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

Location of Spinal Cord Pathways That Control Hindlimb Movement Amplitude and Interlimb Coordination During Voluntary Swimming in Turtles

We performed mechanical lesions of the midbody (D2–D3; second to third postcervical spinal segments) spinal cord in otherwise intact turtles to locate spinal cord pathways that 1) activate and control the amplitude of voluntary hindlimb swimming movements and 2) coordinate hindlimb swimming with the movement of other limbs. Pre- and postlesion turtles were held by a band clamp around the carapace just beneath the water surface in a clear Plexiglas tank and videotaped from below so that kinematic measurements could be made of voluntary forward swimming with motion analysis software. Movements of the forelimbs (wrists) and hindlimbs (knees and ankles) were tracked relative to stationary reference points on the plastron to obtain bilateral measurements of hip and forelimb angles as functions of time along with foot trajectories. We measured changes in limb movement amplitude, cycle period, and interlimb phase before and after spinal lesions. Our results indicate that locomotor command signals that activate and regulate the amplitude of voluntary hindlimb swimming travel primarily in the dorsolateral funiculus (DLF) at the D2–D3 level and cross over to drive contralateral hindlimb movements. This suggests that electrical stimulation of the D3 DLF, which was previously shown to evoke swimming movements in the contralateral hindlimb of low-spinal turtles, activated the same locomotor command pathways that the animal uses during voluntary behavior. We also show that forelimb–hindlimb coordination is maintained by longitudinal spinal pathways that are largely confined to the ventrolateral funiculus (VLF) and mediate phase coupling of ipsilateral limbs, presumably by interenlargement propriospinal fibers.

Immunohistochemical and hodological characterization of calbindin-D28k-containing neurons in the spinal cord of the turtle,Pseudemys scripta elegans

Neurons and fibers containing the calcium-binding protein calbindin-D28k (CB) were studied by immunohistochemical techniques in the spinal cord of adult and juvenile turtles, Pseudemys scripta elegans. Abundant cell bodies and fibers immunoreactive for CB were widely and distinctly distributed throughout the spinal cord. Most neurons and fibers were labeled in the superficial dorsal horn, but numerous cells were also located in the intermediate gray and ventral horn. In the dorsal horn, most CB-containing cells were located in close relation to the synaptic fields formed by primary afferents, which were not labeled for CB. Double immunohistofluorescence demonstrated distinct cell populations in the dorsal horn labeled only for CB or nitric oxide synthase, whereas in the dorsal part of the ventral horn colocalization of nitric oxide synthase was found in about 6% of the CB-immunoreactive cells in this region. Choline acetyltransferase immunohistochemistry revealed that only about 2% of the neurons in the dorsal part of the ventral horn colocalized CB, whereas motoneurons were not CB-immunoreactive. The involvement of CB-containing neurons in ascending spinal projections to the thalamus, tegmentum, and reticular formation was demonstrated combining the retrograde transport of dextran amines and immunohistochemistry. Similar experiments demonstrated supraspinal projections from CB-containing cells mainly located in the reticular formation but also in the thalamus and the vestibular nucleus. The revealed organization of the neurons and fibers containing CB in the spinal cord of the turtle shares distribution and developmental features, colocalization with other neuronal markers, and connectivity with other tetrapods and, in particular with mammals.

Monoaminergic systems in the brainstem and spinal cord of the turtlePseudemys scripta elegansas revealed by antibodies against serotonin and tyrosine hydroxylase

With the aim of gaining more insight into the monoaminergic regulation of spinal motor systems in the turtle, we have studied the distribution of 5-HT (5-HTir) and tyrosine hydroxylase immunoreactivity (THir) in the brainstem and spinal cord of Pseudemys scripta elegans. 5-HTir cell bodies were located in the midline in nucleus raphe inferior, nucleus raphe superior, and laterally in nuclei reticularis superior and inferior and nucleus reticularis isthmi. THir cell bodies were located in the commissural nucleus, nucleus tractus solitarii, the locus coeruleus-subcoeruleus complex, nuclei reticularis superior and inferior, the pretectal area, and substantia nigra. 5-HTir and THir tracts were found in lateral and ventral bundles superficially in the brainstem. 5-HTir fibers in the spinal cord were located in a large dorsolateral and a smaller ventrolateral tract. In the gray matter, a high concentration of 5-HTir fibers were observed in areas I-IV and in the lateral motor column of cervical and lumbar enlargements. Areas V-VIII and area X were less intensively innervated, with the lowest fibre concentration in areas VII-VIII and area X. Throughout the spinal cord, THir nerve fibres were located in the same areas but with a lower density. Small bipolar 5-HTir and THir cell bodies were found ventromedially to the central canal especially in cervical and lumbosacral segments. Large THir cells were found in area IX in the caudal sacral and coccygeal spinal cord. THir cerebrospinal fluid-contacting cells were also found in the most caudal part of the brainstem and the upper cervical spinal cord. The well developed spinal 5-HT system and the less developed THir system provides an anatomical explanation for the monoaminergic modulation of turtle motoneuron membrane properties, which has been observed in electrophysiological experiments.

