Superior colliculus: visuotopic-somatotopic overlap.

A laminar organization was present in the superior colliculus of the cat, with upper layer cells exclusively visual, lower layer cells primarily somatic (or acoustic), and intermediate layers showing significant modality overlap. The close topographic correspondence between the visual and somatic representations observed within this laminar pattern and the similarities in visual and somatic response specificity may be consistent with the hypothesis that the colliculus combines several sensory modalities to facilitate tracking of a given stimulus.

Interactions among converging sensory inputs in the superior colliculus.

The responses of superior colliculus cells to a given sensory stimulus were influenced by the presence or absence of other sensory cues. By pooling sensory inputs, many superior colliculus cells seem to amplify the effects of subtle environmental cues in certain conditions, whereas in others, responses to normally effective stimuli can be blocked. The observations illustrate the dynamic, interactive nature of the multisensory inputs which characterize the deeper laminae of the superior colliculus.

Descending efferents from the superior colliculus relay integrated multisensory information.

By means of their efferent projections to motor and premotor structures, the cells in the deep superior colliculus are intimately involved in behaviors that control the orientation of the eyes, pinnae, and head. These same efferent cells receive multiple sensory inputs, thereby apparently enabling an animal to orient its receptor organs in response to a wide variety of cues. This sensory convergence also provides a system in which motor responses need not be immutably linked to individual stimuli but can vary in reaction to the multitude of stimuli present in the environment at any given moment.

Reptiles and mammals use similar sensory organizations in the midbrain.

Striking similarities were observed between the overlapping visual and tactile maps of the mammalian superior colliculus and of its homolog in reptiles, the optic tectum. This topographic pattern probably represents a plan of sensory representation that existed in ancient reptiles and that was retained during the evolution to mammalian forms more than 180 million years ago.

Superior colliculus: control of eye movements in neonatal kittens.

Activation of the neonatal cat superior colliculus can produce organized eye movements before visual stimuli are capable of activating visual neurons in the colliculus. These findings are consistent with the hypothesis that eye movement development precedes, and is necessary for, visuomotor integration.

Neurofilament proteins are preferentially expressed in descending output neurons of the cat the superior colliculus: a study using SMI-32.

Physiological studies indicate that the output neurons in the multisensory (i.e. intermediate and deep) laminae of the cat superior colliculus receive converging information from widespread regions of the neuraxis, integrate this information, and then relay the product to regions of the brainstem involved in the control of head and eye movements. Yet, an understanding of the neuroanatomy of these converging afferents has been hampered because many terminals contact distal dendrites that are difficult to label with the neurochemical markers generally used to visualize superior colliculus output neurons. Here we show that the SMI-32 antibody, directed at the non-phosphorylated epitopes of high molecular weight neurofilament proteins, is an effective marker for these superior colliculus output neurons. It is also one that can label their distal dendrites. Superior colliculus sections processed for SMI-32 revealed numerous labeled neurons with varying morphologies within the deep laminae. In contrast, few labeled neurons were observed in the superficial laminae. Neurons with large somata in the lateral aspects of the deep superior colliculus were particularly well labeled, and many of their secondary and tertiary dendrites were clearly visible. Injections of the fluorescent biotinylated dextran amine into the pontine reticular formation revealed that approximately 80% of the SMI-32 immunostained neurons also contained retrogradely transported biotinylated dextran amine, indicating that SMI-32 is a common cytoskeletal component expressed in descending output neurons. Superior colliculus output neurons also are known to express the calcium-binding protein parvalbumin, and many SMI-32 immunostained neurons also proved to be parvalbumin immunostained. These studies suggest that SMI-32 can serve as a useful immunohistochemical marker for detailing the somatic and dendritic morphology of superior colliculus output neurons and for facilitating evaluations of their input/output relationships.

Early establishment of adult-like nigrotectal architecture in the neonatal cat: a double-labeling study using carbocyanine dyes.

