The organization of the efferent projections of the thalamic intralaminar nuclei: past, present and future of the anatomical approach

Neuronal plasticity of interrelated cerebellar and cortical networks

The comprehension of the cerebellar physiology is rapidly changing in particular because of the demonstration of the cerebellar importance on cognition. In the present paper, recent data on cerebro cerebellar interactions is reviewed, particularly focusing on cerebellar influences over the neurophysiology of primary motor and primary sensory cortices. The cerebellar role in implicit learning and in sensory data processing is analysed and discussed. It is proposed that the cerebellum could control cortical plastic changes by modulating cortical excitability in a discrete topographic manner and that this mechanism could induce the coupling between significant sensory inputs and definite motor outputs considered as the neurobiological substrate for implicit learning.

Parabrachial nucleus projections to midline and intralaminar thalamic nuclei of the rat

The projections from the parabrachial nucleus to the midline and intralaminar thalamic nuclei were examined in the rat. Stereotaxic injections of the retrograde tracer cholera toxin-beta (CTb) were made in each of the intralaminar nuclei of the dorsal thalamus (the lateral parafascicular, medial parafascicular, oval paracentral, central lateral, paracentral, and central medial nuclei), as well as the midline thalamic nuclei (the paraventricular, intermediodorsal, mediodorsal, paratenial, rhomboid, reuniens, parvicellular part of the ventral posterior, and caudal ventral medial nuclei). The retrograde cell body labeling pattern within the parabrachial subnuclei was then analyzed. The paracentral thalamic nucleus received an input only from the internal lateral parabrachial subnucleus. However, this subnucleus also projected to all the other intralaminar thalamic nuclei, except for the central lateral thalamic nucleus, which received no parabrachial afferent inputs. The external lateral parabrachial subnucleus projected to the lateral parafascicular, reuniens, central medial, parvicellular part of the ventral posterior, and caudal ventromedial thalamic nuclei. Following CTb injections in the paraventricular thalamic nucleus, retrogradely labeled cells were found in the central lateral, dorsal lateral, and external lateral parabrachial subnuclei. The medial and ventral lateral parabrachial subnuclei projected to the oval paracentral, parafascicular, and rhomboid thalamic nuclei. Finally, the waist area of the parabrachial nucleus was densely labeled after CTb injections in the parvicellular part of the ventral posterior thalamic nucleus. Nociceptive, visceral, and gustatory signals may reach specific cortical and other forebrain sites via this parabrachial-thalamic pathway.

Vascular dementia of Binswanger's type: clinical, neuroradiological and 99mTc-HMPAO SPET study

In 24 patients with vascular dementia of Binswanger's type (VDBT) and 14 age-matched neurologically normal volunteers, we investigated the relationship between clinical features, white matter lesions (leuco-araiosis) and cerebral atrophy on computed tomographic (CT) scan, and regional cerebral blood flow. All subjects underwent the Mini-Mental State Examination of Taiwan, version 1 (MMSE-T1), for assessing the severity of cognitive impairment. The patients were subdivided into two groups, one with mild to moderate (group I, MMSE-T1 scores: 11-24, n=11), and the other with severe dementia (group II, MMSE-T1 scores: below 10, n=13). White matter degeneration was evaluated with densitometric methods. Loss of brain parenchyma was estimated with seven linear measurements (Evan's ratio, third ventricle ratio, width of temporal horn tip, anterior-posterior length of temporal horn, anterior-posterior length of Sylvian fissure and width of frontal interhemispheric fissure) by CT scans. Regional cerebral blood flow was determined with technetium-99m hexamethylpropylene amine oxime (HMPAO) single-photon emission tomography (SPET). In neuroimaging studies, subcortical leuco-araiosis was localized at the frontal region in group I patients and scattered diffusely in group II patients. 99mTc-HMPAO SPET analysis revealed reduction of regional cerebral blood flow in the frontal lobe in group I patients and widespread reduction of regional cerebral blood flow in group II patients. A correlation between frontal leuco-araiosis and perfusion defect of the frontal pole was demonstrated in group I patients, showing findings typical of subcortical dementia. There was no difference in frontal atrophic measurements between group I patients and controls. Ratios of volumes of lost brain parenchyma and leuco-araiosis were significantly higher in group II patients than in the age-matched controls, corresponding to a diffuse cerebral perfusion defect. These results suggest that patients with VDBT have early frontal lobe involvement with posterior progression. Patients with mild VDBT are more likely to show reduction of frontal cerebral blood flow and leuco-araiosis, while those with severe VDBT are more likely to have diffuse leuco-araiosis, cerebral hypoperfusion and brain atrophy.

