ON THE CEREBELLAR NUCLEI IN THE CAT

Modular output circuits of the fastigial nucleus for diverse motor and nonmotor functions of the cerebellar vermis

The cerebellar vermis, long associated with axial motor control, has been implicated in a surprising range of neuropsychiatric disorders and cognitive and affective functions. Remarkably little is known, however, about the specific cell types and neural circuits responsible for these diverse functions. Here, using single-cell gene expression profiling and anatomical circuit analyses of vermis output neurons in the mouse fastigial (medial cerebellar) nucleus, we identify five major classes of glutamatergic projection neurons distinguished by gene expression, morphology, distribution, and input-output connectivity. Each fastigial cell type is connected with a specific set of Purkinje cells and inferior olive neurons and in turn innervates a distinct collection of downstream targets. Transsynaptic tracing indicates extensive disynaptic links with cognitive, affective, and motor forebrain circuits. These results indicate that diverse cerebellar vermis functions could be mediated by modular synaptic connections of distinct fastigial cell types with posturomotor, oromotor, positional-autonomic, orienting, and vigilance circuits.

Retrograde analysis of the cerebellar projections to the posteroventral part of the ventral lateral thalamic nucleus in the macaque monkey

The organization of cerebellothalamic projections was investigated in macaque monkeys using injections of retrograde tracers (cholera toxin B and fluorescent dextrans) in the posteroventral part of the ventrolateral thalamic nucleus (VLpv), the main source of thalamic inputs to the primary motor cortex. Injections that filled all of VLpv labeled abundant neurons that were inhomogeneously distributed among many unlabeled cells in the deep cerebellar nuclei (DCbN). Single large pressure injections made in face-, forelimb-, or hindlimb-related portions of VLpv using physiological guidance labeled numerous neurons that were broadly dispersed within a coarse somatotopographic anteroposterior (foot to face) gradient in the dentate and interposed nuclei. Small iontophoretic injections labeled fewer neurons with the same somatotopographic gradient, but strikingly, the labeled neurons in these cases were as broadly dispersed as in cases with large injections. Simultaneous injections of multiple tracers in VLpv (one tracer per somatic region with no overlap between injections) confirmed the general somatotopography but also demonstrated clearly the overlapping distributions and the close intermingling of neurons labeled with different tracers. Significantly, very few neurons (<2%) were double-labeled. This organizational pattern contrasts with the concept of a segregated "point-to-point" somatotopy and instead resembles the complex patterns that have been observed throughout the motor pathway. These data support the idea that muscle synergies are represented anatomically in the DCbN by a general somatotopography in which intermingled neurons and dispersed but selective connections provide the basis for plastic, adaptable movement coordination of different parts of the body. Indexing terms:

Projections of glutamate decarboxylase positive and negative cerebellar neurons to the pretectum in the cat

The pretectum is one of the primary visual centers, and plays an important role in the visuomotor reflexes. It also receives projections from the cerebellar nuclei that are considered to regulate these reflexes. Gamma aminobutylic acid (GABA) and glutamate are supposed to be two major neurotransmitters of the projection neurons of the cerebellar nuclei. We double labeled the projecting neurons with a tracer, biotinylated dextran amine (BDA), and with an antiserum to glutamate decarboxylase (GAD), the enzyme that synthesizes GABA. The results indicated that about 40% of the pretectal-projecting neurons of the cerebellar nuclei were GAD immunoreactive. The GAD positive pretectal-projecting neurons were significantly smaller than the GAD negative projecting neurons. Our findings thus suggest the existence of two distinct cerebello-pretectal projection systems: one is mediated by GABAergic inhibitory projections, while the other is mediated by non-GABAergic, probably glutamatergic excitatory ones.

Zonal organization of the vestibulo-cerebellum in the control of horizontal extraocular muscles using pseudorabies virus: I. Flocculus/ventral paraflocculus

Much literature has studied the relationship between the organization of neurons in the flocculus/ventral paraflocculus and vestibulo-ocular reflex pathways. Although activation of a flocculus central zone produces ipsilateral horizontal eye movement, anatomical tracing evidence in rats suggests that there may not be a simple one-to-one correspondence between flocculus/ventral paraflocculus zones and control of single extraocular muscles or coplanar pairs of antagonistic extraocular muscles. This study used the retrograde transynaptic transport of pseudorabies virus to identify the topographical organization of Purkinje cells in the flocculus/ventral paraflocculus that control the lateral rectus (LR) and medial rectus (MR) muscles in rats. A survival time of 80 h and 84 h was necessary to observe consistent transynaptically labeled cells in the flocculus/ventral paraflocculus following injections of pseudorabies virus into the MR and LR, respectively. The organization of Purkinje cells in the dorsal flocculus and ventral paraflocculus abided by the traditional boundaries, whereas the labeling pattern in the ventral flocculus showed a more complex, interdigitated arrangement. In agreement with prior studies, transynaptically labeled neurons were also observed in specific vestibular nuclear regions within the medial and superior vestibular nuclei and dorsal Y group. The distribution of labeled neurons in ipsilateral and contralateral vestibular nuclei was associated with features of ipsilateral and contralateral retrograde labeling of Purkinje cells in flocculus/ventral paraflocculus. Importantly, this study provides the first evidence of vestibulo-cerebellar zones controlling individual extraocular muscles and also overlapping distribution of neurons in flocculo-vestibular zones that influence the LR and MR motoneuron pools. This suggests that some of these neurons may be responsible for controlling both muscles.

Diverging projections of the C2 and D2 olivocorticonuclear cerebellar pathways of the rat

A divergent mediolateral projection to the cerebellar nuclei of the C2 and the D2 olivocorticonuclear cerebellar pathways was found after segregate injections of a tracer (either WGA-HRP or FR or BDA) in the rostral (D2 area) or caudal side (C2 area) of the rat paraflocculus. The C2 olivary area of the cerebellar cortex sends most of its nuclear projection to the nucleus interpositus posterior (classically perceived as the nuclear target of the C2 olivocorticocerebellar pathway) and a smaller contingent of fibres to the parvocellular region of the nucleus lateralis (classically perceived as the nuclear target of the D2 olivocorticocerebellar pathway). The D2 olivary area of the cerebellar cortex sends most of its nuclear projection to the parvocellular region of the nucleus lateralis (classically perceived as the nuclear target of the D2 olivocorticocerebellar pathway) and a smaller contingent of fibres to the magnocellular region of the nucleus lateralis (classically perceived as the nuclear target of the D1 olivocorticocerebellar pathway). The lateral interaction of the D2 and the C2 olivocerebellar pathways could represent the anatomical substrate for the functional integration of different olivocerebellar compartments.

