Optic tectum of the eastern garter snake, Thamnophis sirtalis. I. Efferent pathways

Motor functions of the superior colliculus

The mammalian superior colliculus (SC) and its nonmammalian homolog, the optic tectum, constitute a major node in processing sensory information, incorporating cognitive factors, and issuing motor commands. The resulting action-to orient toward or away from a stimulus-can be accomplished as an integrated movement across oculomotor, cephalomotor, and skeletomotor effectors. The SC also participates in preserving fixation during intersaccadic intervals. This review highlights the repertoire of movements attributed to SC function and analyzes the significance of results obtained from causality-based experiments (microstimulation and inactivation). The mechanisms potentially used to decode the population activity in the SC into an appropriate movement command are also discussed.

Vocal premotor activity in the superior colliculus

Chronic neural recordings were taken from the midbrain superior colliculus (SC) of echolocating bats while they were engaged in one of two distinct behavioral tasks: virtual target amplitude discrimination (VTAD) and real oscillating target tracking (ROTT). In the VTAD task, bats used a limited range of sonar call features to discriminate the amplitude category of echoes, whereas in the ROTT task, the bat produced dynamically modulated sonar calls to track a moving target. Newly developed methods for chronic recordings in unrestrained, behaving bats reveal two consistent bouts of SC neural activity preceding the onset of sonar vocalizations in both tasks. A short lead bout occurs tightly coupled to vocal onset (VTAD, -5.1 to -2.2 ms range, -3.6 +/- 0.7 ms mean lead time; ROTT, -3.0 to + 0.4 ms range, -1.2 +/- 1.3 ms mean lead time), and this activity may play a role in marking the time of each sonar emission. A long lead bout in SC activity occurs earlier and spreads over a longer interval (VTAD, -40.6 to -8.4 ms range, -22.2 +/- 3.9 ms mean lead time; ROTT, -29.8 to -7.1 ms range, -17.5 +/- 9.1 ms mean lead time) when compared with short lead events. In the goal-directed ROTT task, the timing of long lead event times vary with the bat's sonar call duration. This finding, along with behavioral studies demonstrating that bats adjust sonar call duration as they track targets at changing distance, suggests the bat SC contributes to range-dependent adjustments of sonar call duration.

Afferents of the lamprey optic tectum with special reference to the GABA input: combined tracing and immunohistochemical study

The optic tectum in the lamprey midbrain, homologue of the superior colliculus in mammals, is important for eye movement control and orienting responses. There is, however, only limited information regarding the afferent input to the optic tectum except for that from the eyes. The objective of this study was to define specifically the gamma-aminobutyric acid (GABA)-ergic projections to the optic tectum in the river lamprey (Lampetra fluviatilis) and also to describe the tectal afferent input in general. The origin of afferents to the optic tectum was studied by using the neuronal tracer neurobiotin. Injection of neurobiotin into the optic tectum resulted in retrograde labelling of cell groups in all major subdivisions of the brain. The main areas shown to project to the optic tectum were the following: the caudoventral part of the medial pallium, the area of the ventral thalamus and dorsal thalamus, the nucleus of the posterior commissure, the torus semicircularis, the mesencephalic M5 nucleus of Schober, the mesencephalic reticular area, the ishtmic area, and the octavolateral nuclei. GABAergic projections to the optic tectum were identified by combining neurobiotin tracing and GABA immunohistochemistry. On the basis of these double-labelling experiments, it was shown that the optic tectum receives a GABAergic input from the caudoventral part of the medial pallium, the dorsal and ventral thalamus, the nucleus of M5, and the torus semicircularis. The afferent input to the optic tectum in the lamprey brain is similar to that described for other vertebrate species, which is of particular interest considering its position in phylogeny.

Afferent connectivity to different functional zones of the optic tectum in goldfish

This work studies the afferent connectivity to different functionally identified tectal zones in goldfish. The sources of afferents contributed to different degrees to the functionally defined zones. The dorsocentral area of the telencephalon was connected mainly with the ipsilateral anteromedial tectal zone. At diencephalic levels, neurons were found in three different regions: preoptic, thalamic, and pretectal. Preoptic structures (suprachiasmatic and preoptic nuclei) projected mainly to the anteromedial tectal zone, whereas thalamic (ventral and dorsal) and pretectal (central, superficial, and posterior commissure) nuclei projected to all divisions of the tectum. In the mesencephalon, the mesencephalic reticular formation, torus longitudinalis, torus semicircularis, and nucleus isthmi were, in the anteroposterior axis, topographically connected with the tectum. In addition, neurons in the contralateral tectum projected to the injected zones in a symmetrical point-to-point correspondence. At rhombencephalic levels, the superior reticular formation was connected to all studied tectal zones, whereas medial and inferior reticular formations were connected with medial and posterior tectal zones. The present results support a different quantitative afferent connectivity to each tectal zone, possibly based on the sensorimotor transformations that the optic tectum carries out to generate orienting responses.

