Further observations on the cerebellar projections from the pontine nuclei and the nucleus reticularis tegmenti pontis in the rhesus monkey.

The projections from the pontine nuclei and the necleus reticularis tegmenti pontis (N.r.t.) onto the flocculus, uvula, and the paramedian lobule were studied with retrograde transport of horseradish peroxidase n the rhesus monkey. The main findings are as follows: There is a conspicuous tendency for labeled cells to occur in numerous discrete clusters in the pontine nuclei after injections of these parts of the cerebellum. There appears to be very limited overlap between pontine cell groups projecting to the flocculus, the uvula, and the paramedian lobule, respectively. The flocculus appears to receive a substantial projection from the pontine nuclei. The projection is almost totally crossed (3% ipsilateral), and arises mainly laterally in the rostral half of the pons but in addition from a minor group dorsomedially. The flocculus receives a bilateral projection (slight contralateral preponderance) from medial and dorsomedial parts of the NRT. The number of labeled cells in the NRT was 13% of the number in the pontine nuclei. the uvula is amply supplied from the pontine nuclei. The projection takes origin throughout the rostrocaudal extent of the pons, from one medial and one dorsolateral region. Labeled cells are found in greatest number dorsolaterally in the rostral half of the pons. In the caudal N.r.t., one medial and one lateral cell group were labeled after injection of the uvula. The number of labeled cells in the N.r.t. was only 4% of the number in the pontine nuclei. Findings with regard to the paramedian lobule confirm and extend earlier observations in the monkey (Brodal, '79, '80). The present results are discussed in relation to HRP studies of the pontocerebellar projection in lower animals. Several possible species differences are noted--for example, with regard to projections to the flocculus. There is some evidence that the pontocerebellar projection is more precisely organized in the monkey than in lower animals.

The olivocerebellar projection in the monkey. Experimental studies with the method of retrograde tracing of horseradish peroxidase.

Following injections of horseradish peroxidase (HRP) in various lobes and lobules of the macaque cerebellum the occurrence of retrogradely labeled cells in the inferior olive was mapped. Only cortical areas showing staining of the molecular layer were considered as sites of uptake of HRP. To facilitate comparisons between cases and presentation of findings, a diagram of the macaque inferior olive as imagined unfolded was constructed (Fig. 1). Attempts were made to compare the findings made with data on the olivocerebellar projection in the cat and the pattern of a longitudinal zonal subdivision of the cerebellum. In general there appears to be a remarkably close correspondence between the organization of the olivocerebellar projection in the monkey and the cat. The projection is precisely organized and appears to be purely crossed. Within the projections to some of the cerebellar cortical zones a topical pattern can be demonstrated. Olivary afferents to vermal lobules V, VII, and VIII are derived from the caudal half of the medial accessory olive, projecting to Voogd's zone A. The topical pattern resembles that in the cat (Fig. 8). after injections covering the lateral zone of the anterior lobe vermis (zone B), labeled cells are seen in the caudal part of the dorsal accessory olive. In some cases staining of the intermediate part of the anterior lobe and of the paramedian lobule is followed by labeling of cells in the rostral part of the dorsal accessory olive (zones C1 and C3) or in the rostral half of the medial accessory (zone C2). When the injected area covers lateral parts of the cerebellum, there is labeling in the principal olive (projecting to zones D1 and D2). Although not entirely decisive, the findings lend support to the view that the ventral lamella of the principal olive supplies zone D2, whereas the dorsal lamella supplies zone D1. The relatively sparse data in the literature on the afferents to the monkey olive are briefly considered. On may points the projections appear to be as in the cat. However, there is possibly a species difference between cat and monkey as concerns their receipt of afferents from the red nucleus.

Demonstration of topographically organized projections from the hypothalamus to the pontine nuclei: an experimental anatomical study in the cat.

