Basal Optic Nucleus Research Articles

Visual-response properties of neurons in turtle basal optic nucleus in vitro

1. The spike responses of 105 cells to visual-stimulus movement were analyzed in the turtle's basal optic nucleus (BON) in vitro in the absence of the telencephalon. All cells were direction sensitive (DS) and were driven solely by stimulation of the contralateral eye. These cells had large receptive fields and had vigorous responses to moving, textured patterns. Small moving spots generated only weak responses from these cells, as did the onset or offset of diffuse light flashes. 2. The direction tuning of BON cells was quite broad with most back and forth responses being DS. In fact, for 86% of the cells, there were seven to nine axes (out of 9 total, in 20 degrees increments) for which response to movement in one direction was at least twice that for the opposite direction. In instances where spontaneous activity was relatively high, a suppression of that spike firing was evident when the stimulus moved in directions opposite to preferred stimulus directions. 3. Cells preferring many different directions are found in the BON. More cells preferred inferior-temporal directed motion (49%), compared to superior-temporal (35%) and nasal stimuli (13%). 4. BON cells remained DS over 3 log units of velocity, with their strongest responses between 1 and 50 degrees/s. Responses were often non-DS for stimuli moving slower than 0.1 degrees/s. 5. The receptive fields of BON cells were large and occupied different parts of the retina. When different subregions of a receptive field were stimulated, the cell's directional tuning always remained the same as the full field direction tuning. 6. Thus, BON cells seem well-suited for the analysis of global, visual-field motion in any direction, performed by the accessory optic system. Other brain stem pathways necessary for optokinetic reflexes can be elucidated with the use of this whole-brain, eyes-attached in vitro preparation.

Somatomotor connectivity in the midbrain of Rana pipiens.

The mesencephalic oculomotor nuclei of Rana pipiens and their surrounding cell groups were investigated using the anterograde and retrograde transport of horseradish peroxidase and Golgi techniques. The cell groups surrounding the oculomotor and trochlear nuclei were divided into the nucleus interstitialis (nInt) groups A, B, and C, the basal optic nucleus, and the nucleus reticularis tegmenti. Afferents to the ventral mesencephalon originate from the retina and from vestibular, cerebellar, visual, and accessory oculomotor nuclei. These afferents produce a sequence of terminal arborizations in which visual afferents are found in the outer neuropil, and accessory oculomotor, vestibular, and cerebellar afferents are found along the inner neuropil and central gray. The oculomotor neurons in anurans have extensive dendritic fields, extending to the outer margins of the neuropils, as do many large cells along the margin of nInt. Other neurons in nInt have dendritic fields restricted to the proximal portions of the neuropil. Efferents from nInt area A project to the cerebellum and bilaterally to the spinal cord. Area B nInt projects to the ipsilateral spinal cord, contralateral nInt, pretectal nucleus lentiformis mesencephali, and ipsilateral trochlear nucleus. Efferents from area C nInt reach the deep tectal layers and ipsilateral spinal cord. The outer portions of the neuropil contain the nucleus of the basal optic root which comprises ganglionic elongate and stellate neurons and projects to the pretectum. In the center of the neuropil peri-nBOR neurons have dendrites directed towards the visual terminal fields and axons towards the central gray and oculomotor neurons. The nucleus reticularis tegmenti receives afferents from the tectum and lateral forebrain bundle and projects to the deep tectal layers. In anurans, the oculomotor neurons receive a variety of visual, somatic, and vestibular afferents and appear relatively undifferentiated, whereas the nInt appears more developed.

Distribution of serotonin immunoreactivity in the forebrain and midbrain of the lizard Gekko gecko.