Virtually nothing is known about the ontogeny of substantia nigra, pars reticulata projections to the midbrain superior colliculus, even though this pathway is critical for the basal ganglia modulation of midbrain-mediated visuomotor behaviors. The present studies used the lipophilic carbocyanine dyes 1,1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate and 1,1'-dioctodecyl-3,3,3',3'-tetramethylindodi, 4-chlorobenzenesulfonate salt to examine the crossed and uncrossed nigrotectal projections in neonatal cats, from parturition to 14 days postnatal (the technical limits of the tracing technique). In retrograde experiments, paired placement of the dyes in each superior colliculus produced numerous retrogradely-labeled nigrotectal neurons, with the uncrossed neurons far out numbering their crossed counterparts. No double-labeled neurons were observed, indicating that crossed and uncrossed nigrotectal neurons are segregated at birth. In anterograde experiments, dye placements into each substantia nigra, pars reticulata resulted in an iterative series of labeled patches, aligned medial-to-lateral across the intermediate and deep superior colliculus, a pattern reminiscent of the adult. Uncrossed neonatal axons had simple linear morphologies with few branch points; by contrast, crossed axons displayed more extensive terminal arbors that were distributed diffusely throughout the rostrocaudal extent of the contralateral superior colliculus In the final series of experiments, one dye was placed unilaterally in the substantia nigra, pars reticulata, while the second dye was positioned in the predorsal bundle, in order to bilaterally label superior colliculus output neurons. Although both crossed and uncrossed axons appeared to have contacted superior colliculus output neurons, crossed axons preferentially targeted the soma and proximal dendrites, whereas uncrossed terminals were distributed more distally. Throughout this early postnatal period, no significant changes in cellular morphologies or gross modification of terminal projection patterns were observed; however, the presence of growth cones in even the oldest animals studied suggests that the refinement of the nigrotectal projections extends well into postnatal life. Nevertheless, the segregation of crossed and uncrossed nigrotectal neurons into a highly organized afferent mosaic that has established synaptic contacts with superior colliculus output neurons indicates that many of the salient features characterizing nigrotectal projections are established prior to the onset of visual experience.

A direct projection from superior colliculus to substantia nigra pars compacta in the cat.

Dopaminergic neurons exhibit a short-latency, phasic response to unexpected, biologically salient stimuli. The midbrain superior colliculus also is sensitive to such stimuli, exhibits sensory responses with latencies reliably less than those of dopaminergic neurons, and, in rat, has been shown to send direct projections to regions of the substantia nigra and ventral tegmental area containing dopaminergic neurons (e.g. pars compacta). Recent electrophysiological and electrochemical evidence also suggests that tectonigral connections may be critical for relaying short-latency (<100 ms) visual information to midbrain dopaminergic neurons. By investigating the tectonigral projection in the cat, the present study sought to establish whether this pathway is a specialization of the rodent, or whether it may be a more general feature of mammalian neuroanatomy. Anterogradely and retrogradely transported anatomical tracers were injected into the superior colliculus and substantia nigra pars compacta, respectively, of adult cats. In the anterograde experiments, abundant fibers and terminals labeled with either biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin were seen in close association with tyrosine hydroxylase-positive (dopaminergic) somata and processes in substantia nigra pars compacta and the ventral tegmental area. In the retrograde experiments, injections of biotinylated dextran amine into substantia nigra produced significant retrograde labeling of tectonigral neurons of origin in the intermediate and deep layers of the ipsilateral superior colliculus. Approximately half of these biotinylated dextran amine-labeled neurons were, in each case, shown to be immunopositive for the calcium binding proteins, parvalbumin or calbindin. Significantly, virtually no retrogradely labeled neurons were found either in the superficial layers of the superior colliculus or among the large tecto-reticulospinal output neurons. Taken in conjunction with recent data in the rat, the results of this study suggest that the tectonigral projection may be a common feature of mammalian midbrain architecture. As such, it may represent an additional route by which short-latency sensory information can influence basal ganglia function.

Sensory and multisensory responses in the newborn monkey superior colliculus.

Superior colliculus (SC) neurons have the ability to synthesize information from different sensory modalities, resulting in enhancements (or depressions) of their activity. This physiological capacity is, in turn, closely tied to changes in overt attentive and orientation responses. The present study shows that, in contrast to more altricial species, many deep layer SC neurons in the rhesus monkey are multisensory at birth. Such neurons can respond to stimuli from different sensory modalities, and all convergence patterns seen in the adult are represented. Nevertheless, these neurons cannot yet synthesize their multisensory inputs. Rather, they respond to combinations of cross-modal stimuli much like they respond to their individual modality-specific components. This immature property of multisensory neurons is in contrast to many of the surprisingly sophisticated modality-specific response properties of these neurons and of their modality-specific neighbors. Thus, although deep SC neurons in the newborn have longer latencies and larger receptive fields than their adult counterparts, they are already highly active and are distributed in the typical adult admixture of visual, auditory, somatosensory, and multisensory neurons. Furthermore, the receptive fields of these neurons are already ordered into well organized topographic maps, and the different receptive fields of the same multisensory neurons show a good degree of cross-modal spatial register. These data, coupled with those from cat, suggest that the capacity to synthesize multisensory information does not simply appear in SC neurons at a prescribed maturational stage but rather develops only after substantial experience with cross-modal cues.

