Zonal organization of cortico-nuclear and nucleo-cortical projections of the paramedian lobule of the cat cerebellum. 1. the C1 zone

Variations on the theme: focus on cerebellum and emotional processing

The cerebellum operates exploiting a complex modular organization and a unified computational algorithm adapted to different behavioral contexts. Recent observations suggest that the cerebellum is involved not just in motor but also in emotional and cognitive processing. It is therefore critical to identify the specific regional connectivity and microcircuit properties of the emotional cerebellum. Recent studies are highlighting the differential regional localization of genes, molecules, and synaptic mechanisms and microcircuit wiring. However, the impact of these regional differences is not fully understood and will require experimental investigation and computational modeling. This review focuses on the cellular and circuit underpinnings of the cerebellar role in emotion. And since emotion involves an integration of cognitive, somatomotor, and autonomic activity, we elaborate on the tradeoff between segregation and distribution of these three main functions in the cerebellum.

Multizonal Cerebellar Influence Over Sensorimotor Areas of the Rat Cerebral Cortex

The cerebral cortex requires cerebellar input for optimizing sensorimotor processing. However, how the sensorimotor cortex uses cerebellar information is far from understood. One critical and unanswered question is how cerebellar functional entities (zones or modules) are connected to distinct parts of the sensorimotor cortices. Here, we utilized retrograde transneuronal infection of rabies virus (RABV) to study the organization of connections from the cerebellar cortex to M1, M2, and S1 of the rat cerebral cortex. RABV was co-injected with cholera toxin β-subunit (CTb) into each of these cortical regions and a survival time of 66-70 h allowed for third-order retrograde RABV infection of Purkinje cells. CTb served to identify the injection site. RABV+ Purkinje cells throughout cerebellar zones were identified by reference to the cerebellar zebrin pattern. All injections, including those into S1, resulted in multiple, zonally arranged, strips of RABV+ Purkinje cells. M1 injections were characterized by input from Purkinje cells in the vermal X-zone, medial paravermis (C1- and Cx-zones), and lateral hemisphere (D2-zone); M2 receives input from D2- and C3-zones; connections to S1 originate from X-, Cx-, C3-, and D2-zones. We hypothesize that individual domains of the sensorimotor cortex require information from a specific combination of cerebellar modules.

Computational Principles of Supervised Learning in the Cerebellum

Supervised learning plays a key role in the operation of many biological and artificial neural networks. Analysis of the computations underlying supervised learning is facilitated by the relatively simple and uniform architecture of the cerebellum, a brain area that supports numerous motor, sensory, and cognitive functions. We highlight recent discoveries indicating that the cerebellum implements supervised learning using the following organizational principles: ( a) extensive preprocessing of input representations (i.e., feature engineering), ( b) massively recurrent circuit architecture, ( c) linear input-output computations, ( d) sophisticated instructive signals that can be regulated and are predictive, ( e) adaptive mechanisms of plasticity with multiple timescales, and ( f) task-specific hardware specializations. The principles emerging from studies of the cerebellum have striking parallels with those in other brain areas and in artificial neural networks, as well as some notable differences, which can inform future research on supervised learning and inspire next-generation machine-based algorithms.

Cerebellar Modules and Their Role as Operational Cerebellar Processing Units: A Consensus paper [corrected]

The compartmentalization of the cerebellum into modules is often used to discuss its function. What, exactly, can be considered a module, how do they operate, can they be subdivided and do they act individually or in concert are only some of the key questions discussed in this consensus paper. Experts studying cerebellar compartmentalization give their insights on the structure and function of cerebellar modules, with the aim of providing an up-to-date review of the extensive literature on this subject. Starting with an historical perspective indicating that the basis of the modular organization is formed by matching olivocorticonuclear connectivity, this is followed by consideration of anatomical and chemical modular boundaries, revealing a relation between anatomical, chemical, and physiological borders. In addition, the question is asked what the smallest operational unit of the cerebellum might be. Furthermore, it has become clear that chemical diversity of Purkinje cells also results in diversity of information processing between cerebellar modules. An additional important consideration is the relation between modular compartmentalization and the organization of the mossy fiber system, resulting in the concept of modular plasticity. Finally, examination of cerebellar output patterns suggesting cooperation between modules and recent work on modular aspects of emotional behavior are discussed. Despite the general consensus that the cerebellum has a modular organization, many questions remain. The authors hope that this joint review will inspire future cerebellar research so that we are better able to understand how this brain structure makes its vital contribution to behavior in its most general form.

