The fastigio-tectal projections. An anatomical experimental study

Recent Advances in the Understanding of Specific Efferent Pathways Emerging From the Cerebellum

The cerebellum has a long history in terms of research on its network structures and motor functions, yet our understanding of them has further advanced in recent years owing to technical developments, such as viral tracers, optogenetic and chemogenetic manipulation, and single cell gene expression analyses. Specifically, it is now widely accepted that the cerebellum is also involved in non-motor functions, such as cognitive and psychological functions, mainly from studies that have clarified neuronal pathways from the cerebellum to other brain regions that are relevant to these functions. The techniques to manipulate specific neuronal pathways were effectively utilized to demonstrate the involvement of the cerebellum and its pathways in specific brain functions, without altering motor activity. In particular, the cerebellar efferent pathways that have recently gained attention are not only monosynaptic connections to other brain regions, including the periaqueductal gray and ventral tegmental area, but also polysynaptic connections to other brain regions, including the non-primary motor cortex and hippocampus. Besides these efferent pathways associated with non-motor functions, recent studies using sophisticated experimental techniques further characterized the historically studied efferent pathways that are primarily associated with motor functions. Nevertheless, to our knowledge, there are no articles that comprehensively describe various cerebellar efferent pathways, although there are many interesting review articles focusing on specific functions or pathways. Here, we summarize the recent findings on neuronal networks projecting from the cerebellum to several brain regions. We also introduce various techniques that have enabled us to advance our understanding of the cerebellar efferent pathways, and further discuss possible directions for future research regarding these efferent pathways and their functions.

Monosynaptic inputs to specific cell types of the intermediate and deep layers of the superior colliculus

The intermediate and deep layers of the midbrain superior colliculus (SC) are a key locus for several critical functions, including spatial attention, multisensory integration, and behavioral responses. While the SC is known to integrate input from a variety of brain regions, progress in understanding how these inputs contribute to SC-dependent functions has been hindered by the paucity of data on innervation patterns to specific types of SC neurons. Here, we use G-deleted rabies virus-mediated monosynaptic tracing to identify inputs to excitatory and inhibitory neurons of the intermediate and deep SC. We observed stronger and more numerous projections to excitatory than inhibitory SC neurons. However, a subpopulation of excitatory neurons thought to mediate behavioral output received weaker inputs, from far fewer brain regions, than the overall population of excitatory neurons. Additionally, extrinsic inputs tended to target rostral excitatory and inhibitory SC neurons more strongly than their caudal counterparts, and commissural SC neurons tended to project to similar rostrocaudal positions in the other SC. Our findings support the view that active intrinsic processes are critical to SC-dependent functions, and will enable the examination of how specific inputs contribute to these functions.

Cerebellar fastigial neurons send bifurcating axons to both the left and right superior colliculus in cats

Anesthetized cats were injected with 2% Fast Blue and 0.5% Nuclear Yellow into the intermediate and deep layers of the left and right superior colliculus, respectively. In the right caudal part of the cerebellar fastigial nucleus (cFN), double-labeling was found in 38.5% of the neurons labeled with Fast Blue, and in 11.5% of the neurons labeled with Nuclear Yellow. In the left cFN, 52.2% of the neurons labeled with Fast Blue and 11.0% of the neurons labeled with Nuclear Yellow were double-labeled. The results suggest a role of bifurcating fastigial fibers in cerebellar visual control.

Gaze shifts evoked by electrical stimulation of the superior colliculus in the head-unrestrained cat. II. Effect of muscimol inactivation of the caudal fastigial nucleus

