Cerebrocerebellar Projections Research Articles

Cerebro-cerebellar interactions in nonhuman primates examined by optogenetic functional magnetic resonance imaging.

Functional magnetic resonance imaging (fMRI) is a promising approach for the simultaneous and extensive scanning of whole-brain activities. Optogenetics is free from electrical and magnetic artifacts and is an ideal stimulation method for combined use with fMRI. However, the application of optogenetics in nonhuman primates (NHPs) remains limited. Recently, we developed an efficient optogenetic intracortical microstimulation method of the primary motor cortex (M1), which successfully induced forelimb movements in macaque monkeys. Here, we aimed to investigate how optogenetic M1 stimulation causes neural modulation in the local and remote brain regions in anesthetized monkeys using 7-tesla fMRI. We demonstrated that optogenetic stimulation of the M1 forelimb and hindlimb regions successfully evoked robust direct and remote fMRI activities. Prominent remote activities were detected in the anterior and posterior lobes in the contralateral cerebellum, which receive projections polysynaptically from the M1. We further demonstrated that the cerebro-cerebellar projections from these M1 regions were topographically organized, which is concordant with the somatotopic map in the cerebellar cortex previously reported in macaques and humans. The present study significantly enhances optogenetic fMRI in NHPs, resulting in profound understanding of the brain network, thereby accelerating the translation of findings from animal models to humans.

Disynaptic cerebrocerebellar pathways originating from multiple functionally distinct cortical areas

The cerebral cortex and cerebellum both play important roles in sensorimotor processing, however, precise connections between these major brain structures remain elusive. Using anterograde mono-trans-synaptic tracing, we elucidate cerebrocerebellar pathways originating from primary motor, sensory, and association cortex. We confirm a highly organized topography of corticopontine projections in mice; however, we found no corticopontine projections originating from primary auditory cortex and detail several potential extra-pontine cerebrocerebellar pathways. The cerebellar hemispheres were the major target of resulting disynaptic mossy fiber terminals, but we also found at least sparse cerebrocerebellar projections to every lobule of the cerebellum. Notably, projections originating from association cortex resulted in less laterality than primary sensory/motor cortices. Within molecularly defined cerebellar modules we found spatial overlap of mossy fiber terminals, originating from functionally distinct cortical areas, within crus I, paraflocculus, and vermal regions IV/V and VI - highlighting these regions as potential hubs for multimodal cortical influence.

New Corticopontine Connections in the Primate Brain: Contralateral Projections From the Arm/Hand Area of the Precentral Motor Region.

The ipsilateral corticopontine projection (iCPP) represents a massive descending axon system terminating in the pontine nuclei (PN). In the primate, this projection is well known for its dominant influence on contralateral upper limb movements through the classical cerebrocerebellar circuity system. Although a much weaker contralateral corticopontine projection (cCPP) from motor cortex to the paramedian region has been reported in the non-human primate brain, we provide the first comprehensive description of the cCPP from the lateral motor cortex using high resolution anterograde tract tracing in Macaca mulatta. We found a relatively light cCPP from the hand/arm area of the primary motor cortex (M1), comparatively moderate cCPP from ventrolateral premotor cortex (LPMCv) and a more robust and widespread cCPP from the dorsolateral premotor cortex (LPMCd) that involved all nine contralateral PN. The M1 projection primarily targeted the dorsal pontine region, the LPMCv projection targeted the medial pontine region and LPMCd targeted both regions. These results show the first stage of the primate frontomotor cerebrocerebellar projection is bilateral, and may affect both ipsilateral and contralateral limbs. Clinically, the cCPP originating in the non-injured hemisphere may influence the recovery process of the more affected upper extremity following subtotal unilateral damage to the lateral cortical region. The cCPP may also contribute to the mild impairment of the upper limb contralateral to a unilateral cerebellar injury.

Organization of cerebral projections to identified cerebellar zones in the posterior cerebellum of the rat.

