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Corticothalamic connections from the second somatosensory area and neighboring regions in the lateral sulcus of macaque monkeys
BrainResearch, 309(1984)367-372 Elsevier
367
BRE20355
Corticothalamic connections from the second somatosensory area and neighboring regions in the lateral sulcus of macaque monkeys H. BURTON
Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, MO 63110 (U.S.A.) (Accepted May 8th, 1984)
Key words: second somatosensory area - - primates - - corticothalamic connections
Corticothalamic connections were shown between the second somatosensory area in primates and the ventroposterior nuclei of the thalamus. These projections were topographically arranged with those from the hindlimb portions of SII traced to the most lateral and posterior parts of the ventroposterior lateral nucleus (VPLc) and those from the forelimb located medially within VPLc. The densest labeling was found ventrally in VPLc and dorsally within ventroposterior inferior n. (VPI) only after injections of the forelimb. A more scattered, dorsal distribution of labeling was seen in the rest of VPLc from injections involving more proximal parts of the body representation in SII. Studies of the interconnections between the thalamus and second somatosensory cortex (SII) in primates have produced conflicting results. According to one set of observations, the ventrobasal complex (as defined by its cutaneous inputs and connections with SI 7) is reciprocally interconnected with SII1,s. Recently, however, Friedman and colleagues 4 suggested that the predominant source of thalamocortical projections to the spinal parts of SII may be the ventroposterior inferior nucleus and not the ventroposterior lateral nucleus. A similar argument regarding VPI has also been made with respect to the projections to SII in the raccoon 5. In contrast, the ventrobasal complex in the cat has reciprocal, precisely topographic connections with SII 2. Consequently, it seemed necessary to review the source of these connections in primates. A primary goal of this re-examination was to determine whether a topographical organization exists in these connections similar to the arrangement that characterizes the distribution of projections to SII in the cat. In these experiments anterogradely transported tritiated amino acids were injected into the cortex of the lateral sulcus following physiological mapping of SII in order to label corticothalamic fibers. This procedure was preferred over
retrograde labeling of thalamocortical cells with H R P in order to avoid possible problems from injecting fibers of passage in the white matter underlying SII and because it was felt that, given the reciprocity between thalamocortical and corticothalamic connections, the responsible thalamic nuclei would be more readily visualized. These experiments were conducted on 7 Macaca fascicularis monkeys that were anesthetized with an initial intraperitoneal injection of 25 mg/kg sodium pentobarbital; this was supplemented as needed to maintain an areflexive status by 5 mg intravenous injections throughout all procedures. The animals were held in a stereotaxic device and the exposed lateral sulcus was explored with glass-coated tungsten or platinum-iridium electrodes for evoked responses to electrical stimulation of the hand or foot. When an active site was located, manually applied tactile stimuli were used to determine the location of the most effective cutaneous receptive field for the cluster of cells in the vicinity of the electrode tract. In these experiments, injections consisted of a cocktail of equal parts of tritiated proline and leucine. The amino acids were dried in a vacuum evaporator, reconstituted in saline to 50/~Ci for each for a total of 100
Correspondence: H. Burton, Dept. of Anatomy and Neurobiology, Washington University School of Medicine, 4566 Scott Avenue, St. Louis, MO 63110, U.S.A. 0006-8993/84/$03.00 (~) 1984 Elsevier Science Publishers B.V.
368
2 mm M31
~
(
face M25
insula
iteral sulcus ( i n t e r i o r lip)
M26
Fig. 1. Distribution of injected tritiated amino acids within the cortex of the lateral sulcus in five hemispheres. The rows are arranged to show the most posteriorly placed single injection of 100 nl in the top row (M31), two 100-nl injections into the fundal region that was centered near the posterior extent of the insula (M22) in the second row, four injections of 400-600 nl each spaced at 1-mm AP intervals along the upper bank of the lateral sulcus posterior to the insula (M24) in the middle row, two injections of 300 nl each spaced 1.5 mm apart along the lateral sulcus that covered the outer face (M25), and one injection of 450 nl that was mostly confined to the inner face of the parietal operculum (M26) in the bottom two rows, respectively. In each row the indicated boundaries of the injection sites were obtained from sections viewed with bright-field illumination on a projection microscope at a magnification of x 15. The AP extent and exposure duration of these injections was as follows: M31, 1.8 mm after 9 weeks; M22, 3.4 mm after 8 weeks; M24, 4 mm after 6 weeks; M25, 3.2 mm after 10.5 weeks; M26, 1.6 mrn after 8 weeks. Inset on lower right shows an expanded and flattened view of the lateral sulcus6 with a diagrammatic indication of the location of the injections taken from planimetric reconstructions in the different cases and a generalized summary of the location of SII in this sulcus.