The distribution of serotonin (5-hydroxytryptamine, 5-HT) in the forebrain and midbrain of the lizard Gekko gecko was studied by means of antibodies against serotonin. In the diencephalon, serotonin-immunoreactive (5-HTi) cell bodies were found in the hypothalamic periventricular organ and the ependymal wall of the infundibular recess. In the midbrain, 5-HTi cells were observed in the nucleus raphes superior and the lateral portion of the nucleus reticularis superior. In addition, 5-HTi cell bodies were found lateral to the ventral interpeduncular nucleus and around the ventral aspect of the medial longitudinal fasciculus. Serotonin-immunoreactive fibers and varicosities are present throughout the forebrain and the midbrain, but particularly in the nucleus accumbens, the septal area, the dorsal cortex, the dorsal thalamus, the lateral geniculate body, the ventromedial hypothalamic nucleus, the pretectal nucleus, and the basal optic nucleus. The medial habenular nucleus contains a dense 5-HTi plexus that shows a patchlike pattern. A laminar organization of 5-HTi fibers and varicosities is present in the midbrain tectum. When compared with data obtained in other vertebrates, the present study has confirmed that in the phylogenetic series fishes-amphibians-reptiles-birds-mammals there appears to be (1) a gradual decrease in the number of cerebrospinal-fluid-contacting serotoninergic cells in the hypothalamic periventricular layer and (2) a remarkable increase in number of serotoninergic cells in the midbrain tegmentum. As in mammals, a strong serotoninergic innervation of structures related to sensory, in particular visual, pathways could be recognized.

Specific cell surface labels in the visual centers of Xenopus laevis tadpole identified using monoclonal antibodies

Monoclonal antibodies (MAbs) against the optic tectum of Xenopus tadpoles were generated and screened by the immunofluorescent staining of frozen sections of tadpole brains. MAb-A5 stains the 8th and 9th plexiform layers of the optic tectum, whereas MAb-B2 stains all but the eighth and ninth plexiform layers of the optic tectum. MAb-A5 antigen is also detectable in the nucleus of Belonci, the corpus geniculatum thalamicum, the pretectal area, and the basal optic nucleus, all targets of the optic nerve, but is not detectable in the optic nerve or the optic tract. On the other hand, MAb-B2 does not stain any of these visual centers, though many fibers surrounding them are stained. Eyeenucleation experiments showed that MAb-A5 antigen is expressed in the optic tectum even when it is not innervated by optic nerves. Staining of viable brains with these MAbs indicates that these antigens are cell surface molecules. Immunoadsorption followed by SDS-PAGE suggests that proteins are constituents of these antigens. The MAb-A5 antigen in the diencephalon and the mesencephalon is not detectable at stage 35 36 , but is detectable at stage 39 when the optic nerves begin to innervate the optic tectum. The spatial as well as the temporal patterns of the expression of the MAb-A5 antigen suggest that this molecule may be involved in the target recognition of optic nerve fibers.

Cholinergic innervation of the optic tectum in the frog Rana pipiens

An immunohistochemical method for choline acetyltransferase (ChAT) identifies presumably cholinergic axons in two retino-receptive laminae in the optic tectum of the frog Rana pipiens. Following eye enucleation there is no loss of immunoreactive axons in the optic tectum. Following unilateral ablation of the nucleus isthmi there is a near-total loss of ChAT-positive axons in the superficial cholinergic lamina contralaterally and in the deeper cholinergic lamina ipsilaterally. Thus, the cholinergic innervation of the tectum appears to derive from the nucleus isthmi. However, ChAT-positive staining of the basal optic nucleus does depend upon an intact retinal input and could derive from either retinal axons or some system trophically dependent on them.

Retinofugal and retinopetal projections in the cichlid fish Astronotus ocellatus.

