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 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.     

The representation of the body surface in S-I of cats

In both cats and monkeys, the traditional region of the first somatosensory area of cortex, S-I, has been described as containing four strip-like architectonic fields, areas 3a, 3b, 1, and 2. In monkeys, a number of recent studies have provided evidence that each of these architectonic fields constitutes a separate representation of the body. Because of the observations in monkeys, we decided to re-examine the S-I region of cats to determine whether the evidence supported the traditional concept of a single representation, or the existence of several representations related to the described architectonic fields. Microelectrode multiunit mapping techniques were used to determine the somatotopic organization of the S-I region of 10 cats. The results indicate that a single representation of the body surface occupies most or all of the traditional S-I region including cortex defined as area "3b," area "1," and much of area "2," but excluding area "3a." Neurons throughout this single representation were activated by cutaneous stimuli, indicating that all parts of S-I receive input from cutaneous receptors. Neurons in area 3a were activated by inputs from deep receptors, as reported by others. Neurons caudal to S-I were activated by cutaneous stimuli and appeared to constitute the additional body surface representations of S-II and possibly S-III. Thus, the significance of the architectonic fields "3b," "1," and "2" is quite different in cats than in monkeys. We propose that most or all of these three fields, as described in cats, constitute the homologue of area 3b in monkeys.     

Thalamic projections to sensorimotor cortex in the macaque monkey: use of multiple retrograde fluorescent tracers.

We used several fluorescent dyes (Fast Blue, Diamidino Yellow, Rhodamine Latex Microspheres, Evans Blue, and Fluoro-Gold) in each of eight macaques, to examine the patterns of thalamic input to the sensorimotor cortex of macaques 12 months or older. Inputs to different zones of motor, premotor, and postarcuate cortex, supplementary motor area, and areas 3b/1 and 2/5 in the postcentral cortex, were examined. Coincident labeling of thalamocortical neuron populations with different dyes (1) increased the precision with which their soma distributions could be related within thalamic space, and (2) enabled the detection by double labeling, of individual thalamic neurons that were common to the thalamic soma distributions projecting to separate, dye-injected cortical zones. Double-labeled thalamic neurons projecting to sensorimotor cortex were rarely seen in mature macaques, even when the injection sites were only 1-1.5 mm apart, implying that their terminal arborizations were quite restricted horizontally. By contrast, separate neuron populations in each thalamic nucleus with input to sensorimotor cortex projected to more than one cytoarchitecturally distinct cortical area. In ventral posterior lateral (oral) (VPLo), for example, separate populations of cells sent axons to precentral medial, and lateral area 4, medial premotor, and postarcuate cortex, as well as to supplementary motor area. Extensive convergence of thalamic input even to the smallest zones of dye uptake in the cortex (approximately 0.5 mm3) characterized the sensorimotor cortex. The complex forms of these projection territories were explored using 3-dimensional reconstructions from coronal maps. These projection territories, while highly ordered, were not contained by the cytoarchitectonic boundaries of individual thalamic nuclei. Their organization suggests that the integration of the diverse information from spinal cord, cerebellum, and basal ganglia that is needed in the execution of complex sensorimotor tasks begins in the thalamus.     

