Cerebellar connections in Xenopus laevis. An HRP study

Red nucleus structure and function: from anatomy to clinical neurosciences

The red nucleus (RN) is a large subcortical structure located in the ventral midbrain. Although it originated as a primitive relay between the cerebellum and the spinal cord, during its phylogenesis the RN shows a progressive segregation between a magnocellular part, involved in the rubrospinal system, and a parvocellular part, involved in the olivocerebellar system. Despite exhibiting distinct evolutionary trajectories, these two regions are strictly tied together and play a prominent role in motor and non-motor behavior in different animal species. However, little is known about their function in the human brain. This lack of knowledge may have been conditioned both by the notable differences between human and non-human RN and by inherent difficulties in studying this structure directly in the human brain, leading to a general decrease of interest in the last decades. In the present review, we identify the crucial issues in the current knowledge and summarize the results of several decades of research about the RN, ranging from animal models to human diseases. Connecting the dots between morphology, experimental physiology and neuroimaging, we try to draw a comprehensive overview on RN functional anatomy and bridge the gap between basic and translational research.

Molecular Logic of Spinocerebellar Tract Neuron Diversity and Connectivity

Coordinated motor behaviors depend on feedback communication between peripheral sensory systems and central circuits in the brain and spinal cord. Relay of muscle- and tendon-derived sensory information to the CNS is facilitated by functionally and anatomically diverse groups of spinocerebellar tract neurons (SCTNs), but the molecular logic by which SCTN diversity and connectivity is achieved is poorly understood. We used single-cell RNA sequencing and genetic manipulations to define the mechanisms governing the molecular profile and organization of SCTN subtypes. We found that SCTNs relaying proprioceptive sensory information from limb and axial muscles are generated through segmentally restricted actions of specific Hox genes. Loss of Hox function disrupts SCTN-subtype-specific transcriptional programs, leading to defects in the connections between proprioceptive sensory neurons, SCTNs, and the cerebellum. These results indicate that Hox-dependent genetic programs play essential roles in the assembly of neural circuits necessary for communication between the brain and spinal cord.

Baek et al. show that Hox-transcription factor-dependent programs govern the specification and connectivity of spinal interneurons that relay muscle-derived sensory information to the cerebellum. These findings shed light on the development of neural circuits required for proprioception—the perception of body position.

Indirect pathway to pectoral fin motor neurons from nucleus ruber in the Nile tilapia Oreochromis niloticus

Supraspinal motor control systems of pectoral fins remain unclear in teleosts. Nucleus ruber of Goldstein (1905; NRg), which has been identified as the probable homologue of nucleus ruber of tetrapods, is a candidate structure serving for such functions. In the present study, we investigated possible involvement of the NRg in the control of pectoral fin movement by tract-tracing experiments in the Nile tilapia Oreochromis niloticus. Tracer injections into the NRg revealed the fiber course of rubrospinal tract. Rubrospinal fibers crossed the midline at the level of midbrain, descended through the tegmentum, and terminated in a region ventrally adjacent to the dorsal horn at the spinomedullary junction, without reaching the ventral horn where pectoral fin motor neurons are present. Tracer injection experiments into the dorsal horn region resulted in labeled terminals in proximities of presumed pectoral fin motor neurons in the ventral horn. Tracer injection experiments into the ventral horn resulted in retrogradely labeled neurons ventrally adjacent to the dorsal horn, where labeled terminals were detected following rubral injections. These anatomical analyses suggest that the NRg of actinopterygians is involved in the control of pectoral fin motor neurons through an indirect pathway via interneurons in the dorsal horn.

Neural connections of the pretectum in zebrafish (Danio rerio)