The retinofugal and retinopetal projections of the cichlid fish Astronotus ocellatus were studied by applying cobaltous-lysine to the optic nerve. Retinal axons terminate bilaterally in a preoptic-suprachiasmatic region between the base of the third ventricle and the anterior genu of the horizontal commissure and among periventricular cells along the sides of the ventricle. Other retinal axons innervate the tuberal region of the hypothalamus anterior to the infundibulum. Targets innervated in the pretectum include nucleus lateralis geniculatus and dorsal, medial, and ventral pretectal nuclei. Three other targets (nucleus opticus dorsolateralis, nucleus opticus commissurae posterior, nucleus opticus ventrolateralis) are innervated by fibers that leave the medial edge of the dorsal optic tract. Two other targets (basal optic nucleus and accessory optic nucleus) are innervated by fibers from the ventral optic tract. These retinal projections are similar to those previously reported for goldfish in an experiment that used the cobaltous-lysine method (Springer and Gaffney, J. Comp. Neurol. 203:401-424, '81). Retinotectal optic axons were found in a superficial lamina just above the stratum opticum, in the stratum opticum, in three layers of the stratum fibrosum et griseum superficiale, in a lamina just beneath the stratum fibrosum et griseum superficiale, and in the stratum album centrale just above the stratum periventriculare. This result is similar to that previously reported for goldfish; however, the spatial relationships between the various retinorecipient laminae differ for goldfish and Astronotus ocellatus. Efferents to the retina originate in two nuclei and both project contralaterally. The first is the nucleus olfactoretinalis, which is located ventrally between the olfactory lobe and telencephalon. It consists of about 400 cells, of which, approximately 200 cells project to the retina. The second retinopetal nucleus, nucleus thalamoretinalis, is a diffuse group of about 200 cells that project to the retina.

The organization of retinal projections to the diencephalon and pretectum in the cichlid fish, Haplochromis burtoni.

The organization of retinofugal projections was studied in a cichlid fish by labelling small groups of retinal ganglion cell axons with either horseradish peroxidase or cobaltous lysine. Two major findings resulted from these experiments. First, optic tract axons show a greater degree of pathway diversity than was previously appreciated, and this pathway diversity is related to the target nuclei of groups of axons. The most striking example is the formation of the medial optic tract. Fibers that will become the medial optic tract move abruptly away from their neighbors, at about the level of the optic chiasm, and coalesce at the dorsomedial edge of the marginal optic tract. The medial optic tract projects to the thalamus, the dorsal pretectum, and the deep layer of the optic tectum. The axial optic tract is a group of fibers which segregates from the most medial portion of the marginal optic tract, at about the level of the optic chiasm. The axial tract stays medial to the marginal optic tract for a few hundred microns and then curves laterally to rejoin the marginal optic tract. At least some axial trat axons terminate in the suprachiasmatic nucleus. Within the marginal optic tract, retinal ganglion cell axons from a given retinal quadrant are always segregated into at least two groups. The smaller group projects to the superficial pretectal nucleus. The larger group projects to the superficial layer of the optic tectum. Second, each nontectal retinal termination site receives a unique pattern of retinal input. Within the pretectum the parvocellular superficial pretectal nucleus receives a highly retinotopically organized input from all retinal regions; the basal optic nucleus receives a roughly retinotopically organized input from all retinal regions; the dorsal pretectum receives an input from all retinal regions; and the central pretectal nucleus receives input only from the ventral hemiretina. Within the diencephalon the thalamus receives an input from all retinal regions, but this input is not retinotopically organized; the suprachiasmatic nucleus receives input from the region of central retina that lies just dorsal to the optic nerve head, via the axial optic tract. The accessory optic nucleus receives input from the dorsal hemiretina.