Receptive field properties of somatosensory neurons in the cat superior colliculus.

In general, knowledge of the internal organization of receptive fields has played an important role in shaping current understanding of sensory physiology. Such knowledge is particularly important for understanding the function of the superior colliculus, since this structure is at once implicated in spatial localization and has relatively large receptive fields. While this issue has been addressed in the visual and auditory modalities represented in the superior colliculus, there are no previous studies of its somatosensory receptive field organization. Here, the properties of somatosensory receptive fields in the cat superior colliculus were studied quantitatively to determine whether they contain internal non-homogeneities that might aid in the determination of stimulus detail. Of special interest was the possibility that these comparatively large receptive fields would contain areas of differential excitability that could aid in spatial resolution, that within-field spatial summation and/or inhibition would be exhibited, and that the borders of the excitatory receptive field would be flanked by inhibitory regions. The data demonstrate that while inhibition beyond the receptive field borders is a rarity, these somatosensory receptive fields nearly always contain a well-defined area of maximal sensitivity within which the size of the stimulus is a critical feature in determining the magnitude of the response. These best areas are systematically distributed across receptive fields as a function of their location in the structure, and indicate that the resolution of stimulus location and size may be greater than expected on the basis of receptive field size alone.

Properties of superior colliculus neurons in the golden hamster.

A laminar representation of sensory modalities was present in the hamster superior colliculus (SC) with upper laminar cells being exclusively visual, while intermediate and deeper layer cells were visual, somatic, acoustic or multimodal. Receptive field (RF) properties of visual SC cells were studied quantitatively with stationary spots and moving bars of light of various sizes, velocities and directions of movement. The most effective stimuli were usually less than half of the diameter of the RF. Increasing stimulus size beyond a critical range produced progressively lower discharge rates even though the stimulus was confined within the borders of the RF. Low velocities (10-50 degrees/sec) were most frequently optimal for both upper and lower laminar cells and the majority of cells were directionally selective. Movements in the upper nasal direction usually elicited the highest discharge rates and were thus preferred most frequently. However, directional preferences often could be minimized, or obliterated, by employing nonoptimal stimulus sizes and/or velocities. Most intermediate and deeper laminar somatic cells could be activated by gentle cutaneous stimuli and a general somatotopic plan, which was in register with the overlying visuotopy, was noted. Cells optimally, or solely, activated by noxious stimuli were also located, but RFs of such cells were extensive and a somatotopic plan was not apparent. Although species differences are apparent, the similarities between the organization and the RF properties of SC cells of the hamster and distantly related species are striking. Apparently the same SC system is adaptive in diverse species despite the very different behavioral repertories of these animals and their different ecological niches.

Sensory representation in reptilian optic tectum: some comparisons with mammals.

The sensory representations in the tectum of Iguana iguana were studied with electrophysiological recording techniques, and visual, somatic, and auditory cells were found to be represented here. These cells were not equally distributed throughout the tectal laminae. Upper tectal laminae were populated exclusively by visual cells, and deeper laminae were primarily nonvisual. The intermediate laminae had nonvisual, as well as visual, cells. Maps of the visual and somatic representations were constructed, and both representations were topographic and in register with no another. When electrical stimulation was presented via implanted electrodes, orientation responses were evoked that were predictable on the basis of the visuotopic and somatotopic maps. The organizational features of the iguana tectum are strikingly similar to those described in various mammalian species. It is suggested that the pattern of sensory and motor representation used in the midbrain of mammals is an ancient scheme that was retained during the transition from reptilian to mammalian forms more than 180 million years ago.

Transient projections from the lateral geniculate to the posteromedial lateral suprasylvian visual cortex in kittens.