Evolutionary significance of different neurochemical organisation of the internal and external regions of auditory centres in the reptilian brain: an immunocytochemical and reduced NADPH-diaphorase histochemical study in turtles

An immunocytochemical and histochemical study was undertaken of the torus semicircularis and nucleus reuniens, the mesencephalic and diencephalic auditory centres, in two chelonian species, Testudo horsfieldi and Emys orbicularis. The nucleus centralis of the torus semicircularis receives few 5-HT-, TH-, substance P-, and menkephalin-immunoreactive fibres and terminals, in marked contrast to the external nucleus laminaris of the torus semicircularis, in which 5-HT-, TH-, substance P-, and menkephalin-immunoreactive elements and cell bodies show a laminar distribution. Dense NPY-positive terminal-like profiles and cell bodies were observed in both the nuclei centralis and laminaris, and many NADPH-d-positive cell bodies were observed in the cell layers of the latter. In the nucleus reuniens, the distribution of 5-HT-, TH-, substance P-, and menkephalin-immunolabelling resembles that seen in the torus semicircularis, but at a lower density. The dorsorostral regions of the nucleus reuniens, as in the nucleus centralis, is insignificantly labelled, in contrast to the ventrocaudal regions in which labelled elements abound. NPY-positive elements are uniformly distributed throughout the nucleus, but no labelled cell bodies were observed. NADPH-d-positive fibres and terminals were observed in both dorsal and ventral regions of the nucleus reuniens, but the few labelled cell bodies to be observed were located in the peripheral regions of the nucleus. These findings are discussed in terms of the evolution of the core-and-belt organisation of sensory nuclei observed in other vertebrate 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.

Anatomical study of vestibulospinal neurons in lampreys

The present study was carried out to characterize anatomically the vestibulospinal (VS) system of lampreys. Cobalt-lysine or Texas Red dextran amines were applied in vitro to the rostral spinal cord. Two distinct populations of VS neurons were labeled in the ventral nucleus of the area octavolateralis. The rostral group, comprising the intermediate octavomotor nucleus (ION), contained between 100 and 150 neurons, having somata of variable size and morphology. Intracellular injections of Lucifer Yellow in single neurons revealed ION VS neurons with dendrites extending in the ventrolateral alar plate as well as medially in the basal plate. The caudal group, comprising the posterior octavomotor nucleus (PON), contained approximately 65 neurons, most of which were unipolar with round or oval somata. To study the projections of VS axons, cobalt-lysine was injected into the ION or PON regions in the brainstem. Axons from the ION projected to the ipsilateral spinal cord, whereas PON axons decussated within the basal plate giving out descending and ascending branches. The descending branch projected to the contralateral spinal cord. Injections of two fluorescent dextran-amines, each restricted to one side of the spinal cord, did not double-label VS cells in either octavomotor nuclei, indicating that the projections of each nucleus are restricted to one side. Injections of horseradish peroxidase further caudally in the spinal cord revealed that VS axons from the ION reached past the gill region. Our results indicate that the organization of the VS system of lampreys is similar to that observed in other vertebrates.

Afferent and efferent connections of the torus semicircularis in the sea lamprey: an experimental study

The afferent and efferent connections of the torus semicircularis (TS) of larval sea lampreys were studied with horseradish peroxidase, carbocyanine dye (DiI) and fluorescein-coupled or Texas-Red-coupled dextran amine tract-tracing methods. Application of tracers to the TS or to the octavolateral area revealed the presence of bilateral projections from the octavolateral area to the torus semicircularis, mainly from the mechanoreceptive regions (medial and ventral octavolateral nuclei) though also from the electroreceptive (dorsal octavolateral nucleus) region. The nucleus of the descending root of the trigeminal nerve projects to the contralateral TS, mostly from neurons located rostral to the obex. Fairly numerous reticular cells of the rhombencephalon project to the torus semicircularis. In the mesencephalon, scattered cells in the tegmentum, and some in the tectum, have toral projections, mostly ipsilateral. Numerous thalamic neurons, as well as fairly numerous neurons of the posterior tubercle, hypothalamus and preoptic region, and a few neurons in the ventral telencephalon (striatum, septum), were labeled after tracer application to the TS. The torus semicircularis mainly projects to the thalamus, the hypothalamus and the reticular rhombencephalic nuclei. Our results reveal for the first time a complex pattern of connections of the lamprey TS, which suggests that it is a multisensory center integrating head cutaneous sensitivity with mechano- and electrosensory information from the octavolateral area and with visual information. A number of afferents from the forebrain also appear to contribute to TS function.

Brainstem neurons with descending projections to the spinal cord of two elasmobranch fishes: thornback guitarfish, Platyrhinoidis triseriata, and horn shark, Heterodontus francisci

We studied two cartilaginous fishes and described their brainstem supraspinal projections because most nuclei in the reticular formation can be identified that way. A retrogradely transported tracer, horseradish peroxidase or Fluoro-Gold, was injected into the spinal cord of Platyrhinoidis triseriata (thornback guitarfish) or Heterodontus fransisci (horn shark). We described labeled reticular cells by their position, morpohology, somatic orientation, dendritic processes, and laterality of spinal projections. Nineteen reticular nuclei have spinal projections: reticularis (r.) dorsalis, r. ventralis pars alpha and beta, r. gigantocellularis, r. magnocellularis, r. parvocellularis, r. paragigantocellularis lateralis and dorsalis, r. pontis caudalis pars alpha and beta, r. pontis oralis pars medialis and lateralis, r. subcuneiformis, r. peduncularis pars compacta, r. subcoeruleus pars alpha, raphe obscurus, raphe pallidus, raphe magnus, and locus coeruleus. Twenty nonreticular nuclei have spinal projections: descending trigeminal, retroambiguus, solitarius, posterior octaval, descending octaval, magnocellular octaval, ruber, Edinger-Westphal, nucleus of the medial longitudinal fasciculus, interstitial nucleus of Cajal, latral mesencephalic complex, periventricularis pretectalis pars dorsalis, central pretectal, ventromedial thalamic, posterior central thalamic, posterior dorsal thalamic, the posterior tuberculum, and nuclei B, F, and J. The large number of distinct reticular nuclei with spinal projections corroborates the hypothesis that the reticular formation of elasmobranches is complexly organized into many of the same nuclei that are found in frogs, reptiles, birds, and mammals.

