Somatotopical organisation within the climbing fibre projection to the paramedian lobule and copula pyramidis of the rat cerebellum

Modulation of neural spiking in motor cortex-cerebellar networks during sleep spindles

Sleep spindles appear to play an important role in learning new motor skills. Motor skill learning engages several regions in the brain with two important areas being the motor cortex (M1) and the cerebellum. However, the neurophysiological processes in these areas during sleep, especially how spindle oscillations affect local and cross-region spiking, are not fully understood. We recorded activity from the M1 and cerebellar cortex in 8 rats during spontaneous activity to investigate how sleep spindles in these regions are related to local spiking as well as cross-region spiking. We found that M1 firing was significantly changed during both M1 and cerebellum spindles and this spiking occurred at a preferred phase of the spindle. On average, M1 and cerebellum neurons showed most spiking at the M1 or cerebellum spindle peaks. These neurons also developed a preferential phase-locking to local or cross-area spindles with the greatest phase-locking value at spindle peaks; however, this preferential phase-locking wasn't significant for cerebellar neurons when compared to cerebellum spindles. Additionally, we found the percentage of task-modulated cells in the M1 and cerebellum that fired with non-uniform spike-phase distribution during M1/ cerebellum spindle peaks were greater in the rats that learned a reach-to-grasp motor task robustly. Finally, we found that spindle-band LFP coherence (for M1 and cerebellum LFPs) showed a positive correlation with success rate in the motor task. These findings support the idea that sleep spindles in both the M1 and cerebellum recruit neurons that participate in the awake task to support motor memory consolidation.Significance Statement Neural processing during sleep spindles is linked to memory consolidation. However, little is known about sleep activity in the cerebellum and whether cerebellum spindles can affect spiking activity in local or distant areas. We report the effect of sleep spindles on neuron activity in the M1 and cerebellum-specifically their firing rate and phase-locking to spindle oscillations. Our results indicate that awake practice neuronal activity is tempered during local M1 and cerebellum spindles, and during cross-region spindles, which may support motor skill learning. We describe spiking dynamics in motor networks spindle oscillations that may aid in the learning of skills. Our results support the sleep reactivation hypothesis and suggest that awake M1 activity may be reactivated during cerebellum spindles.

Cortico-cerebellar coordination facilitates neuroprosthetic control

Temporally coordinated neural activity is central to nervous system function and purposeful behavior. Still, there is a paucity of evidence demonstrating how this coordinated activity within cortical and subcortical regions governs behavior. We investigated this between the primary motor (M1) and contralateral cerebellar cortex as rats learned a neuroprosthetic/brain-machine interface (BMI) task. In neuroprosthetic task, actuator movements are causally linked to M1 "direct" neurons that drive the decoder for successful task execution. However, it is unknown how task-related M1 activity interacts with the cerebellum. We observed a notable 3 to 6 hertz coherence that emerged between these regions' local field potentials (LFPs) with learning that also modulated task-related spiking. We identified robust task-related indirect modulation in the cerebellum, which developed a preferential relationship with M1 task-related activity. Inhibiting cerebellar cortical and deep nuclei activity through optogenetics led to performance impairments in M1-driven neuroprosthetic control. Together, these results demonstrate that cerebellar influence is necessary for M1-driven neuroprosthetic control.

Emergent Low-Frequency Activity in Cortico-Cerebellar Networks with Motor Skill Learning

The motor cortex controls skilled arm movement by recruiting a variety of targets in the nervous system, and it is important to understand the emergent activity in these regions as refinement of a motor skill occurs. One fundamental projection of the motor cortex (M1) is to the cerebellum. However, the emergent activity in the motor cortex and the cerebellum that appears as a dexterous motor skill is consolidated is incompletely understood. Here, we report on low-frequency oscillatory (LFO) activity that emerges in cortico-cerebellar networks with learning the reach-to-grasp motor skill. We chronically recorded the motor and the cerebellar cortices in rats, which revealed the emergence of coordinated movement-related activity in the local-field potentials as the reaching skill consolidated. Interestingly, we found this emergent activity only in the rats that gained expertise in the task. We found that the local and cross-area spiking activity was coordinated with LFOs in proficient rats. Finally, we also found that these neural dynamics were more prominently expressed during accurate behavior in the M1. This work furthers our understanding on emergent dynamics in the cortico-cerebellar loop that underlie learning and execution of precise skilled movement.

Neuronal activity patterns in microcircuits of the cerebellar cortical C3 zone during reaching

The cerebellum is the largest sensorimotor structure in the brain. A fundamental organizational feature of its cortex is its division into a series of rostrocaudally elongated zones. These are defined by their inputs from specific parts of the inferior olive and Purkinje cell output to specific cerebellar and vestibular nuclei. However, little is known about how patterns of neuronal activity in zones, and their microcircuit subdivisions, microzones, are related to behaviour in awake animals. In the present study, we investigated the organization of microzones within the C3 zone and their activity during a skilled forelimb reaching task in cats. Neurons in different microzones of the C3 zone, functionally determined by receptive field characteristics, differed in their patterns of activity during movement. Groups of Purkinje cells belonging to different receptive field classes, and therefore belonging to different microzones, were found to collectively encode different aspects of the reach controlled by the C3 zone. Our results support the hypothesis that the cerebellar C3 zone is organized and operates within a microzonal frame of reference, with a specific relationship between the sensory input to each microzone and its motor output. KEY POINTS: A defining feature of cerebellar organization is its division into a series of zones and smaller subunits termed microzones. Much of how zones and microzones are organized has been determined in anaesthetized preparations, and little is known about their function in awake animals. We recorded from neurons in the forelimb part of the C3 zone 'in action' by recording from single cerebellar cortical neurons located in different microzones defined by their peripheral receptive field properties during a forelimb reach-retrieval task in cats. Neurons from individual microzones had characteristic patterns of activity during movement, indicating that function is organized in relation to microcomplexes.