Tonotopic organization in auditory cortex of the cat

Microelectrode mapping techniques were employed in the cat's auditory cortex to relate the best frequencies of a large population of neurons with their spatial loci. Based upon the best-frequency distribution, the auditory region was divided into four complete and orderly tonotopic representations and a surrounding belt of cortex in which the tonotopic organization was more complex. The four auditory fields occupy a crescent-shaped band of tissue which comprises portions of both the exposed gyral surfaces and sulcal banks of the ectosylvian cortex. The anterior auditory field (A) is situated most rostrally upon the anterior ectosylvian gyrus. It extends upon the ventral bank of the suprasylvian sulcus and upon the banks of the anterior ectosylvian sulcus. Adjoining field A caudally is the primary auditory field (AI), which extends across the middle ectosylvian gyrus and portions of both banks of the posterior ectosylvian sulcus. The representations of the highest best frequencies in fields A and AI are contiguous. Caudal and ventral to AI are located the posterior (P) and ventroposterior (VP) auditory fields. They lie mainly upon the caudal bank of the posterior ectosylvian sulcus but also extend upon the rostral bank and upon the posterior ectosylvian gyrus. The low best-frequency representations of fields AI and P are contiguous, whereas the low best-frequency representation of field VP lies near the ventral end of the posterior ectosylvian sulcus. Fields P and VP are joined along their middle and high best-frequency representations. Within each auditory field isofrequency lines defined by the spatial loci of neurons with similar best frequencies are oriented orthogonal to the low-to-high best-frequency gradients.     

Afferent fibers to rat cingulate cortex

Afferent fibers to the rat cingulate cortex were studied by the retrograde labeling technique using horseradish peroxidase-wheat germ agglutinin conjugate as the tracer. The results showed that the posterior cingulate cortex, but not the anterior, received input from the anterior dorsal and anterior ventral nuclei of the anterior thalamic group of nuclei (part of the so-called limbic thalamus), and from the subicular complex. The anterior cingulate cortex, but not the posterior, received input from the mediodorsal and ventral thalamic nuclei. Both posterior and anterior cingulate cortex received input from the hippocampus pars anterior; claustrum; globus pallidus; nucleus of the diagonal band of Broca (a particularly reliable source of afferent fibers); anterior medial, lateral, rhomboid, and reuniens nuclei of the thalamus; region of the medial forebrain bundle; periventricular nucleus of the hypothalamus; the dorsal and median raphe; and the locus ceruleus. Corticocortical projections were seen anterior, posterior, and lateral to the injection site, and in the homologous contralateral cingulate cortex. The results demonstrate a prominent source of cingulate afferent fibers from the subicular complex, provide evidence for a functional division of anterior and posterior cingulate cortices in the rat, and provide information about the relative anatomic importance of cingulate afferent fibers from those different regions.     

Representation of the body surface in somatosensory area I of tree shrews,Tupaia glis

Microelectrode mapping methods were used to determine the organization of the first somatosensory area, S-I, of tree shrews. Tree shrews were chosen for study because of their generalized body form, phylogenetic relationship to primates, and smooth, easily mapped cortex. A systematic representation of the contralateral body surface was found in an architectonically distinct zone identified as somatic koniocortex. Overall features of somatotopic organization were similar to S-I of other mammals, S-I of prosimian primates, and the Area 3b “S-I proper” representation of monkeys. Like Area 3b in monkeys and the somatic koniocortex in galagos, S-I in tree shrews is bordered caudally by cortex also responsive to somatosensory stimuli. Several aspects of S-I organization in tree shrews appear to be primitive and generalized. These include the representation of the trunk with the ventrum at the caudal margin of S-I, the restriction of the glabrous digits of the hand and foot to the rostral half of the representation and pointed rostralward, the representation of an anterior strip of the forelimb lateral to the hand, and a posterior strip of hindlimb medial to the foot representation. As in a number of other mammals, a large portion of S-I in tree shrews is devoted to the head. However, the proportion of S-I activated from the glabrous nose is greater in tree shrews than in any previously studied mammal. We conclude that S-I of tree shrews has both specialized and generalized features, and that the generalized features importantly relate to an understanding of the evolution of the primate somatosensory system.     

