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Structural mechanisms of postlesional remodelling in the central nervous system
No full textNumerous recent experimental studies have evidenced that considerable remodelling takes place in the central nervous system following an injury. The postlesional reorganization is particularly active in the central circuits when the injury occurs during development. The basic structural postlesional mechanisms, such as axonal sprouting and synaptic rearrangement, are here briefly outlined. Experimental evidence of postlesional structural reorganization of central neurons is provided, with special reference to the remodelling that follows cerebellar lesions
Trends in the anatomical organization and functional significance of the mammalian thalamus
No full textThe last decade has witnessed major changes in the experimental approach to the study of the thalamus and to the analysis of the anatomical and functional interrelations between thalamic nuclei and cortical areas. The present review focuses on the novel anatomical approaches to thalamo-cortical connections and thalamic functions in the historical framework of the classical studies on the thalamus. In the light of the most recent data it is here discussed that: a) the thalamus can subserve different functions according to functional changes in the cortical and subcortical afferent systems; b) the multifarious thalamic cellular entities play a crucial role in the different functional states
[The organization of thalamic connections]
No full textConnections ascending to the thalamus. Contrary to classical opinion, all thalamic nuclei receive extrathalamic afferents. Segregation or convergence within a topographically defined nucleus represent two modalities of thalamic afferents. In addition, certain topographically organized thalamic afferents possess "privileged" or primary "targets" in the thalamic nucleus while others possess supplementary "targets" in other thalamic nuclei (see cerebellar, pallidal and spinothalamic projections). Ascending connections from several brain stem structures can converge on the same nucleus or diverge to several thalamic nuclei. Thalamic connections with the telencephalon. Methods for determining axonal transport have demonstrated that all thalamic nuclei, with the exception of the reticular nucleus and the ventral part of the lateral geniculate body, project towards the cerebral cortex. Four nuclear complexes can be recognized in the cat as a function of the different modalities of localization, concentration and lamination of the projections towards the cortex and the central grey nuclei. In general, the thalamocortical connections have reciprocal ipsilateral corticothalamic projections originating in the infragranular layers of the cerebral cortex. The reticular nucleus and the ventral part of the lateral geniculate body, which is not projected to the cerebral cortex, are exceptions. Each cortical area receives a "privileged" connection from a thalamic nucleus and a supplementary connection- from one or several other thalamic nuclei. The "privileged" connections usually pass to the fourth and third layers of the neocortex, and sometimes also to the first layer. In contrast, the supplementary connections pass to different superficial or deep cortical layers. Each nucleus is formed of subunits which possess different hodologic and topographic characteristics as a function of the nucleus considered. Convergence or divergence of thalamocortical and corticothalamic projections on the different thalamic nuclei, as well as the laminar distribution of efferents in the cerebral cortex, are related strictly to the hodologic organization of different cellular subunits constituting the nuclei. Concentration or diffusion of thalamic projections on cerebral cortex is related more to the single or multiple projection of cell populations belonging to a thalamic nucleus than to widespread collateralization of thalamocortical axons
Efferent fibers from the motor cortex terminate bilaterally in the thalamus of rats and cats
No full textThe anterograde transport of lectin-conjugated horseradish peroxidase (WGA-HRP) was here employed in order to visualize crossed corticothalamic efferents of the motor cortex in rats and cats. After WGA-HRP cortical injections in the rat retrogradely labeled cells were observed in the ipsilateral thalamus, and heavy anterograde labeling was observed both in the ipsi- and contralateral thalamus. The contralateral anterograde labeling was less intense than the ipsilateral one and it was distributed in the anterior intralaminar structures, in the parafascicular nucleus, in the ventromedial, ventrolateral and ventrobasal nuclei and in the posterior complex, symmetrically to the labeling observed on the ipsilateral side. Further experiments were made in the rat in order to ascertain that the bilateral anterograde labeling in the thalamus derived unilaterally from the cortex. To this purpose, kainic acid was injected unilaterally either into the frontal cortex or into the thalamus, and WGA-HRP was later injected on the same side in the frontal cortex. Moreover, WGA-HRP was injected into the frontal cortex after splitting of the corpus callosum. The results obtained in these experiments confirmed that cortical neurons projected bilaterally upon the thalamus. Further, these experiments indicated that at least the majority of the contralateral fronto-thalamic fibers crossed the midline in the thalamic massa intermedia. WGA-HRP injections into the pericruciate cortex in the cat confirmed the presence of anterogradely labeled terminals in the contralateral anterior and posterior intralaminar, ventral anterior, ventromedial and ventrolateral nuclei. The labeling was in all cases heavier in the intralaminar nuclei than in the other structures, but it was less intense than that observed in the rat.(ABSTRACT TRUNCATED AT 250 WORDS
Claustroneocortical projections studied in the cat by means of multiple retrograde fluorescent tracing
No full textThe topographical interrelations of claustroneocortical cells and their degree of divergent collateralization were investigated in cat by means of retrograde fluorescent double labeling. The tracers Fast Blue (FB) and Nuclear Yellow (NY) were injected in several combinations in two different cortical fields. FB-labeled and NY-labeled cells were found in all cases in the insular portion of the ipsilateral claustrum (CII). The distributions of labeled cells confirmed the organization reported in previous studies. In some cases the two labeled cell populations were segregated in different parts of the CII; in other cases there was more or less overlap in the two distributions. The degree of overlap was not simply related to the topographic proximity of the injected cortical territories. In all cases the vast majority of cells were single labeled. In one case both FB and NY were injected in the same cortical area in order to control that claustral axons can retrogradely transport two tracers back to the same parent cell bodies. In this control case CII cells were double labeled. All together these data indicate that widely divergent claustroneocortical projection take origin mainly from separate cells
