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

Immunoreactivity for GAD and three peptides in somatosensory cortex and thalamus of the raccoon

Immunocytochemical methods were used to determine the distributions of glutamic acid decarboxylase (GAD), vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), and somatostatin (SOM) in the primary somatosensory cortex and somatosensory thalamus of adult raccoons. The cortex showed extensive immunoreactivity for GAD, revealing a large population of GABAergic neurons. GAD-labeled cells were numerous in all cortical layers, but were most concentrated in laminae II-IV. The cells were nonpyramidal and of varying morphology, typically with somata of small or medium size. GAD-immunoreactive puncta, presumably synaptic terminals, were widespread and often appeared to end on both GAD-negative and GAD-positive neurons. Immunoreactivity for the peptides was much less extensive than that for GAD, with the number of labeled neurons for VIP > CCK > SOM. Peptidergic cells were preferentially located in the upper and middle cortical layers, especially laminae II and III. The cells were nonpyramidal, often bitufted or bipolar in morphology, and small to medium in size. Their processes formed diffuse plexuses of fibers with terminal-like varicosities that occasionally surrounded nonpeptidergic neurons. The thalamus showed a clearly differentiated pattern of immunoreactivity for GAD, but little or no labeling for the three peptides. Nuclei adjoining the ventral posterior lateral (VPL)/ventral posterior medial (VPM) complex--including the reticular nucleus--contained many GAD-positive neurons and fibers. In contrast, the VPL and VPM nuclei displayed considerably less GAD immunoreactivity, somewhat surprising given the raccoon's highly developed somatosensory system. However, the ventral posterior inferior (VPI) nucleus revealed rather dense GAD labeling, perhaps related to a specialized role in sensory information processing. Thus, the primary somatosensory cortex of the raccoon showed patterns of immunoreactivity for GAD and peptides that were similar to those of other species; the somatosensory thalamus revealed a distinctive profile of GAD immunoreactivity, with labeling that was light to moderate in the VPL/VPM complex and relatively extensive in VPL.

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.

Parallel thalamic activation of the first and second somatosensory areas in prosimian primates and tree shrews

In Tupaia belangeri and Galago senegalensis, microelectrode recordings immediately after ablation of the representation of the forelimb in the midportion of the first somatosensory area, S-I, revealed that all parts of the second somatosensory area, S-II, remained highly responsive to cutaneous stimuli. In this way, prosimian primates, close relatives of simian primates, and tree shrews differ markedly from monkeys in which S-II is deactivated by comparable ablations, and resemble such mammals as cats and rabbits in which S-II also remains highly responsive following ablations in S-I. Thus, it appears that the generalized mammalian condition is that S-I and S-II are independently activated via parallel thalamocortical pathways. A dependence of S-II on serial connections from the thalamus to the S-I region and then to S-II apparently evolved with the advent of anthropoid primates, and may be present only in monkeys and perhaps other higher primates.

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)

Thalamic and corticocortical connections of the second somatic sensory area of the mouse

Thalamic and corticocortical connections of the second somatic sensory area (SII) in the mouse cerebral cortex were investigated by means of the retrograde transport of horseradish peroxidase. Focal injections of the enzyme were made in physiologically determined locations within the parietal cortex. Results show that SII receives substantial inputs from topographically appropriate regions within the ipsilateral ventrobasal nucleus and from the ipsilateral posterior group. The limb representation, which was previously found to be responsive to auditory stimulation, received inputs also from the medial division of the medial geniculate body. The SII face representation, which is largely unresponsive to auditory stimuli, received little or no input from the medial geniculate body. SII injections yielded retrograde labeling in the topographically appropriate region in the first somatic sensory area (SI), and SI injections retrogradely labeled cells in SII in a pattern consistent with previous electrophysiological maps. Homotypical regions within SI and SII therefore appear to be reciprocally interconnected. SII also receives inputs from the ipsilateral motor cortex and from contralateral SI and SII. Finally, injections into the SI paw but not face regions yielded retrograde labeling in the thalamic ventrolateral nucleus. Thus, the distal limb representations in SI and SII each receive inputs from a third major relay nucleus (i.e., medial geniculate to SII, ventrolateral nucleus to SI) whereas the face representations do not. These results indicate a close functional interrelationship between homotypical areas in SI and SII, though the two areas differ in several important respects. It is proposed that SII in mice may complement the function of SI by helping to define the overall sensory context in which detailed tactile discriminations are made.

