Discontinuous spatial distribution of the tectal afferents from the trigeminal nucleus in the cat

Honeycomb-like structure of the intermediate layers of the rat superior colliculus: afferent and efferent connections

There is increasing evidence that acetylcholinesterase is organised in a lattice-like fashion in the intermediate layers of the mammalian superior colliculus. In a recent study, we described this organisation in rat by showing that it comprises a well formed honeycomb-like lattice with about 100 cylindrical compartments or modules occupying both the intermediate collicular layers. Considering this enzyme domain as a reference marker for comparing the organisation of collicular input-output systems, the present study investigates whether the principal sensori-motor systems in intermediate layers also have honeycomb-like arrangements. In 33 animals, the distributions of afferents (visual from extrastriate cortex; somatic from the primary somatosensory cortex, the trigeminal nucleus and the cervical spinal cord) and efferents (cells of origin of the crossed descending bulbospinal tract and uncrossed pathway to the pontine gray, the ascending system to the medial dorsal thalamus) were examined in a tangential plane following applications of horseradish peroxidase-wheatgerm agglutinin conjugate (used as an anterograde and retrograde tracer). In 22 of the 33 rats, axonal tracing was made within single tangential sections also stained for cholinesterasic activity in order to compare the neuron profiles with the cholinesterasic lattice.The results show that these afferent and efferent systems are also organised in honeycomb-like networks. Moreover, those related to the cortical, trigeminal and some of the spinal afferents are aligned with the cholinesterasic lattice. Likewise most of colliculo-pontine, colliculo-bulbospinal and half of colliculo-diencephalic projecting cells also tend to be in spatial register with the enzyme lattice. This indicates that the honeycomb-like arrangement is a basic architectural plan in the superior colliculus for the organisation of both acetylcholinesterase and major sensori-motor systems for orientation.

Honeycomb-like structure of the intermediate layers of the rat superior colliculus, with additional observations in several other mammals: AChE patterning

The aim of the present study was to reinvestigate the stereometric pattern of acetylcholinesterase (AChE) activity staining in the intermediate layers of the superior colliculus in several mammalian species. A pioneering study in the cat and the monkey by Graybiel (1978) stressed the regular arrangement of AChE staining in the deep collicular layers. According to her description, made in the frontal plane, the enzyme was arranged in a mediolateral series of patches, the cores of which tended to line up in the longitudinal axis of the structure, so they formed roughly parallel bands. As exhaustive a description as possible of the AChE distribution was undertaken in the rat by compiling observations in the frontal, sagittal, and tangential planes. It emerged that AChE-positive elements are organized in the form of a conspicuous honeycomb-like network that is divided into about 100 rounded compartments, over virtually the full extent of the intermediate layers. The generality of the rat model was then tested in other rodents such as mouse and hamster and also in cat and monkey. For these species we resorted to a single tangential cutting plane, which proved to be more appropriate for disclosing such a modular arrangement. The data revealed that in all species AChE staining followed the same architectural plan and identified the striking similarity in the number of compartments that compose the various honeycomb-like lattices. In conclusion, the present findings support a unified model of the AChE arrangement within the intermediate layers of the mammalian colliculus; the model comprehensively incorporates the classical description of the patchy and stripy features of the enzyme distribution. We hypothesize here that the modular AChE arrangement might be the anatomical basis for collicular vectorial encoding of orienting movements.

Differential projections to the superior collicular layers from the perihypoglossal nuclei in the cat

The primary objective of the present study is to demonstrate the presence of a projection to the superficial layers of the superior colliculus (SC) from the perihypoglossal nuclei, specifically from the nucleus intercalatus (INT) in the cat. Iontophoretic application of WGA-HRP into the perihypoglossal complex produced orthogradely labeled terminals in the SC contralaterally forming two bands: one is in the superficial gray layer, and the other in the intermediate gray layer. The superficial band was evenly distributed in the upper portion of the superficial gray layers (layers II1-2) and the deeper band existed in the intermediate gray layer (layer IV) being arranged in a discontinuous manner. Injections of the tracer into the superficial layers of the SC yielded retrogradely labeled cells only in the rostral part of the contralateral INT; by contrast, the injection confined to the deep layers produced labeling of cells exclusively in the nucleus prepositus hypoglossi (PH). Thus, the INT and the PH each project separately to the functionally different superficial and intermediate layers of the SC, respectively. On the basis of the present anatomical findings, it is suggested that the perihypoglossal nuclei as a whole contribute not only to the oculomotor but also to the visuosensory regulatory function in the SC.