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.

Excitatory Cerebellar Nucleocortical Circuit Provides Internal Amplification during Associative Conditioning

Closed-loop circuitries between cortical and subcortical regions can facilitate precision of output patterns, but the role of such networks in the cerebellum remains to be elucidated. Here, we characterize the role of internal feedback from the cerebellar nuclei to the cerebellar cortex in classical eyeblink conditioning. We find that excitatory output neurons in the interposed nucleus provide efference-copy signals via mossy fibers to the cerebellar cortical zones that belong to the same module, triggering monosynaptic responses in granule and Golgi cells and indirectly inhibiting Purkinje cells. Upon conditioning, the local density of nucleocortical mossy fiber terminals significantly increases. Optogenetic activation and inhibition of nucleocortical fibers in conditioned animals increases and decreases the amplitude of learned eyeblink responses, respectively. Our data show that the excitatory nucleocortical closed-loop circuitry of the cerebellum relays a corollary discharge of premotor signals and suggests an amplifying role of this circuitry in controlling associative motor learning.

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Cerebellar nuclei provide modular corollary discharge to the cerebellar cortex

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Nucleocortical afferents have unique molecular and ultrastructural features

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Eyeblink conditioning induces structural plasticity of nucleocortical mossy fibers

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Nucleocortical afferents amplify the amplitude of conditioned eyeblink responses

Cerebellar loops: a review of the nucleocortical pathway

Feedback pathways are a common circuit motif in vertebrate brains. Reciprocal interconnectivity is seen between the cerebral cortex and thalamus as well as between basal ganglia structures, for example. Here, we review the literature on the nucleocortical pathway, a feedback pathway from the cerebellar nuclei to the cerebellar cortex, which has been studied anatomically but has remained somewhat obscure. This review covers the work examining this pathway on a number of levels, ranging from its existence in numerous species, its organization within cerebellar circuits, its cellular composition, and a discussion of its potential roles in motor control. Recent interest in cerebellar modular organization raises the profile of this neglected cerebellar pathway, and it is hoped that this review will consolidate knowledge gained over several decades of research into a useful format, spurring new investigations into this evolutionarily conserved pathway.

Effect of inactivation of the intermediate cerebellum on overground locomotion in the rat: a comparative study of the anterior and posterior lobes

The importance of the cerebellum in control of locomotion is demonstrated by the ataxic gait of cerebellar patients. The intermediate cerebellum contains somatotopical representations for forelimbs and hindlimbs in both anterior and posterior lobes. However, it is not known whether these separate regions have discrete roles in control of limb movements during locomotion. Here we compared the effect of muscimol-induced inactivation of the anterior or posterior intermediate cerebellum on limb movements in walking rats. Inactivation of the anterior intermediate cerebellum had clear effects on limb movements during overground locomotion, resulting in excessive toe elevation and hyperflexion of joints in the swing phase. Inactivation of the posterior region resulted in similar but less pronounced deficits. Postural defects were not present in either group of rats. These findings suggest that the intermediate cerebellum of the anterior lobe has a greater influence on the ability to control limb movements during overground locomotion than the posterior lobe.