Organization of intracortical and commissural connections in somatosensory cortical areas I and II in the raccoon

The organization of intracortical and callosal projecting cell bodies was examined in somatosensory representation areas I (SI) and II (SII) of the raccoon by use of horseradish peroxidase (HRP) or horseradish peroxidase-wheat germ agglutin (HRP-WHA). HRP and HRP-WHA were injected into commissurally and noncommissurally connected subdivisions of SI and SII. Injection sites in SII were identified electrophysiologically. Results were obtained from transverse sections in which the HRP was visualized with the aid of the substrates dihydrochlorobenzidine or tetramethyl benzidine in the presence of hydrogen peroxidase. The principal findings were the following: (1) there are reciprocal connections between SI and SII; (2) in SI the intracortically projecting cell bodies and terminals are located primarily in sulcal cortex; (3) intracortically projecting neurons in SI are located primarily in layers III whereas in SII they are located principally in layers III and V; (4) there are connections between disparate areas within SI; and (5) there are intracortical connections between callosum-connected and acallosal regions in SII. These results are discussed with regard to the results of mapping studies of the SI, the significance of intracortical connections to the formation of sulci in SI, and the possible roles of nonhomotopic connections in the intermanual transfer of learning.

Patterns of reciprocity in auditory thalamocortical and corticothalamic connections: study with horseradish peroxidase and autoradiographic methods in the rat medial geniculate body

The patterns of reciprocity between retrogradely labeled thalamocortical cells of origin and anterogradely projecting corticothalamic axon terminals were studied in the subdivisions of the adult rat medial geniculate body following auditory cortical injections of mixtures of horseradish peroxidase and [3H]leucine. The labeling produced by each method was examined independently, both qualitatively and quantitatively, in adjacent series of tetramethylbenzidine-processed sections and in autoradiographs after 24-96 hour survivals. The distribution and number of labeled cells and axon terminals were assessed separately for each method and compared systematically throughout the rostro-caudal extent of the medial geniculate complex. The principal finding was that zones containing many retrogradely labeled neuronal somata are not completely coextensive with areas of heavy terminal labeling within the medial geniculate body, although there is a gross congruence of thalamocortical-corticothalamic projections. Conversely, we found many zones of autoradiographic silver grains without retrogradely labeled somata in the adjacent sections; in general, the autoradiographic zones of non-reciprocity were more extensive and marked than were retrograde zones of non-reciprocity. The rat medial geniculate complex could be subdivided on the basis of its neuronal organization, cytoarchitecture, fiber architecture, and thalamocortical and corticothalamic connections into three major parts: the ventral, dorsal, and medial divisions. This pattern of organization was comparable, though not identical, to that of the corresponding subdivisions in the cat medial geniculate body (Winer: Adv. Anat. Embryol. Cell Biol. 86:1-98, '85). While the retrograde labeling appeared to mark many of the different types of neurons in each of the three divisions, there were distinct local and quantitative and qualitative differences in the distribution of autoradiographic terminal labeling. The ventral division received the heaviest cortical input, the medial division the least labeling, while the dorsal division was intermediate. Thus, corticogeniculate projections to the ventral division often produced values 20-100 times above background (absolute values: 2,001-10,000 silver grains/14,400 micron2; background: less than 100 silver grains/14,400 micron2); the same projection to the dorsal division usually resulted in grain counts no more than 5-20 times above background (501-2,000/14,400 micron2), while in the medial division the number of silver grains rarely exceeded two to five times the background (201-500/14,400 micron2).(ABSTRACT TRUNCATED AT 400 WORDS)