~Ci//A, l o a d e d i n t o m i c r o p i p e t t e s w i t h tip o p e n i n g s o f
a n i m a l s w e r e r e t u r n e d to t h e i r c a g e s for p o s t - o p e r a -
25-50/~m, and injected hydraulically into the vicinity
tive s u r v i v a l s of o n e w e e k in o r d e r to p e r m i t m a x i -
of t h e f o r e l i m b or t h e h i n d l i m b r e p r e s e n t a t i o n in SII.
m u m a c c u m u l a t i o n o f r a d i o a c t i v i t y in t h e t h a l a m u s
F o l l o w i n g single o r m u l t i p l e i n j e c t i o n s (Fig. 1), t h e
f r o m slow a × o p l a s m i c t r a n s p o r t . S u b s e q u e n t p r o c e -
369 dures involving perfusion of the brains, tissue prepaYations for autoradiography and data collations of the distribution of radioactivity on the autoradiograms have been documented previously1.6. The locations of the injection sites for five of the cases are presented in Fig. 1. In these experiments the injected tracers covered a substantial portion of the parietal operculum starting from the posterior end of the insula. In two cases the injections spread posterior from the forelimb representations on the outer face of the parietal operculum to involve substantially the buried part of area 7 that invades the. lateral sulcus behind SII (M24 and M25); part of the face area of SII was also covered by the injection in M25. In one case (M26) only the distal forelimb digit area on the inner face of the parietal operculum was heavily labeled; in two cases (M22 and M31) the fundal regions were covered by the injections. In M31 the injection primarily affected cortex lying posterior to the foot and tail representations in SII. In this case, however, the tracers spread rostrally along the fundus of the lateral sulcus to just touch on the hindlimb representation. In M22, the injection was placed near the posterior end of the insula, along the superior limiting sulcus, where portions of the trunk and leg representations in SII are located 11. The distribution of anterograde labeling on selected coronal sections through the thalamus from each of these cases has been illustrated in Fig. 2. The results indicated that autoradiographically labeled processes were located in several ventral posterior (VP) nuclei including ventroposterior lateral pars caudalis (VPLc), ventroposterior medial (VPM) and ventroposterior inferior (VPI). In addition, labeling was also seen in central lateral (CL), posterior nucleus (Po), anterior pulvinar nucleus (Pla) and the dorsomedial part of medial geniculate (MG) in some of the experiments. Comparisons between the five cases on the distribution of labeling in the ventroposterior nuclei showed that the region with the densest patches of silver grains differed. In M31, labeling was located furthest lateral in VPLc, while in M25 and M26, it was most intense in more medial parts of VPLc. In M26, in which the injection was particularly delimited within the distal digit zone on the inner face of the parietal operculum, the labeling was located furthest medial along the lateral margin of VPM (Fig. 2Q, R). In these cases involving the forelimb, a
triangular zone of intense labeling extended down to the ventralmost margins of the ventroposterior nuclei to a portion of the thalamus that includes the VPI region (Fig. 2M, N, Q, R). The cells in this region are somewhat smaller than those normally conspicuous in VPLc. However, additional labeling was seen in each of these cases further dorsal in the region of VPLc that has larger cells (Fig. 2 M - O , Q, R). The topography of these projections can be seen by comparing the location of the dense zones of labeling in M24-M26. In M24 the labeling in VPLc was found dorsal, lateral and posterior to VPI (Fig. 21, J); it was, therefore, lateral to the labeling in M25 and M26. The injection in M24, which was medial and posterior to the injections in the other forelimb cases, mostly involved the proximal parts of the arm and hand areas of SII. In contrast, the injection in M25 was placed medial and posterior to that in M26 and so probably involved more proximal parts of the digit and hand representation9,11. The labeling in M25 overlapped with that seen in M26, but did not extend as far medial in VPLc. The injection in M25 also extended further anterior on the outer face of the parietal operculum to the