Topography of primary visual centres in the brain of the chick,Gallus gallus

In a previous paper (Ehrlich and Mark,'83b) the primary visual centres of the chick were described. In this paper patterns of retinotopy are examined by means of silver degeneration and autoradiographic techniques following discrete laser lesions of the retina. Well-defined and complete retinotopic maps are found in each of the following visual centres: tectum, lateral anterior thalamus, lateroventral geniculate nucleus, superficial synencephalic nucleus, ectomammillary nucleus, and tectal grey. In the dorsolateral anterior thalamus, pars lateralis, and external nucleus there is some evidence of a retinotopic innervation, yet not as well defined as those nuclei mentioned previously. Retinotopic maps were not observed in other retinorecipient regions. These include the ventrolateral thalamus, dorsolateral anterior thalamus, magnocellular part and rostrolateral part, pretectal optic area, and diffuse pretectal nucleus. Within the lateroventral geniculate, lateral anterior thalamus, superficial synencephalic nuclei, and the tectal grey, the ventral to dorsal retinal axis is mapped along the rostrocaudal axis, the reverse of the orientation seen in the tectum, which may have implications for explanations of how retinotopic maps are formed. The poor retinotopy in dorsolateral anterior thalamus, lateral part, is discussed with respect to the view that it may be the avian homologue of the mammalian lateral geniculate nucleus.     

Retinal projections in lamprey (Lampetra fluviatilis)

Visual projections in lamprey were investigated using two methods,--one by revealing transport of horseradish peroxidase, and the other by silver impregnation of degenerating axons and terminals after enucleation of the eye. Both methods produced similar results. The chiasm showed incomplete crossing of retinal fibres, the major part of which, as an optic tract, proceed along the contralateral thalamus up to the entry into the optic tectum, while the smaller part takes the same course on the ipsilateral side. Besides, from the posterior part of the optic chiasm an axial optic tract branches off, which proceeds through the central part of the contralateral thalamus up to the pretectal nucleus, individual fibres of which enter the central grey layer of the optic tectum. On the contralateral side, the visual projections are localized in the lateral geniculate body, pretectal nucleus, in the three upper layers of the optic tectum, in the ventrolateral area of the optic tectum and as solitary diffuse projections in the mesencephalic tegmentum. Innervation of thalamic and pretectal nuclei are realized by two tracts--the tractus opticus proper, and tractus opticus axialis. On the ipsilateral side visual projections, excepting the optic tract, are scarce and in the thalamus appear as small areas of the lateral geniculate body and pretectum adjacent to the optic tract. Solitary visual projections were found in two upper layers of the rostral optic tectum and in larger numbers in the 3rd and 4th layers of the caudal part and in ventrolateral area of the optic tectum. Projections in mesencephalic tegmentum were single. Diffuse visual projections in the lateral part of hypothalamus could be revealed only by the silver impregnation method. Using the peroxidase method two types of cells were observed in mesencephalic tegmentum where, possibly, the centrifugal fibres proceeding to retina, originate. A comparison is made of central visual projections in lampreys and other representatives of nonmammalian vertebrates.     

An autoradiographic study of the efferent connections of the superior colliculus in the cat

The differential projections of the three main cellular strata of the superior colliculus have been examined in the cat by the autoradiographic method. The stratum griseum superficiale projects caudally to the parabigeminal nucleus and rostrally to several known visual centers: the nucleus of the optic tract and the olivary pretectal nucleus in the pretectum; the deepest C laminae of the dorsal lateral geniculate nucleus; the large-celled part of the ventral lateral geniculate nucleus; the posteromedial, large-celled part of the lateral posterior nucleus of the thalamus. Several of these projections are topographically organized. The stratum griseum profundum gives rise to most of the descending projections of the superior colliculus. Ipsilateral projections pass to both the dorsolateral and lateral divisions of the pontine nuclei, the cuneiform nucleus, and the raphe nuclei, and to extensive parts of the brainstem reticular formation: the tegmental reticular nucleus, and the paralemniscal, lateral, magnocellular, and gigantocellular tegmental fields. Contralateral projections descending in the predorsal bundle pass to the medial parts of the tegmental reticular nucleus and of some of the tegmental fields, the dorsal part of the medial accessory nucleus of the inferior olivary complex, and to the ventral horn of the cervical spinal cord. Ascending projections of the stratum griseum profundum terminate in several nuclei of the pretectum, the magnocellular nucleus of the medial geniculate complex and several intralaminar nuclei of the thalamus, and in the fields of Forel and zona incerta in the subthalamus. The strata grisea profundum and intermediale each have projections to homotopic areas of the contralateral superior colliculus, to the pretectum, and to the central lateral and suprageniculate nuclei of the thalamus. However, the stratum griseum intermediale has few or no descending projections.     

