The projection from nucleus reticularis tegmenti pontis onto the cerebellum in the cat. A study using the methods of anterograde degeneration and retrograde axonal transport of horseradish peroxidase

Neurophysiology of the optokinetic system

This chapter provides a review of early studies into the neural substrate for optokinetic-vestibular responses. Properties and connections of retinal and brainstem neurons contributing to optokinetic responses in the afoveate rabbit are summarized. Electrophysiological and lesion studies provide support for confluence of optokinetic and vestibular signals in the vestibular nucleus to provide the brain's estimate of self-rotation. Evidence for optokinetic-vestibular symbiosis in humans comes from the observation that individuals who have lost vestibular function show no optokinetic after-nystagmus in darkness, following full-field stimulus motion. An anatomical scheme for brainstem elaboration of optokinetic responses is proposed and cerebellar contributions are reviewed.

Regional (14C) 2-deoxyglucose uptake during forelimb movements evoked by rat motor cortex stimulation: pons, cerebellum, medulla, spinal cord, muscle

Electrical stimulation of the right forelimb motor (MI) sensory (SI) cortex in normal, adult rats produced repetitive left forelimb movements. Regions of increased (14C) 2-deoxyglucose (2DG) uptake were mapped auto-radiographically during these movements. MI stimulation activated the ipsilateral reticular tegmental pontine nucleus (RTP) and the middle (rostral-caudal) third of the pontine nuclei including pyramidal (P), medial (POM), ventral (POV), and lateral (POL) pontine nuclei. The ipsilateral inferior olivary complex was activated including dorsal accessory olive (DAO), principal olive (PO), and medial accessory olive (MAO). The contralateral lateral reticular (LR) nucleus and nucleus cuneatus (CU) were activated. Lateral vermal, paravermal, and hemispheric portions of the contralateral cerebellum were also activated. Parts of vermian lobules IV, V, VI, VII, and VIII, and lobulus simplex, crus I, crus II, paramedian lobule, and copula pyramidis were activated. Granule cell layers were activated much more than molecular layers. Discrete microzones of high granule cell 2DG uptake alternated with zones of low uptake in left paramedian lobule and copula pyramidis and may correlate with the fractured cerebellar somatotopy described physiologically by Welker and his associates. Portions of the left lateral and interpositus nuclei were metabolically activated. Medial portions of laminae I-VI were activated in the dorsal horn of cervical spinal cord. The 2DG uptake was either unchanged or decreased in the ventral horn. Thoracic and lumbar spinal cord were not activated. Monsynaptic MI and SI connections to P, POM, POV, POL, RTP, DAO, PO, MAO, LR, CU, and spinal cord could account for activation of those structures. However, there are no direct MI or SI connections to the deep cerebellar nuclei, the cerebellar hemisphere, or the muscles. Activation of these structures must be due to activation of polysynaptic pathways, sensory feedback from the moving forelimb, or both. The present experiments cannot distinguish these possibilities. Comparison of the regions activated during forelimb MI stimulation (FLMIS) to those activated during vibrissae MI stimulation (VMIS) suggests that the pontine nuclei, cerebellar hemisphere, and possibly the deep cerebellar nuclei are somatotopically organized. RTP, LR, CU, and spinal cord were activated during FLMIS but were not activated during VMIS. The failure to activate the ventral horn of cervical spinal cord may be due to known inhibition of alpha-motor neurons during motor cortex stimulation.

Oculomotor cerebellum

The anatomical, physiological, and behavioral evidence for the involvement of three regions of the cerebellum in oculomotor behavior is reviewed here: (1) the oculomotor vermis and paravermis of lobules V, IV, and VII; (2) the uvula and nodulus; (3) flocculus and ventral paraflocculus. No region of the cerebellum controls eye movements exclusively, but each receives sensory information relevant for the control of multiple systems. An analysis of the microcircuitry suggests how sagittal climbing fiber zones bring visual information to the oculomotor vermis; convey vestibular information to the uvula and nodulus, while optokinetic space is represented in the flocculus. The mossy fiber projections are more heterogeneous. The importance of the inferior olive in modulating Purkinje cell responses is discussed.

Axonal ramification of neurons in the nucleus reticularis tegmenti pontis projecting to the paramedian lobule in the rabbit cerebellum

Projections of the nucleus reticularis tegmenti pontis (NRTP) to the cerebellar paramedian lobule were examined in the rabbit by means of the double fluorescent retrograde tract-tracing method. The rabbit NRTP is composed of a medial, large part comprising zones A (dorsomedial), B (central) and C (lateral), and of a lateral, small part (the processus tegmentosus lateralis; PTL). Following unilateral injections of Fast Blue (FB) into the rostral part of the paramedian lobule (rPML) and of Diamidino Yellow (DY) into the caudal part (cPML), known to receive spinal inputs from forelimb and hindlimb, respectively, substantial numbers of single labeled neurons were found in all bilateral NRTP divisions, apart from the zone C. Most projection neurons to the PML were located in the medial and medioventral regions of the zone B. Smaller numbers of projection neurons were located in the PTL, zone A and outside the zone B among fibers of the medial lemniscus. The pattern of FB and DY labeling suggested that neurons projecting to the rPML and cPML originated in common rather than separate regions within the NRTP. In addition, a small percentage (mean 1.3%) of double FB+DY labeled neurons were detected with a clear contralateral preponderance, among single labeled FB or DY cells. In spite of the rarity, all the NRTP neurons giving rise to intralobular collateral projections can be regarded as potential sources of simultaneous modulating influences upon two functional different forelimb (rPML) and hindlimb (cPML) regions. The findings have been discussed in relation to earlier studies in other species and commented on with respect to the possible functional meaning of these projections.

The basilar pontine nuclei and the nucleus reticularis tegmenti pontis subserve distinct cerebrocerebellar pathways

Previous studies often considered the basilar pontine nuclei (BPN) and the nucleus reticularis tegmenti pontis (NRTP) as relays of a single cerebro-(ponto)-cerebellar pathway. Conversely, the different cortical afferences to the BPN and the NRTP, as well as the anatomical and functional features of the cerebellopetal projections from these pontine nuclei, support the different, and for some aspect, complementary arrangement of the cerebrocerebellar pathways relayed by the BPN or NRTP. Both the BPN and the NRTP are innervated from the cerebral cortex, but with regional prevalence. The NRTP is principally innervated from motor or sensori-motor areas while the BPN are principally innervated from sensory, mainly teloceptive, and associative area. Projections from sensory-motor areas were also traced to the BPN. The BPN and NRTP project to all parts of the cerebellar cortex with a similar pattern. In fact, from single areas of them projections were traced to set of sagittal stripes of the cerebellar cortex. In variance to such analogies, the projections to the cerebellar nuclei differed between those traced from the NRTP and from BPN. In fact, BPN and NRTP have private terminal areas in the cerebellar nuclei with relatively little overlaps. The BPN innervated the lateroventral part of the nucleus lateralis and the caudoventral aspect of the nucleus interpositalis posterioris. The NRTP principally innervated the mediodorsal part of the nucleus lateralis, the nucleus interpositalis anterioris, the nucleus medialis. Since the single cerebellar nuclei have their specific targets in the extracerebellar brain areas, it follows that the BPN and the NRTP, passing through their cerebellar nuclei relays, are devoted to control different brain areas and thus likely to play different functional roles. From single pontine regions (of both BPN and NRTP) projections were traced to the cerebellar cortex and to the cerebellar nuclei. In some cases these projections reached areas which are likely anatomically connected (by Purkinje axons). This pattern of the pontine projections was termed as coupled projection. In some other cases, the projections reached areas of the cerebellar cortex but not the nuclear regions innervated by them. We termed this as uncoupled projection. The existence of both coupled and uncoupled projections, open new vistas on the functional architecture of the pontocerebellar pathway. More in detail, this study showed the different quantitative and topographic distribution of the coupled and uncoupled projections visualized in the cerebellar projections from BPN and NRTP. All these evidences strongly support the anatomical and the functional differences that characterise the cerebrocerebellar pathways relayed by the BPN and the NRTP.

