The origin of brainstem-spinal pathways in the north american opossum (didelphis virginiana). Studies using the horseradish peroxidase method

Brainstem neurons which project to lumbar, thoracic and cervical spinal levels have been identified in the North American opossum by the horseradish peroxidase (HRP) technique. Neurons which relay to all of the levels studied are located within the medullary and pontine reticular formation as well as within the nucleus cuneatus, the nucleus of the tractus solitarius, the lateral reticular nucleus, the medullary and caudal pontine raphe nuclei, the lateral, medial and inferior vestibular nuclei, the nucleus “F,” the nucleus coeruleus, and the nucleus coeruleus, para α, the red nucleus, and the interstitial nucleus of Cajal. The lateral vestibulospinal and rubrospinal systems are topographically organized, although neurons projecting to different cord levels show considerable intermingling. Our material also provides evidence that raphe-spinal and reticulospinal connections are organized to some degree. Neurons which backfill after cervical and thoracic placements, but not after lumbar injections, are distributed within the caudal spinal trigeminal nucleus, the nucleus intercalatus, the dorsal vagal nucleus, the cuneiform area of Castaldi, the fields of Forel, and the nucleus of Darkschewitsch. Reactive neurons are present within the lateral, dorsal and posterior hypothalamic areas as well as within the periventricular and paraventricular nuclei after thoracic placements and within the superior colliculus after injections within the cervical cord. Additionally, neurons are reactive in the nucleus ambiguus, the interpolar division of the spinal trigeminal nucleus and the rostral division of the oculomotor nucleus (Oswaldo-Cruz and Rocha-Miranda, '68) after HRP placements into the third and fourth cervical segments.     

Collateral branching in axonal projections to spinal cord from paramedian reticular nucleus neurons

Experiments were done in cats to identify neurons in the paramedian reticular nucleus (PRN) sending collateral axons to the region of the intermediolateral nucleus (IML) at different levels of the thoracic cord by using lectin-conjugated horseradish peroxidase (HRP) and double-labeling fluorochrome histochemistry to retrogradely label PRN neurons. Injections of Fast blue (FB) into the spinal cord at the T2 level centered in the region of the IML were coupled with injections of Nuclear yellow (NY) into the ipsilateral cord at either the T4 or T7 levels centered in the region of the IML. Neurons in the PRN retrogradely labeled after diffusion of HRP into the region of the IML at the T2 level were observed throughout the rostrocaudal extent of the ventral PRN. In addition, a few labeled neurons were noted in the ventral portion of the dorsal PRN. About 40% of the neurons in the PRN which were labeled with FB after an injection at the T2 level were also labeled with NY injected into the cord in further caudal segments. These data suggest that the PRN may exert its influence on the cardiovascular system partly through collateral axonal branches to widely separated populations of sympathetic preganglionic neurons in different spinal segmental levels.     

The mormyrid brainstem. I. Distribution of brainstem neurones projecting to the spinal cord in Gnathonemus petersii. An HRP study

The sources of the descending spinal tracts were identified in the teleost fish Gnathonemus petersii by retrograde HRP transport. HRP injections were made at two spinal levels, either at level of the caudal end of the dorsal fin, anterior to the electric organ, or at the pectoral fin. In both cases all labeled cells were found in the rhombencephalon and the mesencephalic tegmentum. No labeled cells were observed either in the cerebellum and lateral line lobes or in the dorsal mesencephalon i.e. torus semicircularis and mesencephalic tectum or in the telencephalon. Following caudal spinal injections, the majority of the labeled cells were grouped in a median and a ventrolateral column of the rhombencephalic reticular formation. The latter is composed of three parts corresponding to the nucleus reticularis inferior, medius and superior. Both Mauthner cells, all the cells in the medullary relay nucleus controlling the electric organ discharge and a few cells in the posterior part of the magnocellular octaval nucleus were labeled. In the mesencephalon, four nuclei were identified by HRP labeling: the nucleus of the medial longitudinal fasciculus, the nucleus reticularis mesencephali and the anterior and posterior tegmental mesodiencephalic nuclei. The rostral injections revealed several additional spinal projections from the descending vestibular and tangential nuclei, from the medial part of the magnocellular nucleus and, finally, from the rostral periventricular gray of the mesencephalon. Also, after such injections, a greater number of cells were labeled in the reticular formation, especially in the median column and in the inferior reticular nucleus. The results suggest that the rostral spinal cord has a larger connection with the acoustico-vestibular area and the medullary reticular formation than the caudal spinal cord. In contrast, the mesencephalic nuclei, probably linked to the mesencephalic tectum and the pretectal area, appears to be a coordinating apparatus between the visual system and the trunk/tail musculature. Thus, it appears that teleost fish possess the same basic equipment of descending spinal pathways as higher vertebrates.     