Descending neural projections to the spinal cord in the channel catfish, Ictalurus punctatus

Retrograde transport of horseradish peroxidase was used to determine the descending projections to the spinal cord in an otophysan fish, the channel catfish, Ictalurus punctatus. The majority of cells projecting to the spinal cord are located in the reticular formation, which is organized into rhombomeric segments. Vestibulospinal neurons are located in the descending, magnocellular, and tangential octaval nuclei, as well as in the medial octavolateralis nucleus of the lateral line system. Cells in the facial lobe project to the spinal cord. Additionally, axons of cells of the trigeminal system and the nucleus of the lateral lemniscus project caudally into the spinal cord. In the midbrain, descending spinal projections arise from cells of the medial longitudinal fasciculus and the red nucleus. More rostrally, cells of the ventrolateral thalamus, dorsal periventricular hypothalamus, central pretectal and magnocellular preoptic nuclei also project to the cord. The results of this study indicate that there are a number of homologies in the descending systems of bony fishes and other vertebrate taxa, including tetrapods. We also provide further evidence that a red nucleus is present in the brains of bony fishes and is therefore a primitive vertebrate character antedating the evolution of tetrapods.

Efferents from the lateral frontal cortex to spinomedullary target areas, trigeminal nuclei, and spinally projecting brainstem regions in the hedgehog tenrec

This study was done in the Madagascan lesser hedgehog tenrec, an insectivore with a very poorly differentiated neocortex. The cortical region, known to give rise to spinal projections, was injected with tracer, and the cortical efferents to brainstem and spinal cord were analyzed. Bulbar reticular fields, in addition, were identified according to their cells of origin and the laterality of their spinal projections after injection of tracer. Only few cortical fibers could be traced from the bulbar pyramid into the ipsilateral spinal cord, particularly to the lateral funiculus. The projections to the dorsal column nuclei and the classical spinally projecting brainstem regions were also weak. Faint projections were demonstrated to the nucleus of the posterior commissure and the nucleus of Darkschewitsch. In comparison to other mammals, there was no evidence that the contralateral cortico-bulbo-spinal pathway was strengthened, substituting for the almost non-existent contralateral corticospinal projection. Unlike the sensorimotor apparatus controlling limb and body movements, the brainstem regions controlling the head and neck received prominent cortical projections. Direct corticotrigeminal projections and indirect pathways were well represented. The projections to the trigeminal nuclei and the lateral reticular fields were clearly bilateral; those to the superior colliculus were predominantly ipsilateral. The corticobulbar fibers left the pyramid along its entire extent; the principal trigeminal nucleus and the dorsolateral pontine tegmentum were supplied by additional fibers of the corticotegmental tract. The lateral frontal cortex also projected densely to the dorsolateral hypothalamus, the periaqueductal gray, and the adjacent mesencephalic tegmentum, components of the emotional motor system.

Organisation of the cerebellar nucleus of the dogfish, Scyliorhinus canicula L.: a light microscopic, immunocytochemical, and ultrastructural study

Elasmobranchs possess a well-developed cerebellum with an associated cerebellar nucleus. To determine whether the organization of this nucleus is comparable with that of the deep cerebellar nuclei of mammals, we studied the dogfish cerebellar nucleus with light microscopic methods (Nissl stain, Golgi method, reduced silver stain, NADPH-diaphorase histochemistry and immunocytochemistry) and with electron microscopy. We found the dogfish cerebellar nucleus to consist of about 1,050 large neurons, the ratio of Purkinje cells to cerebellar nucleus neurons being about 17:1. Immunocytochemistry showed large glutamatergic neurons in the main portions of the nucleus and small glutamate- and/or alpha-aminobutyric acid (GABA)-immunoreactive cells in the subventricular region of the nucleus. Large glutamatergic neurons corresponded to bipolar or triangular cells revealed by Golgi methods. Application of horseradish peroxidase to the cerebellar cortex produced the labelling of beaded fibres of Purkinje cells in the cerebellar nucleus. Unlike in mammals, GABAergic innervation of the cerebellar nucleus was scare: Purkinje cell axon terminals in the cerebellar nucleus did not appear to be GABA-immunoreactive, most GABAergic fibres being found in the subventricular neuropile. Some fibres immunoreactive to serotonin and somatostatin were also observed in the subventricular neuropile of the cerebellar nucleus. Three neuron types were distinguished with electron microscopy (types A to C). Type A cells were abundant and smooth-surfaced, and appeared to correspond to Golgi-impregnated neurons and large glutamate-immunoreactive cells. Type B neurons were scarce and possessed dendrites covered by sessile or stalked spines. Type C neurons were small cells located mainly in the medialmost region of the nucleus and corresponded to subventricular glutamate- and GABA-immunoreactive cells. Six types of synaptic bouton were observed (types I to VI). The most abundant (type I boutons) made symmetrical contacts and appeared to correspond to Purkinje cell axons. Type I boutons were the only type observed on perikarya and initial axon segments of type A cells. Type IV and type V boutons made complex glomerular-like asymmetrical contacts with spines of type B cells. Type VI boutons appeared to correspond to peptidergic and/or monoaminergic axons. The functional significance of these results is discussed.