face region of SII; consequently, labeling was noted in the posterior parts of VPM (Fig. 2P). A comparison between the distributions of labeling in M31 vs M22 also indicated different topographies in VPLc. In M31 the labeling in VPLc was particularly confined to the ventrolateral edge of the nucleus (Fig. 2A-C), but in M22 labeling was distributed more medially (Fig. 2 F - H ) , especially in its most posterior extent (Fig. 2H). This portion of VPLc is known to be concerned with the trunk and leg representations of the body, and these are the regions that were probably represented in the vicinity of the injection site in M22. Dense labeling was seen in Po in those cases (M31, M24) in which the injection invaded the retroinsular cortex. Additional labeling in this nucleus in other cases (M26, Fig. 2T) may indicate that SII also sends some projections to Po. However, in some of these experiments the posterior border of VPM was not well demarcated on the thionine-stained sections and some of the labeling situated close to the lateral edge of CM (Fig. 2K, S) could be interpreted as label in the trigeminal relay. In several cases some labeling was distributed
~y~i!
,~
~
~
371 within the anterior pulvinar nucleus (Pla). The most extensive example of this was seen in M24 in which the injection was centered along the posterior portion of SII in the vicinity of the arm and hand region. In this case the tracer clearly spread posterior and medial (Fig. 1) into area 7b as defined previously 6. Traces of labeling in Pla in the other experiments can also be attributed to area 7b since in M24 and M31 some of area 7b was also injected. In M26 the micropipettes passed through area 7b to reach SII and some leakage was seen in the superificial cortex at the point where the penetrations were made. The primary finding of these experiments is that projections from SII to the ventroposterior nuclei are topographically organized. This result confirms similar observations in the cat in which topographically organized thalamocortical and corticothalamic connections were demonstrated 2. It is likely that the thalamocortical connections in the primate are also topographically arranged from the same nuclei 1. The densest projections were traced from the forelimb portions of SII to the most ventral and posterior aspects of VPLc. In this location at more rostral levels, many peri-cellular aggregations of silver grains occupied the dorsolateral segment of a paler, small-celled area that has previously been designated VPI in the monkey (see review in ref. 1). Consequently, the present observations are consistent with reports that SII receives thalamic connections from this nucleus inasmuch as corticothalamic and thalamocortical connections are reciprocal 4,5. Several factors, however, urge some precaution in
concluding that VPI provides the primary relay to SII. First, the present experiments have shown that labeling occurred in other parts of VPLc in cases involving the forelimb area of SII and also in cases where VPI was not labeled at all. The locations of the labeling in these different cases varied according to the segments of the SII body representation affected by the injections. In addition, in a previous study, a similar localization of labeled cells was seen in the middle of VPLc following an injection of H R P into the hand and arm area of SII as was shown here for M24 and M253. The projections to the more central parts of VPLc are more scattered than those to the more ventral, distal limb areas of the thalamus. A similar scattering of SII's connections has been described in the cat10. Consequently, in cases of small injections that were nearly confined to the distal limb representation, it would be likely that only the most ventral portions of VPLc and VPI contained label as in M26 of the present study. Projections to the remainder of VPLc can be seen when other components of the SII representation are covered by the injected tracers. We greatly appreciated Dr. A. D. Craig's participation in some of the initial recordings from SII, Dr. E. G. Jones' review of some of the labeling distributions in the thalamus, and M. Kopf's technical assistance. We are also grateful to R. Freund for his photographic assistance and J. Hoffmann for typing. This study was supported by funds from N I H Grant NS 09809.
