Medial Cerebellar Nucleus Research Articles

Experimental study on the eye movements induced by the electrical stimulation of the cerebellar nucleus in the rabbit

The purpose of this study is to investigate the induced eye movements by the cerebellar nuclei stimulation in the rabbits. Forty-five pigmented rabbits were used in this experiments. The cerebellar nuclei, especially medial nucleus, were stimulated electrically and the induced eye movements were observed. The results were as follows. 1. The horizontal nystagmus was produced by the electrical stimulation of the medial, anterior interposed, or posterior interposed nuclei. 2. The horizontal nystagmus towards the ipsilateral to the stimulated side was observed when the caudal portion of the medial nucleus was stimulated. On the other hand, the horizontal nystagmus towards the contralateral to the stimulated side was observed when the rostral portion of the medial nucleus was stimulated. 3. The latency of the field potential in the vestibular nucleus by stimulating the medial cerebellar nucleus, and that in the reverse situation, were 0.7 to 1.1 msec. 4. After the destruction of the lateral vestibular nucleus, no nystagmus was elicited by the stimulation of the medial cerebellar nucleus. From these results, it can be concluded that there is the monosynaptic connection between the medial cerebellar nucleus and the lateral vestibular nucleus. This indicates that the stimulation of the medial cerebellar nucleus produces the excitation of the vestibular nucleus via the cerebello-vestibular fibers and produces the nystagmus in the pigmented rabbits.

Collateral branches of cerebellopontine axons reach the thalamus, superior colliculus, or inferior olive: A double-fluorescence and combined fluorescence-horseradish peroxidase study in the rat

Retrograde double-labeling methods that used two different fluorescent dyes or a fluorescent dye in combination with wheat germ agglutinin-horseradish peroxidase were used in the rat to study the collateralization of cerebellopontine fibers to the thalamus, the superior colliculus, or the inferior olive. In cases with combined basilar pontine nuclei and thalamus injections, double-labeled neurons were located in the rostral part of the lateral cerebellar nucleus as well as within the interpositus anterior and interpositus posterior nuclei. These cells are medium to large in size and multipolar-shaped. A much smaller number of double-labeled cells was observed in the combined basilar pontine nuclei and superior colliculus injections. In these cases most of the double-labeled cells were intermediate- to large-sized and either bipolar- or multipolar-shaped. Such neurons were distributed throughout the rostrocaudal extent of the lateral cerebellar nucleus, with only a few double-labeled cells located in the interpositus anterior and posterior nuclei. Finally, in the cases with combined basilar pontine nuclei and inferior olive injections, double-labeled cells were located in interpositus anterior and posterior nuclei and the medial portion of the lateral cerebellar nucleus. The double-labeled cells were relatively small in size and most were spindle-shaped. No double-labeled cells were observed in the medial cerebellar nucleus in any of the three injection combinations. Based upon the observation of double-labeled neurons in the deep cerebellar nuclei in each of the three injection combinations involving the basilar pontine nuclei, we conclude that cerebellar projections to the basilar pons arise in part as collaterals of axons that project to the thalamus, superior colliculus, or the inferior olive.

Immunohistochemical localization of γ-aminobutyric acid- and aspartate-containing neurons in the rat deep cerebellar nuclei

The immunohistochemical localization of γ-aminobutyric acid (GABA)- and aspartate-containing neurons was evidenced in the rat deep cerebellar nuclei, using purified antisera to GABA and aspartate, respectively. Most GABA-containing neurons had small cell bodies and were scattered unevenly throughout the deep cerebellar nuclei. The medial cerebellar nucleus had a few GABA-containing cell bodies and interpositus and lateral nucleu contained many GABA-containing cell bodies. GABA-containing terminals were distributed throughout the nuclei. A large number of various-sized aspartate-containing cells were present in the deep cerebellar nuclei and most of these cells were large or medium-size. Aspartate-containing cells in the medial, interpositus and lateral nucleus were surrounded by GABA-like immunoreactive terminals, thereby suggesting the modulation of aspartate-containing neurons by GABAergic fibers from Purkinje cells.

Topographic organization of the corticonuclear and corticovestibular projections from the pyramis and copula pyramidis in the albino rat. An autoradiographic orthograde tracing study.