SII-projecting neurons in the rat thalamus: a single- and double-retrograde-tracing study

Experiments were performed on adult albino rats, using single-labeling (free horseradish peroxidase [HRP] or wheatgerm agglutinin conjugated to HRP [WGA:HRP]) and double-labeling (fluorescent dyes) techniques to investigate the thalamic projections to the secondary somatosensory cortex (SII) and to demonstrate the presence and location of thalamic neurons projecting to both the primary somatosensory cortex (SI) and SII by way of branching axons. In single-labeling experiments, the tracer was injected in SI or SII with or without electrophysiological control; in double-labeling experiments, fast blue and diamidino yellow were injected into the electrophysiologically identified forelimb areas of SI and SII. Single-tracer experiments showed that after injections in SI, focused in the forelimb representation area, retrogradely labeled neurons were present mainly in the ventral third of the nucleus ventralis posterolateralis (VPL) and in the anterior part of the posterior nuclear complex (PO); labeled neurons were also present consistently in the caudal portion of PO. Injection of tracers in the forelimb or forelimb and hindlimb representation areas of SII resulted in labeling of neurons in the posterior part of PO and in the caudal part of VPL. Double-labeling experiments confirmed the distribution of neurons projecting to SI or to SII, as observed in single-labeling experiments. Some neurons labeled with both tracers were also present. These neurons are interpreted as projecting to both SI and SII by means of axon collaterals and were observed in areas of overlap of the two single-labeled population of neurons--that is, at the border between PO and the ventroposterior complex, and in the medial part of caudal PO. Comparison of these data with those obtained after injections of tracers in SI and SII of cats (Spreafico et al., 1981b) suggests that in both species thalamic neurons projecting to these two areas are largely segregated, though partially overlapping; and that thalamic neurons projecting simultaneously to SI and SII, modest in number in cats, are even sparser in rats.

Callosal and ipsilateral cortical connections of the body surface representations in SI and SII of tree shrews

Injections of horseradish peroxidase (HRP) were used to study the connections of the first and second somatosensory areas (SI and SII) in tree shrews. The locations of callosally projecting neurons in SI were determined by placing large injections of HRP in the SI region of one cerebral hemisphere and determining the organization of SI of the other cerebral hemisphere with microelectrode mapping. Many callosally projecting neurons were revealed in lateral SI representing the face, especially the glabrous nose. A sparse scattering of callosally projecting neurons were located more centrally in SI in portions representing the forepaw; these neurons tended to be in cortex devoted to the dorsal hand and pads of the palm rather than the digits. Part of medial SI, representing the forelimb and trunk, had a moderately dense distribution of callosally projecting neurons. More restricted injections in SI indicated that callosally projecting neurons were largely within comparable portions of contralateral SI, although a few neurons projecting callosally to SI were located in SII and cortex caudal and rostral to SI. Large injections of HRP in SII labeled neurons throughout contralateral SII, including representations of the forepaw and hindpaw. More restricted injections in SII labeled neurons in somatotopically comparable parts of the contralateral SII. A few labeled neurons were also seen in somatotopically matched parts of contralateral SI. The results also demonstrated strong somatotopically organized connections between SI and SII of the same hemisphere, and connections of SI and SII with adjoining subdivisions of parietal and frontal cortex. The major thalamic projections to both SI and SII originated in the ventroposterior nucleus.

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.