Visual and auditory association areas of the cat's posterior ectosylvian gyrus: thalamic afferents

The feline posterior ectosylvian gyrus contains a broad band of association cortex that is bounded anteriorly by tonotopic auditory areas and posteriorly by retinotopic visual areas. To characterize the possible functions of this cortex and to throw light on its pattern of internal divisions, we have carried out an analysis of its thalamic afferents. Deposits of differentiable retrograde tracers were placed at 17 cortical sites in nine cats. The deposit sites spanned the crown of the posterior ectosylvian gyrus and adjacent cortex in the suprasylvian sulcus. We compiled counts of retrogradely labeled neurons in 12 thalamic nuclei delineated by use of Nissl and acetylcholinesterase stains. We then employed a statistical clustering algorithm to identify groups of injections that gave rise to similar patterns of thalamic labeling. The results suggest that the posterior ectosylvian gyrus contains 3 fundamentally different cortical districts that have the form of parallel vertical bands. Very anterior cortex, overlapping previously identified tonotopic auditory areas (AI, P and VP) receives a dense projection from the laminated division of the medial geniculate body (MGl). An intermediate strip, to which we refer as the auditory belt, is innervated by axons from nontonotopic divisions of the medial geniculate body (MGds, MGvl, MGm, and MGd), from the lateral division of the posterior group (Pol), and from the posterior suprageniculate nucleus (SGp). A posterior strip, to which we refer as EPp, receives strong projections from the LM-SG complex (LM-SGa and LMp), and lighter projections from the intralaminar and lateroposterior (LPm and LPl) nuclei. On grounds of thalamic connectivity, EPp is not obviously distinguishable from adjacent retinotopic visual areas (PLLS, DLS, and VLS), and may be regarded as forming, together with these areas, a connectionally homogeneous visual belt.     

Connections of area 2 of somatosensory cortex with the anterior pulvinar and subdivisions of the ventroposterior complex in macaque monkeys

The principal goal of the present study was to determine the thalamic connections of area 2 of postcentral somatosensory cortex of monkeys. The placement of injections of anatomical tracers (horseradish peroxidase, wheat germ agglutinin, or ³H-proline) was guided by extensive microelectrode maps of cortex in the region of the injection site. These maps identified the body parts represented in the cortex included in the injection site, and provided information about the physiological boundaries of area 2, which was related later to the cortical architecture. Most injections were placed in the representation of the hand in area 2, which was highly responsive to cutaneous stimuli and could be mapped in detail. Injections were also placed in other parts of area 2, area 1, or area 5, and some injections involved more than one area. As other investigators have determined, regions of retrograde and anterograde thalamic label overlapped, demonstrating that connections with cortex are reciprocal. Injections completely confined to area 2 consistently produced label in two locations: the anterior pulvinar (Pa) and a dorsal capping zone of the ventroposterior complex that we term the ventroposterior superior nucleus (VPS). Single restricted injection sites resulted in one region of label in VPS, and multiple foci of label in Pa. In some cases where the injection was confined to the representation of the hand in area 2, label was also found more ventrally in the ventroposterior complex in ventroposterior nucleus proper (VP). Thus, area 2 receives input from Pa, VPS, and, at least in some locations and individuals, VP. Injections of tracers into area 1 confirmed previous findings that area 1 is densely interconnected with VP. In addition, there appear to be sparse connections with VPS. There was no evidence of connections with Pa. Evidence from injection sites that extended from area 2 into areas 5 and 7, and from injection sites in area 5, indicates that the lateral posterior nucleus (LP) projects to rostral areas 5 and 7. The results support the conclusion that area 2 is a functionally distinct subdivision of somatosensory cortex, and indicate that area 2 has thalamic connections that are characteristic of both “sensory” (VP and VPS) and “association” (Pa) cortical fields.     

Barrels and septa: separate circuits in rat barrels field cortex.

