E. G. Jones

Distribution of cerebellar terminations and their relation to other afferent terminations in the ventral lateral thalamic region of the monkey

The efferent projections of the deep cerebellar nuclei were studied and their fiber trajectories and thalamic termination zones described. The thalamic termination zones for the dentate, interposed and fastigial nuclei are identical and coincide with the cytoarchitectonically unique cell-sparse region of the ventral lateral complex. This region includes nuclei VPLo, VLc, VLps, X and extensions of these between the cell clusters of nucleus VLo. The inputs from dentate and interpositus are contralateral, dense, and their termination patterns extend continuously throughout all nuclear components of the cell-sparse zone. Interdigitation of these two inputs within the cell-sparse region is directly demonstrated. The fastigial input is more restricted but bilateral. Each of the deep cerebellar nuclei also projects to the central lateral nucleus of the intralaminar complex. The strong interconnection of the cell-sparse zone with cortical area 4 is confirmed. The patterns of retrogradely labeled thalamocortical cells and of anterogradely labeled corticothalamic terminations following cortical injections of horseradish peroxidase and of tritiated amino acids, extend continuously through the VPLo-VLc region and its extensions, but do not invade the posteriorly situated VPLc nucleus. Thalamic inputs from the dorsal column nuclei terminate independently within the morphologically distinct VPLc nucleus adjacent to the cell-sparse cerebellar terminal zone. The dorsal column-lemniscal terminations do not overlap the cerebellar terminations. The clear segregation of the two sets of terminations is demonstrated directly using an anterograde double labeling method. Spinothalamic terminations end in VPLc but extend into the cerebellar terminal zone. Another ascending input, from the vestibular nuclei, is also shown to terminate within the cell-sparse zone. Comparison with other studies implies that cerebellar, pallidal and substantia nigral inputs do not converge in the monkey thalamus and that the nuclei in which they terminate project to different cortical areas. The relation of these and of sensory influences ascending to motor cortex from the periphery are discussed.

Anatomical evidence for segregated focal groupings of efferent cells and their terminal ramifications in the cerebellothalamic pathway of the monkey

Patterns of termination of the cerebellothalamic pathway were investigated using anterograde tracing techniques. The thalamic projections from each of the deep cerebellar nuclei are topographically organized in two and possibly in three dimensions. First, the caudo-rostral cerebellar nuclear dimension is mapped onto the mediolateral dimension within the cell-sparse ventral lateral thalamic region (VPLo, VLc, VLps, and nucleus X). By correlating this topographic ordering with the previously established lamellar organization of the cell-sparse thalamic region a somatotopy is inferred within the deep cerebellar nuclei, with caudal body parts represented anteriorly and rostral body parts represented posteriorly in each nucleus. A second topography consists of the mapping of the mediolateral dimension of the dentate and interpositus nuclei onto the ventrodorsal dimension of the lamellae in the thalamus. Since the thalamic connections with motor cortex predict a somatotopic organization with distal body parts ventral and axial parts dorsal in thalamus, each cerebellar nucleus should, therefore, represent axial body parts laterally and distal parts medially. A third mapping dimension is shown for the dentatothalamic projection: dorsal parts of the dentate nucleus project posteriorly within the cell-sparse thalamic region, and ventral parts project anteriorly. The significance of this as regards representation of the body is not known. Subsidiary foci of terminations within the cell-sparse thalamic region are visible following tritiated amino acid injections into each of the deep cerebellar nuclei. Following dentate injections these foci appear as anteroposteriorly elongated, rod-like aggregations of terminations which are similar to the rod-like aggregations of thalamocortical relay cells which have been demonstrated following focal injections of horseradish peroxidase into the motor cortex. The interpositothalamic and the fastigiothalamic terminations are elongated and appear as focal clusters in all planes of section. The interpositothalamic clusters are distributed within posterodorsally curving planar sheets. An anterograde double labeling technique, using a combination of the autoradiographic technique with the axonal degeneration technique, was used to investigate the interrelations of the terminations from different nuclei and from different parts of the same nucleus. Rods from different parts of the dentate nucleus terminate independently of one another. Dentatothalamic rods and interpositothalamic clusters, though interdigitating within the same thalamic region, do not overlap. This topographic and modular organization of the cerebellothalamic pathway suggests that the cerebellar input may reflect both the somatotopic and the columnar organization of the motor cortex.