Projections of the basilar pontine nuclei and nucleus reticularis tegmenti pontis to the cerebellar nuclei of the rat

This study showed the precise projection pattern of the basilar pontine nuclei (BPN) and the nucleus reticularis tegmenti pontis (NRTP) to the cerebellar nuclei (CN), as well as the different anatomic features of BPN and NRTP projections. The staining of BPN or NRTP with biotinylated dextran labeled projection fibers to complementary topographic areas in the CN. In fact, BPN principally project to a rostrocaudally oriented column of the nucleus lateralis (NL), which at the midcentral level shifts to the lateroventral part of the nucleus, as well as to the caudolateral part of the nucleus interpositus posterioris. The NRTP projects to a rostrocaudal column of the NL, which at the midcentral level shifts medially, as well as to the nucleus interpositalis and to the caudal part of the nucleus medialis. BPN axons in the CN usually branch into short collaterals of simple morphology that involve small terminal areas, whereas NRTP axons branch into longer collaterals of complex morphology involving terminal areas of different sizes. Each site of injection is at the origin of a set of terminal areas in the CN. The set of projections from different BPN or NRTP areas were partially, but never completely, overlapping. Thus, the set of terminal areas in the CN was specific for each area of both BPN and NRTP. Injection of tetramethyl-rhodamine-dextran-amine into the CN stained cell bodies of BPN and NRTP with different repartition on the two sides. The study showed that CN are innervated by the contralateral BPN and not very much by the ipsilateral BPN, whereas they are innervated by NRTP bilaterally, even if with a contralateral prevalence. In conclusion, this study supports the hypothesis that both BPN and NRTP are concerned in the central program for skilled movements, even if they are probably involved in different functional roles.

Multiple zonal projections of the nucleus reticularis tegmenti pontis to the cerebellar cortex of the rat

Compartmentalization (alternating labelled and unlabelled stripes) of mossy fibre terminals was found in the cerebellar cortex after iontophoretic injections of biotinylated dextran amine into discrete regions of the nucleus reticularis tegmenti pontis (NRTP). The zonal pattern was only observed when volumes of nuclear tissue ranging from 4.5 x 106 to 17.66 x 106 microm3 were impregnated. Up to nine compartments (i.e. up to five stripes separated by four interstripes) were found in crus I and in vermal lobule VI. Up to seven compartments (four stripes and three interstripes) were found in crus II; up to five compartments (three stripes and two interstripes) were identified in the lobulus simplex, the paraflocculus and vermal lobules IV, V and VII; up to three compartments (two stripes and one interstripe) were identified in the paramedian lobule and, finally, up to two compartments (one stripe and one interstripe) were identified in the copula pyramidis, in the flocculus and in vermal lobules II, III, VIII and IX. The projections of the NRTP are arranged according to a divergent/convergent projection pattern. From single injections in the NRTP, projections were traced to a set of cortical stripes widely distributed over the cerebellar cortex. The set of stripes labelled from different regions of the NRTP partially overlapped but complete overlap was never found. This finding revealed that the topographic combination of the projections of the NRTP to the cerebellar cortex is specific for each region of the NRTP. Finally, the projections to single cortical areas were arranged according to a pattern of compartmentalization that is specific for each cortical area, independent of the site of injection in the NRTP and of the number of stripes evident in the cortex.

Laterality of the pontocerebellar projections in the rat

This study aimed to investigate the trajectory of fibres from the pontine nuclei that reach the two sides of the cerebellum. Injections of biotinylated dextran amine (BDA) were made within the basilar pontine nuclei (BPN) and the nucleus reticularis tegmenti pontis (NRTP) in one side of rats with electrolytic injury of the middle cerebellar peduncle (MCP), ipsilateral or contralateral to the side of injection. Fibres were traced from the pontine nuclei (BPN and NRTP) to both sides of the cerebellum passing through the respective MCPs. The study carried out in rats with injury to one peduncle showed projections segregated to the half-side of the cerebellum innervated by the intact peduncle. The laterality observed was confirmed by a retrograde tracer study. In fact, injections of different fluorescent tracers in rats with injury of single MCP showed that in the pontine nuclei only cell bodies stained by the tracer injected in the half-cerebellum ipsilateral to the intact peduncle. Finally, similar injections (i.e. different fluorescent tracers in symmetric areas of the cerebellar cortex) in the cerebellum of intact brain rats showed that BPN and NRTP differ for the laterality of their projections. In fact, 82% of BPN cells project contralaterally and 18% ipsilaterally, whereas 60% of NRTP cells project contralaterally and 40% ipsilaterally. In conclusion, this study showed that the MCPs receive fibres from the pontine nuclei of both sides and project to the ipsilateral half of the cerebellum and that different contingents of projections to the two sides of the cerebellum arise from BPN and NRTP.

Pontine neurons which relay projections from the superior colliculus to the posterior vermis of the cerebellum in the cat: distribution and visual properties

Extracellular unit recording revealed that the dorsolateral pontine nucleus (DLPN) and nucleus reticularis tegmenti pontis (NRTP) of the cat constituted pontine relays for transmission of visual information from the superior colliculus (SC) to the posterior vermis (lobules VI and VII) of the cerebellum. The relay neurons in DLPN/NRTP responded to moving visual stimuli with large receptive fields (23-75 degrees ) which occupied mainly the contralateral hemifield including the fovea. These neurons were usually more responsive to large-sized stimuli than to discrete-spot stimuli, had direction selectivity, and showed preference to visual motion at a relatively high speed. No clear differences in visual properties were observed between DLPN and NRTP. After a local injection of lidocaine into SC, the visual responses in DLPN/NRTP transiently disappeared, indicating that DLPN/NRTP received the visual inputs through SC.

Differences of the primate flocculus and ventral paraflocculus in the mossy and climbing fiber input organization

Potential sources of cerebellar cortical afferent fibers were identified in the vestibular ganglion, medulla oblongata, pons, and cerebellar nucleus of seven anesthetized Macaca fuscata after local injections of wheat germ agglutinin-conjugated horseradish peroxidase or Fast Blue into the flocculus (FL) or ventral paraflocculus (VP). There were differences in the sources of mossy fibers to the FL and VP. Labeled neurons, after injections into the FL, were located mainly in the ipsilateral vestibular ganglion, bilaterally in the vestibular and prepositus hypoglossal nuclei, nucleus reticularis tegmenti pontis, and the central part of the mesencephalic reticular formation including the raphe nuclei. Labeled neurons were rarely seen in the pontine nuclei after injections into the FL. By contrast, after injections into the VP, numerous labeled neurons were located in the contralateral pontine nuclei, but relatively few in the vestibular nuclei bilaterally. Sources of climbing fibers to the FL and VP were completely contralateral to the injection side. After the injection into the FL and VP, labeled neurons were located in the dorsal cap, ventrolateral outgrowth, and ventral part of the medial accessory olivary nucleus. The projections from these three olivary areas were generally consistent with a zonal pattern of terminations in the FL and VP. The present results are consistent with a hypothesis that the FL is mainly involved in the control of vestibulo-ocular reflex and that the VP is mainly involved in the control of smooth pursuit eye movements.