Consensus paper: current views on the role of cerebellar interpositus nucleus in movement control and emotion

In the present paper, we examine the role of the cerebellar interpositus nucleus (IN) in motor and non-motor domains. Recent findings are considered, and we share the following conclusions: IN as part of the olivo-cortico-nuclear microcircuit is involved in providing powerful timing signals important in coordinating limb movements; IN could participate in the timing and performance of ongoing conditioned responses rather than the generation and/or initiation of such responses; IN is involved in the control of reflexive and voluntary movements in a task- and effector system-dependent fashion, including hand movements and associated upper limb adjustments, for quick effective actions; IN develops internal models for dynamic interactions of the motor system with the external environment for anticipatory control of movement; and IN plays a significant role in the modulation of autonomic and emotional functions.

Structural basis of cerebellar microcircuits in the rat

The topography of the cerebellar cortex is described by at least three different maps, with the basic units of each map termed "microzones," "patches," and "bands." These are defined, respectively, by different patterns of climbing fiber input, mossy fiber input, and Purkinje cell (PC) phenotype. Based on embryological development, the "one-map" hypothesis proposes that the basic units of each map align in the adult animal and the aim of the present study was to test this possibility. In barbiturate anesthetized adult rats, nanoinjections of bidirectional tracer (Retrobeads and biotinylated dextran amine) were made into somatotopically identified regions within the hindlimb C1 zone in copula pyramidis. Injection sites were mapped relative to PC bands defined by the molecular marker zebrin II and were correlated with the pattern of retrograde cell labeling within the inferior olive and in the basilar pontine nuclei to determine connectivity of microzones and patches, respectively, and also with the distributions of biotinylated dextran amine-labeled PC terminals in the cerebellar nuclei. Zebrin bands were found to be related to both climbing fiber and mossy fiber inputs and also to cortical representation of different parts of the ipsilateral hindpaw, indicating a precise spatial organization within cerebellar microcircuitry. This precise connectivity extends to PC terminal fields in the cerebellar nuclei and olivonuclear projections. These findings strongly support the one-map hypothesis and suggest that, at the microcircuit level of resolution, the cerebellar cortex has a common plan of spatial organization for major inputs, outputs, and PC phenotype.

The mysterious microcircuitry of the cerebellar nuclei

The microcircuitry of cerebellar cortex and, in particular, the physiology of its main element, the Purkinje neuron, has been extensively investigated and described. However, activity in Purkinje neurons, either as single cells or populations, does not directly mediate the cerebellar effects on the motor effector systems. Rather, the result of the entire cerebellar cortical computation is passed to the relatively small cerebellar nuclei that act as the final, integrative processing unit in the cerebellar circuitry. The nuclei ultimately control the temporal and spatial features of the cerebellar output. Given this key role, it is striking that the internal organization and the connectivity with afferent and efferent pathways in the cerebellar nuclei are rather poorly known. In the present review, we discuss some of the many critical shortcomings in the understanding of cerebellar nuclei microcircuitry: the extent of convergence and divergence of the cerebellar cortical pathway to the various cerebellar nuclei neurons and subareas, the possible (lack of) conservation of the finely-divided topographical organization in the cerebellar cortex at the level of the nuclei, as well as the absence of knowledge of the synaptic circuitry within the cerebellar nuclei. All these issues are important for predicting the pattern-extraction and encoding capabilities of the cerebellar nuclei and, until resolved, theories and models of cerebellar motor control and learning may err considerably.

A trans-spinal loop between neurones in the reticular formation and in the cerebellum

Voluntary limb movements are initiated in the brain but the neurones responsible for activating the muscles (motoneurones and interneurones) are located in the spinal cord. The spinal cord also contains neurones that provide the brain, and especially the cerebellum, with continuous information on effects of the descending commands. We show that one population of such neurones provide the cerebellum with information on how likely the brain's commands (mediated by descending reticulospinal neurones) are to be executed as planned, depending on the degree of inhibition of motoneurones. They may therefore play an important role in preventing errors in activation of motoneurones and thereby help the brain to correct its signals to the spinal cord before such errors have been committed.