Brainstem and spinal projections of the deep cerebellar nuclei in the monkey, with observations on the brainstem projections of the dorsal column nuclei

Abstract The brainstem and spinal projections of fibers arising in each of the deep cerebellar nuclei were compared. The brainstem terminations of fibers arising in the dentate and interposed nuclei are similar. Both cerebellofugal systems follow similar intracerebellar trajectories, leave the cerebellum via the brachium conjunctivum, decussate and distribute terminal ramifications contralaterally in the red nucleus, the nucleus reticularis tegmenti pontis and the inferior olivary complex. Although the zones innervated by the dentate and interposed nuclei are similar within the nucleus reticularis tegmenti pontis, these two deep cerebellar nuclei project differentially to the red nucleus, and to the inferior olivary complex. The dentate nucleus projects only to the parvocellular red nucleus and the principal olivary nucleus, whereas the interposed nucleus projects only to the magnocellular red nucleus and the two accessory olivary nuclei. No terminations were detected in the oculomotor nuclei following injections into the dentate nucleus. The fastigial projections to the brainstem are extensive, bilateral, and independent of the dentatofugal and interpositofugal systems. The majority of the fastigio-brainstem fibers decussate within the cerebellum, then leave the cerebellum via the uncinate fasciculus. Contralaterally, the fastigio-brainstem projections terminate in the nucleus reticularis tegmenti pontis, Deiters nucleus, portions of the descending vestibular nucleus, the pontine nuclei, the pontine raphe, and parts of the reticular formation from the midbrain caudally. The fastigial projection to the nucleus reticularis tegmenti pontis terminates in a zone clearly independent of the dentate and interpositus projections. The ipsilateral fastigio-brainstem terminations are less extensvie than the contralateral. They occur in Deiters nucleus, in portions of the descending vestibular nucleus, and at the rostralmost end of the mesencephalic reticular formation near the pretectal area. The terminations of fibers projecting from the dorsal column nuclei to the inferior olivary complex occur in zones which are coextensive with cerebellar termination zones. Both the fibers of the dorsal column nuclei and of the anterior interposed nucleus terminate in the dorsal accessory olive. Other than the dorsal accessory olive, the only other brainstem projection from the dorsal column nuclei is to the external nucleus of the inferior colliculus. Projections to the spinal cord arise in the interposed and fastigial nuclei. The interpositospinal projection leaves the cerebellum via the ipsilateral brachium conjunctivum, decussates, travels in the ventral funiculus of the spinal cord, and terminates on interneurons in the intermediate zone. By contrast, the fastigiospinal projection decussates within the cerebellum, leaves the cerebellum via the contralateral uncinate fasciculus, travels in the lateral funiculus, and terminates around motoneurons in the ventral horn. No spinal projections were detected from the dentate nucleus.

Morphological diversity of immunocytochemically identified GABA neurons in the monkey sensory-motor cortex

SummaryGABAergic neurons have been identified in monkey sensory-motor cerebral cortex by light microscopic, immunocytochemical localization of the GABA synthesizing enzyme, glutamic acid decarboxylase (GAD). All GAD-positive neurons are non-pyramidal cells. Their somata are present in all layers and are evenly distributed across layers II-VI of the motor cortex (area 4), but are found in greater concentrations in layers II, IV and VI of all areas of first somatic sensory cortex (SI; areas 3a, 3b and 1–2). GAD-positive puncta (putative axon terminals) are present throughout the sensory-motor cortex, and they are found immediately adjacent to the somata, dendrites and presumptive axon initial segments of GAD-negative pyramidal cells. In addition, they are observed in close approximation to the somata of both large and small GAD-positive neurons. In area 4, the density of puncta is highest in the superficial cortical layers (layers I-III) and gradually declines throughout the deeper layers. In SI, the highest densities of puncta are present in layer IV, while moderately high densities are found in layers I-III and VI. In areas 3a and 3b, the puncta in layers IV and VI are particularly numerous and form foci that exhibit greater density than adjacent regions.GAD-positive neurons withlarge somata, 15–33 μ in diameter, are present in layers IIIB-VI of all areas. Such cells have many primary dendrites that radiate in all directions. In addition they have axons that ascend either from the superficial aspect of the somata or from primary dendrites, and that exhibit horizontal collateral branches. These neurons closely resemble the large basket cells (Marin-Padilla, 1969; Jones, 1975), and they may give rise to many of the GAD-positive endings surrounding the somata and proximal dendrites of pyramidal cells in layers III-VI. In addition,small GAD-positive somata are present in all layers, but they are most numerous in layers II and IIIA of all areas and in layer IV of SI. The somata and proximal dendrites of these cells vary from a multipolar shape with small, beaded dendrites, found primarily in layer IV, to bitufted and multipolar shapes with larger, smooth dendrites. The diversity of somal sizes and locations, the variety of dendritic patterns, and the different distributions of GAD-positive puncta, all combine to suggest that several different morphological classes of intrinsic comprise the GABA neurons of monkey cerebral cortex.