Sensory and motor electrophysiological mapping of the cerebellum in humans

Cerebellar damage during posterior fossa surgery in children can lead to ataxia and risk of cerebellar mutism syndrome. Compartmentalisation of sensorimotor and cognitive functions within the cerebellum have been demonstrated in animal electrophysiology and human imaging studies. Electrophysiological monitoring was carried out under general anaesthesia to assess the limb sensorimotor representation within the human cerebellum for assessment of neurophysiological integrity to reduce the incidence of surgical morbidities. Thirteen adult and paediatric patients undergoing posterior fossa surgery were recruited. Sensory evoked field potentials were recorded in response to mapping (n = 8) to electrical stimulation of limb nerves or muscles. For motor mapping (n = 5), electrical stimulation was applied to the surface of the cerebellum and evoked EMG responses were sought in facial and limb muscles. Sensory evoked potentials were found in two patients (25%). Responses were located on the surface of the right inferior posterior cerebellum to stimulation of the right leg in one patient, and on the left inferior posterior lobe in another patient to stimulation of left forearm. No evoked EMG responses were found for the motor mapping. The present study identifies challenges with using neurophysiological methods to map functional organization within the human cerebellum and considers ways to improve success.

Motor learning rapidly increases synaptogenesis and astrocytic structural plasticity in the rat cerebellum

Motor-skill learning is associated with cerebellar synaptogenesis and astrocytic hypertrophy, but most of these assessments of cerebellar ultrastructure have been completed after one month of training. After one month of training, the motor-skills necessary to complete these tasks have been acquired for weeks. This experiment aimed to characterize cerebellar ultrastructure during the acquisition phase of motor-skill learning, at a point when performance is still improving. Male and female rats trained for four days on the acrobatic motor learning task, which involved traversing challenging obstacles such as narrow beams and ladders. Concurrently, rats in the motor control condition walked a flat alleyway requiring no skilled movements. After training was complete, all rats were euthanized, and tissue was prepared for electron microscopy. Unbiased stereology techniques were used to assess synaptic and astrocytic plasticity. Results indicated that during the initial days of training, female rats made fewer errors and had shorter latencies on the acrobatic course compared to male rats. However, there were no sex differences in cerebellar ultrastructure. Male and female rats that completed four days of acrobatic training displayed an increase in the density of parallel fiber-Purkinje cell synapses per Purkinje cell and an increase in astrocytic volume, relative to rats in the motor control group.

Sensorimotor, language, and working memory representation within the human cerebellum

The cerebellum is involved in a wide range of behaviours. A key organisational principle from animal studies is that somatotopically corresponding sensory input and motor output reside in the same cerebellar cortical areas. However, compelling evidence for a similar arrangement in humans and whether it extends to cognitive functions is lacking. To address this, we applied cerebellar optimised whole‐brain functional MRI in 20 healthy subjects. To assess spatial overlap within the sensorimotor and cognitive domains, we recorded activity to a sensory stimulus (vibrotactile) and a motor task; the Sternberg verbal working memory (VWM) task; and a verb generation paradigm. Consistent with animal data, sensory and motor activity overlapped with a somatotopic arrangement in ipsilateral areas of the anterior and posterior cerebellum. During the maintenance phase of the Sternberg task, a positive linear relationship between VWM load and activity was observed in right Lobule VI, extending into Crus I bilaterally. Articulatory movement gave rise to bilateral activity in medial Lobule VI. A conjunction of two independent language tasks localised activity during verb generation in right Lobule VI‐Crus I, which overlapped with activity during VWM. These results demonstrate spatial compartmentalisation of sensorimotor and cognitive function in the human cerebellum, with each area involved in more than one aspect of a given behaviour, consistent with an integrative function. Sensorimotor localisation was uniform across individuals, but the representation of cognitive tasks was more variable, highlighting the importance of individual scans for mapping higher order functions within the cerebellum.

The entire trajectories of single pontocerebellar axons and their lobular and longitudinal terminal distribution patterns in multiple aldolase C-positive compartments of the rat cerebellar cortex

The mammalian cerebellar cortex is compartmentalized, both anatomically and histochemically, into multiple parasagittal bands. To characterize the multiple zonal patterns of pontocerebellar mossy fiber projection, single neurons in the basilar pontine nucleus (BPN) were labeled by injecting biotinylated dextran amine into the BPN, and the entire axonal trajectory of single labeled neurons (n = 25) was reconstructed in relation to aldolase C compartments of Purkinje cells in rats. Single pontocerebellar axons, after passing through the contralateral middle cerebellar peduncle, ran transversely in the deep cerebellar white matter toward and often across the midline, and on their ways, gave rise to 2-10 primary collaterals at almost right angles in specific lobules only contralaterally or bilaterally with contralateral predominance. Each primary collateral further branched in a parasagittal plane to form a strip-shaped longitudinal termination zone with rosette-type swellings clustered in aldolase C-positive compartments in a single or multiple lobules, mainly in compartment 4+//5+, 5+//6+, and 6+//7+. Axons arising from the central, rostral, and lateral part of the BPN projected with multiple branches, mainly to simple lobule, crus II and paramedian lobule, to crus I and dorsal paraflocculus, and to ventral paraflocculus and lobule IXc, respectively. The results showed the pontocerebellar projection is closely related to lobular and compartmental organization of the cerebellum. A comparison of single axon morphologies of different mossy fiber systems indicates that the projection pattern of single pontocerebellar neurons with multiple collaterals innervating different longitudinal compartments arranged in a mediolateral direction represents a general feature of mossy fiber projection.

Spino-olivary projections in the rat are anatomically separate from postsynaptic dorsal column projections

The gracile nucleus (GN) and lateral part of rostral dorsal accessory olive (rDAO) are important relays for indirect, postsynaptic dorsal column, and direct ascending pathways, respectively, that terminate as climbing fibers in the “hindlimb-receiving” parts of the C1 and C3 zones in the cerebellar cortex. While the spinal cells of origin of that project to GN and rDAO are from largely separate territories in the spinal cord, previous studies have indicated that there could be an area of overlap between these two populations in the medial dorsal horn. Given the access of these two ascending tracts to sensory (thalamic) versus sensorimotor (precerebellar) pathways, the present study therefore addresses the important question of whether or not individual neurons have the potential to contribute axons to both ascending pathways. A double-fluorescent tracer strategy was used in rats (red Retrobeads and Fluoro-Ruby or green Retrobeads and Fluoro-Emerald) to map the spatial distribution of cells of origin of the two projections in the lumbar spinal cord. The two pathways were found to receive input from almost entirely separate territories within the lumbar cord (levels L3–L5). GN predominantly receives input from lamina IV, while rDAO receives its input from three cell populations: medial laminae V–VI, lateral lamina V, and medial laminae VII–VIII. Cells that had axons that branched to supply both GN and rDAO represented only about 1% of either single-labeled cell population. Overall, the findings therefore suggest functional independence of the two ascending pathways. J. Comp. Neurol. 522:2179–2190, 2014. © 2013 Wiley Periodicals, Inc.