The neural circuitry within sensory cortex determines its functional properties, and different solutions have evolved for integrating the activity that arises from an array of sensory inputs to cortex. In rodent, circumscribed receptors, such as whiskers, are represented in somatic sensory (S-I) cortex in islands of cells in layer IV called "barrels" surrounded by narrow channels that separate barrels called "septa." These two cortical domains were previously shown to receive sensory inputs through parallel subcortical pathways. Here, by using small biocytin injections, we demonstrate that distinct intrinsic and corticocortical circuitries arise from barrel and septal columns. The intracortical S-I projections originating from barrel columns are rather short-ranged, terminating for the most part within the far boundaries of the most immediate neighboring barrel columns, whereas corticocortical projections reach the second somatosensory (S-II) cortex. In contrast, the intrinsic projections arising from septal columns extend two to three barrels' distance along the row of whisker representation, producing terminals preferentially in other septal columns. Septal corticocortical projections terminate in the dysgranular cortex anterior to E-row barrels and in the posteromedial parietal cortex in addition to S-II. Whereas layer IV barrels are largely isolated from lateral connections, septa are the main conduits of intracortical projections arising from neighboring barrel and septal columns. These results indicate that the two subcortical pathways from whiskers to cortex continue as two distinct partially segregated pathways in cortex.     

Connections of the posterior thalamic region with the auditory ectosylvian cortex in the dog

The purpose of the present study was to define auditory cortical areas in the dog on the basis of thalamocortical connectivity patterns. Connections between the posterior thalamic region and auditory ectosylvian cortex were studied using axonally transported tracers: fluorochromes and biotinylated dextran amine. Cyto- and chemoarchitecture provided grounds for the division of the posterior thalamic region into three complexes, medial geniculate body (MGB), posterior nuclei (Po), and lateromedial and suprageniculate nuclei (LM-Sg). Distinctive cytoarchitectonic features and the distribution of dominant thalamocortical connections (determined quantitatively) allowed us to define four ectosylvian areas: middle (EM), anterior (EA), posterior (EP), and composite (CE). We found that each area was a place of convergence for projections from five to eleven nuclei of the three thalamic complexes, with dominant projections derived from one or two nuclei. Dominant topographical projections from the ventral nucleus to area EM confirmed physiological reports that it may be considered a primary auditory area (AI). We found the anterior part of the EM to be distinct in having unique strong connections with the deep dorsal MGB nucleus. Area EA, which receives dominant projections from the lateral Po (Pol) and medial MGB nuclei, as well as area EP, which receives dominant connections from the dorsal caudal MGB nucleus, compose two parasensory areas. Area CE receives dominant projections from the extrageniculate nuclei, anterior region of the LM-Sg, and Pol, supplemented with an input from the somatosensory VP complex, and may be considered a polymodal association area.     

Somatotopically organized transient projections from the primary somatosensory cortex to the cerebellar cortex

The organization of transient projections from the primary somatosensory cortex (S-I) to the cerebellar cortex in neonatal kittens was examined using orthograde intraaxonal labeling techniques. Tritiated amino acid injections into face, forelimb and hindlimb areas of representation in S-I labeled mossy fiber-like terminals of cerebrocerebellar axons in different areas of the cerebellar cortex bilaterally. The hindlimb area of S-I projected to lobules I-IV in the anterior lobe and to ventral folia of the paramedian lobule (PML). Injections into forelimb areas of S-I labeled terminals in lobules IV and V and in intermediate and dorsal folia of the PML. The face area of S-I projected to the lobules V and VI, to medial folia in the ansiform and simplex lobules and to dorsal PML folia. Labeled terminals were more numerous in the cerebellar cortex contralateral to the S-I injections, except in lobules I and II and the ventral PML where the density of hindlimb input was approximately the same on both sides. These observations were supplemented by findings that small wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections into the dorsal or ventral PML resulted in retrogradely labeled layer V pyramidal neurons in lateral (face and forelimb) and medial (hindlimb) areas of S-I respectively. The somatotopic organization of transient S-I cerebrocerebellar projections is very similar to the topography of cerebellar somatosensory afferent pathways in adult cats.     

