R. de Boer-van Huizen

Cells of origin of descending pathways to the spinal cord in the clawed toad (Xenopus laevis).

Abstract The cells of origin of pathways descending to the spinal cord in the clawed toad Xenopus laevis have been demonstrated with the horseradish peroxidase technique. A technique has been used taking advantage of the phenomenon that damaged axons can take up horseradish peroxidase and transport this enzyme to their parent cell bodies. The following descending supraspinal pathways could be demonstrated: a striatospinal pathway to the rostral part of the cord; distinct hypothalamospinal projections; a projection as far as the lumbar cord from the ventral thalamic nucleus; distinct projections from the mesencephalic tegmentum; a contralateral cerebellospinal projection from the cerebellar nucleus; a projection from neurons directly medial to the nucleus isthmi which shows resemblance to the coeruleospinal pathway of higher veretebrates; massive reticulospinal projections; a vestibulospinal projection arising in the nucleus ventralis VIII and pathways arising in nuclei receiving lateral line afferents. Furthermore, spinal projections from the nucleus of the solitary tract and the nucleus descendons nervi trigemini were observed. Rather massive projections were found to arise in the midbrain tegmentum: a mainly ipsilateral projection from the interstitial nucleus of the fasciculus longitudinalis medialis, a contralateral projection as far as the lumbar cord from a cell group which presumably represents the anuran homologue of the red nucleus of higher vertebrates, and projections from various other parts of the midbrain tegmentum, mainly to more rostral levels of the cord. Only a very small tectospinal projection could be demonstrated. A comparison with experimental data in higher vertebrates makes it likely that the pathways demonstrated from the hypothalamus and brain stem in Xenopus laevis show remarkable similarities to pathways in reptiles, birds and mammals.

Basal ganglia projections to the brain stem in the lizard Varanus exanthematicus as demonstrated by retrograde transport of horseradish peroxidase

Abstract In the present study the cells of origin of basal ganglia projections to the brain stem have been studied with the horseradish peroxidase technique in the lizard, Varanus exanthematicus . Injections of horseradish peroxidase were made at various levels of the brain stem from the mesodiencephalic border to the obex as well as in the tectum mesencephali. Efferent libers from the telencephalon to the diencephalon and the brain stem were found to arise predominantly from the striatum. From the present data it seems likely that the basal ganglia in Varanus exanthematicus as in other reptiles consist of two parts, a rostral ‘striatal’ part with projections mainly to the diencephalon and mesencephalon including the substantia nigra and a caudal ‘pallidal’ part with projections to the intercollicular nucleus and the rhombencephalic reticular formation. Injections of horseradish peroxidase into various parts of the rhombencephalic reticular formation have shown rather extensive projections from diencephalic and mesencephalic structures which receive afferents from the striatum: the posterior entopeduncular nucleus, the intercollicular nucleus and the substantia nigra were found to project as far caudal as the nucleus reticularis inferior. The substantia nigra shows, as regards its fiber connections, striking similarities to the mammalian substantia nigra, whereas the intercollicular nucleus possibly represents the reptilian homologue of the mammalian pedunculopontine nucleus. Injections of horseradish peroxidase into the tectum mesencephali have shown labeled cells in the nucleus of the posterior commissure, the posterior entopeduncular nucleus and the substantia nigra, all centers which are known to receive afferents from the striatum. Thus, the striatum can influence bisynaptically the reptilian homologue of the mammalian superior colliculus. It can be concluded that the striatum of the lizard, Varanus exanthematicus , has extensive direct as well as indirect projections to centers which influence the motor apparatus of the brain stem and spinal cord. Thus in reptiles it seems likely that the striatum exerts its influence on motor activity mainly via descending projections, in contrast to mammals where both descending and ascending striatal efferent pathways occur.

Dorsal root projections in the clawed toad (Xenopus laevis) as demonstrated by anterograde labeling with horseradish peroxidase