The topographic organization of the corticonuclear and corticovestibular fibers from the pyramis and copula pyramidis in the albino rat was investigated by the autoradiographic orthograde tracing technique. Cortical efferent fibers from the medial part of the pyramis project to the caudoventral part of the caudomedial and middle subdivisions of the medial cerebellar nucleus, whereas those arising from the lateral part of the pyramis project to the caudomedial part of the posterior interpositus nucleus and also to the dorsal part of the lateral vestibular nucleus via the juxtarestiform body. Fibers from the medial part of the copula pyramidis project to the medial part of the anterior interpositus nucleus. On the other hand, fibers from the medial portion of the lateral part of the copula pyramidis project to the posterior interpositus nucleus, while those from the lateral portion, including the lateral limit of the copula pyramidis, project to the rostroventral part of the lateral cerebellar nucleus. These results indicate that the cortical efferent projections from the pyramis and the copula pyramidis are clearly oriented mediolaterally in the albino rat.

Collateralization of cerebellar efferent projections to the paraoculomotor region, superior colliculus, and medial pontine reticular formation in the rat: a fluorescent double-labeling study.

Collateralization of cerebellar efferent projections to the oculomotor region, superior colliculus (SC), and medial pontine reticular formation (mPRF) was studied in rats using fluorescent tracer substances. In one group, True Blue (TB) was injected into the oculomotor complex (OMC), including certain paraoculomotor nuclei and supraoculomotor ventral periaqueductal gray (PAG), and Diamidino Yellow (DY) was injected into the medial pontine reticular formation (mPRF) or pontine raphe. The largest number of single-TB-labeled (paraoculomotor-projecting) cells was observed in the medial cerebellar nucleus (MCN) and posterior interposed nucleus (PIN), whereas the largest number of single-DY-labeled (mPRF-projecting) cells was in the MCN. Double-TB/DY-labeled cells were present in the caudal two-thirds of the MCN, suggesting that some MCN neurons send divergent axon collaterals to the paraoculomotor region and mPRF. In another group, TB was injected into the SC and DY into the mPRF. The largest number of single-TB-labeled (SC-projecting) cells was in the PIN, although a considerable number of cells was observed in the caudal MCN, and ventral lateral cerebellar nucleus (LCN). Single-DY-labeled (mPRF-projecting) neurons were primarily located in the central and ventral MCN, but were also present in the lateral anterior interposed (AIN) and in the LCN. Double-TB/DY-labeled neurons were observed in the caudal two-thirds of the MCN and in the central portion of the LCN. The most significant new findings of the study concerned the MCN, which not only contained neurons that projected independently to the paraoculomotor region, SC, and mPRF, but also contained a considerable number of cells which collateralized to project to more than one of these nuclei. The possibility that the MCN projects to the supraoculomotor ventral PAG (containing an oculomotor interneuron system) and to the mPRF, which in the cat and monkey contain neural elements essential to the production of saccadic eye movements, is discussed. The anatomical findings suggest that the MCN in the rat plays an important role in eye movement.

The origin of brainstem afferents of the paramedian pontine reticular formation in the cat

Transcannular microinjections of horseradish peroxidase (HRP) were made into the paramedian pontine reticular formation (PPRF) in adult cats to determine the origin of the principal sources of inputs to this important preoculomotor center for the production of saccadic eye movements. Retrogradely labeled cells were observed in numerous oculomotor-related structures, including the prerubral field (rostral interstitial nucleus of the medial longitudinal fasciculus), nucleus of Darkschewitsh, nucleus of the posterior commissure, deep superior colliculus, supraoculomotor ventral periaqueductal gray, contralateral paramedian pontine reticular formation, pontine raphe and dorsal medial pontine tegmentum medial to the abducens nucleus (purported to contain omnipause neurons), cell group Y, and the perihypoglossal complex (nucleus prepositus hypoglossi). Other sources of afferents to the region included the zona incerta, lateral and medial habenular nuclei, medial hypothalamus, medial mammillary nucleus, nucleus cuneiformis, medial medullary reticular formation, and the medial and lateral cerebellar nuclei. The results are discussed in terms of the potential influence of these nuclei on the control of eye movement.

