Cells of origin of pathways descending to the spinal cord in some quadrupedal reptiles

A crossed rubrobulbar projection in the snake Python regius

In the present study a distinct crossed rubrobulbar projection has been demonstrated in the snake Python regius, a limbless vertebrate which lacks a rubrospinal tract. This rubrobulbar projection is presumably involved in the neural control of mastication. The red nucleus may relay cerebellar influence to the trigeminal and facial nuclei.

Some afferent and efferent connections of the vestibular nuclear complex in the red-eared turtle Pseudemys scripta elegans

In the present study some afferent, commissural, and efferent connections of the vestibular nuclear complex in the turtle Pseudemys scripta elegans were demonstrated with the HRP tracing technique. Afferent projections to the vestibular nuclei were found to arise in the nucleus of the basal optic root, the interstitial nucleus of the fasciculus longitudinalis medialis, the medial and lateral cerebellar nuclei, the perihypoglossal nuclear complex, and the reticular formation. Distinct commissural projections appeared to arise in the dorsolateral, ventromedial, and descending vestibular nuclei. The commissural projection arising in the ventrolateral vestibular nucleus appeared to be only sparsely developed. Both ascending and descending efferent projections were demonstrated to arise from the vestibular nuclear complex. The ascending vestibulo-oculomotor projection was found to be organized in an ipsilateral pathway arising in the dorsolateral vestibular nucleus and in a contralateral pathway, arising mainly in the medial vestibular nucleus. These projections appeared to be directed to the interstitial nucleus of the fasciculus longitudinalis medialis, the oculomotor, trochlear, and abducens nuclei. Also the perihypoglossal nuclear complex appeared to be an important target of vestibular efferents. The origin and course in the brainstem of the descending vestibular projections, i.e., the lateral and medial vestibulospinal tracts, as demonstrated in previous anatomical and experimental studies in reptiles, were confirmed. However, in addition a direct projection of the vestibulospinal tracts to presumably neck motoneurons was found. THe organization of the vestibular connections observed in the turtle Pseudemys scripta elegans appeared to be basically comparable to the organization of the vestibular connections in birds and mammals.

Reticulospinal and vestibulospinal pathways in the snake Python regius

In 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.

Long descending projections of the hypothalamus in the pigeon, Columba livia

An autoradiographic analysis was performed on the descending projections of nucleus periventricularis magnocellularis (PVM) of the hypothalamus in the pigeon. A PVM-medullospinal pathway was observed coursing posteriorly through the lateral hypothalamus, ventrolateral midbrain tegmentum, and into the spinal lemniscus (ls) in the ventrolateral pons and medulla. In the pons, some fibers course dorsomedially from ls and terminate at the lateral border of the locus coeruleus. At medullary levels, fibers from ls sweep dorsomedially in the plexus of Horsley and project to certain regions of the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus (NX). Specifically, PVM fibers project heavily into NTS subnuclei medialis superficialis, medialis ventralis, and lateralis (sulcalis) dorsalis as well as into the ventral parvocellular subnucleus of NX. Fibers in ls were traced caudally into the lateral funiculus as far as upper cervical levels of the spinal cord. Although autoradiographs of lower cervical or thoracic spinal cord sections were not available, PVM fibers do descend to thoracic spinal cord levels, as evidenced by the retrograde transport of horseradish peroxidase. In addition to the medullospinal pathway, the autoradiographs demonstrated PVM projections to septum, diencephalon, and midbrain. Labeled PVM fibers are found in the lateral septal nucleus, nucleus of the anterior pallial commisure, dorsomedial thalamic nucleus, dorsolateral anterior thalamic nucleus (pars ventralis), median eminence, medial and lateral hypothalamus, medial mammillary area, and nucleus intercollicularis and central gray of the midbrain. The projection of fibers to medullospinal regions and median eminence suggests that PVM is homologous to the mammalian paraventricular nucleus. These projections to specific subnuclei of NTS and NX denote hypothalamic control over certain autonomic functions.