Second somatic sensory cortical area (SII) in a prosimian primate, Galago crassicaudatus

The cortex adjacent to and along the upper bank of the lateral sulcus (UB-LS) of a prosimian primate, Galago crassicaudatus, was explored to determine the topographical representation of low-threshold cutaneous inputs to this region. The somatic sensory projections to this cortex were considered homologous to those defined in other species as the second somatosensory cortical area (SII). Multiple and single neuron recordings were obtained with tungsten microelectrodes in animals anesthetized with sodium pentobarbital or ketamine hydrochloride; receptive fields were determined by means of manually applied tactile stimuli. The area of SII was located approximately 1-1.5 mm rostral to the posterior limit of LS, extended rostrally approximately 4 mm, and occupied nearly all of the upper bank of the sulcus throughout this region. Receptive fields (RFs) in SII were primarily contralateral except for some bilateral input in the cortex representing portions of the trunk, head, and face. The boundaries of RFs were well defined, especially where recordings were located in the middle layers of the cortex. The distribution of RFs across SII was somatotopically organized into a single, relatively erect representation of the body that involved inputs from the face rostral and medial (superficially along the UB-LS) surrounding an enlarged forelimb area; the latter, in turn, lies rostral and medial to the hindlimb zone. Projections from the tail and sacrum are located furthest caudal and lateral (deeper along UB-LS). Separate regions that were devoted to the glabrous skin surfaces of the distal limbs formed the rostral and lateral boundaries of the distal fore- and hindlimb representations, respectively. In the zone for the glabrous surfaces of the forelimb digits, individual digits dominated discrete components of the SII map, especially medially where digit 1 was represented. The glabrous tip of digit 5 was represented caudal and lateral to the tip of digit 1. A similar radial to ulnar medial to lateral sequence was noted in the area representing the palm. Except for a possible medially located toe 1 zone in the hindlimb representation, separated representations for the glabrous skin of individual toes were not noted. The dorsal hairy surfaces of the digits and toes were, respectively, amalgamated within the representations for the dorsal surfaces of the hand and foot. In these regions, which were found superficial and slightly caudal to their respective glabrous zones, some RFs were found that were devoted only to the distal extremities, but most RFs included more proximal portions of the hand or foot dorsum.(ABSTRACT TRUNCATED AT 400 WORDS)

Non-overlapping thalamocortical projections for separate forepaw digits before and after cortical reorganization in the raccoon

The fluorescent dye retrograde tracing technique was used to examine the projection from ventroposterior lateral thalamus to primary somatosensory cortex in the raccoon. Results are presented from fifteen experiments using Fast Blue in combination with either Nuclear Yellow or Diamidino Yellow Dihydrochloride. Injections of two dyes into adjacent but separate digit regions of cortex produced single-labeling of thalamic neurons, suggesting that projections for each digit are separate and independent with no branching to adjacent parts of the somatotopic map. Double-labeling was only seen when the cortical injection sites were overlapping. Similar results were seen in six cases in which amputation of the 5th digit of the contralateral forepaw was carried out four months earlier. These results suggest that neither the strengthening of existing collaterals nor growth of new collaterals in the thalamocortical projection are likely to be responsible for the physiological reorganization that is seen in raccoon cortex after peripheral deafferentation.

The distribution and topographical organization in the thalamus of anterogradely-transported horseradish peroxidase after spinal injections in cat and raccoon

The distribution of anterogradely-transported horseradish peroxidase (HRP) was examined in the rostral mesencephalon and thalamus of cats and raccoons that had received injections of HRP in the cervical and/or lumbosacral enlargements of the spinal cord. Labeling was consistently observed in a large number of loci. All regions previously identified as targets of spinomesencephalic or spinothalamic fibers were included. Evidence of topographical organization was obtained in several regions. Adjacent fields of labeling were often separable on the basis of the distribution, appearance and topographical organization of the labeling. Subject to the methodological constraints imposed by the possibilities of transneuronal and/or collateral labeling, we conclude that a wide variety of loci in the thalamus receive direct spinal input. The organization of these projections suggests that each terminal region may be associated with different aspects of spinal cord function.