Limb specific connections of the cat magnocellular red nucleus.

Afferent and efferent connections of the limb specific divisions of the cat magnocellular red nucleus (RNm) were traced using the bidirectional transport of wheatgerm agglutinin-horseradish peroxidase complex (WGA-HRP). Injection sites within forelimb or hindlimb RNm regions were identified by microelectrode recording and confirmed by the position of labeled rubrospinal terminals. Additional injections into structures that project to, or receive input from, RNm confirmed the somatotopic organization of these pathways. The forelimb region of RNm receives input from the posteriolateral part of the anterior interpositus nucleus (NIA) and the intermediate part of the posterior interpositus nucleus (NIP). The hindlimb region of RNm receives input from anteriomedial NIA and medial NIP. Terminals of NIA cells densely fill all of RNm, but terminals of NIP cells form a half shell on the medial, ventral, and posterior borders of RNm without encroaching on RNm's lateral edge or central core. Forelimb and hindlimb RNm are reciprocally connected with the caudal cuneate and gracile nuclei respectively. There is little or no input to RNm from the medial or lateral cerebellar nuclei. Forelimb RNm, which also contains a face representation, projects to the lateral reticular nucleus, cell group f of the inferior vestibular nucleus, the facial nucleus, the main sensory nucleus of the trigeminal nerve, the caudal cuneate nucleus, the parvicellular reticular formation, and cervical segments of the spinal cord. A few fibers from forelimb RNm project directly to motor neurons in the lower cervical cord. Hindlimb RNm projects to only the lateral reticular nucleus, gracile nucleus, and lower spinal segments. Forelimb and hindlimb RNm project to different regions of the lateral reticular nucleus with some overlap.

Neuronal migration and dendritic maturation of the medial cerebellar nucleus in rat embryos: an HRP in vitro study using cerebellar slabs.

The morphological maturation of medial nuclear neurons of fetal rat cerebella was studied using an in vitro assay. Neurons of this nucleus were identified in isolated preparations of rhombencephalon between embryonic days 16 and 20 (E16-E20) by the intracerebellar decussation of their outgrowing axons within the uncinate fascicle. A small crystal of horseradish peroxidase (HRP) applied either in the region containing the inferior cerebellar peduncle or, preferably, in the lateral cerebellum retrogradely labeled contralateral medial nuclear neurons. In the youngest embryos (E16-E17), HRP-marked neurons were situated rostrally at the dorsal surface of the cerebellum. By E18, the cell mass containing labeled neurons had shifted in a rostrocaudal and dorsoventral direction and finally reached the adult position in E19-E20 embryos. Dendritic differentiation of these neurons followed a similar positional gradient, closely corresponding to the pattern of temporal development. From the most immature monopolar forms located dorsally to the virtually adult stellate neurons in a ventral position, it was possible to trace a continuum of intermediary forms grouped into six well-defined stages. Immature monopolar cells first became transversely bipolar. Then, they changed orientation, assuming a longitudinal radial direction. During this stage, neurons sank into the cerebellar parenchyma. As they reached their final destination, these neurons gradually developed dendrites which radiated from the cell body in an adult-like pattern. It is concluded that the medial nuclear neurons occupy a superficial dorsal position in early phases of cerebellar ontogeny, thereafter undergoing a second, inward migration. The main stages of neuronal dendritic differentiation occur between E16 and E20, indicating that the ingrowth of afferent in puts to the medial nucleus most probably occurs rather early and is concomitant with dendritic development.

Cerebellar corticonuclear and corticovestibular fibers from the posterior lobe of the albino rat, with comments on zones.