Synaptic organization of immunocytochemically identified GABA neurons in the monkey sensory-motor cortex

SummaryNeurons in the monkey somatic sensory and motor cortex were labelled immunocytochemically for the GABA synthesizing enzyme, glutamic acid decarboxylase (GAD), and examined with the electron microscope. The somata and dendrites of many large GAD-positive neurons of layers III–VI receive numerous asymmetric synapses from unlabelled terminals and symmetric synapses from GAD-positive terminals. Comparisons with light and electron microscopic studies of Golgi-impregnated neurons suggest that the large labelled neurons are basket cells. Small GAD-positive neurons generally receive few synapses on their somata and dendrites, and probably conform to several morphological types. GAD-positive axons form symmetric synapses on many neuronal elements including the somata, dendrites and initial segments of pyramidal cells, and the somata and dendrites of non-pyramidal cells. Synapses between GAD-positive terminals and GAD-positive cell bodies and dendrites are common in all layers. Many GAD-positive terminals in layers III–VI arise from myelinated axons. Some of the axons form pericellular terminal nests on pyramidal cell somata and are interpreted as originating from basket cells while other GAD-positive myelinated axons synapse with the somata and dendrites of non-pyramidal cells. These findings suggest either that the sites of basket cell terminations are more heterogeneous than previously believed or that there are other classes of GAD-positive neurons with myelinated axons. Unmyelinated GAD-positive axons synapse with the initial segments of pyramidal cell axons or formen passant synapses with dendritic spines and small dendritic shafts and are interpreted as arising from the population of small GAD-positive neurons which appears to include several morphological types.

The early formation of the corpus callosum: a light and electron microscopic study in foetal and neonatal rats.

SummaryA light and electron microscopic study of the developing corpus callosum was carried out in foetal and neonatal rats in order to determine the mode of growth of the earliest callosal axons across the midline and to investigate the potential role played by non-neuronal cells during the formation of the tract. The axons of the corpus callosum first cross the midline between the 18th and 19th days of gestation by traversing the anterodorsal aspect of the pre-existing hippocampal commissure. Prior to the appearance of the callosal axons at the midline, there is an aggregation of astrocyte processes anterior and dorsal to the hippocampal commissure. Careful examination of these processes in different planes of section shows that they are not organized in any obvious way that would provide a clearly defined path for the growing axons; nor are there any preferentially oriented extracellular spaces at the midline. No specialized membrane contacts could be seen between non-neuronal cell processes and the early callosal axons. Thus, there is no overt morphological evidence for an active role of non-neuronal cells in axon guidance in the initial formation of the corpus callosum.The development of the corpus callosum is accompanied by the formation of a temporary cavum septi pellucidi, which is always closed to the subarachnoid space. The cavum persists during the first postnatal week, after which time it becomes populated by cells of the lateral septal nuclei. Macrophages are present within the cavum and may play a role in its formation. Macrophages are also found within parts of the corpus callosum. No obvious degeneration of axons is seen in the corpus callosum during its early development.

Cytoarchitectonic delineation of the ventral lateral thalamic region in the monkey

The cytoarchitecture of the ventral lateral region of the primate thalamus has been appraised in the frontal, parasagittal and horizontal planes. A morphologically distinct region, possessing a sparse and diffuse distribution of large and small neurons is identified. The region includes several nuclei previously separately named by Olszewski. These are nuclei VPLo, VLc, X, VLps, and some cellular extensions into the VLo nucleus. The whole zone is continuous, and it is shown that no clear separation exists between any of the previously identified subnuclei. Connectional grounds are given for suggesting that this region should be considered as a common cerebellar relay nucleus to motor cortex. Morphological criteria for distinguishing the cell-sparse nucleus from adjacent nuclei are given. These cytological criteria provide a basis for the experimental analysis of cortical and subcortical connectivity of the ventral lateral thalamic region. Close attention was paid to the border between the VPLo nucleus and the VPLc nucleus. VPLc is separated from VPLo by a clear border, and no transitional zone can be detected in the parasagittal or horizontal planes. Previous ambiguities in the delineation of the VPLo-VPLc border probably stem from analysis in the frontal plane, in which the border is not clear.