The medioposterior cerebellum [vermian lobules VI and VII and caudal fastigial nucleus (cFN)] is known to play a major role in the control of saccadic gaze shifts toward a visual target. To determine the relative contribution of the cFN efferent pathways to the brainstem reticular formation and to the superior colliculus (SC), we recorded in the head-unrestrained cat the effects of cFN unilateral inactivation on gaze shifts evoked by electrical microstimulation of the deeper SC layers. Gaze shifts evoked after muscimol injection still exhibited the typical qualitative features of normal saccadic gaze shifts. Nevertheless, consistent modifications in amplitude and latency were observed. For ipsiversive movements (evoked by the SC contralateral to the inactivated cFN), these changes depended on the locus of stimulation on the motor map: for the anterior 2/3 of the SC, amplitude increased and latency tended to decrease; for the posterior 1/3 of the SC, amplitude decreased and latency increased. For the contraversive direction, amplitude moderately decreased and latency tended to increase for all but the caudal-most stimulated SC site. These modifications of SC-evoked gaze shifts during cFN inactivation differed from the ipsiversive hypermetria/contraversive hypometria pattern observed for visually triggered gaze shifts recorded during the same recording sessions. We conclude that (i) the topographical organization of gaze shift amplitude in the deeper SC layers is influenced by the cerebellum and is either severely distorted or demonstrates an amplitude reduction during inactivation of the contralateral or ipsilateral cFN, respectively; (ii) gaze shifts evoked by SC microstimulation and visually triggered gaze shifts either rely on distinct cerebellar-dependent control processes or differ by the location of the caudal-most active SC population. We present a functional scheme providing several predictions regarding the modulatory influence of the cerebellum on SC neuronal activities and on the topographical organization of fastigial-SC projections.

Bifurcating projections from the cerebellar fastigial neurons to the thalamic suprageniculate nucleus and to the superior colliculus

A double-labeling fluorescence microscopic study was performed on the mesencephalic and thalamic distribution of fastigial efferents. Anesthetized cats were injected with 2% fast blue into the suprageniculate nucleus and with 0.5% nuclear yellow into the superior colliculus. Analysis of serial sections through the cerebellar fastigial nucleus revealed that 25% of the neurons projecting to the superior colliculus and 10% of those projecting to the thalamus were double labeled. The results suggest that bifurcating fastigial fibers to the mesencephalon and to the visual thalamus may play a role in cerebellar visual control.

Cerebellotectal projection in the rat: anterograde and retrograde WGA-HRP study of individual cerebellar nuclei

Cerebellotectal projections were studied in the rat by the anterograde and retrograde tracing methods using wheat-germ-agglutinin-conjugated horseradish peroxidase. The pathway arises from all four cerebellar nuclei on the contralateral side; mainly from the posterior interpositus nucleus and lateral nucleus and to a lesser extent from the medial nucleus and anterior interpositus nucleus. The fibers arising from the medial nucleus and the posterior interpositus nucleus terminate mainly in the deeper zone of layer IV and in layer VI throughout the entire rostrocaudal extent of the contralateral superior colliculus. Those arising from the anterior interpositus nucleus and the lateral nucleus terminate mainly in the superficial zone of layer IV in the rostral three-fourths of the contralateral superior colliculus. In addition, the fibers from the lateral nucleus terminate densely in a zone extending from the deep part of layer III through layer VII in the lateral portion of the rostral half of the superior colliculus. In comparison with data on other species the present findings are discussed with respect to the evolutional changes from monocular to binocular vision.

Afferent and efferent connections of the oculomotor region of the fastigial nucleus in the macaque monkey