Augmentation of postural muscle tone induced by the stimulation of the descending fibers in the midline area of the cerebellar white matter in the acute decerebrate cat

In a reflexively standing acute decerebrate cats, the cerebellar white matter was systematically stimulated and the effects on the level of postural muscle tone were studied. A stimulating microelectrode was placed systematically at 0.1-0.5 mm increments from H + 2 to H - 2 at levels ranging from P7.0 to P8.0 rostrocaudally and mediolaterally from LR0 to L1.5 or R1.5. Stimuli delivered to the restricted region of the cerebellar white matter along its midline resulted in simultaneous and bilateral augmentation of tonic activities in the neck, lumbar back, fore- and hindlimb extensor muscles along with increased levels in the forces exerted by each of the left and the right fore- and hindlimbs. Effective stimulus regions were located in the cerebellar white matter rostral and ventral to the most rostral part of the fastigial nucleus. Microinjection of a retrograde neural tracer, cholera-toxin b subunit conjugated horseradish peroxidase (CTb-HRP), into the lesioned effective stimulus sites resulted in a retrograde labeling of cells in the fastigial nuclei, bilaterally. All these results suggest that the augmentation of postural muscle tone was evoked by a selective activation of fastigiofugal fibers which course through the 'hook bundle'.

Cerebellar influences on accessory oculomotor nuclei of the rat: a neuroanatomical, immunohistochemical, and electrophysiological study

With the aim to evaluate a possible neocerebellar control on eye movements, the projections from the cerebellar lateral nucleus (LN) to the accessory oculomotor nuclei (i.e., the nucleus of posterior commissure, the nucleus of Darkschewitsch, and the interstitial nucleus of Cajal), the putative neurotransmitters subserving this pathway, and the nature of the synaptic influences exerted by these projections were studied in adult rats. We used the orthograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) to identify the mesencephalic areas where cerebellofugal fibers terminate, and retrograde labeling with the fluorescent dye fluoro-gold to estimate the incidence of cerebellar neurons projecting to the accessory oculomotor nuclei. Orthograde labeling showed that only a small contingent of cerebellofugal fibers reaches the contralateral accessory oculomotor nuclei. The retrogradely labeled cells were located primarily in the small-celled part of LN. By immunohistochemistry, we observed that all the cells retrogradely labeled from the accessory oculomotor area were also stained by using glutamate or aspartate antisera, but none of them were double-stained with a GABA antiserum. Electrical stimulation of the contralateral LN elicited changes in firing rate of a significant fraction of cells belonging to the accessory oculomotor nuclei (36.4% in the nucleus of posterior commissure, 47.1% in the nucleus of Darkschewitsch, and 44.6% in the interstitial nucleus of Cajal). In 57.8% of the cases, the responses were excitations, most of which had latencies and response characteristics compatible with a monosynaptic linkage. The remaining 42.2% of the cases were inhibitions with latencies ranging between 5 and 22 ms. Extracellular field potential recordings within the contralateral accessory oculomotor nuclei were interpreted as arising from impulses propagating along excitatory axons projecting in a bundle from the cerebellum. Stimulation of LN area in rats following intranuclear injection of kainic acid was not capable of evoking short latency excitations, so these responses can be considered to depend on the activation of LN efferents. The LN projection on accessory oculomotor nuclei could be part of the final precise control exerted by the neocerebellum on those brain structures concerned with movements of the eyes.

Evidence of an x zone in lobule V of the squirrel monkey (Saimiri sciureus) cerebellum: the distribution of corticonuclear fibers

The distribution of corticonuclear fibers to medial-most parts of the posterior interposed nucleus (NIP) from lateral areas of the vermis was studied in the squirrel monkey (Saimiri sciureus), using a silver impregnation method. The origin and course of degenerated fibers were studied in serial sections. The distribution pattern of corticonuclear fibers from a series of small well localized lesions placed in the vermis and paravermal cortex of lobule V is compatible with the interpretation that an x zone is present in Saimiri. A comparison of the positions of lesions and the trajectory of fibers arising therein suggests that corticonuclear input to medial-most parts of the NIP originated from a narrow cortical area (about 0.5-0.7 mm wide) located between a cortical area projecting into the medial cerebellar nucleus (the A zone) and a laterally adjacent area (the B zone) which related to the lateral vestibular nucleus. This NIP-projecting cortical area, located about 1.7 mm to 2.5 mm off the midline in lobule V, is interpreted as the x zone in this primate; it extends from lobule IV into lobule VI in squirrel monkey. Corticonuclear fibers of zone x in this primate form a comparatively small terminal field in the medial-most portions of NIP. This contrasts with the distribution of corticonuclear fibers of the C2 zone which consistently distribute to terminal fields that are shifted into more central areas of NIP. There appears to be no overlap of the corticonuclear terminal fields in the NIP for zone x versus the C2 zone. These results were correlated with data from the literature on the distribution of olivocerebellar fibers to the x zone and the C2 zone and the arrangement of cerebellar nucleoolivary projections into the inferior olive from the NIP. The x zone and the C2 zone both receive input from the contralateral medial accessory olive (MAO), both zones project into the NIP, and the NIP projects into those regions of the MAO which, in turn, project to these respective cortical zones and into the NIP. This suggest that the x zone is a component of the NIP-MAO circuit. Furthermore the proposed function of the x zone would support the view that this sagittal strip may have a more extensive rostrocaudal distribution in primates as compared to the cat.

Cerebellar projections to the somatic pretectum in the cat

Neurons in the somatic pretectum receive input from the dorsal column nuclei (DCN) and project to a comparable "somatic" portion of the dorsal accessory nucleus of the inferior olive (DAO). This somatic DAO is reciprocally connected with the anterior interpositus nucleus of the cerebellum. One question that arises is whether this circuitry is further controlled by an output specifically from the anterior interpositus nucleus to the somatic pretectum. Wheatgerm agglutinin conjugated to horseradish peroxidase was injected into various parts of the cat pretectum. Injection sites were interpreted as including the somatic pretectum if neurons in the DCN were retrogradely labeled and if anterograde terminal labeling occurred in somatic DAO. The locations of retrogradely labeled neurons within the deep cerebellar nuclei were then compared in cases in which the injection sites included or excluded the somatic pretectum. In all cases in which the injection site included the somatic pretectum, retrogradely labeled neurons were observed in the anterior interpositus nucleus as well as in the lateral cerebellar nuclei. In some of these cases, neurons in the posterior interpositus and medial nuclei were also labeled. In contrast, in cases in which the pretectal injection site was located outside or at the border of the somatic pretectum, retrogradely labeled neurons were observed only in the lateral, posterior interpositus, and medial nuclei. Thus, the somatic pretectum appears to receive input primarily from neurons in the anterior interpositus nucleus, along with some input from neurons in the lateral nucleus. These results provide additional evidence for a pathway through the DCN in which sequentially processed somatic information has access to and is modulated by cerebellar circuitry. The existence of such a pathway supports the conclusion that neurons in the DCN convey somatic information important not only for cutaneous, kinesthestic, and other bodily sensations, but also for the control of movement.

Identification of the Purkinje cell/climbing fiber zone and its target neurons responsible for eye-movement control by the cerebellar flocculus

We identified 3 Purkinje cell/climbing fiber zones in the cat cerebellar flocculus. The zones were perpendicular to the long axes of the crooked floccular folia, forming the crooked zones. Each zone was different in axonal projection areas of its target neurons. From the neuronal networks it is theoretically expected that activity changes of a particular zone control eye movement in a particular plane: (1) the rostral and caudal zones on one side control movement in the anterior canal plane on the side of the activity changes and those on both sides control movement in all vertical planes from sagittal to transverse planes; and (2) the middle zone controls movement in the horizontal plane by reciprocal activity changes on both sides. The zone-specific climbing fiber input to a particular zone may contribute to activity changes of the zone in response to mossy fiber input spreading across several zones. Electrical stimulation of each zone evoked the same pattern of eye movement as that theoretically expected from the neuronal networks. This is the first indication that there are indeed functional differences between the Purkinje cell zones in the cerebellum. Our findings support Oscarsson's proposal that each Purkinje cell/climbing fiber zone plus its target neurons may be an operational unit for control of a given motor function.