In 22 cats implantations and injections of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) or Fluoro-Gold were placed in the pontine nuclei or the hypothalamus. The occurrence and distribution of labeled cells in the hypothalamus and of labeled terminal fibers in the pontine nuclei were mapped. Following implantations of HRP-WGA ventromedially in rostral parts of the pontine nuclei, 22-44% of all labeled cells in the brainstem and diencephalon are found in the medial mamillary nucleus ipsilateral to the implantation. Some labeled cells are also found in the supramamillary, premamillary, anterior mamillary, and tuberomamillary nuclei. Thus, labeled cells in the hypothalamus make up 33-54% of all labeled cells in the brainstem and diencephalon in such cases. In contrast, implantations and injections in mediocaudal parts of the pontine nuclei result in labeling of cells mainly in the posterior, dorsal, and lateral hypothalamic areas (terminology of Bleier: The Hypothalamus of the Cat. Baltimore: Johns Hopkins Press, '61). In these cases the labeled cells in the hypothalamus make up 16-25% of all labeled cells in the brainstem and diencephalon. Implantations in more lateral parts of the pontine nuclei label only a few cells in the hypothalamus. Following implantations of HRP-WGA in restricted parts of the hypothalamus, fibers from the medial mamillary nucleus were found to terminate ventromedially at all rostrocaudal levels of the pontine nuclei, ipsilateral to the implantation. In the rostralmost part of the pontine nuclei, the terminal labeling forms a dense, transversely oriented, c-shaped band. Fibers from the posterior and dorsal hypothalamic areas terminate medially and dorsomedially in the caudal third of the pontine nuclei. Sparse terminal labeling is also seen in lateral parts of the pontine nuclei and medially at more rostral levels. In two cases with small implantations of HRP-WGA ventromedially in rostral parts of the pontine nuclei, labeled cells are found both in the medial mamillary nucleus and the cingulate gyrus. Thus, it seems possible that fibers from the medial mamillary nucleus and the cingulate gyrus converge upon a restricted area ventromedially in rostral parts of the pontine nuclei.

Distribution in area 17 of neurons projecting to the pontine nuclei: a quantitative study in the cat with retrograde transport of HRP-WGA.

The number and distribution of corticopontine neurons within area 17 of the cat were studied quantitatively with the use of retrograde transport of horseradish peroxidase-wheat germ agglutinin. Eight cats received stereotactic injections in the pontine nuclei; in three of these complete staining of the parts of the pontine nuclei receiving fibers from the visual cortex was achieved. Labeled cells were counted in frontal sections through the hemisphere, spaced at 0.5 mm. The borders of area 17 were determined cyto- and myeloarchitectonically and a flat map was produced for each animal. A map of the representation of the visual field in 10 degrees X 10 degrees blocks in the first visual area (Tusa et al., '78, '81) was transferred to our maps of area 17. The density and number of labeled corticopontine cells could then be determined within blocks of the cortex representing 10 degrees X 10 degrees of the visual field. The cell density (number of labeled cells per mm2 cortex) was found in general to be highest in parts of the cortex representing peripheral parts of the visual field. The cell density is low in cortex representing the central visual field, but the lowest density was found in the representation of a para-central region in the upper visual field. Furthermore, cortical regions representing the lower part of the visual field have a higher cell density than those representing the upper part; in four cases, 68-86% of all labeled cells were found in parts of area 17 representing the visual field below the horizontal meridian. Since there is an enlarged cortical representation of central vision, the much lower cell densities in "central" parts of area 17 than in "peripheral" parts may mean that all parts of the visual field are represented with equal numbers of corticopontine neurons ("linear" representation). This is not the case, however, since the number of labeled cells per 10 degrees X 10 degrees is considerably higher in the cortex representing the central 10 degrees and medial parts of the lower visual field than in the rest of area 17. Assuming that the corticopontine cells in the visual cortex transmit spatially relevant information, we conclude that there is an overrepresentation of central vision and the medial parts of the lower visual field in the corticopontine projection from area 17.

Projections to the pontine nuclei from choline acetyltransferase-like immunoreactive neurons in the brainstem of the cat.