Direction and specificity of the axonal and transcellular transport of nucleosides

The axoplasmic transport of nucleosides or their derivatives has been examined autoradiographically in avian and mammalian brains. Following injections of [3H]-adenosine or [3H]uridine intravitreally in chicks and rats or into the thalamus or neocortex of rats, three findings emerge: (1) axoplasmic transport of these materials occurs both in the anterograde and retrograde direction in birds and mammals; (2) anterograde transport in the axons of injected cells results in considerable cellular labeling in the region in which the axons terminate. This labeling is not exclusively transsynaptic, but probably results from some less specific, trancellular transport, since glial cells at the terminal site and along the path of the axons also become labeled; (3) injection of [3H]uridine in the chick optic and rat thalamocortical systems results in a pattern of labeling which differs considerably from that seen after injection of [3H]adenosine. After [3H]adenosine injections in the chick eye or rat thalamus, retrograde cell labeling is far more obvious than anterograde, transcellular labeling; after injections of [3H]uridine, anterograde, transcellular labeling is more intense than retrograde cell labeling.

Thalamic inputs to identified commissural neurons in the monkey somatic sensory cortex

SummaryCommissurally projecting neurons were identified in the monkey first somatic sensory area (SI) by the retrograde axonal transport of horseradish peroxidase (HRP) injected into the contralateral cortex. Neurons identified in this way have large pyramidal somata primarily in layer IIIB of the SI area. Their basal dendrites lie within the terminal plexus of thalamocortical afferents.Electron microscopy was used to examine the synaptic relations of the labelled commissural cells, in particular to determine whether they receive monosynaptic thalamic connections. To do this, retrogradely labelled commissural cells and Golgi-impregnated large pyramidal neurons from layer IIIB were examined ultrastructurally in material in which thalamocortical terminals were degenerating due to a prior lesion of the thalamus. In a significant number of cases degenerating terminals were found to make synapses on the spines or shafts of labelled dendrites.Injections of HRP into SI or into the white matter adjacent to the corpus callosum labelled callosal axons and terminals in the opposite SI. These axons terminated mainly near the somata of the layer IIIB pyramidal cells. Some of their terminals were found to synapse with dendrites receiving synaptic contacts from thalamocortical axon terminals.

Nucleus interpositus projection to spinal interneurons in monkeys.

Direct spinal projections from the deep cerebellar nuclei have recently received considerable attention in catsS, 10, and some other species1,3, 9. These reports have focused primarily on the fastigiospinal neurons, which are thought to influence upper cervical motoneurons monosynaptically 10. Retrograde transport studies z have indicated that in monkeys, the nucleus interpositus also projects to the spinal cord. During the course of more detailed studies of the output connections of the monkey deep cerebellar nuclei, we have identified the course and mode of termination of interpositospinal axons. Their terminations are dorsally situated, on the somata and proximal dendrites of interneurons in the intermediate zone. Injections of [3H]leucine and [aH]proline were made into various foci within the deep cerebellar nuclei of cynomolgus and rhesus monkeys s. After survival periods of 5-8 days, serial sections of the brains and spinal cords were prepared for autoradiography 4. In experiments in which the injections were confined to the nucleus interpositus, all labeled efferent fibers leave the cerebellum via the superior cerebellar peduncle and decussate at mesencephalic levels. Although many of these labeled fibers, after decussation, either terminate in the reticulotegmental nucleus, the magnocellular red nucleus, or proceed rostrally towards the diencephalon, a substantial number project caudally. In the ventrolateral tegmentum there is a diffuse array of labeled fibers, most of them descending to terminate in the inferior olivary complex. However, clusters of concentrated label, indicative of a few large diameter axons are segregated dorsomedially and proceed caudally along the lateral surface of the pyramidal decussation into the medial part of the ventral funiculus of the spinal cord (Fig. 1). Up to 20 labeled axons can be identified in this position in each experiment. Labeled axons can be traced into the central gray from the caudal medulla to the third cervical segment (they were not traced further). Grain concentrations indicative of labeling of terminal