Organization of the projections from the trigeminal brainstem complex to the superior colliculus in the rat and hamster: anterograde tracing with Phaseolus vulgaris leucoagglutinin and intra-axonal injection

Anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA-L) and intra-axonal recording and injection techniques were employed to describe the projection from the trigeminal (V) brainstem complex to the deep laminae of the superior colliculus (SC) in the hamster and the rat. The organization of these projections was the same in the two species. Deposits of PHA-L into V nucleus principalis (PrV) produced labelled axons and boutonlike swellings in the lower stratum griseum intermediale (SGI) and upper stratum album intermedium (SAI) in the SC bilaterally. Plots of boutonlike swellings indicated that the terminals of this projection were arrayed in clusters. Nucleus principalis also projected to the stratum griseum profundum (SGP) and stratum album profundum (SAP). This deeper projection did not terminate in clusters and it was most prominent in the lateral SC. The ipsilateral PrV-SC projection appeared to arise mainly from axons that recrossed the midline at the level of the SC commissure. Reconstruction of individual PHA-L labelled fibers demonstrated that single axons gave rise to terminals on both sides of the midline. Deposits of PHA-L into V subnucleus interpolaris (SpI) yielded results that were identical to those obtained with PrV injections with one exception: none of these deposits produced any labelled terminals in the ipsilateral SC. Deposits of PHA-L into V subnucleus caudalis (SpC) produced only sparse labelling in SC. Most labelled swellings were located in the SGP and SAP and they were visible only in the SC contralateral to the PHA-L injection site. Single axons arising from cells in SpI were recorded and injected with horseradish peroxidase (HRP) in the hamster's SC. These fibers all responded to stimulation of multiple mystacial vibrissae and gave rise to 2-5 clusters of bouton-like swellings in the lower SGI and upper SAI.

Trigeminotectal and other trigeminofugal projections in neonatal kittens: an anatomical demonstration with horseradish peroxidase and tritiated leucine

The trigeminal projection to the superior colliculus in neonatal kittens was studied by using both anterograde and retrograde neuroanatomical tracing techniques. Trigeminothalamic observations also were made. In the first series of experiments, horseradish peroxidase was injected into the superior colliculus in kittens on the day of parturition and in adult cats. Retrogradely labeled cells were found throughout the contralateral sensory trigeminal complex: the greatest numbers of cells were concentrated in pars oralis, with fewer in the principal nucleus, and fewer still in pars interpolaris and pars caudalis. Thus, the distribution pattern of trigeminotectal cells in neonates is similar to that in adult animals. In the second series of experiments, we injected tritiated leucine into the rostral portion of the spinal trigeminal nucleus in neonatal kittens and adult cats and compared the laminar and spatial distribution of anterogradely transported label in the superior colliculus and thalamus. Terminal label was observed in both structures in animals as young as 1-2 days postpartum. The label in the superior colliculus was overwhelmingly contralateral and formed a tier of discontinuous patches in the stratum griseum intermediale and, in a more diffuse manner, in the stratum griseum profundum. Most of the patches were located in the rostral 80% of the superior colliculus and were 60-280 micron in width. Although the size of the patches was smaller in the neonates, their distribution was similar to that in adult cats. Thus, with the exception of the difference in patch size, the terminal pattern of trigeminotectal projections is essentially adultlike at birth. The dense pattern of contralateral terminal label in the arcuate division of the ventrobasal complex also was similar to that of the adult cat, as was the trigeminal projection to the supraoculomotor gray. These data indicate that the development of the spatial organization of a major ascending somatosensory pathway to the superior colliculus (and to the thalamus) is largely a prenatal event. It is likely that the further maturation of these systems during postnatal life is limited to fine changes in axonal terminals and synaptic formation within prenatally determined terminal territories. The in utero maturation of these trigeminofugal projections is necessary to enable the newborn kitten to utilize the perioral tactile cues necessary for early orientation and suckling behaviors.

