Corticonuclear projections of the cerebellum preserve both anteroposterior and mediolateral pairing patterns

Gating by Functionally Indivisible Cerebellar Circuits: a Hypothesis

The attempt to understand the cerebellum has been dominated for years by supervised learning models. The central idea is that a learning algorithm modifies transmission strength at repeatedly co-active synapses, creating memories stored as finely calibrated synaptic weights. As a result, Purkinje cells, usually the de facto output cells of these models, acquire a modified response to input in a remembered pattern. This paper proposes an alternative model of pattern memory in which the function of a match is permissive, allowing but not driving output, and accordingly controlling the timing of output but not the rate of firing by Purkinje cells. Learning does not result in graded synaptic weights. There is no supervised learning algorithm or memory of individual patterns, which, like graded weights, are unnecessary to explain the evidence. Instead, patterns are classed as simply either known or not, at the level of input to a functional population of 100s of Purkinje cells (a microzone). The standard is strict. If only a handful of Purkinje cells receive a mismatch output of the whole circuit is blocked. Only if there is a full and accurate match are projection neurons in deep nuclei, which carry the output of most circuits, released from default inhibitory restraint. Purkinje cell firing at those times is a linear function of input rates. There is no effect of modification of synaptic transmission except to either allow or block output.

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.

The fate of spontaneous synchronous rhythms on the cerebrocerebellar loop

How does the cerebellum participate in neocortical rhythms? Neocortical signals destined for the cerebellum are integrated in the pontine nuclei (PN) with cerebellar output signals via a direct, reciprocal feedback loop with the cerebellar nuclei (CN). The present study investigated the fate of two spontaneously occurring rhythms in rat neocortex under ketamine anesthesia-slow wave activity at around 1 Hz and gamma oscillations-within this pontonuclear feedback loop. Coordinated oscillatory neuronal activity was studied using simultaneous multineuron recordings in primary motor cortex (M1), PN, and lateral CN. It was revealed that slow burst firing-known in neocortex as "up and down states"-is readily conveyed within the pontonuclear feedback loop and thus engages the entire cerebropontocerebellothalamic loop. In contrast, gamma band synchronous oscillations reached only the PN under the present experimental conditions. Surprisingly, many CN single units were actually found to oscillate in the gamma range, but they completely failed to synchronize with other units in either CN or PN. These results show firstly that slow concerted activity can readily engage the entire cerebrocerebellar loop. Secondly, they raise the possibility that fast gamma oscillations may be incompatible with cerebellar processing and get blocked out. Future studies in behaving animals are needed to answer the question whether signals coded in gamma band frequency are converted to another carrier code using the feedback control exerted by the pontonuclear loop.

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.

Coding of tactile response properties in the rat deep cerebellar nuclei

In the lateral hemispheres of the cerebellar cortex, somatosensory responses are represented in a finely grained fractured somatotopy. It is unclear, however, how these responses contribute to the ultimate output of the cerebellum from the deep cerebellar nuclei (DCN). Robust responses of DCN neurons to somatosensory stimuli have been described, but a detailed examination of their somatotopic arrangement and stimulus coding properties is lacking. To address these questions, we recorded extracellular, single-unit activity in the DCN of ketamine-anesthetized rats in response to air-puff stimuli aimed at six different orofacial and forelimb locations. In additional experiments, the duration and intensity of air-puff stimuli to the ipsilateral upper lip were systematically varied. Overall, we found that DCN neuron responses to air puff stimuli showed combinations of three distinct response components: a short-latency spike response, a pronounced inhibition, and a long-latency increase in firing. Individual neurons responsive to air-puff stimulation exhibited any combination of just one, two, or all three of these response components. The inhibitory response was most common and frequently consisted of a complete cessation of spiking despite a high spontaneous rate of baseline firing. In contrast to published findings from cerebellar cortical recordings, the receptive fields of all responsive neurons in the DCN were large. In fact, the receptive field of most neurons covered the ipsi- and contralateral face as well as forepaws. Response properties of individual neurons did not reliably indicate stimulus intensity or duration, although as a population DCN neurons showed significantly increasing response amplitudes as air-puff intensity or duration increased. Overall, the responses were characterized by a distinct temporal profile in each neuron, which remained unchanged with changes in stimulus condition. We conclude that the responses in the DCN of rats to air-puff stimuli differ substantially from cerebellar cortical responses in their receptive field properties and do not provide a robust code of tactile stimulus properties. Rather, the characteristic temporal response profile of each neuron may be tuned to control the timing of a specific task to which its output is linked.

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.

The distribution of climbing and mossy fiber collateral branches from the copula pyramidis and the paramedian lobule: congruence of climbing fiber cortical zones and the pattern of zebrin banding within the rat cerebellum

Individual cerebellar cortical zones defined by the somatotopy of climbing fiber responses and by their olivo-cortico-nuclear connections located in the paramedian lobule and the copula pyramidis of the rat cerebellum were microinjected with cholera toxin B subunit. Collateral branches of climbing and mossy fibers were mapped and related to the pattern of zebrin-positive and -negative bands of Purkinje cells. Climbing fiber collaterals from the copula distribute to the anterior lobe: from the paramedian lobule mainly to lobulus simplex and rostral crus I. Climbing fibers terminating in particular zones (X, A2, C1, CX, C2, C3, D1, and D2) in the paramedian lobule or the copula collateralize to one or two corresponding zones in lobulus simplex, crus I and II, the paraflocculus, and/or the anterior lobe. These zones can be defined by their relationship to the pattern of zebrin banding. Collaterals from mossy fibers, labeled from the same injection sites in the copula and paramedian lobule, often distribute bilaterally in a symmetrical pattern of multiple but ill-defined longitudinal strips in the anterior lobe and/or lobulus simplex. One or more of these longitudinal aggregates of mossy fiber collaterals was always found subjacent to the strip(s) of labeled climbing fiber collaterals arising from the same locus in the paramedian lobule or the copula. Corticonuclear projections focused on the target nucleus of each zone, although a bilateral plexus of thinner axons, presumably of mossy fiber collateral origin, was sometimes also present in several other regions of the cerebellar nuclei. Overall, these results suggest that climbing fiber zones and zebrin banding reflect a common organizational scheme within the cerebellar cortex.

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.