Somatosensory thalamocortical connections in the raccoon: an HRP study

In the North American raccoon (Procyon lotor), representations of the glabrous surfaces of the hand digits are found within separate subnuclei of the thalamic ventrobasal complex (VB) and on separate subgyri of the somatosensory cortex (SmI). In the present study, the retrograde transport of horseradish peroxidase from SmI to VB was utilized to study relationships between physiologically identified cortical subgyri and somatotopically corresponding thalamic subnuclei. Single large or multiple small injections confined to a single gyral crown led to retrograde labeling of large groupings of cells filling the entire VB subnucleus for the appropriate digit. In the aggregate, the regions of label appeared as thin, wedge-shaped sheets extended in the dorsoventral and anteroposterior dimensions, but flattened mediolaterally, and curving to form a laterally directed convexity; these appear to correspond to the lamellae of monkey VB described by others. These large injections led to labeling of approximately 80% of all large (18-30-micron diameter) cells within the lamella. Single, small, focal injections of a gyral crown led to variable amounts of labeling, ranging from an entire digital lamella to only a small focal cluster of cells. No evidence was obtained for the existence of anteroposteriorly extending "rods" of cells, as reported in primates. Finally, there was a sparse, but consistent labeling of cells of the posterior nuclear group (Po) following gyral crown injections. These results are in agreement with expectations based on prior electrophysiological studies of raccoon VB and SmI, as well as prior anatomical studies of thalamocortical relationships.

Callosal mechanism for the interhemispheric transfer of hand somatosensory information in the monkey

The retrograde transport of horseradish peroxidase (HRP) was combined with extracellular microelectrode recording from single and multiple-neurones to study the anatomical and functional organization of the callosal connections of the hand sensory projection field in the parietal operculum of monkeys (Macaca Irus). In 3 animals anaesthetized with ketamine, a single injection of HRP (0.5 microliter) was delivered into the cortex forming the upper bank of the sylvian sulcus at a site where neuronal responses to somatic sensory stimulation of the hand were recorded. In the ipsilateral hemisphere, retrogradely HRP-labelled cells were found in the cortex of the post-central gyrus and in the thalamic nuclei ventralis posteroinferior and pulvinar oralis. In the contralateral hemisphere HRP-labelled neurones were present in the opercular cortex lying dorsal, and slightly caudal, to the posterior pole of the insula. Few scattered callosal neurones were also found in the post-central gyrus. In 3 other animals, multiple injections (5-8; 0.5 microliter each) of HRP were performed in the parietal operculum. In the ipsilateral hemisphere, retrogradely labelled cells were present in the post-central gyrus and in the following thalamic nuclei: ventralis posteroinferior, pulvinar oralis and medialis, ventralis posteromedialis and posterior complex. Few labelled cells were also present in the ventral part of the nucleus ventralis posterolateralis. In the contralateral hemisphere, numerous callosal cells were labelled with HRP. These cells were found, with regional variations in density, in wide regions of the buried and exposed cortex of the parietal operculum and in the post-central gyrus. These 3 monkeys were subjected to microelectrode mapping experiments (N2O and halothane anaesthesia) to explore the peripheral receptive fields of neurones in the parietal operculum and post-central gyrus contralateral to the injected side. HRP labelled callosal neurones were found in regions of the second and first somatosensory cortical areas which also contained units driven from the contralateral hand.

The terminations of the spinothalamic tract in the cat

The termination sites of the spinothalamic tract (STT) in the cat have been examined using the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP). Cats which received an injection of WGA-HRP at the spinal cervical level (C7-C8) displayed anterograde labelling in the ventral posterior lateral (VPL), ventrolateral, lateral posterior, limitans, suprageniculate, medial geniculate pars magnocellularis, central lateral, paracentral, centrum medianum, submedius, central medial, reuniens, rhomboid and periventricular thalamic nuclei. Lumbar injection (L2-L3) produced a similar although less intense pattern of labelling, with only the VPL displaying an obvious topological organization. Although the STT terminations in the cat appear to be less dense than those observed in the rat or monkey the present results demonstrate that in the cat a wide variety of thalamic nuclei receive a direct STT projection and that the cat VPL, like rat, monkey and man, receives a direct STT input.