The pretectum is a complex region of the caudal diencephalon which in adult zebrafish comprises both retinorecipient (parvocellular superficial, central, intercalated, paracommissural, and periventricular) and non-retinorecipient (magnocellular superficial, posterior, and accessory) pretectal nuclei distributed from periventricular to superficial regions. We conducted a comprehensive study of the connections of pretectal nuclei by using neuronal tracing with fluorescent carbocyanine dyes. This study reveals specialization of efferent connections of the various pretectal nuclei, with nuclei projecting to the optic tectum (paracommissural, central, and periventricular pretectal nuclei), the torus longitudinalis and the cerebellar corpus (paracommissural, central, and intercalated pretectal nuclei), the lateral hypothalamus (magnocellular superficial, posterior, and central pretectal nuclei), and the tegmental regions (accessory and superficial pretectal nuclei). With regard to major central afferents to the pretectum, we observed projections from the telencephalon to the paracommissural and central pretectal nuclei, from the optic tectum to the paracommissural, central, accessory and parvocellular superficial pretectal nuclei, from the cerebellum to the paracommissural and periventricular pretectal nuclei and from the nucleus isthmi to the parvocellular superficial and accessory pretectal nuclei. The parvocellular superficial pretectal nucleus sends conspicuous projections to the contralateral magnocellular superficial pretectal nucleus. The composite figure of results reveals large differences in connections of neighbor pretectal nuclei, indicating high degree of nuclear specialization. Our results will have important bearings in functional studies that analyze the relationship between specific circuits and behaviors in zebrafish. Comparison with results available in other species also reveals differences in the organization and connections of the pretectum in vertebrates.

Embryonic stages in cerebellar afferent development

The cerebellum is important for motor control, cognition, and language processing. Afferent and efferent fibers are major components of cerebellar circuitry and impairment of these circuits causes severe cerebellar malfunction, such as ataxia. The cerebellum receives information from two major afferent types – climbing fibers and mossy fibers. In addition, a third set of afferents project to the cerebellum as neuromodulatory fibers. The spatiotemporal pattern of early cerebellar afferents that enter the developing embryonic cerebellum is not fully understood. In this review, we will discuss the cerebellar architecture and connectivity specifically related to afferents during development in different species. We will also consider the order of afferent fiber arrival into the developing cerebellum to establish neural connectivity.

Anatomy of zebrafish cerebellum and screen for mutations affecting its development

The cerebellum is important for the integration of sensory perception and motor control, but its structure has mostly been studied in mammals. Here, we describe the cell types and neural tracts of the adult zebrafish cerebellum using molecular markers and transgenic lines. Cerebellar neurons are categorized to two major groups: GABAergic and glutamatergic neurons. The Purkinje cells, which are GABAergic neurons, express parvalbumin7, carbonic anhydrase 8, and aldolase C like (zebrin II). The glutamatergic neurons are vglut1(+) granule cells and vglut2(high) cells, which receive Purkinje cell inputs; some vglut2(high) cells are eurydendroid cells, which are equivalent to the mammalian deep cerebellar nuclei. We found olig2(+) neurons in the adult cerebellum and ascertained that at least some of them are eurydendroid cells. We identified markers for climbing and mossy afferent fibers, efferent fibers, and parallel fibers from granule cells. Furthermore, we found that the cerebellum-like structures in the optic tectum and antero-dorsal hindbrain show similar Parvalbumin7 and Vglut1 expression profiles as the cerebellum. The differentiation of GABAergic and glutamatergic neurons begins 3 days post-fertilization (dpf), and layers are first detectable 5 dpf. Using anti-Parvalbumin7 and Vglut1 antibodies to label Purkinje cells and granule cell axons, respectively, we screened for mutations affecting cerebellar neuronal development and the formation of neural tracts. Our data provide a platform for future studies of zebrafish cerebellar development.

Afferent and efferent connections of the cerebellum of a salmonid, the rainbow trout (Oncorhynchus mykiss): A tract-tracing study

The connections of the cerebellum of the rainbow trout were studied by experimental methods. The pretectal paracommissural nucleus has reciprocal connections with the cerebellum. Three additional pretectal nuclei project to both the corpus and valvula cerebelli, and seem to receive cerebellar afferents. A large number of cells of the lateral nucleus of the valvula project to wide regions of the cerebellum, including the valvula, the corpus, the granular eminences, and the caudal lobe, whereas the contralateral inferior olive and scattered reticular cells project only to the corpus and valvula cerebelli. Afferents to the corpus were also observed from the ventral tegmental nucleus, the "paraisthmic nucleus," the perilemniscal nucleus, the central gray, and the octavolateral area. Valvular afferents were also observed from the torus semicircularis and the midbrain tegmental areas. In most cases of cerebellar application, labeled fibers were seen in the thalamus, the pretectum, the torus longitudinalis and torus semicircularis, the nucleus of the medial longitudinal fascicle, and midbrain and rhombencephalic reticular areas. From the corpus cerebelli some fibers also project to the posterior tubercle and the hypothalamus. Moreover, the granular eminences project to the cerebellar crest. DiI application to most of the areas showing labeled fibers after cerebellar tracer application led to the labeling of characteristic eurydendroid cells, mainly in the valvula cerebelli and the caudal lobe. A few putative eurydendroid cells were labeled from the octavolateralis regions. These results in a teleost with a generalized brain indicate several differences with respect to the cerebellar connections reported in other teleost fishes that have specialized brains.