Mesencephalic and diencephalic afferent connections to the thalamic nucleus rotundus in the lizard, Psammodromus algirus

The present work is an analysis of the afferent projections to the thalamic nucleus rotundus in a lizard, both at the light- and electron-microscopic level, using biotinylated dextran amine (BDA) as a neuroanatomical tracer. This study has confirmed previously reported afferent projections to nucleus rotundus in reptiles and has also identified a number of new cellular aggregates projecting to this dorsal thalamic nucleus. After BDA injections into nucleus rotundus, retrogradely labelled neurons were observed consistently within the following neuronal groups in the midbrain and the diencephalon: (i) the stratum griseum centrale of the optic tectum; (ii) the nucleus subpretectalis in the pretectum; (iii) the nucleus ansa lenticularis posterior, the posterior nucleus of the ventral supraoptic commissure, and the posteroventral nucleus, in the dorsal thalamus and (iv) the lateral suprachiasmatic nucleus and part of the reticular complex in the ventral thalamus. Tectal axons entering nucleus rotundus were fine and varicose and formed exclusively asymmetric synaptic contacts, mainly on small dendritic profiles. Rotundal neurons had symmetric synapses made by large boutons probably of nontectal origin. After comparing our results with those in other reptiles, birds and mammals, we propose that the sauropsidian nucleus rotundus forms part of a visual tectofugal pathway that conveys mesencephalic visual information to the striatum and dorsal ventricular ridge, and is similar to the mammalian colliculo-posterior/intralaminar-striatoamygdaloid pathway, the function of which may be to participate in visually guided behaviour.

Pretectal connections in turtles with special reference to the visual thalamic centers: a hodological and gamma-aminobutyric acid-immunohistochemical study

Projections of the pretectal region to forebrain and midbrain structures were examined in two species of turtles (Testudo horsfieldi and Emys orbicularis) by axonal tracing and immunocytochemical methods. Two ascending gamma-aminobutyric acid (GABA)ergic pathways to thalamic visual centers were revealed: a weak projection from the retinorecipient nucleus lentiformis mesencephali to the ipsilateral nucleus geniculatus lateralis pars dorsalis and a considerably stronger projection from the nonretinorecipient nucleus pretectalis ventralis to the nucleus rotundus. The latter is primarily ipsilateral, with a weak contralateral component. The interstitial nucleus of the tectothalamic tract is also involved in reciprocal projections of the pretectum and nucleus rotundus. In addition, the pretectal nuclei project reciprocally to the optic tectum and possibly to the telencephalic isocortical homologues. Comparison of these findings with previous work on other species reveals striking similarities between the pretectorotundal pathway in turtles and birds and in the pretectogeniculate pathway in turtles, birds, and mammals.

Morphology, axonal projection pattern, and response types of tectal neurons in plethodontid salamanders. I: tracer study of projection neurons and their pathways

In three salamander species (Hydromantes italicus, H. genei, Plethodon jordani), the tectobulbospinal and tectothalamic pathways and their cells of origin were studied by means of anterograde and retrograde biocytin and tetramethylrhodamine tracing. In plethodontid salamanders, five types of tectal projection neurons were identified. TO1 neurons have widefield dendritic trees that arborize in the layers of retinal afferents and form a neuropil in the superficial layer; axons constitute the crossed tectospinal tract. Dendrites of TO2 cells have the largest dendritic trees that arborize in the intermediate and deep layers of retinal afferents; axons constitute a lateral uncrossed tectospinal tract. TO3 cells have widefield dendritic trees that arborize in the deep layer of retinal afferents and in the layer of tectal efferents; axons constitute a superficial uncrossed tectospinal tract. TO4 cells have slender primary dendrites and small-field dendritic trees that arborize in the intermediate layers of retinal afferents; axons constitute another lateral uncrossed tectospinal tract. TO2, TO3, and TO4 cells also have ascending axons that run to the ventral and dorsal thalamus. TO5 cells have slender primary dendrites and small-field dendritic trees that extend into the superficial layers of retinal afferents; their fine axons constitute the bulk of the pathways ascending to the ipsilateral and contralateral thalamus. These morphological types of projection neurons and their ascending and descending axonal pathways closely resemble those found in frogs, reptiles, and birds. Their role in visual and visuomotor functions is discussed.