The location of spinal projection neurons in the cerebellar nuclei (cerebellospinal tract neurons) of the cat. A study with the horseradish peroxidase technique

The distribution of spinal projections neurons was studied in the cerebellar nuclei of the cat following injections of horseradish peroxidase (HRP) into the cervical, thoracic and lumbar cord. HRP-positive (labeled) neurons were found in the medial (fastigial) and the posterior interpositus nuclei on the side contralateral to the cervical injections, being most numerous in cases with injections between the C2 and the C3 segments.In the medial nucleus (M) labeled neurons were distributed in the central to the caudal portions, and there was a conspicuous group of labeled small neurons extending from the ventolateral part to the intermediate zone between the M and the anterior interpositus nucleus. With an increasing number of medium-sized neurons, this neuronal group persisted caudally in a similar position, ventromedial to the posterior interpositus nucleus (IP). Labeled large neurons were seen in the medial third of the IP. In the two cases labeled neurons of medium and small sizes were equal iin number, and the neurons of the IP constituted about 10% of the total number of the spinal projection neurons. The present study suggests that the neurons of the M and the IP, including those of the intermediate group located between the two, project the bulk of the crossed descending fibers as far caudally as the C2 and the C3 segments.     

The origins of supraspinal projections to the cervical and lumbar spinal cord at different stages of development in the gray short-tailed Brazilian opossum, Monodelphis domestica.

We have used the retrograde transport of Fast blue (FB) to study the origins of supraspinal projections to the lumbar and cervical spinal cord at different stages of development in the Brazilian, short-tailed opossum, Monodelphis domestica. Monodelphis was chosen for study because its young are born in a very immature state, 14-15 days after copulation, making it possible to manipulate its nervous system in an embryonic state without intra-uterine surgery. When injections of FB were made into the lumbar cord at postnatal day (PD) 1, neurons were labeled within several areas of the reticular formation (the retroambiguus nucleus, the ventral and dorsal reticular nuclei of the medulla, the gigantocellular reticular nucleus, the lateral paragigantocellular reticular nucleus, and the pontine reticular nucleus), the presumptive coeruleus complex, and the lateral vestibular nucleus. In many cases, labeled neurons were also found within the caudal raphe and the presumptive interstitial nucleus of the medial longitudinal fasciculus. The results of immunocytochemical studies provided evidence for catecholaminergic and serotoninergic neurons in the brainstem at PD1 and for axons of both phenotypes in the spinal cord. By PD3, labeled neurons were found within the ventral gigantocellular and ventral pontine nuclei of the reticular formation, the spinal trigeminal nucleus, and the presumptive paraventricular nucleus of the hypothalamus. When injections were made at PD4, neurons were also labeled within the medial and inferior vestibular nuclei, the red nucleus, the mesencephalic nucleus of the trigeminal nerve, the presumptive nucleus of Edinger-Westphal and the lateral hypothalamus. By at least PD7, the pattern of supraspinal labeling was similar to that obtained at older ages and in the adult animal. When FB was injected into the cervical cord at PD1, neurons were labeled in all of the areas labeled by lumbar injections at the same age and in larger numbers. In addition, labeled neurons were found within the ventral gigantocellular and spinal trigeminal nuclei. When cervical injections were made at PD15, labeled neurons were found within the deep cerebellar nuclei and amygdala and by PD17 they were also present within the superior colliculus and cerebral cortex. In some cases, cortical labeling was present outside the areas labeled by comparable injections in adult animals.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cells of origin of the spinoreticular tract in the monkey