By use of retrograde transport of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) in combination with monoclonal antibodies against choline acetyltransferase (ChAT), we show that putative cholinergic inputs to the feline pontine nuclei originate from cells in the dorsolateral pontine tegmentum. These cells form a loosely arranged continuum that nevertheless may be subdivided into two groups on the basis of differences in cell morphology. One group consists of double-labeled cells in the periventricular gray substance medial to, and partly merging with, the nucleus locus coeruleus. The other group consists of double-labeled cells surrounding the brachium conjunctivum. In two cats with tracer injections in the pontine nuclei, 81% and 84%, respectively, of the retrogradely labeled cells in the dorsolateral pontine tegmentum are ChAT-like immunoreactive (ChAT-LI). In the same experiments, many ChAT-LI cells, but no retrogradely labeled cells, are seen in the basal telencephalon. The pontine nuclei contain a plexus of thin ChAT-LI fibers with varicosities resembling en passant as well as terminal boutons. These ChAT-LI fibers appear to branch extensively and cover all parts of the pontine nuclei. Following injections of rhodamine-B-isothiocyanate (RITC) in the thalamus and Fluoro-Gold in the pontine nuclei and surrounding regions in the same animal, all retrogradely labeled cells in the dorsolateral pontine tegmentum are labeled with both tracers, whereas most cells in the paramedian pontine reticular formation are labeled either with RITC or Fluoro-Gold. Thus it appears that all cells in the dorsolateral pontine tegmentum that project to the pontine nuclei also project to the thalamus. In analogy with findings by others in the dorsal lateral geniculate nucleus, we suggest that the putative cholinergic projections to the pontine nuclei may serve to modulate transmission of cerebellar afferent information in accordance with the behavioral state of the animal.

Principles of organization of the corticopontocerebellar projection to crus II in the cat with particular reference to the parietal cortical areas.

In 13 cats injections of horseradish peroxidase-wheat germ agglutinin in various parts of the cerebral cortex were combined with injections in the cerebellar crus II in the same animal in order to study the cortical regions that may influence the crus II via the pontine nuclei. In 2 cats lesions in the cerebral cortex were combined with horseradish peroxidase injections in the crus II. In the pons terminal regions (anterogradely labelled from the cerebral cortex or containing terminal degeneration) and cell groups retrogradely labelled from crus II were carefully plotted. The pontocerebellar projection to crus II is mainly crossed, on the average 26% of the labelled cells were found in the ipsilateral pons. Some overlap between sites of ending of cortical fibres and sites of origin of fibres to crus II was present in all cases, but the degree of overlap varied considerably, depending on which cortical region was injected. Typically, partial overlap between terminal patches and groups of labelled cells occurred at multiple sites in the pontine nuclei. A major input to crus II appears to come from the parietal region. Experiments with bilateral cortical injections showed that the pontine projection from the parietal region is topographically organized in a precise mosaic pattern of adjacent but apparently non-overlapping patches of termination. Area 6 also has strong connections with crus II, while only very few of the corticopontine fibres from the sensorimotor region overlap with cell groups labelled from crus II. The second somatosensory area and the visual cortex both seem able to influence a small but significant proportion of cells projecting to crus II. In contrast to other cortical regions, the auditory cortex appears to send fibres mainly to cell groups projecting to the ipsilateral crus II. It is concluded that the input to crus II originates in wide areas of the cerebral cortex. Small subgroups of neurons projecting to crus II can be differentiated on the basis of their cortical afferents. It appears likely that each subgroup receives fibres mainly or in some instances only from one cortical site. The corticopontocerebellar projection to crus II probably exhibits a high degree of spatial order providing a specific pattern of convergence and divergence in the cerebellar cortex, in agreement with recent physiological evidence from micromapping studies.

Visual pathways to the cerebellum: segregation in the pontine nuclei of terminal fields from different visual cortical areas in the cat.