Complementary and non-matching afferent compartments in the cat's superior colliculus: innervation of the acetylcholinesterase-poor domain of the intermediate gray layer

Three tectal afferent-fiber systems were experimentally labeled in the cat to learn how their distributions within the superior colliculus were related to the prominent compartments of high acetylcholinesterase activity found in the intermediate gray layer. Presumptive somatic sensory afferents were labeled by injections of horseradish peroxidase-wheatgerm agglutinin conjugate placed at the bulbospinal junction and in the ventral anterior ectosylvian cortex corresponding to somatic sensory area SIV. Vision-related afferents were labeled by injections of the same tracer substance into the lateral suprasylvian visual area. In each animal, a single type of injection was made and a detailed study was carried out to compare the patterns of anterograde labeling and acetylcholinesterase staining in serially adjoining sections through the superior colliculus. Fibers labeled by the three types of injection were distributed in clusters that resembled the acetylcholinesterase-positive patches in the intermediate gray layer. In no case, however, were the afferent-fiber clusters in register with the histochemically defined patches. Instead, the innervations derived from the bulbospinal junction, anterior estosylvian sulcus and lateral suprasylvian visual area all formed patchworks within the acetylcholinesterase-poor domain of the intermediate gray layer. In some instances, the afferent-fiber clusters and enzyme-positive patches appeared to have complementary distributions. In other instances, the afferent-fiber clusters seemed to be arranged in the acetylcholinesterase-poor parts of the intermediate layer in a fashion independent of, but not significantly overlapping, the acetylcholinesterase-positive patches. Not all of the space between the acetylcholinesterase-positive patches was taken up by any one of the afferent-fiber systems labeled. The complementary and non-matching distribution of these afferent systems in relation to the acetylcholinesterase-rich patches of the intermediate gray layer stands in contrast to the spatial registration of two other tectal afferent systems with the zones of high acetylcholinesterase activity. Both nigrotectal and frontotectal afferents converge on the acetylcholinesterase-positive patches. We conclude that afferent systems projecting to the intermediate gray layer can be divided into at least two groups: those innervating the acetylcholinesterase-rich compartments and those avoiding them.(ABSTRACT TRUNCATED AT 400 WORDS)

Laminar distribution and patchiness of cytochrome oxidase in mouse superior colliculus

The cytochrome oxidase (CO), acetylcholinesterase (AChE), myelin, and Nissl stains were studied and compared to develop an anatomical system identifying the laminar architecture of the mouse superior colliculus. The CO and myelin stains are shown to define collicular laminae more distinctly than does the Nissl stain. The layer of large rostrocaudally coursing fiber bundles that has formerly been referred to in the rodent literature as stratum album intermediale (SAI; layer V) is renamed as a sublayer of the stratum griseum intermediale (SGI; layer IV) to conform with the nomenclature for the cat superior colliculus of Kaneseki and Sprague ('74, J. Comp. Neurol. 158:319-338). Patches of CO activity in layer IV (SGI) are shown that contain intensely stained, large, multipolar cell bodies. The CO patches do not correspond to those previously reported for AChE. The CO, myelin, and AChE stains all indicate the presence of a large lateral extension termed the flank of layer IV (SGI). In contrast to the classical lamination pattern of the superior colliculus, the flank has no overlying layer II (stratum griseum superficiale, SGS) or layer III (stratum opticum, SO).

Cytoarchitectonic coincidence with the discontinuous connectional pattern in the deep layers of the superior colliculus in the rat

The aims of the present study are to demonstrate cytoarchitectonically columnar structures in the deep layers of the rat's superior colliculus, and to show experimentally the existence of a clear correlation between the cytoarchitectonically defined columnar structures and the discontinuous patterns of the tectal connections in the deep layers. Injections of horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA) into the prefrontal cortex produced orthograde labeling in the columnar structures in the deep layers of the superior colliculus, while HRP-WGA injections into the somatic sensory cortex resulted in orthograde labeling in the areas outside the columnar structures, so that the distribution patterns of terminals from these two different cortical areas are complementary in the deep layers. Cells of origin of the tectal efferents are also differentiated in terms of the columnar structures; HRP-WGA injections into the dorsal medial nucleus of the thalamus yielded retrograde labeling of spindle-cells within the columns, whereas the injections into the trigeminal sensory nuclei produced retrograde labeling of polygonal cells in the areas outside of the columns. These results suggest that as in the dorsoventral laminar coincidence with the tectal connections, there is a well organized mediolateral registration of the tectal connections with the cytoarchitectonically defined cell arrangement in the deep layers of the superior colliculus.