Cerebellar corticonuclear projections in the red-eared turtle Pseudemys scripta elegans

In the present study the organization of the corticonuclear projections of the cerebellum in the red-eared turtle Pseudemys scripta elegans was investigated. To find out whether a zonal pattern exists in the cerebellar cortex, a topological analysis was made of the Purkinje cell layer of a Nissl-stained series of the cerebellum. This analysis showed a distinct, longitudinally oriented zonal pattern of Purkinje cells. In each cerebellar half a medial, an intermediate, and a lateral zone could be distinguished. This pattern appeared to correspond in part to an earlier subdivision of the reptilian cerebellum suggested by Larsell ('26, '32, '67). In the experimental part of this study, the corticonuclear projections to the cerebellar nuclei and the vestibular nuclear complex were demonstrated with the horseradish peroxidase (HRP) technique. All projections of the Purkinje cells appeared to be strictly ipsilateral. After HRP slow-release gel implantations at various levels of the vestibular nuclear complex, longitudinally oriented areas of labeled Purkinje cells were found in the lateral part of the medial zone, the intermediate zone, and the lateral zone. Two strips of Purkinje cells remained unlabeled: One in the rostrolateral part of the intermediate zone, and another in the medial part of the medial zone. After HRP gel implantations in the cerebellar peduncle aimed at the medial and lateral cerebellar nuclei, respectively. Purkinje cells in the latter two strips were labeled. The present results indicate that the corticonuclear projections to the medial cerebellar nucleus arise in the medial part of the medial zone and those to the lateral cerebellar nucleus in the rostrolateral part of the intermediate zone. It can be concluded that cerebellar zones as demonstrated in the turtle Pseudemys scripta elegans, are phylogenetically very old and may represent the basic functional circuit of the cerebellum.

The sources of supraspinal afferents to the spinal cord in a variety of limbed reptiles. I. Reticulospinal systems

Horseradish peroxidase was injected into various levels of the spinal cord of turtles (Pseudemys and Chrysemys), lizards (Tupinambis, Iquana, Gekko, Sauromelus, and Gerrhonotus), and a crocodilian (Caiman). The results suggest that brainstem reticulospinal projections in limbed reptiles rival mammalian reticulospinal systems in complexity. The reptilian myelencephalic reticular formation can be divided into four distinct reticulospinal nuclei. Reticularis inferior pars dorsalis (RID) contains multipolar neurons which project bilaterally to the spinal cord. Reticularis inferior pars ventralis (RIV), which is only found in lizards and crocodilians, contains fusiform neurons with horizontally running dendrites and it projects ipsilaterally to the spinal cord. Reticularis ventrolateralis (RVL), which is found only in field lizards, contains triangular neurons whose dendrites parallel the ventrolateral edge of the brainstem and it projects ipsilaterally to the spinal cord. The myelencephalic raphe (RaI) varies considerably. RaI of turtles contains large reticulospinal neurons which form a continuous population with more laterally situated RID cells. RaI of lizards contains a few small reticulospinal neurons. RaI of the crocodilian Caiman contains giant reticulospinal neurons with laterally directed dendrites. The caudal metencephalic reticular formation of reptiles can be divided into two distinct reticulospinal nuclei. Reticularis medius (RM) contains large neurons with long, ventrally directed dendrites; it projects ipsilaterally to the spinal cord. Reticularis medius pars lateralis (RML) contains small neurons with laterally directed dendrites; it projects contralaterally to the spinal cord. The rostral mesencephalic and caudal mesencephalic reticular formation of reptiles can be divided into three distinct reticulospinal nuclei. Reticularis superior pars medialis (RSM) consists mostly of small, spindle-shaped neurons which project bilaterally to the spinal cord. In the lizard Tupinambis, however, large multipolar, ipsilaterally projecting neurons are occasionally seen in RSM. Reticularis superior pars lateralis (RSL) contains large, ipsilaterally projecting neurons with long, ventrolaterally directed dendrites. SRL in lizards can be divided into a dorsomedial portion, which projects ipsilaterally to the spinal cord, and a ventrolateral portion which projects contralaterally. The locus ceruleus-subceruleus field (LC-SC) contains small spindle-shaped neurons which project bilaterally to the spinal cord. Labelled reticulospinal neurons were also observed in the rostral metencephalic raphe (RaS) of the turtle brainstem. These cells are small, spindle-shaped neurons which resemble the small cells of the adjacent RSM field.