Horseradish peroxidase was applied to the proximal stumps of severed cervical, thoracic and lumbar dorsal roots in the clawed toad, Xenopus laevis, in order to study the course, distribution and site of termination of dorsal root fibers in the spinal cord and brain stem. The anterograde transport of horseradish peroxidase as applied in the present study proved to be a useful and reliable technique. Results show that on entering the spinal cord, dorsal root fibers segregate into a medially placed component entering the dorsal funiculus and a more laterally situated bundle in the dorsal part of the lateral funiculus. As regards its position the latter bundle presumably represents the anuran homologue of the mammalian tract of Lissauer. Moreover, a small intermediate bundle of fibers directly enters the spinal gray matter. The labeled fibers entering the dorsal funiculus and the tract of Lissauer ascend and descend in the spinal cord, displaying a longitudinal arrangement resembling that of higher vertebrates. In the spinal gray, dorsal root fibers terminate in the dorsal, central and lateral fields of Ebbesson, with the last field being a major terminus for dorsal root fibers originating in the limb-innervating segments. No dorsal root fibers were found to project to the motoneuron fields. A dorsal column nucleus, which is divisible into medial and lateral compartments, is present in the obex region and extends from the level of the second spinal nerve to that of the entrance of the vagus and glossopharyngeal nerves. Dorsal root fibers from the lumbar and all thoracic segments project to the medial compartment of the dorsal column nucleus, whereas those of the cervical enlargement project to the lateral compartment. Although the anuran dorsal column nucleus appears to be less differentiated than that of higher vertebrates, its medial and lateral compartments can be considered to be the forerunners of the mammalian nucleus gracilis and nucleus cuneatus, respectively.

Cells of origin of pathways descending to the spinal cord in a lizard (Larcerta galloti)

The pathways descending to the spinal cord have been studied in the lizard Lacerta galloti with the HRP-technique. Cells of origin of such pathways have been demonstrated in the hypothalamus and throughout the brain stem. In the brain stem, besides labeled cell groups in the reticular formation and in the vestibular nuclear complex, labeled neurons have been found in the nucleus ruber, in the locus coeruleus, in the two deep cerebellar nuclei, in two somatosensory relay nuclei (the nucleus funiculi dorsalis and the descending nucleus of the trigeminus) and in two presumably parasympathetic nuclei (the nucleus of Edinger-Westphal and the dorsal motor nucleus of the vagus).

The fasciculus longitudinalis medialis in the lizard Varanus exanthematicus

SummaryIn the present study the vestibular components of the fasciculus longitudinalis medialis (flm) were investigated in the lizard Varanus exanthematicus with various tracing techniques: anterograde transport of horseradish peroxidase to study vestibulo-oculomotor and vestibulospinal projections, the multiple retrograde fluorescent tracer technique for the cells of origin of such projections. Internuclear projections between the oculomotor and abducens nuclei could also be studied in this way.Rather extensive vestibulo-ocular projections passing via the flm were demonstrated. Mainly ipsilateral ascending projections arise in the dorsolateral vestibular nucleus, mainly contralateral ascending projections in the ventromedial vestibular nucleus and adjacent parts of the ventrolateral and descending vestibular nuclei. Furthermore, distinet bilateral ascending projections of the nucleus prepositus hypoglossi were demonstrated. Extensive vestibulospinal projections pass via the flm and form the medial vestibulospinal tract. This largely contralateral descending pathway arises predominantly in the ventromedial and descending vestibular nuclei. Terminal structures presumably arising in the ventromedial and descending vestibular nuclei were found on contralateral neurons, probably motoneurons innervating neck muscles.Vestibular neurons with both ascending (presumably to extra-ocular motoneurons) and descending projections to the spinal cord are present in all vestibular nuclei, although preferentially in the ventromedial vestibular nucleus and adjacent parts of the ventrolateral and descending vestibular nuclei. However, also in the dorsolateral vestibular nucleus a substantial number of double labeled neurons were found. These vestibular neurons with both vestibulomesencephalic and vestibulospinal projections are probably involved in combined movements of eyes and head.Evidence for reciprocal internuclear connections between the oculomotor and abducens nuclei was found. Neurons in the dorsal part of the oculomotor nucleus probably project to the ipsilateral abducens nucleus, while neurons in the abducens nucleus most likely project to the contralatcral oculomotor nucleus. These recpprocal internuclear connections between the oculomotor and abducens nuclei probably play an important role in conjugate horizontal eye movements.

Funicular Trajectories of Descending Brain Stem Pathways in a Lizard(Varanus exanthematicus)

Publisher Summary This chapter discusses funicular trajectories of descending brain stem pathways in a lizard. Horseradish peroxidase (HRP) slow-release gels have been applied to various parts of the lateral and ventral funiculi of the cervical enlargement in the lizard Varunus exanthematicus to obtain more conclusive evidence on funicular trajectories of various descending pathways. In an earlier study, the preparation of the HRP slow-release gel was slightly modified to obtain a slower release of the enzyme and a less fragile dried gel, which would be easier to handle and to apply. Following the implantation of HRP slow-release gel in the dorsal part of the lateral funiculus, many labeled neurons were observed throughout the brain stem. No labeled neurons were found in the hypothalamus. A few labeled cells were present in the ipsilateral interstitial nucleus of the fasciculus longitudinalis medialis (flm). At the isthmic level, labeled cells were observed in the locus coeruleus and in two reticular cell groups.