Systematic regional variations in Purkinje cell spiking patterns

In contrast to the uniform anatomy of the cerebellar cortex, molecular and physiological studies indicate that significant differences exist between cortical regions, suggesting that the spiking activity of Purkinje cells (PCs) in different regions could also show distinct characteristics. To investigate this possibility we obtained extracellular recordings from PCs in different zebrin bands in crus IIa and vermis lobules VIII and IX in anesthetized rats in order to compare PC firing characteristics between zebrin positive (Z+) and negative (Z-) bands. In addition, we analyzed recordings from PCs in the A2 and C1 zones of several lobules in the posterior lobe, which largely contain Z+ and Z- PCs, respectively. In both datasets significant differences in simple spike (SS) activity were observed between cortical regions. Specifically, Z- and C1 PCs had higher SS firing rates than Z+ and A2 PCs, respectively. The irregularity of SS firing (as assessed by measures of interspike interval distribution) was greater in Z+ bands in both absolute and relative terms. The results regarding systematic variations in complex spike (CS) activity were less consistent, suggesting that while real differences can exist, they may be sensitive to other factors than the cortical location of the PC. However, differences in the interactions between SSs and CSs, including the post-CS pause in SSs and post-pause modulation of SSs, were also consistently observed between bands. Similar, though less strong trends were observed in the zonal recordings. These systematic variations in spontaneous firing characteristics of PCs between zebrin bands in vivo, raises the possibility that fundamental differences in information encoding exist between cerebellar cortical regions.

Spatial localization and projection densities of brainstem mossy fibre afferents to the forelimb C1 zone of the rat cerebellum

The present study uses a double retrograde tracer technique in rats to examine the spatial localization and pattern of axonal branching in mossy fibres arising from three major sources in the medulla-the external cuneate nucleus, the sensory trigeminal nucleus and the reticular formation, to two electrophysiologically-identified parts of the cerebellar cortex that are linked by common climbing fibre input - the forelimb-receiving parts of the C1 zone in lobulus simplex and the paramedian lobule. In each experiment a small injection of rhodamine-tagged beads was injected into one cortical region and an injection of fluorescein-tagged beads was injected into the other region. The main findings were: (i) the proportion of double-labelled cells in each of the three precerebeller sources of mossy fibres was positively correlated with those in the inferior olive; and (ii) the C1 zone in lobulus simplex was found to receive a greater density of projections from all three sources of mossy fibres than the C1 zone in the paramedian lobule. These data suggest that two rostrocaudally separated but somatotopically corresponding parts of the C1 zone receive common mossy fibre and climbing fibre inputs. However, the differences in projection densities also suggest that the two parts of the zone differ in the extent to which they receive mossy fibre signals arising from the same precerebellar nuclei. This implies differences in function between somatotopically corresponding parts of the same cortical zone, and could enable a higher degree of parallel processing and integration of information within them.

Ins and outs of cerebellar modules

The modular concept of cerebellar connections has been advocated in the lifetime work of Jan Voogd. In this concept, a cerebellar module is defined as the conglomerate of one or multiple and non-adjacent, parasagittally arranged zones of Purkinje cells, their specific projection to a well-defined region of the cerebellar nuclei, and the climbing fiber input to these zones by a well-defined region of the inferior olivary complex. The modular organization of these olivo-cortico-nuclear connections is further exemplified by matching reciprocal connections between inferior olive and cerebellar nuclei. Because the different regions of the cerebellar nuclei show highly specific output patterns, cerebellar modules have been suggested to constitute functional entities. This idea is strengthened by the observation that anatomically defined modules adhere to the distribution of chemical markers in the cerebellar cortex suggesting that modules not only differ in their input and output relations but also may differ in operational capabilities. Here, I will briefly review some recent data on the establishment of cerebellar modules in rats. Furthermore, some evidence will be shown suggesting that the other main afferent system (i.e., mossy fibers), at least to some extent, also adheres to the modular organization. Finally, using retrograde transneuronal tracing with rabies virus, some evidence will be provided that several cerebellar modules may be involved in the control of individual muscles.

Projection patterns of single mossy fiber axons originating from the dorsal column nuclei mapped on the aldolase C compartments in the rat cerebellar cortex

Although cerebellar mossy fibers are the most abundant cerebellar afferents and are deeply involved in cerebellar function, the organization of their projection has remained obscure, particularly in relation to cerebellar compartmentalization. The dorsal column nuclei (DCN) are a major source of cerebellar mossy fibers and possess distinct somatotopic representations of specific somatosensory submodalities. We reconstructed individual dextran-labeled DCN axons completely from serial sections and mapped their terminals on the longitudinal cerebellar compartments that were visualized by aldolase C immunostaining to clarify their projection pattern. Individual axons branched and formed about 100 rosette terminals in the cerebellar cortex, but infrequently projected to the cerebellar nuclei (1 out of 15 axons). Cortical terminals were clustered in multiple areas in the vermis and pars intermedia mostly, but not exclusively, ipsilateral to the origin of the axon. The gracile, cuneate, and external cuneate nuclei (ECuN) mainly projected to the copula pyramidis and lobule V, paramedian and simple lobules, and lobules I-V and VIII-IX, respectively, although there was some overlap. The majority of terminals were located within aldolase C negative or lightly positive compartments. However, terminals of a single axon can be located on aldolase C-negative as well as on aldolase C-positive compartments. In particular, the rostral ECuN, which is responsive to shoulder movements, projected consistently to lobule IX, which were mostly aldolase C-positive. In sum, DCN-cerebellar axons project to multiple compartments with terminals clustered mainly in the conventional spinocerebellar region with a coarse topography, which shows some relationship to the cortical compartments defined by aldolase C.