Anatomical organization of the limb premotor network in the turtle (Chrysemys picta) revealed by in vitro transport of biocytin and neurobiotin

The in vitro turtle brainstem-cerebellum preparation has been a valuable tool in the study of central motor programs. In the present study, we investigate the anatomical organization of the turtle rubrocerebellar limb premotor network and its sensory connections in vitro by combining the rapid anterograde and retrograde transport of neurobiotin and biocytin with the extended viability of the isolated turtle brainstem-cerebellum. These compounds retrogradely labeled soma, dendrites, and axons, and orthogradely labeled axons and, to a lesser extent, terminals. The chelonian red nucleus receives a dense input from the contralateral lateral cerebellar nucleus and projects heavily to the contralateral spinal cord. Rubral axons sparsely innervate the lateral cerebellar nucleus and project heavily to the lateral reticular nucleus. Lateral reticular axons heavily innervate the lateral cerebellar nucleus before terminating in the pars lateralis of the cerebellar cortex as mossy fibers. These prominent, recurrent loops among the lateral cerebellar nucleus, red nucleus, and lateral reticular nucleus constitute the turtle rubrocerebellar limb premotor network. Sensory inputs to the red nucleus originate in the contralateral dorsal column nuclei, the principal trigeminal nucleus, and the spinothalamic system. These sites project bilaterally to the lateral reticular nucleus. The lateral cerebellar nucleus receives a contralateral input from the dorsal column nuclei. The red nucleus projects sparsely to the dorsal column nuclei. The red nucleus also receives an ipsilateral descending projection from the suprapeduncular nucleus, located in the diencephalon, and an ascending input from the rostral rhombencephalic reticular formation. An ipsilateral descending pathway originating in the red nucleus is likely to be the rubro-olivary tract.

Cytoarchitecture of spinal-projecting neurons in the brain of the larval sea lamprey

The descending spinal projecting system of the lamprey is of interest because it includes axons that activate swimming pattern generators and because regeneration of this system is involved in the behavioral recovery of lampreys following spinal transection. However, little is known about the true size of this projection and of the distribution of its terminations along the spinal cord. Brain neurons with spinal projections were studied in larval sea lampreys by using wholemount preparations labeled retrogradely with horseradish peroxidase (HRP) from spinal injections at 10%, 15%, 25%, 50%, 70%, and 75% of body length from the anterior end. Neurons projecting to different levels of the spinal cord were mapped. A large number of descending axons terminated within nine segments caudal to the last gill. The spinal projection system was divided into 10 bilateral groups based on cytoarchitectural landmarks. All of the lateral nuclear groups had contralateral spinal projections. In addition to the 12 pairs of Müller cells, the pair of Mauthner cells, and the pair of auxiliary Mauthner cells described by previous authors, the study revealed four pairs of smaller neurons that were individually identifiable.

Anatomy of the turtle cerebellorubral circuit studied in vitro using neurobiotin and biocytin

We have combined the rapid anterograde and retrograde transport of neurobiotin and biocytin with the extended viability of the isolated turtle brainstem-cerebellum to conduct in vitro studies of the chelonian cerebellorubral circuit. Tracers were pressure injected in 15-25 nl quantities and the optimal transport time was 16 h. Tissue sections were incubated with avidin-biotin-HRP complex and reacted with DAB. Retrogradely labeled soma, dendrites and axons, and anterogradely labeled axons and to a lesser extent terminals were visible with both tracers. Red nucleus injections resulted in dense retrograde label in the contralateral lateral cerebellar nucleus and a heavily labeled contralateral rubrospinal tract. Cerebellar nucleus injections revealed light retrograde and dense terminal label in the contralateral red nucleus, together with retrograde label in a cell cluster in the ipsilateral ventrolateral medullary reticular formation, an area we identify as the lateral reticular nucleus. Injections into this medullary region resulted in heavy mossy fiber input to the ipsilateral cerebellum and moderate retrograde label in the contralateral red nucleus. These results identify prominent recurrent projections between the lateral cerebellar nucleus, red nucleus and lateral reticular nucleus, in addition to revealing other features of the cerebellorubral circuit.

A telencephalospinal projection in the Tegu lizard (Tupinambis teguixin)

Tegu lizards (Tupinambis teguixin) were studied to determine the presence of a homologue of the mammalian corticospinal tract. The sources of telencephalic efferent projections to the spinal cord were determined by evaluating the localization of retrogradely transported horseradish peroxidase applied in the cervical spinal cord. Labeled cells were present in subtelencephalic sites reported previously by other authors and, in addition, were found in the principal sensory and motor nuclei of the trigeminal nerve and in the nucleus of the posterior commissure. A telencephalospinal projection was identified, originating in the ventral caudal telencephalon. Histochemical staining revealed a high concentration of acetylcholinesterase in cells and neuropil in the same area. This tract is suggested to be homologous to the mammalian amygdalospinal tract. No reptilian homologue of the corticospinal tract was identified.