Organization of dye-coupled cerebellar granule cells labeled from afferent vestibular and dorsal root fibers in the frogRana esculenta

Application of neurobiotin to the nerves of individual labyrinthine organs and dorsal root fibers of limb-innervating segments of the frog resulted in labeling of granule cells in the cerebellum showing a significant overlap with a partial segregation in the related areas of termination. In different parts of the cerebellum, various combinations of different canal and otolith organ-related granule cells have been discerned. The difference in the extension of territories of vertical canals vs. horizontal canals may reflect their different involvement in the vestibuloocular and vestibulospinal reflex. Dye-coupled cells related to the lagenar and saccular neurons were localized in more rostral parts of the cerebellum, whereas cells of the utricle were represented only in its caudal half. This separation is supportive of the dual function of the lagena and the saccule. The territories of granule cells related to the cervical and lumbar segments of the spinal cord were almost completely separated along the rostrocaudal axis of cerebellum, whereas their territories were almost entirely overlapping in the mediolateral and ventrodorsal directions. The partial overlap of labyrinthine organ-related and dorsal root fiber-related granule cells are suggestive of a convergence of sensory modalities involved in the sense of balance. We propose that the afferent input of vestibular and proprioceptive fibers mediated by gap junctions to the cerebellar granule cells subserve one of the possible morphological correlates of a very rapid modification of the motor activity in the vestibulocerebellospinal neuronal circuit.

Projections of the sensory trigeminal nucleus in a percomorph teleost, tilapia (Oreochromis niloticus)

The sensory trigeminal nucleus of teleosts is the rostralmost nucleus among the trigeminal sensory nuclear group in the rhombencephalon. The sensory trigeminal nucleus is known to receive the somatosensory afferents of the ophthalmic, maxillar, and mandibular nerves. However, the central connections of the sensory trigeminal nucleus remain unclear. Efferents of the sensory trigeminal nucleus were examined by means of tract-tracing methods, in a percomorph teleost, tilapia. After tracer injections to the sensory trigeminal nucleus, labeled terminals were seen bilaterally in the ventromedial thalamic nucleus, periventricular pretectal nucleus, medial part of preglomerular nucleus, stratum album centrale of the optic tectum, ventrolateral nucleus of the semicircular torus, lateral valvular nucleus, prethalamic nucleus, tegmentoterminal nucleus, and superior and inferior reticular formation, with preference for the contralateral side. Labeled terminals were also found bilaterally in the oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, medial funicular nucleus, and contralateral sensory trigeminal nucleus and inferior olive. Labeled terminals in the oculomotor nucleus and trochlear nucleus showed similar densities on both sides of the brain. However, labelings in the trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, and medial funicular nucleus showed a clear ipsilateral dominance. Reciprocal tracer injection experiments to the ventromedial thalamic nucleus, optic tectum, and semicircular torus resulted in labeled cell bodies in the sensory trigeminal nucleus, with a few also in the descending trigeminal nucleus.

Plurisegmental vestibulocerebellar projections and other hindbrain cerebellar afferents in midterm chick embryos: biotinylated dextranamine experiments in vitro

The vestibular neuronal groups that project to the cerebellum were mapped in midterm chick embryos (10-11 days in "ovo") through "in vitro " retrograde tracing experiments. Massive unilateral deposits of biotin-dextranamine were placed at the basis of the cerebellum to label the cerebellar peduncles. Separate rostral and caudal vestibulo-cerebellar groups were identified, with predominance of contralateral neurons. We tentatively identified the rhombomeric location of both groups, as well as their topography within the conventional cytoarchitectonically-defined vestibular nuclei, by comparison with previously established segmental fate maps. The rostral group extended from rhombomeres 1-4 (r1-r4) and was restricted mainly to the superior vestibular nucleus. The caudal group stretched from r6 to pseudorhombomere "r8" and was related to the descending and medial vestibular nuclei. The less abundant ipsilateral vestibulocerebellar neurons had a similar topography. The crossing axons of the rostral vestibulocerebellar neurons formed a distinct rostral vestibulocerebellar decussation, restricted to the floorplate of rhombomere 2. The axons of the caudal vestibulocerebellar population mostly decussated associated to the deep cochlear commissure. The present results extend the "segmental hodological mosaic" of defined projection-neuron groups identified within the avian vestibular nuclear complex: The vestibulocerebellar projecting neurons as a type appear iterated from r1 to r4 and from r6 to pseudorhombomere "r8," albeit showing in their arrangement peculiarities related to single segmental domains, particularly rostrally. In contrast, the vestibulospinal groups are located more restrictedly in r4-r6, while the vestibulo-ocular projecting neurons extend from r1 to "r7." Only r4 and r6 contain elements of all three hodological types. The organization of the three vestibular projection populations studied to date seems comparable in chicken and frogs and may be a conserved feature in vertebrates.