Current source density correlates of cerebellar Golgi and Purkinje cell responses to tactile input

The overall circuitry of the cerebellar cortex has been known for over a century, but the function of many synaptic connections remains poorly characterized in vivo. We used a one-dimensional multielectrode probe to estimate the current source density (CSD) of Crus IIa in response to perioral tactile stimuli in anesthetized rats and to correlate current sinks and sources to changes in the spike rate of corecorded Golgi and Purkinje cells. The punctate stimuli evoked two distinct early waves of excitation (at <10 and ∼ 20 ms) associated with current sinks in the granular layer. The second wave was putatively of corticopontine origin, and its associated sink was located higher in the granular layer than the first trigeminal sink. The distinctive patterns of granular-layer sinks correlated with the spike responses of corecorded Golgi cells. In general, Golgi cell spike responses could be linearly reconstructed from the CSD profile. A dip in simple-spike activity of coregistered Purkinje cells correlated with a current source deep in the molecular layer, probably generated by basket cell synapses, interspersed between sparse early sinks presumably generated by synapses from granule cells. The late (>30 ms) enhancement of simple-spike activity in Purkinje cells was characterized by the absence of simultaneous sinks in the granular layer and by the suppression of corecorded Golgi cell activity, pointing at inhibition of Golgi cells by Purkinje axon collaterals as a likely mechanism of late Purkinje cell excitation.

Electrophysiological characterization of the cerebellum in the arterially perfused hindbrain and upper body of the rat

In the present study, a non-pulsatile arterially perfused hindbrain and upper body rat preparation is described which is an extension of the brainstem preparation reported by Potts et al., (Brain Res Bull 53(1):59-67), 1. The modified in situ preparation allows study of cerebellar function whilst preserving the integrity of many of its interconnections with the brainstem, upper spinal cord and the peripheral nervous system of the head and forelimbs. Evoked mossy fibre, climbing fibre and parallel fibre field potentials and EMG activity elicited in forelimb biceps muscle by interpositus stimulation provided evidence that both cerebellar inputs and outputs remain operational in this preparation. Similarly, the spontaneous and evoked single unit activity of Purkinje cells, putative Golgi cells, molecular interneurones and cerebellar nuclear neurones was similar to activity patterns reported in vivo. The advantages of the preparation include the ability to record, without the complications of anaesthesia, stabile single unit activity for extended periods (3 h or more), from regions of the rat cerebellum that are difficult to access in vivo. The preparation should therefore be a useful adjunct to in vitro and in vivo studies of neural circuits underlying cerebellar contributions to movement control and motor learning.

Mechanisms of synchronous activity in cerebellar Purkinje cells

Complex spike synchrony is thought to be a key feature of how inferior olive climbing fibre afferents make their vital contribution to cerebellar function. However, little is known about whether the other major cerebellar input, the mossy fibres (which generate simple spikes within Purkinje cells, PCs), exhibit a similar synchrony in impulse timing. We have used a multi-microelectrode system to record simultaneously from two or more PCs in the posterior lobe of the ketamine/xylazine-anaesthetized rat to examine the relationship between complex spike and simple spike synchrony in PC pairs located mainly in the A2 and C1 zones in crus II and the paramedian lobule. PC pairs displaying correlations in the occurrence of their complex spikes (coupled PCs) were usually located in the same zone and were also more likely to exhibit correlations in the timing of their spontaneous simple spikes and associated pauses in activity. In coupled PCs, synchrony in both complex spike and simple spike activity was enhanced and the relative timing in the occurrence of complex spikes could be altered by peripheral stimulation. We conclude that the functional coupling between PC pairs in their complex spike and simple spike activity can be significantly modified by sensory inputs, and that mechanisms besides electrotonic coupling are involved in generating PC synchrony. Synchronous activity in multiple PCs converging onto the same cerebellar nuclear cells is likely to have a significant impact on cerebellar output that could form important timing signals to orchestrate coordinated movements.

Cerebellar cortex contributions to the expression and timing of conditioned eyelid responses

We used micro-infusions during eyelid conditioning in rabbits to investigate the relative contributions of cerebellar cortex and the underlying deep nuclei (DCN) to the expression of cerebellar learning. These tests were conducted using two forms of cerebellum-dependent eyelid conditioning for which the relative roles of cerebellar cortex and DCN are controversial: delay conditioning, which is largely unaffected by forebrain lesions, and trace conditioning, which involves interactions between forebrain and cerebellum. For rabbits trained with delay conditioning, silencing cerebellar cortex by micro-infusions of the local anesthetic lidocaine unmasked stereotyped short-latency responses. This was also the case after extinction as observed previously with reversible blockade of cerebellar cortex output. Conversely, increasing cerebellar cortex activity by micro-infusions of the GABA(A) antagonist picrotoxin reversibly abolished conditioned responses. Effective cannula placements were clustered around the primary fissure and deeper in lobules hemispheric lobule IV (HIV) and hemispheric lobule V (HV) of anterior lobe. In well-trained trace conditioned rabbits, silencing this same area of cerebellar cortex or reversibly blocking cerebellar cortex output also unmasked short-latency responses. Because Purkinje cells are the sole output of cerebellar cortex, these results provide evidence that the expression of well-timed conditioned responses requires a well-timed decrease in the activity of Purkinje cells in anterior lobe. The parallels between results from delay and trace conditioning suggest similar contributions of plasticity in cerebellar cortex and DCN in both instances.

The periaqueductal grey modulates sensory input to the cerebellum: a role in coping behaviour?

The paths that link the periaqueductal grey (PAG) to hindbrain motor circuits underlying changes in behavioural responsiveness to external stimuli are unknown. A major candidate structure for mediating these effects is the cerebellum. The present experiments test this directly by monitoring changes in size of cerebellar responses evoked by peripheral stimuli following activation of the PAG. In 22 anaesthetized adult Wistar rats, climbing fibre field potentials were recorded from the C1 zone in the paramedian lobule and the copula pyramidis of the cerebellar cortex evoked, respectively, by electrical stimulation of the ipsilateral fore- and hindlimb. An initial and a late response were attributable to activation of Abeta and Adelta peripheral afferents respectively (hindlimb onset latencies 16.9 and 23.8 ms). Chemical stimulation at physiologically-identified sites in the ventrolateral PAG (a region known to be associated with hyporeactive immobility) resulted in a significant reduction in size of both the Abeta and Adelta evoked field potentials (mean reduction relative to control +/- SEM, 59 +/- 7.5 and 66 +/- 11.9% respectively). Responses evoked by electrical stimulation of the dorsal or ventral funiculus of the spinal cord were also reduced by PAG stimulation, suggesting that part of the modulation may occur at supraspinal sites (including at the level of the inferior olive). Overall, the results provide novel evidence of descending control into motor control centres, and provide the basis for future studies into the role of the PAG in regulating motor activity in different behavioural states and in chronic pain.