Descending pathways to the spinal cord: II. Quantitative study of the tectospinal tract in 23 mammals

To study the early evolution of the mammalian motor systems, we have collected quantitative data on the nuclear origins of tracts descending into the spinal cord in 99 individuals representing 23 species of mammals and one species of reptile. In each individual, the spinal cord was hemisected at the C1-C2 junction and raw HRP immediately applied to the cut fibers. After a 3-day survival period, brain and spinal cord sections were treated with conventional tetramethylbenzidine procedures. In every case, this procedure resulted in heavy retrograde labeling of neural somata throughout the neuraxis from coccygeal cord to cerebral neocortex. Many thousands of supraspinal neurons were vividly labeled within at least 27 discrete cell groups in every mammal (Nudo and Masterton, '88). Despite the vast number and wide diversity of heavily labeled neurons, however, relatively few labeled somata were found in the superior colliculus. The total number of labeled cells in the tectum contralateral to the hemisection was highest in the cat (909) and second highest in the raccoon (628). In the remaining animals, the number was considerably less--averaging only 243 in the 23 mammalian species, 193 in the 21 noncarnivores, and 95 in the iguana. In 7 species of primates the average was 220, and in 3 species of Old World monkeys the average was 142. This wide variation in the number of tectospinal neurons is not related to body size, brain size, or absolute and relative tectum size. Arranging the animals in order of their kinship or recency-of-last-common-ancestor with Man, the average number of labeled tectal cells tends to decrease slightly, whereas arranging the same animals in order of their kinship with the cat or raccoon shows a marked and statistically reliable increase. Neither the evolutionary increase in the tectospinal tract along the Carnivora lineage nor the slight decrease along Man's lineage is altered by mathematical corrections for allometric or scaling factors. Of an array of morphological, visual, motor, and ecological traits tested statistically as a possible source of the variation in size of the tectospinal tract, only a primarily carnivorous feeding preference was found to be reliably related. The relatively small number of tectospinal fibers in most mammals in our sample, including the primates, suggests that the tectospinal tract in Man may be quite small, perhaps far too small to warrant continuing description as a "major descending tract."

The organization of descending tectofugal pathways underlying orienting in the frog, Rana pipiens. II. Evidence for the involvement of a tecto-tegmento-spinal pathway

In the frog, identical orienting deficits, involving a failure to turn toward stimuli in the ipsilateral hemifield, can be produced by small white matter lesions either in the caudal mesencephalon (Kostyk and Grobstein, 1987a) or in the caudal medulla (Masino and Grobstein, 1989). These findings suggest that descending turn signals may run uninterrupted from the midbrain to the spinal cord, and that something other than tectospinal axons may carry such signals. We here report studies to determine whether there is a tecto-recipient structure whose axons pass through the known critical lesion sites in the caudal mesencephalon and medulla, and whether damage to such a structure, sparing tectospinal pathways, produces an orienting deficit. Horseradish peroxidase (HRP) was applied to behaviorally effective lesions in the caudal medulla and the resulting labelling patterns compared with those resulting from application of HRP to nearby but behaviorally ineffective lesions at the same rostrocaudal level. A column of large cells in the ventrolateral midbrain tegmentum (including nMLF as well as parts of AV and PV) was robustly labelled in all effective lesion cases, and less frequently labelled in ineffective cases. A quantitative analysis showed labelling in this region to be more highly correlated with the existence of a behavioral deficit than that in any other brain region. Reconstructions of single retrogradely labelled cells in the rostral part of the column (nMLF) showed that they have dendrites in a position to receive tectal input and axons which pass through the critical lesion sites in both the caudal mesencephalon and the caudal medulla. Tegmental lesions, sparing the tectospinal tracts, produced ipsilateral turning deficits in cases where the large cell column was completely removed but did not when the column was spared. The findings support the hypothesis that tectofugal signals involved in orienting turns descend uninterrupted to the spinal cord on something other than tectospinal axons, and suggest that the critical projections derive from the large cell column of the ventral tegmentum.

Origins of the descending spinal projections in petromyzontid and myxinoid agnathans

The origins of the descending spinal pathways in sea lampreys (Petromyzon marinus), silver lampreys (Ichthyomyzon unicuspis), and Pacific hagfish (Eptatretus stouti) were identified by the retrograde transport of horseradish peroxidase (HRP) placed in the rostral spinal cord. In lampreys, the majority of HRP-labeled cells were located along the length of the brainstem reticular formation in the inferior, middle, and superior reticular nuclei of the medulla, mesencephalic tegmentum, and nucleus of the medial longitudinal fasciculus. Labeled reticular cells included the Mauthner and Müller cells. Horseradish-peroxidase-filled cells were also present in the descending trigeminal tract, intermediate and posterior octavomotor nuclei, and a diencephalic cell group, the nucleus of the posterior tubercle. As in lampreys, the reticular formation of the Pacific hagfish was the largest source of descending afferents to the spinal cord. Labeled cells were found in the dorsolateral and ventromedial reticular nuclei, the dorsal tegmentum at the juncture of the medulla and midbrain, and the nucleus of the medial longitudinal fasciculus. Additional medullary cells projecting to the cord were located in the perivagal nucleus, the central gray, and the anterior and posterior magnocellular octavolateralis nuclei. The existence of reticulospinal and possible vestibulo-, trigemino-, and solitary spinal projections in lampreys and hagfishes and the wide distribution of these pathways in jawed vertebrates suggest that they evolved in the common ancestor of gnathostomes and both groups of jawless fishes. However, descending spinal pathways from the cerebellum, red nucleus, and telencephalon appear to be gnathostome characters.