Primary afferent projections to the spinal cord and the dorsal column nuclear complex in the turtle Pseudemys

Primary afferent projections from cervical and lumbar levels were studied in the turtle Pseudemys scripta elegans. Injections of radioactive amino acids, wheat germ agglutinin and horseradish peroxidase were made into the dorsal root ganglia or the spinal cord. Previous reports on the terminal distribution of primary afferents within the ipsilateral segment of entry were confirmed (Kusuma and ten Donkelaar 1979, 1980) and additional dorsal root projections were demonstrated to the contralateral side and to several neighboring spinal segments. The primary afferent projections to the brainstem were essentially restricted to a dorsolateral area that appears to be homologous to the main dorsal column nuclei (n. gracilis and n. cuneatus medialis) in mammals. While exhibiting a similarly extensive rostro-caudal span, the projections originating from lumbar injections terminated more medially, those from cervical injections more laterally. The labeling pattern suggested that terminations are mainly on dorsally extending dendrites.

Mesodiencephalic and other target regions of ascending spinal projections in the turtle, Pseudemys scripta elegans

Ascending spinal projections were investigated in turtle Pseudemys scripta elegans following injections of radioactive amino acids into the spinal cord at various levels. Experiments using S35-methionine were most successful in demonstrating various mesodiencephalic target areas. Ascending projections from lumbar and cervical segments terminated predominantly in caudal and lateral reticular fields including the perihypoglossal complex. These spinal regions also projected for a lesser extent to rostrolateral and caudomedial reticular fields and to the nucleus (n.) raphe inferior. Afferents terminated consistently within the peritoral region, the optic tectal layers, the mesodiencephalic periventricular white matter, and the ovalis complex. Occasional labeling was noted in the diencephalic white matter adjacent to the optic tract, the n. supraopticus, and in the n. commissuralis anterior. Projections to the so-called rostrolateral perirotundal complex following high cervical injections were less prominent following low cervical and lumbar injections. Cervical afferents terminated in a variable manner in the vestibular complex, the torus semicircularis n. centralis, the mesodiencephalic periventricular gray (griseum centrale, n. interstitialis commissuralis posterior and hypothalamic areas), the n. suprapenduncularis, and the n. reuniens.

Distribution and structural characterization of neurons giving rise to descending spinal projections in the turtle, Pseudemys scripta elegans

Descending spinal projections were investigated in the turtle Pseudemys scripta elegans following injections of horseradish peroxidase and/or radioactive wheat germ agglutinin into the spinal cord at various levels. Using various planes of section the cells of origin in the brainstem, cerebellum, and diencephalon were characterized according to their size, dendritic tree, and precise location. Projections to levels as far caudal as the lumbar spinal cord were found to arise from medial and lateral rhombencephalic reticular fields, including the perihypoglossal complex, the nucleus raphe inferior, and the locus coeruleus; from certain subdivisions of the vestibular complex (ipsilateral subnucleus (subn.) ventrolateralis, contralateral subn. ventromedialis, and possibly subn. tangentialis); from the motor trigeminal nucleus; from the contralateral red nucleus, the ipsilateral nucleus (n.) interstitialis of the fasciculus longitudinalis medialis (flm) and from the hypothalamus. Fibers to high cervical levels arose from neurons within the dorsolateral and superior vestibular nuclei, the lateral cerebellar nucleus, the mesencephalic trigeminal nucleus, and from neurons of the optic tectum. Low cervical and thoracic spinal levels were reached by fibers from the torus semicircularis n. laminaris, the n. of the flm, the medial cerebellar nucleus as well as from the n. vestibularis inferior and the principal and descending trigeminal nuclei.

The organization of the reptilian formation: a comparative study using Nissl and Golgi techniques

The brainstem reticular formation has been studied in 16 genera representing 11 families of reptiles. Measurements of Nissl-stained reticular neurons revealed that they are distributed along a continuum, ranging in length from 10 micrometer to 95 micrometer. Reticular neurons in crocodilians and snakes tend to be larger than those found in lizards and turtles. Golgi studies revealed that reticular neurons possess long, rectilinear, sparsely branching dendrites. Small reticular neurons (less than 31 micrometers length) possess fusiform or triangular somata which bear two or three primary dendrites. These dendrites have a somewhat simpler ramification pattern when compared with those of large reticular neurons ( greater than 30 micrometers length). Large reticular neurons generally possess perikarya which are triangular or polygonal in shape. The somata of large reticular neurons bear an average of four primary dendrites. The dendrites of reptilian reticular neurons ramify predominantly in the transverse plane and are devoid of spines or excrescences. The dendritic ramification patterns observed in the various repitilian reticular nuclei were correlated with known input and output connections of these nuclei. Nissl and golgi techniques were used to divide the reticular formation into seven nuclei. A nucleus reticularis inferior (RI) is found in the myelencephalon, a reticularis medius (RM) in the caudal two-thirds of the metencephalon, and a reticularis superior (RS) in the rostral metencephalon and caudal mesencephalon. Reticularis inferior can be subdivided into a dorsal portion (RID) and a ventral portion (RIV). All reptilian groups possess RID and RM but RIV is lacking in turtles. Reticularis superior can be subdivided into a large-celled lateral portion (RSL) and a small-celled medial portion (RSM). All reptilian groups possess RSM and RSL, but RSL is quite variable in appearance, being best developed in snakes and crocodilians. The myelencephalic raphe nucleus is also quite variable in its morphology among the different reptilian families. A seventh reticular nucleus, reticularis ventrolateralis (RVL), is found only in snakes and in teiid lizards. It was noted that the reticular formation is simpler (fewer numbers of nuclei) in the representative of older reptilian lineages and more complex (greater numbers of nuclei) in the more modern lineages. Certain reticular nuclei are present or more extensive in those families which have prominent axial musculature.