The organization of the ipsi- and contralateral claustrocortical system in rat with notes on the bilateral claustrocortical projections in cat
No full textThe organization of the claustrocortical system was investigated in rat by means of cortical injections of either lectin-conjugated horseradish peroxidase or retrograde fluorescent tracers. The latter were also employed in cat. Evans Blue, Fast Blue, True Blue, Nuclear Yellow and Diamidino Yellow were used in different combinations and were injected, uni- or bilaterally, in different cortical fields. Cells retrogradely labeled from each cortical injection were observed in the ipsi- and contralateral claustrum. Anterogradely labeled terminals were also seen in the claustra of both sides in the horseradish peroxidase experiments. The topographic and quantitative study of the distribution of labeled neurons showed a topographic organization of the rat's claustrocortical system, although a certain degree of overlap of the cell populations projecting to frontal and occipital fields was also evident. Four types of branched claustrocortical neurons were observed in the double labeling experiments: neurons branch ing ipsilaterally (A) or contralaterally (B) to anterior and posterior cortical fields; neurons branching bilaterally to homotopic (C) or heterotopic (D) cortical fields. Each population of branched neurons was equivalent to a different percent value of the total labeled cell populations; the percent value decreased from type A to type D. Type C branched neurons were also identified in the claustrofrontal system of the cat. The intricate organization of the claustral-ascending projections suggests that the nucleus is involved in different cortical activities and that its efferents may also provide the substrate of a powerful subcortical mechanism of interhemispheric communication
GABAergic interneurons and neuropil of the intralaminar thalamus: an immunohistochemical study in the rat and the cat, with notes in the monkey
No full textImmunohistochemistry using antibodies to glutamic acid decarboxylase (GAD) was used to investigate the intralaminar nuclei of the thalamus in rat, cat and monkey. Antibodies to gamma aminobutyric acid (GABA) were also used in the cat. Intralaminar immunoreactive cell bodies were not detected in the rat, but were clearly present in cat and monkey. In the latter species, GABA- or GAD-immunopositive perikarya were distributed throughout the anterior intralaminar nuclei, whereas in the posterior intralaminar complex they prevailed in the lateral part of the centre median nucleus and around the fasciculus retroflexus. Measurements of the area of immunostained intralaminar cell bodies in cat and monkey indicated that they are represented by small neurons. Experiments in the cat, based on retrograde tracers injections involving large sectors of the frontal and parietal cortices and the head of the caudate nucleus, revealed that the GABA- or GAD-immunoreactive cells and the retrogradely labeled projection neurons represented two separate intralaminar cell populations, although the latter also included small cells. Considerable differences were observed in the immunoreactive GABAergic neuropil of the anterior and posterior intralaminar nuclei. Clusters of densely packed bouton-like immunoreactive elements were detected in the former structures in the rat, cat and monkey, and were especially evident in the central lateral nucleus; immunopositive varicose fibers and puncta were diffusely distributed in the posterior intralaminar structures. Taken together with data from the literature, the present findings indicate that in cat and monkey local circuit inhibitory cells regulate not only the activity of principal thalamic nuclei which project densely upon restricted cortical fields, but also of the intralaminar structures which are widely connected with the cerebral cortex and the striatum. Regional variations in the distribution of GABAergic fibers and terminals suggest major differences in the organization of inhibitory circuits and synaptic arrangements of the anterior and posterior intralaminar thalamus
Increased collateralization of the cerebellothalamic pathway following neonatal hemicerebellectomy
No full textThe postlesional reorganization of the cerebellothalamic cells was here studied using a multiple retrograde tracing technique. To this purpose, the cerebellothalamic cell population was investigated in 3 groups of adult rats: (a) cases hemicerebellectomized at birth; (b) cases hemicerebellectomized in adulthood; and (c) control unlesioned ones. In all of the groups the fluorescent tracers Diamidino Yellow and Fast Blue were injected bilaterally in the thalamus and the retrograde labeling obtained in the cerebellar nuclei was analyzed quantitatively. In the adult unlesioned rats the cerebellar cells projecting to the ipsilateral thalamus represented 3-4% of the cell population projecting to the contralateral thalamus. Furthermore, in agreement with previous results, it was demonstrated that the ipsilateral component was almost totally formed by axon collaterals of the main contralateral cerebellothalamic pathway. The organization of the bilateral cerebellothalamic pathway was unaffected by a hemicerebellec tomy performed in adulthood. However, when the hemicerebellectomy had been performed at birth, the number of cerebellar cells that project to the ipsilateral thalamus was 3 times higher than in the controls. In these lesioned cases, as in the other ones, the ipsilateral projecting cell population was mainly represented by branched cerebellar cells which project bilaterally upon the thalamus. These results indicate that the bilaterality of the cerebellothalamic pathway is enhanced after an early hemicerebellectomy, and that this phenomenon is responsible for the reinnervation of the deafferented thalamus. Further, the present study shows that the increase in the bilaterality of the system is sustained by cerebellar cells which bifurcate bilaterally upon the thalamus, and therefore that the reinnervation of the deafferented thalamus is performed by axon collaterals of the contralateral spared cerebellothalamic pathway
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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