The distribution of the cells of origin of the primate spinoreticular tract was determined following injections of horseradish peroxidase (HRP) into the pontomedullary reticular formation in Macaca fascicularis. Five animals received large bilateral injections which included the raphe nuclei and seven monkeys received smaller, unilateral injections. Sections sampled were from upper cervical levels, the cervical enlargement, upper and lower thoracic levels, and lumbosacral levels. The laminar distribution of spinoreticular cells in all spinal cord levels was comparable. More than half of the labeled cells were located ventromedially, in laminae VII and VIII. HRP-labeled cells were also found in the dorsal horn, primarily in the lateral reticulated part of lamina V. Some cells were also found in laminae I and X. Spinoreticular cells in the lumbosacral spinal cord mainly projected to the contralateral brainstem. In the cervical enlargement, however, a bilateral distribution of cells was observed following unilateral injections of HRP. Most spinoreticular cells were multipolar neurons with extensive dendritic ramifications. The distribution of spinoreticular cells is similar to the distribution of spinal cord neurons that project to the medial thalamus, but different from that of spinal neurons projecting to the ventrobasal complex. The anatomical organization of the spinoreticular tract is consistent with a role for this pathway in nociception.     

Projections to the spinal cord from medullary somatosensory relay nuclei.

Descending projections to the spinal card from the dorsal column nuclei were studied in the cat, rat and monkey with the retrograde horseradish peroxidase (HRP) technique, and in the cat with the autoradiographic anterograde axonal transport technique. Retrogradely labeled neurons were seen in the dorsal column nuclei after HRP injections at all levels of the spinal cord and additionally in the magnocellular division of the spinal caudalis nucleus of the trigeminal nerve after injections into cervical spinal segments in all three species. HRP-positive neurons were predominantly located along the middle of the rostro-caudal axis of the dorsal column nuclei and amongst the fusiform, triangular and polygonal cells that surround, especially ventrally, the cell nest zone containing thalamic relay neurons. The labeled neurons are densely concentrated in those portions of the dorsal column nuclei where most corticofugal and non-primary afferent projections terminate and where the terminal distribution of primary afferent fibers is overlapping and diffuse. Previous studies have shown that most neurons in this middle and ventral region do not project to the thalamus or cerebellum. The majority of the cells in the dorsal column nuclei with descending axons or axon collaterals project by way of the ipsilateral dorsal columns, but some fibers project into the dorsolateral funiculus; the descending trigeminal fibers course in the dorsolateral funiculus. The terminal fields for these fibers in the cervical spinal cord include the lateral cervical nucleus, laminae IV and V, and possibly lamina I. These results indicate that the dorsal column nuclei may contribute to a feedback mechanism regulating the flow of sensory information ascending along other somatosensory spinal pathways.     

The location of spinal neurons with long descending axons (long descending propriospinal tract neurons) in the cat: a study with the horseradish peroxidase technique.

The distribution spinal neurons with long descending axons was studied in the cat by means of retrograde transport of horseradish peroxidase. Labeled neurons appeared bilaterally in the cervical and the thoracic cord following injections in the lumbosacral cord. In some cases hemisections were made rostrally and contralaterally to the injections in an attempt to determine whether or not the axons crossed. Neurons with uncrossed descending axons were located in laminae I, V, VII and VIII. Lamina I neurons were present in all the spinal segments. In lamina V labeled neurons were distributed mainly laterally in the cervical cord but medially and laterally in the thoracic cord. In the upper cervical and the thoracic cord laminae VII and VIII neurons were distributed very densely along the lateral cord, accounting for 30 and 40 of the total labeled neurons, respectively. In the cervical enlargement they were located in the middle part of lamina VII and in lamina VIII, accounting for about 25% of the total labeled neurons. Neurons with crossed descending axons were found in laminae V, VII and VIII, in the medial part of lamina VII including the intermediomedial nucleus of the thoracic levels and close to the central canal. Lamina V neurons were very small in number. The largest collections of labeled neurons were present in the medial part of laminae VII and VIII. They accounted for about 45% to 55% and 37% of the total in the cervical and the thoracic cord. These neurons may function as the long spinal reflex paths for forelimb-hindlimb synergies and the intercalated paths between the supraspinal descending tracts and the spinal motor centers.     