The cerebellum receives input from visual cortical areas via a relay in the pontine nuclei. We have compared the location in the pontine nuclei of terminal fields of fibres from visual areas 18 and 20, and the posteromedial lateral suprasylvian visual area. Due to individual variations in the precise location of terminal fields, comparisons were performed in individual animals. Horseradish peroxidase-wheat germ agglutinin conjugate was used as an anterograde tracer in combination with the Fink and Heimer method for visualization of anterograde degeneration. Most of the terminal fields of area 20 are widely separated from those of area 18. Fibres from the posteromedial lateral suprasylvian visual area and area 20 terminate close to each other but overlap of terminal fields is limited. Area 18 and the posteromedial area have in some places completely overlapping terminal fields; in other places, however, there is only partial overlap or complete separation. Generally, segregation of terminal fields from different areas is most pronounced in the caudal part of the recipient zone of the pontine nuclei. The terminal fields of fibres from the three cortical areas studied appear as numerous patches arranged in a complicated mosaic that tend to form concentric lamellae around the ventromedial aspect of the peduncle. Within these lamellae, area 18 projects mainly to the innermost one, area 20 to the outermost, and the posteromedial area to an intermediate lamella. Whether terminal fibres from different areas are segregated (non-overlapping) or overlapping in the pontine nuclei is of relevance for the functional organization of the cerebrocerebellar pathway. Segregation of terminal fields from different areas would mean that the areas in question influence different sets of pontocerebellar neurons and thereby relay information to the cerebellum in separate channels. Overlap of terminal fields from different areas could mean that convergence on the same pontocerebellar neurons occurs (although convergence cannot be proved with the techniques employed in this study). This study indicates that information from visual areas is relayed at least in part in separate channels from the cortex to the cerebellum.

GABA and glycine as putative transmitters in subcortical pathways to the pontine nuclei. A combined immunocytochemical and retrograde tracing study in the cat with some observations in the rat.

Using the retrograde tracers horseradish peroxidase-wheatgerm agglutinin and gold particles conjugated to wheatgerm agglutinin apo-horseradish peroxidase in combination with an antiserum against glutaraldehyde-fixed GABA, it was examined whether the pontine nuclei of the cat receive projections from GABA-like immunoreactive neurons in the brainstem, diencephalon, or deep cerebellar nuclei, contributing to the GABA-like immunoreactive fibre plexus previously demonstrated in the pontine nuclei [Brodal et al. (1988) Neuroscience 25, 27-45]. Following tracer injections that covered both the pontine nuclei and the reticular tegmental nucleus in two cats, it was found that 125 out of 1166 (10.7%) and 29 out of 294 (9.9%) retrogradely labelled neurons in the cerebellar nuclei were GABA-like immunoreactive. In the same two experiments only six out of 2029 (0.3%) and 10 out of 1398 (0.7%) retrogradely labelled neurons in the brainstem and diencephalon were GABA-like immunoreactive. Among the regions in the brainstem and diencephalon known to project to the pontine nuclei, double-labelled cells were seen in the reticular formation, the periaqueductal gray, and the nucleus praepositus hypoglossi, but not in the zona incerta or the anterior pretectal nucleus, regions that have been shown to contain glutamate decarboxylase-like immunoreactive neurons projecting to the pontine nuclei in the rat [Border et al. (1986) Brain Res. Bull. 17, 169-179]. In order to test whether this is due to species differences, the same experimental approach was used in the rat, and it was found that 54 out of 3249 (1.7%) retrogradely labelled neurons in the brainstem and diencephalon were double-labelled. Notably, in the zona incerta 2% of the retrogradely labelled cells were also GABA-like immunoreactive, and in the reticular formation there was a higher proportion of double-labelled cells than was found in the cat. Additional sources were identified, that may contribute to the GABA-like immunoreactive fibre plexus in the pontine nuclei of the rat. This, in conjunction with the previous finding that the pontine nuclei of the rat contain only very few putative GABAergic neurons [Border and Mihailoff (1985) Expl Brain Res. 59, 600-614; Brodal et al. (1988) Neuroscience 25, 27-45], lead to the suggestion that the GABA-like immunoreactive fibre plexus in the pontine nuclei of the rat is predominantly of extrinsic origin, possibly representing a mosaic of the terminal fields of several subcorticopontine projections.(ABSTRACT TRUNCATED AT 400 WORDS)

The corticopontine projection: axotomy-induced loss of high affinity L-glutamate and D-aspartate uptake, but not of gamma-aminobutyrate uptake, glutamate decarboxylase or choline acetyltransferase, in the pontine nuclei.