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

Our previous study has indicated that accommodative responses can be evoked with weak currents applied to a circumscribed area of the superior colliculus in the cat. We investigated efferent projections from this area with biocytin in the present study. The accommodation area in the superior colliculus was identified by systematic microstimulation in each of five anesthetized cats. Accommodative responses were detected by an infrared optometer. After mapping the superior colliculus, biocytin was injected through a glass micropipette into the accommodation area, where accommodative responses were elicited with low-intensity microstimulation. In addition, accommodative responses to stimulation of the superior colliculus were compared before and after an injection of muscimol, an agonist of inhibitory neurotransmitter, into the pretectum. Following the injection of biocytin, in the ascending projections, labeled terminals were seen mainly in the caudal portion of the nucleus of the optic tract, the nucleus of the posterior commissure, the posterior pretectal nucleus, the olivary pretectal nucleus, the mesencephalic reticular formation at the level of the oculomotor nucleus, and the lateral posterior nucleus of the thalamus on the ipsilateral side. Less dense terminals were seen in the anterior pretectal nucleus, the zona incerta, and the centromedian nucleus of the thalamus. In the descending projections, labeled terminals were observed mainly in the paramedian pontine reticular formation, the nucleus raphe interpositus, and the dorsomedial portion of the nucleus reticularis tegmenti pontis on the contralateral side. Less dense terminals were also seen in the nucleus of the brachium of the inferior colliculus, the cuneiform nucleus, the medial part of the paralemniscal tegmental field, and the dorsolateral division of the pontine nuclei on the ipsilateral side. Following the injection of muscimol into the pretectum, including the nucleus of the optic tract, the posterior pretectal nucleus, and the nucleus of the posterior commissure, accommodative responses evoked by microstimulation of the superior colliculus were reduced to 33–55% of the value before the injections. These findings suggest that the accommodation area in the superior colliculus projects to the oculomotor nucleus through the ipsilateral pretectal area, especially the nucleus of the optic tract, the nucleus of posterior commissure, and the posterior pretectal nucleus, and also projects to the pupilloconstriction area (the olivary pretectal nucleus), the vergence-related area (the mesencephalic reticular formation), and the active visual fixation-related area (the nucleus raphe interpositus). © 1996 Wiley-Liss, Inc.     

The effects of ablation of visual cortex in neonatal rabbits on the organization of retinothalamic and retinopretectal projections

Primary visual cortex was ablated unilaterally in neonatal rabbits. Following a survival of 2-4 months, retrograde degeneration of the dorsal lateral geniculate nucleus (LGd) was assessed, and reorganization of retinofugal pathways was studied using methods of anretrograde transport of [3H]proline or of horseradish peroxidase. A complete lesion of primary visual cortex resulted in complete retrograde degeneration of the LGd with no sparing of any class of neurons. The terminations of retinofugal axons in the pretectum and thalamus were compared with those observed in normal animals. No major reorganization of ipsilateral retinofugal projections was observed in either the thalamus and pretectum ipsilateral to the ablated cortex, or in the thalamus and pretectum contralateral to the ablated cortex. However, contralateral retinofugal projections to the thalamus and to the pretectum ipsilateral to the ablated cortex were significantly different from normal. In the thalamus, the projections to the lateral posterior nucleus were expanded in area and increased in density. In the pretectum, the projections to the rostral pretectal areas were greatly increased in area, especially in the region of the olivary pretectal nucleus and posterior pretectal nucleus. However, the density of these projections was not increased relative to normal. Consideration of these results in relation to other published data on the anatomical consequences of neonatal visual cortex lesions, both in mammals which show behavioral sparing following neonatal visual cortex lesions and in mammals which, like the rabbit, show no behavioral sparing, suggests that: (1) behavioral sparing may correlate with patterns of survival or death of neurons in the thalamus and retina; and (2) reorganization of retinofugal pathways is not necessarily associated with behavioral sparing.     