Distribution of corticotropin-releasing factor in the cerebellum and precerebellar nuclei of the cat

The present study analyzes the distribution of corticotropin-releasing factor-immunoreactive (CRF-IR) fibers and neuronal cell bodies within the cerebellum and brainstem, respectively, of the cat. Within the cerebellum, CRF is present in climbing fibers, mossy fibers, and a population of varicose fibers which traverses the lower molecular layer. CRF-IR fibers are present throughout all lobules of the cat cerebellar cortex, though the density and immunostaining intensity of each fiber system vary. Bands of intensely immunoreactive climbing fibers are prominent within the vermis, intermediate cortex, and crus II. Bands of intensely immunoreactive mossy fiber terminals accompany the climbing fiber bands within the vermis. Collaterals of climbing and mossy fibers contribute to a beaded fiber plexus localized to the Purkinje cell layer. Varicose fibers containing CRF immunoreactivity are present in all deep cerebellar nuclei. CRF-IR neuronal cell bodies are prominent within several brainstem nuclei known to project to the cerebellum: all divisions of the inferior olivary complex, the lateral reticular nucleus, paramedian reticular nucleus, gigantocellular reticular nucleus, raphe nuclei, perihypoglossal complex, medial and inferior vestibular nuclei and cell groups f and x, locus ceruleus, and nucleus subceruleus. This study confirms and extends a previous study of CRF distribution within cerebellar afferent systems of the cat (Cummings et al.: J. Neurosci. 8:543-554, '88) and compares this distribution with previous descriptions in other species. The ubiquitous distribution of CRF throughout the cat cerebellum suggests a primary role for this peptide in signal transduction.

Sagittal organisation of the olivocerebellonuclear pathway in the rat. III. Connections with the nucleus dentatus

The organization of olivary afferents and nuclear efferents of the D-zone of the rat cerebellum was studied by means of tracing with wheat-germ agglutinin-coupled peroxidase using tetramethylbenzidine as a chromagen. The tracer was injected iontophoretically within the cerebellar cortex. This allowed us to study both afferent and efferent pathways of the cerebellar lobules concerned with retrograde and anterograde tracing, respectively. Retrograde cellular labelling in the inferior olive was restricted to the principal olive (PO). Anterograde terminal labelling was found only within the various subdivisions of the nucleus lateralis or dentatus (ND). For any one of our small cortical injections there was a corresponding sagittal band of retrogradely labelled cells in the contralateral PO, and a sagittal band of terminal labelling through the ND. Based on both their olivary and nuclear connections, 3 sagittal subzones can be distinguished within the D-zone of the rat. From medial to lateral, we call them D0, D1 and D2. The 3 subzones run through part of the anterior and posterior lobes. D1 and D2 run continuously from their rostral to their caudal extents whereas D0 is discontinuous. It is interrupted through lobule VIc (crus I). The olivary projections to D0 arise within the medial half of the ventral lamella of the PO, including the dorsomedial cell column. Those to D1 arise within the dorsal lamella of the PO. Those to D2 arise within the lateral half of the ventral lamella of the PO. Rostrocaudally, widely distant cells of the same subdivision of the PO project to the same cerebellar lobule. This indicates extensive convergence of the olivary afferents within each of the 3 hemispheric compartments, D0, D1 and D2. Each of the 3 hemispheric subzones specifically projects to one of the 3 subdivisions distinguished within the ND of the rat, without apparent mediolateral overlapping. The medialmost D0 projects onto the dorsolateral hump; D1 projects more laterally onto the main, magnocellular part of the ND, and D2 projects ventrally onto the parvicellular subdivision of the ND. Thus the sagittal partition of the hemispheric cortex is reflected at the nuclear level. In contrast, Purkinje cell axons from individual lobules appear to branch extensively in the rostrocaudal direction. Therefore, within each of the 3 compartments D0, D1 as well as D2, the nuclear projection of the anterior lobe and the posterior lobe are largely coextensive.

Anatomical mapping of the cerebellar nucleocortical projections in the rat: a retrograde labeling study

An analysis of the cerebellar nucleocortical projections was made by means of retrograde cellular labeling with wheat germ agglutinin-horseradish peroxidase conjugate. Each of the main nuclear subregions appears to give rise to nucleocortical projections. The cortical distribution of the projections is referred to here in term of sagittal zones. Zones A, B, and C conform to the recent description in the rat (Buisseret-Delmas, '88a,b) on the basis of their olivocortical and corticonuclear projections. A corresponding description of zone D is given here. According to their distribution, three types of nucleocortical projections have been distinguished: 1) ipsilateral, reciprocal; 2) nonreciprocal; and 3) contralateral, symmetrical to the corticonuclear afferent. Reciprocal projections are strictly arranged in the sagittal direction, with the following zonal distribution. Zone A is subdivided into two subzones. Medial A zone receives its nuclear afferents from the medial aspect of the nucleus medialis (NM). The lateral A zone of the anterior lobe and lobule VI and that of the posterior lobe receive their reciprocal nuclear afferents from the ventrolateral NM and the dorsolateral protuberance, respectively. Zone B does not seem to receive nucleocortical projections. Zone C has three subzones in the rat. C1 is supplied from the medial third of the anterior and posterior subdivisions of the nucleus interpositus (NIA and NIP, respectively). C2 is supplied from the central third of the NIA and NIP. Rostrocaudally, the anterior lobe and lobule VIII are connected to the NIA, and lobules VI and VII to the NIP. C3 appears to be connected to the lateral third of NIA. Zone D contains three subzones mediolaterally in the rat. D0, not previously described, is defined on the basis of both its olivary afferent from the medial half of the ventral lamella of the principal olive and its corticonuclear projections onto the dorsolateral hump of Goodman et al. ('63). It receives a reciprocal nucleocortical afferent from the dorsolateral hump. D1 receives its olivary afferent from the dorsal lamella of the principal olive. It is reciprocally connected with the lateral, magnocellular part of the nucleus lateralis (NL). D2 is the most lateral subzone of the hemisphere. Its olivary afferent comes from the lateral half of the ventral lamella of the principal olive. D2 is reciprocally connected with the ventral, parvicellular subdivision of NL. The main cortical recipients for the nonreciprocal projections are the lateral A zone, the C3, and the D1 subzones.(ABSTRACT TRUNCATED AT 250 WORDS)

Neocerebellar control of the motor activity: experimental analysis in the rat. Comparative aspects

