First somatosensory cortical columns and associated neuronal clusters of nucleus ventralis posterolateralis of the cat: An anatomical demonstration

Microlesions (30–275 microns in diameter) were placed in VLPm of the cat and the terminal axonal degeneration in SI cortex was stained using the Fink-Heimer I technique. Following each of these microlesions, small, localized patches or subcolumns of degeneration, relativly light in density, were observed within laminae IIIb and IV of SI when viewed in the coronal plane. In addition, a few degenerating fibers ascended to lamina I. These multiple subcolumns had distinct radial boundaries and were narrow in the mediolateral plane (80–120 microns in width) but elongated rostro-caudally (2500–3000 microns in length). Localized patches of degeneration were separated at their widest points by a distance of 500 microns medio–laterally, but at various rostro–caudal levels of SI were observed to merge into larger columns of degeneration (250–400 microns) and then separate again into smaller multiple patches (i.e., a “zebra-like” pattern). Small injections of HRP into the forelimb region of area 3b or rostral area 1–2 of SI resulted in the labeling of small, discrete clusters of neurons in the ventral regions of VPLm. The clusters examined ranged in size from 140–350 microns in medio–lateral diameter and were elongated rostro–caudally (up to 500 microns in extent); virtually all cells within a cluster appeared labeled, but not equally so. A pattern of HRP labeling different from that observed following area 3b and rostral area 1–2 injections was observed following injections into more caudal regions of area 1–2 and into SII cortex. The labeling that resulted from these injections was not in the form of neuronal clusters but instead labeled cells tended to be scattered in more dorsal regions of VPLm. This scattering did not appear to be random since the labeled neurons were grouped within the same general area of VPLm. Labeling was distributed throughout a number of cell clusters, comprising only a small proportion of cells within each cluster. The pattern of labeling seen after caudal area 1–2 and SII injections differed only in its rostro–caudal extent within VPLm. SII injections generally resulted in labeling along the full rostro–caudal dimension of VPLm. A differential organization of the anatomy of thalamocortical projections to the various subdivisions of SI and to SII was noted in this study. It is postulated that the multiple, discrete patches of degeneration in laminae IIIb and IV of SI represent a portion of the somatosensory cortical columns and that the HRP-labeled clusters seen in VPLm following area 3b and rostral area 1–2 injections are the subcortical equivalents of these subcolumns.     

Connections of the fronto-parietal operculum and the postcentral gyrus with the posterior ventral thalamic nucleus, especially its medial nucleus, in monkeys

In 22 crab-eating monkeys (Macaca irus), either lesions or HRP injections were made in the frontol parietal operculum and, for the purpose of contrast, in the postcentral gyrus to observe their fiber connectionl with the posterior ventral nucleus (VP). The exposed opercular part, reported to consist of areas 3b, 1, and 2 showed scanty fiber connections with VP. The buried parietal operculum had connections with the media (VPM) and lateral (VPL) nuclei of VP, corresponding to the second somatic sensory cortex (SII). This cortica area was connected, in VPM, to its ventromedial part, while the base of the postcentral gyrus, the ordinary face area of SI, was connected to its lateral-most part. This suggests that the ventromedial part of VPM is more intimately related to SII, and its lateral-most part to SI. In contrast, the SI and SII areas of VPL overlapped more extensively with each other. The cortical area which received fibers from the parvocellular part of VPM (VPMpc) lay in the frontal operculum, especially in its buried part. This area extended caudally beyond the precentral dimple and included areas 1 and 2 of the buried frontal operculum, indicating that its posterior extent was wider than the pure taste cortical area. Spinothalamic terminations in VPL and the PO nuclear group were discussed in relation to cortical connections of these thalamic nuclei. It was suggested that spinothalamic input could be relayed to both postcentral and opercular cortices. The relation of the spinothalamic input to the PO group appeared to be minor.     