Topographical mapping of the thalamocortical projections in rodents and comparison with that in primates

The general topographical organization of the thalamo-cortical projection of two rodents, the Siberian hamster (Phodopus sungorus) and the Guinea pig (Cavia aperta) was investigated with the HRP-method and compared with that of the new world primate marmoset (Cal-lithrix jacchus) as shown in a companion study by Brysch et al. (1990). HRP was injected into various regions of the cortex in different animals and hemispheres, and plots were made of the retrogradely stained thalamic projection neurons. The thalamocortical projection is virtually identical in both rodent species. It is topological throughout in that nearby cortical injections label nearby, though overlapping cell groups in the thalamus. Cortical injections in a rostro-caudal progression labelled thalamic projection zones on top of each other, layered like tiles on a roof or fish scales, beginning in the rostromedial and ending in the caudo-dorsal thalamus. The progression vector of thalamic zones projecting successively from more rostral to more caudal cortical zones is twisted and turns from a predominantly mediolateral direction in the anterior thalamus to an essentially ventro-dorsal direction in the posterior thalamus In the marmoset, the thalamo-cortical topography follows the same topological rule, with the exception of the lateral geniculate body which is translocated latero-ventrally and separated from the rest of the thalamus as in all primates. This suggests a general thalamo-cortical mapping rule common to all mammals which can be related to gradients and timing of cell birth in the thalamus. It is proposed that this mapping rule is the consequence of successive appositions of neurons in the medio-ventral thalamus during ontogenetic development.

Restricted cortical termination fields of the midline and intralaminar thalamic nuclei in the rat

The projections from the midline and intralaminar thalamic nuclei to the cerebral cortex were studied in the rat by means of anterograde tracing with Phaseolus vulgaris-leucoagglutinin. The midline and intralaminar nuclear complex taken as a whole projects to widespread, predominantly frontal, cortical areas. Each of the constituent thalamic nuclei has a restricted cortical projection field that overlaps only slightly with the projection fields of adjacent midline and intralaminar nuclei. The projections of the intralaminar nuclei cover a larger cortical area than those of the midline nuclei. The laminar distributions of fibres from individual midline and intralaminar thalamic nuclei are different and include both deep and superficial cortical layers. The parataenial, paraventricular and intermediodorsal midline nuclei each project to circumscribed parts of the prefrontal cortex and the hippocampal and parahippocampal regions. In the prefrontal cortex, the projections are restricted to the medial orbital, infralimbic, ventral prelimbic and agranular insular fields, and the rostral part of the ventral anterior cingular cortex. In contrast to the other midline nuclei, the rhomboid nucleus projects to widespread cortical areas. The rostral intralaminar nuclei innervate dorsal parts of the prefrontal cortex, i.e. the dorsal parts of the prelimbic, anterior cingular and dorsal agranular insular cortical fields, the lateral and ventrolateral orbital areas, and the caudal part of the ventral anterior cingular cortex. Additional projections are aimed at the agranular fields of the motor cortex and the caudal part of the parietal cortex. The lateral part of the parafascicular nucleus sends fibres predominantly to the lateral agranular field of the motor cortex and the rostral part of the parietal cortex. The medial part of the parafascicular nucleus projects rather sparsely to the dorsal part of the prelimbic cortex, the anterior cingular cortex and the medial agranular field of the motor cortex. Individual midline and intralaminar thalamic nuclei are thus in a position to directly influence circumscribed areas of the cerebral cortex. In combination with previously reported data on the organization of the midline and intralaminar thalamostriatal projections and the prefrontal corticostriatal projections the present results suggest a high degree of differentiation in the convergence of thalamic and cortical afferent fibres in the striatum. Each of the recently described parallel basal ganglia-thalamocortical circuits can thus be expanded to include projections at both the cortical and striatal levels from a specific part of the midline and intralaminar nuclear complex. The distinctive laminar distributions of the fibres originating from the different nuclei emphasize the specificity of the midline and intralaminar thalamocortical projections.