Convergence of afferents from frontal cortex and substantia nigra onto acetylcholinesterase-rich patches of the cat's superior colliculus

The patterns of distribution of frontotectal and nigrotectal fibers were studied with the anterograde horseradish peroxidase method in the cat. Direct serial-section comparisons were made between the afferent-fiber patterns and the compartmentalized arrangements of acetylcholinesterase staining within the intermediate and deep collicular layers. Many of the patches of high acetylcholinesterase activity in the intermediate gray layer proved to be zones in which labeled frontotectal and nigrotectal fibers converged. These acetylcholinesterase-rich patches may thus represent sites at which functional influences from the basal ganglia and frontal cortex are coordinated. In the deeper tiers of the intermediate gray layer and layers ventral to it, there were also zones of heightened and diminished acetylcholinesterase staining. Much of this histochemical patterning was reflected in the arrangement of fibers labeled by large rostromedial frontal injections, but these deeper tiers were not strongly labeled after more lateral frontal injections or after injections placed in the substantia nigra. The deeper parts of the acetylcholinesterase-positive gridwork in the superior colliculus are thus distinct from its upper tier of acetylcholinesterase-positive patches. We conclude that the compartmentalized patterning of dense acetylcholinesterase staining in the intermediate and deep collicular layers represents a mosaic architecture to which collicular afferent circuitry is tightly related. This gridwork may serve to set up functional domains within which different aspects of collicular processing are accommodated.

Vibrissae tactile stimulation: (14C) 2-deoxyglucose uptake in rat brainstem, thalamus, and cortex

The right mystacial vibrissae of awake, adult rats were stroked at 4-6 times/second and brain regions which increased (14C) 2-deoxyglucose (2DG) uptake were mapped autoradiographically. The ventral parts of the ipsilateral spinal trigeminal nuclei pars caudalis (Sp5c), pars interpolaris (Sp5i), pars oralis (Sp5o), and the principal trigeminal sensory (Pr5) nuclei were activated. The lateral part of the ipsilateral facial (VII) nucleus (the region which innervates the vibrissae muscles) was also activated possibly via excitatory, trigeminal (Sp5c, Sp5i, Sp5o, Pr5) sensory afferents. A number of regions were activated contralateral to the sensory stimulus. Discrete patches of (14C) 2DG uptake occurred in deep layers of the superior colliculus (SCsgp). Dorsolateral and dorsomedial parts of the ventrobasal nucleus (VB), and posterior, dorsolateral parts of the reticular nucleus (R) of thalamus were activated, along with broad portions of the primary somatosensory cortex (SI) and second somatosensory cortex (SII). Though all layers of SI and SII cortex increased 2DG uptake, VB thalamic afferents to layers IV and Vc-Vla presumably accounted for the greater activation of these cortical layers during repetitive sensory stimulation of the vibrissae (RSSV). Activation of the above structures fits well with known anatomical data. However, the pattern of activation during RSSV was very different from that previously described during vibrissae motor cortex stimulation (VMIS). RSSV and VMIS both produced similar repetitive movements of all the mystacial vibrissae. However, only a few overlapping brain regions were activated during both RSSV and VMIS. These RSSV-VMIS overlap zones included Sp5o; rostral Sp5i; lateral VII; SCsgp; ventrobasal-posteromedial and ventrobasal-ventrolateral zones in thalamus; and a rostral region of SI probably anterior to the Woolsey vibrissae barrelfield in the dysgranular somatosensory (SI) cortex. Since RSSV and VMIS would both be expected to activate vibrissae proprioceptors, we have hypothesized that vibrissae proprioceptive input was processed in part in the RSSV-VMIS overlap zones. Convergence of motor-sensory inputs and other types of processing could have also occurred in these overlap zones.

The termination of spinomesencephalic fibers in cat. An experimental anatomical study