Quantitative comparisons of corticothalamic topography within the ventrobasal complex and the posterior nucleus of the rodent thalamus

To compare the topographic precision of corticothalamic projections to the ventrobasal (VB) complex and the medial part of the posterior (POm) complex, different anterograde tracers were placed in neighboring parts of the primary (SI) and secondary (SII) somatosensory cortical areas. The location of labeled corticothalamic terminals and their beaded varicosities were plotted, and the digital reconstructions were analyzed quantitatively to determine the extent of overlapping projections from the cortical injection sites. Among animals that received all tracer injections in SI cortex, tracer overlap in the thalamus varied according to the proximity of the cortical injection sites. Regardless of which combination of somatic representations were injected in SI, within each animal the amount of tracer overlap in POm was similar to that observed in VB, and a matched-sample statistical analysis failed to reveal significant differences in the proportion of the labeled regions that contained overlapping projections from the injected cortical sites. Among those animals in which the tracers were injected into the whisker representations of SI and SII, the amount of tracer overlap in the thalamus was not affected by the proximity of the cortical injection sites. Instead, tracer overlap appeared to be related to the degree of somatotopic correspondence. Furthermore, within each of these animals, the amount of tracer overlap in POm was similar to that found in the VB complex. These results indicate that POm has a well-defined topographic organization that is comparable to the degree of topography observed in the VB complex.     

Connections of areas 3b and 1 of the parietal somatosensory strip with the ventroposterior nucleus in the owl monkey (Aotus trivirgatus).

Anatomical tracers were injected into electrophysiologically defined sites in somatosensory cortical Area 3b (SI proper) and Area I (posterior cutaneous field) of owl monkeys after these cortical subdivisions had been extensively explored in microelectrode mapping experiments. These mapping experiments revealed that both Areas 3b and 1 contain complete and separate representations of the body surface (Merzenich et al., '78). Restricted injections of the retrograde tracer, horseradish peroxidase (HRP), into either Area 3b or Area 1 labeled neurons within a band of cells in the ventroposterior nucleus (VP). The location of the labeled band in VP varied with the location of the injection site in both representations, and the labeled region of VP was overlapping for injections in corresponding body parts in the two representations. Neurons projecting to the hand and foot cortical representations were in architectonically identified subnuclei. Because injections into either Area 3b or Area 1 labeled over half of the neurons in the appropriate regions of VP, it appears that some neurons in VP project to both cortical representations. Finally, injections of HRP combined with the anterograde tracer, 3H-proline, indicate that VP neurons are reciprocally interconnected with both Areas 3b and 1.     

Glutamate-positive corticocortical neurons in the somatic sensory areas I and II of cats

Combined retrograde transport-immunocytochemical experiments were carried out on cats to study the morphology, laminar distribution, and percentages of corticocortical projecting neurons of somatosensory area I (SI) and II (SII) showing immunoreactivity to an antiserum raised against the amino acid glutamate (Glu). A previously characterized anti-Glu serum (Conti et al., 1987a, b; Hepler et al., 1987) was used in conjunction with HRP. This tracer was injected either in SI to label retrogradely neurons in ipsilateral SII (SII-SI association neurons) and contralateral SI (SI-SI callosal neurons) or in SII to label retrogradely neurons in ipsilateral SI (SI-SII association neurons) and contralateral SII (SII-SII callosal neurons). In sections from SI and SII processed for simultaneous visualization of Glu and HRP (Bowker et al., 1982), and containing the cells from which every one of the 4 corticocortical projections arise, 3 types of labeled neurons were observed: (1) single-labeled neurons showing the homogeneous brown immunoreaction product of Glu (Glu-positive neurons); (2) single-labeled neurons containing the granular black reaction product of retrogradely transported HRP (Glu-negative, association or callosal neurons); and (3) double-labeled neurons in which both the black HRP granules and the brown immunostaining were present (Glu-positive, association or callosal neurons). Double-labeled neurons were all pyramidal in shape and were distributed intermingled with Glu-negative corticocortical neurons in all layers of SI and SII known to give rise to association and callosal projections. Counts from 25-micron-thick sections showed that of 432 association and callosal neurons sampled from SI and SII, 214 (49.5%) were Glu-negative and 218 (50.5%) Glu-positive. In counts carried out on 5-micron-thick sections, the percentage of Glu-positive corticocortical neurons raised to about 70%. The 2 populations of single- and double-labeled corticocortical neurons showed no difference in their perikaryal cross-sectional areas. The present results show that a large fraction of association and callosal neurons of SI and SII are immunoreactive for Glu, and, therefore, these neurons probably use this excitatory amino acid, or a closely related compound, as neurotransmitter.     