The postnatal maturation of the projection from the lateral geniculate nucleus to the posteromedial lateral suprasylvian visual cortex (PMLS) was studied with injections of fluorescent dyes into the PMLS at various postnatal ages. Labeled neurons projecting to the PMLS were present in all laminae of the ipsilateral lateral geniculate on the day of birth. However, there was a conspicuous change in the distribution of labeled geniculo-PMLS neurons by 11 days of age: now very few labeled neurons were present in lamina A, indicating a loss of geniculo-PMLS connections. The loss of connections began at the peripheral margins of lamina A and proceeded through other laminae toward laminae C1-3. By adulthood, labeled geniculo-PMLS neurons were largely confined to laminae C1-3; they were never observed in lamina A or A1 and were rarely observed in lamina C. To determine whether the lateral geniculate neurons survived after their projections to PMLS were lost, injections of fast blue were made at 1 or 2 days postnatally and the animals were allowed long postinjection survival times. Labeled neurons were found in all lateral geniculate laminae, thereby indicating that for many neurons the loss of connections could be attributed to a loss of their axon collaterals rather than to the death of the neurons themselves. After injections of fast blue into the PMLS and diamidino yellow dihydrochloride into area 17 shortly after birth, many double-labeled neurons were present in all laminae, indicating that they have collaterals to both targets. Thus, the survival of many of the geniculo-PMLS neurons contributing to the transient geniculo-PMLS projection seems to be due to sustaining collateral projections to area 17 or other cortical targets.

Small lateral suprasylvian cortex lesions produce visual neglect and decreased visual activity in the superior colliculus.

Previous experiments in cats have shown that complete contralateral visual neglect is produced by removing all known visual cortex on one side of the brain, which can then be reversed by damaging the opposite superior colliculus. Presumably, descending facilitatory influences from the visual cortex to the ipsilateral superior colliculus are counterbalanced by intercollicular inhibition (Sprague: Science 153:1544-1546, '66). However, not all of visual cortex or all of the superior colliculus needs to be involved in this circuit. It is the deep rather than the superficial laminae of the superior colliculus that are primarily involved in visual attentive and orientation behaviors, and these laminae are largely independent of primary visual cortex. However, they do depend on corticotectal influences from a comparatively small extraprimary visual area of the posterior region of the lateral suprasylvian cortex (PSSC-Ogasawara et al: J. Neurophysiol. 52:1226-1245, '84). The present experiments demonstrate that lesions only a few millimeters in diameter in this corticotectal zone of the PSSC can produce profound visual neglect. While damage to this area has little, if any, effect on superficial laminae visual activity, it produces a dramatic decrease in the visual activity of the deep laminae. These cats with PSSC lesions fail to orient to a visual stimulus that is introduced suddenly into the contralateral visual field, yet they respond on nearly 100% of the trials to this same stimulus when it is presented in the ipsilateral visual field. The lesion-induced visual neglect produced by PSSC lesions is long-lasting but can be abruptly ameliorated by a midbrain lesion that primarily involves, or undercuts, the deep laminae of the contralateral superior colliculus. Thus, 1) visual neglect can be produced by depriving the deep laminae of the superior colliculus of visual inputs from the cortex, even when the principal visual cortical regions (17, 18, and 19) and their target structures are intact, and 2) visually guided behavior can be restored by eliminating afferents originating in, or passing through, the deep laminae of the contralateral superior colliculus.

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.

Somatotopic component of the multisensory map in the deep laminae of the cat superior colliculus.

The topographic organization of the somatosensory representation in the deep layers of the cat superior colliculus was reexamined using methods previously used to examine the visuotopy in these layers. This technique identified the distribution of neurons in the superior colliculus that represent a designated region of the body surface (i.e., a dermal image), as well as assessed the differential distribution of deep layer neurons representing different body regions (e.g., face, forelimb, hindlimb, etc.). When the area of densest representation within a dermal image was considered, a well-ordered somatotopy was evident that was similar to the one previously described (Stein et al., '76: J. Neurophysiol. 39:401-419). Each region of the body surface, however, was represented within a surprisingly broad area of the deep layers, which often had considerable overlap with the representations of adjacent body regions. This organization was similar to that of the deep layer visuotopy and emphasizes that the representation of a peripheral stimulus is accomplished by the simultaneous activation of a large population of deep layer neurons. Furthermore, an examination of the convergence patterns on somatosensory-responsive neurons demonstrated that the somatotopy was formed primarily by multisensory neurons. These data indicate that the somatosensory representation is best considered as a component of a comprehensive multisensory functional unit that plays a critical role in effecting behavioral responses to a wide variety of stimuli.