The corticopontine fibres were severed in the crus cerebri in rats and mice by a stereotaxically operated retractable wire-knife. The pontine nuclei were microscopically dissected from fresh slices of rats and synaptosome-containing homogenates were prepared. The high affinity uptake of radiolabelled L-glutamate (L-Glu) and D-aspartate (D-Asp) was heavily reduced five days after the lesions. The uptake was further reduced after bilateral (-75% for D-Asp and -65% for L-Glu) than after unilateral lesions (-55% for D-Asp and -45 to 50% for L-Glu on the lesioned side.) The molar ratio of the uptakes of D-Asp and L-Glu was consistently lower in pons after transection of the cortical afferents than normally (-28% after bilateral lesions). gamma-Aminobutyrate uptake and glutamic acid decarboxylase were not changed. Choline acetyltransferase was increased (+53%) after unilateral lesions, but not altered after bilateral lesions. Autoradiograms of slices from mice, incubated with tritium-labelled amino acids and fixed in glutaraldehyde, showed high affinity uptake sites for D-Asp to be enriched in the pontine nuclei, compared to neighbouring structures. After partial lesion of the crus cerebri the uptake was reduced in the area with degenerated corticopontine afferents. gamma-Aminobutyrate uptake sites were relatively less concentrated in the pontine nuclei than D-Asp uptake sites. The results indicate, along with the previous demonstration of Ca-dependent K-induced release of D-[3H]aspartate from the corticopontine terminals, that glutamate and/or aspartate may be transmitters in this pathway. The results also suggest that acidic amino acid uptake sites may differ in their relative transport rates for aspartate and glutamate.

GABA-containing neurons in the pontine nuclei of rat, cat and monkey. An immunocytochemical study.

Putative GABAergic elements in the pontine nuclei have been studied in the rat, cat and two old world monkeys (Macaca mulatta and Papio papio) using an antiserum against GABA-glutaraldehyde-protein conjugates and the peroxidase-antiperoxidase method. In addition, an antiserum against glutamate decarboxylase has been used in the cat. For comparison, Golgi impregnated material from cat and macaque has been studied. In all species there is a moderately dense plexus of fibres with GABA-like immunoreactivity with only minor regional differences between different parts of the pontine nuclei. The number of cell bodies showing GABA-like immunoreactivity is, however, strikingly different. Thus, in the rat there are very few such neurons. In the cat, they make up about 1% of the total cell population, while the corresponding number in the two primate species is about 5%. The number is consistently somewhat higher in rostral than in caudal parts of the pontine nuclei. Numbers in the cat are essentially the same with the glutamate decarboxylase antiserum as with the GABA antiserum. The size of GABA-like immunoreactivity positive somata is very similar in cat, macaque and baboon, averaging about 160 micron2 in cross-sectional area. The average cross-sectional area of the total neuronal population as measured in adjacent thionin-stained sections is about 280 micron2. However, the range of sizes for GABA-like immunoreactivity positive cells is wide, so that size alone is not a good criterion for their identification. Although their dendritic morphology is varied, a significant proportion of GABA-like immunoreactivity positive cells have very long and straight dendrites. A few examples were found in the primate species of GABA-like immunoreactivity positive cells with processes tentatively identified as axons. Such processes could be seen to divide several times. No such branching processes could be identified, however, in Golgi impregnated material from the same species. In order to determine whether GABA-like immunoreactivity positive cells project to the cerebellum, retrograde tracing of horseradish peroxidase-wheat germ agglutinin was combined with immunocytochemistry. No double labelled cells could be found in the pontine nuclei. Comparison of size distribution of retrogradely labelled pontocerebellar and GABA-like immunoreactivity positive cell bodies showed a high degree of overlap, although the average size of projection neurons and GABA-like immunoreactivity positive ones is clearly different.(ABSTRACT TRUNCATED AT 400 WORDS)

Salient anatomic features of the cortico-ponto-cerebellar pathway.