The ventral lateral geniculate nucleus and the intergeniculate leaflet: interrelated structures in the visual and circadian systems

The ventral lateral geniculate nucleus (vLGN) and the intergeniculate leaflet (IGL) are retinorecipient subcortical nuclei. This paper attempts a comprehensive summary of research on these thalamic areas, drawing on anatomical, electrophysiological, and behavioral studies. From the current perspective, the vLGN and IGL appear closely linked, in that they share many neurochemicals, projections, and physiological properties. Neurochemicals commonly reported in the vLGN and IGL are neuropeptide Y, GABA, enkephalin, and nitric oxide synthase (localized in cells) and serotonin, acetylcholine, histamine, dopamine and noradrenalin (localized in fibers). Afferent and efferent connections are also similar, with both areas commonly receiving input from the retina, locus coreuleus, and raphe, having reciprocal connections with superior colliculus, pretectum and hypothalamus, and also showing connections to zona incerta, accessory optic system, pons, the contralateral vLGN/IGL, and other thalamic nuclei. Physiological studies indicate species differences, with spectral-sensitive responses common in some species, and varying populations of motion-sensitive units or units linked to optokinetic stimulation. A high percentage of IGL neurons show light intensity-coding responses. Behavioral studies suggest that the vLGN and IGL play a major role in mediating non-photic phase shifts of circadian rhythms, largely via neuropeptide Y, but may also play a role in photic phase shifts and in photoperiodic responses. The vLGN and IGL may participate in two major functional systems, those controlling visuomotor responses and those controlling circadian rhythms. Future research should be directed toward further integration of these diverse findings.

Intertectal commissural projection in the lizard Gallotia stehlini: origin and midline topography

The retinotectal projection of reptiles is largely crossed. The intertectal commissure is an important pathway that interconnects directly the two sides of the optic tectum. The rostrocaudal topography of intertectal commissural fibers at the dorsal midplane was examined by means of the in vitro horseradish peroxidase (HRP) labelling technique in the lizard Gallotia stehlini. Unilateral large deposits of tracer in the optic tectum as well as smaller deposits restricted to one quadrant were used to map the intertectal fibers anterogradely. Most commissural axons reached the contralateral side grouped into a dense bundle at the transition between two structurally distinct parts of the midbrain dorsal midline. The smaller rostral zone relates laterally to the griseum tectale, whereas the larger caudal zone relates to the tectum. The intertectal fibers seem to converge on the rostralmost part of the latter midline region, even though they originate throughout the optic tectum. A rough rostrocaudal tectotopic order was detected at the midline. Retrogradely labelled neurons were best obtained by depositing HRP directly within the compact commissure at the midline. These belong to pyriform cells in the periventricular layers 3 and 5. Axons labelled from the tectum did not enter the posterior commissure nor the intervening commissural region related to the griseum tectale.

Anatomical pathways from the optic tectum to the spinal cord subserving orienting movements in the barn owl

Electrical stimulation of the optic tectum in many vertebrate species elicits eye, head or body orienting movements in the direction of the receptive field location recorded at the site of stimulation; in the barn owl, tectal stimulation produces short latency saccadic head movements (du Lac and Knudsen 1990). However, the barn owl, like other avians, lacks a direct projection from the tectum to the spinal cord, implying that less direct connections underlie tectally mediated head movements. In order to determine the pathways by which the tectum gains access to spinal cord circuitry, we searched for overlap regions between tectal efferent projections and the locations of cells afferent to the spinal cord. Tectal efferent pathways and terminal fields were revealed by anterograde labeling using horseradish peroxidase (HRP) or tritiated amino acids injected into the optic tectum. Cells afferent to the spinal cord were identified by means of retrograde labeling using HRP, rhodamine, or rhodamine-coupled latex beads injected into the cervical spinal cord. A comparison of results from the anterograde and retrograde labeling experiments demonstrated several areas of overlap. All of the cell groups that both received heavy tectal input and contained a high proportion of cells projecting to the spinal cord were located in the medial half of the midbrain and rhombencephalic tegmentum, and included the red nucleus, the interstitial nucleus of Cajal, the medial reticular formation, the nucleus reticularis pontis giganto-cellularis, and the nucleus reticularis pontis oralis. All of these cell groups receive their tectal input from the medial efferent pathway, one of three major output pathways from the tectum. The other two output pathways (the rostral and the caudal) project to regions containing no more than a few scattered cells that are afferent to the spinal cord. Based on these data and on the functions of homologous cell groups in other vertebrates, we hypothesize that the medial efferent pathway and its brainstem target nuclei are primarily responsible for tectally mediated orienting head movements in the barn owl.