Ascending and descending projections of the lateral vestibular nucleus in the frog Rana esculenta

The lectin Phaseolus vulgaris leucoagglutinin was injected into the frog lateral vestibular nucleus (LVN) to study its antero- and retrograde projections. The following new observations were made. 1) In the diencephalon, vestibular efferents innervate the thalamus in a manner similar to that of mammalian species. The projections show a preference for the anterior, central, and ventromedial thalamic nuclei. 2) In the mesencephalon, vestibular fibers terminate in the tegmental nuclei and the nucleus of medial longitudinal fascicle. 3) In the rhombencephalon, commissural and internuclear projections interconnect the vestibular nuclei. Some of the termination areas in the reticular formation can be homologized with the mammalian inferior olive and the nucleus prepositus hypoglossi. Another part of the vestibuloreticular projection may transmit vestibular impulses toward the vegetative centers of the brainstem. A relatively weak projection is detected in the spinal nucleus of the trigeminal nerve, dorsal column nuclei, and nucleus of the solitary tract. 4) In the spinal cord, vestibular terminals are most numerous in the ipsilateral ventral horn and in the triangular area of the dorsal horn. 5) The coincidence of retrogradely labeled cells with vestibular receptive areas suggests reciprocal interconnections between these structures and the LVN. 6) In seven places, the LVN projections overlap the receptive areas of proprioceptive fibers, suggesting a convergence of sensory modalities involved in the sense of balance.

Rhombomeric organization of vestibular pathways in larval frogs

Rhombencephalic subnuclei and projection pathways related to vestibular function were mapped in larval ranid frogs. The retention of overt postembryonic rhombomeres (r) allowed direct visualization of the locations of neurons retrogradely labeled with fluorescent dextran amines from the midbrain oculomotor complex, cerebellum, vestibular nuclei, and spinal cord. Oculomotor projecting vestibular neurons were mainly located in bilateral r1/2, ipsilateral r3, and contralateral r5-8, and spinal projecting vestibular neurons mainly in ipsilateral r4 and contralateral r5. Vestibular commissural neurons were located in r1-3 and r5-7 and were largely excluded from r4. Cerebellar projecting neurons included contralateral inferior olivary neurons in r8 and vestibular neurons in bilateral r6/7 and contralateral r1/2. Mapping these results onto adult anuran vestibular organization indicates that the superior vestibular nucleus derives from larval r1/2, the lateral vestibular nucleus from r3/4, and the major portions of the medial and descending vestibular nuclei from r5-8. The lateral vestibulospinal tract projects from an origin in r4, whereas a possible ascending tract of Deiters arises in r3. Rhombomere 5 contains a nuclear group that appears homologous to the tangential nucleus of fish, reptiles, and birds and thus likely serves gravistatic and linear vestibulomotor reflexes. Comparisons between frogs and other vertebrates suggest that vestibular neurons performing similar computational roles during head movements originate from the same segmental locations in different species.

Descending supraspinal pathways in amphibians. I. A dextran amine tracing study of their cells of origin