Projection from the cerebellar lateral nucleus to precerebellar nuclei in the mossy fiber pathway is glutamatergic: a study combining anterograde tracing with immunogold labeling in the rat

The pontine nuclei (PN) and the nucleus reticularis tegmenti pontis (NRTP) are sources of an excitatory projection to the cerebellar cortex via mossy fibers and a direct excitatory projection to the cerebellar nuclei. These precerebellar nuclei, in turn, receive a feedback projection from the cerebellar nuclei, which mostly originate in the lateral nucleus (LN). It has been suggested that the feedback projection from the LN partially uses gamma-aminobutyric acid (GABA) as a transmitter. We tested this hypothesis by using a combination of anterograde tracing (biotinylated dextran amine injection into the LN) and postembedding GABA and glutamate immunogold histochemistry. The pattern of labeling in the PN and the NRTP was compared with that of cerebellonuclear terminals in two other target structures, the parvocellular part of the nucleus ruber (RNp) and the ventromedial and ventrolateral thalamus (VM/VL). The projection to the inferior olive (IO), which is known to be predominantly GABAergic, served as a control. A quantitative analysis of the synaptic terminals labeled by the tracer within the PN, the NRTP, and the VL/VM revealed no GABA immunoreactivity. Only one clearly labeled terminal was found in the RNp. In contrast, 72% of the terminals in the IO were clearly GABA immunoreactive, confirming the reliability of our staining protocol. Correspondingly, glutamate immunohistochemistry labeled the majority of the cerebellonuclear terminals in the PN (88%), the NRTP (90%), the RNp (93%), and the VM/VL (63%) but labeled only 5% in the IO. These data do not support a role for GABAergic inhibition either in the feedback systems from the LN to the PN and the NRTP or within the projections to the RNp and the VM/VL.

Comparison of projection neurons in the pontine nuclei and the nucleus reticularis tegmenti pontis of the rat

Dendritic features of identified projection neurons in two precerebellar nuclei, the pontine nuclei (PN) and the nucleus reticularis tegmenti pontis (NRTP) were established by using a combination of retrograde tracing (injection of fluorogold or rhodamine labelled latex micro-spheres into the cerebellum) with subsequent intracellular filling (lucifer yellow) in fixed slices of pontine brainstem. A multivariate analysis revealed that parameters selected to characterize the dendritic tree such as size of dendritic field, number of branching points, and length of terminal dendrites did not deviate significantly between different regions of the PN and the NRTP. On the other hand, projection neurons in ventral regions of the PN were characterized by an irregular coverage of their distal dendrites by appendages while those in the dorsal PN and the NRTP were virtually devoid of them. The NRTP, dorsal, and medial PN tended to display larger somata and more primary dendrites than ventral regions of the PN. These differences, however, do not allow the differentiation of projection neurons within the PN from those in the NRTP. They rather reflect a dorso-ventral gradient ignoring the border between the nuclei. Accordingly, a cluster analysis did not differentiate distinct types of projection neurons within the total sample. In both nuclei, multiple linear regression analysis revealed that the size of dendritic fields was strongly correlated with the length of terminal dendrites while it did not depend on other parameters of the dendritic field. Thus, larger dendritic fields seem not to be accompanied by a higher complexity but rather may be used to extend the reach of a projection neuron within the arrangement of afferent terminals. We suggest that these similarities within dendritic properties in PN and NRTP projection neurons reflect similar processing of afferent information in both precerebellar nuclei.

Projection from the accommodation-related area in the superior colliculus of the cat

Our previous study has indicated that accommodative responses can be evoked with weak currents applied to a circumscribed area of the superior colliculus in the cat. We investigated efferent projections from this area with biocytin in the present study. The accommodation area in the superior colliculus was identified by systematic microstimulation in each of five anesthetized cats. Accommodative responses were detected by an infrared optometer. After mapping the superior colliculus, biocytin was injected through a glass micropipette into the accommodation area, where accommodative responses were elicited with low-intensity microstimulation. In addition, accommodative responses to stimulation of the superior colliculus were compared before and after an injection of muscimol, an agonist of inhibitory neurotransmitter, into the pretectum. Following the injection of biocytin, in the ascending projections, labeled terminals were seen mainly in the caudal portion of the nucleus of the optic tract, the nucleus of the posterior commissure, the posterior pretectal nucleus, the olivary pretectal nucleus, the mesencephalic reticular formation at the level of the oculomotor nucleus, and the lateral posterior nucleus of the thalamus on the ipsilateral side. Less dense terminals were seen in the anterior pretectal nucleus, the zona incerta, and the centromedian nucleus of the thalamus. In the descending projections, labeled terminals were observed mainly in the paramedian pontine reticular formation, the nucleus raphe interpositus, and the dorsomedial portion of the nucleus reticularis tegmenti pontis on the contralateral side. Less dense terminals were also seen in the nucleus of the brachium of the inferior colliculus, the cuneiform nucleus, the medial part of the paralemniscal tegmental field, and the dorsolateral division of the pontine nuclei on the ipsilateral side. Following the injection of muscimol into the pretectum, including the nucleus of the optic tract, the posterior pretectal nucleus, and the nucleus of the posterior commissure, accommodative responses evoked by microstimulation of the superior colliculus were reduced to 33-55% of the value before the injections. These findings suggest that the accommodation area in the superior colliculus projects to the oculomotor nucleus through the ipsilateral pretectal area, especially the nucleus of the optic tract, the nucleus of posterior commissure, and the posterior pretectal nucleus, and also projects to the pupilloconstriction area (the olivary pretectal nucleus), the vergence-related area (the mesencephalic reticular formation), and the active visual fixation-related area (the nucleus raphe interpositus).

Mossy fiber projections from the brain stem to the nodulus in the cat. An experimental study comparing the nodulus, the uvula and the flocculus

Mossy fiber projections from the brainstem to the nodulus were studied by means of the retrograde transport of horseradish peroxidase (HRP) in the cat. The findings indicate exclusive secondary vestibular projections to the nodulus (96.5% of the total number of labeled cells in cat 1). Major sources of such projections include the caudal half of the medial and inferior vestibular nuclei, and the dorsal half of the superior vestibular nucleus. Groups-x and -z of the vestibular nuclei give rise to minor projections. The projections from groups-f and -y, and the interstitial nucleus of the eighth nerve are very few, if any. Minor extravestibular projections originate from the prepositus hypoglossal nucleus and the infratrigeminal nucleus. All these projections are bilateral. No other nuclei in the brainstem were labeled following HRP injection in the nodulus. No indications of mediolateral and dorsoventral differences in mossy fiber projections were found. There are quantitative differences in the sources of projections to the nodulus, the ventral uvula and the flocculus, all of which belong to the vestibulocerebellum. The largest sources for projections to the nodulus and the ventral uvula are from the vestibular nuclei. Among the vestibular nuclei, group-x provides the major projections to the ventral uvula. For the flocculus, in contrast, the major sources of input are the reticular formation and raphe nuclei. These quantitative differences may play an important role for differential functions of the nodulus, the ventral uvula and the flocculus.