The topographic organization of cerebellar cortical efferent fibers from the posterior lobe of the albino rat was studied by silver impregnation methods. The corticonuclear fibers from the vermis of the posterior lobe project to the caudomedial and middle parts of the medial cerebellar nucleus (MN) with medio-lateral localization; fibers from the medial portion of the vermis project to the caudomedial part of the MN (MNcm) and those from the lateral portion project to the middle part of the MN (MNm). Corticovestibular fibers originate in the caudal vermis lateral to the area projecting to the MNcm and MNm, and terminate in the dorsal part of the lateral vestibular nucleus (LVN). The origins of corticonuclear fibers to the dorsolateral protuberance of the MN (MNdlp), the posterior interpositus nucleus (PIN) and the anterior interpositus nucleus (AIN) are located latero-medially in the intermediate cortex. Fibers from the area adjacent to the vermis and rostral to the prepyramidal fissure project to the MNdlp, while those from the area lateral to the origin of the corticovestibular fibers and caudal to the fissure project to the medial AIN. Together, these areas comprise a medial portion of the intermediate cortex. Corticonuclear fibers to the PIN and lateral AIN originate from the lateral portion of the intermediate cortex. The corticonuclear fibers to the dorsolateral hump and lateral nucleus originate from the medial and lateral portions of the lateral cortex, respectively.

Immunocytochemical localization of glutamate-, glutaminase- and aspartate aminotransferase-like immunoreactivity in the rat deep cerebellar nuclei

Althougthe anatomy and the connectivity of the deep cerebellar nuclei have been well documented, little is known about the neurotransmitter systems mediating cerebellar efferent pathways. The present study utilizes immunohistochemical procedures in conjunction wita novel monoclonal antibody specific for carbodiimide-fixed glutamate and polyclonal antisera against glutaminase (GLNase) and aspartate aminotransferase (AATase) to examine the presence of putative excitatory amino acid transmitters in neurons of the deep cerebellar nuclei. Carbodiimide-fixed glutamate-like, GLNase-like and AATase-like immunoreactivities were observed in neurons of the lateral, posterior interpositus, anterior interpositus and medial deep cerebellar nuclei. More neurons were stained witAATase antiserum than witthe GLNase antiserum or the monoclonal antibody. These results suggest glutamate, GLNase and AATase are present in neurons of the deep cerebellar nuclei and raise the possibility that glutamate may be an excitatory transmitter in these structures.

Topographical organization of the afferent connections of the principal ventromedial thalamic nucleus in the cat

The afferent connections to the principal division of the ventromedial thalamic nucleus (VMP) were studied in the cat by means of the HRP retrograde transport technique. The large (40 nl) and small (20 nl) injections of this enzyme were delivered into the VMP using different stereotaxic approaches. The main afferents to VMP emanated bilaterally from the prefrontal, premotor, and rostral agranular insular cortices. Another important group of afferents to the VMP were those originating in the rostral third of the reticular thalamic nucleus, the entopeduncular nucleus, the substantia nigra pars reticulata, and the deep cerebellar nuclei. From the cerebellar nuclei, the contralateral lateral nucleus and the caudal third of both (ipsi and contralateral) medial cerebellar nuclei were the origin of afferents to the VMP. Other cortical areas projecting (in a lower density) to the VMP were the motor cortex, the cortex along the anterior ectosylvian sulcus, the granular insular cortex, the posterior agranular insular area, the prelimbic area, and the cortex along the posterior rhinal sulcus (SRP). Among other subcortical prosencephalic structures projecting to the VMP are the dorsal claustrum, substantia innominata, hypothalamic formations, and the zona incerta. Projections originated from the brainstem in the lateral part of the intermediate and deep layers of the superior colliculus, the central gray matter, the locus coeruleus, and the reticular formation. The nucleus tegmenti pedunculopontinus pars compacta, parabrachial nuclei, the vestibular complex, and the spinal trigeminal nucleus were also origins of projections to the VMP. We conclude by emphasizing the important bilateral cortical modulation of the different functions attributed to the VMP: recruiting-response mediation, reticular-activating system participation, and extrapyramidal motor integration. In light of the connections just described, the VMP may be considered as a point for impulses coming from complex association cortical areas and limbic formations to converge with those emanating from cortical and subcortical motor structures.

An HRP study of hypothalamo-cerebellar and cerebello-hypothalamic connections in squirrel monkey (Saimiri sciureus).