Organization of ascending projections from the optic tectum and mesencephalic pretectal gray in Rana pipiens

The ascending projections from the dorsal mesencephalon to the thalamus and pretectum in Rana pipiens were investigated by using the anterograde and retrograde transport of HRP with regard to two major issues: (1) the degree of tectotopic organization in the projections, and (2) their cells of origin. The results indicate that the spatial organization of the tecto-thalamic tract is specifically related to the laminar organization of the contributing tectal efferent neurons. Axons of neurons in the superficial portion of tectal layer 8 exit the tectum through layer 9 and travel in the superficial portion of the dorsal and ventral tecto-thalamic tracts and innervate the nucleus lentiformis mesencephali, the posterior lateral dorsal nucleus, and corpus geniculatum. The distribution of terminals within these structures varied with the tectal HRP-injection site. HRP injections in the ventral tecto-thalamic tract retrogradely labeled neurons in the superficial portion of tectal layer 8 across the lateral and caudal portion of the tectal lobe. HRP injections into the dorsal tecto-thalamic tract, at the level of the pretectum, retrogradely labeled pyriform neurons in the superficial portion of tectal layer 8 in the rostral and medial portions of the tectal lobe. With regard to the deep tectal layers, axons from pyramidal neurons in layer 6 and ganglionic neurons in layer 8 leave the tectum through layer 7, travel in both the dorsal and ventral tecto-thalamic tracts, and are located internal to the axons of the pyriform neurons of superficial tectal layer 8. The majority of the ganglionic neurons project to the posterior lateral ventral nucleus and the anterior lateral nucleus. The distribution of terminals within these nuclei did not display a tectotopic organization. A second major projection to the thalamus originates from the mesencephalic pretectal gray and innervates the nucleus lentiformis mesencephali, the posterior lateral dorsal nucleus, the anterior lateral nucleus, dorsal and ventral divisions of the ventral lateral thalamus, and the nucleus of Bellonci. Other axons from the mesencephalic pretectal gray terminate in the contralateral, medial portions of the posterior lateral dorsal thalamus, the ventral lateral thalamus, and the anterior lateral nucleus. The isthmo-tectal projection was also retrogradely labeled following tectal injections of HRP. This pathway travels in the most ventral portion of the ventral tecto-thalamic tract; its axons passed over the lateral margin of the endopeduncular nucleus bilaterally, and crossed the midline in the caudal portion of the optic chiasm. Extensive, bead-like varicosities were observed on these axons both in the endopeduncular nucleus and in the posterior optic chiasm.(ABSTRACT TRUNCATED AT 400 WORDS)

The serotoninergic system of the brain of the viper, Vipera aspis. An immunohistochemical study

Serotoninergic cell bodies and fibers in the brain of the viper, Vipera aspis, were visualized by immunohistochemistry. Immunoreactive cell bodies were observed in the diencephalic hypothalamic periventricular organ and in the dorsal wall of the infundibular recess, in the nuclei raphe superior and inferior of the midbrain and hindbrain, and to a lesser extent in the nuclei reticularis superior, reticularis inferior and reticularis lateralis. In contrast to other reptilian species, serotoninergic cells were also observed in the central gray matter of the midbrain in the neighbourhood of the nucleus of the trochlear nerve. Immunoreactive fibers are widely distributed throughout the brain of the viper. In the olfactory bulb, fibers were observed in the internal plexiform layer and mitral cell layer. The cerebral cortex contains the highest density of fibers in the dorsal region. The distribution of immunoreactive fibers in the dorsal ventricular ridge is extremely heterogeneous, and five subcomponents of this structure can be distinguished. The majority of diencephalic and mesencephalic structures that contain immunoreactive fibers are also primary visual centres: the nuclei geniculatus lateralis pars dorsalis, the n. posterodorsalis and n. opticus tegmenti, and the optic tectum. Serotoninergic fibers in the nuclei of the oculomotor and motor cranial nerves (III, IV, V, VII, X) are disposed in a tightly woven basket around the non-immunoreactive cell bodies of the motoneurons. These findings, together with the available literature, suggest that the serotoninergic system in snakes is comparable to that in lizards, with a massive ascending projection of fibers from the n. raphe superior to mesencephalic and prosencephalic structures, and a descending projection from the n. raphe inferior to the spinal cord.

The parabigeminogeniculate projection: connectional studies in eight mammals

Anterograde and retrograde tracing methods have been used to analyze the origin and distribution of parabigeminogeniculate axons in the gray squirrel, the gopher, the rat, the opossum, the cat, the greater bushbaby, the squirrel monkey and the macaque monkey. Our findings reveal that parabigeminogeniculate axons most heavily innervate regions of the lateral geniculate that are also targeted by axons arising from the superior colliculus (tectogeniculate). These geniculate layers and zones of parabigeminal and tectal overlap contain small cells, and in several species are associated with the small W cell retino-geniculocortical pathway. In addition to the dense input to small-celled layers and zones, parabigeminal axons in several species also innervate regions of the lateral geniculate nucleus that are relatively free of tectogeniculate axons and that are associated with the medium (X) and large (Y) cell streams. Finally, our data reveal that the laterality of parabigeminogeniculate pathways varies across mammals, being primarily crossed in the gray squirrel, the gopher, the rat, and the opossum, bilateral in the cat, and primarily ipsilateral in the three primates.