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.

Brainstem mossy fiber projections to lobules VIa, VIb,c, VII and VIII of the cerebellar vermis in the rat

The brainstem mossy-fiber projections to lobules VIa, VIb,c, VII and VIII of the cerebellar vermis were studied by retrograde transport of horseradish peroxidase in the rat. The distribution of labeled cells indicated that these lobules received major projections from topographically different locations of the basilar pontine nuclei and the nucleus reticularis tegmenti pontis. Lobules VIa and VIII received an additional strong projection from the lateral reticular nucleus. Moderate projections were found to reach lobule VIa from the raphe pontis and external cuneate nucleus; lobules VIb,c from the raphe pontis, lateral reticular nucleus, and a group of cells in the lateral tegmentum; lobule VII from the spinal vestibular nucleus and a lateral tegmental cell group; and lobule VIII from the medial and spinal vestibular nuclei, nucleus intercalatus and Roller of the perihypoglossal nuclei, and the main cuneate nucleus. The quantitative and topographical differences in the origin of mossy fibers suggest that these lobules may subserve slightly different functions.

Organization of cingulo-ponto-cerebellar connections in the cat

This study deals with three different aspects of the organization of connections from the cingulate gyrus to the cerebellum. (1) With the use of wheat germ agglutinin-horseradish peroxidase as a retrograde tracer, the distribution of cingulate neurons projecting to the pontine nuclei was studied. Retrogradely labeled cells were found in layer 5 in all parts of the cingulate gyrus. Average densities of cingulo-pontine cells were similar in the different cytoarchitectonic subdivisions, although some density gradients were observed. The projection was found to be remarkably strong. Average densities of corticopontine cells in the cingulate gyrus ranged from 500-700 cells per mm2 cortical surface, and the total number of neurons was in the range of 75,000-105,000 (n = 4). (2) A topographical organization of terminal fields of fibers originating in different parts of the cingulate gyrus was demonstrated with the combined use of anterograde degeneration and anterograde transport of wheat germ agglutinin-horseradish peroxidase. Terminal fibers originating in different zones of the cingulate gyrus were distributed in a patchy mosaic within a narrow band along the ventromedial aspect of the pontine nuclei. (3) We confirm, with the combined use of lesions in the cingulate gyrus and injections of wheat germ agglutinin-horseradish peroxidase in the ventral paraflocculus, that there is considerable overlap between terminal fibers originating in the cingulate gyrus, and cells retrogradely labeled from the ventral paraflocculus. The role of the ventral paraflocculus as a receiver of "limbic" input is discussed.

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

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

Afferents to the cerebellar flocculus in cat with special reference to pathways conveying vestibular, visual (optokinetic) and oculomotor signals

Horseradish peroxidase (HRP) was injected into the cerebellar flocculus of 20 cats to determine: (a) the proportions of afferents from the various brain stem nuclei; (b) possible projections from the basilar pontine nuclei; and (c) sources of saccadic eye movement signals recorded from flocculus Purkinje cells. Results confirm earlier findings that the flocculus receives large numbers of mossy fibre afferents from the vestibular and perihypoglossal nuclei, bilaterally, and climbing fibres from the contralateral inferior olive (dorsal cap, ventrolateral outgrowth, medial accessory olive, ventral bend of principal olive). In addition, large numbers of HRP-labeled neurons have been identified within: (i) the basilar pontine nuclei, bilaterally, where they are distributed in columns in the dorsolateral, lateral, ventral medial and dorsomedial nuclei; (ii) the nucleus reticularis tegmenti pontis; (iii) several of the cranial motor nuclei, VI, VII, X (retrofacial n.), XI (n. ambiguus), and XII; (iv) the raphe magnus, pontis and obscurus; (v) the lateral reticular nucleus, pars subtrigeminalis. Finally, new information is presented which shows that large numbers of flocculus projecting neurons are located within the medial longitudinal fasciculus at two locations; one just rostral to the hypoglossal nucleus and another group extends 2-3 mm rostral to the abducens nucleus. These groups are bilateral, and have been termed, respectively, the caudal and intermediate interstitial nucleus of the medial longitudinal fasciculus. Both groups correspond in location to physiologically identified neurons in cat which fire in relation to saccadic eye movements. Their projection to the flocculus, in part, explains the saccadic discharge of Purkinje cells in the flocculus.

Subcortical projections to the pontine nuclei in the cat

In a systematic attempt to trace all projections from the brainstem and diencephalon to the pontine nuclei of the cat, implantations and injections of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) or Fluoro-Gold were placed in the pontine nuclei of 21 cats. In most of the cases there was no evidence of spread of tracer outside the pontine nuclei. Retrogradely labeled cells in the brainstem and diencephalon were carefully mapped and counted. The number labeled cells in the brainstem and diencephalon ranged from 24 in cases with very small implantations to 3,490 in cases with large injections in the pontine nuclei (counts from every fifth section). The labeled cells are located bilaterally with an ipsilateral preponderance. After large injections, 25-38% of the labeled cells were located in the brainstem reticular formation, 10-16% in the pretectal nuclei, 10-15% in the hypothalamus, 7-9% in the zona incerta, 3-9% in the fields of Forel, 4-5% in the nucleus locus coeruleus, 3-5% in the ventral lateral geniculate body, 2-4% in the superior colliculus, 3% in the periaqueductal gray, and 14-15% in other parts of the brainstem. Judging from cases with small tracer deposits entirely confined to the pontine nuclei, there appear to be two types of subcortical inputs: Projections from the reticular formation, the nucleus locus coeruleus, the periaqueductal gray, and the raphe nuclei are widespread, presumably reaching all parts of the pontine nuclei, while projections from a diversity of other sources are localized, reaching limited parts of the pontine nuclei only or predominantly.

The cerebellar nuclear afferent and efferent connections with the lateral reticular nucleus in the cat as studied with retrograde transport of WGA-HRP

The cerebellar nuclear projection from the lateral reticular nucleus (NRL) was studied in 29 cats by means of retrograde axonal transport after implantation of the crystalline wheat germ agglutinin-horseradish peroxidase (WGA-HRP) complex in the cerebellar nuclei. It was confirmed that all the cerebellar nuclei receive afferent fibres from the NRL with the strongest termination in the ipsilateral interposed nuclei. In addition, these experiments give evidence of a previously unrecognized topical pattern in the projection to the interposed nuclei, arranged according to the same principle as in the projection to the immediately overlying cerebellar cortex. Thus, the anterior interposed nucleus receives fibres from all parts of the main NRL, its rostral part especially from laterally situated neurons, while subsequent more caudal parts from more medially situated neurons, while the posterior interposed nucleus receives fibres mainly from the dorsomedial part of the main NRL. The cerebellar nuclear projection to the NRL was investigated in 15 cats using retrograde transport after ventral microiontophoretical ejections of the WGA-HRP complex in the main NRL. The contralateral rostral fastigial nucleus was confirmed as the main origin of this projection, but projecting neurons were, in addition, discovered rostrally in the anterior interposed and dentate nuclei on the same side. No topical differences could be observed following ejections in different parts of the NRL; the majority of the projecting neurons were always concentrated along the ventral and lateral borders of the fastigial nucleus and in the adjacent medial part of the anterior interposed nucleus.