Afferent and efferent connections of the fastigial oculomotor region (FOR) were studied in macaque monkeys by using axonal transport of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). When injected HRP is confined to the FOR, retrogradely labeled cells appear in lobules VIc and VII of the ipsilateral vermis and in group b of the contralateral medial accessory olive (MAO). In reference to the maps of topographical organization, the extent of the effective site in the fastigial nucleus (FN) could be assessed from the distributions of labeled Purkinje cells (P cells) in the vermis and labeled olivary neurons in the MAO. In contrast to the unilateral nature of the P-cell and climbing-fiber projections, those from the other brainstem regions to the FOR were bilateral. Following the injection of HRP into the FOR, the largest number of retrogradely labeled cells appeared in the pontine nuclei. Although the number of labeled cells was greater on the contralateral side in both the peduncular and dorsomedial pontine nuclei (DMPN), the number of each side was virtually identical in the dorsolateral pontine nucleus (DLPN). In the nucleus reticularis tegmenti pontis (NRTP), labeled cells were located only in its medial and dorsolateral portions bilaterally. In the vestibular complex, labeled cells appeared in the superior (SVN), medial (MVN), and inferior vestibular nuclei (IVN) bilaterally. The lateral vestibular nucleus (LVN), including y group and the ventrolateral vestibular nucleus, were free of labeled cells. Labeled cells appeared also in the perihypoglossal nucleus (PHN) bilaterally. In the pontine raphe (PR) and paramedian pontine reticular formation (PPRF), labeled cells appeared bilaterally in the caudal third of the area between the oculomotor and abducens nuclei. Labeled cells appeared also in the mesencephalic and medullary reticular formation. Tracing of anterogradely labeled axons demonstrated that most fibers from the FOR decussated within the cerebellum and entered the brainstem via the contralateral uncinate fasciculus. Some crossed fibers ascended with the contralateral brachium conjunctivum and terminated in the midbrain tegmentum. A small contingent of fibers advanced further to the thalamus. In the mesodiencephalic junction, labeled terminals were found contralaterally in the rostral interstitial nucleus of medial longitudinal fasciculus (riMLF) and a medial portion of FOrel's H Field. They appeared also in the central mesencephalic reticular formation (cMRF), the periaqueductal gray (PAG), the posterior commissure nucleus, and the superior colliculus. The oculomotor and trochlear nuclei, the red nucleus, and the interstitial nucleus of Cajal were free of labeled terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

The descending auditory pathway and acousticomotor systems: connections with the inferior colliculus

In this review the following major points are emphasized. First, the descending auditory system includes 3 separate, but parallel pathways connecting the AC, MGB and IC. Each pathway makes a strong set of connections with a distinctive area from each of 3 auditory centers. The three sets of connections are mutually exclusive, such that the pathways describe 3 separate corticocolliculo-geniculate systems. Thus, multiple feedback loops between the AC and the IC are formed which create a great capacity for parallel processing of auditory information. Second, the IC projects to the SOC and, in particular, to the source of olivocochlear efferent neurons. The connections of the IC with the AC rostrally, and with the olivocochlear neurons caudally, imply a descending trisynaptic pathway from the cortex to the cochlea whose travel time could better that of the ascending pathway and thus provide an efficient feedback mechanism. It is probable that the IC influences cochlear signal processing. The reciprocal connectivity between any two of either the IC, SOC or the CN, again, affords to the auditory system remarkable parallel processing capabilities. Finally, the descending auditory, and 'extra-auditory' connections of the IC bestow a functional separateness to the 3 nuclei of the IC, a view that is best illustrated by description of the ICX as an acousticomotor nucleus, having connections with the SC, cerebellum and somatosensory and vocalization systems. More sophisticated questions about the descending auditory system will incorporate these present observations and test functional implications to which they allude.

Cerebellotectal pathways in the macaque: implications for collicular generation of saccades

The cerebellum is thought to modulate saccadic activity in the primate in order to maintain targeting accuracy, and the cerebellotectal pathway has been posited to play a role in this modulation. However, anatomical descriptions of this pathway in primates are sketchy and conflicting. To determine whether the organization of the cerebellotectal projection in primates is similar to that found in other species, neuroanatomical tracer transport techniques were utilized in two species of macaque monkey to label cerebellotectal somata and fiber terminations. Two pathways were found. One, the fastigiotectal pathway, is derived from cells in the caudal fastigial nucleus and projects bilaterally to the rostral end of the intermediate gray layer. The other pathway is derived from cells in the posterior interposed nucleus and the adjacent posterior wing of the dentate nucleus, and it terminates contralaterally throughout the ventral half of the intermediate gray and the deep gray layers. Both of these pathways terminate within the layers of the superior colliculus containing premotor, saccade-related neurons, but the differences in the distribution of their terminals and cells of origin suggest that these two pathways have different functions. Furthermore, the pattern of connections of these two pathways indicates that they do not function as a traditional feedback circuit. We suggest that the cerebellotectal pathways may instead modulate collicular activity in a more complex manner. For example, it may provide signals necessary for corrective saccades or for maintaining spatial registry between the different sensory representations supplied to the superior colliculus and its presaccadic output, which is organized into a motor map.