A light microscope-based double retrograde tracer strategy to chart central neuronal connections

This protocol describes a double retrograde tracing method to chart divergent projections in the CNS using light microscope techniques. It is based on immunohistochemical visualization of retrograde transport of cholera toxin b-subunit (CTb) and silver enhancement of a gold-lectin conjugate. Production of the gold-lectin is explained in detail, and a technique is offered to record through the injection pipettes, to help guide accurate placement of injections. Visualization of the two tracers results in light brown staining of CTb-labeled neurons and labeling by black particles of gold-lectin-containing neurons. Both types of label are easily recognized in the same neuron. The labeling is permanent and is well suited for studies in which large areas of the brain need to be surveyed. The whole procedure (excluding survival time) takes approximately 5-7 d to complete.

Topographical organization of pathways from somatosensory cortex through the pontine nuclei to tactile regions of the rat cerebellar hemispheres

The granule cell layer of the cerebellar hemispheres contains a patchy and noncontinuous map of the body surface, consisting of a complex mosaic of multiple perioral tactile representations. Previous physiological studies have shown that cerebrocerebellar mossy fibre projections, conveyed through the pontine nuclei, are mapped in registration with peripheral tactile projections to the cerebellum. In contrast to the fractured cerebellar map, the primary somatosensory cortex (SI) is somatotopically organized. To understand better the map transformation occurring in cerebrocerebellar pathways, we injected axonal tracers in electrophysiologically defined locations in Sprague-Dawley rat folium crus IIa, and mapped the distribution of retrogradely labelled neurons within the pontine nuclei using three-dimensional (3-D) reconstructions. Tracer injections within the large central upper lip patch in crus IIa-labelled neurons located centrally in the pontine nuclei, primarily contralateral to the injected side. Larger injections (covering multiple crus IIa perioral representations) resulted in labelling extending only slightly beyond this region, with a higher density and more ipsilaterally labelled neurons. Combined axonal tracer injections in upper lip representations in SI and crus IIa, revealed a close spatial correspondence between the cerebropontine terminal fields and the crus IIa projecting neurons. Finally, comparisons with previously published three-dimensional distributions of pontine neurons labelled following tracer injections in face receiving regions in the paramedian lobule (downloaded from http://www.rbwb.org) revealed similar correspondence. The present data support the coherent topographical organization of cerebro-ponto-cerebellar networks previously suggested from physiological studies. We discuss the present findings in the context of transformations from cerebral somatotopic to cerebellar fractured tactile representations.

A novel site of synaptic relay for climbing fibre pathways relaying signals from the motor cortex to the cerebellar cortical C1 zone

The climbing fibre projection from the motor cortex to the cerebellar cortical C1 zone in the posterior lobe of the rat cerebellum was investigated using a combination of physiological, anatomical and neuropharmacological techniques. Electrical stimulation of the ipsilateral fore- or hindimbs or somatotopically corresponding parts of the contralateral motor cortex evoked climbing fibre field potentials at the same cerebellar recording sites. Forelimb-related responses were located in the C1 zone in the paramedian lobule or lobulus simplex and hindlimb-related responses were located in the C1 zone in the copula pyramidis. Microinjections of anterograde axonal tracer (Fluoro-Ruby or Fluoro-Emerald) were made into the fore- or hindlimb parts of the motor cortex where stimulation evoked the largest cerebellar responses. After a survival period of 7-10 days, the neuraxis was examined for anterograde labelling. No terminal labelling was ever found in the inferior olive, but labelled terminals were consistently found in a well-localized site in the dorso-medial medulla, ventral to the gracile nucleus, termed the matrix region. Pharmacological inactivation of the matrix region (2 mm caudal to the obex) selectively reduced transmission in descending (cerebro-olivocerebellar) but not ascending (spino-olivocerebellar) paths targeting fore- or hindlimb-receiving parts of the C1 zone. Transmission in spino-olivocerebellar paths was either unaffected, or in some cases increased. The identification of a novel pre-olivary relay in cerebro-olivocerebellar paths originating from fore- and hindlimb motor cortex has implications for the regulation of transmission in climbing fibre pathways during voluntary movements and motor learning.

Organization of pontocerebellar projections to identified climbing fiber zones in the rat

The organization of pontocerebellar projections to the paravermis and hemisphere of the posterior cerebellum of the rat was studied in relation to the organization of climbing fibers. Small injections of cholera toxin subunit B were placed in the cerebellar cortex at locations predetermined by evoked climbing fiber potentials from selected body parts or based on coordinates. The injection site was characterized with respect to the zebrin pattern and by the distribution of retrogradely labeled neurons in the inferior olive. The following zones were studied: hindlimb-related zones C1 and C2 of lobule VIII; forelimb-related zones C1, C2, and D0/D1 of the paramedian lobule; and face-related zones A2 of the paramedian lobule and C2 and D0 of crus 2B. The results show that the distribution of pontine neurons is closely related to the climbing fiber somatotopy. Injections centered on face-related zones result in distribution of pontine neurons within the pontine core region. Forelimb regions surround this core, whereas hindlimb regions are mostly supplied by caudal pontine regions and by a single patch of more rostrally located neurons. This distribution fits well with published data on the somatotopy of the corticopontine projection from the rat primary somatosensory cortex. However, apart from differences in the participation of ipsilaterally projecting cells, the distribution of pontine neurons does not change significantly when the injection covers different zones of the same lobule such as C1 and C2 of lobule VIII; C1, C2, and D0/D1 of the paramedian lobule; A2 of the paramedian lobule; and C2 and D0 of crus 2B.