The cerebrocerebellar connection makes use of two of the largest fiber tracts in the mammalian brain, i.e., the cerebral and medial cerebellar peduncles. Neuroanatomical approaches aimed to elucidate the organization of this important connection have been hindered by its multisynaptic nature, the complex organization of its components, and the dependency of conventional tracers on precisely placed injections. To overcome these problems, we used rabies virus (RV) as a retrograde transneuronal tracer. RV was injected simultaneously with cholera toxin β subunit (CTb) into selected areas of the cerebellar cortex of 18 male Wistar rats. A survival time of 48-50 h resulted in first- and second-order labeling of RV in combination with first-order labeling of CTb. The distribution of CTb-labeled neurons in the inferior olive established the zonal identity of the injection site. In this way, it was possible to examine the cortical distribution of neurons from which disynaptic cerebrocerebellar projections to specific cerebellar loci originate. The results show that this distribution covaries with the identity of the injected cerebellar lobule. More subtle changes were present when different zones of the same lobule were injected. The C1 zone of lobule VIII receives a more prominent projection from the somatosensory cortex compared with the C2/D zones. The laterally positioned D zones receive information from more rostral regions of the cerebral cortex. The vermis of lobule VII receives a prominent input from the retrosplenial and orbitofrontal cortices. Different injection sites also result in differences in laterality of the connections.

Functional topography in the human cerebellum: A meta-analysis of neuroimaging studies

Clinical, experimental and neuroimaging studies indicate that the cerebellum is involved in neural processes beyond the motor domain. Cerebellar somatotopy has been shown for motor control, but topographic organization of higher-order functions has not yet been established. To determine whether existing literature supports the hypothesis of functional topography in the human cerebellum, we conducted an activation likelihood estimate (ALE) meta-analysis of neuroimaging studies reporting cerebellar activation in selected task categories: motor ( n = 7 studies), somatosensory ( n = 2), language ( n = 11), verbal working memory ( n = 8), spatial ( n = 8), executive function ( n = 8) and emotional processing ( n = 9). In agreement with previous investigations, sensorimotor tasks activated anterior lobe (lobule V) and adjacent lobule VI, with additional foci in lobule VIII. Motor activation was in VIIIA/B; somatosensory activation was confined to VIIIB. The posterior lobe was involved in higher-level tasks. ALE peaks were identified in lobule VI and Crus I for language and verbal working memory; lobule VI for spatial tasks; lobules VI, Crus I and VIIB for executive functions; and lobules VI, Crus I and medial VII for emotional processing. Language was heavily right-lateralized and spatial peaks left-lateralized, reflecting crossed cerebro-cerebellar projections. Language and executive tasks activated regions of Crus I and lobule VII proposed to be involved in prefrontal-cerebellar loops. Emotional processing involved vermal lobule VII, implicated in cerebellar-limbic circuitry. These data provide support for an anterior sensorimotor vs. posterior cognitive/emotional dichotomy in the human cerebellum. Prospective studies of multiple domains within single individuals are necessary to better elucidate neurobehavioral structure–function correlations in the cerebellar posterior lobe.

A direct cerebrocerebellar projection in adult birds and rats

The rostral Wulst of birds, like the somatosensory cortex of mammals, receives somatosensory information from the thalamus and projects to the brainstem and spinal cord via a pyramidal-like tract. Using anterograde and retrograde tract-tracers, we show here, in adult zebra finches, that the rostral Wulst also projects directly to the cerebellar cortex and deep nuclei. In the cortex, the cerebrocerebellar fibers resemble neither mossy nor climbing fibers, but more closely resemble the multilayer fibers shown to originate from the hypothalamus in mammals. We also show that a sparse projection to the cerebellum from the mammalian neocortex, originally thought to be lost during early development, is present in the adult rat. Although the functional implications of these results are obscure, they suggest a revision of the concept of the “cerebrocerebellar system”, which is generally considered to involve a pontine relay.