Descending pathways to the spinal cord: a comparative study of 22 mammals

In order to estimate the qualitative commonalities and range of variation among major descending spinal pathways relevant to mankind's ancestral lineage, the supraspinal cell groups originating fibers descending directly to the spinal cord were examined in 22 mammalian species. In a standardized retrograde tract-tracing procedure, flakes of raw HRP were applied directly to the freshly cut fibers of the spinal cord after it had been hemisected at the C1-C2 junction. After a 72-hour survival period, brain and spinal cord tissues were processed by conventional HRP-processing techniques. This procedure was performed on 94 individual animals. Of this total, 41 individual cases were eliminated by a rigorous culling procedure. The results are based on 53 individuals representing 15 species selected for their successive kinship with mankind and seven species in two other lineages selected for the convergence of their visual or sensorimotor systems with anthropoids. The 22 species represent 19 genera, 14 families, eight orders, and two subclasses of Mammalia. The results show that at least 27 supraspinal cell groups, each containing intensely labeled cells, can be readily identified in each of the species. Despite vast quantitative differences in cell number and cell size, this qualitative uniformity among the relatively large number of diverse taxa suggests that the same pathways were probably present in the extinct ancestors throughout mankind's mammalian lineage and are probably still present in extant viviparous mammals as well. If so, these pathways are as old in phylogenetic history as the last common ancestor of marsupial and placental mammals--dating from the late Jurassic to early Cretaceous, perhaps 145-120 million years ago. Further comparison of the results with similar experimental findings in members of other vertebrate classes supports the notion that several of these same pathways can be traced to even more remote ancestry, with some possibly as old as the entire vertebrate subphylum--dating from the early Devonian or before, perhaps 430 million years ago. Within mankind's ancestral lineage, from the appearance of vertebrates to the appearance of mammals, there seems to have been an irregular stepwise augmentation of the set of descending pathways until the full mammalian complement was finally attained with the appearance of the corticospinal tract.

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.

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.

Descending projection neurons to the spinal cord of the goldfish, Carassius auratus

The sources of descending spinal tracts in the goldfish, Carassius auratus, were visualized by retrograde transport of horseradish peroxidase (HRP) administered to the hemisected spinal cord. In the diencephalon, HRP-positive neurons were identified in the nucleus preopticus magnocellularis pars magnocellularis and ventromedial nucleus of the thalamus of the ipsilateral side. In the mesencephalic tegmentum, a few somata of the contralateral nucleus ruber and several ipsilateral neurons of the nucleus of the median longitudinal fasciculus were labeled. The reticular formation of the rhombencephalon was the major source of descending afferents to the spinal cord. A larger number of neurons were retrogradely labeled in the ipsilateral superior, middle, and inferior nuclei than in the contralateral nuclei. A few raphe neurons and the contralateral Mauthner neuron were also HRP-positive. The octaval area showed retrogradely labeled neurons in the anterior, magnocellular, descending, and posterior octaval nuclei of the ipsilateral side. A large number of neurons in the facial lobe and a few somata located adjacent to the descending trigeminal tract were labeled on the ipsilateral side. The pattern of descending spinal projections in goldfish is comparable to that of tetrapods and suggests that the spinal tracts have originated quite early in the course of vertebrate evolution.

Autoradiographic study of descending pathways from the pontine reticular formation and the mesencephalic trigeminal nucleus in the rat

Descending projections were studied in autoradiographically prepared material after injections of tritiated leucine in the pontine tegmentum of rats. Injections involving the medial pontine reticular formation resulted not only in labeling commissural fibers, the medial reticulospinal tract, and the dorsal cap of the inferior olive, but also, in two cases, in labeling a cerebellar projection that originated from a region near the midline and clearly dorsal to the nucleus reticularis tegmenti pontis. The labeled fibers passed ventral in the midline to the pontine gray, then laterally through the gray and into the middle cerebellar peduncle to terminate as mossy fibers primarily in the flocculus, lobulus simplex, and Crus I of the ansiform lobule. Injections involving the mesencephalic nucleus of the trigeminal nerve (Vmes), resulted in labeling of Probst's tract, which descends in the dorsolateral reticular formation. Probst's tract gave off extensive terminal branches to the lateral medullary reticular formation and weaker projections to restricted portions of the descending trigeminal nucleus, the solitary nucleus, and the hypoglossal nucleus. In one case, fibers could be traced into the dorsal horn of the upper cervical cord.

