Ventroposterior Inferior Nucleus Research Articles

Cortical and thalamic connections of the parietal ventral somatosensory area in marmoset monkeys (Callithrix jacchus).

Microelectrode mapping methods were used to define the parietal ventral somatosensory area (PV) on the upper bank of the lateral sulcus in five marmosets (Callithrix jacchus). In the same animals, neuroanatomical tracers were placed into electrophysiologically identified sites in PV and/or the second somatosensory area (S2). Foci of anterograde and retrograde label were related to electrophysiological maps of cortical areas and cortical and thalamic architecture. The results lead to the following conclusions: (1) Multiunit recordings from cortex on the upper bank of the lateral sulcus demonstrate that PV is somatotopically organized, with the face representation adjoining area 3b and the hindlimb and tail representations away from this border in cortex deep on the upper bank of the lateral sulcus. The forelimb representation is caudal in PV adjacent to the S2 forelimb representation. The body surface representation in PV approximates a mirror image of that in S2; (2) Areas PV and S2 are less myelinated and have less cytochrome oxidase enzyme activity than area 3b; (3) The ventroposterior inferior nucleus (VPI) of the thalamus provides the major somatosensory projections to PV. PV is reciprocally connected with VPI and anterior pulvinar; (4) PV has ipsilateral cortical connections with areas 3a, 3b, 1, and M1 and higher order somatosensory fields, and at least most of these connections are somatotopically matched; and (5) Callosal connections of PV are with S2 and PV of the other cerebral hemisphere. These results further establish PV as one of at least four somatosensory areas of the lateral sulcus of primates.

Distribution of neurons immunoreactive for parvalbumin and calbindin in the somatosensory thalamus of the raccoon

The aim of this study was to assess the distribution of neurons immunoreactive for parvalbumin (PV), calbindin (CaBP), glutamic acid decarboxylase (GAD), and gamma-aminobutyric acid (GABA) in the somatosensory thalamus of the raccoon and to compare these features to those of other species, especially primates. Immunohistochemistry was used to study the location of these neurons in the ventroposterior nucleus (VP), ventroposterior inferior nucleus (VPI), posterior group of nuclei (Po), and reticular nucleus (Rt). A consistent differential pattern of PV-positive (PV+) and CaBP-positive (CaBP+) cells was found in the somatosensory thalamus. Many PV+ neurons were observed in VP and Rt, but very few were found in VPI or Po. In contrast, CaBP+ neurons were distributed throughout VP, VPI, and Po but were very sparse or absent in Rt. In the VP nucleus, PV+ cells were distributed in clusters separated by interclusteral regions with a sparse distribution of PV+ cell bodies. The distributions of PV+ and CaBP+ cells tended to be complementary to each other in VP; regions with a high density of PV+ neurons had a low density of CaBP+ cell bodies. Double-labeling experiments revealed very few neurons in which PV and CaBP immunoreactivities were colocalized. Cells immunoreactive for GAD or GABA were found in PV-dense clusters of VP; fewer GABAergic neurons were present in the CaBP-dense interclusteral regions of VP and in VPI and Po. GAD+ and GABA+ neurons were most prominently distributed in Rt. We conclude that the distributions of PV+ and CaBP+ cell bodies in the raccoon somatosensory thalamus are very similar to those in primates. The density of GABAergic neurons in the somatosensory thalamus of the raccoon is less than that in the cat and monkey, but the relative distribution of GABAergic neurons in the different somatosensory nuclei is very similar to that in the cat and monkey. These results are discussed in relation to findings in other species and are related to the functions of lemniscal and nonlemniscal somatosensory pathways.

The somatosensory thalamus of monkeys: cortical connections and a redefinition of nuclei in marmosets.

Thalamic connections of three subdivisions of somatosensory cortex in marmosets were determined by placing wheatgerm agglutinin conjugated with horseradish peroxidase and fluorescent dyes as tracers into electrophysiologically identified sites in S-I (area 3b), S-II, and the parietal ventral area, PV. The relation of the resulting patterns of transported label to the cytoarchitecture and cytochrome oxidase architecture of the thalamus lead to three major conclusions. 1) The region traditionally described as the ventroposterior nucleus (VP) is a composite of VP proper and parts of the ventroposterior inferior nucleus (VPi). Much of the VP region consists of groups of densely stained, closely packed neurons that project to S-I. VPi includes a ventral oval of pale, less densely packed neurons and finger-like protrusions that extend into VP proper and separate clusters of VP neurons related to different body parts. Neurons in both parts of VPi project to S-II rather than S-I. Connection patterns indicate that the proper and the embedded parts of VPi combine to form a body representation paralleling that in VP. 2) VPi also provides the major thalamic input into PV. 3) In architecture, location, and cortical connections, the region traditionally described as the anterior pulvinar (AP) of monkeys resembles the medial posterior nucleus, Pom, of other mammals and we propose that all or most of AP is homologous to Pom. AP caps VP dorsomedially, has neurons that are moderately dense in Nissl staining, and reacts moderately in CO preparations. AP neurons project to S-I, S-II, and PV in somatotopic patterns.