The mormyrid mesencephalon. III. Retinal projections in a weakly electric fish, Gnathonemus petersii

The optic nerve and the retinal projections were studied in a mormyrid fish, Gnathonemus petersii, by using Fink-Heimer, HRP, cobalt labeling, and autoradiographic tracing techniques. The retinal fibers terminate bilaterally in the following places: suprachiasmatic nucleus, dorsolateral optic nucleus, optic nucleus of the posterior commissure, cortical nucleus, ventral pretectal area, optic tectum, and the accessory optic terminal field. The number of uncrossed fibers is relatively high in the suprachiasmatic nucleus, but negligibly small in the other retinal terminal fields. In the lateral geniculate nucleus and pretectal nucleus only crossed retinal fibers could be detected. The visual system of Gnathonemus is compared to that of other fishes, amphibians, and reptiles and the possible homologies are proposed. The comparison points to the conclusion that the visual system is less developed in Gnathonemus. This nocturnal species lives in turbid waters and has a special electric sense which may permit compensation for the reduced visual capacity.     

The ascending tectofugal visual system in amniotes: new insights

Ascending tectal axons carrying visual information constitute a fiber pathway linking the mesencephalon with the dorsal thalamus and then with a number of telencephalic centers. The sauropsidian nucleus rotundus and its mammalian homologue(s) occupy a central position in this pathway. The aim of this study was analyzing the rotundic connections in reptiles and birds in relation with comparable connections in mammals, by using biotinylated dextran amines and the lipophilic carbocyanine dye DiI as tracing molecules. In general, rotundic connections in reptiles and birds are quite similar, especially with regards to pretectal and tectal afferences; as a novel finding, we describe varicose fibers arising from nucleus rotundus that reached the developing chick striatum. In addition, this study described the dorsal claustrum as a novel telencephalic target for the suprageniculate nucleus in mammals. Overall, telencephalic projections from the posterior/intralaminar complex of the mammalian thalamus can be compared with the telencephalic projections of the reptilian nucleus rotundus. With the exception of the isocortical connections, the mouse suprageniculate nucleus shares a number of afferent and efferent connections with the sauropsidian nucleus rotundus. Especially significant were the suprageniculate fibers reaching the striatum and then following to reach pallial derivatives such as the lateral amygdala (ventral pallium) and the dorsal claustrum (lateral pallium). These connections can be compared with the rotundic fibers reaching the ventromedial part of the anterior dorsal ventricular ridge in reptiles/entopallium in birds (ventral pallium) and the dorsolateral part of the anterior dorsal ventricular ridge in reptiles (lateral pallium), and probably the mesopallium in birds.     

Retinal projections in gymnotid fishes

Retinal projections were studied in four species of gymnotid fishes, Gymnotus carapo, Hypopomus artedi, Eigenmannia virescens and Sternopygus sp. with the aid of cobalt or HRP labelling and autoradiographic techniques. The optic tract gives off a small branch, the axial optic tract and then, after crossing in the midline, splits into a dorsomedial, dorsal and ventral fascicle. E. virescens and Sternopygus sp. display in addition an accessory optic tract. In all four species retinal projections are bilateral; ipsilateral projections, however, are extremely sparse. In all four species, the retinal fibres terminate bilaterally in the suprachiasmatic nucleus, dorsolateral optic nucleus of the thalamus and the optic nucleus of the posterior commissure; a bilateral retinotectal projection was only found in E. virescens and G. carapo. Retinal projections are only contralateral to the ventromedial nucleus of the thalamus, the central pretectal nucleus and the accessory optic nucleus. The contralateral retinotectal fibres terminate in the stratum fibrosum and griseum superficiale, and in the stratum album centrale and stratum periventriculare. A small accessory optic tract and nucleus were detected in E. virescens and Sternopygus sp. but not in G. carapo and H. artedi. The results indicate that the visual system of gymnotid fish is as simple as that of mormyrids. The poor visibility in the environment where these animals live and the additional sensory system which these animals possess may explain the poor development of the visual system.     