Studies on the evolution of multiple somatosensory representations in primates: the organization of anterior parietal cortex in the New World Callitrichid, Saguinus

Because members of the New World family, Callithricidae, are generally regarded as the most primitive of monkeys, we studied the organization of somatosensory cortex in the tamarin (Saguinus) in hopes of better understanding differences in the organization of anterior parietal cortex in primates and how these differences relate to phylogeny. In most prosimian primates only one complete representation of cutaneous receptors has been found in the region of primary cortex, S-I, while in all Old and New World monkeys studied to date, two cutaneous representations exist in distinct architectonic fields, areas 3b and 1. In detailed microelectrode mapping studies in anesthetized tamarins, only one complete representation responsive to low-threshold cutaneous stimulation was evident in the S-I region. This topographic representation was in a parietal koniocortical field that architectonically resembles area 3b of other monkeys, and the general somatotopic organization of the field was similar to that of area 3b of other monkeys. Cortex rostral to the single representation was generally unresponsive to somatosensory stimuli, or required more intense stimulation for neural activation. Cortex caudal to the representation, in the region of area 1 of other monkeys, was generally either unresponsive or responded to only high-threshold stimulation, although some recording sites were activated by low-threshold tactile stimulation. The present evidence, together with that from previous studies, suggests that the single, complete body surface representation in Saguinus is homologous to the S-I representation found in some prosimians (Galago, Perodicticus) and the area 3b cutaneous representation found in New World Cebidae (Aotus, Saimiri, and Cebus) and Old World Macaca. Cortex rostral to S-I in Saguinus has the appearance of areas 3a and 4 of other primates. The cortex caudal to S-I in Saguinus, while resembling area 1 in some ways, does not have all of the features of area 1 of other monkeys. In particular, the field was not easily activated by low-threshold cutaneous stimuli, as area 1 is in other monkeys, and therefore a second cutaneous representation of all body parts was not demonstrated. Thus, cortex in the expected location of area 1 of Saguinus was not as responsive as area 1 of other monkeys, and it somewhat resembled the high-threshold fringe zones found caudal to S-I in anesthetized prosimians and some nonprimates. The results raise the possibility that the area 1 cutaneous representation that is characteristic of other New World monkeys and Old World monkeys evolved from a less responsive precursor along the caudal border of S-I in early monkeys.     

Thalamic projections to the posteromedial cortex in the macaque

The medial parietal, posterior cingulate, and retrosplenial cortices collectively constitute a region of cortex referred to as the posteromedial cortices (PMC). In an effort to shed light on the neuroanatomical organization of the PMC, we undertook a study to identify and analyze the thalamocortical connections of these cortices. Retrograde tracer injections were placed in the posterior cingulate (PCC), retrosplenial (RSC), medial parietal cortices (MPC), and posterior cingulate sulcus (PCS), and the labeling patterns within the thalamus were analyzed. Three afferent projection patterns were observed to the PMC from the thalamus: a PCC/RSC pattern that involved the anterior thalamic nuclei, an MPC pattern that involved the lateral posterior and pulvinar nuclei, and a PCS pattern that involved the ventral thalamic nuclei. Additionally, a shared pattern of projections from the anterior intralaminar nuclei (AILN) and posterior thalamic nuclei (PTN) to all cortical regions of the PMC was observed. Our findings suggest that distinct regions within the PMC are supplied by distinctive patterns of thalamic input, but also share common projections from intralaminar and posterior thalamic sources. In addition, we relate our findings to functional abnormalities in aging and dementia, and address a domain-like pattern of thalamocortical labeling of the PMC that is drawn selectively and collectively from multiple thalamic nuclei.     

Thalamic connectivity of the second somatosensory area and neighboring somatosensory fields of the lateral sulcus of the macaque

