes (SSs). The modulation of SSs is out of phase with the modulation of vestibular primary afferents. Modulation of SSs persists, even after vestibular primary afferents are destroyed by a unilateral labyrinthectomy, suggesting that an interneuronal network, triggered by CFRs is responsible for SS modulation. The vestibulo-cerebellum, imposes a vestibular coordinate system on postural responses and permits adaptive guidance of movement.

Zonal organization of the vestibulocerebellum in pigeons (Columba livia): II. Projections of the rotation zones of the flocculus

Previous neurophysiologic research in birds and mammals has shown that there are two types of Purkinje cells in the flocculus. The first type shows maximal modulation in response to rotational optokinetic stimulation about the vertical axis (rVA neurons). The second type shows maximal modulation in response to rotational optokinetic stimulation about a horizontal axis oriented 45 degrees to contralateral azimuth (rH45c neurons). In pigeons, the rVA and rH45c are organized into four alternating parasagittal zones. In this study we investigated the projections of Purkinje cells in the rVA and rH45c zones by using the anterograde tracers biotinylated dextran amine and cholera toxin subunit B. After iontophoretic injections of tracers into the rH45c zones, heavy anterograde labeling was found in the infracerebellar nucleus and the medial margin of the superior vestibular nucleus. Some labeling was also consistently observed in the lateral cerebellar nucleus and the dorsolateral vestibular nucleus. After injections into the rVA zones, heavy anterograde labeling was found in the medial and descending vestibular nuclei, the nucleus prepositus hypoglossi, and the central region of the superior vestibular nucleus. Less labeling was seen in the tangential nucleus, the dorsolateral vestibular nucleus, and the lateral vestibular nucleus, pars ventralis. These results are compared and contrasted with findings in mammalian species.