Origin of cerebellar projections to the region of the oculomotor complex, medial pontine reticular formation, and superior colliculus in New World monkeys: a retrograde horseradish peroxidase study

Cerebellar projections to oculomotor-related brainstem regions were studied in four groups of New World (capuchin, squirrel) monkeys by using the retrograde transport of horseradish peroxidase (HRP) to determine the origin of the principal cerebellar influence on eye movement. Group A monkeys had HRP injections or transcannular HRP gel implants into the oculomotor complex (OMC), the largest of which involved adjacent paraoculomotor nuclei (e.g., ventral periaqueductal gray, PAG; nucleus of Darkschewitsch, ND; medial accessory nucleus of Bechterew, MAB; dorsomedial parvicellular red nucleus, dmPRN). All of these cases contained large numbers of retrogradely labeled cells in cell group Y. Whereas the smallest OMC injection only labeled a few cells in the dentate nucleus (DN), injections involving paraoculomotor nuclei produced labeling in all of the cerebellar nuclei except the basal interstitial nucleus (BIN). Injections extending into the ND and MAB produced particularly heavy labeling within the interposed nuclei. Group B monkeys had injections/implants into the medial pontine tegmentum and dorsomedial basilar pons. The pontine tegmental cases contained labeled cells in all cerebellar nuclei, but the DN was the most heavily labeled when the implant involved the nucleus reticularis tegmenti pontis (NRTP). Cases with injections into the caudal medial pontine tegmentum (nucleus reticularis pontis caudalis, NRPC), including the physiological paramedian pontine reticular formation (PPRF), but not NRTP, contained the largest number of labeled cells in the fastigial nucleus (FN) and lacked retrograde labeling in the DN. Dorsomedial basilar pontine cases contained almost no labeled cells in the FN, anterior interpositus nucleus (AIN), and posterior interpositus nucleus (PIN) but did contain DN labeling when the injection involved the NRTP. Two dorsomedial pontine tegmental cases and one dorsomedial basilar pontine case had more labeled cells in the BIN than in other cases. Tegmental cases also contained a few labeled cells in cell group Y. Group C monkeys had injections into the parvicellular red nucleus (PRN) and had their heaviest labeling in the DN, although the AIN and PIN also contained labeled cells. The FN, BIN, and cell group Y, on the other hand, contained almost no labeling. Group D consisted of monkeys which had injections into the intermediate and deep superior colliculus (SC). These cases contained the largest numbers of labeled cells in the PIN and a lesser number in the ventrolateral FN. The DN, AIN, BIN, and cell group Y lacked labeled neurons in these cases.(ABSTRACT TRUNCATED AT 400 WORDS)

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 projection from the superior colliculus to the lateral reticular nucleus in the cat as studied with retrograde transport of WGA-HRP

Medium-sized and large superior collicular neurons were retrogradely labelled after small ejections of the wheat germ agglutinin-horseradish peroxidase complex in the lateral reticular nucleus of the feline medulla. The projection from the superior colliculus to the lateral reticular nucleus is bilateral with a contralateral predominance. It originates mainly from the intermediate, but also from the deep gray layer of the superior colliculus. Our observations provide evidence that the lateral reticular nucleus is an important target of tectal efferents. The findings are discussed in relation to the organization of other fiber connections of the superior colliculus.

An EM-autoradiographic and EM-HRP study of the commissural projection of the superior colliculus in the cat

Terminals of the commissural projection in the cat were characterized ultrastructurally by autoradiographic and horseradish peroxidase methods. The results of the two studies are complementary. Terminals of commissural cells are present in the intermediate and deep layers of the cat superior colliculus. Two distinct populations of terminals are present: one containing mostly round vesicles and forming asymmetric specializations, and a second containing mostly pleomorphic vesicles and forming symmetric specializations. Both populations contact small dendrites or dendritic appendages. The two populations, mostly round and mostly pleomorphic, are present in the ratio of 2:1. Terminals measure approximately 1.1 micron in mean diameter and contact profiles ranging in size from 0.2 to 4.6 micron. There is no significant difference between the two populations in either pre- or postsynaptic profile size. The colocalization of terminals of commissural neurons with other afferent and efferent projections of the intermediate layers of the superior colliculus is discussed.