The present study is the first of a series on descending supraspinal pathways in amphibians in which hodologic and developmental aspects are studied. Representative species of anurans (the green frog, Rana perezi, and the clawed toad, Xenopus laevis), urodeles (the Iberian ribbed newt, Pleurodeles waltl), and gymnophionans (the Mexican caecilian, Dermophis mexicanus) have been used. By means of retrograde tracing with dextran amines, previous data in anurans were largely confirmed and extended, but the studies in P. waltl and D. mexicanus present the first detailed data on descending pathways to the spinal cord in urodeles and gymnophionans. In all three orders, extensive brainstem-spinal pathways were present with only minor representation of spinal projections originating in forebrain regions. In the rhombencephalon, spinal projections arise from the reticular formation, several parts of the octavolateral area, the locus coeruleus, the laterodorsal tegmental nucleus, the raphe nucleus, sensory nuclei (trigeminal sensory nuclei and the dorsal column nucleus), and the nucleus of the solitary tract. In all species studied, the cerebellar nucleus and scattered cerebellar cells innervate the spinal cord, predominantly contralaterally. Mesencephalic projections include modest tectospinal projections, torospinal projections, and extensive tegmentospinal projections. The tegmentospinal projections include projections from the nucleus of Edinger-Westphal, the red nucleus, and from anterodorsal, anteroventral, and posteroventral tegmental nuclei. In the forebrain, diencephalospinal projections originate in the ventral thalamus, posterior tubercle, the pretectal region, and the interstitial nucleus of the fasciculus longitudinalis medialis. The most rostrally located cells of origin of descending spinal pathways were found in the suprachiasmatic nucleus, the preoptic area and a subpallial region in the caudal telencephalic hemisphere, probably belonging to the amygdaloid complex. Our data are discussed in an evolutionary perspective.

Distribution of choline acetyltransferase immunoreactivity in the brain of anuran (Rana perezi, Xenopus laevis) and urodele (Pleurodeles waltl) amphibians

Because our knowledge of cholinergic systems in the brains of amphibians is limited, the present study aimed to provide detailed information on the distribution of cholinergic cell bodies and fibers as revealed by immunohistochemistry with antibodies directed against the enzyme choline acetyltransferase (ChAT). To determine general and derived features of the cholinergic systems within the class of Amphibia, both anuran (Rana perezi, Xenopus laevis) and urodele (Pleurodeles waltl) amphibians were studied. Distinct groups of ChAT-immunoreactive cell bodies were observed in the basal telencephalon, hypothalamus, habenula, isthmic nucleus, isthmic reticular formation, cranial nerve motor nuclei, and spinal cord. Prominent plexuses of cholinergic fibers were found in the olfactory bulb, pallium, basal telencephalon, ventral thalamus, tectum, and nucleus interpeduncularis. Comparison of these results with those obtained in other vertebrates, including a segmental approach to correlate cell populations, reveals that the cholinergic systems in amphibians share many features with amniotes. Thus, cholinergic pedunculopontine and laterodorsal tegmental nuclei could be identified in the amphibian brain. The finding of weakly immunoreactive cells in the striatum of Rana, which is in contrast with the condition found in Xenopus, Pleurodeles, and other anamniotes studied so far, has revived the notion that basal ganglia organization is more preserved during evolution than previously thought.

Anuran dorsal column nucleus: organization, immunohistochemical characterization, and fiber connections in Rana perezi and Xenopus laevis

As part of a research program on the evolution of somatosensory systems in vertebrates, the dorsal column nucleus (DCN) was studied with (immuno)histochemical and tract-tracing techniques in anurans (the large green frog, Rana perezi, and the clawed toad, Xenopus laevis). The anuran DCN contains some nicotinamide adenine dinucleotide phosphate diaphorase-positive neurons, very little calbindin D-28k, and a distinct parvalbumin-positive cell population. The anuran DCN is innervated by primary and non-primary spinal afferents, by primary afferents from cranial nerves V, VII, IX, and X, by serotonin-immunoreactive fibers, and by peptidergic fibers. Non-primary DCN afferents from the spinal cord appear to arise throughout the spinal cord, but particularly from the ipsilateral dorsal gray. The present study focused on the efferent connections of the DCN, in particular the targets of the medial lemniscus. The medial lemniscus could be traced throughout the brainstem and into the diencephalon. Along its course, the medial lemniscus gives off collaterals to various parts of the reticular formation, to the octavolateral area, and to the granular layer of the cerebellum. At mesencephalic levels, the medial lemniscus innervates the lateral part of the torus semicircularis as well as various tegmental nuclei. A striking difference between the two species studied is that while in R. perezi medial lemniscal fibers do not reach the tectum mesencephali, in X. laevis, intermediate and deep tectal layers are innervated. Beyond the midbrain, both dorsal and ventral thalamic areas are innervated by the medial lemniscus. The present study shows that the anuran "lemniscal pathway" is basically similar to that of amniotes.