Segregation and heterogeneity of thalamic cell populations projecting to superficial layers of posterior parietal cortex: a retrograde tracer study in cat and monkey

The thalamic neurons projecting to the superficial layers of areas 5 and 7 in the cat, and area 5 in the monkey, were investigated by using superficial deposits of either horseradish peroxidase or Fast Blue in one hemisphere. In the contralateral hemisphere injections of the same tracer involving the full cortical depth were made in homotopical locations, and the distribution and soma size of retrogradely labeled thalamocortical neurons in each side of the thalamus were compared. It was found that, in the cat, labeled neurons in the lateral posterior pulvinar complex, and in paralaminar regions of the ventrolateral complex, were fewer in number and smaller in size in cases of superficial deposits than in cases of deep injection. In more lateral portions of the ventrolateral complex, however, there were no size differences. In the monkey, similar differences in number and size appeared in the caudal division of the ventrolateral complex and in the lateral posterior and pulvinar nuclei, whereas no such differences were found for neurons labeled in the oral and medial divisions of the ventrolateral complex, and in the ventral posteroinferior nucleus. In all cases the intralaminar and midline nuclei exhibited retrogradely labeled neurons only when deep layers were injected. These and previous findings point to the existence of a widely distributed layer I-projecting system of neurons which, in most nuclei, are interspersed among neurons projecting mainly to middle or deep layers. In some nuclei, however, as is the case with the ventromedial nucleus proper, layer I-projecting system neurons would make up the whole nucleus. The cell groups located in a paralaminar position, which would be but a part of this system, could provide through their projections to layer I in the posterior parietal and frontal cortical regions a final path for recruiting responses and spontaneous spindling activities.

Topographical organization of the thalamic afferent connections to the motor cortex in the cat

The topographical distribution of the cortical afferent connections to the different subdivisions of the motor cortex (MC) was studied in adult cats. The retrograde axonal transport of horseradish peroxidase technique was used. Small single injections of the enzyme were made in the entire MC, including the hidden regions in the depth of the sulcus cruciatus. The areal location and density of the subsequent thalamic neuronal labeling were evaluated in each case. Comparison of the results obtained in the various cases shows that the following: (1) The ventral anterior-ventral lateral complex is the principal thalamic source of afferents to the MC. (2) The ventral medial, dorsal medial, the different components of the posterior thalamic group (lateral, medial, and ventral posteroinferior and suprageniculate nuclei), and the intralaminar, lateral anterior, lateral intermediate, lateral medial, and anteromedial thalamic nuclei are also thalamic sites in which neural projections to the MC arise. (3) The thalamocortical projections to the MC are sequentially organized. The connections arising from the lateral part of the thalamus end in the region of area 4 that is situated medially in the superior lip of the sulcus cruciatus and in the posterior sigmoid gyrus. The projections originating in the most medial thalamic regions terminate in that region of area 6a beta which is located in the medial part of the inferior lip of the cruciate sulcus, and in the anterior sigmoid gyrus. Moreover, the ventral thalamic areas send connections to the most anteriorly located zones of the MC, while the most dorsal thalamic ones project to the most posteriorly located parts of the MC. (4) This shift in the thalamocortical connections is not restrained by cytoarchitectonic boundaries, either in the thalamus or in the cortex. (5) The populations of thalamocortical cells which project to neighboring MC subdivisions exhibit consistent overlapping among themselves. (6) These findings suggest, moreover, that the basal ganglia and the cerebellar projections to the MC through the thalamus are arranged in a number of parallel pathways, which may occasionally overlap.

Spinal afferents and cortical efferents of the anterior intralaminar nuclei: an anterograde-retrograde tracing study

The topographical relations among the terminal field of spinothalamic fibers and the cells projecting upon areas 4 and 5 were studied in the anterior intralaminar nuclei of the cat. Terminals anterogradely labeled from the spinal cord and cell populations retrogradely labeled from the lateral pericruciate and anterior suprasylvian cortex were simultaneously observed by means of a multiple fluorescent tracing strategy. The present findings confirm that spinal afferents in the central lateral and paracentral nuclei overlap with the cells projecting to area 4. Further, the present data demonstrate that spinal terminals are largely segregated from the intralaminar cell population projecting to area 5.

Calbindin immunoreactivity alternates with cytochrome c-oxidase-rich zones in some layers of the primate visual cortex

Calcium ions have a pivotal role in many neuronal activities, but little is known about their involvement in the cortical processing of visual information. Using immunohistochemical methods, we have now detected a calcium-binding protein, calbindin-D-28K, which may confer on certain compartments of cortical area 17 the ability to modulate Ca2+ metabolism. Thus, calbindin occurs in the primate striate cortex in a pattern almost complementary to that displaying strong cytochrome c-oxidase activity. From this and other observations, we deduce that the distribution of calbindin-immunoreactive sites corresponds mainly to extra-geniculocortical connections of the primary visual cortex. This implies that the geniculocortical and extra-geniculocortical compartments of area 17 differ in an intracellular system for Ca2+ homeostasis.