Post-lesion transcommissural olivocerebellar reinnervation improves motor function following unilateral pedunculotomy in the neonatal rat

In the adult mammalian central nervous system, reinnervation and recovery from trauma are limited. During development, however, post-lesion plasticity may generate alternate paths providing models to investigate reinnervation and repair. Sometimes, these paths are maladaptive, although the relationship between dysfunction and anatomical abnormality remains unknown. After unilateral transection of the neonatal rat olivocerebellar path (pedunculotomy), axons from the remaining inferior olive reinnervate Purkinje cells in the denervated hemicerebellum with appropriate topography and synaptic function. However, whether this new pathway confers beneficial behavioural effects remains unknown. We studied the behavioural sequelae in rats with and without transcommissural reinnervation using righting and vestibular-drop reflexes, simple locomotion (bridge), complex locomotion (wire) and motor coordination (rotarod) tests. In animals pedunculotomised on day 3 (Px3), which develop olivocerebellar reinnervation, dynamic postural adjustments and complex motor skills develop normally, whereas simple gait is broad-based and slightly delayed. In contrast, Px11 animals, which do not develop reinnervation, have delayed maturation of postural reflexes, gait and complex locomotor skills. In addition, when compared to control animals, their performance in locomotory tasks was slower and the complex task impaired. On the rotarod, control and Px3 animals learned to coordinate their gait and walked for longer at 10 and 20 rpm than Px11 animals. These results show that transcommissural olivocerebellar reinnervation is associated with almost normal motor development and the ability to synchronise gait at slow and moderate speeds, i.e. this reinnervation confers significant behavioural function and is therefore truly compensatory.

Differential effects oftrans-crotononitrile and 3-acetylpyridine on inferior olive integrity and behavioural performance in the rat

The inferior olive climbing fibre projection is key to cerebellar contributions to motor control. Here we present evidence for a novel tool, trans-crotononitrile (TCN), to selectively inactivate the olive to study its functions. Anatomical, electrophysiological and behavioural techniques have been used in rats to assess the CNS effects of TCN, with a focus on the olivocerebellar projection. These findings were compared with those obtained with 3-acetylpyridine (plus nicotinamide administered 3.5 h later, 3AP + 3.5 h). Fluoro-Jade B cell labelling showed that TCN and 3AP + 3.5 h induce neurodegeneration primarily within the inferior olive, with no other targets in common. Recordings of evoked field potentials on the cerebellar cortical surface showed that both neurotoxins can reduce transmission in climbing fibre but not mossy fibre pathways. Both histological and electrophysiological differences suggest that TCN and 3AP have distinct mechanisms of action. Estimates of the numbers of surviving cells within individual subdivisions of the olive indicate that TCN and 3AP + 3.5 h cause different patterns of subtotal olivary lesion: most surviving neurons are present in the rostral (TCN) or caudal (3AP + 3.5 h) parts of the medial accessory olive, which are associated with two different cerebellar modules: the C2 and A modules, respectively. In behavioural studies, TCN and 3AP + 3.5 h produced differences in motor deficits consistent with the notion that these cerebellar modules have distinct functional responsibilities. Thus, studies using TCN as compared with 3AP + 3.5 h have the potential to shed light on the contributions of different cerebellar modules in motor control.

Anatomical and physiological foundations of cerebellar information processing

A coordinated movement is easy to recognize, but we know little about how it is achieved. In search of the neural basis of coordination, we present a model of spinocerebellar interactions in which the structure-functional organizing principle is a division of the cerebellum into discrete microcomplexes. Each microcomplex is the recipient of a specific motor error signal - that is, a signal that conveys information about an inappropriate movement. These signals are encoded by spinal reflex circuits and conveyed to the cerebellar cortex through climbing fibre afferents. This organization reveals salient features of cerebellar information processing, but also highlights the importance of systems level analysis for a fuller understanding of the neural mechanisms that underlie behaviour.

Axonal ramification of neurons in the nucleus reticularis tegmenti pontis projecting to the paramedian lobule in the rabbit cerebellum

Projections of the nucleus reticularis tegmenti pontis (NRTP) to the cerebellar paramedian lobule were examined in the rabbit by means of the double fluorescent retrograde tract-tracing method. The rabbit NRTP is composed of a medial, large part comprising zones A (dorsomedial), B (central) and C (lateral), and of a lateral, small part (the processus tegmentosus lateralis; PTL). Following unilateral injections of Fast Blue (FB) into the rostral part of the paramedian lobule (rPML) and of Diamidino Yellow (DY) into the caudal part (cPML), known to receive spinal inputs from forelimb and hindlimb, respectively, substantial numbers of single labeled neurons were found in all bilateral NRTP divisions, apart from the zone C. Most projection neurons to the PML were located in the medial and medioventral regions of the zone B. Smaller numbers of projection neurons were located in the PTL, zone A and outside the zone B among fibers of the medial lemniscus. The pattern of FB and DY labeling suggested that neurons projecting to the rPML and cPML originated in common rather than separate regions within the NRTP. In addition, a small percentage (mean 1.3%) of double FB+DY labeled neurons were detected with a clear contralateral preponderance, among single labeled FB or DY cells. In spite of the rarity, all the NRTP neurons giving rise to intralobular collateral projections can be regarded as potential sources of simultaneous modulating influences upon two functional different forelimb (rPML) and hindlimb (cPML) regions. The findings have been discussed in relation to earlier studies in other species and commented on with respect to the possible functional meaning of these projections.

Topography of olivo-cortico-nuclear modules in the intermediate cerebellum of the rat

This study provides a detailed anatomical description of the relation between olivo-cortico-nuclear modules of the intermediate cerebellum of the rat and the intrinsic zebrin pattern of the Purkinje cells. Strips of climbing fibers were labeled using small injections of biotinylated dextran amine into either the medial or dorsal accessory olives, while, in some cases, simultaneous retrograde labeling of Purkinje cells was obtained using gold-lectin injections into selected parts of the interposed nuclei. Our data are represented in a new, highly detailed, cortical surface reconstruction of the zebrin pattern and in relation to the collateral labeling of the climbing fibers to the cerebellar nuclei. We show that the somatotopic regions of the dorsal accessory olive behave differently in their projections to essentially zebrin-negative regions that represent the C1 and C3 zones of the anterior and posterior parts of the cortex. The rostral part of the medial accessory olive projects to zebrin-positive areas, in particular to the P4+ band of the anterior lobe and lobule VI and to the P5+ band of the posterior lobe, indicating that C2 has two noncontiguous representations in the SL and crus 1. By relating the areas of overlap that resulted from the injections in the accessory olives, i.e., labeling of climbing fiber strips and patches of climbing fiber nuclear collaterals, with the results from the injections in the interposed nuclei, i.e., retrograde labeling of Purkinje cells and of inferior olivary neurons, direct verification of the concept of modular cerebellar connections was obtained.