Behavior of floccular Purkinje cells correlated with adaptation of horizontal optokinetic eye movement response in pigmented rabbits

Single unit spike activities of Purkinje cells in the cerebellar flocculus were examined during sustained horizontal sinusoidal oscillation (0.33 Hz, 2.5 degrees peak-to-peak) of a striped screen around an alert pigmented rabbit. The floccular area specifically related to horizontal reflex eye movement (H-zone) was identified by means of local stimulation that induced abduction of the ipsilateral eye. In control states, simple spike discharge of most of the H-zone Purkinje cells was enhanced by backward screen movement and depressed by forward screen movement, while complex spike discharge was modulated reciprocally. After one-hour sustained oscillation of the screen, the gain of horizontal optokinetic eye movement response (HOKR) increased by 0.16 on average. Correspondingly, simple spike modulation in most of H-zone Purkinje cells tested significantly increased in amplitude, while complex spike modulation tended to decrease. No such systematic changes were observed in other Purkinje cells. These results are consistent with the hypothesis that the floccular H-zone Purkinje cells adaptively control the optokinetic eye movement through modification of the visual mossy fiber responsiveness under the influence of the retinal error signals conveyed by the visual climbing pathway.

The feline oculomotor nucleus: morphological subdivisions and projection to the cerebellar cortex and nuclei

The cytoarchitecture of the feline oculomotor nucleus was examined in sections stained with thionin and neutral red. Five different subdivisions (caudal central, paramedian, ventral, dorsomedial and dorsolateral divisions) can be identified on each side of the midline. This observation is discussed, and our findings are compared to previous studies of the cytoarchitecture or central muscular representation of the oculomotor nucleus in which different subgroups have been distinguished. Implants or injections of the wheat germ agglutinin-horseradish peroxidase complex have revealed that all five subdivisions project to different parts of the cerebellar cortex and nuclei. Retrogradely labelled cells were found in the oculomotor nucleus in 18 cases following deposition of tracer in the fastigial and interposed nuclei and certain regions of the anterior, posterior and flocculonodular lobes. The projection is bilateral and appears to have its main termination in flocculus. It originates from small neurons, especially from those located along the dorsal border of the oculomotor nucleus.

Influence of long-term optokinetic stimulation on eye movements of the rabbit

Two kinds of optokinetic afternystagmus (OKAN) have been studied in rabbits; positive and negative OKAN. Positive OKAN is the persistence of eye movements evoked by optokinetic stimulation following the termination of the stimulus, with the slow phase of the eye movements in the same direction as the inducing stimulus. Negative OKAN is evoked by long duration optokinetic stimulation, and has a slow phase of opposite direction to the inducing stimulus. The stimulus conditions which are optimal for inducing and maintaining negative OKAN were characterized. Rabbits were placed in an optokinetic drum for periods of 12-96 h (with appropriate intervening periods for food and water). Eye movements were recorded during and after the termination of optokinetic stimulation. The optimum optokinetic stimulus velocity for the induction of negative OKAN was 5 degrees/s. The minimum duration of stimulation for the induction of negative OKAN of maximum velocity was 48 h. Once induced, the slow phase of negative OKAN attained velocities of 50-100 degrees/s. Three conditions of restraint of the rabbits were studied after negative OKAN was induced during the intervening periods when eye movements were not being recorded. These conditions were: (1) unrestrained (full freedom of movement) without visual stimulation (in a dark enclosure); (2) restrained (horizontal head and body movement prevented) without visual stimulation; and (3) restrained with visual stimulation (in the stationary optokinetic drum). Conditions 1 and 2 caused negative OKAN to dissipate within 24 h. Condition 3 caused negative OKAN to be maintained for more than 70 h. The velocity imbalance of the horizontal vestibuloocular reflex (HVOR) was measured at different times following the induction of negative OKAN. It provided a more sensitive index of the central imbalance which caused negative OKAN, than did spontaneous nystagmus. One of the consequences of optokinetic stimulation measured over a 16 h period was a decrease in the gain of the optokinetic reflex. This reduction in gain could represent a central adaptation to maintained stimulation which in the absence of continued optokinetic stimulation is expressed as a nystagmus.

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

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

Excitatory inputs to cerebellar dentate nucleus neurons from the cerebral cortex in the cat

1. In anesthetized cats, we investigated excitatory and inhibitory inputs from the cerebral cortex to dentate nucleus neurons (DNNs) and determined the pathways responsible for mediating these inputs to DNNs. 2. Intracellular recordings were made from 201 DNNs whose locations were histologically determined. These neurons were identified as efferent DNNs by their antidromic responses to stimulation of the contralateral red nucleus (RN). Stimulation of the contralateral pericruciate cortex produced excitatory postsynaptic potentials (EPSPs) followed by long-lasting inhibitory postsynaptic potentials (IPSPs) in DNNs. The most effective stimulating sites for inducing these responses were observed in the medial portion (area 6) and its adjacent middle portion (area 4) of the precruciate gyrus. Convergence of cerebral inputs from area 4 and area 6 to single DNNs was rare. 3. To determine the precerebellar nuclei responsible for mediation of the cerebral inputs to the dentate nucleus (DN), we examined the effects of stimulation of the pontine nucleus (PN), the nucleus reticularis tegmenti pontis (NRTP) and the inferior olive (IO). Systematic mapping was made in the NRTP and the PN to find effective low-threshold stimulating sites for evoking monosynaptic EPSPs in DNNs. Stimulation of either the PN or the NRTP produced monosynaptic EPSPs and polysynaptic IPSPs in DNNs. Using a conditioning-testing paradigm (a conditioning stimulus to the cerebral peduncle (CP) and a test stimulus to the PN or the NRTP) and intracellular recordings from DNNs, we tested cerebral effects on neurons in the PN and the NRTP making a monosynaptic connection with DNNs. Conditioning stimulation of the CP facilitated PN- and NRTP-induced monosynaptic EPSPs in DNNs. This spatial facilitation indicated that the excitatory inputs from the cerebral cortex to DNNs are at least partly relayed via the PN and the NRTP. 4. Stimulation of the contralateral IO produced monosynaptic EPSPs and polysynaptic IPSPs in DNNs. These monosynaptic EPSPs were facilitated by conditioning stimulation of the CP, strongly suggesting that the IO is partly responsible for mediating excitatory inputs from the cerebral cortex to the DN. A comparison was made between the latencies of IO-evoked IPSPs in DNNs and the latencies of IO-evoked complex spikes in Purkinje cells. Such a comparison indicated that the shortest-latency IPSPs evoked from the IO were not mediated via the Purkinje cells and suggested the pathway mediated by inhibitory interneurons in the DN.(ABSTRACT TRUNCATED AT 400 WORDS)

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

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

Autoradiography of mossy fiber terminals in the fastigial nucleus of the cat

Terminal boutons of mossy fiber collaterals in the fastigial nucleus originating from the nucleus reticularis tegmenti pontis, the bulbar reticular formation, and the medial vestibular nucleus were studied with high-resolution autoradiography in order to examine their ultrastructural features and synaptic relations. Labeled mossy fiber boutons ranged in size from 0.5 to 5 micron in diameter, and they all contained clear and spherical vesicles in an electron-lucent matrix, mitochondria, and some fine tubular elements. These boutons form asymmetric synapses with dendritic profiles of different sizes. No evidence was found for mossy fiber termination on the soma of fastigial neurons. Two types of mossy fiber terminals were distinguished on the basis of the aggregation of synaptic vesicles: one type with clustered vesicles and one type with densely packed vesicles, occurring in equal number from all sources. Furthermore, the applicability of the congruity hypothesis is confirmed for the general identification of terminals.

