Functional localization and cortical architecture in the nine-banded armadilli (Dasypus novemcinctus mexicanus).

A functional map of the armadillo neocortex was produced by cortical stimulation and recording evoked potentials following somatic, auditory and visual stimuli. The results obtained were then correlated with the cortical architecture as revealed by Nissl, Golgi and myelin-stained sections. Cortex rostral to the supraorbital sulcus has a wide layer IV and is mostly silent, except for a motor eye field and a part of the tongue sensory region in its caudal part. Two types of motor-sensory cortex are present caudal to the supraorbital sulcus. Postsupraorbital I is mostly motor and has prominent pyramidal layers. Layer V is particularly well developed and in rostral sections its superficial zone is broken up into clusters similar to the solid "barrels" seen in layer IV of other species. Postsupraorbital II has less prominent pyramidal layers and layers II and III are organized into clusters. This region corresponds to the sensory area for the limbs and trunk and the partially overlapping (surface recordings) sensory and motor areas for head, snout and tongue. Digits and limbs are rostral to the trunk representation in both the sensory and motor "homunculi." Even though surface recording was employed, potentials evoked by visual stimuli could only be recorded from a small caudal area with a very thin layer IV. Although striate and peristriate areas appear similar in Nissl stained preparations, they can be readily differentiated in Weil stained sections. The stellate character of neurons in layer IV of the visual cortex is particularly apparent in Golgi material. Auditory evoked surface potentials were recorded from a broad oval region in the caudal lateral cortex which has a wide layer IV and aggregates of neurons in layers II and III. A Weil stain demonstrates inner and outer bands of Baillarger in this same region. The presumptive insular cortex is electrically silent to sensory stimulation and presents as a narrow band just dorsal to the rhinal fissure with indefinite cell lamination and little myelin.     

The projections of the midbrain periaqueductal grey to the pons and medulla oblongata in rats

It is now established that stimulation of the ventrolateral midbrain periaqueductal grey (PAG) evokes inhibition of nociceptive spinal neurons, which results in analgesia and a powerful attenuation of pain behaviour. It is postulated that the PAG exerts this inhibitory effect on spinal nociceptive functions through the activation of descending serotonergic and noradrenergic pathways that arise from the rostral ventromedial medulla (RVM) and pontine noradrenergic nuclei. To investigate the neuroanatomical substrate of this functional link between the PAG and RVM, as well as the pontine noradrenergic nuclei in the rat, we labelled axons that project from the ventrolateral PAG to various regions of the pons and medulla oblongata using the anterograde tracing substance, Phaseolus vulgaris leucoagglutinin. We demonstrated that some of PAG efferents really do terminate in the RVM and pontine noradrenergic nuclei, but a substantial proportion of them project to the intermediate subdivision of the pontobulbar reticular formation. Combining the axonal tracing with serotonin- and tyrosine-hydroxylase-immunohistochemistry, we also found that, in contrast to previous results, PAG efferents make relatively few appositions with serotonin- and tyrosine-hydroxylase-immunoreactive neurons in the RVM and pontine noradrenergic nuclei; most of them terminate in nonimmunoreactive territories. The results suggest that the ventrolateral PAG may activate a complex pontobulbar neuronal assembly including neurons in the intermediate subdivision of the pontobulbar reticular formation, serotonin- and tyrosine-hydroxylase-immunoreactive and nonimmunoreactive neurons in the RVM and pontine noradrenergic nuclei. This pontobulbar neural circuitry, then, may mediate the PAG-evoked activities towards the spinal dorsal horn resulting in the inhibition of spinal nociceptive functions.     

Corticobulbar projections of the marsupial phalanger (Trichosurus vulpecula). I. Projections to the pons and medulla oblongata

A total of 27 adult phalangers was employed to investigate the pattern of neocortical projections to the pontine and medullary portions of the brain stem. Lesions restricted to neocortical areas rostral to the orbital sulcus resulted in fiber degeneration which distributed mainly to midline and medial areas of the pontine and medullary reticular formation. The greatest amount of fiber degeneration was located within the superior central nucleus, the nucleus of the pontine raphe, the nucleus pontis centralis oralis and the nucleus pontis centralis caudalis. However, a few degenerating fibers were present within the nucleus gigantocellularis and the magnocellular portion of the medullary raphe. In contrast, lesions which were located just caudal to the orbital sulcus resulted in fiber degeneration chiefly within the more lateral parvocellular reticular formation and within the subnucleus dorsalis of the nucleus medullae oblongatae centralis. In such cases, additional degenerating fibers were present within the dorsal column nuclei and within more medial areas of the reticular formation. In those brains with ventral parietal ablations, degenerating fibers were present within the chief sensory and spinal nuclei of the trigeminal complex and the closely adjacent reticular formation. All of the above neocortical lesions resulted in fiber degeneration within the basilar pontine gray. In those specimens subjected to caudal (striate and peristriate) or ventrocaudal (temporal) lesions, degenerating fibers were present within the basilar pontine gray, but not within other areas of the pons or the medulla oblongata.     