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.     

Spinal projections from the lower brain stem in the cat as demonstrated by the horseradish peroxidase technique. II. projections from the dorsolateral pontine tegmentum and raphe nuclei

The descending projections to the spinal cord arising from the dorsolateral pontine tegmentum and brain stem raphe nuclei have been investigated by means of the horseradish peroxidase (HRP) technique. Particular attention was taken to clarify the cells of origin and the funicular trajectory of these spinal projections.After injections of HRP into the spinal cord, a significant number of HRP labeled neurons were observed in the following dorsolateral pontine tegmental structures: (1) an area ventral to the nucleous cuneiformis; (2) principal locus coeruleus; (3) locus coeruleus α; (4) locus subcoeruleus; (5) Kölliker-Fuse nucleus; and (6) nucleus parabrachialis lateralis. As a rule, the projections are ipsilateral and the descending fibers course in the ventral part of the lateral funiculus.As concerns the raphe-spinal projections, we have demonstrated that the nucleus raphe dorsalis also sends axons to the cervical segment of the spinal cord. Furthermore, in accord with previous reports, HRP labeled cells were also identified in the nucleus raphe magnus, pallidus and obscurus, but not in the nucleus raphe centralis superior and pontis.On the whole the present study further clarified the organization of spinal projections from the dorsolateral pons and raphe nuclei and provided some additional anatomical data for the physiology of the tegmentospinal and raphe-spinal projections.     

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.     

Distribution of neurons in the lateral pontine tegmentum projecting to thoracic, lumbar and sacral spinal segments in the cat

The course of descending fibers projecting to the spinal cord and the arrangement of their parent cells located in various nuclei of the dorso-lateral pontine tegmentum were studied using the horseradish peroxidase (HRP) retrograde axonal transport technique. Retrogradely labeled neurons were found in the locus coeruleus (LC), subcoeruleus (SC), K?lliker-Fuse nucleus (KF) and in the lateral parabrachial nucleus (LPB) after HRP injections into various spinal segments. Neurons innervating the thoracic spinal cord were found to be arranged in the ventral portion of the LC and in the entire SC; their axons descended ipsilaterally. Neurons with descending axons to lumbar segments were seen mainly in the ventral portion of the LC and in the medial portion of SC. Most of their axons were also seen to descend ipsilaterally. Neurons projecting to sacral segments occurred in the entire LC and in the medial portion of the SC. Large part of descending fibers crossed the midline at the level of (or near) the termination site. Neurons of all portions of the KF and LPB projected to the thoracic spinal cord only ipsilaterally, while many descending fibers innervating the sacral segments crossed the midline.     

Descending projections to the rat sacrocaudal spinal cord.

Descending supraspinal and propriospinal neurons projecting to the female rat sacrocaudal spinal cord, the portion of the spinal cord that innervates the tail, were identified following injection of Fluoro-Gold into the S1-Ca2 spinal cord segments. This study attempted to determine anatomical substrates for propriospinal and supraspinal control of the tail. Propriospinal neurons were identified throughout laminae V-VIII and X at all levels of the spinal cord. The greatest density of labeling was in the lumbar enlargement, followed by the cervical enlargement, with least in the thoracic spinal cord. Within a given cord level, labeling was greatest within the intermediate zone. In addition, other prominent spinal cord collections included neurons in 1) lamina V of the lumbar enlargement, 2) dorsal lamina X of the cervical enlargement, and 3) the lateral spinal nucleus within the cervical enlargement. Supraspinal cells were identified within raphe nuclei, reticular formation nuclei, dorsal column nuclei, vestibular nuclei, noradrenergic groups, the red nucleus, the periaqueductal gray, the hypothalamus, and the motor cortex. These data indicate that there are significant descending projections to the sacrocaudal spinal cord, with distributions similar to those of other cord levels. Functionally, important supraspinal and propriospinal influences on tail, pelvic viscera and limbs, such as with locomotion, balance, defense, micturition, defecation, and sexual functions, may be mediated by these connections.     