Neuronal typology of the thalamic area triangularis of Gallotia galloti (reptilia, sauria)

In a Golgi study of the area triangularis (AT), a rostral nucleus of the ventral thalamus of Gallotia galloti, we have identified four major neuronal types on the basis of their morphological characteristics: medium-sized fusiforms with two processes, medium-sized fusiforms with three or four processes, small bipolars, and small and medium-sized multipolars. These neurons are characterized by a simple morphology and radial arrangement. Cell size varies from small to medium, and all axons project laterally. These characteristics distinguish AT neurons from those of neighboring nuclei. In addition, we found some evidence of differential topographic distribution of each neuronal type within the nucleus. Medium-sized fusiform neurons with two processes are located in the most ventral part, where they constitute the ventral nuclear limit. Small multipolar neurons prevail in the dorsal and ventromedial parts, and in the rest of the nucleus medium-sized neurons, including both fusiform with three or four processes and multipolar types, are normally found. Finally, we discuss a putative homology of the reptilian AT with a part of the mammalian zona incerta.

A light- and electron-microscopic investigation of the optic tectum of the frog, Rana pipiens, II: The neurons that give rise to the crossed tecto-bulbar pathway

The superficial layers of the frog's optic tectum, Potter's (1969) layers A-G, comprise a complex neuropil made up of many afferent axons, the somata of a few neurons, and many dendrites from the neurons located in the deeper layers. Different types of retinal axons are believed to terminate in different layers (Maturana et al., 1960; Kuljis & Karten, 1988; Sargent et al., 1989), but little is known about the relationships between each type of input and the dendrites of the deep tectal neurons that extend into these superficial layers. The present study used the method of retrograde transport of horseradish peroxidase to study the synaptic contacts on the dendrites of the neurons that give rise to the crossed tecto-bulbar pathway. These cells have apical dendrites that ascend through the superficial retino-recipient layers. The somata of the cells that give rise to the crossed tecto-bulbar pathway are located in the superficial half of layer 6, preferentially clustered along the caudal, lateral, and rostral margins of the tectum. The somata of these cells range from 8-30 microns in diameter. Their axons are large (2-4 microns in diameter) myelinated fibers that arise from either their somata or proximal dendrites. Their axons travel within the deep medullary layer to leave the tectum at the lateral margin. Their dendritic arbors extend obliquely through the superficial layers to reach layer B where they turn and extend within the layer for up to 0.5 mm. The somata of these cells receive only a scant synaptic input. In contrast, their dendrites receive input in every layer, but the nature of this input varies from layer to layer. Synaptic terminals that resemble retinal ganglion cell boutons contact the labeled dendrites in layers B, F, and G. This indicates that the dendrites may receive monosynaptic input from several types of retinal ganglion cells. Terminals with small, flattened vesicles also contact the dendrites of these cells in each layer. In layer F and below, the terminals with flattened vesicles constitute 15% of the contacts; above layer F they constitute only 5-8% of the contacts. Terminals with medium-sized, flattened vesicles also contact the dendrites of these cells in every layer and constitute a large proportion of their input (33-95%). The latter terminals resemble those that are often postsynaptic to retinal terminals.

Tectal connectivity in the frog Rana pipiens: tectotegmental projections and a general analysis of topographic organization

Recent studies of visually elicited orienting in the frog Rana pipiens suggest that tectofugal signals important in this behavior relay in the midbrain tegmentum before descending to the spinal cord. They also suggest that the high degree of topographic organization displayed by the retinotectal projection may be less characteristic of other tectal afferent and efferent pathways. To explore these possibilities, we have studied patterns of retrograde and anterograde labelling following multiple and single injections of horseradish peroxidase into the tectum. We have found that the midbrain tegmentum is a major terminal zone for tectal efferent projections. Our material also provided a description of the boundaries of other structures which project to and receive input from the tectum. With this background, we studied topographic organization by analyzing for each structure the distribution of labelling following multiple injections, and comparing it with the label distribution following single injections at tectal loci with known visual field input. Multiple injections produced patchy anterograde and retrograde labelling in the nucleus isthmi, with the number of patches corresponding to the number of tectal sites injected. Single injections produced labelling in restricted regions of the nucleus isthmi, the location of which varied systematically with the location of the tectal injection site. In all other structures studied, labelling was more evenly distributed following multiple injections. In none of these structures could we detect systematic variations in the location of labelling associated with variations in the location of single tectal injection sites, and the labelling following single injections was frequently coextensive with that following multiple injections. We also found no evidence that there exist structures which project to or receive input from particular tectal regions and not others. We conclude that there exist adequate neuroanatomical substrates for a tectotegmentospinal pathway believed to be important for visually elicited orienting in the frog. We also conclude that a high degree of topographic organization is more the exception than the rule in considering tectal connections generally in the frog. Topographic organization was readily apparent in connections related to the nucleus isthmi but not in connections related to any other nonretinal structure.