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.     

Connections between the thalamus and the somatosensory areas of the anterior ectosylvian gyrus in the cat

The thalamocortical and corticothalamic connections of the second somatic sensory area (SII) and adjacent cortical areas in the cat were studied with anterograde and retrograde tracers. Injections consisted of horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA) or a mixture of equal parts of tritiated leucine and proline. The cortical regions to be injected were electrophysiologically studied with microelectrodes to determine the localization of the selected components of the body representation in SII. The distribution of recording points was correlated in each case with the extent of the injection mass in the cortex. Distributions of retrograde and anterograde labeling in the thalamus were reconstructed from serial coronal sections. The results from cases with injections of tracers exclusively confined to separate parts of the body map in SII indicated a fairly precise topographical organization of projections from the ventrobasal complex (VB) to SII. The labeled cells and fibers were located within a series of lamella-like rods that curved throughout the dorsoventral and rostrocaudal axis of VB. The position and extent of these lamellae shifted from medial and ventral, in the medial subdivision of ventral posterior lateral nucleus (VPLm) for radial forelimb digit zones of SII, to dorsal, Posterior, and lateral, in the lateral subdivision of ventral posterior lateral nucleus (VPLl) for proximal leg and trunk regions in SII. For every injected area in SII the densest clustering of labeled cells and fibers was usually more posteriorly represented in VB. The distribution in these dense zones of labeling often extended through the central core of VB. SII projecting neurons were also consistently noted in the extreme rostral portion of the medial subdivision of the posterior nuclei (Pom) that lies dorsal to VB. Corticothalamic and thalamocortical connections for SII Were entirely reciprocal. Injections of tracers into cortical areas surrounding SII labeled other parts of the posterior complex but failed to label any part of VB except when the injection mass also diffused into SII. Injections into the somatic sensory cortex located lateral to SII, within the lips and depth of the upper bank of the anterior ectosylvian sulcus (AES), heavily labeled the central and posterior portions of Pom. Substantial labeling was noted in the lateral (Pol) and intermediate (Poi) divisions of Po only when the injections involved some part of the auditory area that occupies the most posterior part of the AEG and both banks of the immediately adjoining AES. The magnocellular nucleus of the medial geniculate (MGmc) was labeled only when some part of the auditory cortex was injected. The suprageniculate nucleus (SG) was labeled from the insula and lower bank of the AES. These results indicated that medial (rostral and caudal Pom) and lateral components (Poi, Pol, MGmc) of the Posterior complex have separate cortical projection zones to somatic sensory and auditory cortical regions, respectively. SIV and the lateral extent of area 5a located in the medial bank of the anterior suprasylvian sulcus sent projections to the deep layers of the supe- rior colliculus and the ventrolateral periaqueductal gray. No cortico-tectal projections were seen from SII.     

Long-lasting potentiation of field potentials in primary and secondary somatosensory cortex

Effects of tetanic bursts (200 Hz, 10 pulses) on field potentials elicited by ventral posterolateral thalamic nucleus (VPL) stimulation were investigated in the feline somatosensory cortex. In the first experiments, field potentials elicited by VPL stimulation (test pulse) were simultaneously recorded in the primary (SI) and the secondary (SII) somatosensory cortex in six animals. Potentiation of field potentials recorded in SII was induced by tetanic stimulation of VPL in all six animals, whereas the same tetanic bursts failed to produce significant changes in SI in four of the six animals. The results suggest that plastic changes in somatosensory processing take place in SII rather than SI. In subsequent experiments, features of the potentiation observed in SII were examined in 20 animals. The field potentials were simultaneously recorded at 16 points placed vertically at 150-μm intervals from the cortical surface. The potentiation of field potentials (to 110–170% of control values) observed at depths between 600 and 1350 μm lasted more than 90 min after tetanic stimulation. Poststimulus histograms of multiple-unit activities revealed a long-lasting increase in the number of unit discharges evoked by VPL stimulation. This change in the number of activated cellsis regarded as a cause of potentiation of SII field potentials. In the last session, the effects of N-methyl-d-aspartate (NMDA) receptor antagonists on the potentiation of SII field potentials were investigated. Cortical intraventricular injection ofd-2-amino-5-phosphonovalerate (APV) anddl-2-amino-7-phosphonoheptanoic acid (APH) prevented induction of the potentiation in SII. NMDA receptor activation participates in forming this SII potentiation.     