Basic optokinetic-ocular reflex pathways in the frog

Frogs (Rana temporaria) have two midbrain nuclei that receive contralateral retinal afferents, and whose neurons respond to optokinetic stimulation. The basal optic nucleus is composed of direction-selective neurons with different response types. One type is activated exclusively by upward moving optokinetic targets; another type is activated only by downward moving targets. Two other types of basal optic neurons show this vertical preference, but each is also activated by patterns moved horizontally from the nasal to temporal visual field. No activation of these cells was found with patterns moved horizontally from the temporal to nasal visual fields. Rather, cells in a discrete pretectal region have this type of sensitivity: they increase their resting rate with temporal to nasal stimulation and decrease it with nasotemporal stimulation. Oculomotor neurons (antidromically identified) have similar optokinetic sensitivities. As with basal optic neurons, these cells have exclusively upward or downward sensitivity, and some also have nasotemporal sensitivity. An additional type of oculomotor neuron and abducens motoneurons are activated by temporonasal pattern movement. In general, the extraocular motoneurons have similar velocity and pattern size preferences, as have the sensory nuclei. Investigations of the connectivity between the sensory and motor nuclei were primarily restricted to the relation between the pretectum and the abducens. A monosynaptic connection between the pretectum and the abducens is suggested by four points: (1) excitatory postsynaptic potential onset latency in antidromically identified abducens motoneurons, following optic nerve stimulation, is consistent with the interpretation of a disynaptic pathway to the abducens from the retina; (2) pretectal cells, sensitive to optokinetic stimulation, can be activated antidromically from stimulation of the abducens nucleus; (3) horseradish peroxidase injections into the pretectum result in labeling of axons, which terminate in the abducens nucleus; (4) horseradish peroxidase injections into the abducens result in labeling of cells in the pretectal region, where optokinetically sensitive cells are found. In the frog, there seem to be three-neuronal retino-ocular reflexes mediating optokinetic slow phase behavior as there are three-neuronal vestibulo-ocular reflexes that also mediate compensatory spatial behavior. It is suggested that these direct connections act to initiate ocular movements and accelerate the eye, whereas more indirect pathways may act to maintain eye position.

Transection of the basal optic root in the frog abolishes vertical optokinetic head-nystagmus

Bilateral disruption, or one-sided transection of the basal optic root with a partial injury of this tract of retinal fibres on the other side, abolished vertical, but spared horizontal optokinetic head-nystagmus. Horizontal nystagmus could not be evoked when the ventral part of the pretectum was injured and the basal optic root was transected on the same side. The lesions were checked by the cobalt-filling technique. It is concluded that the basal optic nucleus mediates stimuli which evoke vertical optokinetic head-nystagmus in frogs.

Retinal afferents and efferents of an infrared sensitive snake, Crotalus viridis

The retinal afferents and efferents were examined in Crotalus viridis. Retinofugal fibers were traced by injecting horseradish peroxidase (HRP) or tritiated leucine into the eye, or by removing the eye and staining degenerating axons with silver methods. Terminations were seen contralaterally in the suprachiasmatic nucleus, the dorsal and ventral lateral geniculate nuclei (extensive), the pretectal nuclei, including the nucleus posterodorsalis (a very heavy input), the nucleus lentiformis mesencephali, nucleus geniculatus pretectalis, and nucleus pretectalis, the superficial layers of the optic tectum, including the stratum zonale, the stratum opticum, the stratum griseum et fibrosum centrale and the upper portion of stratum griseum centrale, and the basal optic nucleus. Ipsilateral input reaches the intermediate portion of the dorsal lateral geniculate nucleus, a small portion of the pretectal nucleus and nucleus posterodorsalis, and the basal optic nucleus (very minimally). Retinopedal fibers were traced with the HRP method. The cell bodies lie in the ventral thalamus within the nucleus of the ventral supraoptic decussation. These neurons project primarily to the contralateral retina, but some more rostrally located neurons project to the ipsilateral retina.

Primary retinal targets in the Atlantic loggerhead sea turtle, Caretta caretta.