The results collected by electrical microstimulation of the nucleus lateralis of the cerebellum in anaesthetized rats may be summarized as follows. The stimulations evoked motor effects in head and forelimb principally whereas hindlimb was only occasionally involved. The movements were prevalently segregated to only one joint (simple movements), in a lesser degree they involved two or three segments (complex movements). Simple and complex movements were apparently distributed in the nuclear mass without topographical segregation or preferentiality. The electromyographic records suggest that the neocerebellar movements are of synergistic nature. A somatotopical organization was evidenced within the nucleus lateralis: 3 specific functional regions were identified in the caudorostral nuclear extension. They concern the forelimb (caudally), head (centrally) and hindlimb (rostrally). This somatotopical organization persisted unmodified following elimination of either the cerebral motor cortex alone or in addition to that of the red nucleus. The nuclear subdivisions of the cerebellar nucleus lateralis showed functional differences: (1) the dorsolateral hump of Goodman et al. was principally involved in lip movements; (2) the subnucleus lateralis parvocellularis elicited movements of single vibrissae, neck and medio-distal segments of the forelimb, prevalently; (3) the magnocellular subdivision essentially controlled both limbs with large prevalence for their medio-proximal segments. To identify the functional role of the different descending pathways which relay the neocerebellum to the cord, the motor effects evoked in intact rats were compared with those elicited in rats submitted to cortical ablation and/or to lesion of the red nucleus region. The integrity of the cerebral cortex was essential only for distalmost forelimb motor activities. After lesion of the rubral region (which concomitantly eliminates corticospinal output), the stimulation of the nucleus lateralis evoked motor effects of the proximo-axial segments prevalently with intensity thresholds increased above two-fold those obtained in intact/decorticated rats. The movements elicited in rats with injury of the red nucleus region, including the ascending fibers of the brachium conjunctivum, are presumably mediated to the spinal cord through the reticulospinal pathway. The proportion of simple and complex movements decreased and increased respectively after cortical ablation and further on after injury of the red nucleus region. The discussion on the motor effects elicited in rats by the neocerebellum focussed on the possible role of 3 descending pathways.(ABSTRACT TRUNCATED AT 400 WORDS)

Demonstration of a Mamillo-Ponto-Cerebellar Pathway

The pathway from the mamillary complex to the cerebellum via the pontine nuclei has been studied using several anterograde and retrograde tracing techniques in the cat. We have also compared the pontine terminal regions of fibres from the mamillary complex and from the cingulate gyrus. Implantations of crystalline horseradish peroxidase wheat germ agglutinin (HRP-WGA) in the mamillary complex and lesions of the cingulate gyrus were combined in the same animal with injections of HRP-WGA, rhodamine-B-isothiocyanate (RITC), and Fluoro-Gold in different parts of the cerebellar hemisphere. Fibres from both the mamillary complex and the cingulate gyrus terminate mainly within a transversely oriented, c-shaped band in the ipsilateral, rostral pontine nuclei. Within this band the terminal fields of fibres from the mamillary complex and the cingulate gyrus form a mosaic-like pattern of partly overlapping patches. Pontine regions receiving a mamillary input project mainly to the ventral paraflocculus, and to a lesser degree to the dorsal paraflocculus, but apparently not to the uvula or crus II. Judging from the literature it seems highly unlikely that other parts of the cerebellar hemispheres received projections from these pontine regions. Fibres from the ventral paraflocculus were shown to terminate in the parvicellular part of the lateral cerebellar nucleus only. The present findings would seem to imply that inputs from the mamillary complex and a related cortical region, the cingulate gyrus, are partly integrated, partly kept separate at the precerebellar level. This would ensure that small groups of cells in the rostral pontine nuclei receive a specific set of afferents. Conceivably, the information transmitted to the cerebellum by these groups of pontine cells might be related to functions of the mamillary complex, such as learning, motivation, and spatial memory.

Target neurons of floccular middle zone inhibition in medial vestibular nucleus

Unitary activities of 288 neurons were recorded extracellularly in the medial vestibular nucleus (MV) in anesthetized cats. In 19 neurons, located in the rostral part of the MV adjacent to the stria acustica, floccular middle zone stimulation resulted in cessation of spontaneous discharges. Systematic microstimulation in the brainstem during recording of 16 of 19 target neurons of floccular middle zone inhibition revealed that the target neurons projected to the ipsilateral abducens nucleus (ABN), and not to the contralateral ABN nor the oculomotor nucleus. The conjugate ipsilateral horizontal eye movement elicited by middle zone stimulation may be mediated by this pathway to motoneurons and internuclear neurons in the ipsilateral ABN. In additional experiments, the MV neurons responding antidromically to ipsilateral ABN stimulation and orthodromically to ipsilateral 8 nerve stimulation were recorded extracellularly. In only 7 of 36 recorded neurons, middle zone stimulation depressed the orthodromic and spontaneous activities. Many neurons were free of floccular inhibition. As to the route of floccular inhibitory control over the vestibulo-ocular reflex (VOR) during visual-vestibular stimulation, we propose that the interaction of target and VOR relay neurons takes place at the ipsilateral ABN and modulates the VOR, in addition to well known Ito's proposal that the interaction of the floccular output and the VOR takes place at secondary vestibular neurons and modulates the VOR.

The dentatorubral projection in the rat: an autoradiographic study

The dentatorubral projection has been mapped in rats using autoradiography. Any part of lateral cerebellar nucleus (NL) projects throughout the contralateral parvocellular red nucleus (NRp) rostrocaudally; the projection is topographically organized: (1) a caudorostral shift in the NL corresponds to a dorsoventral displacement through the NRp; matching of this arrangement with the origin of rubrospinal projections is discussed; (2) only ventral parts of the NL, including the parvocellar subnucleus, project to the lateral edge of the NRp.

Afferent and efferent connections of the oculomotor cerebellar vermis in the macaque monkey

Saccadic eye movements were evoked with weak currents applied to a circumscribed vermal area. The area was confined to lobule VII in the majority of the monkeys and coincided with the distribution of saccade-related neural activity. We defined this area as the oculomotor vermis and studied its anatomical connections with wheat germ-agglutinin conjugated horseradish peroxidase (WGA/HRP) and HRP. When injected HRP was confined to the oculomotor vermis, most labeled Purkinje axons terminated ipsilaterally in an ellipsoidal region in the mediocaudal aspect of the fastigial nucleus. Retrogradely labeled cells were found in two relatively circumscribed regions in the fastigial nucleus: one group was in the lateral half of the ellipsoidal terminal region and the other group was in a spherical region near the lateral margin of the nucleus. Following the injection of HRP into the oculomotor vermis, the largest population of retrogradely labeled neurons was found in the nucleus reticularis tegmenti pontis. Labeled cells were located only in the medial and dorsolateral portions of the nucleus. The cell aggregates in the dorsolateral portion merged with densely labeled cells of the processus tegmentosus lateralis. The second largest population of labeled cells was found in the pontine nuclei. Approximately 28% of the labeled pontine cells aggregated in the paramedian pontine nucleus, whereas the other labeled pontine cells were widely distributed in the dorsal part of the pontine peduncular nucleus and the dorsolateral pontine nucleus. Labeled cells were scattered also in the pontine raphe, the paramedian pontine reticular formation, and the interfascicular nucleus at the rostral level of the hypoglossal nucleus. Fewer labeled cells were discovered in the vestibular nuclear complex and the prepositus hypoglossi. In the inferior olivary nucleus, labeled cells were located in the subnucleus b of the medial accessory nucleus.