Barbiturate spindle activity in the thalamic lateral ventro-posterior nucleus and the second somato-sensory area of the cortex.

(1) Barbiturate spindles recorded from the second somato-sensory cortical area (SII) were similar to spindles in the primary somato-sensory area (SI) both with respect to incidence, duration of each spindle and per cent spindle time. The spindle wave amplitude was smaller in SII. The highest spindle wave amplitude was observed in the anterior part of SII which receives input from nucleus ventralis postero-lateralis (VPL). No spindle activity was observed in the posterior part of SII which receives input from the posterior nuclear group (PO) of the thalamus. (2) Barbiturate spindles recorded from a locus in VPL and its projection area in SII were cross-correlated. The analysis resulted in high cross-correlation factors, indicating that a considerable degree of spindle wave synchrony existed between the spindles. This wave synchrony was reduced by moving the cortical electrode a short distance. (3) Cortical spindles recorded from corresponding sites in SI and SII were cross-correlated, and gave a high cross-correlation coefficient. This synchrony was markedly reduced if one of the electrodes was moved a few millimetres away from the optimal point. (4) Spindles started simultaneously in corresponding sites of SI and SII. A high degree of coincidence was found also between spindles in a VPL locus and the corresponding projection site in SII. Local anaesthesia applied to or total removal of SI failed to influence the spindle activity in SII and vice versa. Similarly, the SI-SII synchrony survived a deep incision cutting all connections between the two areas. (6) It is suggested that spindles in corresponding sites of SII and SI have a common thalamic pacemaker which probably projects to both areas by axonal branching.     

Overlapping corticostriatal projections from the rodent vibrissal representations in primary and secondary somatosensory cortex

To determine whether the neostriatum receives overlapping projections from two somatosensory cortical areas, the anterograde tracers biotinylated dextran amine (BDA) and fluoro-ruby (FR) were injected into the whisker representations of primary (SI) and secondary (SII) somatosensory cortex. Reconstructions of labeled terminals and their beaded varicosities in the neostriatum and thalamus were analyzed quantitatively to compare the extent of overlapping projections to both subcortical structures. Corticostriatal projections from focal sites in both somatosensory areas exhibited substantial amounts of divergence within the dorsolateral neostriatum. Most of the labeled terminals were concentrated in densely packed arborizations that occupied lamellar-shaped regions along the dorsolateral edge of the neostriatum. Tracer injections in both cortical areas also produced dense anterograde and retrograde labeling in the thalamus, especially in the ventrobasal complex (VB) and in the medial part of the posterior (POm) nucleus. Because these thalamic regions are topographically organized and have reciprocal connections with corresponding representations in both SI and SII, the amount of labeled overlap in the thalamus was used to indicate the degree of somatotopic correspondence at the SI and SII injection sites. We found that the proportion of overlapping projections to the neostriatum was moderately correlated with the amount of overlap observed in the thalamus. This result strongly indicates that specific sites in the dorsolateral neostriatum receive convergent projections from corresponding somatotopic representations in SI and SII, but also suggests that some of the corticostriatal divergence may reflect neostriatal integration of somatosensory information from noncorresponding representations in SI and SII.     

Overlapping corticostriatal projections from the rodent vibrissal representations in primary and secondary somatosensory cortex