Connections between area 3b of the somatosensory cortex and subdivisions of the ventroposterior nuclear complex and the anterior pulvinar nucleus in squirrel monkeys

The goal of this study was to determine whether somatosensory thalamic nuclei other than the ventroposterior nucleus proper (VP) have connections with area 3b of the postcentral cortex in squirrel monkeys. Small injections of the anatomical tracers wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) or 3H-proline were placed in electrophysiologically identified representations of body parts. The results indicate that, besides the well-established somatotopically organized connections with VP, area 3b has connections with three other nuclei of the somatosensory thalamus: the ventroposterior superior nucleus (VPS ["shell" of VP]), the ventroposterior inferior nucleus (VPI), and the anterior pulvinar nucleus (Pa). Injections confined to area 3b or involving adjacent parts of area 3a or area 1 indicate that connections between VPS, VPI, and Pa and the postcentral cortex are somatotopically organized. In VPS, connections related to the hand were found medially, and connections related to the foot were lateral. In VPI, connections with the cortical representations of the mouth, hand, and foot were successively more lateral. In Pa, connections related to the mouth, hand, and foot were successively more ventral, lateral, and caudal, and the trunk region was caudomedial. The findings suggest that VPI contains a representation of all parts of the body, including the face. The connections of Pa with the primary somatosensory cortex, area 3b, the location of Pa relative to the ventroposterior nucleus, and the high degree of topographic order in the connections of Pa with the postcentral cortex suggest that Pa is an integral part of the somatosensory thalamus in monkeys and is homologous to the medial nucleus of the posterior group (Pom) in other mammals. Overall, the results contribute to the growing evidence that individual somatosensory cortical areas in monkeys receive inputs from multiple thalamic sources, and that a single thalamic nucleus has several cortical targets.

Thalamic connections of three representations of the body surface in somatosensory cortex of gray squirrels

The anatomical tracer, wheat germ agglutinin, was used to determine the connections of electrophysiologically identified locations in three architectonically distinct representations of the body surface in the somatosensory cortex of gray squirrels. Injections in the first somatosensory area, S-I, revealed reciprocal connections with the ventroposterior nucleus (VP), a portion of the thalamus just dorsomedial to VP, the posterior medial nucleus, Pom, and sometimes the ventroposterior inferior nucleus (VPI). As expected, injections in the representation of the face in S-I resulted in label in ventroposterior medial (VPM), the medial subnucleus of VP, whereas injections in the representation of the body labeled ventroposterior lateral (VPL), the lateral subnucleus of VP. Furthermore, there was evidence from connections that the caudal face and head are represented dorsolaterally in VPM, and the forelimb is represented centrally and medially in VPL. The results also support the conclusion that a representation paralleling that in VP exists in Pom, so that the ventrolateral part of Pom represents the face and the dorsomedial part of Pom is devoted to the body. Because connections with VPI were not consistently revealed, the possibility exists that only some parts or functional modules of S-I are interconnected with VPI. Two separate small representations of the body surface adjoin the caudoventral border of S-I. Both resemble the second somatosensory area, S-II, enough to be identified as S-II in the absence of evidence for the other. We term the more dorsal of the two fields S-II because it was previously defined as S-II in squirrels (Nelson et al., '79), and because it more closely resembles the S-II identified in most other mammals. We refer to the other field as the parietal ventral area, PV (Krubitzer et al, '86). Injections in S-II revealed reciprocal connections with VP, Pom, and a thalamic region lateral and caudal to Pom and dorsal to VP, the posterior lateral nucleus, Pol. Whereas major interconnections between S-II and VPI have been reported for cats, raccoons, and monkeys, no such interconnections were found for S-II in squirrels. The parietal ventral area, PV, was found to have prominent reciprocal interconnections with VP, VPI, and the internal (magnocellular) division of the medial geniculate complex (MGi). The pattern of connections conforms to the established somatotopic organization of VP and suggests a crude parallel somatotopic organization in VPI. Less prominent interconnections were with Pol.(ABSTRACT TRUNCATED AT 400 WORDS)

The ventroposterior inferior nucleus in the thalamus of cats: a relay nucleus in the Pacinian pathway to somatosensory cortex.