The organization of descending tectofugal pathways underlying orienting in the frog, Rana pipiens. I. Lateralization, parcellation, and an intermediate spatial representation

We have studied the visually triggered orienting behavior of frogs following complete unilateral transection of the neuraxis at the junction of the medulla and spinal cord, as well as after smaller lesions at the same level. Complete transection produces the same behavioral deficit as previously reported (Kostyk and Grobstein 1982, 1987a) for a similar lesion at the junction between midbrain and medulla. Lesioned frogs failed to turn toward stimuli at all locations in the ipsilateral visual hemifield, responding instead with forwardly directed movements in which there was a persistance of variations related to stimulus elevation and distance. Responses to stimuli in the contralateral visual hemifield were normal. Similar deficits were seen after smaller lesions restricted to a medial white tract. Partial damage to the tract resulted in turns of reduced amplitude for stimuli throughout the ipsilateral hemifield. Lesions to adjacent tissue were without effect on the behaviors studied. In all animals, we observed a strong correlation between turn amplitude for lateral stimuli and the distance at which the animals switched from snapping to hopping. These observations provide new evidence that a transformation from a retinocentric to a lateralized and parcellated form of spatial representation occurs in going from the retinotectal projection to the descending tectofugal pathway in the caudal midbrain, and that this form of representation remains stable until the spinal cord. A second transformation involved in determining the actual movement to be triggered must occur subsequently. Our findings also suggest that the signals underlying orienting turns may not descend into the spinal cord on tectospinal axons, and suggest that the lateralization of descending signals probably occurs coincidentally with a synaptic relay in the midbrain tegmentum.

Origins of descending projections to the medulla oblongata and rostral medulla spinalis in the urodele Salamandra salamandra (amphibia)

Descending projections to the medulla oblongata and rostral medulla spinalis have been examined in the urodele Salamandra salamandra with retrograde horseradish peroxidase tracing. Ipsilateral projections originate from the striatum and the nucleus ventrolateralis thalami and reach the medulla oblongata. The ipsilateral nucleus praeopticus magnocellularis reaches the medulla spinalis. The rostral part of the nucleus tuberculi posterioris projects to the ipsilateral medulla oblongata; its caudal part projects further caudally. Tectal efferents and the efferents of the nucleus praetectalis profundus project bilaterally, the nucleus praetectalis superficialis, nucleus mesencephalicus nervi trigemini, torus semicircularis, nucleus Darkschewitsch, and nucleus fasciculi longitudinalis medialis project ipsilaterally to the medulla oblongata. The nucleus mesencephalicus nervi trigemini, nucleus fasciculi longitudinalis medialis, and tectal efferents reach the rostral medulla spinalis. The nucleus ruber projects mainly via the contralateral dorsolateral funiculus to the medulla spinalis. A largely crossed medullary projection arises in the nucleus dorsalis tegmenti pars anterior, a bilateral projection arises in the nucleus dorsalis tegmenti pars posterior, and an ipsilateral projection arises in the nucleus ventralis tegmenti pars anterior. Cerebellar and statoacoustic efferents descend to the medulla spinalis. The nucleus reticularis isthmi, superior, medius and inferior as well as the nucleus raphes exhibit spinal trajectories. The nucleus vestibularis magnocellularis projects bilaterally, the nucleus vestibularis medialis projects ipsilaterally spinalward. The supposed nucleus descendens nervi trigemini descends mainly contralaterally. A small spinal projection arises in the nucleus tractus solitarii. The results indicate that salamander brains display elaborate descending connections which are similar to those in other vertebrates despite their scarcely differentiated neuronal cytoarchitecture.

The optic tectum of the gymnotiform electric fish, Eigenmannia: labeling of physiologically identified cells

A total of 47 tectal neurons of the weakly electric fish, Eigenmannia, were studied physiologically and labelled by intracellular injection of Lucifer Yellow. With the exception of two cell types, all cells could be classified in accordance with the Golgi studies of Sas and Maler. The dominant stimulus modality of neurons was correlated with their laminar location. Neurons of the stratum opticum only responded to visual stimuli, such as modulations of the light level or the motion of an object. They showed, however, no directional preferences for motion. Neurons of the stratum griseum centrale were predominantly driven by electrosensory stimuli, most often those associated with the movement of an object, and generally were very sensitive to the direction of motion. Integration of different sensory modalities was found in neurons with dendrites invading laminae with different sensory inputs. In addition, small axons of interneurons appear to relay information across laminae. Large multipolar neurons in the deep tectum responded to the motion of objects, often preferring a particular direction of motion. Some of these large multipolar neurons of the deep tectum also discriminated the sign of the frequency difference between a mimic of a neighbor's sinusoidal electric organ discharge and the animal's own signal. These neurons are potential candidates for the control of the jamming avoidance response. These neurons were morphologically indistinguishable from large multipolar neurons of the deep tectum that either responded to moving objects or to acoustical stimuli. Individual large cells of the deep tectum project to various targets (Fig. 1) and probably contribute to the control of different behavioral responses. This suggests that the nature of such responses would then depend upon the constitution of sets of neurons recruited by a given stimulus situation, and the role of individual tectal neurons would neither be particularly specific nor very significant.