To determine whether the neostriatum receives overlapping projections from two somatosensory cortical areas, the anterograde tracers biotinylated dextran amine (BDA) and fluoro-ruby (FR) were injected into the whisker representations of primary (SI) and secondary (SII) somatosensory cortex. Reconstructions of labeled terminals and their beaded varicosities in the neostriatum and thalamus were analyzed quantitatively to compare the extent of overlapping projections to both subcortical structures. Corticostriatal projections from focal sites in both somatosensory areas exhibited substantial amounts of divergence within the dorsolateral neostriatum. Most of the labeled terminals were concentrated in densely packed arborizations that occupied lamellar-shaped regions along the dorsolateral edge of the neostriatum. Tracer injections in both cortical areas also produced dense anterograde and retrograde labeling in the thalamus, especially in the ventrobasal complex (VB) and in the medial part of the posterior (POm) nucleus. Because these thalamic regions are topographically organized and have reciprocal connections with corresponding representations in both SI and SII, the amount of labeled overlap in the thalamus was used to indicate the degree of somatotopic correspondence at the SI and SII injection sites. We found that the proportion of overlapping projections to the neostriatum was moderately correlated with the amount of overlap observed in the thalamus. This result strongly indicates that specific sites in the dorsolateral neostriatum receive convergent projections from corresponding somatotopic representations in SI and SII, but also suggests that some of the corticostriatal divergence may reflect neostriatal integration of somatosensory information from noncorresponding representations in SI and SII.     

Corticofugal influence upon cat thalamic ventrobasal complex

The influence of somatosensory cortex upon transmission through its specific thalamic relay nucleus, the ventrobasal complex (VB), was studied in the paralyzed, unanesthetized cat. The medial lemniscus was electrically stimulated, and evoked responses were recorded from the thalamic radiations projecting respectively to the first and second somatosensory cortex (SITR) and SIITR) and from the pial surface of SI and SII. Cortical influence was assessed by cooling so as to produce a functional and reversible ablation. This technique avoided the ambiguity usually associated with direct electrical stimulation of cortex. Such stimulation, as used by several other authors, may lead to uncontrolled transsynaptic effects upon VB neurons via antidromic activation of thalamocortical fibers and resultant invasion of VB recurrent collaterals.Cooling of SI and SII together resulted in greatly augmented evoked activity in thalamocortical projection fibers concurrent with cessation of cortical EEG at an intracortical temperature of 21 °C. This is interpreted to mean that under normal conditions the somatosensory cortex exhibits a net tonic inhibitory influence upon VB transmission. The same results were obtained in thecerveau isolé preparation; thus, the net cortical inhibitory influence could not be mediated by the brain stem reticular formation, but must be a direct corticofugal influence exerted upon VB. Antidromic activation of ML terminals in VB was unaltered by cooling of somatosensory cortex. This suggests that the corticofugal inhibition is mediated via a postsynaptic mechanism, rather than a presynaptic one.Cooling SI alone resulted in increased responses in SITR but not in SIITR. On the other hand, separate cooling of SII resulted in increased responses in both SITR and SIITR. This suggests that each somatosensory receiving area exerts inhibitory control over its own thalamic input but that, in addition, SII exerts control over SI input.     

Mechanical vibratory stimulation of feline forepaw skin induces long-lasting potentiation in the secondary somatosensory cortex

We investigated the long-lasting effects of mechanical vibratory stimulation of the skin on the excitability of feline cortical neurons in the forelimb areas of the primary (SI) and secondary (SII) somatosensory cortices. Conditioning mechanical stimuli were 300 bursts of 10 pulses at 200 Hz delivered with a 10-s interburst interval from a mechanical stimulator. Test field potentials and unit discharges were evoked by electrical stimulation to the ventral posterolateral thalamic nucleus (VPL) or by single mechanical stimuli applied to the skin. In SII, the mechanical burst stimulation to the skin increased the amplitudes of field potentials and the frequency of unit discharges elicited by single mechanical stimuli applied to the skin. The vibratory conditioning stimulus also produced a similar potentiation of the VPL-evoked field potentials (126-139% increase in amplitude, P < 0.05) with an associated increase in firing rates of extracellularly recorded neuronal activity (117%, P < 0.001). These potentiations persisted through the entire experimental period of 120 min. The translaminar current source density analysis calculated from the VPL-evoked field potentials increased to 127% of the control value (P < 0.01). In contrast, in SI we observed no significant changes in the field potential amplitudes or in the currents generated in superficial layers (91-117%). Taken together with the previous finding that tetanic electrical stimulation of VPL induces long-lasting potentiation of the VPL-evoked cortical responses in SII but not of those in SI, the present results suggest that SII has a large capacity for the rapid functional plasticity involved in the learning that occurs during repeated tactile experiences.     