Autoradiographic analysis distinguished twelve primary retinal targets in the diencephalon and the mesencephalon of the Atlantic loggerhead sea turtle, Caretta caretta. While the majority of fibers terminate contralaterally, sparse labelling is seen over ipsilateral thalamic nuclei. The dorsal optic nucleus is the most expansive retinal target in the dorsal thalamus. Four nuclei ventral and one dorsal, to the dorsal optic nucleus, receive retinal input. Before terminating in the optic tectum, labelled fibers pass through the pretectum terminating in four nuclei. Within the superficial zone of the optic tectum, three terminal zones are recognized. A distinct accessory tegmental tract separates from the main optic tract terminating in the basal optic nucleus. While such a multiplicity of retinal targets occurs among other reptiles, birds and mammals, it is presently impossible to accurately recognize visual homologies among amniotic vertebrates.

Retinal recipient nuclei in the painted turtle, Chrysemys picta: an autoradiographic and HRP study.

Retinofugal pathways in the painted turtle were examined with autoradiographic and HRP methods. The majority of the retinal fibers decussate at the optic chiasm and course caudally to terminate in 12 regions of the diencephalon and mesencephalon. The pars dorsalis of the lateral geniculate nucleus is the densest target in the thalamus. Two nuclei dorsal to pars dorsalis--the dorsal optic and dorsal central nuclei--receive optic input. Three nuclei ventral to pars dorsalis and retinal targets--the ventral geniculate nucleus, nucleus ventrolateralis pars dorsalis, and nucleus ventrolateralis pars ventralis. Contralateral fibers course through the pretectum where they terminate in nucleus geniculatis pretectalis, nucleus lentiformis mesencephali, nucleus posterodorsalis, and the external pretectal nucleus. Retinal fibers also terminate within the superficial zone of the optic tectum. HRP material demonstrates three optic fiber layers--laminae 9, 12, and 14. Optic fibers leave the main optic tract as a distinct accessory tegmental optic pathway and terminate in the basal optic nucleus. Ipsilateral retinal terminals occur in a pars dorsalis and a pars ventralis of the lateral geniculate nucleus, the dorsal optic nucleus, nucleus posterodorsalis, the basal optic nucleus, and in laminae 9 and 12 of the optic tectum. Rostrally, the ipsilateral tectal fibers occupy two zones along the medial and lateral tectal roof; these zones converge caudally and are continuous along the caudal wall of the tectum.

Two visual pathways to the telencephalon in the nurse shark (Ginglymostoma cirratum). I. Retinal projections.

The central projections of the retina in the nurse shark were studied by anterograde transport of horseradish peroxidase and tritiated proline. With regard to efferent retinal fibers, both techniques gave completely identical results. Projections were found to pretectal area, dorsal thalamus, basal optic nucleus, and optic tectum, all at the contralateral side. The retinal target cells in the dorsal thalamus are restricted to the ventrolateral optic nucleus and the posterior optic nucleus. No evidence was found for an earlier-reported projection to the lateral geniculate nucleus. The present findings show that the ventrolateral optic nucleus exhibits homological features of the dorsal lateral geniculate nucleus in other vertebrate groups, whereas the lateral geniculate nucleus of the nurse shark is much more comparable to the nucleus rotundus of teleosts and birds and would be more appropriately so named. The application of the HRP technique also allowed us to study afferents to the retina by retrograde transport of tracer. Retrogradely labeled cells were observed in the contralateral optic tectum and are apparently similar to those reported for teleosts and birds.

Anomalous ipsilateral optic fibre projection in Xenopus induced by larval tectal ablation

Unilateral left tectal ablation was carried out in Xenopus between stage 48 and 1 month after metamorphosis. Six to 12 weeks after metamorphosis the retinal projection from the right eye was examined with the use of [3H]proline autoradiography. The autoradiographs indicated that optic fibres whose tectal target was destroyed recrossed to the ipsilateral tectum and basal optic nucleus via the posterior and pretectal commissures. No anomalous recrossing occurred if the tectal ablation was carried out at stage 58 or later. The aberrant optic fibres were restricted to the rostrolateral and central parts of the ipsilateral tectum and they terminated in a discontinuous manner. It is concluded that available surfaces serving as contact guidance cues are needed to direct aberrant optic fibres to the ipsilateral tectum.