Response properties of nociceptive and low-threshold neurons in rat trigeminal pars caudalis.

There is little doubt that trigeminal nociceptive neurons play a critical role in signaling the presence of harmful, or potentially harmful, orofacial stimuli. Unfortunately, there is only a limited understanding of how these neurons code such stimuli and whether this code is maintained in those structures responsible for generating overt reactions. The present series of experiments were designed to quantitatively document the response properties of nociceptive neurons in the rat trigeminal pars caudalis using the same electrical and innocuous and/or noxious mechanical and thermal stimuli employed in the characterization of nociceptive neurons with orofacial receptive fields in the rat superior colliculus. Neurons were classified as either low-threshold mechanoreceptive, wide-dynamic-range, or nociceptive-specific (type I, II) depending on their responsiveness to these stimuli. Nociceptive pars caudalis neurons (92/135, 68%) had receptive field organizations and input fibers (as indicated by latencies to electrical stimuli) quite different from those of low-threshold neurons (43/135, 32%). Nociceptive stimulus-response relationships for the population of wide-dynamic-range and nociceptive-specific type I neurons to contact heat stimuli were positively accelerating power functions with exponents of 3.9 and 4.4, respectively. This contrasted sharply with the low-threshold component of wide-dynamic-range neurons which was a negatively accelerating power function with an exponent of 0.7. All categories of nociceptive neurons also responded vigorously to cold stimuli. The thresholds of both hot and cold stimuli were often below psychophysical estimates of thermal pain, suggesting that "nociceptive" neurons process far more information than that required to signal potentially harmful stimuli. The fundamental similarities in nociceptive properties in pars caudalis and other structures of the central nervous system suggest that there is little transformation of the information encoded at successive levels of the neuraxis. This is consistent with the idea that the functional role of nociceptive neurons is reflected more in which circuits they are integrated and less in differences in their physiological properties.

The organization of trigeminotectal and trigeminothalamic neurons in rodents: a double-labeling study with fluorescent dyes.

Retrogradely transported fluorescent dyes (fast blue and diamidino-dihydrochloride yellow) were used to compare the distributions of trigeminofugal neurons that project to the superior colliculus and/or the thalamus in three rodent species. The objective was to determine what the projection and collateralization patterns of these trigeminofugal pathways are and whether they are similar among different species. In each anesthetized animal, one dye was injected into the superior colliculus and the other into the topographically congruent area of the thalamus. Counts of the numbers of yellow, blue, and double-labeled neurons were made throughout the trigeminal complex: principalis, pars oralis, pars interpolaris, and pars caudalis. Trigeminothalamic projections were similar in each of the rodent species studied. The densest concentration of retrogradely labeled neurons was in principalis, with substantially fewer neurons in pars interpolaris, and fewer still in pars oralis and pars caudalis. These neurons were generally small and tended to have round or fusiform somata. A common pattern was also noted among the three species for trigeminotectal neurons. Most trigeminotectal projections originated from neurons in pars interpolaris, somewhat fewer from pars oralis, and the fewest from principalis and pars caudalis. These neurons tended to be the largest in each subdivision and were often multipolar. Following paired injections of the tracers, double-labeled neurons were scattered throughout the sensory trigeminal complex and had morphologies characteristic of single-labeled trigeminotectal neurons. Although comparatively few double-labeled neurons were observed in any species, most of those seen were restricted to the ventrolateral portion of pars interpolaris, a position that corresponds to the representation of the vibrissae. These data indicate that, regardless of the rodent species, the vast majority of labeled trigeminal neurons project either to the superior colliculus or the thalamus, but not to both targets. This might be expected on the basis of the very different behavioral roles these structures play. On the other hand, a subpopulation of trigeminal neurons exists (mainly in pars interpolaris) that does project to both the superior colliculus and the thalamus, perhaps because both structures require some of the same somatosensory information to perform their behavioral functions.

Postnatal development of acetylcholinesterase in, and cholinergic projections to, the cat superior colliculus.