Neuronal organization underlying visually elicited prey orienting in the frog--III. Evidence for the existence of an uncrossed descending tectofugal pathway

A complete transverse hemisection of the neuraxis just caudal to the optic tectum in the frog, Rana pipiens, results in a failure to orient toward stimuli in one visual hemifield [Kostyk and Grobstein (1986) Neuroscience 21, 41-55]. This finding indicates that each tectal lobe gives rise to a crossed descending pathway adequate to cause turns in a direction contralateral to that tectal lobe, and suggests that each may also give rise to an uncrossed descending pathway adequate to cause turns in the ipsilateral direction. To determine whether there is in fact such an uncrossed pathway, we have studied the orienting behavior of frogs after lesions which interrupt crossed pathways. Two groups of animals were studied. In one group we made midline lesions of the ansulate commissure, through which run the major crossed descending projections from both tectal lobes. In the other group, we combined a complete transverse hemisection with removal of the tectal lobe on the same side of the brain, leaving intact only an uncrossed pathway from one tectal lobe. A persistence of orienting turns was observed in both groups of animals. In both, the direction of the turns was that expected on the assumption that an uncrossed pathway would cause ipsilateral turns. We conclude that such a pathway exists. While both groups of animals turned in the expected directions, they did so for stimuli at unexpected locations. Increasingly eccentric stimulus locations to one side of the mid-sagittal plane were associated with increasing amplitude turns to the other. The observation suggests that tectal regions mapping areas of visual space to one side of the mid-sagittal plane are capable of triggering turns not only in that direction but in the opposite direction as well. In the case of ansulate commissure section, mirrored orienting responses were observed for tactile stimuli as well. These and other behavioral anomalies described in the preceding papers [Kostyk and Grobstein (1986) Neuroscience 21, 41-55 and 57-82] suggest that between the topographic retinotectal projection and the premotor circuitry for orienting there may exist an intermediate processing step, one in which stimulus location is represented in a generalized spatial coordinate frame.

Optic tectum of the eastern garter snake, Thamnophis sirtalis. V. Morphology of brainstem afferents and general discussion

Brainstem neurons that project to the optic tectum of the eastern garter snake were identified by retrograde transport of horseradish peroxidase. The distribution and morphology of tectal afferent axons from the thalamus, pretectum, nucleus isthmi, and midbrain reticular formation were then studied by anterograde transport of horseradish peroxidase. Diencephalic projections to the tectum arise from the ventral lateral geniculate complex ipsilaterally and the ventrolateral nucleus, suprapeduncular nucleus, and nucleus of the ventral supraoptic decussation bilaterally. Three pretectal groups (the lentiform thalamic nucleus, the lentiform mesencephalic-pretectal complex and the geniculate pretectal nucleus) give rise to heavy, bilateral tectal projections. Small neurons in nucleus isthmi and large reticular neurons in nucleus lateralis profundus mesencephali also give rise to bilateral projections. Caudal to the tectum, projections arise bilaterally from the pontine and medullary tegmentum, nuclei of the lateral lemniscus, the posterior colliculus, and the sensory trigeminal nucleus. A small contralateral projection arises from the medial vestibular complex. Tectal afferents from the thalamus, pretectum, nucleus isthmi, and midbrain reticular formation had characteristic morphologies and laminar distributions within the tectum. However, these afferents fall into two groups based on their spatial organization. Afferents from the thalamus and nucleus isthmi arise from small neurons with spatially restricted, highly branched dendritic trees. Their axons terminate in single, highly branched and bouton-rich arbors about 100 micron in diameter. By contrast, afferents from the midbrain reticular formation and the pretectum arise from large neurons with long, radiate, and sparsely branched dendritic trees. Their axons course parallel to the tectal surface and emit numerous collateral branches that are distributed widely through the mediolateral and rostrocaudal extent of either the central or superficial gray layers. Each collateral bears several small, spatially disjunct clusters of boutons.