The ventroposterior region of the thalamus of mongrel cats was searched to locate zones activated by somatic stimuli. By using stimuli that selectively excited Pacinian corpuscles, areas activated by this class of afferent fibers were differentiated from regions activated by other classes of cutaneous mechanoreceptors. The results showed that Pacinian inputs excite neurons in the ventroposterior inferior nucleus (VPI) of the thalamus, whereas other more dorsal zones within the ventroposterior thalamus receive inputs from other mechanoreceptor classes. This definition of the VPI tended to be larger and to extend further lateral than some published descriptions. Horseradish peroxidase (HRP) was injected into ventroposterior zones shown by electrophysiological recordings to receive inputs from Pacinian afferents. Subsequently, labeled cell bodies were observed in the caudal poles of the dorsal column nuclei, a region previously shown to be activated by Pacinian afferents. Very few labeled cells were found in the central region of these nuclei, a region previously shown to be activated by other classes of cutaneous mechanoreceptors. Electrophysiological recordings were used to locate a small portion of the second somatosensory cortex driven by Pacinian stimuli. When HRP was injected into this region cell bodies in the VPI and the lateral part of the posterior group were labeled, but few or no labeled cells were found in ventroposterior lateral nucleus. We hypothesize that the VPI receives Pacinian information from a cytoarchitecturally distinct region in the caudal poles of the dorsal column nuclei. Further, we suggest that a major cortical target for the VPI is a subdivision of the second somatosensory cortex. These studies do not exclude the possibility that Pacinian inputs have other thalamic and cortical targets.

A thalamic terminus of the lateral cervical nucleus: the lateral division of the posterior nuclear group.

The submodality and receptive field properties of single units in the lateral cervical nucleus (LCN) of barbiturate anesthetized cats were studied with glass microelectrodes. In other experiments, a region of the posterior thalamus containing neurons with properties comparable to those seen in the LCN was examined with tungsten microelectrodes. The responses of most units in the LCN reflected a major input from large myelinated afferent fibers innervating guard hairs but no input from Pacinian afferent fibers. The large size of the receptive fields indicated that excitatory input converged selectively from afferent fibers serving hairs over large areas of the body. In the posterior thalamus rapidly adapting neurons characterized by very large receptive fields and driven by the movement of guard hairs were observed to a region identified histologically as the rostral extension of the lateral division of the posterior nuclear group (POl). Caudally this region was located immediately adjacent to the dorsolateral part of the ventroposterior inferior nucleus (VPI). In more rostral parts of the thalamus it was located more dorsally and the ventroposterior lateral nucleus intervened between it and the VPI. This region was less than 1 mm wide in the frontal plane but extended rostrocaudally for several millimeters. Horseradish peroxidase injected into the region of the VPI and the POl labeled many cells in the LCN and the caudal pole of the dorsal column nuclei demonstrating that neurons in the LCN relay information to this part of the thalamus. These data, plus previous experiments showing that the VPI receives a major projection from the caudal poles of the dorsal column nuclei, suggest that the rostral portion of the POl receives an important afferent supply from the LCN. The responses of neurons in the POl appear to arise from specific classes of sensory receptors and cannot be considered less precise or more primitive than responses observed in the ventroposterior nucleus of the thalamus.

The organization and connections of somatosensory cortex in primates

The use of microelectrode mapping methods, in conjunction with studies of cortical architecture, patterns of connections, and the response properties of single neurons, has led to a greatly increased understanding of the functional organization of somatosensory cortex in mammals. All mammals appear to have the traditionally defined first and second somatosensory representations, SI and SII, but at least some mammals have additional body representations as well. Furthermore, the area traditionally regarded as “SI” in monkeys actually includes four functionally distinct representations. Within “SI,” four striplike architectonic fields, areas 3a, 3b, 1, and 2 have long been recognized. It is now clear that areas 3b and 1 form separate parallel and largely mirror‐image representations of the body surface, each with major inputs from a single representation of the body surface in the ventroposterior nucleus (VP) of the thalamus. VP appears to relay information from both slowly and rapidly adapting peripheral neurons to area 3b, while information from rapidly adapting neurons is relayed to area 1. Neurons with properties that could reflect Pacinian receptor influences are found in area 1, but not area 3b. The anatomical pathways mediating Pacinian‐like responses are unknown. Area 3a contains a representation of largely deep receptors, probably muscle spindle afferents, relayed from part of the thalamus dorsal to VP. Area 2 forms a representation of both deep and cutaneous body receptors. The somatosensory cortex receives input from other regions as well, including the anterior pulvinar and ventroposterior inferior nucleus of the thalamus. Other portions of the somatosensory cortex are less well understood.