The postnatal development of cholinergic afferents to the superior colliculus in neonatal cats was studied by using acetylcholinesterase (AChE) histochemistry, choline acetyltransferase (ChAT) immunohistochemistry, and retrograde transport of horseradish peroxidase (HRP). In the adult cat, the pattern of AChE staining was laminar specific. AChE was distributed continuously in the stratum griseum superficiale (SGS) but was organized as patches in the stratum griseum intermediate (SGI). Diffuse AChE staining also was present in the stratum griseum profundum (SGP) and the dorsolateral periaqueductal gray (PAG). At birth, however, AChE staining was barely detectable in the SGS and, aside from a few isolated labeled neurons, was absent from the SGI, SGP, and PAG. By 7 days postnatal (dpn), staining in the SGS was more apparent but did not change appreciably in the deeper laminae. A substantial increase in AChE staining occurred in the SGS at 14 dpn (several days after eye opening), at which time patches in the SGI first became apparent. By 28 dpn, the complete laminar-specific adult AChE staining pattern was present, though the staining intensity did not reach the adult level until 56 dpn. A protracted maturation of both AChE staining and ChAT immunoreactivity also was observed in the sources of cholinergic afferents to the superior colliculus, which include the parabigeminal nucleus, and the pedunculopontine (PPN) and lateral dorsal tegmental (LDTN) nuclei. AChE and ChAT-immunoreactive staining in each nucleus was weak at birth but increased during the ensuing 2 weeks. At 21 dpn, however, ChAT immunoreactivity virtually disappeared in the parabigeminal nucleus and significantly decreased in PPN and LDTN. The ChAT immunoreactivity in these nuclei then gradually increased reaching maximum levels by 28 dpn. At 35 dpn, AChE staining showed a significant, though temporary (4 weeks), decrease in the parabigeminal nucleus, but not in the PPN and LDTN, that subsequently increased to the adult level of staining at 70 dpn. The absence of AChE in the SGI in neonatal animals was correlated, at least in part, with a paucity of neurons in the brainstem cholinergic cell groups labeled by retrograde transport of HRP from the superior colliculus. Injections of HRP into the superior colliculus retrogradely labeled many neurons in the parabigeminal nucleus, but few, if any, neurons in the PPN or LDTN at 1 dpn. Retrogradely labeled neurons also were observed in the substantia nigra pars reticulata, albeit fewer in neonates than in adults.(ABSTRACT TRUNCATED AT 400 WORDS)

Corticothalamic and corticotectal somatosensory projections from the anterior ectosylvian sulcus (SIV cortex) in neonatal cats: an anatomical demonstration with HRP and 3H-leucine.

Corticothalamic and corticotectal projections from the anterior ectosylvian sulcus (AES) in neonatal cats were studied with anterograde and retrograde neuroanatomical techniques. When the injection site was relatively restricted to the sulcal walls and fundus of the rostral AES (i.e., the SIV cortex), heavy ipsilateral thalamic label was observed in the medial subdivision of the posterior group, in the suprageniculate nucleus, and in the external medullary lamina. No terminal label was seen in the contralateral thalamus although the contralateral homotopic cortex was heavily labeled. Within the ventrobasal complex (VB), dense axonal label was observed in fascicles that traversed VB, but only light terminal label was observed within VB itself. However, in cases where the tracer spread into adjacent SII, terminal label in VB was pronounced. Similarly, when the injection site extended into auditory cortex, terminal label was observed in the lateral and intermediate subdivisions of the posterior group. Rostral AES injections produced distinct, predominantly ipsilateral, terminal label in the superior colliculus that was distributed in two tiers: a discontinuous band in the stratum griseum intermedium and a more diffuse band in stratum griseum profundum. Caudally, dense terminal label was seen in the intercollicular zone and dorsolateral periaqueductal gray. When the injection site did not include rostral AES, no label was observed in the superior colliculus. Horseradish peroxidase injections into the superior colliculus of neonates produced retrogradely labeled neurons throughout the AES, but none was found on the crown of the gyrus where SII is located. Thus, the neonatal corticotectal somatosensory projection arises exclusively from AES and parallels that found in adults. These data indicate that the elaboration of a major descending somatosensory pathway from AES to the thalamus and midbrain is largely a prenatal event. The in utero anatomical maturation of the corticofugal projections from SIV cortex to the superior colliculus contrasts with the protracted postnatal development of the corticotrigeminal projections from SI cortex but is consistent with the mature anatomical state of ascending trigeminotectal projections.