Optic tectum of the eastern garter snake, Thamnophis sirtalis. III. Morphology of intrinsic neurons

Extracellular iontophoretic injections of horseradish peroxidase and Golgi preparations were used to study the distribution and morphology of intrinsic neurons of the garter snake optic tectum. Four morphologically distinct classes of neurons were identified. The type A neuron is found throughout the retinorecipient tectal layers. It has a large, fusiform soma and infrequently branching dendrites that radiate in the horizontal plane and are studded with varicose appendages. An axon arises from the soma or proximal dendrite and gives rise to widely spreading branches that overlap the cell's dendritic field. The type B neuron has a small, spherical soma in sublayer b of the stratum fibrosum et griseum superficiale. Thick, varicose dendrites ascend from the soma and form a bushy arbor in the overlying sublayer a. A thin axon descends vertically from the soma and arborizes in vertical alignment with the cell's dendritic field in sublayer c of the stratum fibrosum et griseum superficiale and the upper third of the stratum griseum centrale. The type C neuron is a bipolar cell with a small, vertically fusiform soma situated at the upper border of the stratum griseum centrale. Thin, sparsely branching dendrites extend vertically into the superficial and central gray layers. An axon arises from the soma and courses ventrally into the stratum griseum centrale where it gives rise to a plexus of widely spreading branches that extend medially from the cell's dendritic field. The type D neuron is a small, stellate cell with a spherical soma and fine, appendage-laden dendrites that are restricted to the stratum griseum centrale. The axon of the type D cell courses in the central gray where it gives rise to widely spreading branches that extend laterally from the cell's dendritic field.

Optic tectum of the eastern garter snake, Thamnophis sirtalis. IV. Morphology of afferents from the retina

The morphology of single retinal terminals in the optic tectum of the eastern garter snake was demonstrated by orthograde filling from extracellular injections of horseradish peroxidase (HRP) into the optic tract. HRP-filled terminals share a characteristic shape and structure. Their parent axons course caudally in the stratum opticum within fascicles of 200-300 fibers of varying diameters. Single axons exit a fascicle and course into either the stratum fibrosum et griseum superficiale, ventrally, or the stratum zonale, dorsally, where they bifurcate successively two or three times into preterminal branches. Each preterminal branch gives rise to many thin, terminal branchlets laden with boutons. The arbors are ellipsoidal with their long axes oriented mediolaterally and their short axes oriented rostrocaudally. Arbors vary in their overall size (from 45 to 150 micron), in the diameters of their parent axons (from less than 0.5 to 3.0 micron), and in the size of their terminal boutons (from 0.5 to 3.5 micron). Bouton size increased with increasing diameter of the parent axon. The great majority of arbors are confined to one of three retinorecipient sublayers in the superficial tectum. However, the full range of arbor sizes and axon diameters is present in each sublayer.

Optic tectum of the eastern garter snake, Thamnophis sirtalis. II. Morphology of efferent cells

Tectal efferent neurons were retrogradely filled from extracellular injections of horseradish peroxidase (HRP) into pathways efferent from the tectum. Tectorotundal neurons have cylindrical dendritic trees, 80-100 microns in diameter, that extend vertically across the central and superficial tectal layers. Apical and basal dendrites are laden with complex appendages. The axon gives rise to an intratectal, collateral arbor that extends horizontally into the stratum griseum centrale beyond the cell's dendritic tree. The parent axon exits the tectum laterally in the tectothalamic tract. Tectogeniculate neurons also have narrow, radially oriented, and highly branched apical dendrites, but their basal dendrites are infrequently branched and lack appendages. An intratectal axon collateral forms a small, spherical arbor overlapping the apical dendrites in sublayer c of the stratum fibrosum et griseum superficiale. The parent axon ascends vertically and just below the stratum opticum turns rostrad to follow the optic fibers to the diencephalon. Tectoisthmi neurons have small somata and thin, radial dendrites that arborize below the pial surface in the stratum zonale. An intratectal axon collateral forms a spatially restricted arbor ventral to the soma in register with the dendritic tree. Tectoisthmobulbar neurons have dendrites that arborize extensively in sublayer a of the stratum fibrosum et griseum superficiale. The axon exits the tectum without collateralizing and joins a small-caliber component of the ventral tectobulbar tract. Ipsilateral tectobulbar neurons have stellate dendritic fields, 150-250 microns in diameter, that are restricted to the deep layers of the tectum. Sparsely branched dendrites are appendage-free but bear many short, fine spicules. The axon initially ascends from the soma and recurves into the stratum album centrale without collateralizing before joining a medium-caliber component of the ventral tectobulbar tract. Crossed tectobulbar neurons have large, stellate dendritic trees with diameters ranging from 200 to 500 microns. Like ipsilateral tectobulbar neurons, their dendrites are appendage-free but bear spicules. Their thick-caliber axons exit the tectum without collateralizing and course deep in the stratum album centrale to reach the dorsal tectobulbar tract.