Multiple cortical targets of one thalamic nucleus: the projections of the ventral medial nucleus in the cat studied with retrograde tracers

The organization of the cortical projections of the ventral medial thalamic nucleus (VM) was studied in the cat with retrograde tracers. The extent of the VM-cortical projections was first investigated with horseradish peroxidase injected in different cortical fields. The results obtained in the experiments indicated that the main target of VM efferents is represented by a large territory anterior to the cruciate sulcus involving area 6 and the gyrus proreus and extending into the anterior part of the medial cortical surface. The afferents to these precruciate fields arise from throughout the VM. In addition, the lateral third of VM projects upon the lateral precruciate cortex that is coextensive with the precruciate part of area 4, whereas VM efferents do not extend into the posterior sigmoid gyrus. A second major target of VM efferents is represented by the insular cortex in the anterior sylvian gyrus. VM projections also reach the prepyriform cortex and the cingulate gyrus. An anteroposterior decrease of density was found in the VM-cingulate projections. Sparse VM projections reach the temporal cortex, the adjacent posterior sylvian and ectosylvian fields, and the anterior ectosylvian gyrus. No VM projections were found either upon the visual areas 17 and 18 or upon the primary auditory cortex. The interrelations between some VM-cortical cell populations and their divergent collateralization were studied by using double retrograde labeling with fluorescent tracers. The results of these experiments demonstrated that a relatively high number (at least 20%) of VM cells projecting to the insula are also connected to the precruciate fields by means of axon collaterals. This finding indicates that VM is a highly collateralized structure of the cat's thalamus. Very few branched cells were found in the other combinations of cortical fields here examined (precruciate vs. posterior sylvian fields, lateral precruciate vs. proreal cortex, anterior vs. posterior cingulate fields). Altogether these data indicate that VM branched cells preferentially interconnect the two main cortical targets of the nucleus, i.e., precruciate and insular fields. The results of the present study are discussed in regard to the literature on the VM projections in the rat and the previously available data in the cat, to the afferent VM organization in the cat, to the relationships between VM and the nucleus submedius, and to the anatomical and functional role of VM in relation to the so-called "nonspecific" thalamocortical system.

Organization of thalamic projections in the nucleus accumbens and the caudate nucleus in cats and its relation with hippocampal and other subcortical afferents

The organization of thalamic projections in the nucleus accumbens (NA) and the caudate nucleus of cats and its relation to other subcortical striatal afferents were studied with a retrograde tracing technique by use of lectin-conjugated horseradish peroxidase. The study showed that the paraventricular and medial parafascicular nuclei (PF) of the thalamus project to the medial NA and the parataenial and medial PF project to the lateral NA. The ventral tegmental area and substantia nigra pars dorsalis (SNpd) project to medial and lateral NA. The midline thalamic nuclei, rostral intralaminar nuclei, ventroanterior nucleus, medial and lateral PF, lateral posterior complex, and nucleus limitans project to medial caudate nucleus. The most medial substantia nigra pars compacta (SNpc) and rostral SNpd project to medial caudate nucleus. The center median, ventrolateral, and the central lateral nuclei of thalamus, SNpc, and SNpd project to lateral caudate nucleus. These results suggest that the thalamic and subcortical nuclei known to connect with the limbic and frontal cortices project to NA and medial caudate nucleus. Those thalamic nuclei connected with the motor system project to lateral caudate nucleus. The hippocampus projects selectively to medial NA. The amygdala, raphe, and other mesencephalic nuclei project only to NA and medial caudate nucleus. The organization of hippocampal, amygdala, and other subcortical afferents suggests that NA and caudate nucleus can be separated into medial "limbic" and lateral nonlimbic "sensory-motor" compartments. A brief review of the distribution pattern of some neurotransmitters, neuropeptides, and their receptors and behavior studies provides additional support to the concept that the striatum can be divided into several subcompartments.

The second, intralaminar thalamo-cortical projection system

In the marmoset (Callithrix jacchus), HRP and 3H-apo-HRP were injected into various cortical regions and the positions of labelled neurons in the non-specific, intralaminar thalamic nuclei (N. centralis and centre m edian ) were investigated. Although neuron populations projecting to the different cortical regions overlap widely, a coarse topology exists inasmuch as intralaminar neurons projecting to the posterior cortex were located more rostrally and those projecting to the anterior cortex were located more caudally in the intralaminar complex. With injections into nearby cortical regions of the parieto-temporal association cortex with HRP and 3H-apo-HRP, respectively, no double labelled cells were found in the intralaminar nuclei, although the fields of labelled cells completely overlapped. Also in the specific projection nuclei no double labelled cells were encountered. About 10-20% of the thalamo-cortical projection cells are located in the intralaminar nuclei. Some functional aspects of this second thalamo-cortical projection system are discussed.