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.     

The origin of reticulospinal fibers in the rat: a HRP study

The distribution as well as morphological characteristics of brain stem reticular neurons projecting to spinal cord both of aminergic and non-aminergic natures in the rat was investigated by means of the retrograde horseradish peroxidase (HRP) method. For the identification of aminergic neurons, a combination of HRP technique and monoamine-oxidase staining as well as a pretreatment of 6-hydroxydopamine and 5,6-dihydroxytryptamine (5,6-DHT) was applied. Following the injection of HRP to the cervical, thoracic and lumbar cord, a remarkable number of neurons in the nuclei reticularis ventralis (rv), reticularis lateralis (RI; Meesen & Olszewski), reticularis gigantocellularis, reticularis pontis caudalis, reticularis pontis oralis (rpo), and a small number of cells in the nucleus reticularis dorsalis dorsalis as well as the mesencephalic reticular nuclei were found to be labeled. In the case of cervical injection, HRP labeled cells were also found in the nucleus reticularis parvocellularis, adjacent to the nucleus tractus spinalis n. trigemini oralis, in which labeled neurons were observed. Within Rl, ventral division of rv and lateral part of rpo, the labeled noradrenaline neurons in A 1, 3 and 7, respectively, were found intermingled with the non-aminergic labeled neurons. Many neurons in the nucleus raphe magnus (ram), obscurus (rao) and pallidus (rap) were labeled. From the fact that there was a marked decrease in the number of the labeled cells in rao, rap, and a slight decrease in ram after 5,6-DHT treatment, it was suggested that the majority of labeled cells in rao, rap and a partial number of labeled cells in ram are serotonergic.     

Horseradish peroxidase labeling of the efferent and afferent pathways of the avian tangential vestibular nucleus

The efferent and afferent pathways of the chick tangential nucleus were studied by using horseradish peroxidase (HRP: Sigma type VI) to label nerve cell bodies and fibers. Depositions of HRP into the tangential nucleus, as well as into the second cervical level of the spinal cord, show that the axons of tangential neurons on leaving the nucleus form an anteriorly coursing tract that passes through the ventrolateral vestibular nucleus without branching and then to the contralateral medial longitudinal fasciculus (MLF). Within the MLF, the tangential axons course posteriorly, forming collaterals that innervate the abducens nucleus, and then proceed to the cervical spinal cord. This pathway was demonstrated for the axons of the two main neurons, the principal and elongate cells, in 1-day, 1-week, and 7-week-old animals. In addition, we propose the existence of an unidentified, ipsilateral pathway to the spinal cord for the tangential axons, since HRP injections into one side of the spinal cord resulted in the bilateral labeling of tangential neurons. No labeled cells were found in the tangential nucleus following HRP depositions into the uvula, flocculus, pontine reticular formation, nucleus piriformis, nucleus jumeaux, vestibulocerebellar nucleus, retrotangential nucleus, or the dorsomedial part of the medial vestibular nucleus. The tangential nucleus receives afferents from the colossal vestibular fibers (spoon endings), small collaterals of fine vestibular ampullary fibers, flocculus, and high cervical levels of the spinal cord. From our small sample, it appears that the spinal cord fibers form most of the afferent terminals in the tangential nucleus in 1-day, 1-week, and 7-week-old animals.     