Thalamic connections of the insula in the rhesus monkey and comments on the paralimbic connectivity of the medial pulvinar nucleus.

Thalamic connections of the insula in the rhesus monkey were studied with axonal transport methods. Tritiated amino acid injections limited to the insula revealed autoradiographic label in the principal and parvicellular components of the ventroposterior medial nucleus, the ventroposterior inferior nucleus, the oral and medial pulvinar nuclei, the nucleus reuniens, the parvicellular and magnocellular components of the medial dorsal nucleus, the centromedian-parafasicularis nuclei, and the reticular nucleus. In additional animals, tritiated amino acids and horseradish perioxidase injections were made within different regions of the insula. Although the injection sites in these additional cases may have included minor extensions into claustrum and adjacent structures, several tentative conclusions emerged with respect to the antero-posterior gradient in insulothalamic connectivity. The anterior insula appears to have a more extensive relationship with the ventroposterior medial complex, the medial dorsal nucleus, the centromedian-parafasicularis nuclei and with some midline nuclei. In contrast, the posterior insula is more extensively connected with the ventroposterior inferior nucleus, the oral and medial pulvinar nuclei, and the suprageniculate nucleus. The patterns of insulothalamic connections support conclusions derived from observations on the cortical connectivity of the primate insular cortex indicating that the anterior insula is related to olfactory, gustatory, and viscero-autonomic behavior, whereas the posterior insula is related to auditory-somesthetic-skeletomotor function (Mesulam and Mufson, '82b). The medial pulvinar nucleus has extensive connections with many paralimbic cortical regions including the insula as well as with high order polymodal association cortex. These findings suggest that the medial pulvinar may provide a region for the convergence of multisensory association input with limbic information.

The connections of cortical somatosensory areas I and II with separate nuclei in the ventroposterior thalamus in the raccoon

The thalamocortical afferents to cortical somatosensory areas I (SI) and II (SII) were investigated in the raccoon using the horseradish peroxidase technique. The purpose of this study was to determine if the cell bodies of origin for thalamocortical afferents to these cortical regions were localized in the same or different nuclei in the ventroposterior region of the thalamus. Horseradish peroxidase was injected into subdivisions of SI or SII and after post-injection survival periods of 12–72 hours the horseradish peroxidase in the tissue was reacted with the chromogens dihydrochlorobenzidine or tetramethylbenzidine in the presence of hydrogen peroxide. The results show that SI and SII receive projections from neurons in separate and distinct nuclei in the ventroposterior thalamus. Following injections into subdivisions of area I, a topographical distribution of retrogradely-labelled cell bodies was observed in the ventrobasal complex. Following injections of horseradish peroxidase into subdivisions of area II, a topographical distribution of labelled cell bodies was observed in the ventroposterior inferior nucleus. No labelled cell bodies were observed in the ventrobasal complex. The thalamocortical connections of somatosensory cortices I and II in raccoon are compared with those in other animals and it is suggested that these two cortical areas may be involved in differential processing of tactile information.

Regional segregation of neurons responding to quickly adapting, slowly adapting, deep and pacinian receptors within thalamic ventroposterior lateral and ventroposterior inferior nuclei in the squirrel monkey ( Saimiri sciureus)

The ventroposterior nuclei and the ventroposterior inferior nucleus of the thalamus were mapped in squirrel monkeys ( Saimiri sciureus) using low-impedance platinum-iridium microelectrodes. The body site and receptor category from which the optimum response was obtained was recorded every 50 μm in orthogonal penetrations through these nuclei. The results suggest that the neurons are grouped according to submodality in this area and that discrete and spatially separate volumes of the thalamus are devoted to input from deep, Pacinian, cutaneous rapidly adapting and cutaneous slowly adapting receptors. The Pacinian responses were limited to the ventroposterior inferior nucleus and deep responses were limited to a region just above the ventroposterior nuclei. Within ventroposterior nuclei the cutaneous rapidly adapting and cutaneous slowly adapting volumes were interspersed in a pattern that could not be readily defined. The results indicate that there is more than one representation of the body in this area of the thalamus. Discontinuities in receptive field sequences make it difficult to see how any representation can be a spatially continuous map of the body.