Somatostatin (SRIF)-like immunoreactivity in subcortical and cortical visual centers of the rat

The distribution of neuronal elements containing immunoreactive somatostatin (I-SRIF) in the rat central visual pathway was examined by light-microscopic immunocytochemistry. These studies were concerned with the location and morphology of neurons and innervated cells and the distribution of fiber and terminal plexuses in the primary visual cortex (area 17), visual association areas 18 and 18a, the superior colliculus, the lateral geniculate nucleus, and the pretectum. In the superior colliculus, I-SRIF-containing fibers and perikarya were distributed predominantly in the superficial, or visual, layers; these elements were moderately dense and occupied the entire mediolateral extent of these layers. In the intermediate and deep layers, immunoreactive neurons were widely scattered, and fibers were located mainly in the medial third. Immunoreactive cell populations in the superior colliculus included small bipolar neurons with fusiform perikarya and multipolar neurons with round to ovoid perikarya. In the pretectum, the peptide was demonstrable in large and small multipolar neurons of the nucleus of the optic tract and in the posterior and olivary pretectal nuclei. I-SRIF-containing neurons were also present in the nucleus of the posterior commissure, the nucleus of Edinger-Westphal, and the ventral division of the lateral geniculate nucleus. In the visual cortex, the peptide was present in all layers and in a variety of morphologically defined cell populations, including some which are presumed excitatory (pyramidal and bipolar cells) and others which are presumed inhibitory (bitufted and stellate cells). Our data suggest that somatostatin is involved in visual and visuomotor reflex pathways and in the horizontal optokinetic nystagmus reflex pathway. These results provide a foundation for further studies to evaluate the role of this peptide in visual processes.

Nigral and cerebellar synaptic terminals in the intermediate and deep layers of the cat superior colliculus revealed by lesioning studies

The presence of degenerating nigral and cerebellar synaptic terminals in the intermediate and deep layers of the cat superior colliculus was demonstrated by electron microscopy following lesions of the substantia nigra or brachium conjunctivum. The superior colliculus was taken for analysis 4-5 days after operation. Nigral terminals underwent a dark type of degeneration following kainic acid lesion of the pars reticulata of the substantia nigra. The majority of nigral degenerating terminals and axons were found in the stratum griseum intermediale with a few in the stratum griseum profundum. Two kinds of cerebellar terminals were distinguished by general appearances such as size, type of synaptic contact and type of synaptic vesicle and by the pattern of degenerative changes following electrical lesion of the brachium conjunctivum. Large elongated synaptic terminals 4-7 microns in diameter, were found mainly in the stratum griseum profundum. They often had double termination with conventional dendrites and with vesicles containing dendrites. This kind of terminal had a filamentous type of degeneration. A second type of degenerating cerebellar terminal, characterized by an electron-lucent type of degeneration, was predominantly located in the stratum griseum intermediale. These terminals were circular, about 4 microns in diameter, and did not have synaptic contact with vesicle-containing profiles. The finding of the two types of degenerating terminal after lesion of the brachium conjunctivum can be considered as evidence of the coexistence of at least two kinds of cerebellar terminals in the superior colliculus. The presence of nigral and cerebellar terminals in the intermediate and deep layers of the superior colliculus implicates the involvement of the substantia nigra and cerebellum in control of collicular visuomotor function.

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.