This study describes the distribution of labeled hypothalamic neurons in squirrel monkey following pressure injections of horseradish peroxidase (HRP) into cerebellar cortex and the pattern of labeling in the cerebellar nuclei subsequent to iontophoretic injections localized in the hypothalamus. Two types of HRP (HRP and a wheat germ agglutinin conjugate, HRP-WGA) were used as tracers; tetramethylbenzidine was the chromogen. Retrogradely filled neurons were found in lateral (LHAr) and posterior (PHAr) hypothalamic areas, and in the lateral mammillary (LMNu) and supramammillary (SMNu) nuclei following injections into ansiform and paramedian lobules and into the paraflocculus. Labeled cells were occasionally seen in the medial mammillary nucleus (parafloccular cases) and among fascicles of the mammillothalamic tract (all posterior lobe cases) immediately above the medial nucleus. After injections into the anterior lobe, labelling was again found in the LHAr, PHAr, LMNu, and SMNu. In addition, retrogradely filled cells were present in ventromedial, dorsomedial, and dorsal hypothalamic nuclei and in the dorsal hypothalamic area. Labeled cells were occasionally found among fascicles of the fornix along its intrahypothalamic course. In general, labeling extends into slightly more rostral hypothalamic levels in anterior lobe cases when compared to posterior lobe experiments. We interpret these data as indicating that some hypothalamic neurons project directly to the cerebellar cortex (i.e., hypothalamo-cerebellar fibers); this projection is bilateral with an ipsilateral preponderance. In experiments with injections of HRP-WGA into the cerebellar nuclei, anterogradely filled axons were traced into the contralateral PHAr and LHAr; this was suggestive of a direct cerebello-hypothalamic projection. Following iontophoretic injections localized in the LHAr and the medial mammillary nucleus, labeling was seen in the medial (NM), posterior interposed (NIP), and lateral (NL) cerebellar nuclei; this is essentially a contralateral projection. Retrogradely labeled cells were found in the rostral and ventral NM, the ventral and dorsocaudal NL, and diffusely throughout the NIP. On the basis of the known distribution of cerebello-thalamic fibers and other criteria, these labeled cells are representative of a true cerebello-hypothalamic projection. It is suggested that the cerebellum, through these pathways, may have a relatively direct influence on visceral centers in the brainstem and spinal cord.

Muscarinic cholinergic receptors in the rat deep cerebellar nuclei: A quantitative autoradiographic study

The distribution of muscarinic cholinergic receptors within the rat deep cerebellar nuclei was analyzed using in vitro receptor binding of [ 3H]quinuclidinylbenzilate (QNB) in conjunction with autoradiography. The highest density of QNB binding sites occurred in the lateral cerebellar (dentate) nucleus. Interpositus nuclei displayed an intermediate density of muscarinic cholinergic binding sites with the posterior interpositus nucleus demonstrating higher binding than the anterior nucleus. The fastigial (medial) cerebellar nucleus exhibited the lowest levels of QNB binding among the four cerebellar nuclei. These results indicate that muscarinic cholinergic receptors are present in the deep cerebellar nuclei and that differences in receptor density occur among the four nuclear groups.

Some afferent and efferent connections of the vestibular nuclear complex in the red-eared turtle Pseudemys scripta elegans.

In the present study some afferent, commissural, and efferent connections of the vestibular nuclear complex in the turtle Pseudemys scripta elegans were demonstrated with the HRP tracing technique. Afferent projections to the vestibular nuclei were found to arise in the nucleus of the basal optic root, the interstitial nucleus of the fasciculus longitudinalis medialis, the medial and lateral cerebellar nuclei, the perihypoglossal nuclear complex, and the reticular formation. Distinct commissural projections appeared to arise in the dorsolateral, ventromedial, and descending vestibular nuclei. The commissural projection arising in the ventrolateral vestibular nucleus appeared to be only sparsely developed. Both ascending and descending efferent projections were demonstrated to arise from the vestibular nuclear complex. The ascending vestibulo-oculomotor projection was found to be organized in an ipsilateral pathway arising in the dorsolateral vestibular nucleus and in a contralateral pathway, arising mainly in the medial vestibular nucleus. These projections appeared to be directed to the interstitial nucleus of the fasciculus longitudinalis medialis, the oculomotor, trochlear, and abducens nuclei. Also the perihypoglossal nuclear complex appeared to be an important target of vestibular efferents. The origin and course in the brainstem of the descending vestibular projections, i.e., the lateral and medial vestibulospinal tracts, as demonstrated in previous anatomical and experimental studies in reptiles, were confirmed. However, in addition a direct projection of the vestibulospinal tracts to presumably neck motoneurons was found. THe organization of the vestibular connections observed in the turtle Pseudemys scripta elegans appeared to be basically comparable to the organization of the vestibular connections in birds and mammals.