The projections to the midbrain from the spinal cord have been investigated in the cat with the degeneration technique and by using horseradish peroxidase (HRP) as an anterograde tracer. Two types of spinal cord lesions were performed: 1) Cordotomies at cervical or thoracic levels transecting the ventral and lateral funiculi. 2) Transections of the ventral, ventrolateral, dorsolateral or dorsal funiculus, respectively, at cervical levels. In the anterograde tracing experiments HRP was injected into the spinal cord at cervical, lumbar or sacral levels. The results show large projections to the lateral and ventrolateral parts of the periaqueductal gray (PAG1), the posterior pretectal nucleus (PP) and the nucleus of Darkschewitsch (D). More moderate projections go to the medial division of the periaqueductal gray (PAGm), the cuneiform nucleus (CF), the mesencephalic reticular formation (MRF), lateral part of the deep layer of the superior colliculus (SP) and magnocellular medial geniculate nucleus (GMmc), while scattered spinal fibers are present in the dorsal part of the periaqueductal gray (PAGd), the external inferior collicular nucleus (IX), the intermediate layer of the superior colliculus (SI), the lateral part of the red nucleus (NR) and in the Edinger-Westphal portion of the oculomotor nucleus (3). In addition a few fibers are present in the interstitial nucleus of Cajal (CA) and anterior pretectal nucleus (PAc). The results indicate that at midcervical levels most of the spinomesencephalic fibers ascend in the ventral funiculus, with only a moderate fraction ascending in the ventral half of the lateral funiculus. Almost no fibers ascend in the dorso-lateral funiculus and none appear to pass in the dorsal funiculus. No distinct somatotopic pattern was found in the spinomesencephalic projections, but more fibers from cervicobrachial segments terminate in the rostral than in the caudal parts of the terminal fields in PAG, CF, SP and IX, while the lumbar fibers were more numberous in the caudal parts. PP seems to receive spinal fibers mainly from the caudal half of the cord.

Topographical linkage of tecto-thalamo-anterior ectosylvian sulcal cortex in the cat: an 125I-WGA autoradiographic study

Following injection of 125I-WGA into various parts of the caudal thalamus in the cat, the distribution of orthograde and retrograde labels in the cortex around the anterior ectosylvian sulcus (AESS) and the superior colliculus (SC) was examined autoradiographically. When 125I-WGA injections involved the medial part of nucleus lateralis posterior (Lp) of the thalamus, both orthograde and retrograde labels consistently appeared in the cortex around AESS, and retrograde labels in the SC. The topographical organization of the cortical connections with the medial part of Lp can be well correlated with that of the tecto-thalamic projections, in such a way that the dorsal portion of the medial part of Lp which receives fibers from the rostromedial part of SC is connected reciprocally with the lateral lip of AESS and the crown of the anterior sylvian gyrus; whereas, the most ventral portion of the medial part of Lp which receives tectal afferents from the caudolateral part of SC is connected with the dorsal bank and fundus of AESS. These results suggest the existence of retinotopically ordered linkage between the tecto-Lp and the Lp-AESS connections in the cat.

Cluster-and-sheet pattern of enkephalin-like immunoreactivity in the superior colliculus of the cat

The distribution of enkephalin-like immunoreactivity in the superior colliculus has been studied in the cat with the peroxidase-antiperoxidase method. Two striking patterns of immunoreactivity were observed. In the superficial layers there is a thin, dense horizontal band of immunoreactivity in the neuropil of the most dorsal tier of the superficial gray layer (sublamina 1). Because this sublayer corresponds to the zone of densest contralateral retinotectal projection, an intraocular injection of horseradish peroxidase was made in one cat to allow direct comparison of the distributions of opiate-like immunoreactivity and transported tracer in the contralateral superior colliculus. There was a detailed similarity between the two, including the presence of a gap in both at the presumptive site of the optic disc representation. The presence of enkephalin-like immunoreactivity in neural perikarya in and near sublamina 1 of the superficial gray layer, however, raised the possibility that the immunoreactive band is part of an intrinsic opiate system. Deeper in the superficial gray layer there was appreciable but weaker immunoreactivity in the neuropil and fewer immunoreactive neurons. In the intermediate gray layer and, especially medially, even deeper in the superior colliculus, enkephalin-like immunoreactivity was organized into small (100-300 micron wide) patches. In the intermediate gray layer these tended to be arranged periodically, five-seven patches being spaced at 200-600 micron intervals in caudal transverse sections. In some sections adjoining patches appeared to be fused. The patches were absent or difficult to detect in rostral sections. Caudally, they sometimes were adjacent to blood vessels penetrating the intermediate gray layer, but other times were not. Serial section reconstructions suggested that the patches observed in individual sections are part of larger arrays which have the form of anastomotic bands running in longitudinal directions somewhat oblique to the sagittal plane. It is concluded that an opiate mechanism may play a part in controlling the effects of incoming retinal information in the superficial gray layer, directly or indirectly, and that opiate peptides may also act in modulating one or more afferent or efferent systems of the deep collicular layers. Accordingly, from the functional standpoint, enkephalin-like peptides may influence both visual and sensory motor processing in the superior colliculus.