Spinal cord development in anuran larvae: II. Ascending and descending pathways

The ontogeny of ascending and descending spinal pathways was examined in bullfrog (Rana catesbeiana) tadpoles using the transported histochemical marker, horseradish peroxidase (HRP). The adult pattern of brainstem projections to lumbar spinal cord is evident as early as larval stage I (Taylor and Kollros, Anat. Rec., 94:7-24, 1946), although the number and size of projecting cells increases as the animal matures. These projections arise from presumptive hypothalamic neurons at the diencephalic-mesencephalic border as well as from neurons of the vestibular nucleus, oculomotor nucleus, and reticular formation. In contrast to the stability of the pattern of descending projections, the sources of fibers ascending to the brainstem change during larval life. In early larval stages, brainstem projections from lumbar spinal cord arise primarily from Rohon-Beard cells and neurons of the superficial dorsal horn. In later stages, neurons in the intermediate and ventral areas of the spinal gray can also be retrogradely labeled by HRP application to the brainstem at the level of the VIIIth nerve. Evidence of the existence of dorsal column and lateral cervical nuclei in adult frog and tadpoles older than stage VIII is presented. The ascending projections of embryonically born primary neurons were also investigated. Rohon-Beard cells, which are sensory neurons with their cell bodies in the spinal cord, were found to send ascending processes as least as far rostral as the level of the VIIIth nerve entry zone. Anterolateral and dorsal marginal cells, probable homologs, respectively, of mammalian spinal border cells and cells of Waldeyer (1888), were also found to project rostrally at least to the rhombencephalon. These marginal cells persisted through metamorphosis into adulthood.

Afferent connections of the cerebellum in various types of reptiles

The origin of cerebellar afferents was studied in various types of reptiles, viz., the turtles Pseudemys scripta elegans and Testudo hermanni, the lizard Varanus exanthematicus, and the snake Python regius, with retrograde tracers (the enzyme horseradish peroxidase and the fluorescent tracer "Fast Blue"). Projections to the cerebellum were demonstrated from the nucleus of the basal optic root, the interstitial nucleus of the fasciculus longitudinalis medialis, the vestibular ganglion, and the vestibular nuclear complex, two somatosensory nuclei, viz., the descending nucleus of the trigeminal nerve and the nucleus of the dorsal funiculus, the nucleus of the solitary tract, the reticular formation, and throughout the spinal cord. A distinct bilateral projection to the cerebellum was found to arise in a nucleus previously called nucleus parvocellularis medialis (Ebbesson, '67). In the present study this cell mass is termed the perihypoglossal nuclear complex, considering its comparable position and fiber connections to the perihypoglossal nuclei in mammals. In all reptilian species studied a contralateral cerebellar projection of a cell mass located in the caudal brainstem adjacent to the nucleus raphes inferior was observed. It seems likely that this cell mass represents the reptilian homologue of the mammalian inferior olive. Most of the spinocerebellar fibers appeared to arise in neurons located in area VII-VIII of the gray matter. In this respect the origin of the spinocerebellar projection in reptiles resembles the origin of the rostral and ventral spinocerebellar tracts in mammals. No indications for the existence of a column of Clarke or a central cervical nucleus in the reptilian spinal cord were obtained. On comparison of the cerebellum afferents in reptiles with the known connections of the cerebellum in amphibians, birds, and mammals, a basic pattern of cerebellar afferent projections appears to exist in these vertebrate classes, including retinal, vestibular, precerebellar, somatosensory, and spinal afferents.