Branching projection of the nucleus “k” neurons to the rabbit cerebellar paramedian lobule: a retrograde fluorescent tracing study

The nucleus "k", in the reticular core of the rabbit caudal pons, is divided into a large medial (composed of dorsal k1 and ventral k2) and a small lateral (k3) subdivision. In this study, the nucleus "k" subdivisions were examined in the rabbit with respect to projections to the cortex of rostral (rPML; face-forelimb region) and caudal (cPML; hindlimb region) paramedian lobule of the cerebellum. The retrograde fluorescent labeling method with Fast Blue (FB) and Diamidino Yellow (DY) was used. Numerous single FB or DY labeled neurons were found in defined regions of all nucleus "k" subdivisions bilaterally, with an ipsilateral preponderance. The distribution of these neurons indicated that afferents originating from different nucleus "k" subdivisions terminated in overlapping regions within the rPML and the cPML rather than in separate domains. Apart from this, double FB + DY labeled neurons (n = 104) were intermingled within a common region of single labeling, but exclusively on the ipsilateral side. Such neurons occupied predominantly the central and lateral regions of the caudal two thirds of the k1 subdivision, and were scattered in the caudal half of k2 as well as throughout the entire rostrocaudal extent of the k3 subdivision. The size of labeled perikarya varied from 20 to 40 microm in diameter. The number of neurons with branching axons was considerably lower than those with single projections to the rPML and the cPML. It amounted to about 3% in k1 and k3, and 2% in the k2 subdivision. However, this population may form an intralobular link between two somatotopically non-corresponding PML regions. The present study provides a morphological basis for further investigations for comparison with other species using both anatomical and electrophysiological methods, also with respect to other connections of the nucleus "k".

Cerebellar responses evoked by nociceptive leg withdrawal reflex as revealed by event-related FMRI

The aim of the present study was to examine nociceptive leg withdrawal reflex-related areas in the human cerebellum using event-related functional brain imaging (fMRI). Knowledge about cerebellar areas involved in unconditioned limb withdrawal reflex control has some relevance in understanding data of limb withdrawal reflex conditioning studies. Sixteen healthy adult subjects participated. Nociceptive leg withdrawal reflexes were evoked by electrical stimulation of the left tibial nerve behind the medial malleolus. An event-related fMRI paradigm was applied with a total of 30 stimuli being delivered pseudorandomly during 500 consecutive MR scans. Surface electromyographic (EMG) recordings were performed from the left anterior tibial muscle. Only trials with significant reflex EMG activity were used as active events in fMRI statistical analysis. The specified contrasts compared the active event condition with rest. Leg withdrawal reflex-related areas were located within the vermis, paravermis, and lateral posterior cerebellar hemispheres bilaterally. Vermal and paravermal areas in lobules III/IV in the anterior lobe and in lobule VIII in the posterior lobe agree with the cerebellar representation of climbing and mossy fiber hindlimb afferents and voluntary leg movements. They are likely related to efferent modulation of the leg withdrawal reflex and/or sensory processing of afferent inputs from the reflex and/or the noxious stimulus. Additional activation within vermal lobule VI and hemispheral lobules VI/Crus I may be related to other pain-related processes (e.g., facial grimacing, fear, and startlelike reactions).

Axonal collateral branching of neurones in the inferior olive projecting to the cerebellar paramedian lobule in the rabbit

The double fluorescent retrograde technique was employed to examine the distribution of the inferior olive (IO) neurones projecting to the cortex of the rostral and caudal parts of the paramedian lobule (PML) in the rabbit cerebellum, known to be the face-forelimb and hindlimb receiving areas, respectively. Moreover, this technique was also used to investigate the possibility that IO projections reaching these two somatotopically non-homologous PML regions are collaterals of the same axones. No other reports have addressed this question. After non-overlapping unilateral injections of the cytoplasmic tracer fast blue (FB) and the nuclear dye diamidino yellow (DY) into the rostral and caudal PML, respectively, numerous single FB- or DY-labelled cells were found in the defined regions of the contralateral IO. These regions showed considerable overlap, apart from the dorsal accessory olive where a clear spatial separation of labelled cell groups was observed. Furthermore, double FB + DY-labelled neurones (n = 310) were seen in the medial accessory olive, the dorsal and ventral laminas of the principal olive, in the dorsomedial cell column and the beta nucleus. It suggests that IO neurones may branch to supply the two functionally different PML regions and in this way participate in the mechanisms of forelimb-hindlimb coordination.

Activation of cerebellar climbing fibres to rat cerebellar posterior lobe from motor cortical output pathways

1. The activation of climbing fibres projecting to the posterior lobe cerebellar cortex by focal stimulation of the cerebral corticofugal pathway was investigated in anaesthetised rats. Large climbing fibre responses were evoked in parts of crus II and paramedian lobule by stimulation of corticofugal fibres. Lesions of the pyramidal tract just rostral to the inferior olive substantially reduced these responses, suggesting that they were not mediated by relays in the rostral brainstem. 2. By comparison of latencies of climbing fibre responses evoked from different locations in the corticofugal pathway, the conduction velocities of the corticofugal fibres that mediate the responses were estimated to be 1.9 +/- 0.3 m s(-1) (mean +/- S.E.M.). The fastest conducting corticofugal fibres were estimated to conduct significantly faster (18.7 +/- 2.3 m s(-1)). 3. Climbing fibre responses with similar form and cerebellar distribution were evoked from sites in the pyramidal tract rostral and caudal to the inferior olive. This suggests that at least a proportion of the fibres that activate climbing fibres are corticospinal fibres. 4. Lesions of the dorsal column nuclei did not affect the climbing fibre responses evoked in crus II, and produced a relatively small reduction of the responses in the paramedian lobule. This implies that the climbing fibre responses were not exclusively mediated via the dorsal column nuclei. 5. Corticofugal evoked climbing fibre responses were mapped across the cerebellar hemisphere. At some sites they were co-localised with responses evoked by limb afferents. On the basis of limb afferent inputs and other work, these zones were tentatively identified as being functionally equivalent to the c1, c2 and d zones described in the cat.