Retinofugal pathways in fetal and adult spiny dogfish, Squalus acanthias

Retinofugal pathways in fetal and adult spiny dogfish were determined by intraocular injection of [ 3H]proline for autoradiography. Distribution and termination of the primary retinal efferents were identical in pups and adults. The retinal fibers decussate completely, except for a sparse ipsilateral projection to the caudal preoptic area. The decussating optic fibers terminate ventrally in the preoptic area and in two rostral thalamic areas, a lateral neuropil area of the dorsal thalamus and more ventrally in the lateral half of the ventral thalamus. At this same rostral thalamic level, a second optic pathway, the medial optic tract, splits from the lateral marginal optic tract and courses dorsomedially to terminate in the rostral tectum and the central and periventricular pretectal nuclei. The marginal optic tract continues caudally to terminate in a superficial pretectal nucleus and also innervates the superficial zone of the optic tectum. A basal optic tract arises from the ventral edge of the marginal optic tract and courses medially into the central pretectal nucleus, as well as continuing more caudally to terminate in a dorsal neuropil adjacent to nucleus interstitialis and in a more ventrally and medially located basal optic nucleus. Comparison of the retinofugal projections of Squalus with those of other sharks reveals two grades of neural organization with respect to primary vusula projections. Squalomorph sharks possess a rostral dorsal thalamic nucleus whose visual input is primarily, if not solely, axodendritic, and an optic tectum in which the majority of the cell bodies are located deep to the visual terminal zone. In contrast, galeomorph sharks are characterized by an enlarged and migrated rostrodorsal thalamic visual nucleus, and an optic tectum in which the majority of the cell bodies are located within the visual terminal zone. These data suggest that evolution of primary visual pathways in sharks occurs by migration and an increase in neuronal number, rather than by the occurence of new visual pathways.

Application of cobalt-filling technique to show retinal projections in the frog

Cobaltous-lysine complex was used to fill the axons of the cut optic nerve in the frog, Rana esculenta. After intensification in a physical developer, fibres containing black precipitate could be detected in the nucleus of Bellonci, in the lateral geniculate body, in the pretectal region, in the optic tectum and in the basal optic nucleus on both sides. The axial optic tract was described in detail. Fibres of this tract terminate in the posterior entopeduncular nucleus, in the ventrolateral area of the thalamus, in the lateral geniculate nucleus, in the posterolateral nucleus and in layers F, G and 6 of the optic tectum contralateral to the cobalt-filled optic nerve. The existence of a hypothalamic optic root was also verified. Cobaltous-lysine complex proved to be an excellent marker to show the projections of a central pathway. This compound covered longer distances and gave heavier precipitate than cobaltous chloride alone. Fibres of 0.6–3 μm in diameter were well-filled in the majority of cases. In the best preparations, fibres with a calibre of 0.3–0.5 μm were also found. The thinnest fibres are probably unmyelinated.

The paths and destinations of the induced ipsilateral retinal projection in goldfish

Adult goldfish had one tectal lobe removed surgically, and several months later, the eye contralateral to the missing tectum was injected with radioactive proline. Radioautographs of the brains were studied to trace the paths and termination sites of the optic fibers. The optic tract decussated at the chiasm, as normally, but then ran caudally in a large neuroma on the tectum-less side of the brain. Substantial numbers of fibers left this neuroma to enter two or more of five commissures, through which they recrossed the midline. These commissures: transverse, minor, horizontal, posterior and ansate, ordinarily contain few or no optic fibers. All are normally linked with the tectum. Negligible numbers of aberrant optic fibers recrossed the midline elsewhere. On the intact side of the brain, ipsilateral to the injected eye, the optic fibers innervated some or all of the nuclei and areas normally served by contralateral retinal fibers. An earlier behavioral study of these same fish had shown that some of them made reversed optokinetic nystagmus in response to stripe movement seen by the eye projecting ipsilaterally; others failed to respond to stimuli through this eye. In all the reversed responders, a caudal group of retinal projection sites was labeled ipsilaterally. This included the basal optic nucleus and the caudal portions of nucleus dorsolateralis thalami and area pretectalis. In the non-responders, these targets were not labeled ipsilaterally. Together, these results suggest that one or more of these three sites is or are responsible for optokinetic nystagmus in normal goldfish.