Collateralization of descending pathways from the brainstem to the spinal cord in a lizard, Varanus exanthematicus

With the multiple fluorescent retrograde tracer technique, the collateralization in the spinal cord of descending supraspinal pathways was studied in a lizard, Varanus exanthematicus. Fast Blue (FB) gels were implanted unilaterally in the spinal gray matter of the cervical enlargement and Nuclear Yellow (NY) gels were implanted ipsilaterally in two series of experiments in all spinal funiculi of the lumbar enlargement or in midthoracic spinal segments, respectively. All brainstem nuclei known to project to the spinal cord in reptiles were found to give rise to branching axons that may influence widely separate levels of the spinal cord. The number of double-labeled FB-NY cells varied in these brainstem nuclei from none to half the number of neurons projecting to the cervical enlargement. Highly collateralizing projections (expressed as the percentage of double-labeled neurons, DL) were found to arise from the nucleus raphes inferior, the contralateral nucleus reticularis superior pars lateralis, the contralateral nuclei vestibulares ventromedialis and descendens, and the ipsilateral nucleus reticularis inferior pars ventralis. A lower percentage of DL neurons was noted for the contralateral nucleus ruber and bilaterally for the nucleus reticularis medius and nucleus reticularis inferior. Extensive brainstem projections directed to cervical and high thoracic spinal levels originate from the area lateralis hypothalami, the nucleus of the fasciculus longitudinalis medialis, the contralateral nucleus cerebellaris medialis, and from the nucleus tractus solitarii. Projections preferentially directed to midthoracic or lower levels of the spinal cord were found to arise from the ipsilateral locus coeruleus, the contralateral nucleus reticularis superior pars lateralis, the nucleus reticularis inferior pars ventralis, the nucleus reticularis inferior, and the nucleus raphes inferior. In contrast to findings in mammals, in Varanus exanthematicus the red nucleus, the nucleus vestibularis ventrolateralis, and certain parts of the reticular formation did not display a clear-cut somatotopic organization. In general two different patterns of collateralization can grossly be discerned: a gradual decrease of spinal collaterals caudalward, which can be interpreted as a certain specificity of such projections; and a constant number of collateral nerve fibers throughout the spinal cord that can be interpreted as either a nonspecific or, in contrast, a highly specific system, focussed exclusively on the cervical and lumbar enlargements.

Projections from the cochlear nuclear complex to rhombencephalic auditory centers and torus semicircularis in the turtle

The projections arising from the cochlear nucleus complex in the turtle were investigated using [35S]methionine as a tracer. Commissural connections between the auditory tubercles terminate in both the laminar and the magnocellular nuclei. The cochlear projections to the superior olivary complex, the lateral lemniscus complex and the torus semicircularis were predominantly contralateral. Terminal labeling in the torus semicircularis was found in the peritoral region (TSP) and in the central nucleus (TSC), particularly its caudoventral and rostromedial portions. The caudal dorsolateral portion of TSC, which had previously been shown to receive ascending spinal fibers was not labeled following injections of tracer into the cochlear nuclear complex. Ascending spinal and cochlear projections, however, appeared to overlap in the rostral portions of TSC and in TSP.

The cerebellar and vestibular nuclear complexes in the turtle. II. Projections to the prosencephalon

Prosencephalic projections from the cerebellar and vestibular nuclear complexes in the turtle Pseudemys scripta elegans were investigated with anterograde tracing. Following injections of 35S-methionine at various locations within the cerebellar and vestibular nuclear complexes, labeled ascending fibers were found to arise from the lateral cerebellar and the rostral (superior and/or dorsolateral) vestibular nuclei. The great majority of these fibers coursed within the ipsilateral ascending periventricular tract. There were possible terminations in the hypothalamosuprapeduncular region, the ovalis-complex, and the nucleus commissuralis anterior, but scarcely any indication of terminal labeling within the dorsal thalamus. The labeled fibers, however, continued rostralward, entered the lateral forebrain bundle, and terminated in the anterior dorsal ventricular ridge--in all but one case, exclusively ipsilaterally. The terminal area within the lateral division (referred to as area L) of the anterior dorsal ventricular ridge was sharply delimited, being situated ventrolateral to the visually oriented area D of the anterior dorsal ventricular ridge (Balaban and Ulinski, '81), medial to the lateral cortex, and ventral to the pallial thickening (motor pallium of Johnston, '16). The findings are compared with related ones in mammals, particularly those pertaining to telencephalic somatosensorimotor regions and their interactions with the vestibular nuclear complex and the cerebellum.

The cerebellar and vestibular nuclear complexes in the turtle. I. Projections to mesencephalon, rhombencephalon, and spinal cord

Cerebellar and vestibular projections were investigated in the turtle Pseudemys scripta elegans following injection of 35S-methionine into the cerebellar and vestibular nuclear complexes at various locations. Fibers arising from the cerebellar nuclei were traced via the cerebellar commissure to the contralateral vestibular nuclear complex (particularly the n. vestibularis inferior and n. vestibularis ventrolateralis) and caudal rhombencephalic tegmentum. Ascending projections crossing the midline in the ventral isthmomesencephalic tegmentum terminated in the contralateral red nucleus and nuclei of the fasciculus longitudinalis medialis (f lm). Vestibular projections ascending mainly via the f lm terminated in the nuclei of the f lm, the nuclei of the posterior commissure, and particularly the extraocular motor nuclei. Vestibulo-ocular projections arising from the rostral vestibular nuclear complex were almost exclusively ipsilateral; those from the caudal vestibular nuclear complex were bilateral. Evidence for a topographic organization of the projections to the trochlear and oculomotor nuclei was also obtained. There were some vestibular projections to the contralateral rhombencephalic tegmentum and n. vestibularis inferior. Spinal projections coursing within the ipsilateral ventral descending tract and the ipsilateral fasciculus longitudinalis medialis were found to arise from both rostral and caudal vestibular regions. The caudal vestibular nuclear complex in addition gave rise to fibers descending in the contralateral fasciculus longitudinalis medialis. Evidence for the existence of labeled fibers crossing at spinal levels was also obtained. Vestibulospinal terminations appeared restricted to the ventral horn.