Further studies on the use of the fluorescent tracers fast blue and diamidino yellow: effective uptake area and cellular storage sites

Some basic methodological issues concerning the use of the fluorescent tracers Fast blue (FB) and Diamidino yellow (DY) were studied using the projections of the red nucleus to the nucleus interpositus anterior (NIA) of the cerebellum of the cat. On standard Nissl-stained sections, it was possible to delineate 4 distinct zones at the FB and DY injection sites. Correlative studies of injection sites in the NIA and retrograde labeling of cell bodies in the contralateral red nucleus showed that effective uptake occurred only from the zone mechanically damaged by the injection needle (termed zone 0). The tracer remains in this zone during the post-injection survival. The limited uptake area for both tracers is a valuable feature for studies of restricted neuronal projections. However, the tracers are not suitable for use in quantitative studies, especially those concerning axonal collateralization. Perfusion with water-soluble fixatives did not alter the cellular storage site. In double-labeling experiments using horseradish peroxidase and DY, the HRP histochemistry induced an important "washing out" of DY and consequently, an underestimation of the number of labeled neurons.

The cerebellar corticonuclear projection from lobule Vb/c of the cat anterior lobe: a combined electrophysiological and autoradiographic study. I. Projections from the intermediate region

The present study examines the projection to the cerebellar nuclei of Purkinje cells in particular sagittal zones within the intermediate region of the cerebellar cortex. The boundaries between the zones were delimited electrophysiologically on the basis of their climbing fibre input so that a small volume (10-120 nl) of 3H-leucine could be injected into the centre of a chosen zone. The subsequent uptake and orthograde transport of labelled material by the Purkinje cells was studied autoradiographically. It was found that the smallest injections resulted in injection sites restricted to a single cortical zone and extremely reproducible results could be obtained using such a combined electrophysiological/autoradiographic technique. Larger injections sometimes spread to a neighbouring zone but the resultant terminal labelling within the deep nuclei was invariably consistent with the results obtained from smaller injections. The c1 and c3 olivocerebellar zones, which are known to receive climbing fibre input transmitted from the ipsilateral forelimb via a dorsal funiculus spino-olivo-cerebellar pathway (DF-SOCP), were found to project to partially overlapping regions within nucleus interpositus anterior (NIA). No projection to nucleus interpositus posterior (NIP) was demonstrated for either zone. No distinction could be seen between the terminal fields for the medial and lateral halves of the c1 zone which are, however, known to receive their climbing fibre input from quite separate regions within the inferior olive. The c2 zone, which was delimited on the basis of its climbing fibre input which is transmitted from both forelimbs via a lateral funiculus SOCP, was found to project exclusively to interpositus posterior. The hemispheral d1 zone was found to project to the transitional region where interpositus anterior and the dentate nucleus adjoin.

The projection of the nucleus reticularis tegmenti pontis and adjacent regions of the pontine nuclei to the central cerebellar nuclei in the cat

The projection of the nucleus reticularis tegmenti pontis (NRTP) and the pontine nuclei (NP) to the central cerebellar nuclei (CCN) was investigated by means of anterograde transport of tritiated leucine. Although termination was found in all the CCN, it was most pronounced in the lateral nucleus and the lateral aspect of the posterior interposed nucleus. The extreme lateral aspect of the anterior interposed nucleus and the caudal part of the fastigial nucleus received a projection of modest intensity. Termination in the infracerebellar nucleus and group Y is likely to be present but could not be confirmed with certainty from the light microscopical material. The contribution from the NP was small and originated from the dorsolateral and dorsal paramedian subdivisions of the NP. Within the NRTP the total area giving rise to projections to the CCN was extensive, and the origin of the projections to the individual CCN overlapped considerably. The projection of the NRTP to the ventrocaudal part of the lateral nucleus was found in conjunction with a projection to the ventrolateral part of the posterior interposed nucleus. Both projections seemed to branch off the fiber bundle terminating in the ventral paraflocculus. Similar correlations could be established in the projection of the NRTP to the dorsal paraflocculus and crus II of the ansiform lobule with other parts of the lateral and posterior interposed nuclei. It was concluded that the transverse, lobular organization of mossy fibers, which differs fundamentally from the longitudinal, modular organization of climbing fibers, is maintained in the collateral projection to the CCN. The results are further discussed in relation to the corticonuclear projection and the engagement of the NRTP and different parts of the CCN in pontocerebellar circuits.

Functional organization of thalamic projections to the motor cortex. An anatomical and electrophysiological study in the rat

In rats, horseradish peroxidase crystals were injected in motor cortical foci functionally identified by means of the motor effects evoked by electrical stimulations. The location in the thalamus of the neurons linked to different motor cortical foci was studied. Thalamic neurons were retrogradely labeled in both "motor" (ventralis lateralis and ventralis medialis) and "non-motor" nuclei: centralis lateralis, lateralis posterior, mediodorsalis and posterior thalamic nuclear group, as well as the ventrobasal complex. The ventrobasal complex was labeled after horseradish peroxidase injections in hindlimb and trunk motor areas. The ascending projections toward the motor cortex from both "motor" and "non-motor" thalamic nuclei are organized more precisely and more elaborately than previously reported. The motor cortical afferents from the nucleus ventralis lateralis are organized in three planes, rostrocaudally, dorsoventrally and mediolaterally. An inverted relation exists in the rostrocaudal plane between the nucleus ventralis lateralis and the motor cortex: the caudal motor cortex region (hindlimb) receives fiber inputs from the rostral region of the nucleus ventralis lateralis, whereas the caudal zone of the nucleus ventralis lateralis projects to the rostral motor cortex region (forelimb and vibrissae). A dorsoventral organization has also been observed in the rostral region of the nucleus ventralis lateralis: the ventral aspect is the source of fibers directed to the distal hindlimb region, whereas fibers originating from the dorsal aspect are directed to the proximal hindlimb area. A mediolateral relationship exists between medial and lateral sides of the nucleus ventralis lateralis and, respectively, proximal and distal forelimb cortical areas. There is some overlap between the various nuclear regions thus delineated. Four functional zones were found in the lateral half of the nucleus ventralis medialis and were classified according to their projection to the motor cortex; these are involved in motor control of the proximal and distal forelimb, vibrissae and ocular movements. The projection is topographically organized according to both an inverted rostrocaudal and a direct dorsoventral-mediolateral arrangement. Caudally, dorsal and ventral nuclear parts project to rostromedial (vibrissae) and rostrolateral (distal forelimb) regions of the motor cortex, respectively. More rostral nuclear zones project to more caudal (proximal forelimb, eye) cortical regions. There is little overlap between these four nuclear subdivisions. The nucleus centralis lateralis projects to vibrissae and proximal, as well as distal, forelimb areas.(ABSTRACT TRUNCATED AT 400 WORDS)

Topographic organization of the cerebellothalamic projections in the rat. An autoradiographic study