Absence of impulse activity in cortical neurons with transient projections to the cerebellum

During the second postnatal week of development in cats, neurons in layer V of the primary sensorimotor cortex project transiently, by way of collaterals of pyramidal tract axons, to the cerebellum. All cerebrocerebellar collaterals are subsequently eliminated, while the collaterals in the pyramidal tract persist into the adult. To determine if the transience of the projection to the cerebellum could be due to the lack of functional activity in cerebrocerebellar projection neurons, single-unit extracellular recordings were made from neurons in the primary somatosensory cortex (S-I) in 8–14-day-old kittens. Projection neurons were identified by their antidromic activation from pyramidal tract or cerebellum. Collision experiments confirmed that some neurons had collateral projections to both structures. Recordings from both generally anesthetized as well as locally anesthetized, but awake preparations, indicated that pyramidal tract and cerebrocerebellar projection neurons never fired action potentials spontaneously or were orthodromically activated following stimulation of the medial lemniscus. Stimulation of the medial lemniscus did synaptically activate neurons in the cortex, but these were always located superficial to the antidromically activated projection neurons. These findings indicate that pyramidal tract and/or cerebrocerebellar S-I projection neurons are physiologically silent during the period of development that cortical axons are transiently present in the cerebellum, suggesting that cerebrocerebellar projections may be eliminated because of the lack of impulse activity.

Somatotopically organized transient projections from the primary somatosensory cortex to the cerebellar cortex

The organization of transient projections from the primary somatosensory cortex (S-I) to the cerebellar cortex in neonatal kittens was examined using orthograde intraaxonal labeling techniques. Tritiated amino acid injections into face, forelimb and hindlimb areas of representation in S-I labeled mossy fiber-like terminals of cerebrocerebellar axons in different areas of the cerebellar cortex bilaterally. The hindlimb area of S-I projected to lobules I-IV in the anterior lobe and to ventral folia of the paramedian lobule (PML). Injections into forelimb areas of S-I labeled terminals in lobules IV and V and in intermediate and dorsal folia of the PML. The face area of S-I projected to the lobules V and VI, to medial folia in the ansiform and simplex lobules and to dorsal PML folia. Labeled terminals were more numerous in the cerebellar cortex contralateral to the S-I injections, except in lobules I and II and the ventral PML where the density of hindlimb input was approximately the same on both sides. These observations were supplemented by findings that small wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections into the dorsal or ventral PML resulted in retrogradely labeled layer V pyramidal neurons in lateral (face and forelimb) and medial (hindlimb) areas of S-I respectively. The somatotopic organization of transient S-I cerebrocerebellar projections is very similar to the topography of cerebellar somatosensory afferent pathways in adult cats.

Cerebro-cerebellar projection from the frontal eye field onto the cerebellar cortex

Cerebro-cerebellar projection from the frontal eye field onto the cerebellar cortex

The transience of cerebrocerebellar projections is due to selective elimination of axon collaterals and not neuronal death

Fluorescent dyes were used to determine firstly if the transience of cerebrocerebellar projections in neonatal kittens is due to the selective elimination of axon collaterals or to neuronal death; and secondly, if the cerebrocerebellar projection neurons lived, did any maintain a projection to the brainstem or spinal cord. Injections of Fast Blue were made into the cerebellar cortex and deep nuclei in 7–9 postnatal days old kittens, the age in which cortical axons grow into the cerebellum. Later, at 31–71 postnatal days of age, when the transient cerebrocerebellar projections have disappeared, injections of Nuclear Yellow were made into the brainstem or the spinal cord. In the frontoparietal cortex, numerous neurons were labeled with Fast Blue suggesting that the disappearance of cerebrocerebellar projections is due primarily to the selective elimination of axon collaterals and not neuronal death. Moreover, many of the cortical neurons labeled with Fast Blue also were labeled with Nuclear Yellow which shows that many of the cortical neurons with transient collateral projections to the cerebellum in the neonate maintain a projection to brainstem or spinal targets in older animals.