Development of the precerebellar nuclei in the rat: IV. The anterior precerebellar extramural migratory stream and the nucleus reticularis tegmenti pontis and the basal pontine gray

Sequential thymidine radiograms from rats injected on days E16, E17, E18, and E19 and killed 2 hours after injection and at daily intervals up to day E22 were used to establish the site of origin, migratory route, and settling patterns of neurons of the nucleus reticularis tegmenti pontis and basal pontine gray. The nucleus reticularis tegmenti pontis neurons, which are produced predominantly on days E15 and E16, derive from the primary precerebellar neuroepithelium. These cells, unlike those of the lateral reticular and external cuneate nuclei, take an anteroventral subpial route, forming the anterior precerebellar extramural migratory stream. This migratory stream reaches the anterior pole of the pons by day E18. In rats injected on day E16 and killed on day E18 some of the cells that reach the pons are unlabeled, indicating that they represent the early component of neurons generated on day E15. The cells labeled on day E16 begin to settle in the pons on day E19, 3 days after their production. These cells, migrating in an orderly temporal sequence, form a posterodorsal-to-anteroventral gradient in the nucleus reticularis tegmenti pontis. Unlike the neurons of all the other precerebellar nuclei, the basal pontine gray neurons derive from the secondary precerebellar neuroepithelium. The secondary precerebellar neuroepithelium forms on day E16 as an outgrowth of the primary precerebellar neuroepithelium, and it remains mitotically active through day E19, spanning the entire period of basal pontine gray neurogenesis. The secondary precerebellar neuroepithelium is surrounded by a horizontal layer of postmitotic cells, representing the head-waters of the anterior precerebellar extramural migratory stream. In rats injected on day E18 and killed on day E19 the cells are labeled in the proximal half of the stream around the medulla but those closer to the pons are unlabeled, indicating an orderly sequence of migration. In rats injected on day E18 and killed on day E20 the labeled cells reach the pole of the pons. In the basal pontine gray the sequentially generated neurons settle in a precise order. The neurons generated on day E16 form a small core posteriorly and the neurons generated on days E17, E18, and E19 form regular concentric rings around the core in an inside-out sequence.

Cerebellar nuclear afferents--where do they originate? A re-evaluation of the projections from some lower brain stem nuclei

Cerebellar nuclear afferents from some caudal brain stem nuclei in the cat were studied by means of retrograde transport after implantation of the wheat germ agglutinin-horseradish peroxidase complex in crystalline form in the cerebellar nuclei. The findings give evidence that projections to the cerebellar nuclei from certain nuclei of the reticular formation proper (e.g., from the gigantocellular reticular nucleus) are very modest, while there appears to be no or extremely few cerebellar nuclear afferents from the paramedian reticular, spinal trigeminal, gracile, cuneate and external cuneate nuclei. Previous tracer studies have given evidence that also the pontine and red nuclei send very few, if any, fibres to the cerebellar nuclei. All these brain stem regions are known to project to the cerebellar cortex. This relative lack of mossy fibre collaterals to the cerebellar nuclei is discussed with references to previous literature on the distribution of cerebellar nuclear afferents, and the problem of how the cerebellar nuclei are facilitated is considered.

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.

Lesions in the cat prepositus complex: effects on the optokinetic system

The effects of bilateral lesions within and around the prepositus hypoglossi (p.h.) nuclei on the optokinetic system were studied. The pure optokinetic nystagmus (o.k.n.) was evoked by a step of velocity (60 deg/s, 30 s duration) of the surrounding. The visual-vestibular interaction was investigated by measuring the gain and phase of the vestibulo-ocular reflex (v.o.r.) as a function of frequency before and after lesion under three different conditions of testing: basic v.o.r. tested in the dark, v.o.r. tested in the light and v.o.r. suppressed by vision. The tested amplitude was +/- 20 deg. A posterior vermectomy was performed for controls in two cats. A bilateral electrolytic p.h. lesion including the rostral pole of this nucleus was added to the posterior vermectomy in three cats. A lesion similar but sparing the rostral pole of the nucleus was carried out in three other cats. In one cat a bilateral electrolytic lesion of the medial vestibular nuclei (m.v.n.) was combined with a posterior vermectomy. In two cats the medulla was cut on the mid line after a posterior vermectomy. The posterior vermectomy affected neither the optokinetic response nor the visual-vestibular interactions. In cats where p.h. lesion included its rostral pole and in the cat with m.v.n. lesion, all the tested optokinetic effects (step o.k.n., and visual-vestibular interactions) were abolished. In the three cats where p.h. lesion spared its rostral pole, the optokinetic effects were quite normal in one cat, mildly reduced in the second one, and seriously affected but not completely abolished in the third one. The surgical cut of the medulla on the mid line did not dramatically disturb the various optokinetic effects. The most marked deficit was the loss of the optokinetic after-nystagmus (o.k.a.n.). From the comparison of these results with the neuroanatomical data and with the Robinson's model concerning the optokinetic processing, it was suggested that: (a) the rostral p.h. could be the location of the o.k.n. integrator or could be an essential link on the o.k.n. pathway, (b) the posterior four-fifths of the p.h. could not be an essential relay on the o.k.n. pathway, (c) the loss of o.k.a.n. after mid-line lesion could be due to the interruption of the positive feed-back loop formed by the reciprocal inhibitory connexions between the two m.v.n.

Collaterals of corticospinal and pyramidal fibres to the pontine grey demonstrated by a new application of the fluorescent fibre labelling technique

Selective visualization of collaterals of corticospinal and pyramidal fibres to the pons in cat was obtained by retrograde transport of the fluorescent tracer fast blue (FB) through the stem fibres. Unilateral FB injections in the cervical cord and the pyramidal tract respectively produced soft blue fluorescent labelling of pyramidal fibres and of fibres and structures resembling 'terminals' in the pontine grey: contralateral to the spinal injections and ipsilateral to the pyramidal injections. These labelled elements were concluded to represent collaterals of corticospinal and pyramidal fibres because (a) their distribution corresponded to that of the pericruciate corticopontine fibres, (b) their labelling was prevented when the FB injections were preceded by a transection of either the cerebral peduncle or the pyramidal tract which lesions also prevented the FB labelling of the distal parts of the transected axons. Similar findings were obtained when using wheat germ agglutinin-horseradish peroxidase. In other experiments FB-labelling of pyramidal collaterals was combined with retrograde labelling of pontine neurones projecting to the contralateral anterior lobe of the cerebellum using diamidino yellow dihydrochloride as the second tracer. The distributions of the retrogradely labelled neurones and of the pyramidal collaterals in the pontine grey showed an almost complete overlap indicating that these collaterals mainly establish connections with the cerebellar anterior lobe.