Thalamic connections of two functional subdivisions of the somatosensory forepaw cerebral cortex of the raccoon

The purpose of this study was to compare the thalamic interconnectivities of 2 functionally distinct subdivisions of the somatosensory (Sml) forepaw cortex of the raccoon--the somatotopic subdivision representing the glabrous skin of the digits and the more heterogeneous subdivision representing the hairy skin and claws. Injections of HRP were made into one or the other functional subdivision of a specific digit subgyrus of Sml cortex in 10 adult raccoons. The distribution of HRP-labeled neurons and axon terminals in the thalamus revealed that the 2 sectors have different patterns of thalamic projections. The glabrous skin region of each cortical digit zone was interconnected with a specific crescent-shaped lamella of neurons that extended rostrocaudally through the ventral posterior lateral (VPL) nucleus and typically was separated from adjacent lamellae by small bundles of myelinated fibers. The VPL lamellae constituted relatively distinct digit subnuclei that were connected somatotopically with the glabrous subdivisions of the corresponding cortical digit areas. The projections were dense, topographic, and reciprocal; labeled neurons and axon terminals within a particular lamella overlapped considerably and tended to be arranged in clusters. In contrast, the heterogeneous region of each cortical digit zone was reciprocally connected with the somatotopically appropriate VPL digit subnucleus and with adjoining subnuclei as well. The projections were comparatively sparse, less topographic, and more widely distributed than those of the glabrous skin sectors; groups of HRP-positive neurons and terminals in VPL tended to straddle the borders of the appropriate lamella and extended into adjacent lamellae. Furthermore, small clusters of labeling were found in the dorsal, presumed kinesthetic region of VPL and in portions of the ventral posterior inferior nucleus and the posterior nucleus. These results indicate that the glabrous cortical subdivisions have precise, somatotopically organized connections with specific VPL subnuclei, whereas the heterogeneous cortical subdivisions have more diffuse and scattered connections with several subregions of VPL and other thalamic nuclei as well. These 2 thalamocortical projection patterns may account for many of the differing functional properties of neurons residing within the 2 cortical sectors. Finally, the convergent thalamic projections to the heterogeneous cortical regions could contribute, at least indirectly, to the functional reactivation that occurs within Sml cortex of the raccoon following peripheral nerve transection (Kelahan and Doetsch, 1984).     

Spinothalamocortical projections to the secondary somatosensory cortex (SII) in squirrel monkey

Anterograde labeling of the cervical spinothalamic tract was combined with retrograde labeling of thalamocortical cells projecting to the hand region of the second somatosensory cortex (hSII) to identify likely sites in the thalamus for processing and transmitting nociceptive information to hSII. Anterograde labeling of terminals was done with 2% WGA-HRP injections in the cervical enlargement; thalamocortical cells were retrogradely labeled with fluorescent tracers. In one experiment, the contralateral primary somatosensory cortex hand region (hSI) was injected to provide a direct comparison with hSII thalamic label. Both labeled cells and terminal-like structures were visualized in single thalamic sections and their numbers and positions quantitatively analyzed. The number of labeled cells within 100 microns from the STT terminals were counted as overlapping cells. Four thalamic nuclei, ventroposterior inferior (VPI), ventroposterior lateral (VPL), posterior nucleus (PO) and centrolateral nucleus (CL) combined to contain 86.5% of all hSII-projecting overlapping cells. Of all hSII-projecting thalamic overlapping cells, VPI contained the largest number (36.4% of the total) followed by the anterior portion of the posterior nuclear complex (POa; 20.4%), VPL (18.3%) and CL (11.4%). Results of the hSI injection show a different pattern of overlap in agreement with our earlier study. The relative distribution of overlapping cells was dependent on the antero-posterior position of the SII injections. The most anterior injections resulted in small numbers of labeled cells, with the majority of overlapping cells located in PO and CL. The more posterior injections resulted in overlapping cells mainly in VPI and VPL. The results indicate that, in the squirrel monkey, VPI, VPL, POa and CL relay nociceptive information from the spinal cord to the second somatosensory cortex.     

The organization of thalamocortical relay neurons in the rat ventrobasal complex studied by the retrograde transport of horseradish peroxidase.