The thalamocortical relations of the somatic fields in and around the lateral sulcus of the macaque were studied following cortical injections of tritated amino acids and horseradish peroxidase (HRP). Special attention was paid to the second somatosensory area (S2), the connections of which were also studied by means of thalamic isotope injections and retrograde degeneration. S2 was shown to receive its major thalamic input from the ventroposterior inferior thalamic nucleus (VPI) and not, as previously reported, from the caudal division of the ventroposterior lateral nucleus (VPLc). Following small injections of isotope or HRP into the hand representation of S2, only VPI was labeled. Larger injections, which included the representations of more body parts, led to heavy label in VPI, with scattered label in VPLc, the central lateral nucleus (CL), and the posterior nucleus (Po). In addition, small isotope injections into VPLc did not result in label in S2 unless VPI was also involved in the injection site, and ablations of S2 led to cell loss in VPI. Comparison of injections involving different body parts in S2 suggested a somatotopic arrangement within VPI such that the trunk and lower limb representations are located posterolaterally and the hand and arm representations anteromedially. The location of the thalamic representations of the head, face, and intraoral structures that project to S2 may be in the ventroposterior medial nucleus (VPM). The granular (Ig) and dysgranular (Id) fields of the insula and the retroinsular field (Ri) each receive inputs from a variety of nuclei located at the posteroventral border of the thalamus. Ig receives its heaviest input from the suprageniculate-limitans complex (SG-Li), with additional inputs from Po, the magnocellular division of the medial geniculate n. (MGmc), VPI, and the medial pulvinar (Pulm). Id receives its heaviest input from the basal ventromedial n. (VMb), with additional inputs from VPI, Po, SG-Li, MGmc, and Pulm. Ri receives its heaviest input from Po, with additional input from SG-Li, MGmc, Pulm, and perhaps VPI. Area 7b receives its input from Pulm, the oral division of the pulvinar, the lateral posterior n., the medial dorsal n., and the caudal division of the ventrolateral n. These results indicate that the somatic cortical fields, except for those comprising the first somatosensory area, each receive inputs from an array of thalamic nuclei, rather than just one, and that individual thalamic somatosensory relay nuclei each project to more than one cortical field.     

Thalamic projections to areas 3a, 3b,and 4 in the sensorimotor cortex of the mature and infant macaque monkey

Area 3a in the macaque monkey, located in the fundus of the central sulcus, separates motor and somatosensory cortical areas 4 and 3b. The known connections of areas 4 and 3b differ substantially, as does the information which they receive, process, and transfer to other parts of the central nervous system. In this analysis the thalamic projections to each of these three cortical fields were examined and compared by using retrogradely transported fluorescent dyes (Fast Blue, Diamidino Yellow, Rhodamine and Green latex microspheres) as neuron labels. Coincident labeling of projections to 2–3 cortical sites in each monkey allowed the direct comparison of the soma distributions within the thalamic space of the different neuron populations projecting to areas 3a, 3b, and 4, as well as to boundary zones between these cortical fields. The soma distribution ofthalamic neurons projecting to a small circumscribed zone (diameter = 0.5–1.0 mm) strictly within cortical area 3a (in region of hand representation) filled out a “territory” traversing the dorsal half of the cytoarchitectonically defined thalamic nucleus, VPLc (abbreviations as in Olszewski [1952] The Thalamus of the Macaca mulatta. Basel: Karger). This elongate, rather cylindrical, territory extended caudally into the anterior pulvinar nucleus, but not forward into VPLo. The rostrocaudal extent of the thalamic territory defining the soma distribution of neurons projecting to small zones of cortical area 3b was similar, but typically extended into the ventral part of VPLc, filling out a medially concavo-convex laminar space. Two such territories projecting to adjacent zones of areas 3a and 3b, respectively, overlapped and shared thalamic space, but not thalamic neurons. Contrasting with the 3a and 3b thalamic territories, the soma distribution of thalamic neurons projecting to a circumscribed zone in the nearby motor cortex (area 4) did not penetrate into VPLc, but instead filled out a mediolaterally flattened territory extending from rostral VLo, VLm, VPLo to caudal and dorsal VLc, LP, and Pulo. These territories skirted around VPLc. All three cortical areas (4, 3a, and 3b) also received input from distinctive clusters of cells in the intralaminar Cn.Md. It is inferred that, in combination, the thalamic territories in areas 3a, 3b, and 4 (and also area 1 and 2), which would be coactive during the execution of a manual task, constituted a lamellar space extending from VLo rostrally to Pul.o caudally. How Pul.o neuron populations relate to the more rostral populations within the same thalamic territory projecting to a localized cortical zone remains uncertain. Within the medially located territories the distribution of the neuron population in Pul.o was spatially continuous with the more rostral thalamic cells projecting to the same localized cortex, but in lateral thalamic territories these 2 populations were usually spatially discontinous. In the newborn macaque an orderly change in the territorial projections to localized zones in area 4, 3a, and 3b was also demonstable. However, thalamic nuclear projections were more expansive than in the mature animal. As well as the VPLc input, a third of the thalamic input to area 3a was now from VLo, VPLo, and VLm. Area 4 also had a significant input from VPLc, an input not observed in the mature macaque. © 1993 Wiley-Liss, Inc.     