Recent studies of the primate corticopontine projection show that the neocerebellum--in addition to connections from motor and sensory areas--receives connections from various association areas of the cerebral cortex, some of which are thought to be primarily engaged in cognitive tasks. The quantities of such connections in relation to those from more clearly motor-related parts of the cortex need to be more precisely determined, however. Furthermore, the anatomic data on origin of corticopontine fibers needs to be supplemented with physiological experiments to clarify their functional properties at the single-cell level. For example, nothing is known of the functional role of the large input from the cingulate gyrus, nor is the input from the posterior parietal cortex physiologically characterized. Finally, the scarcity of corticopontine connections from the prefrontal cortex in the monkey (and probably also in man) may not seem readily compatible with a prominent role of the neocerebellum in certain cognitive tasks. We discuss data--in particular from three-dimensional reconstructions--indicating that both corticopontine projects and pontocerebellar neurons are arranged in a lamellar pattern. Corticopontine and pontocerebellar lamellae have similar shapes and orientations but appear to differ in other respects. Corticopontine terminal fields are sharply delimited, apparently without gradual overlap between projections from different sites in the cortex, whereas pontocerebellar lamellae are more fuzzy and exhibit gradual overlap of neuronal populations projecting to different targets. In spite of the sharpness of the corticopontine projection, there may be many opportunities for convergence of inputs from different parts of the cortex. Thus, the wide divergence of corticopontine projections produces many sites of overlap, and extensive interfaces between different terminal fields enabling convergence of inputs onto each neuron. We suggest that the lamellar arrangement of corticopontine terminal fields and of pontocerebellar neurons serve to create diversity of pontocerebellar neuronal properties. Thus, each small part of the cerebellar cortex would receive a specific combination of messages from many different sites in the cerebral cortex. The spatial arrangement of cerebrocerebellar connections have to be understood both in terms of fairly simple large-scale, gradual topographic relationships and an apparently highly complex pattern of divergence and convergence. Developmental studies of corticopontine and of pontocerebellar projections together with three-dimensional reconstructions in adults suggest that the highly complex adult connectional pattern may be created by simple rules operating during development.

Corticopontine terminal fibres form small scale clusters and large scale lamellae in the cat.

We investigated whether terminal fibres in the pontine nuclei are arranged in a lamellar pattern like that demonstrated earlier for pontocerebellar neurones. Following tracer injections in visual and parietal cortices and subsequent computer-based 3-D analysis, we found that labelled corticopontine terminal fibres form numerous sharply delimited aggregates of variable shape. Several of the aggregates are cylindroids (diameter 200-300 microns, length 1-3 mm). The aggregates are confined to a lamellar subspace, the position of which depends on the anteroposterior location of the cortical injections. These findings suggest that the cerebroponto-cerebellar system may be organized according to fairly simple, topographical rules. We discuss the implications of our results in relation to the development of corticopontine topographical organization.

Cat pontocerebellar network: numerical capacity and axonal collateral branching of neurones in the pontine nuclei projecting to individual parafloccular folia.

We have studied the convergence and divergence in the pontocerebellar pathway. Two or three different fluorescent tracers were injected in separate folia of the parafloccular complex. Retrogradely labelled cells were quantitatively recorded. The estimated total number of labelled neurones in the pontine nuclei contralateral to the injection sites was 18000 (median; range 5000-46000; 14 cell populations, six animals). Using stereological principles, the total number of neurones on one side in the pontine nuclei was estimated to be 490000 (mean; n = 6). Thus, approximately 4% of the total number of neurones in the pontine nuclei would project to a single parafloccular folium. Assuming that the highest estimates of labelled cells are the most representative, the proportion would be 9%. Considering that the volume injected makes up a tiny fraction of the total cerebellar cortical volume, these figures reflect an extreme convergence. After injections in adjacent folia we observed 19-27% double labelling. The double labelling frequency dropped steeply with increasing distance between injections. The strong convergence and limited local axonal branching suggest the existence of extensive branching to widely separated cerebellar regions.