The organization of thalamocortical relay neurons in the thalamic ventrobasal complex (VB) of the rat was investigated by the use of the retrograde axonal transport of horseradish peroxidase (HRP). Injections of HRP into somatosensory cortex (SI) resulted in a distinctive gradient of neuronal and non-neuronal HRP reaction product. Electrophysiologically characterized points of SI injected with small volumes of HRP labeled a sector of neurons in VB ipsilateral to the injection. This zone of labeled neurons consisted of a complex curvilinear array or lamina with a rostral hollow or solid expansion of densely packed HRP positive neurons which continues caudally as a less dense tapering wing. Despite this complex arrangement, an approximate pattern of somatotopy was determined indicating the direction of shifts in peripheral receptive fields moving in any axis of VB. The number of labeled neurons projecting upon a point in SI was also determined. Injections of the cortical vibrissae, face and forepaw representations labeled a greater number of neurons in VB per unit area of cortex than did injections in the hindlimb or body representations. The total number of HRP positive neurons in VB increased proportionately with areal increase of the HRP injections into overlapping cortical representations. The number of HRP positive neurons in the rostral half of the laminae increased almost linearly with the area of injection, while the number of HRP positive neurons in the caudal half of the laminae showed relatively smaller increases.     

D-[3H]aspartate retrograde labelling of callosal and association neurones of somatosensory areas I and II of cats

Experiments were carried out on cats to ascertain whether corticocortical neurones of somatosensory areas I (SI) and II (SII) could be labelled by retrograde axonal transport of D-[3H]aspartate (D-[3H]Asp). This tritiated enantiomer of the amino acid aspartate is (1) taken up selectively by axon terminals of neurones releasing aspartate and/or glutamate as excitatory neurotransmitter, (2) retrogradely transported and accumulated in perikarya, (3) not metabolized, and (4) visualized by autoradiography. A solution of D-[3H]Asp was injected in eight cats in the trunk and forelimb zones of SI (two cats) or in the forelimb zone of SII (six cats). In order to compare the labelling patterns obtained with D-[3H]Asp with those resulting after injection of a nonselective neuronal tracer, horseradish peroxidase (HRP) was delivered mixed with the radioactive tracer in seven of the eight cats. Furthermore, six additional animals received HRP injections in SI (three cats; trunk and forelimb zones) or SII (three cats; forelimb zone). D-[3H]Asp retrograde labelling of perikarya was absent from the ipsilateral thalamus of all cats injected with the radioactive tracer but a dense terminal plexus of anterogradely labelled corticothalamic fibres from SI and SII was observed, overlapping the distribution area of thalamocortical neurones retrogradely labelled with HRP from the same areas. D-[3H]Asp-labelled neurones were present in ipsilateral SII (SII-SI association neurones) in cats injected in SI. In these animals a bundle of radioactive fibres was observed in the rostral portion of the corpus callosum entering the contralateral hemisphere. There, neurones retrogradely labelled with silver grains were present in SI (SI-SI callosal neurones). Association and callosal neurones labelled from SI showed a topographical distribution similar to that of neurones retrogradely labelled with HRP. The laminar patterns of corticocortical neurones labelled with D-[3H]Asp or with HRP were also similar, with one exception. In the inner half of layer II, SII-SI association neurones and SI-SI callosal neurones labelled with the radioactive marker were much less numerous than those labelled with HRP. In cats injected in SII, D-[3H]Asp retrogradely labelled cells were present in ipsilateral SI (SI-SII association neurones). Their topographical and laminar distribution overlapped that of neurones labelled with HRP but, as in cats injected in SI, association neurones labelled with silver grains were unusually rare in the inner layer III.(ABSTRACT TRUNCATED AT 400 WORDS)     

The lateral cervical nucleus in the cat: anatomic organization of cervicothalamic neurons.