Thalamocortical connections of anterior and posterior parietal cortical areas in New World titi monkeys

We examined the thalamocortical connections of electrophysiologically identified locations in the hand and forelimb representations in areas 3b, 1, and 5 in the New World titi monkeys (Callicebus moloch), and of area 7b/AIP. Labeled cells and terminals in the thalamus resulting from the injections were related to architectonic boundaries. As in previous studies in primates, the hand representation of area 3b has dense, restricted projections predominantly from the lateral division of the ventral posterior nucleus (VPl). Projections to area 1 were highly convergent from several thalamic nuclei including the ventral lateral nucleus (VL), anterior pulvinar (PA), VPl, and the superior division of the ventral posterior nucleus (VPs). In cortex immediately caudal to area 1, what we term area 5, thalamocortical connections were also highly convergent and predominantly from nuclei of the thalamus associated with motor, visual, or somatic processing such as VL, the medial pulvinar (PM), and PA, respectively; with moderate projections from VP, central lateral nucleus (CL), lateral posterior nucleus (LP), and VPs. Finally, thalamocortical connections of area 7b/AIP were from a range of nuclei including PA, PM, LP/LD, VL, CL, PL, and CM. The current data support two conclusions drawn from previous studies in titi monkeys and other primates. First, cortex caudal to area 1 in New World monkeys is more like area 5 than area 2. Second, the presence of thalamic input to area 5 from both motor nuclei and somatosensory nuclei of the thalamus, suggests that area 5 could be considered a highly specialized sensorimotor area.     

Distribution of large terminal inputs from the primary and secondary somatosensory cortices to the dorsal thalamus in the rodent

The present study was undertaken to determine the precise projection pattern from the primary (S1) and secondary (S2) somatosensory cortices to the posterior nuclear proper (POm) and ventroposterior thalamic nuclei (VP). The POm was previously shown to receive large boutons arising exclusively from layer V of the S1 barrel region. This descending input was proposed to play a key role, namely, as a driver, in shaping the receptive property of POm neurons. To determine whether other body parts and the S2 also contribute such unique inputs to the dorsal thalamus, anterograde neuroanatomical tracers were focally deposited in the S1 and S2 forepaw and whisker regions of rats and C57BL6‐Tg (GFPm)/Thy1 transgenic mice. Our major findings were that, 1) irrespective of body representations, both the S1 and the S2 provided corticothalamic large terminals to the POm with comparable morphological characteristics and 2) descending large terminals were also noted in particular subzones within the VP, including boundary and caudal areas. We concluded, based on these findings, that the rodent VP has three partitions: the rostral VP innervated by small corticothalamic terminals, the caudal VP with both corticothalamic small and large terminals, and a surrounding shell region, which also contained large terminals. Furthermore, assuming that the large terminal has a driver's role, we propose that particular subzones in the VP may play a role as a multiple‐order thalamic relay so that they can simultaneously coordinate with first‐ and higher‐order relays in the thalamocortical circuitry for processing somatosensory information. J. Comp. Neurol. 518:2592–2611, 2010. © 2010 Wiley‐Liss, Inc.