Spinal projections from the mesencephalon in the toad

Mesencephalic cell groups projecting to the spinal cord have been identified by means of the retrograde axonal transport of the enzyme horseradish peroxidase (HRP). The injections were made either in the cervical or lumbar enlargements of the toad spinal cord. Following injections in the cervical cord, labeled cells are located in the isthmus region, in the ipsilateral laminated nucleus posteroventralis tegmenti mesencephali (Potter). At more rostral levels the labeled cells are in the nucleus of the fasciculus longitudinalis medialis, in the nucleus interstitialis of the fasciculus longitudinalis medialis, in the contralateral red nucleus, in lamina six of the contralateral optic tectum, bilaterally in the nucleus of the posterior commissure and in the mesencephalic nucleus in the Vth nerve. Injections in the lumbar cord label neurons of the nucleus posteroventralis tegmenti mesencephali (Potter) and nucleus interstitialis of the fasciculus longitudinalis medialis. Nuclei that had not been previously identified in anurans but which were labeled after HRP spinal injections (i.e., the nucleus interstitialis of the fasciculus longitudinalis medialis, the nucleus of the posterior commissure and the red nucleus) have been delimited in normal material in Nissl-stained transverse sections. The spinal pathways from the mesencephalon can be classified into four projections: reticulospinal, rubrospinal, tectospinal and trigeminospinal. A comparison of these descending fiber systems with homologous pathways in other vertebrate species has been made.     

Origins of cerebellar mossy and climbing fibers immunoreactive for corticotropin-releasing factor in the rabbit

Corticotropin-releasing factor (CRF) has been implicated by both anatomical and physiological techniques as a potential cerebellar transmitter or modulator. In the present experiment, with the aid of immunohistochemistry, we have described specific cerebellar afferent pathways in the rabbit in which CRF is located. CRF-immunoreactive climbing fibers were present in the molecular layer throughout the cerebellum, but especially in lobules 8–9a. All inferior olivary neurons were CRF-immunoreactive. In lobules 8–9a, CRF-immunoreactive mossy fibers were organized in sagittal bands. The highest density of CRF-immunoreactive mossy fiber terminals was observed in the granule cell layer of lobules 8–9a and the flocculus. No CRF-immunoreactive perikarya were located in rabbit cerebellum. The brainstem origin of CRF-immunoreactive mossy fiber terminals was suggested by numerous CRF-immunoreactive perikarya located in the medial, lateral and descending vestibular nuclei, nucleus prepositus hypoglossi, nucleus x, paramedian reticular nucleus, gigantocellular reticular nucleus, lateral reticular nucleus, and raphe nuclei. Using double label experiments, we investigated the specific CRF afferent projection to the flocculus and posterior vermis. Horseradish peroxidase (HRP) injections into the posterior vermis double labeled CRF-immunoreactive neurons in the caudal medial and descending vestibular nuclei and nucleus prepositus hypoglossi. HRP injections into the flocculus double labeled more CRF-immunoreactive neurons in the nucleus prepositus hypoglossi than in the vestibular nuclei. HRP injections into either the posterior vermis or flocculus double labeled CRF-immunoreactive neurons in the paramedian reticular nucleus, nucleus reticularis gigantocellularis, and raphe nuclei. These data suggest that CRF may play an important role in vestibularly related functions of the cerebellum. © 1993 Wiley-Liss, Inc.     

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.     

Parabrachial nucleus neurons projecting to the lower brain stem and the spinal cord. A study in the cat by the Fink-Heimer and the horseradish peroxidase methods

Direct projections from the parabrachial nucleus (PBN) to the lower brain stem and the spinal cord were examined in the cat by the Fink-Heimer and the horseradish peroxidase (HRP) methods. After placing lesions in the PBN, many fine degenerated fibers were seen contralaterally in the ventromedial portions of the caudal pontine reticular formation, and ipsilaterally in the lateral portions of the facial nucleus, the regions around the hypoglossal nucleus, and the regions around the ambiguus nucleus; some degenerated fibers were traced ipsilaterally down to the spinal cord. Subsequently, HRP injections were attempted into these regions where many fine degenerated fibers were observed. In cats injected with HRP into the lateral portions of the facial nucleus, the regions around the hypoglossal nucleus, the regions around the ambiguus nucleus, or the first cervical cord segment, many HRP-labeled neurons were seen in the ventral portions of the PBN. The mean of the average soma diameters of the PBN neurons labeled with HRP injected into the regions around the hypoglossal nucleus or the first cervical cord segment was significantly larger than that of the PBN neurons labeled with HRP injected into the lateral portions of the facial nucleus or the regions around the ambiguus nucleus.