Organization of the pontine nuclei.

The pontine nuclei provide the cerebellar hemispheres with the majority of their mossy fiber afferents, and receive their main input from the cerebral cortex. Even though the vast majority of pontine neurons send their axons to the cerebellar cortex, and are contacted monosynaptically by (glutamatergic) corticopontine fibers, the information-processing taking place is not well understood. In addition to typical projection neurons, the pontine nuclei contain putative GABA-ergic interneurons and complex synaptic arrangements. The corticopontine projection is characterized by a precise but highly divergent terminal pattern. Large and functionally diverse parts of the cerebral cortex contribute; in the monkey the most notable exception is the almost total lack of projections from large parts of the prefrontal and temporal cortices. Within corticopontine projections from visual and somatosensory areas there is a de-emphasis of central vision and distal parts of the extremities as compared with other connections of these sensory areas. Subcorticopontine projections provide only a few percent of the total input to the pontine nuclei. Certain cell groups, such as the reticular formation, project in a diffuse manner whereas other nuclei, such as the mammillary nucleus, project to restricted pontine regions only, partially converging with functionally related corticopontine connections. The pontocerebellar projection is characterized by a highly convergent pattern, even though there is also marked divergence. Neurons projecting to a single cerebellar folium appear to be confined to a lamella-shaped volume in the pontine nuclei. The organization of the pontine nuclei suggests that they ensure that information from various, functionally diverse, parts of the cerebral cortex and subcortical nuclei are brought together and integrated in the cerebellar cortex.

Do pontocerebellar mossy fibres give off collaterals to the cerebellar nuclei? An experimental study in the cat with implantation of crystalline HRP-WGA.

In 15 cats with implantations of crystalline HRP-WGA in the cerebellar nuclei and tetramethylbenzidine histochemistry, the pontine nuclei were carefully examined for presence of retrogradely labelled cells. Findings in the nucleus reticularis tegmenti pontis and the inferior olive, both known to project to the cerebellar nuclei, served as controls for effectiveness of uptake and transport. After implantations restricted to the lateral cerebellar nucleus in 5 cats altogether two labelled cells were found in the contralateral pontine nuclei in regions receiving afferents from the lateral nucleus. In contrast, many labelled cells occurred in the nucleus reticularis tegmenti pontis and the inferior olive. After implantations in 5 cats restricted to the posterior or anterior interposed nuclei, altogether only one labelled cell was found in the pontine nuclei, while many labelled cells occurred in the inferior olive. The nucleus reticularis tegmenti pontis contained a small number of retrogradely labelled cells after implantations in the anterior interposed nucleus, but none after implantations restricted to the posterior interposed nucleus. After implantations restricted to the medial (fastigial) nucleus, no retrogradely labelled cells were found in the pontine nuclei and nucleus reticularis tegmenti pontis (although many were present in the inferior olive). The present findings support earlier conclusions based on anterograde tracing methods that the cerebellar nuclei receive very few, if any, afferents from the pontine nuclei.

Demonstration of a somatotopically organized projection onto the paramedian lobule and the anterior lobe from the lateral reticular nucleus: an experimental study with the horseradish peroxidase method.

Using the retrograde axonal transport of horseradish peroxidase, the projection from the lateral reticular nucleus (NRL) to the cerebellar anterior lobe and paramedian lobule has been studied in 13 cats. Both the anterior lobe and the paramedian lobule receive a somatotopically organized projection from the NRL. The projection to the paramedian lobule is nearly exclusively ipsilateral and originates mainly in the dorsal part of NRL, while the projection to the anterior lobe is bilateral (with ipsilateral predominance) and takes origin from all parts of the NRL. The lateral part of the NRL (closely coinciding with the parvocellular nucleus) projects to the rostral part of the anterior lobe and caudal parts of the paramedian lobule (both representing the hindlimb), while the medial part of the NRL (magnocellular nucleus) projects to the caudal parts of the anterior lobe and rostral parts of the paramedian lobule (representing the forelimb). The small subtrigeminal nucleus projects onto the anterior lobe as well as the paramedian lobule, but apparently mainly to their forelimb areas.