A possible pain control system in a non-mammalian vertebrate (a lizard, Gekko gecko)

Abstract In a lizard ( Gekko gecko ) the anterograde tracer PHA-L was microiontophoretically applied to the predominantly serotonergic nucleus raphes inferior. Extensive spinal projections from the rostral magnocellular part of this nucleus were demonstrated to the superficial layers of the dorsal horn and to the intermediate zone, more sparsely to the ventral horn. But, in addition, retrogradely labeled neurons were found in and just below a periventricular cell group in the tegmentum mesencephali, i.e. the laminar nucleus of the torus semicircularis, a cell group which receives spinal afferents and projects to the spinal cord as the mammalian periaqueductal gray. These data suggest the presence of a three-tiered pain control system in a lizard composed of projections from the laminar nucleus of the torus semicircularis to the rostral part of the inferior raphe nucleus which in its turn projects to the superficial layers of the dorsal horn of the spinal cord.

Brain stem afferents to the anterior dorsal ventricular ridge in a lizard (Varanus exanthematicus)

SummaryThe anterior dorsal ventricular ridge (ADVR), a large intraventricular protrusion in the reptilian forebrain, receives information from many different sensory modalities and in turn, projects massively onto the striatum. The ADVR possesses functional similarities to the mammalian isocortex and may perform complex sensory integrations. The ADVR in lizards is composed of three longitudinal zones which receive visual, somatosensory and acustic information, respectively. These projections are relayed via thalamic nuclei. Previous retrograde tracer studies also suggested brain stem projections to the ADVR arising in the midbrain reticular formation and in certain monoaminergic brain stem nuclei (substantia nigra, locus coeruleus and nucleus raphes superior). In the present study the powerful retrograde fluorescent tracer. ‘Fast Blue’ was applied as a slow-release gel to the ADVR of the savanna monitor lizard, Varanus exanthematicus. Thalamic projections were confirmed and various direct brain stem projections to the ADVR were demonstrated. Brain stem afferents to the ADVR were found from the laminar nucleus of the torus semicircularis (possibly comparable to the mammalian periaqueductal gray), from the midbrain reticular formation, from the substantia nigra (pars compacta and reticulata) and the adjacent ventral tegmental area, from the nucleus raphes superior, from the locus coeruleus, from the parabrachial region, from the nucleus of the lateral lemniscus and even from the most caudal part of the brain stem (a few neurons in the nucleus of the solitary tract and lateral reticular formation, possibly comparable to the mammalian A2 and A1 groups, respectively). These data strongly suggest direct ADVR projections from the parabrachial region (related to visceral and taste information) as well as distinct catecholaminergic (presumably dopaminergic: substantia nigra, ventral tegmental area and, noradrenergic: locus coeruleus, respectively) and serotonergic projections (nucleus raphes superior).

Reticulospinal and vestibulospinal pathways in the snake Python regius

SummaryIn the present HRP study extensive reticulospinal projections and more modestly developed vestibulospinal pathways have been demonstrated in the snake Python regius. The funicular trajectories of the main reticulospinal pathways have been shown: via the lateral funiculus pass spinal projections of the nucleus reticularis superior pars lateralis, the nucleus reticularis inferior and nucleus raphes inferior; via the ventral funiculus fibers arising in the nucleus reticularis superior and nucleus reticularis medius. Spinal projections of the locus coeruleus and subcoeruleus area reach their targets via both the lateral and ventral funiculi. Two vestibulospinal pathways have been demonstrated: an ipsilateral tractus vestibulospinalis lateralis arising in the ventrolateral vestibular nucleus, and a contralateral tractus vestibulospinalis medialis from the descending and ventromedial vestibular nuclei. After HRP gel implants into the vestibular nuclear complex direct vestibulocollic projections to motoneurons in the rostral spinal cord were observed.Spinal projections from the ventral part of the nucleus reticularis inferior and the descending and ventromedial vestibular nuclei are mainly aimed at the thin “neck area” (approximately the first 50 spinal segments). This area is extensively used in such acts as orientation and prey-catching, requiring a rather delicate brain stem control.

Cells of origin of propriospinal and ascending supraspinal fibers in a lizard (Lacerta galloti)

The location of cells of origin of propriospinal and ascending supraspinal fibers has been determined by injecting horseradish peroxidase (HRP) unilaterally into various parts of the spinal cord of the lizard Lacerta galloti. The distribution of retrogradely labeled cells after unilateral high spinal cord injections suggests that ascending supraspinal fibers are derived from neurons in the following areas: in the cervical intumescence in most areas of the spinal gray, particularly ipsilaterally, in thoracic and lumbar segments, in deeper situated areas, especially contralaterally. The presence of cells of origin of long descending propriospinal pathways has been demonstrated following HRP-injections into the lumbar intumescence.