Distribution of cerebellar fiber terminals in the midbrain visuomotor areas: an autoradiographic study in the cat

Cerebellar fibers to the midbrain visuomotor areas were traced in the cat auto-radiographically after injections of tritiated amino acids into individual cerebellar nuclei. Fibers from the dentate (DN), anterior interpositus (AIN) and posterior interpositus (PIN) nuclei were distributed contralaterally, while those from the fastigial nucleus (FN) bilaterally. The FN fibers appeared to arise mainly from the caudal half of the FN. In the superior colliculus (SC), the FN or DN fibers were more numerous than the PIN fibers, and the areas of termination of the FN fibers were located more medially than those of the DN and PIN fibers. These cerebellotectal fiber terminals were in the intermediate and deep SC layers; clustering of terminal silver grains was noted in the FN and DN fibers-recipient areas in the intermediate gray layer. In the pretectum, the DN fibers terminated ventrally in the reticular part of the anterior pretectal nucleus and the posterior pretectal nucleus. THe AIN fibers terminated ventrally in the compact part of the anterior pretectal nucleus and the posterior pretectal nucleus. The nucleus of the posterior commissure received cerebellar fibers chiefly from the DN, and additionally from the FN. The nucleus of Darkschewitsch and the interstitial nucleus of Cajal received fibers from all cerebellar nuclei. No cerebellar fibers terminated in the extraocular motor nuclei and the Edinger-Westphal and anteromedian nuclei.

Simple spike activity of Purkinje cells in the posterior vermis of awake cats during spontaneous saccadic eye movements

Extracellular recordings were made from 151 cerebellar cortical cells in the posterior vermis of 12 awake cats. Thirty-two percent (n = 48) of these cells modulated their activity with respect to the onset of spontaneous saccadic eye movements. Thirty-five cells in this group were positively identified as Purkinje cells and manifested changes in simple spike activity that were related to saccade onset. These included short excitatory, inhibitory, or biphasic changes that were superimposed on background tonic firing rates (avg. = 54 spikes/sec). Such changes were recorded before as well as after the onset of a saccade. Sixty-five percent (n = 22) of these cells were related to horizontal and vertical saccades in more than one direction of motion. These cells were randomly distributed throughout the posterior vermis and manifested no anatomical topographic organization with respect to the direction of saccadic eye movement. The results of this study suggest that lobules VI and VII of the cerebellar vermis participate in both the initiation and execution of spontaneous saccades in preferred directions.

Mesencephalic and diencephalic afferents to the superior colliculus and periaqueductal gray substance demonstrated by retrograde axonal transport of horseradish peroxidase in the cat

The mesencephalic and diencephalic afferent connections to the superior colliculus and the central gray substance in the cat were examined by means of the retrograde transport of horseradish peroxidase (HRP). After deep collicular injections numerous labeled cells were consistently found in the parabigeminal nucleus, the mesencephalic reticular formation, substantia nigra pars reticulata, the nucleus of posterior commissure, the pretectal area, zona incerta, and the ventral nucleus of the lateral geniculate body. A smaller number of cells was found in the inferior colluculus, the nucleus of the lateral lemniscus, the central gray substance, nucleus reticularis thalami, the anterior hypothalamic area, and, in some cases, in the contralateral superior colliculus, Forel's field, and the ventromedial hypothalamic nucleus. Only the parabigeminal nucleus and the pretectal area showed labeled cells following injections in the superficial layers of the superior colliculus. In the cats submitted to injections in the central gray substance, labeled cells were consistently found in the contralateral superior colliculus, the mesencephalic reticular formation, substantia nigra parts reticulata, zona incerta and various hypothalamic areas, especially the ventromedial nucleus. In some cases, HRP-positive cells were seen in the nucleus of posterior commissure, the pretectal area, Forel's field, and nucleus reticularis thalami. A large injection in the mediodorsal part of the caudal mesencephalic reticular formation, which included the superior colliculus and the central gray substance, resulted in numerous labeled cells in nucleus reticularis thalami. The findings are discussed with respect to the suggested functional division of the superior colliculus into deep and superficial layers. Furthermore, the possible implications of labeled cells in zona incerta and the reticular thalamic nucleus are briefly discussed.