Topographic organization of the projections from cortical areas 17, 18, and 19 onto the thalamus,pretectum and superior colliculus in the cat

The distribution of cortical projections from areas 17, 18, and 19 to the lateral thalamus, pretectum, and superior colliculus was investigated with the autoradiographic tracing method. Cortical areas 17, 18 and 19 were demonstrated to project retinotopically and in register upon the dorsal lateral geniculate nucleus, medial interlaminar nucleus, lateral zone of the lateral posterior complex, nucleus of the optic tract and superior colliculus. Area 19 was shown to project retinotopically upon the pulvinar nucleus. Clear retinotopic organization was not demonstrable in the projections of areas 17, 18 and 19 to the reticular complex of the thalamus and ventral lateral geniculate nucleus, or in the projection of area 19 to the anterior pretectal nucleus. The cortical projections were employed to define the retinotopic organization of the nucleus of the optic tract, pulvinar nucleus, and lateral zone of the lateral posterior complex. The cortical projections show the vertical meridian to be represented caudally, with the lower visual field represented laterally, and the upper visual field medially, within the nucleus of the optic tract. The projections of area 19 to the pulvinar nucleus demonstrate the lower visual field to be represented rostrally and the upper visual field caudally in this nucleus; the vertical meridian to be represented at the lateral border and the visual field periphery to be represented at the medial border of the pulvinar nucleus. Cortical projections to the lateral zone of the lateral posterior complex demonstrate the lower visual field to be represented rostrally and the upper visual field caudally; the vertical meridian to be represented at the medial limit and the visual field periphery at the lateral border of the termination zones. On the basis of the experimental findings, a new terminology is introduced for the feline lateral posterior complex. Divisions are proposed which correspond to zones with demonstrably distinct afferent input. The pulvinar nucleus is defined by the distribution of projections from area 19. Three flanking divisions are defined within the lateral posterior complex; a lateral division recipient of projections from area 17, 18 and 19, an interjacent division recipient of projections of the superficial layers of the superior colliculus, and a medial division flanking the tectorecipient zone medially.     

Subcortical connections of area 17 in the tree shrew: an autoradiographic analysis

The present anterograde autoradiographic study reveals several targets of the striate cortex (area 17) of the tree shrew which were not previously observed in studies which used anterograde degeneration methods; our data also confirm several previous findings. The results are discussed in the context of these projections modulating ascending visual information (claustrum, lateral intermediate nucleus, pulvinar, dorsal lateral geniculate, cells of the external medullary lamina, reticular nucleus of the thalamus, superficial collicular layers, and the anterior and posterior pretectal nuclei) or visuomotor information (putamen, caudate, ventral lateral geniculate, pontine gray, and the anterior and posterior pretectal nuclei).     

Thalamo-telencephalic connections: new insights on the cortical organization in reptiles

Tracer injections into the dorsal tier of the lacertilian dorsal thalamus revealed an extensive innervation of the cerebral cortex. The medial cortex, the dorsomedial cortex, and the medial part of the dorsal cortex received a bilateral projection, whereas the lateral part of dorsal cortex and the dorsal part of the lateral cortex received only an ipsilateral thalamic projection. Thalamocortical fibers were found superficially in all cortical regions, but in the dorsal part of the lateral cortex, varicose axons within the cellular layer were also observed. The bilateral thalamocortical projection originates from a cell population located throughout the dorsolateral anterior nucleus, whereas the ipsilateral input originates mainly from a rostral neuronal subpopulation of the nucleus. This feature suggests that the dorsolateral anterior nucleus consists of various parts with different projections. The dorsal subdivision of the lateral cortex displayed hodological and topological (radial glia processes) features of a dorsal pallium derivative. After tracer injections into the dorsal cortex of lizards, we found long descending projections that reached the striatum, the diencephalic basal plate, and the mesencephalic tegmentum, which suggests that it may represent a sensorimotor cortex.     