The topographical organization of the subnuclear projections towards the thalamus was studied with autographic methods in adult Wistar rats. The four cerebellar deep nuclei give rise to projections to the ventral region of the rostral thalamus. Most of the fibers end contralaterally, according to a topographical pattern; however, some fibers from each of the cerebellar nuclei recross the midline at the thalamic level and terminate ipsilaterally, within regions symmetric to those receiving the densest contralateral projection. These ipsilateral cerebellothalamic components arise in decreasing order from the caudal nucleus lateralis, the ventrocaudal nucleus medialis and the nucleus interpositus, respectively. The projections of the nucleus lateralis directed to the contralateral thalamus are topographically organized. (1) Within the nucleus ventralis lateralis, the rostral and caudal parts of the cerebellar nucleus lateralis project respectively to rostral and caudal regions; lateral and medial zones of the nucleus lateralis project, respectively, to medial and central aspects of the nucleus ventralis lateralis. (2) The nucleus ventralis medialis and particularly its caudal portion appears to receive the bulk of its afferents from the ventromedial portion of the nucleus lateralis including the "subnucleus lateralis parvocellularis". (3) The nucleus centralis lateralis receives fibers from most parts of the nucleus lateralis including the "dorsolateral hump". (4) The nucleus interpositus anterior projects to the dorsomedial aspect of the rostral nucleus ventralis lateralis. In the latter nucleus, the ventrolateral aspect of the central region receives projections in cases in which the nucleus interpositus posterior is largely involved. A particular emphasis is put on the different projections from the various subnuclear regions of the lateral nucleus. A comparison is attempted with the situation in the primates, particularly with regard to the question of the parvocellular subdivision of the lateral nucleus.

Localization of serotonin immunoreactivity in the opossum cerebellum

We have used the indirect antibody peroxidase-antiperoxidase technique to analyze the course of serotonin (5-hydroxytryptamine; 5HT) fibers to the deep cerebellar nuclei; the distribution of serotonin within the nuclei; the continued course of 5HT fibers to the cerebellar cortex; and the lobular and laminar distribution of this indoleamine in the cerebellar cortex. Only rarely are fibers found in either the restiform body or the brachium pontis. However, a distinct bundle of serotoninergic axons is present in the medial aspect of the brachium conjunctivum. Axons arise from this bundle and course dorsally into the neuropil of the deep cerebellar nuclei. The densest immunostaining is present in posterior and ventral regions of all four cerebellar nuclei. Within the nuclei large (24% of total) and small (76% of total) varicosities are present. The average distance between varicosities on individual axons is 3.85 micron (S.D. = 1.2). The innervation of the cerebellar cortex is derived primarily from fibers that course through the deep nuclei. At levels caudal to the deep nuclei a single midsagittal band courses into lobules VIII and IX. In the cerebellar cortex, serotoninergic axons and varicosities are present in all lobules; however, the fiber density is not uniform. The densest distribution is present in vermal lobule VIII and the dorsal folia of lobule IX. Within the granule cell layer of lobules VIII and IX, immunoreactive elements form a midsagittal band, and to a lesser degree, two parasagittal bands. Beaded serotoninergic fibers course through the deep and middle portion of the granule cell layer and give rise to a plexus at the border between the Purkinje cell and granule cell layers. Within this plexus axons extend long distances in the transverse and sagittal planes. Long beaded axons oriented in the transverse plane of the folia are also present in the deep molecular layer. A few radial serotoninergic fibers ascend to the pial surface and give rise to very short tangential branches. In all three cortical layers, both large (19% of total) and small (81% of total) varicosities are present. The average distance between varicosities on individual fibers is 5.3 micron (S.D. = 2.2).(ABSTRACT TRUNCATED AT 400 WORDS)

Basal interstitial nucleus of the cerebellum: cerebellar nucleus related to the flocculus

We have shown that the monkey flocculus is not connected with any of the major, well-demarcated cerebellar nuclei. There is, however, a broadly distributed interstitial population of neurons in the white matter ventral to the cerebellar nuclei and extending into the peduncle of the flocculus; this population, previously undescribed in the monkey, has reciprocal connections with the flocculus (Langer et al., '85a,b). Several lines of evidence indicate that this collection of neurons, called the basal interstitial nucleus of the cerebellum (BIN/Cb), can justifiably be considered a nucleus. (1) Injection of horseradish peroxidase (HRP) into the flocculus always labels a group of neurons that lie immediately ventral to the well-demarcated cerebellar nuclei and extend posteromedially into the lateral margin of the nodulus and rostrolaterally around the caudal surface of the y-group, infiltrating the peduncle of the flocculus. (2) In Nissl-stained material there is a readily seen collection of neurons that are clearly distinct from the overlying cerebellar nuclei, with precisely the same distribution. These neurons have a characteristic morphology: they are intermediate-sized, chromatophilic, multipolar, and fusiform, and have rapidly tapering proximal dendrites. The cell nucleus is generally placed eccentrically in the cell body, against the plasma membrane or in one pole of the cell. The Nissl substance is usually finely granular in the center of the cell body and forms dense clumps adjacent to the cell membrane. (3) Anterograde label from injections of HRP or tritiated amino acids into the flocculus extends over the same group of neurons. In one brain with an HRP injection involving a part of the BIN/Cb there was a patchy, clustered distribution of labeled Purkinje cells extending throughout the flocculus and into the adjacent lateral parts of the simple lobule. The clusters were confined to the medial half of many of the floccular folia.

Synaptic organization of the cerebello-thalamo-cerebral pathway in the cat. I. Projection of individual cerebellar nuclei to single pyramidal tract neurons in areas 4 and 6

The neural connections of the dentate (DN) and the interpositus (IN) nuclei to the motor cortex and area 6 were investigated by recording intracellular postsynaptic potentials from fast and slow pyramidal tract neurons (PTNs) in the anesthetized cat. Localized stimulation of DN and IN produced di- or polysynaptic EPSPs in fast and slow PTNs in the "forelimb area" of the motor cortex and area 6. The effects of stimulation of the two cerebellar projections were essentially the same, although some regional difference of their relative strength was noted. In these cortical areas, the majority of fast and slow PTNs received convergent inputs from both DN and IN. By examining the interaction of DN- and IN-evoked EPSPs, spatial facilitation and occlusion at the level of the thalamus were demonstrated. Therefore, it was concluded that at least a portion of the convergence of the dentate and the interpositus inputs occurred at the level of the ventrolateral nucleus of the thalamus.

Neuronal pathway from floccular caudal zone contributing to vertical eye movements in cats--role of group y nucleus of vestibular nuclei

In ketamine-anesthetized cats, electric microstimulation of the group y nucleus of the vestibular nuclei evoked a slow and smooth upward eye movement. Destruction of the group y nucleus eliminated a slow and smooth downward eye movement evoked by stimulation of the caudal zone of the flocculus. These data support the interpretation that Purkinje cell activity in the caudal zone of the flocculus can evoke vertical eye movements by inhibiting the activity of neurons of the group y nucleus.

Functional localization in the three floccular zones related to eye movement control in the cat

Anatomically, the cat's cerebellar flocculus can be divided into 3 zones on the basis of differences in their efferent projection sites. The functional differences of these 3 zones in relation to eye movement control were investigated by observing the eye movements evoked by electric stimulation of each zone of the flocculus in ketamine-anesthetized cats. Stimulation of the flocculus elicited a slow eye movement. The direction of the slow eye movement was mapped. A downward eye movement was evoked by stimulation of the caudal zone. An ipsilateral horizontal eye movement was induced from the middle zone. An upward eye movement was elicited from the rostral zone. When prolonged stimulation was applied to the flocculus, the slow eye movement was followed by nystagmus in the opposite direction. This nystagmus persisted for many seconds after cessation of stimulation (afternystagmus). Nystagmus and afternystagmus could not be elicited in deeply anesthetized cats. Possibilities as to how the stimulation leads to various eye movements are discussed.