Cells of origin of pathways descending to the spinal cord in two chondrichthyans, the shark Scyliorhinus canicula and the ray Raja clavata

The cells of origin of pathways descending to the spinal cord in the shark Scyliorhinus canicula and in the ray Raja clavata have been demonstrated by using the horseradish peroxidase (HRP) technique. Following HRP injections in the spinal cord of Scyliorhinus (fourth to sixth segment) and of Raja (15th to 20th segment) labeled neurons could be identified in the rhombencephalon, the mesencephalon, and in the diencephalon. Cells of origin of diencephalic nuclei, which project to the spinal cord, were observed in the nucleus periventricularis hypothalami and in the thalamus ventralis pars medialis which can in this respect be considered hypothalamic. Descending pathways from mesencephalic structures originate from the interstitial nucleus of the fasciculus longitudinalis medialis, the tectum mesencephali, the nucleus intercollicularis, the tectotegmental junction zone, and from diffusely arranged tegmental neurons. A contralateral rubrospinal pathway could be recognized in Raja, but not in Scyliorhinus. Rhombencephalic cells of origin of pathways descending to the spinal cord were found in all parts of the reticular formation, i.e., the nucleus raphes inferior, the nucleus reticularis inferior, medius, superior, and isthmi, in two vestibular nuclei, and in three nuclei, which have been tentatively indicated as nucleus B, F, and G. Furthermore cells of origin of descending pathways have been found in the nucleus tractus descendens nervi trigemini, in the nucleus funiculi lateralis, and in the nucleus tractus solitarii. The descending pathways of the two species studied have been compared with those of other vertebrates. It is concluded that the basic pattern in the organization of descending pathways to the spinal cord, as proposed by ten Donkelaar ('76) for terrestrial vertebrates, also holds for cartilaginous fishes.

Ascending projections of the brain stem reticular formation in a nonmammalian vertebrate (the lizard Varanus exanthematicus), with notes on the afferent connections of the forebrain

In the present study an attempt has been made to analyze the ascending reticular projections in the lizard Varanus exanthematicus by means of the horseradish peroxidase (HRP) technique. Reticular projections ascending to the telencephalon were found to arise in the mesencephalon, but not caudal to the mesorhombencephalic border. HRP injections into the dorsal thalamus have demonstrated retrogradely labeled cells in the mesencephalic reticular formation, particularly at the level of the oculomotor nerve and in the medial magnocellular zone of the rhombencephalic reticular formation, predominantly rostrally. HRP infiltrations at the mesodiencephalic border damaged most of the fibers passing beyond this junction, resulting in the uptake of HRP by the damaged axons and subsequent labeling of the cell bodies or origin of ascending reticular projections to the diencephalon and telencephalon. From a comparison of cell-labeling patterns in cases of HRP injections of, respectively, the dorsal thalamus and the mesodiencephalic border, it seems likely that the nucleus reticularis medius and more sparsely the nucleus reticularis inferior project to ventral diencephalic structures (ventral thalamus and hypothalamus), whereas the midbrain reticular formation and the rostral parts of the rhombencephalic reticular formation (nuclei reticulares isthmi and superior) project to both the dorsal thalamus and more ventral diencephalic structures. Projections arising throughout the rhombencephalic reticular formation, but predominantly in the nucleus reticularis inferior, were found to ascend to the midbrain reticular formation. The present experimental data in the lizard Varanus exanthematicus are comparable to the findings in mammals, with the exception of the reticulo-oculomotor pathways which have not been analyzed so far in reptiles. In addition to the aforementioned ascending reticular projections, the present study has demonstrated projections ascending from monoamine cell groups, various diencephalic structures, as well as from neuronal groups involved in somatosensory, auditory, and gustatory systems. Projections were found from the locus coeruleus and the nucleus raphes superior to the telencephalon, as well as from the substantia nigra and the presumable reptilian homologue of the mammalian ventral tegmental area to the basal forebrain and the dorsal thalamus. Bilateral projections were demonstrated from the principal trigeminal nucleus to the telencephalon, reminiscent of the quintofrontal tract of birds. Ascending projections to the diencephalon were found to originate bilaterally in the descending trigeminal nucleus and the dorsal funicular nucleus. Auditory projections to the midbrain arise bilaterally in the superior olivary complex and in the cochlear nuclear complex. Finally, the ascending gustatory pathway arising in the nucleus of the solitary tract was found to project to the "parabrachial region," which in its turn has extensive projections to the forebrain.