Cerebellar saccadic dysmetria is not equal in the two eyes

Differences in the saccadic dysmetria between the two eyes were examined in five trained monkeys. Dysmetria, produced by reversible lesions of the medial cerebellar nuclei on the left side, was qualitatively similar in the two eyes in that both eyes were hypometric or were hypermetric. Quantitatively, however, the dysmetria was different. The right eye made larger amplitude and higher velocity saccades to the right and smaller amplitude and lower velocity saccades to the left than the left eye. Differences in eye position, during a saccade trajectory, were larger than those 50 ms after saccade termination. The differences varied as a function of saccade initial position with the largest differences occurring when the saccade was directed away from the center. The results suggest that the cerebellum can compensate for differences in muscle strength by selectively adjusting the strength of the pulse-step innervation to each muscle of a yoked muscle pair. Thus, Hering's law of equal innervation would appear to be a consequence of appropriate cerebellar compensation and not an invariant property of synaptic connections.

Direct projections from the cerebellar nuclei to the superior colliculus in the rabbit: an HRP study.

Cerebellar projections to the superior colliculus in the rabbit were studied by the anterograde and retrograde HRP methods. Cerebellotectal fibers arise mainly from the anterior and posterior interpositus nuclei and terminate contralaterally in layer VII, layer VI, layer V, and the deep tier of layer IV of the superior colliculus. Cerebellotectal fibers from the posterior interpositus nucleus originate from the lateral part of the nucleus and end chiefly in the caudal part of the superior colliculus. Cerebellotectal fibers from the anterior interpositus nucleus arise from the ventral part of the nucleus and terminate mainly in the rostromedial part of the superior colliculus. Some neurons in the lateral cerebellar nucleus also send fibers contralaterally to the intermediate and deep layers of the superior colliculus, especially to its rostral and lateral parts. Few, if any, cerebellotectal fibers arise from the medial cerebellar nucleus.

Cerebellar corticonuclear projections in the red-eared turtle Pseudemys scripta elegans.

In the present study the organization of the corticonuclear projections of the cerebellum in the red-eared turtle Pseudemys scripta elegans was investigated. To find out whether a zonal pattern exists in the cerebellar cortex, a topological analysis was made of the Purkinje cell layer of a Nissl-stained series of the cerebellum. This analysis showed a distinct, longitudinally oriented zonal pattern of Purkinje cells. In each cerebellar half a medial, an intermediate, and a lateral zone could be distinguished. This pattern appeared to correspond in part to an earlier subdivision of the reptilian cerebellum suggested by Larsell ('26, '32, '67). In the experimental part of this study, the corticonuclear projections to the cerebellar nuclei and the vestibular nuclear complex were demonstrated with the horseradish peroxidase (HRP) technique. All projections of the Purkinje cells appeared to be strictly ipsilateral. After HRP slow-release gel implantations at various levels of the vestibular nuclear complex, longitudinally oriented areas of labeled Purkinje cells were found in the lateral part of the medial zone, the intermediate zone, and the lateral zone. Two strips of Purkinje cells remained unlabeled: One in the rostrolateral part of the intermediate zone, and another in the medial part of the medial zone. After HRP gel implantations in the cerebellar peduncle aimed at the medial and lateral cerebellar nuclei, respectively. Purkinje cells in the latter two strips were labeled. The present results indicate that the corticonuclear projections to the medial cerebellar nucleus arise in the medial part of the medial zone and those to the lateral cerebellar nucleus in the rostrolateral part of the intermediate zone. It can be concluded that cerebellar zones as demonstrated in the turtle Pseudemys scripta elegans, are phylogenetically very old and may represent the basic functional circuit of the cerebellum.

The cerebellopontine system in the rat. I. Autoradiographic studies.