Chemical identification and morphological characterization of the inferior olive in the frog

Tritiated D-aspartate was injected into the cerebellar cortex of grassfrogs, Rana temporaria. Retrograde labeling was observed in a cell column of the contralateral caudal medulla, but not in other areas known to give rise to cerebellar mossy fibers. The aspartate-positive neurons are therefore considered to represent the origin of cerebellar climbing fibers in the inferior olive, as reported earlier for rat and turtle by other investigators. Contrary to earlier reports we found no extraolivary climbing fibers in the VIIIth nerve and in Scarpa's ganglion. Our results support the view that the climbing fiber system of vertebrates is anatomically, physiologically and chemically very distinct and phylogenetically very conservative.

Morphological evidence for a monosynaptic connection between cerebellar Purkinje cells and vestibulospinal tract neurons in the larval clawed toad, Xenopus laevis

A double-labeling technique was used to trace synaptic connections between the efferent neurons of the cerebellum (Purkinje cells) and vestibulospinal tract neurons in larvae of the clawed toad, Xenopus laevis. The efferent cerebellar projection was labeled with horseradish peroxidase (HRP) while in the same preparations the brainstem neurons projecting to the spinal cord were labeled with cobaltous lysine. It was found that the distribution of the Purkinje cell terminal boutons overlaps significantly with the location of vestibulospinal neurons in the brainstem. Moreover, several close appositions were seen between Purkinje cell boutons and the dendrites and somata of these latter neurons. The close appositions seen in light microscopy were confirmed by subsequent electron microscopy. This study shows that early in development (at least well before metamorphosis) a cerebello-vestibulospinal connection exists in Xenopus laevis. This connection is likely to persist throughout metamorphosis to the adult state.

Development of spinocerebellar afferents in the clawed toad, Xenopus laevis

The development of spinocerebellar projections in the clawed toad, Xenopus laevis, was studied with horseradish peroxidase as an anterograde and retrograde tracer. Early in development cells of origin of spinocerebellar projections were found, contralaterally, in or close to the medial motor column. In older tadpoles ipsilaterally projecting spinal neurons were also labeled from the cerebellum. These are virtually indistinguishable from the large primary motoneurons that occupy a very similar position in the spinal cord. Most of the labeled spinal cells were found in the thoracic spinal cord; they lie halfway between the brachial and lumbar secondary motor columns. Surprisingly, no primary spinocerebellar projection arising from dorsal root spinal ganglion cells could be demonstrated in X. laevis tadpoles and adult toads. Therefore, fibers in the cerebellum that were labeled anterogradely from the spinal cord can be expected to originate exclusively from the secondary spinocerebellar tract cells. These fibers appear to cross the cerebellum in or at the border of the granular layer. The present data suggest that in X. laevis early in the development of the cerebellum a distinct secondary spinocerebellar projection is already present, originating in neurons that can be compared with the "spinal border cells" in mammals. The relative sparseness of this secondary spinocerebellar projection and the apparent absence of primary spinocerebellar afferents probably indicate that spinocerebellar pathways are only of minor importance in X. laevis. The possibility remains, however, that the expansion of the secondary spinocerebellar pathway only starts when metamorphosis has been completed.

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.

Central distribution of the efferent cells and the primary afferent fibers of the trigeminal nerve in Pleurodeles waltlii (Amphibia, Urodela)

As part of a study on the organization of the brainstem in a primitive group of vertebrates, the efferent cells and primary afferent fibers of the urodele amphibian Pleurodeles waltlii were examined by means of retrograde and anterograde axonal transport and anterograde degeneration. The trigeminal motor nucleus is located in the periventricular gray just medial to the sulcus limitans. Its rostral part is a band of pear-shaped cells lying parallel to the wall of the ventricle, whereas its caudal part is a round mass consisting of polygonal cells. In addition, a small group of scattered neurons is situated ventral to the rostral part of the nucleus. The primary afferent fibers enter the brainstem in the dorsal two-thirds of the trigeminal root. They diverge into a short ascending and a long descending tract. The former distributes its axons to the principal sensory trigeminal nucleus, which is an ill-defined cell group located at the ventrolateral edge of the periventricular gray. In the descending tract, the fibers of the ophthalmic nerve are predominantly located ventromedially, and those of the maxillomandibular nerve dorsolaterally. A fascicle of the ophthalmic nerve leaves the descending tract and, apparently, makes contact with the accessory abducens nucleus. The descending tract extends caudally into the three upper cervical segments of the spinal cord. The mesencephalic trigeminal nucleus consists of conspicuous large cells, which are scattered through the tectum of the mesencephalon. The cells with peripheral branches in the ophthalmic nerve are mainly located in the caudal half of the tectum, and those with peripheral branches in the maxillomandibular nerve in the rostral half. Collaterals of the central branches of the mesencephalic trigeminal system were traced to an area of the periventricular gray situated between the motor nucleus and the principal sensory nucleus of the trigeminus.