Functional organization of climbing fibre projection to the cerebellar anterior lobe of the rat

1. The input characteristics and distribution of climbing fibre field potentials evoked by electrical stimulation of various parts of the skin were investigated in the cerebellum of barbiturate anaesthetized rats. 2. Climbing fibre responses were recorded in sagittally oriented microelectrode tracks across the mediolateral width of the anterior lobe. Climbing fibres with similar response latencies and convergence patterns terminated in sagittal bands with widths of 0.5-1.5 mm. The principal organization of the anterior lobe with respect to input characteristics and locations of sagittal zones was similar to that in the cat and ferret. Hence, the sagittal bands in the rat were tentatively named the a, b, c1, c2 and d1 zones. 3. In contrast to the cat and ferret, the a zone of the rat was characterized by short latency ipsilateral climbing fibre input. Furthermore, it was divisible into a medial 'a1' zone with convergent, proximal input and a lateral 'ax' zone with somatotopically organized input. A forelimb area with similar location and input characteristics as the X zone of the cat was found, but it formed an integral part of the ax zone. A somatotopic organization of ipsilateral, short latency climbing fibre input was also found in the c1 zone. 4. Rostrally in the anterior lobe, climbing fibres activated at short latencies from the ipsilateral side of the body terminated in a somatotopically organized transverse band which extended from the midline to the lateral end of the anterior lobe. 5. The absence of the C3 and Y zones may be interpreted as a reflection of differences in the organization of the motor systems in the rat as compared with the cat. Skilled movements, which in the cat are controlled by the C1, C3 and Y zones via the anterior interposed nucleus, may in the rat be partly controlled by the ax zone via the rostrolateral part of the fastigial nucleus.

Multiple somatotopic representations in the human cerebellum

The classic view of representation in the cerebellum assumes two homunculi, one in the anterior lobe and one in the posterior lobe. Functional imaging has confirmed this somatotopy in the human anterior lobe but not, so far, in the posterior lobe. Using fMRI, we found separate peaks of activation for finger and toe in three ipsilateral cerebellar regions. In both the anterior and posterior lobe, the toe representation was semicircular around the finger area, with peaks of activation aligned in accord with the classic homunculi. Also, segregated peaks of activation were found in the pyramis vermis. These results confirm the existence of a second homunculus in the posterior lobe of the human cerebellum and suggest a third one.

Projections from the upper lumbar cord to the cerebellar nuclei in the rat, studied by anterograde axonal tracing

The spinocerebellar tracts arising from the upper lumbar cord consist of the dorsal and the ventral spinocerebellar tracts (DSCT and VSCT), which ascend ipsilaterally and contralaterally, respectively. By using anterograde labeling with biotinylated dextran in the rat, this study examined whether the lumbar DSCT and the VSCT project to the cerebellar nuclei. Injections of the tracer were made unilaterally at levels between the L1 and L3 segments, with diffusion to either a rostral or a caudal segment. The injections resulted in bilateral labeling of axon terminals in the cerebellar nuclei. In the medial nucleus, labeled terminals were distributed in medial, ventral, and ventrolateral parts of the middle subdivision and ventral parts of the caudomedial subdivision. In the anterior interpositus nucleus, they were distributed in medial and dorsomedial parts throughout the rostrocaudal extent. Labeled terminals were seen within the dorsomedial crest region. In the posterior interpositus nucleus, labeled terminals were seen in the rostromedial extension, the caudomedial part, and the caudal pole. Labeled terminals were seen in the hilus and the ventral part of the lateral nucleus. Projections of the DSCT and the VSCT to these regions were confirmed after tracer injections preceded by sectioning of either tract. Both tracts projected bilaterally, but the DSCT projected mainly ipsilaterally. The present study suggests that the spinocerebellar tracts originating from the upper lumbar cord (the lumbar DSCT and the VSCT) project to specific areas of the cerebellar nuclei to transmit information about the peripheral and central events during the movement of hindlimbs.

Movement-related gating of climbing fibre input to cerebellar cortical zones

The inferior olive climbing fibre projection and associated spino-olivocerebellar paths (SOCPs) have been studied intensively over the last quarter of a century yet precisely what information they signal to the cerebellar cortex during movements remains unclear. A different approach is to consider the times during a movement when afferent signals are likely to be conveyed via these paths. Central regulation (gating) of afferent transmission during active movements is well documented in sensory pathways leading to the cerebral cortex and the present review examines the possibility that a similar phenomenon also occurs in SOCPs during movements such as locomotion and reaching. Several lines of evidence are considered which suggest that SOCPs are not always open for transmission. Instead, flow of sensory information to the cerebellum via climbing fibre paths is powerfully modulated during active movements. The findings are discussed in relation to the parasagittal zonal organization of the cerebellar cortex and, in particular, evidence is presented that different cerebellar zones are subject to similar patterns of gating during reaching but can differ appreciably in the pattern of modulation their SOCPs exhibit during locomotion. Furthermore, differences in gating can occur at different rostrocaudal loci within the same zone, suggesting that in the awake behaving animal, individual cerebellar zones are not functionally homogeneous. Finally, the data are interpreted in relation to the error detector hypothesis of climbing fibre function and the possibility explored that the gating serves as a task-dependent mechanism that operates to prevent self-generated 'irrelevant' sensory inputs from being relayed via the SOCPs to the cerebellar cortex, while behaviourally 'relevant' signals are selected for transmission.

Cutaneous receptive fields and topography of mossy fibres and climbing fibres projecting to cat cerebellar C3 zone

1. The topographical organization of mossy fibre input to the forelimb area of the paravermal C3 zone in cerebellar lobules IV and V was investigated in barbiturate-anaesthetized cats and compared with the previously described microzonal organization of climbing fibre input to the same part of the cortex. Recordings were made in the Purkinje cell and granule cell layers from single climbing fibre and mossy fibre units, respectively, and the organization of cutaneous receptive fields was assessed for both types of afferents. 2. Based on spatial characteristics, receptive fields of single mossy fibres could be systematized into ten classes and a total of thirty-two subclasses, mainly in accordance with a scheme previously used for classification of climbing fibres. Different mossy fibres displayed a substantial range of sensitivity to natural peripheral stimulation, responded preferentially to phasic or tonic stimuli and were activated by brushing of hairs or light tapping of the skin. 3. Overall, mossy fibres to any given microzone had receptive fields resembling the climbing fibre receptive field defining that microzone. However, compared with the climbing fibre input, the mossy fibre input had a more intricate topographical organization. Mossy fibres with very similar receptive fields projected to circumscribed cortical regions, with a specific termination not only in the mediolateral, but also in some cases in the rostrocaudal and dorsoventral, dimensions of the zone. On the other hand, mossy fibre units with non-identical, albeit usually similar, receptive fields were frequently found in the same microelectrode track.