The organization of subcortical projections of the hamster's visual cortex

The subcortical projections of the hamster's visual cortex were determined by use of injections of tritiated proline and heat lesions placed in different cortical loci. The brains were processed for autoradiography and silver impregnation of degenerating axons. Striate cortex was shown to project ipsilaterally to the dorsocaudal region of the caudate nucleus, a dorsolateral area within the thalamic reticular nucleus (RT), a laterodorsal region of the nucleus lateralis anterior (LA), the rostral half of nucleus lateralis posterior (LP), the whole territory of the dorsal (dLGN) and ventral (vLGN) geniculate nuclei, the anterior (PA) and posterior (PP) pretectal nuclei, the superior colliculus (SC), and the precerebellar pontine nuclei. In addition, the medial visual area (18b) was shown to project to a medial band of LA and part of the caudal half of LP, while the adjoining parietal cortex was seen to terminate in a lateral part of the caudate, a ventral band of LA, and the ventral half of rostral LP. Segregation of different cortical inputs was clear in LA, LP, caudate, and pons. The projections to dLGN, vLGN, SC, LP, and PA were retinotopically organized. Clear evidence of some topography was found within RT, PP, and the pons, although a consisten map could not be derived from the data.     

Autoradiographic study of the retinal projections in the Chinese pangolin, Manis pentadactyla

Autoradiography was used to investigate the optic system of the Chinese pangolin, Manis pentadactyla. The pattern of retinal projections in the Chinese pangolin is similar to that described in other mammals. Each retina projects bilaterally to the suprachiasmatic nucleus, dorsal and ventral lateral geniculate nuclei, pretectal area, and superior colliculus (SC). Only contralateral projections are found to the medial, lateral, and dorsal accessory optic nuclei. The large lateral nucleus receives a dense projection from the retina and forms a compact mass on the dorsolateral area of the cerebral peduncle. The lamination of the SC could not be clearly demonstrated in the brain of the Chinese pangolin.     

Connections of the precommissural nucleus.

The connections of the precomissural nucleus (PRC) have been examined with anterograde and retrograde axonal tracing methods in the rat. Experiments with cholera toxin B subunit (CTb) indicate that the PRC shares a number of common afferent sources with the dorsolateral periaqueductal gray (PAG). Thus, we have shown that the nucleus receives substantial inputs from the prefrontal cortex, specific domains of the rostral part of the lateral septal nucleus, rostral zona incerta, perifornical region, anterior hypothalamic nucleus, ventromedial hypothalamic nucleus, dorsal premammillary nucleus, medial regions of the intermediate and deep layers of the superior colliculus, and cuneiform nucleus. Moreover, the PRC also receives inputs from several PAG regions and from neural sites involved in the control of attentive or motivational state, including the laterodorsal tegemental nucleus and the ventral tegmental area. The efferent projections of the PRC were analyzed by using the Phaseolus vulgaris-leucoagglutinin (PHA-L) method. Notably, the PRC presents a projection pattern that resembles in many ways the pattern described previously for the rostral dorsolateral PAG in addition to projections to a number of targets that also are innervated by neighboring pretectal nuclei, including the rostrodorsomedial part of the lateral dorsal thalamic nucleus, the ventral part of the lateral geniculate complex, the medial pretectal nucleus, the nucleus of the posterior commissure, and the ventrolateral part of the subcuneiform reticular nucleus. Overall, the results suggest that the PRC might be viewed as a rostral component of the PAG, and the possible functional significance of the nucleus is discussed in terms of its connections.     

Projections of the optic tectum in the longnose gar,lepisosteus osseus

Efferent projections of the optic tectum were studied with the anterograde degeneration method in the longnose gar. Ascending projections were found bilaterally to 3 pretectal nuclei — the superficial pretectal nucleus, nucleus pretectalis centralis and nucleus pretectalis profundus — and to a number of targets which lie further rostrally — the central posterior nucleus, dorsal posterior nucleus, accessory optic nucleus, nucleus ventralis lateralis, nucleus of the ventral optic tract, rostral part of the preglomerular complex, suprachiasmatic nucleus, anterior thalamic nucleus, nucleus ventralis medialis, nucleus intermedius, nucleus prethalamicus and rostral entopeduncular nucleus. Projections of the tectum reach the contralateral side via the supraoptic decussation and are less dense contralaterally than ipsilaterally. Descending projections resulting from tectal lesions include: (1) a tectal commissural pathway to the core of the torus longitudinalis bilaterally and the contralateral tectum and torus semicircularis; and (2) a pathway leaving the tectum laterally from which fibers terminate in the ipsilateral torus semicircularis, an area lateral to the nucleus of the medial longitudinal fasciculus, lateral tegmental nucleus, nucleus lateralis valvulae, nucleus isthmi and the reticular formation. A component of this bundle decussates at the level of the lateral tegmental nucleus to project to the contralateral reticular formation.