This study utilized light microscopic autoradiographic procedures to describe the projections from the three major subdivisions of the deep cerebellar nuclei (DCN) to the basilar pontine nuclei (BPN). Although the vast majority of cerebellopontine axons reached the BPN via the descending limb of the brachium conjunctivum (BC) after crossing the midline within the midbrain, a relatively small number of ipsilaterally projecting fibers was also observed. Fascicles of cerebellopontine axons left the main bundle of descending limb fibers throughout much of the rostrocaudal length of the BPN and passed around and through the medial lemniscus and cerebral peduncle to enter the pontine gray. The lateral cerebellar nucleus gave rise to the largest number of cerebellopontine fibers, whose terminal fields exhibited both diffuse and patchlike labeling patterns within each of the major subdivisions of the BPN including medial, ventral, lateral, and dorsal areas. Projections from the interpositus complex exclusive of its posterior division were fewer and less widely distributed than those from the lateral nucleus. Interpositopontine fibers terminated primarily in the caudal one-half of the BPN in medial, ventral, and lateral regions and overlapped somewhat with projections from the lateral cerebellar nucleus. Pontine projections emanating from the medial cerebellar nucleus were the fewest and most restricted in distribution relative to the other two cerebellar efferent systems. Such fibers formed a patchlike network of terminal fields which extended throughout much of the rostrocaudal length of the BPN in medial and dorsomedial regions. A relatively small but considerable number of ipsilateral cerebellopontine fibers terminated in pontine regions, which often mirrored the typical contralateral projection fields. Although it proved difficult to determine the precise origin of the ipsilateral fiber systems, it appeared that each of the three major DCN subdivisions made some contribution. Also it was apparent that considerable overlap existed between cerebellopontine projection zones and those of other pontine afferents including sensorimotor, visual, and auditory cortices, the superior colliculus, and the mammillary nuclei of the hypothalamus. Moreover, cerebellopontine terminal fields were congruent in some instances with discrete clusters of BPN neurons which serve as the source of pontocerebellar fiber systems, reaching portions of the lateral cerebellar hemispheres, posterior vermis, and the paraflocculus.

Distribution and structural characterization of neurons giving rise to descending spinal projections in the turtle, Pseudemys scripta elegans.

Descending spinal projections were investigated in the turtle Pseudemys scripta elegans following injections of horseradish peroxidase and/or radioactive wheat germ agglutinin into the spinal cord at various levels. Using various planes of section the cells of origin in the brainstem, cerebellum, and diencephalon were characterized according to their size, dendritic tree, and precise location. Projections to levels as far caudal as the lumbar spinal cord were found to arise from medial and lateral rhombencephalic reticular fields, including the perihypoglossal complex, the nucleus raphe inferior, and the locus coeruleus; from certain subdivisions of the vestibular complex (ipsilateral subnucleus (subn.) ventrolateralis, contralateral subn. ventromedialis, and possibly subn. tangentialis); from the motor trigeminal nucleus; from the contralateral red nucleus, the ipsilateral nucleus (n.) interstitialis of the fasciculus longitudinalis medialis (flm) and from the hypothalamus. Fibers to high cervical levels arose from neurons within the dorsolateral and superior vestibular nuclei, the lateral cerebellar nucleus, the mesencephalic trigeminal nucleus, and from neurons of the optic tectum. Low cervical and thoracic spinal levels were reached by fibers from the torus semicircularis n. laminaris, the n. of the flm, the medial cerebellar nucleus as well as from the n. vestibularis inferior and the principal and descending trigeminal nuclei.

An autoradiographic study of the cerebellopontine projections in the rat. I. Projections from the medial cerebellar nucleus

The medial cerebellar nucleus of the rat is shown by the autoradiographic technique to project to both the contralateral nucleus reticularis tegmenti pontis and the pontine nuclei proper. The former projection is more concentrated in the medial — parvocellular — region. In the pontine gray, the bulk of the projection concerns the dorsal aspect of the medial nucleus. Rostral parts of the medial cerebellar nucleus project to caudal pontine levels whereas caudal parts seem to project throughout the rostrocaudal extent of the basilar pons.