Neurons in the area postrema are the only catecholamine-synthesizing cells in the medulla or pons with projections to the rostral ventrolateral medulla (C1-area) in the rabbit

We have identified, in the rabbit medulla and pons, neurons which project to the C1-region of the rostral ventrolateral medulla. By combining tyrosine hydroxylase immunohistochemistry with retrograde transport of Fluoro-Gold we determined whether any of the retrogradely labelled neurons synthesize catecholamines. The only doubly labelled cells were located in the area postrema. No other group of catecholamine-synthesizing neurons in either the medulla or the pons was found to project to the C1-area of the rostral ventrolateral medulla. Pharmacological agents which lower arterial pressure by stimulating adrenoceptors in the rostral ventrolateral medulla may act on receptors which are not innervated by catecholamine-synthesizing perikarya located outside the C1-region.     

Distribution of hypocretin (orexin) immunoreactivity in the feline pons and medulla

The distribution of hypocretin-1 (hcrt-1) and hypocretin-2 (hcrt-2) immunoreactivities in the cat brainstem was examined using immunohistochemical techniques. Hcrt-1- and hcrt-2-positive fibers with varicosities were detected in almost all brainstem regions. However, no hcrt-1- or hcrt-2-immunoreactive neuronal somata were observed in the cat brainstem. Both hcrt-1- and hcrt-2-labeled fibers exhibited different densities in distinct regions of the brainstem. In most brainstem regions, the intensity of hcrt-1 immunoreactivity was higher than that of hcrt-2 immunoreactivity. The highest densities of hcrt-1- and hcrt-2-positive fibers were found in the nucleus raphe dorsalis (RD), the laterodorsal tegmental nucleus (LDT) and the locus coeruleus (LC), suggesting an important role for these peptides in functions related to sleep-wake behavior.     

Dorsal mesencephalic projections to pons, medulla, and spinal cord in the cat: limbic and non-limbic components.

The vertebrate dorsal mesencephalon consists of the superior colliculus, the dorsal portion of the periaqueductal gray, and the mesencephalic trigeminal neurons in between. These structures, via their descending pathways, take part in various behavioral responses to environmental stimuli. This study was undertaken to compare the origins and trajectories of these pathways in the cat. Injections of horseradish peroxidase into the cervical spinal cord and upper medullary medial tegmentum retrogradely labeled cells mainly in the contralateral intermediate and deep superior colliculus, and in the ipsilateral dorsal and lateral periaqueductal gray and adjacent tegmentum. Only injections in the medullary lateral tegmental field labeled mesencephalic trigeminal neurons ipsilaterally. Autoradiographic tracing results, based on injections across the dorsal mesencephalon, revealed three efferent fiberstreams. A massive first fiberstream (limbic pathway), consisting of thin fibers, descended ipsilaterally from the dorsal and lateral periaqueductal gray and adjacent superior colliculus through the mesencephalic and pontine lateral tegmentum, terminating in these areas as well as in the ventral third of the caudal pontine and medullary medial tegmentum. A few fibers from the dorsal periaqueductal gray matter (PAG) were distributed bilaterally to the dorsal vagal, solitary, and retroambiguus nuclei. The second fiberstream (the predorsal bundle) descended contralaterally from the superior colliculus (SC) and consisted of both thick and thin labeled fibers. The thin fibers terminated bilaterally in the dorsomedial nucleus reticularis tegmenti pontis and the medial half of the caudal medial accessory inferior olive. The thick fibers targeted the contralateral dorsal two thirds of the caudal pontine and medullary medial tegmental fields, and the facial, abducens, lateral reticular, subtrigeminal, and prepositus hypoglossi nuclei. A few fibers recrossed the midline to terminate in the ipsilateral medial tegmentum. Caudal to the obex, fibers terminated laterally in the tegmentum and upper cervical intermediate zone. From the lateral SC, fibers terminated bilaterally in the lateral tegmental fields of the pons and medulla and lateral facial subnuclei. The third fiberstream (mesencephalic trigeminal or Probst tract) terminated in the supratrigeminal and motor trigeminal nuclei, and laterally in the tegmentum and upper cervical intermediate zone. In summary, neurons in the PAG and in the deep layers of the SC give rise to a massive ipsilateral descending pathway, in which a medial-to-lateral organization exists. A similar topographical pattern occurs in the crossed SC projections. The possibility that these completely different descending systems cooperate in producing specific defensive behaviors is discussed.     

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.     

Unilateral asterixis due to ipsilateral lesions in the pons and medulla

Unilateral asterixis was ipsilateral to the brainstem lesions in 2 patients. The finding of unilateral asterixis has been consistently associated with structural neurological disease, but contrary to previous reports it is not a reliable localizing or lateralizing finding.     