Olivo-cortico-nuclear localizations within crus I of the cerebellum

Retrograde and anterograde tracers were microinjected into the folia of crus I of the cat cerebellum to investigate spatial localization in olivo-cerebellar and cortico-nuclear projections. The folia were shown to be mainly occupied in rostrocaudal succession by three zones receiving their olivo-cerebellar climbing fiber afferents from parts of, respectively, the dorsal lamella of the principal olive, the ventral lamella of the principal olive, and the rostral half of the medial accessory olive. These zones are presumably parts of the D(2), D(1), and C(2) cerebellar cortical zones, as earlier proposed by Rosina and Provini ([1982] Neuroscience 7:2657-2676). Their respective nuclear target territories were found to be in the rostroventral quadrant of nucleus lateralis, the caudoventral quadrant of nucleus lateralis, and the ventral half of nucleus interpositus posterior. The medial-to-lateral width of each zone was shown to be innervated by different groups of olive cells and to project respectively to medial and lateral parts of the nuclear territory for that zone, consistent with the existence in crus I of olivo-cortico-nuclear microcomplexes (cf. Ito [1984] New York: Raven Press). Parts of the length of each zone located within different folia were also shown to relate to different groups of olive cells and to different regions of the zone's overall nuclear territory. Interfolial localizations, which were heavily overlapping in nature, intersected orthogonally with those for zone width. The fine-grain topography implies that individual microzones exist within each of the zones present within crus I. The results also have implications for the possibility that lateral cerebellar pathways are involved in cognition.

Different pontine projections to the two sides of the cerebellum

This study analyzed the projections of the basilar pontine nuclei (BPN) and of the nucleus reticularis tegmenti pontis (NRTP) to the two sides of the cerebellum in the rat. It showed that the two sides of the cerebellar cortex were innervated by different percentages of BPN (about 82% of the cells project to the contralateral cortex and 18% to the ipsilateral) and NRTP cells (some 60% project to the contralateral cortex and 40% to the ipsilateral). In comparison to projections traced to the cortex, only a few fibers were traced to the nuclei of the same animals. Most of the projections of the BPN to the cerebellar nuclei were traced to the lateralis and posterior interpositus nucleus of the contralateral side (95%), while a few were traced to homologous nuclei of the ipsilateral side (5%). Thus, the BPN principally control the activity of the contralateral cerebellum, with a much less important control over the activity of the ipsilateral cerebellum. Vice versa, the NRTP, which project to the lateralis, interpositus, and medialis nuclei of the two sides, with percentages (64% contra- and 36% ipsilateral) similar to those reported for the projections to the cortex, is more concerned in the bilateral control of the cerebellum, although with a moderate contralateral prevalence. The fact that projections of the BPN were principally traced to the contralateral nuclei, from which the efferent projection fibers from the cerebellum originate, suggests that the BPN are principally involved in the motor control of the contralateral body. Conversely, the bilateral projections of the NRTP to the cerebellar nuclei suggest that the NRTP is mainly involved in bilateral motor activities. The comparison of the projections to the cortex and nuclei of the cerebellum of single animals supports the co-existence of coupled (i.e., projections to the cortex and the corresponding nuclei) and uncoupled (i.e., projections to the cortex but not to the nuclei) projection patterns, from both the BPN and the NRTP. These features of the pontocerebellar projections open new vistas on the functional architecture of this pathway.

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.

Lateral cerebellum: functional localization within crus I and correspondence to cortical zones