Studies of the principal sensory and spinal trigeminal nuclei of the rat: projections to the superior colliculus, inferior olive, and cerebellum

We have analyzed the connections between the sensory trigeminal nuclei and two major sensorimotor areas (i.e., the superior colliculus and crura I and II of the cerebellar cortex) in which tactile input from peri-oral and other facial regions is a prominent feature. Following injections of horseradish peroxidase into the superior colliculus, retrogradely labeled cells occupy the ventral one-third of the contralateral principal sensory and spinal trigeminal nucleus; trigeminocollicular neurons are especially numerous within the subnucleus interpolaris (Svi). Injections of either 3H-proline or horseradish peroxidase (HRP) into the Svi reveal that trigeminocollicular axons reach the rostral two-thirds to three-quarters of the contralateral superior colliculus, where they distribute in a nonuniform, patchy manner within layers IV-VI. In addition to demonstrating the trigeminocollicular projection, anterograde and retrograde transport studies of the Svi also reveal a trigeminoolivary projection which terminates primarily within the contralateral rostral dorsal accessory (DAO) and adjacent principal (PO) olives; some of the Svi neurons innervate both the superior colliculus and the DAO-PO via axon collaterals. Data from a final set of retrograde tracing experiments show that the trigeminorecipient zone of the DAO-PO contains neurons which project to crura I and/or II of the cerebellar cortex. Of the various submodalities conveyed by the trigeminal system, it is likely that the trigeminal connections we have demonstrated are carrying tactile information. This is indicated by the fact that responses to tactile stimulation of the face have been reported for cells in (1) the deeper collicular layers, (2) the trigeminorecipient zone of the DAO-PO, and (3) cerebellar targets of this zone, crura I and II. All data are discussed in the context of the anatomical and physiological literature.

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.

Projections of the superior colliculus to the supraspinal nucleus and the cervical spinal cord gray of the cat

Anterograde transport experiments reveal two novel findings regarding the distribution of descending tectofugal axons. First, such axons project to the supraspinal nucleus of the caudal medulla; this nucleus is known to project to the upper cervical spinal cord gray. Second, some tectospinal axons ramify within Rexed's lamina IX of the first 5 cervical spinal cord segments. This zone contains motoneurons which innervate neck musculature. Retrograde data reveal that tectospinal neurons occur in clusters within the intermediate and deep gray layers. A close relationship between the clusters of tectospinal neurons and a modular type of connectional organization of the intermediate and deep gray layers is suggested.

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.

The projection from the nucleus of the posterior commissure to the superior colliculus of the cat: patch-like endings within the intermediate and deep grey layers

The nucleus of the posterior commissure projects to the intermediate (SGI) and deep (SGP) grey layers of the ipsilateral superior colliculus throughout its rostral-caudal dimension. This projection terminates in a patchy, discontinuous, manner. The patches form a tier in the dorsal half of SGI, and are smaller medially than laterally. These results are discussed in relation to other patchy projections to SGI and in relation to a possible modular organization of the superior colliculus.

Trigeminal projections to the superior colliculus of the rat

The deep layers of the rodent superior colliculus contain a vibrissae-related organization that is in "spatial register" with the overlying visuotopic organization (Dräger and Hubel, '76). The distribution of vibrissae-related afferents and their cells of origin were determined with a number of anatomical techniques. The brainstem trigeminal complex afferents to the superior colliculus terminate in the lateral portions of the strata album intermediate and griseum profundum and, to a lesser degree, in deep portions of the stratum griseum intermediate. The cells giving rise to these afferents are located mainly in the ventral portions of the contralateral principal sensory nucleus, subnucleus oralis, and subnucleus interpolaris. The majority of tectal projection cells are found in subnucleus interpolaris, and the fewest in the principal sensory nucleus. Further, the density of projection cells in the three components of the brainstem trigeminal complex can be correlated with the density of their projections to the superior colliculus. The afferents from the somatosensory cortex terminate in a continuous band in the strata album intermediate and griseum intermediate. The cells of origin of this projection are located in layer Vb of the agranular zones of the ipsilateral somatosensory cortex. The present results suggest that the organization of trigeminal afferents to the deep portion of the superior colliculus is similar to that of the visual afferents to the superficial laminae. Further, the results suggest that observations on the nature of afferent termination pattern should be made with care, considering both the techniques employed and the idiosyncrasies of the local neuropil.