Anatomical studies on the nucleus reticularis tegmenti pontis in the pigmented rat. I. Cytoarchitecture, topography, and cerebral cortical afferents

The nucleus reticularis tegmenti pontis (NRTP) is a precerebellar reticular nucleus that has been found to be related to cerebropontocerebellar pathways and, more recently, to eye movements. The present study investigates the cytoarchitecture, the topography, and the cerebral cortical projections to the NRTP in the pigmented rat. The cytoarchitecture and topography of the NRTP was determined by examination of Nissl-stained material sectioned in the transverse and sagittal planes. Two cytoarchitectonically distinct portions of the NRTP are apparent; a central subdivision (NRTPc) composed of large multipolar, small spherical, and fusiform neurons, and a pericentral subdivision (NRTPp) composed of loosely packed small fusiform and spherical neurons. The NRTPc is located dorsal to the medial lemniscus and pyramidal tracts over the caudal two-thirds of the pons. It extends caudodorsally to the region just rostral and ventral to the abducens nucleus. The NRTPp is adjacent to the lateral margins of the NRTPc, rostrally, and lies ventral to the caudal portions of the NRTPc. Large injections of horseradish peroxidase (HRP) were made into the cerebellum in order to determine the degree to which each subdivision of the NRTP contributes to the cerebellar projection. A high percentage of NRTPc neurons and a lower percentage of NRTPp neurons were labeled. These differences in labeling density and neuronal morphology noted above confirm the appropriateness of subdividing the NRTP into central and pericentral subdivisions. The cerebral cortical afferents to the NRTP were examined by placing small iontophoretic injections of HRP into the NRTPc and NRTPp. A systematic examination of all cortical areas revealed that the HRP-labeled neurons are entirely localized within pyramidal layer V of three major cortical areas: the ipsilateral prefrontal cortex (Brodmann areas 8, 8a, 11, and 32); the ipsilateral motor and somatosensory cortices (Brodmann areas 2, 4, 6, and 10), and the bilateral cingular cortex (Brodmann areas 24a, 24b, 29c, and 29d). By far, the heaviest cortical labeling with HRP injections into the medial NRTPc is within the cingular cortex that may, in the rat, be homologous to the frontal eye field of the cat and monkey. In contrast, injections involving the lateral NRTPc or the NRTPp produced labeling within wide regions of the cortex with the greatest number in the somatomotor cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

The cerebellar nucleo-olivary and olivo-cerebellar nuclear projections in the cat as studied with anterograde and retrograde transport in the same animal after implantation of crystalline WGA-HRP. I. The dentate nucleus

The implantation technique described by Mori et al. has been modified for the implantation of crystalline wheat germ agglutinin-horseradish peroxidase (WGA-HRP) complex. This method permits a detailed analysis of the afferent and efferent connections of the cerebellar nuclei without the complication of uptake and transport of the tracer into passing fibres. We have used this method for studies of the olivo-dentate and dentato-olivary projections in the cat. After implantation of WGA-HRP into the dentate nucleus in all our cases, both anterogradely labelled terminal dentato-olivary fibres and retrogradely labelled olivo-dentate neurons were found in the contralateral inferior olive. It appears from our findings that both projections are topically organized. The dorsal part of dentate nucleus is bidirectionally connected with the rostral part of the principal olive, the ventrolateral part of the dentate is connected with the intermediate portion of the principal olive, while its ventromedial part is connected with the caudal portion of the principal olive. The olivo-dentate and dentato-olivary connections appear to be largely reciprocally organized. The advantages and drawbacks encountered with implantation of crystalline WGA-HRP are discussed, and our observations are considered in relation to previous studies on the olivo-cerebellar and cerebello-olivary connections.

Organization of afferent connections to the lateral and interpositus cerebellar nuclei from the brainstem relay nuclei: a horseradish peroxidase study in the cat

Afferent projections to the lateral (dentate) and interpositus cerebellar nuclei from the brainstem relay nuclei were studied in cats using the horseradish peroxidase (HRP) method. In the first series of experiments, HRP was injected into the brachium pontis. Mossy fiber terminals were anterogradely labeled, predominantly in the lateral (hemispherical) part, moderately in the intermediate part, and slightly in the vermal part of the cerebellum. Besides these terminals in the cerebellar cortex, axon terminals labeled anterogradely were also found in the cerebellar nuclei. The labeled terminals appeared almost exclusively in the lateral nucleus and rarely in the interpositus nucleus. Cells labeled retrogradely were found both in the pontine nuclei and the tegmental reticular nucleus, but not in other brainstem nuclei. In the second series of experiments, HRP was injected into the lateral and interpositus nuclei, and retrograde labeling was examined in the brainstem relay nuclei. After HRP injection into the lateral nucleus, the number of labeled cells was significantly large in the pontine nuclei, but fairly small in the reticular or vestibular nuclei. The number of labeled cells was generally large in the inferior olive, mainly in the principal olive. After HRP injection into the interpositus nucleus, the number of labeled cells was moderate in the reticular or vestibular nuclei, but small in the pontine nuclei. The number of labeled cells in the inferior olive was also large, being distributed mainly in the accessory olives. These results indicate that the pontine nuclei and the principal olive provide major afferent inputs to the lateral nucleus, whereas the reticular nuclei, the vestibular nuclei and the accessory olives are the major afferent sources to the interpositus nucleus.

Afferents to the flocculus of the cerebellum in the rhesus macaque as revealed by retrograde transport of horseradish peroxidase

To investigate the afferent projections to the flocculus in a nonhuman primate, we injected horseradish peroxidase into one flocculus of six rhesus macaques (Macaca mulatta) and processed their brains according to the tetramethylbenzidine protocol to reveal retrogradely labeled neurons. Labeled neurons were found in a large set of nuclei within the rostral medulla and the pons. The greatest numbers of labeled neurons were in the vestibular complex and the nucleus prepositus hypoglossi. There were neurons labeled bilaterally throughout all the vestibular nuclei except the lateral vestibular nucleus, but most of the labeled neurons were in the caudal parts of the medial and inferior vestibular nuclei and in the central part of the superior vestibular nucleus; the nucleus prepositus was also labeled bilaterally, primarily caudally. Modest numbers of labeled neurons were found in the y-group, most ipsilaterally, and many neurons were labeled in the interstitial nucleus of the vestibular nerve. No labeled neurons were found in the vestibular ganglion following a large injection into the flocculus. A second large source of afferents to the flocculus was the medial, paramedial, and raphe reticular formation. Dense aggregates of labeled neurons were located in several pararaphe nuclei of the rostral medulla and the rostral pons and in the nucleus reticularis paramedianus of the medulla and several component nuclei of the nucleus reticularis tegmenti pontis bilaterally. Several groups of cells within and abutting upon the medial and rostral aspects of the abducens nucleus were labeled bilaterally. There was a modest projection from two parts of the pontine nuclei. Both a dorsal midline nucleus ventral to the nucleus reticularis tegmenti pontis and a collection of nuclei in a laminar region adjacent to the contralateral middle cerebellar peduncle contained labeled neurons whose numbers, while modest, were large compared to the projections to the flocculus in other animals. This generic difference may be due to the greater development of the smooth pursuit system in monkeys and the consequent need for a more substantial input from the cerebral cortex. As in other genera, the inferior olive projected to the flocculus via the dorsal cap of Kooy and the contiguous ventrolateral outgrowth. The projection was completely crossed and large injections labeled virtually every neuron in the dorsal cap, suggesting that the dorsal cap is the principal source of climbing fiber afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebellar targets of visual pontine cells in the cat