The present study investigates the functional connections of different parts of the medial-most folium of crus I in the cat cerebellar hemisphere. Three areas were identified physiologically by recording on the cerebellar surface climbing fibre (CF) field potentials evoked by electrical stimulation of different body sites. From medial to lateral in relation to the long axis of the folium, area 1 receives convergent input from all body sites tested (optic chiasm, ipsilateral periorbital region, ipsilateral and contralateral forelimbs), area 2 receives input mainly from the ipsilateral periorbital region, while area 3 receives input mainly from the optic chiasm. These physiological differences were used to guide injections of bi-directional tracer material into individual cortical areas. The inferior olive and cerebellar nuclei were then mapped, revealing a precise topography within the olivo-cerebellar and cortico-nuclear projections for each area. On the basis of their anatomical and physiological characteristics areas 1, 2 and 3 correspond to zones C2, C3 and D1, respectively. CF inputs arise from the rostral medial accessory olive (C2), the interface between the rostral dorsal accessory olive and ventral lamella of the principal olive (vlPO, C3), and from vlPO (D1). The corresponding cortico-nuclear projections are nucleus interpositus posterior (C2), the transitional region between the dentate nucleus and nucleus interpositus anterior (C3), and the dentate nucleus (D1). Overall, the results provide a comprehensive description of the functional localization of different zones within crus I (folium 1), and suggest that a potent source of CF input to the C2 and D1 zones within this region of cortex arises from visual pathways.

Structure-function relations of two somatotopically corresponding regions of the rat cerebellar cortex: olivo-cortico-nuclear connections

The somatotopical organization of the climbing fiber input to the paravermal region of lobulus simplex (LS, lobule Vla) was charted in the cerebellar cortex of anaesthetized rats. From medial to lateral in LS, zones a2, c1, c2 and c3 were identified. Forelimb responses were found in both LS and the paramedian lobule (PML) and simultaneous recordings from the c1 zone in both lobules showed that trial-by-trial fluctuations in climbing fiber field size evoked by ipsilateral forelimb stimulation did not occur in synchrony, suggesting that the two parts of the same zone are not closely linked by their climbing fiber input. Electrophysiological mapping in combination with a double fluorescent axonal tracing strategy (mix of Fluoro-Emerald and green beads, and mix of Fluoro-Ruby and red beads) revealed that the two parts of the c1 zone receive climbing fiber input from similar territories in the medial and dorsal accessory olives, but that only 4% of the total population of labelled cells have axons that branch to supply climbing fiber afferents to both regions of cortex. The corticonuclear output of the two parts of the zone was found in mainly overlapping regions of the transitional region between the anterior and posterior divisions of nucleus interpositus. Overall, the results suggest that the olivocerebellar and corticonuclear projections of cerebellar zones are similarly organized in rat and cat, implying that the function of individual zones is conserved between species.

Precise matching of olivo-cortical divergence and cortico-nuclear convergence between somatotopically corresponding areas in the medial C1 and medial C3 zones of the paravermal cerebellum

The paravermal cerebellar cortex contains three spatially separate zones (the C1, C3 and Y zones) which form a functionally coupled system involved in the control of voluntary limb movements. A series of 'modules' has been postulated, each defined by a set of olivary neurons with similar receptive fields, the cortical microzones innervated by these neurons and the group of deep cerebellar nuclear neurons upon which the microzones converge. A key feature of this modular organization is a correspondence between cortical input and output, irrespective of the zonal identity of the microzone. This was tested directly using a combined electrophysiological and bi-directional tracer technique in barbiturate-anaesthetized cats. During an initial operation, small injections of a mix of retrograde and anterograde tracer material (red beads combined with Fluoro-Ruby or green beads combined with biotinylated dextran amine or Fluoro-Emerald) were made into areas of the medial C1 and medial C3 zones in cerebellar lobule V characterized by olivo-cerebellar input from the ventral forelimb. The inferior olive and the deep cerebellar nuclei were then scrutinized for retrogradely labelled cells and anterogradely labelled axon terminals, respectively. For individual experiments, the degree of C1-C3 zone terminal field overlap in the nucleus interpositus anterior was plotted as a function of either the regional overlap of single-labelled cells or the proportion of double-labelled cells in the dorsal accessory olive. The results were highly positively correlated, indicating that cortico-nuclear convergence between parts of the two zones is in close proportion to the corresponding olivo-cerebellar divergence, entirely consistent with the modular hypothesis.