Evolution of the red nucleus and rubrospinal tract

A red nucleus, defined by its relative position in the tegmentum mesencephali, its contralateral rubrospinal or rubrobulbar projections and by crossed cerebellar afferents, is found in terrestrial vertebrates and certain rays. A crossed rubrospinal tract occurs in anurans, limbed urodeles and reptiles, birds and mammals, but is apparently absent in boid snakes, caecilians and sharks. A distinct rubrospinal tract is found in certain rays which use their enlarged pectoral fins for locomotion. A crossed tegmentospinal tract, possibly a rubrospinal tract, is found in lungfishes. Although evidence was presented for a rubrospinal tract in more advanced snakes, the available experimental data in lower vertebrates suggest that the presence of a rubrospinal tract is related to the presence of limbs or limb-like structures. In the connectivity of the red nucleus in terrestrial vertebrates, 'levels' of complexity can be distinguished, paralleled by the development of the cerebellum. These 'grades of organization' are probably related to the type of motor performance the particular terrestrial vertebrates are capable of.

Some connections of the area octavolateralis of Pleurodeles waltlii. A study with horseradish peroxidase under in vitro conditions

Horseradish peroxidase application in exsanguinated, pre-perfused newts, Pleurodeles waltlii, appeared to be very useful to study the connections of the area octavolateralis. Efferents from the octavolateral area course via bilateral lemnisci to the tectum and a presumptive torus semicircularis in the midbrain. Additional projections to the oculomotor nuclei, cerebellum, reticular formation, contralateral octavolateral area, and spinal cord were observed. The area octavolateralis receives inputs from the cerebellum and the contralateral octavolateral area. It is concluded that the octavolateral second order projections in Pleurodeles waltlii resemble in many respects those reported for other vertebrates that possess a complete octavolateralis system.

Observations on the development of cerebellar afferents in Xenopus laevis

The development of cerebellar afferents has been studied in the clawed toad, Xenopus laevis, from stage 46 to 64, with the horseradish peroxidase retrograde tracer technique. Already in stage 48 tadpoles, i.e. before the formation of the limbs, a distinct set of cerebellar afferents was found. Vestibulocerebellar (mainly arising bilaterally in the nucleus vestibularis caudalis) and contralateral olivo-cerebellar projections dominate. Secondary trigeminocerebellar (from the descending nucleus of the trigeminal nerve) and reticulocerebellar connections were also found. At stage 50, spinocerebellar projections appear originating from cervical and lower thoracic/upper lumbar levels. The cells of origin of the spinocerebellar projection can be roughly divided in two neuronal types: ipsilaterally projecting large cells, which show a marked resemblance to primary motoneurones ('spinal border cells') and smaller contralaterally projecting neurons. Primary spinocerebellar projections from spinal ganglion cells could not be demonstrated. At stage 50, a possible anuran homologue of the mammalian nucleus prepositus hypoglossi was found to project to the cerebellum. In only one of the experiments labeled neurons were found in the contralateral mesencephalic tegmentum. At none of the studied stages a raphecerebellar projection could be demonstrated. It appears that already early in cerebellar development, before the formation of the limbs, most of the cerebellar afferents as found in adult Xenopus laevis are present.

Cerebellar efferents in the lizard Varanus exanthematicus. II. Projections of the cerebellar nuclei