The cerebellum receives visual mossy fiber input from the cerebral cortex via visual cells in the pons. We identified the regions of cat cerebellum that receive cerebral visual input by injecting orthograde tracers among physiologically identified visual pontine cells. Cerebellar labeling following these injections indicates that the contralateral paraflocculus and the rostral folium of the uvula (vermal lobule IX) receive the heaviest projection from cortically activated pontine visual cells. Lighter visual input reaches much of the rest of the contralateral posterior lobe. A second experiment combined, in the same animal, orthograde tracing of the visual corticopontine pathway with retrograde tracing of the pontocerebellar projection. The results of this experiment confirm that the paraflocculus and uvula receive more cortical visual input than do other regions of the cerebellum. This experiment also shows that uvula-projecting and paraflocculus-projecting cells occupy different parts of the ventromedial pons. Uvula-projecting cells cluster immediately adjacent to the ventral and medial borders of the pyramidal tract and near the midline. Paraflocculus-projecting cells lie ventral and medial to the pyramidal tract but displaced from its border. There are few paraflocculus-projecting cells near the midline.

The mossy fiber projection of the nucleus reticularis tegmenti pontis to the flocculus and adjacent ventral paraflocculus in the cat

The pontine projection of the flocculus and adjacent ventral paraflocculus was investigated with antegrade and retrograde axonal tracer techniques. Injections of horseradish peroxidase into the floccular complex revealed subsets of labeled neurons in the nucleus reticularis tegmenti pontis, the nucleus raphe pontis and the medial lemniscus. Following injections of tritiated leucine in these subsets, the topographical distribution of labeled mossy fibers in the floccular complex was studied. Cells clustered in the central part of the nucleus reticularis tegmenti pontis project to the rostral flocculus and the rostral part of the caudal flocculus. The terminal field of cells in the nucleus raphe pontis and of cells associated with the lateral aspect of the medial lemniscus covered the same area. The number of mossy fiber terminals arising from these cells is small and concentrated in a medial position. The medial extension of the ventral paraflocculus and its most caudal sublobule do receive a very dense mossy fiber projection from cells associated with the medial edge of the medial lemniscus next to the rostral nucleus reticularis tegmenti pontis and beyond. Concomitantly, a collateral projection terminates in a restricted part of the uvula. Labeled mossy fiber terminals were never observed in the nodulus. The nucleus reticularis tegmenti pontis does not project to any part of the lower brain stem. The connections described in this paper are discussed in relation to the possible role of the nucleus reticularis tegmenti pontis as a relay nucleus in brain stem pathways transmitting visual information. It is concluded that in the cat this nucleus is an exclusively pre-cerebellar relay, not involved as a final link in the non-cerebellar pathway transmitting visual information to the vestibular nuclei.

Effects of vestibulocerebellar lesions upon dynamic characteristics and adaptation of vestibulo-ocular and optokinetic responses in pigmented rabbits

The vestibulo-ocular reflex (VOR) and optokinetic response (OKR) in the horizontal plane were examined in pigmented rabbits, using sinusoidal whole-body rotation and sinusoidal rotation of a striped screen. Sustained rotation of the animal (5 degrees peak-to-peak, 0.1 Hz) for 4 h, under different optokinetic stimulus conditions, induced the following adaptive changes in the VOR: (1) outphase rotation of the screen (5 degrees) increased the VOR gain by 0.3 (on average); (2) with the screen fixed in space, VOR gain increased by 0.2. (3) in-phase rotation of the screen (5 degrees) decreased the VOR gain by 0.16. However, (4) in-phase rotation of the screen at twice (10 degrees) the amplitude of whole-body rotation did not affect the gain. Instead, it induced a significant phase lead (23 degrees) in the VOR, which did not occur in other stimulus conditions. Adaptive increases of the OKR gain occurred under sustained rotation of the screen alone (2.5 degrees, 0.33 Hz). After bilateral destruction of floccular Purkinje cells with kainic acid the VOR gain and phase were affected only very slightly, but adaptive changes in the VOR were abolished. By contrast, the OKR gain was reduced and the OKR phase delayed. OKR adaptation was also affected in such a way that a gain increase initially produced could not be maintained during sustained screen rotation. Ablation of nodulus-uvala caused a gain increase and phase lead in both VOR and OKR, and its only effect on adaptability of the VOR or the OKR was observed for the VOR under stimulus condition (4).

Afferent projections from the brainstem to the three floccular zones in cats. II. Mossy fiber projections

Mossy fiber projections from the brainstem to the flocculus were studied following injections of horseradish peroxidase (HRP) into the flocculus and following microinjections of HRP into each of the three zones of the flocculus. It has been found that the flocculus receives mossy fiber projections from 4 main sources. (1) Perihypoglossal nucleus--dense projections originate from discrete areas of the rostral pole of the intercalated nucleus, the ventral part of the prepositus hypoglossal nucleus and the adjacent reticular formation. (2) Vestibular nuclear complex--secondary vestibular fibers come from discrete areas in the vestibular nuclei: the ventromedial and dorsomedial parts of the medial and inferior nucleus, the central area of the superior nucleus, the ventromedial part of the lateral nucleus, the group y and the interstitial nucleus of the vestibular nerve. (3) Medullary reticular formation--the strongest projection of mossy fibers arises from the accessory group of the paramedian reticular nucleus. (4) Pontine reticular formation and raphe nucleus--dense projections originate from a narrow zone which involves the caudal part of the dorsal nucleus of the raphe, the inferior and superior central nucleus of the raphe and the medial part of the nucleus reticularis tegmenti pontis. No clear indication of a different mossy fiber projection from the nuclei in the brainstem to the 3 zones of the flocculus was found.

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.

Axonal branching in the projections from precerebellar nuclei to the lobulus simplex of the rat's cerebellum investigated by retrograde fluorescent double labeling

The projections to lobulus simplex and Crus I of the cerebellum from various brainstem nuclei have been examined in adult rats by using the retrograde fluorescent double labeling technique. True blue was injected into the lobulus simplex on one side and nuclear yellow on the other and the brainstem was examined for labeled neurons. The lateral reticular nucleus, pontine tegmental reticular nucleus, and nucleus praepositus hypoglossi were similar to the equivalent nuclei in other species but all contained double- as well as single-labeled neurons and it was concluded that these nuclei have neurons whose axons branch to both sides of the cerebellum. More neurons in the rostral part of the lateral reticular nucleus were bilaterally projecting than in the caudal and the significance of this in relation to its afferents is considered. The individual neurons in the pontine nuclei, inferior olivary nucleus, and cuneate nuclei only appear to project to one side and the recent evidence for axonal branching of pontine neurons in the cat is discussed.

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.