On the basis of comparisons of these findings with the pattern of retinal projections in gars and other data, it is argued that the nuclei previously called the lateral geniculate and rotundus in fish are not the homologues of the nuclei of those names in land vertebrates but are rather pretectal cell groups. The overall organization of both retinal and tectal projections in gars is strikingly similar to that in land vertebrates; at present, the best candidate for a rotundal homologue is the dorsal posterior nucleus.     

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.     

Cholinergic systems in the rat brain: III. Projections from the pontomesencephalic tegmentum to the thalamus, tectum, basal ganglia, and basal forebrain

The ascending cholinergic projections of the pedunculopontine and dorsolateral tegmental nuclei, referred to collectively as the pontomesencephalotegmental (PMT) cholinergic complex, were investigated by use of fluorescent tracer histology in combination with choline-O-acetyltransferase (ChAT) immunohistochemistry and acetylcholinesterase (AChE) pharmacohistochemistry. Propidium iodide, true blue, or Evans blue was infused into the anterior, reticular, mediodorsal, central medial, and posterior nuclear areas of the thalamus; the habenula; lateral geniculate; superior colliculus; pretectal/parafascicular area; subthalamic nucleus; caudate-putamen complex; globus pallidus; entopeduncular nucleus; substantia nigra; medial septal nucleus/vertical limb of the diagonal band area; magnocellular preoptic/ventral pallidal area; and lateral hypothalamus. In some animals, separate injections of propidium iodide and true blue were made into two different regions in the same rat brain, usually a dorsal and a ventral target, in order to assess collateralization patterns. Retrogradely transported fluorescent labels and ChAT and/or AChE were analyzed microscopically on the same brain section. All of the above-delimited targets were found to receive cholinergic input from the PMT cholinergic complex, but some regions were preferentially innervated by either the pedunculopontine or dorsolateral tegmental nucleus. The former subdivision of the PMT cholinergic complex projected selectively to extrapyramidal structures and the superior colliculus, whereas the dorsolateral tegmental nucleus was observed to provide cholinergic input preferentially to anterior thalamic regions and rostral portions of the basal forebrain. The PMT cholinergic neurons showed a tendency to collateralize extensively.     

Tectal efferents in the banded water snake,Natrix sipedon

Visual information reaches the dorsal thalamus by two distinct 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 esablished 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 uncrossed projections to the brainstem reticular formation. Commissural connections are effected with the contralateral tectum via the tectal and osterior commissures. tectum projects densely to the ipsilateral basal optic nucleus. Bilateral ascending projections reach the pretectal area, nucleus lentiformis mesencephali, 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, bilaternal 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.     

Efferent connections from the anterior pretectal nucleus to the diencephalon and mesencephalon in the rat

The anterior pretectal nucleus has been described as part of the visual pretectal complex. However, several electrophysiological and behavioural studies showed that this area is involved in somatosensory modulation, more specifically, antinociception. The efferents of the anterior pretectal nucleus have not been identified taking into account the different function of this nucleus in relation to the rest of the pretectal complex. In the study herein described, a sensitive anterograde tracer Phaseolus vulgaris leucoagglutinin was used to trace the mesencephalic and diencephalic efferents of the anterior pretectal nucleus in the rat. The majority of the connections were ipsilateral. Fibres with varicosities were observed in discrete areas of the thalamus (central lateral, posterior complex), hypothalamus (lateral, posterior and ventromedial), zona incerta, parvocellular red nucleus, intermediate and deep layers of the superior colliculus, central grey, deep mesencephalon, pontine parabrachial region, and pontine nuclei. Fibres en passant were detected in the medial lemniscus, from the level of the injection site to rostral medullary levels. Some labelled axons were seen coursing to the contralateral side through the posterior commissure and the decussation of the superior cerebellar peduncle. These results show that the anterior pretectal nucleus projects principally to areas involved in somatosensory and motor control in a manner that permits sensory modulation at higher and lower levels of the brain. These connections may explain the antinociceptive and antiaversive effects of stimulating the anterior pretectal nucleus in freely moving animals.     

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 fibres, 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.