The projections of the cerebellar nuclei have been studied in the lizard Varanus exanthematicus with various experimental anatomical techniques. In anterograde degeneration experiments (lesions of the cerebellar peduncle) both ascending and decending contralateral projections were found. Ascending fibers which could be traced from the cerebellar commissure ventralward decussated at the level of the trochlear and oculomotor nuclei. These fibers coursed rostralward to the mesodiencephalic junction. With anterograde tracing techniques (3H-leucine and HRP) this tract was found to terminate in the nucleus ruber and the interstitial nucleus of the fasciculus longitudinalis medialis. Moreover, retrograde tracer studies (HRP, "Fast Blue") showed that this tract appeared to arise mainly in the lateral cerebellar nucleus. With both anterograde degeneration and tracing techniques (3H-leucine and HRP) a bundle of fibers could be followed, which decussates in the basal part of the cerebellum and passes dorsally around the contralateral medial cerebellar nucleus to the lateral side of the brainstem. This contralaterally descending projection system was found, lateral to the vestibular nuclear complex, and as far caudally as the descending vestibular nucleus, to terminate on various vestibular nuclei. Horseradish peroxidase studies showed that this contralaterally descending projection system originates mainly in the medial cerebellar nucleus, but ipsilaterally descending projections were also found. With the fluorescent double labeling technique ("Fast Blue" and "Nuclear Yellow") the projections of the cerebellar nuclei described above were confirmed. Furthermore, double labeling revealed neurons in both cerebellar nuclei (especially the medial nucleus) that project to both the mesencephalon and the cervical spinal cord. The present results indicate that the efferent connections of the cerebellar nuclei in the lizard Varanus exanthematicus are organized as two main projections, an ascending projection comparable to the mammalian brachium conjunctivum arising in the lateral cerebellar nucleus, and a descending projection comparable to the mammalian hook bundle (fasciculus uncinatus), originating mainly in the medial cerebellar nucleus. Such projections are common for terrestrial vertebrates.

Early development of descending pathways from the brain stem to the spinal cord in Xenopus laevis

The early development of descending pathways from the brain stem to the spinal cord has been studied in Xenopus laevis tadpoles. The relatively protracted development of this permanently aquatic amphibian as well as its transparency during development make this animal particularly attractive for experimental studies. Between the 5th and 10th myotome the spinal cord was crushed with a thin needle and dry horseradish peroxidase (HRP) crystals were applied. After a survival time of one day the tadpoles were fixed and the brain and spinal cord were stained as a whole according to a modification of the heavy metal intensification of the DAB-reaction, cleared in cedarwood oil and examined as wholemounts. At stage 28 (the neural tube has just closed) the first brain stem neurons projecting to the spinal cord were found in what appear to be the nucleus reticularis inferior and -medius. At this stage of development the first, uncoordinated swimming movements can be observed. At stage 30/31 (the tailbud is visible) both Mauthner cells project to the spinal cord as well as the interstitial nucleus of the fasciculus longitudinalis medialis situated in the mesencephalon. Towards stage 35/36 (the tail is now clearly visible), a more extensive reticulospinal innervation of the spinal cord appears, now including cells of the nucleus reticularis superior. At this stage also the first vestibulospinal and raphespinal projections were found. At stage 43/44 (the tadpoles have now a well-developed tail) the pattern of reticulospinal projections appears to be completed with the presence of labeled neurons in the nucleus reticularis isthmi. From stage 43/44 on, the number of HRP-positive cells is steadily increasing. At stage 47/48, when the hindlimb buds appear, the descending projections to the spinal cord are comparable with the adult situation except for the absence of a rubrospinal and a hypothalamospinal projection. The observations demonstrate that already very early in development reticulospinal fibers and, somewhat later, Mauthner cell axons and vestibulospinal fibers innervate the spinal cord. Furthermore, a caudorostral gradient appears to exist with regard to the development of descending projections to the spinal cord. However, the interstitial nucleus of the fasciculus longitudinalis medialis forms an exception to this rule.

Cerebellar efferents in the lizard Varanus exanthematicus. I. Corticonuclear projections

The organization of the cerebellar corticonuclear projections, i.e., the projections from the Purkinje cell layer to the cerebellar nuclei and the vestibular nuclear complex, was investigated with the horseradish peroxidase (HRP) technique in the lizard Varanus exanthematicus. After HRP slow-release gels were implanted in the cerebellar nuclei or various parts of the vestibular nuclear complex, the following longitudinally oriented zones of labeled Purkinje cells could be distinguished: a medial zone projecting to the medial cerebellar nucleus; an intermediate zone projecting to the vestibular nuclear complex, especially the ventrolateral vestibular nucleus, but probably also the dorsolateral vestibular nucleus; a caudolaterally located area of the cerebellar cortex projecting to the lateral cerebellar nucleus; and the flocculus and the adjacent lateral part of the Purkinje cell layer with projections to the middle and caudal parts of the vestibular nuclear complex, i.e., the descending and ventromedial vestibular nuclei. All projections of the Purkinje cells appeared to be strictly ipsilateral. It can be concluded that in reptiles a longitudinal organization of cerebellar corticonuclear projections exists, which may be basic for terrestrial vertebrates.