Tectal efferents in the branded water snake, Natrix sipedon.

Visual information reaches the dorsal thalamus by two distrinct routes in most reptiles. Retinal efferents terminate directly in the dorsal lateral geniculate nucleus (DLGN). Retinal information is also channeled indirectly through the tectum to nucleus rotundus. Retinal projections to DLGN and tectum are also well established in snakes, but the status of the tecto-rotundal link of the indirect visual pathway is uncertain. Thus, tectal efferents were studied with Fink-Heimer methods in banded water snakes (Natrix sipedon). The tectum gives rise to crossed and uncorssed projections to the brainstem reticular formation. Commissural connections are effected with the contralateral tectum via the tectal and posterior commissures. Tectum projects densely to the ipsilateral basal optic nucleus. Bilateral ascending projections reach the pretectal area, nucleus lentiformis mescencephali, lateral habenular nuclei, and posterodorsal nuclei. Ascending projections reach the ventral lateral geniculate and suprapeduncular nuclei. There is a diffuse projection to the central part of the caudal thalamus and a dense, bilateral projection to the DLGN. These results indicate that the relation of the tectum to the dorsal thalamus is different in snakes than in other reptiles. Nucleus rotundus is either absent or poorly differentiated and there is a strong convergence of the direct and indirect visual pathways at DLGN.

Retinofugal pathways in the lingnose gar Lepisosteus osseus (linnaeus).

Retinal projections were studied with autoradiographic and silver methods in the gar, Lepisosteus osseus, one of the two surviving members of the holostean actinopterygians. Contralaterally, the retina projects to the preoptic nucleus of the hypothalamus, and, via the medial optic tract, to the dorsal thalamus, medial ventral thalamic nucleus, nucleus pretectalis profundus pars ventralis and pars dorsalis, and the medial portion of the deep layers of the central zone in the optic tectum. The dorsal optic tract projects to the lateral ventral thalamic nucleus, nucleus pretectalis centralis, and the superficial white and gray zone of the optic tectum. The ventral optic tract terminates in the nucleus of the ventral optic tract, the lateral and medial ventral thalamic nuclei, nucleus pretectalis superficialis, nucleus pretectalis centralis, nucleus pretectalis profundus pars ventralis, the basal optic nucleus, and the superficial white and gray zone of the optic tectum. Ipsilateral projections are to similar sites, except for an absence of inputs to the lateral ventral thalamic nucleus from the dorsal tract and to the nucleus pretectalis superficialis, nucleus pretectalis profundus pars ventralis, and the basal optic nucleus from the ventral tract. The presence of ipsilateral retinal projections in gars is compared to their presumed absence in teleosts, and comparisons of retinorecipient targets in gars are made with teleosts and with non-actinopterygian vertebrates.

Retinal projections in a snake, Thamnophis sirtalis.

AbstractThe retinofugal projections of the snake Thamnophis sirtalis were studied by the method of experimentally induced Wallerian degeneration stained by the Fink‐Heimer method. The retinal ganglion cells project to all parts of the contralateral lateral geniculate complex, nucleus lentiformis mesencephali, nucleus geniculatus pretectalis, nucleus posterodorsalis, basal optic nucleus and superficial layers of the optic tectum. In addition, the retinofugal projections were observed terminating in portions of the ipsilateral lateral geniculate complex and nucleus posterodorsalis.Examination of the morphology of the retinal terminal areas stained for Nissl substance with cresyl violet led to the conclusion that these regions are well differentiated and should not be considered poorly developed when compared with other reptilian forms such as turtles.