The collateral origin of a transient cerebrocerebellar pathway in kittens. A study using fluorescent double-labeling techniques

A transient pathway from the sensorimotor neocortex to the cerebellum recently has been described in the kitten which appears late in the first postnatal week, develops maximally at days 9 and 10 and then gradually disappears. The transience of this pathway has been investigated in the present study using the retrograde transport of fluorescent dyes. In one set of experiments Fast Blue was injected into the caudal medulla of neonatal kittens while Nuclear Yellow was injected into the opposite cerebellum 3–10 days later. Neurons labeled with Fast Blue were found bilaterally throughout lamina V of the frontoparietal cortex. Neurons labeled with Nuclear Yellow also were found in lamina V. However, their distribution was predominantly ipsilateral to the cerebellar injection and generally was limited to the anterior and posterior sigmoid gyri, and the coronal gyrus. Most importantly, many neurons were labeled with both dyes, indicating that at least part of the transient cerebrocerebellar projection is derived from collateral branches of corticobulbar axons. A second set of experiments was done to determine if the transience of the cerebrocerebellar pathway is due to a retraction of collaterals of corticofuggal axons or due to the death of the cortical neurons themselves. In these experiments True Blue was injected into the cerebella of kittens 8–10 days after birth and allowed to survive for 40–75 days. No neurons were labeled with True Blue in the frontoparietal cortex, although numerous neurons were labeled in the precerebellar nuclei of the brainstem. In a control set of experiments, True Blue was injected into the caudal medulla in animals at 7 postnatal days. After 12–55 days, the frontoparietal cortex of these animals also was examined for neurons labeled with True Blue but, again, no cortical neurons were labeled. Thus, although the absence of cortical neurons labeled with True Blue after injections into the cerebellum would suggest that most, if not all, of the cortical neurons which project to the cerebellum in the neonate die during maturation, the negative results of the control experiments leave this question unanswered. These experiments also imply that True Blue may not be the appropriate marker for determining the ultimate fate of immature neurons in all developing systems.

Transient cerebrocerebellar projections in kittens: postnatal development and topography.

Orthograde and retrograde labeling techniques were used to study the ontogenesis of transient cerebrocerebellar projections in kittens. Tritiated amino-acid or horseradish peroxidase injections were made into the coronal gyrus of the primary somatosensory cortex of kittens 1-70 postnatal days old. Orthogradely labeled axons were observed bilaterally in the superior and inferior cerebellar peduncles in kittens between 6 and 49 postnatal days of age. Most cerebrocerebellar axons labeled on the ipsilateral side arise from the pyramidal tract as it courses through the pontine nuclei. These axons descend through the pontine tegmentum as a diffusely organized corticotegmental tract and enter the ipsilateral superior cerebellar peduncle. Fewer cerebrocerebellar axons leave the pyramidal tract caudal to the pontine nuclei and project into the contralateral superior cerebellar peduncle. Cerebrocerebellar projections through the superior cerebellar peduncles terminate primarily in the cerebellar nuclei, where they are localized in the interpositus nuclei and in immediately adjacent areas of the dentate and fastigial nuclei. More caudally, labeled axons exit from the pyramidal tract and take a superficial route around the ventrolateral brainstem into the inferior cerebellar peduncles bilaterally. These projections are more numerous contralaterally and are directed primarily to the internal granule cell layer of the posterolateral folia of the anterior lobe, the posteromedial simplex lobule, and the dorsal paramedian lobule. Horseradish peroxidase injections were made into the cerebellar posterior lobe and deep nuclei and the results from these cases showed that the cerebrocerebellar pathway originates from pyramidal neurons in layer V primarily in the coronal, the precoronal, and the anterior and posterior sigmoid gyri on both sides. In these gyri, many of the HRP-positive neurons were found in clusters of two to five neurons, aligned in anterior-posterior strips. The results from all experiments provide evidence about the ontogeny of cerebrocerebellar projections. Projections through the superior cerebellar peduncles generally develop at 6-8 postnatal days of age, whereas projections through the inferior cerebellar peduncles first are seen at 8-10 days postnatally. Cerebrocerebellar projections reach their maximum development in the second postnatal week but sharply decrease in density during the third postnatal week. No cerebrocerebellar projections were observed after the seventh postnatal week of development. Possible functional implications for this transient projection are discussed.