Preservation of physical dimensions in a model of reactive synaptogenesis in the red nucleus

Collateral sprouting of cerebral cortical fibers in the red nucleus following destruction of the interpositus nucleus may be effective in restoring activity of rubral neurons. Shrinkage of the deafferented red nucleus was measured to estimate its effect on recording neural activity and its contribution as a stimulus for sprouting. The results suggest that rubral morphology is preserved during the early time course of collateral sprouting when electrophysiological changes are evident.

The cerebellar projections to the superior colliculus and pretectum in the cat: an autoradiographic and horseradish peroxidase study

Efferent projections from the cerebellar nuclei to the superior colliculus and the pretectum have been studied using both retrograde and orthograde labeling techniques in the cat. In order to identify what parts of the cerebellar nuclei project to the superior colliculus and the pretectum, the retrograde horseradish labeling technique was employed. In another set of experiments, tritiated amino acids were injected into each of the cerebellar regions from which the cerebello-tectal and cerebello-pretectal projections arise, and the laminar and spatial distributions of orthograde labeling in the superior colliculus and the pretectum were compared. The results showed that the cerebello-tectal projections arise from two different regions of the cerebellar nuclei: the caudal half of the medial nucleus and the ventrolateral part of the posterior interposed nucleus. Fibers arising from the medial nucleus distribute bilaterally in the superficial zone of the intermediate gray layer in the superior colliculus, while those originating from the posterior interposed nucleus terminate contralaterally in the deeper aspect of the intermediate gray layer and in the deep gray and white layers. Although the lateral nucleus does not contribute to the cerebello-tectal projection, it projects profusely to the pretectum contralaterally. The origin of the cerebello-pretectal projection lies in the parvicellular part of the lateral nucleus. Among several pretectal nuclei, the posterior pretectal, the medial pretectal nucleus and the reticular part of the anterior pretectal nucleus receive the cerebellar afferents. The findings of the differential projections from the cerebellum to the superior colliculus and the pretectum suggest that the cerebellum exerts a regulatory influence on visuo-motor and somato-motor transfer in these midbrain structures by differential circuits.

Cerebellar corticonuclear fibers of the paramedian lobule of tree shrew (Tupaia glis) with comments on zones

Following a series of lesions in dorsal (DPML) and ventral (VPML) divisions of tree shrew (Tupaia) paramedian lobule (PML), the distribution of degenerated axons within the deep cerebellar nuclei was determined using the Fink and Heimer ('67) method. Damage to PML produced axonal degeneration in lateral (NL), anterior interposed (NIA), and posterior interposed (NIP) cerebellar nuclei. No degenerated fibers could be traced to either the medial cerebellar nucleus or vestibular complex, via juxtarestiform body, from lesions in PML. Corticonuclear fibers to NL, NIA, and NIP from PML cortex are topographically organized. Subsequent to lesions of lateral DPML, axonal debris is found in rostral and medial NL, while the lateral edge of VPML projects primarily into medial NL. According to the terminology of Voogd ('69) these lateral regions of PML represent the D zone. The NIP receives corticonuclear input from a relatively wide middle area of both portions of PML, interpreted as the C2 zone. There is some evidence which suggests that medial portions of the C2 area of DPML project into more lateral areas of NIP, while lateral regions of this zone in DPML are related to more medial NIP. This projection pattern is invited for the C2 area of VPML; medial C2 to medial NIP, lateral C2 to lateral NIP. Corticonuclear fibers of PML which enter NIA appear to arise from a narrow, irregular, partially discontinuous strip of cortex located at the interface of the D and C2 areas in lateral PML and from a wider, more regular region in the most medial areas of this lobule. These represent, respectively, the C3 and C1 zones. Although an overall pattern of zones is present, there is evidence to suggest that their spatial organization differs from DPML to VPML. The zonal patterns appears to be more obvious in VPML, while this pattern for DPML is less distinct. This is interpreted as indicating that either (1) zones C1--C3 overlap to a greater degree in DPML than in VPML, or (2) zones C1 and C3 may converge in rostral DPML, partially obliterating the intervening zone C2. The different ways in which zonal terminology is applied to both corticonuclear and certain of the afferent cerebellar systems are discussed.

Harmaline-induced tremor. II. Unit activity correlation in the interposito-rubral and oculomotor systems of cat

Units were recorded extracellularly in the cat brainstem under the effect of tremogenic doses of harmaline. They were localized post mortem and the units discharging at the harmaline tremor frequency were mapped. Harmaline-sensitive neurons were found in the bulbo-pontine reticular formation, in particular, in the lateral reticular nucleus and the nucleus reticularis tegmenti pontis. The nucleus interpositus as well as the red nucleus also displayed numerous units discharging at the tremor frequency, indicating that the cerebello-interposito-rubro-spinal system controlling the flexor muscles participate in harmaline tremor. Participation of the oculomotor system in the harmaline-induced tremor was tested at the level of the vestibular neurons relaying the vestibulo-ocular reflex, the motoneurons, the eye muscles and the eye movements. No rhythmic discharge at the tremor frequency nor eye movements could be detected, indicating that harmalie tremor does not affect the oculomotor system.

A light microscopic investigation of the afferent connections of the lateral reticular nucleus in the cat

The topographical organization of the projections to the lateral reticular nucleus (LRN) of the cat was investigated using the horseradish peroxidase (HRP), silver-impregnation and autoradiographic tracing methods. Following injection of HRP into the LRN, labelled cells were found mainly within Rexed's laminae VII and VIII of the spinal cord, the contralateral red nucleus, the ventro-rostral aspect of the contralateral fastigial nucleus and the contralateral anterior sigmoid and coronal gyri of the cerebral cortex. Animals with injections of tritiated amino acids placed within the pericruciate cortex, red nucleus or fastigial nucleus were processed for autoradiography. In a corresponding series of animals, electrolytic lesions were placed selectively into the above sources of reticular afferents, and terminal degeneration within the LRN was studied by light microscopy. An extensive input from the spinal cord was found to terminate predominantly on the ipsilateral side throughout the rostrocaudal extent of the LRN, except for a small ventromedial area of the rostral parvocellular division and a small rostromedial area of the magnocellular division. The cortical projection terminated diffusely within the middle one-half of the contralateral magnocellular division, while the rubral projection terminated extensively within the contralateral subtrigeminal division and the dorsolateral region of the rostral magnocellular and neighbouring parvocellular divisions. The rubral projection did not overlap the cortical projection. The fastigial nucleus projected sparsely to the contralateral LRN, mainly to the medial aspect of the rostral two-thirds of the magnocellular division, with less to the parvocellular and subtrigeminal divisions. The LRN therefore receives spinal and supraspinal projections within at least its rostral one-half, and these terminate within specific areas in a partially overlapping fashion, whereas the caudal one-half is primarily a spinal receiving region. No convergence of the rubral and sensorimotor cortical projections was evident.