The origins of descending spinal projections in lepidosirenid lungfishes

The origins of descending spinal projections in the lepidosirenid lungfishes were identified by retrograde transport of horseradish peroxidase (HRP) introduced into the rostral spinal cords of juvenile African (Protopterus annectans and Protopterus amphibians) and South American (Lepidosiren paradoxa) lungfishes. Standard HRP histochemistry revealed retrogradely labeled neurons in the nucleus of the medial longitudinal fasciculus, midbrain tegmentum, red nucleus, optic tectum, mesencephalic trigeminal nucleus, granule cell layer of the cerebellum, superior, middle, and inferior medullary reticular nuclei, magnocellular and descending octaval nuclei, region of the descending trigeminal tract, solitary complex, and the margins of the spinal gray matter anterior to the spinal HRP implant. A small number of retrogradely labeled neurons were also present in the ventral thalamus of Protopterus. A descending spinal projection from the forebrain was not evident in either genus of lepidosirenid lungfishes. The presence of projections to the spinal cord from the diencephalon, medial reticular formation of the midbrain and medulla, octaval (vestibular) nuclei, solitary complex, and probable nucleus of the descendin trigeminal tract in lungfishes and their overall similarity to comparable projections in other vertebrates suggest that these pathways are among those representative of the primitive pattern of descending spinal projections in vertebrates.

The fasciculus longitudinalis medialis in the lizard Varanus exanthematicus. 2. Vestibular and internuclear components

In 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, distinct 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 contralateral oculomotor nucleus. These reciprocal internuclear connections between the oculomotor and abducens nuclei probably play an important role in conjugate horizontal eye movements.

Distribution of serotonin in the brain stem and spinal cord of the lizard Varanus exanthematicus: an immunohistochemical study

The distribution of serotonin-containing nerve cell bodies, fibers and terminals in the lizard Varanus exanthematicus was studied with the indirect immunofluorescence technique, using antibodies to serotonin. Most of the serotonin-containing cell bodies were found in the midline, in both of the raphe nuclei, i.e. the nuclei raphes superior and inferior. A considerable number of more laterally shifted serotonergic neurons was found particularly at three levels of the brain stem, viz. in the caudal mesencephalic tegmentum, at the isthmic level, and over a long distance in the medulla oblongata. These laterally situated serotonin-positive neurons were partly found within the confines of the substantia nigra, the nucleus reticularis superior and the lateral part of the nucleus reticularis medius and ventrolateral part of the nucleus reticularis inferior, respectively. No serotonergic cell bodies were found in the spinal cord. In the brain stem a dense serotonergic innervation was observed in all of the motor nuclei of the cranial nerves, in two layers of the tectum mesencephali, in the nucleus interpeduncularis pars ventralis, the nucleus profundus mesencephali pars rostralis, the periventricular grey, the nucleus parabrachialis, the vestibular nuclear complex, the nucleus descendens nervi trigemini, the nucleus raphes inferior, and parts of the nucleus tractus solitarii. Descending serotonergic pathways could be traced into the spinal cord via the dorsolateral, ventral and ventromedial funiculi, and were found to innervate mainly three parts of the spinal grey throughout the spinal cord, i.e. the dorsal part of the dorsal horn, the motoneuron area in the ventral horn, and the intermediate zone just lateral to the central canal. The results obtained in the present study suggest a close resemblance of the organization of the serotonergic system in reptiles and mammals, especially as to the serotonergic innervation of the spinal cord.

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.

Distribution of catecholamines in the brain stem and spinal cord of the lizard Varanus exanthematicus: an immunohistochemical study based on the use of antibodies to tyrosine hydroxylase

Antibodies to tyrosine hydroxylase were used to study the distribution of nerve cells, fibers and terminals, containing catecholamines, in the lizard Varanus exanthematicus, by means of the indirect immunofluorescence technique. Tyrosine hydroxylase-containing cell bodies occurred in the hypothalamus, the ventral and dorsal tegmentum mesencephali, the substantia nigra, the isthmic reticular formation, in and ventrolaterally to the locus coeruleus, in the nucleus tractus solitarii and in a lateral part of the nucleus reticularis inferior. In addition tyrosine hydroxylase-containing cell bodies were found throughout the spinal cord, ventral to the central canal. Tyrosine hydroxylase-immunoreactive terminal areas in the brain stem were seen in the nucleus interstitialis of the fasciculus longitudinalis medialis, the nucleus raphes superior, the locus coeruleus, several parts of the reticular formation and the nucleus descendens nervi trigemini. Ascending catecholaminergic pathways could be traced from the ventral mesencephalic tegmentum as well as from the dorsal isthmic tegmentum rostralwards, through the lateral hypothalamus. These pathways correspond to the mesostriatal and isthmocortical projections respectively, as described in mammals. Furthermore, ascending catecholaminergic fibers could be traced from the catecholaminergic cell groups in the medulla oblongata to the isthmus, where they intermingle with the locus coeruleus neurons. These pathways correspond to the medullohypothalamic projection and to the dorsal periventricular system in mammals. Descending catecholaminergic fibers to the spinal cord pass via the dorsomedial part of the lateral funiculus, and mainly terminate in the dorsal horn. The results obtained in the present study have been placed in a comparative perspective, which illustrates the constancy of catecholaminergic innervation throughout phylogeny.

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.

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.