Identification of cells of origin of tectopontine fibers in the cat superior colliculus: an experimental study with the horseradish peroxidase method

The pontine projections from the superior colliculus in the cat have been studied by means of retrograde axonal transport of horseradish peroxidase (HRP). Following injections of HRP in the dorsolateral pontine nucleus, where the tectopontine fibers terminate, a fair number of labeled cells are found throughout the rostrocaudal extent of the ipsilateral superior colliculus. Relatively few of the labeled cells are of medium size (25-40 micron in diameter), more than 80% are small (10-25 micron), but no large cells are labeled. The cell bodies giving rise to tectopontine fibers are distributed in tectal layers deeper than the optic stratum (including this), with only a few in the deeper portion of the superficial gray layer. There are only few labelled cells in the relatively large lateral portion of the intermediate and deep gray layers were the largest neurons (more than 40 micron) are located. Most of these presumably belong to the tectoreticular and the tectospinal projections. The tectal neurons, distributed in various collicular layers, are supposed to receive different kinds of information from other parts of the central nervous system, e.g. from the retina, the cerebral cortex, the brain stem reticular formation, the spinal cord etc. The dorsolateral pontine nucleus appears to have a particular function in the integration of the input from the superior colliculus with those from other sources, especially from the inferior colliculus and the auditory cerebral cortex.

The commissural projection of the superior colliculus in the cat

The origin, course, and termination of the commissural projection of the superior colliculus were studied using the orthograde and autoradiographic tracing method and the retrograde method utilizing horseradish peroxidase. The complementary and mutually confirming sets of data showed that the commissural fibers interconnect a restricted region of the colliculi. This region includes the strata grisea intermedium and profundum and to a lesser degree the stratum opticum. It extends throughout only the rostral part of the colliculus where it ends abruptly at a level slightly less than half the distance from the anterior border of the deep gray layers. By using the needle used for isotope injection to record multiunit responses to somatic and visual stimuli, direct evidence was obtained that this region falls within that functional area of the colliculus devoted to face representation and central vision. The results also suggested that more commissural fibers arise from lateral than medial parts of this region and that many fibers interconnect corresponding points in the colliculi. In addition to intertectal connections, the commissural projection contains decussating axons which terminate in tegmental structures and within a restricted zone of the central gray matter directly overlying the oculomotor complex. The results are discussed in relation to the possible role the commissural projection plays in the regulation of eye and head movement.

The connections and laminar organization ofthe optic tectum in a reptile (lguana iguana)

The goals of this study were: (1) to describe the total pattern of projections from the optic tectum of Iguana iguana and Pseudemys scripta; and (2) to describe the contributions of particular lamina of the Iguana's optic tectum to this total pattern. Lesions were made in the optic tectum of the Iguana which damaged either all or only certain tectal laminae and, for comparison with the Iguana, lesions in the turtle's optic tectum were made which involved all laminae. The anterograde degeneration resulting from these lesions was stained with the Fink-Heimer ('67) method. The total pattern of projections from the optic tectum in the Iguana and the turtle is similar to that reported for representatives of other vertebrate classes. That is, the optic tectum gives rise to ipsilateral ascending projections to pretectal nuclei, to nucleus rotundus and to nucleus geniculatus lateralis pars ventralis of the diencephalon and, in addition, to a contralateral ascending pathway which courses via the supraoptic decussation to the contralateral diencephalon. Tectotectal connections and several descending pathways were also recognized in each species. The descending pathways include ipsilateral tectobulbar and tecto-isthmi pathways and a contralateral predorsal bundle. Lesions which damaged only certain tectal laminae in the Iguana revealed a laminar organization of the efferent projections. A lesion restricted to the superficial retinal-recipient layers, stratum griseum et album superficiale, resulted in degeneration in only nucleus isthmi pars magnocellularis and nucleus geniculatus lateralis pars ventralis. A lesion which involved both the retinal-recipient layers and stratum griseum centrale resulted in degeneration in only one additional structure, nucleus rotundus. A small lesion involving the deep periventricular layers as well as the superficial layers produced degeneration in the predorsal bundle and the ipsilateral tectobulbar tract as well as in the structures receiving input from the more superficial layers. These results are compared to the results of similar analyses of the superior colliculus in mammals.