Cortical projections to the paramedian tegmental and basilar pons in the monkey

The efferent connections of the cerebral cortex to paramedial tegmental and basilar pons were studied in the monkey by using the retrograde and orthograde capabilities of the horseradish peroxidase (HRP) technique. Six capuchin monkeys (Cebus apella) received transcannular pontine HRP gel implants to retrogradely label the cells of origin of corticopontine projections. Four additional capuchin monkeys, one rhesus (Macaca mulatta), and one cynomolgus (Macaca fascicularis) monkey, received HRP gel implants in premotor (area 6), frontal eye field (FEF, area 8), superior (area 5), and inferior (area 7) parietal lobules to orthogradely label the course and termination of corticopontine projections, and thus to confirm the retrograde studies. The brains were processed according to the tetramethylbenzidine (TMB) protocol of Mesulam ('78) and studied with darkfield microscopy. Premotor (area 6) frontal cortex and FEF (area 8) were found to be the main sources of cortical inputs to the ipsilateral paramedian basilar pons, whereas FEF, dorsal prefrontal convexity, and dorsal medial prefrontal (granular frontal association) cortex were the main sources of bilateral projections to the paramedian pontine tegmentum. The medial portion of the nucleus reticularis tegmenti pontis (NRTP), considered to be a tegmental extension of the basilar pontine gray, also received its principal cortical input from the frontal lobe. Parietal cortex, on the other hand, was observed to project to lateral NRTP and lateral basilar pons. Although the possibility exists of convergence of frontal and parietal eye field efferents in the NRTP, the frontal eye field and prefrontal cortex appear to be the principal source of cortical projections to the paramedian pontine tegmentum, which contains the physiologically defined PPRF (paramedian pontine reticular formation), an important preoculomotor center. The results are discussed primarily with regard to their significance for potential cortical influence on the oculomotor system.     

Hypothalamic projections to cardiovascular centers of the medulla

The purpose of this project was to identify hypothalamic neurons having projections to two cardiovascular centers of the medulla, the rostral ventrolateral medulla (RVLM; a vasopressor region) and the nucleus of the solitary tract (NTS; a vasodepressor region). To accomplish this, fluorescent tracers (fast blue and diamidino yellow) were injected into NTS and RVLM, after each site had been physiologically identified in rats. In each case, one of the tracers was injected into the RVLM and another was injected into the NTS. Labelled neurons were subsequently observed along the entire rostral-caudal extent of the hypothalamus, where they were found in nuclei having known cardiovascular functions. Although the two groups of hypothalamomedullary neurons were largely overlapped in their distributions, less than 0.1% of the neurons were double labelled. In addition to this overlapping distribution of neurons, there were some areas within the hypothalamus where the two groups of hypothalamomedullary neurons were somewhat segregated. This clustering pattern was observed in the posterolateral hypothalamus (PLH) and, to a much lesser degree, in the paraventricular nucleus (PVN). Within the PLH, lying medial to the subthalamic nucleus, virtually all the labelled neurons projected exclusively to the NTS. Within the PVN, neurons projecting to the NTS were more numerous ventrally, whereas neurons projecting to the RVLM were more evenly dispersed within the PVN. In addition to hypothalamic labeling, clusters of labelled neurons were also observed in the zona incerta and the interstitial nucleus of the stria terminalis. Within the zona incerta, almost all the labelled neurons projected to the RVLM. Within the interstitial nucleus of the stria terminalis, neurons projecting to NTS were much more abundant in the dorsal portion of this nucleus; whereas, neurons projecting to the RVLM were more abundant ventrally. The findings of this study provide additional support to the notion that hypothalamic influences upon cardiovascular functions are in part mediated through hypothalamomedullary projections.     

Locomotor projections from the pedunculopontine nucleus to the medioventral medulla

Retrograde tracers were injected into the rat medioventral medulla (MED) and the injection site was identified as a locomotion- inducing area by electrical stimulation in the decerebrate preparation. Histological reconstructions showed that about 10% of cholinergic pedunculopontine (PPN) and laterodorsal tegmental (LDT) neurons project to the MED. Also, large numbers of non-cholinergic cells in and around the PPN and LDT were found to project to the MED.     

Serotonergic projections to the rostroventrolateral medulla from midbrain and raphe nuclei

Double-label fluoresence immunohistochemistry was performed to define serotonergic projections from the raphe and midbrain to the sympathoexcitatory region of the rostroventrolateral medulla (RVLM). Immunolabelling of cholera toxin B subunit retrogradely transported from the pressor region of the RVLM was combined with serotonin (5-HT) immunohistochemistry. Major sources of serotonergic input to the RVLM were shown to include the raphe obscurus, raphe pallidus and raphe magnus with a minor contribution from the ventrolateral, lateral and ventral regions of the periaqueductal gray matter, and the dorsal raphe nucleus. Serotonergic modulation of sympathoexcitatory neurons may establish patterns of sympathetic nerve activity evident in many aspects of cardiovascular regulation.     

Vestibulofugal projections to the lateral reticular nucleus of the cat medulla

Certain amount of neurons in vestibular nuclei were labelled after horseradish peroxidase injection in the medullar lateral reticular nucleus of cat. Such projections were strictly homolateral and formed by small and medium-size neurons localized mainly in the Deiters nucleus; their number was, probably, much less in comparison with the number of vestibulo-spinal units. Functional role of vestibular projections to the lateral reticular nucleus in the motor activity control is discussed.