The cerebellar nucleo-olivary projection in the cat

The crossed cerebello-olivary projection in the cat was studied by means of retrograde transport of HRP. The cerebello-olivary connection is organized according to a zonal pattern similar to that of the olivocerebellar projection. However, some labelled neurons are in addition found in cerebellar nuclear areas adjacent to a nuclear zone sending its fibres to the corresponding olivary region. This observation indicates that there is a certain degree of overlapping between the different nuclear zones. The cerebello-olivary fibres from the fastigial nucleus appear to be more widely distributed than those from the other cerebellar nuclei. Nuclear neurons of all sizes project to the inferior olive, but the majority of the cells are medium sized. The findings are discussed and related to previous studies on the cerebello-olivary connection.

A cerebello-pulvino-cortical and a retino-pulvino-cortical pathways in the cat as revealed by the use of the anterograde and retrograde transport of horseradish peroxidase

A cerebello-pulvino-cortical and a retino-pulvino-cortical pathways were revealed in the cat by means of the horseradish peroxidase (HRP) method. The sites of termination of the cerebellofugal and retinofugal fibers in the pulvinar nucleus (Pul) were visualized by the use of the anterograde transport of HRP. The cerebello-pulvinar fibers were found to arise mainly from the parvicellular region of the lateral cerebellar nucleus and to terminate contralaterally in a narrow area at the extreme dorsolateral edge of the Pul at the level of the stereotaxic frontal plane A-7.0. The area of terminal ramification of retino-pulvinar fibers was seen as a thin sheet lying at the extreme lateral edge of the Pul through most of the rostrocaudal extent of the Pul, bilaterally with contralateral predominance. The cerebellorecipient area in the Pul did not seem to overlap with the retinorecipient Pul area; the former appeared to be contiguous ventrolaterally to the latter. The cerebellorecipient and retinorecipient Pul areas were also observed to be connected reciprocally with the cerebral cortical areas; the former was connected with the most posterior part of the area 20, and the latter with the area 19.

An HRP and autoradiographic study of the projection from the cerebellar cortex to the nucleus interpositus anterior and nucleus interpositus posterior of the cat

The recently developed anatomical techniques of retrograde transport of the enzyme horseradish peroxidase (HRP), anterograde transport of tritiated amino acid, and intracellular injections of HRP were used to study the organization of the corticonuclear projection to the nucleus interpositus anterior (NIA) and the nucleus interpositus posterior (NIP) of the cat. Injections of HRP into the NIA and the NIP revealed that the major areas of the cortex which provided afferents to these two nuclei were the intermediate cortex of the anterior lobe (IAL) and the paramedian lobule (PML). There were, however, significant differences in the distribution of Purkinje (Pk) cells which projected to each nucleus. The NIA received afferents from all areas of the IAL while the NIP projection area was restricted to a band located at the medi-almost aspect of the lobe. All areas of the PML, in particular the intermediate folia, projected to the NIP, while the Pk cells which sent axons to the NIA were restricted to the rostral and caudal folia of this lobule. The projection from each area was somatotopically organized. The axons of intracellularly stained Pk cells were followed to their termination in the NIA and NIP confirming the results obtained with the two extracellular techniques. An attempt was made to examine the organization of the corticonuclear projection at the single cell level in the PML. Pk cells located in the same sagittal plane appeared to terminate in the same area of the same nucleus while Pk cells located not more than 500 micrometers medial or lateral to each other terminated in different nuclei. Basically, the organization of the corticonuclear projection from the IAL is longitudinally organized while the PML has a much more complex arrangement in which the Pk cells projecting to the NIA and NIP are interspersed.

Sources of subcortical projections to the superior colliculus in the cat

A comprehensive search for subcortical projections to the cat superior colliculus was conducted using the retrograde horseradish peroxidase (HRP) method. Over 40 different subcortical structures project to the superior colliculus. The more notable among these are grouped under the following categories. Visual structures: ventral lateral geniculate nucleus, parabigeminal nucleus, pretectal area (nucleus of the optic tract, posterior pretectal nucleus, nuclei of the posterior commissure). Auditory structures: inferior colliculus (external and pericentral nuclei), dorsomedial periolivary nucleus, nuclei of the trapezoid body, ventral nucleus of the lateral lemniscus. Somatosensory structures: sensory trigeminal complex (all divisions, but mainly the gamma division of nucleus oralis), dorsal column nuclei (mostly cuneate nucleus), and the lateral cervical nucleus. Catecholamine nuclei: locus coeruleus, raphe dorsalis, and the parabrachial nuclei. Cerebellum: medial, interposed, and lateral nuclei, and the perihypoglossal nuclei. Reticular areas: zona incerta, substantia nigra, midbrain tegmentum, nucleus paragigantocellularis lateralis, and the hypothalamus. Evidence is presented that only the parabigeminal nucleus, the nucleus of the optic tract, and the posterior pretectal nucleus project to the superficial collicular layers (striatum griseum superficiale and stratum opticum), while all other afferents terminate in the deeper layers of the colliculus. Also presented is information concerning the rostrocaudal distribution of some of these afferent connections. These findings stress the multiplicity and diversity of inputs to the deeper collicular layers, and more specifically, identify multiple sources of the physiologically well-known representations of the somatic and auditory modalities in the colliculus.

The cerebellar projection of the vestibular nerve in the cat

The projection of the vestibular nerve to the cerebellum of the cat was examined with silver degeneration methods after complete lesions of the vestibular ganglion. The majority of the primary vestibular afferents were traced to the cortex of the ipsilateral nodulus and uvula, relatively fewer entering the ipsilateral flocculus. Fibers were not traced to the paraflocculus, lingula or lateral cerebellar nucleus. A sparse projection to the ipsilateral fastigial nucleus may exist, but it remains equivocal until confirmed with additional methods. Light microscopic examination of plastic sections confirmed these observations and showed further details of the organization of the primary vestibular projection to the nodulus and uvula. These results show that the region of the cerebellum densely innervated by primary vestibular afferents is smaller than previously believed.

The organization of afferents to the cerebellar cortex in the cat: projections from the deep cerebellar nuclei

The topography of the cerebellar nucleo-cortical projection was investigated in the cat by experiments employing the horseradish peroxidase (HRP) technique or by combined HRP-autoradiographic methods. The results of the HRP studies extend previous findings showing that neurons in the deep nuclei project to the cerebellar cortex in an orderly way. Thus, it appears that the cortex of the vermis-proper receives projections from neurons located predominately in the fastigial nucleus. Intermediate and lateral zones of mid-vermal cerebellar cortex are projected on by neurons located in the interposed and dentate nuclei. Crus II receives input from neurons located predominately in the dentate nucleus, while the paramedian lobule is projected on by neurons located in a large postero-dorsal sector of the interposed nucleus and in a smaller medial strip of the dentate nucleus. Neurons in the ventral part of the dentate nucleus and the lateral part of the interposed nucleus send fibers to the paraflocculus. The nucleo-cortical pathway to the flocculus and nodulus arises largely from a population of neurons located in a ventral region stretching from the medial border of the dentate nucleus to the lateral border of the fastigial nucleus. The results of experiments using the combined HRP-autoradiographic method show that clusters of neurons in the deep cerebellar nuclei project back to the cerebellar cortical areas from which they receive input, establishing a fairly precise feedback loop between the cerebellar cortex and deep nuclei.