The corticopontocerebellar pathway to crus I in the cat as studied with anterograde and retrograde transport of horseradish peroxidase.

In 13 cats injections of horseradish peroxidase (HRP) in various parts of the cerebral cortex were combined with injections of HRP in the cerebellar crus I in the same animal in order to study the cortical regions which may influence the crus I via the pontine nuclei. In the pons terminal regions anterogradely labeled from the cerebral cortex and cell groups retrogradely labeled from crus I were carefully plotted. Overlap between sites of ending of cortical fibers and sites of origin of fibers to crus I was relatively modest in all experiments. On the other hand, partial overlap was usually found at multiple sites. The largest single input to crus I appears to come from the parietal region, particularly the anterior part of the suprasylvian gyrus, while the sensorimotor region contributes much less. Area 6, the second somatosensory cortex and the orbital gyrus, all seem able to influence pontine cells projecting to crus I. Least overlap is found after injections of the visual cortex. The size and orientation of the dendritic fields of pontine cells were studied in Golgi-impregnated material. The dendritic fields average 187 X 339 microns in the transverse plane and are so small that they will only moderately increase the overlap. It is concluded that small subgroups of neurons projecting to crus I receive somewhat different sets of cortical afferents. The input to crus I must originate in wide areas of the cerebral cortex, and probably exhibits a high degree of spatial order, with an intricate pattern of specific divergence and convergence. The present results are compatible with previous physiological evidence from micromapping studies of a precise and complicated mosaic pattern of connections to the cerebellar hemispheres.

Do pontocerebellar fibers send collaterals to the cerebellar nuclei?

Three cats received large injections in the pontine nuclei of horseradish peroxidase labeled wheat germ agglutinin. Pontocerebellar axons were stained throughout their length and dense terminal label was present in the granular layer. The cerebellar nuclei, however, contained only a few scattered labeled fibers without a consistent distribution from case to case. If nuclear collaterals from pontocerebellar fibers exist, they appear to be very few and can be expected to give only a very small contribution to the excitatory input to the cerebellar nuclei.

The projection of the basal nucleus of Meynert upon the neocortex in the monkey.

After injections of horseradish peroxidase into several areas of the neocortex in the macaque monkey longitudinal bands of labeled cells in the basal nucleus of Meynert related to areas of cortex in the frontal lobe have been found to overlap along their long axes with the bands related to widely separated but interconnected areas of the parieto-temporal cortex. The frontal and parietal lobes are related to the anterior and posterior halves respectively of the nucleus, the temporal cortex to the postero-lateral margin of the nucleus and the occipital lobe to its upturned posterior extension.

Is lectin-coupled horseradish peroxidase taken up and transported by undamaged as well as by damaged fibers in the central nervous system?

Uptake and transport of horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) in intact and damaged passing fibers were studied by injections of the medulla and pons in 11 cats. Injections with evidence of damage to olivocerebellar fibers and cranial nerve fibers invariably lead to retrograde labeling of neurons in the inferior olive and cranial motor nuclei. With staining around--but apparently no damage of--cranial nerve root fibers, no labeling was found in their motor nuclei. Injections limited to the medullary pyramid with slight fiber damage and limited staining lead to faint retrograde labeling of a small number of cells in the ipsilateral sensorimotor cortex. More extensive staining and fiber damage of the pyramid gave a higher number of labeled cells in the ipsilateral sensorimotor cortex. From these experiments we conclude that HRP-WGA is taken up and transported retrogradely with subsequent significant cell labeling in damaged but not in intact fibers. Anterograde transport of HRP-WGA in fibers passing through the injected area was found to take place only for a very short distance, as judged from cases with injections of either the pons or the medullary pyramid interrupting many corticospinal fibers.