The morphology of the lateral cervical nucleus (LCN) and the organization of the cervicothalamic projection neurons were studied in cats which had received thalamic injections of horseradish peroxidase (HRP). The boundaries of the LCN were defined following very large thalamic (HRP injections. Roughly 92-97% of LCN cells project contralaterally to thalamus; an additional 1.5% project ipsillaterally. Computer-assisted measurements of perikaryal areas demonstrated that there are two sizes of LCN cells, large (175-900 micrometer 2) and small (less than 175 micrometer 2); the small cells are localized in the medial third of the LCN. LCN cells which are not labeled after large thalamic HRP injections are predomininantly small, medially-located neurons. Small HRP injections into physiologically identified regions of ventroposterior thalamus demonstrated that cervicothalamic neurons are organized in a topography consistent with that observed physiologically in the LCN (Craig and Tapper, '78). Dorsolateral LCN cells are retrogradely labeled from nucleus ventroposterolateralis, pars lateralis (VPL1), ventromedial LCN cells are labeled from pars medialis (VPL m), and a few medial cells are labeled from nucleus ventroposteromedialis (VPM). A few cells in the medial portion of the LCN are also labeled from each part of ventroposterior thalamus. Some interspersion was observed even in the cases with the most well-restricted labeling. We conclude that the LCN maintains a basic somatotographic organization with an inherent variability, certain aspects of which are consistently demonstrable both physiologically and anatomically. Evidence was also obtained suggestive of a rostrocaudal inversion in the cervicothalamic projection. The cervicothalamic projection, the differentiation of the medial LCN subpopulation, and the possible redefinition of the LCN are discussed in light of these results.     

Topography of cortical projections to the dorsolateral neostriatum in rats: multiple overlapping sensorimotor pathways

In rodents, the whisker representation in primary somatosensory (SI) cortex projects to the dorsolateral neostriatum, but the location of these projections has never been characterized with respect to layer IV barrels and their intervening septa. To address this issue, we injected a retrograde tracer into the dorsolateral neostriatum and then reconstructed the location of the labeled corticostriatal neurons with respect to the cytochrome oxidase (CO)-labeled barrels in SI. When the tracer was restricted to a small focal site in the neostriatum, the retrogradely labeled neurons formed elongated strips that were parallel to the curvilinear orientation of layer IV barrel rows. After larger tracer injections, labeled neurons were distributed uniformly across layer V and were aligned with both the barrel and septal compartments. Labeled projections from the contralateral SI barrel cortex, however, were much fewer in number and were disproportionately associated with the septal compartments. A comparison of the labeling patterns in the ipsilateral and contralateral hemispheres revealed symmetric, mirror-image distributions that extended across primary motor cortex (MI) and multiple somatosensory cortical regions, including the secondary somatosensory (SII) cortex, the parietal ventral (PV) and parietal rhinal (PR) areas, and the posteromedial (PM) region. Examination of the thalamus revealed labeled neurons in the intralaminar nuclei, in the medial part of the posterior nucleus (POm), and in the ventrobasal complex. These results indicate that the dorsolateral neostriatum integrates sensorimotor information from multiple sensorimotor representations in the thalamus and cortex.     

Cortical-thalamic relationships in the rat

Single electrolytic lesions were made in the ventrobasal or posterior thalamic nuclear complex of 12 rats with a microelectrode after having first recorded the activity evoked by light tactile stimulation of various body parts. Resultant cortical axonal degeneration was studied with the Fink-Heimer I technique. Lesions in a region of the ventrobasal complex responding to stimulation of contralateral mystacial vibrissae led to somatotopically organized degeneration in the barrel field of the first somatic sensory cortex (SI), i.e., caudoventral vibrissae were represented dorsocaudally in the barrel field while rostrodorsal vibrissae were represented in the rostroventral barrel field. Degenerating axons ascended through deep cortical layers in bundles and terminated densely on the cells of lamina IV, involving neurons within the core and walls of the barrels as well as those in septal areas between barrels. A lesion in a lateral portion of the ventrobasal complex, Emmers' ventrobasal complex SII area, which was responsive to bilateral stroking of the hairs of the rat's dorsum, produced degeneration caudal to and overlapping cortical SI, but not in SII. Lesions of the posterior complex led to axonal degeneration caudal to SI, overlapping both visual and auditory cortices. A lesion in the medial portion of the ventrobasal complex, the thalamic gustatory nucleus, resulted in dense degeneration in a region of cortex dorsal to the rhinal fissure at the rostrocaudal level of the middle cerebral artery. This study answers the question of the cortical target of a portion of Emmers' ventrobasal complex SII region and of the posterior complex and confirms the position of the gustatory area and somatotopic representation of mystacial vibrissae.