Fig. 2. Plots of the distribution of labeling in the thalami of the cases whose injection sites have been shown in Fig. 1. Corresponding rows in the two figures have been matched according to the cases: A-D, M31 (autoradiograms exposed for 9 weeks); E-H, M22 (exposure 6 weeks); I-L, M24 (exposure 6 weeks); M-P, M25 (exposure 7 weeks); Q-T, M26 (exposure 11 weeks). The labeling was plotted with the aid of dark-field illumination and camera lucida. The most anterior sections are shown on the left for each row. Distances between adjacent sections were as follows: 400/~m, A-D, F-H, J-L, M-O, S-T; 600/am, E-F; 800/lm, A-B, O-P, Q-S; 1000/am, I-J. Sections in A - D were plotted at × 30; those in E-T were plotted at × 15. Abbreviations: CL, central lateral n.; CM, centrum median n.; mc, magnoceUular n. of MG; MG, medial geniculate n,; Pla, anterior pulvinar n.; Pli, inferior pulvinar n. ; Plm, medial pulvinar n.; Po, posterior n. ; VMb, ventromedial pars basalis n.; VPI, ventroposterior inferior n.; VPLc, ventroposterior lateral pars caudalis n. ; VPM, ventroposterior medial n.
372 1 Burton, H. and Jones, E. G., The posterior thalamic region and its cortical projectionin new world and old world monkeys, J. comp. Neurol., 168 (1976) 249-302. 2 Burton, H. and Kopf, M., Connections between the thalamus and the somatosensory areas of the anterior ectosylvian gyrus in the cat, J. comp. Neurol., 224 (1984) 173-205. 3 Burton, H. and Robinson, C. J., Organization of the SII pa-. rietal cortex: multiple somatic sensory representation within and near the second somatic sensory area of cynomolgus monkeys. In C. N. Woolsey (Ed.), Multiple Cortical Sensory Areas, Humana Press, Clifton, N J, 1981, pp. 67-119. 4 Friedman, D. P., Murray, E. A. and O'Neill, J. B., Thalamic connectivity of the somatosensory cortical fields of the lateral sulcus of the monkey, Soc. Neurosci. Abstr., 9, Part 2 (1983) 921. 5 Herron, P., The connections of cortical somatosensory areas I and II with separate nuclei in the ventroposterior thalamus in the raccoon, Neuroscience, 8 (1983) 243-257. 6 Jones, E. G. and Burton, H., Areal differences in the laminar distribution of thalamic afferents in cortical fields of the
7
8
9
10
11
insular, parietal and temporal regions of primates, J. comp. Neurol., 168 (1976) 197-248. Jones, E. G. and Friedman, D. P., Projection pattern of functional components of thalamic ventrobasal complex on monkey somatosensory cortex, J. NeurophysioL, 48 (1982) 521-544. Jones, E. G. and Powell, T. P. S., Connections of the somatic sensory cortex of the rhesus monkey. III. Thalamic connections, Brain, 93 (1970) 37-56. Juliano, S. L., Hand, P. J. and Whitsel, B. L., Patterns of metabolic activity in cytoarchitectural area SII and surrounding cortical fields of the monkey, J. Neurophysiol., 50 (1983) 961-980. Kosar, E. and Hand, P. J., First somatosensory cortical columns and associated neuronal clusters of nucleus ventralis posterolateralis of the cat: an anatomical demonstration, J. comp. Neurol., 198 (1981) 515-539. Robinson, C. J. and Burton, H., Somatotopographic organization in the second somatosensory area of M. fascicularis, J. comp. Neurol., 192 (1980) 43-68.
367
BRE20355
Corticothalamic connections from the second somatosensory area and neighboring regions in the lateral sulcus of macaque monkeys H. BURTON
Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, MO 63110 (U.S.A.) (Accepted May 8th, 1984)
Key words: second somatosensory area - - primates - - corticothalamic connections
Corticothalamic connections were shown between the second somatosensory area in primates and the ventroposterior nuclei of the thalamus. These projections were topographically arranged with those from the hindlimb portions of SII traced to the most lateral and posterior parts of the ventroposterior lateral nucleus (VPLc) and those from the forelimb located medially within VPLc. The densest labeling was found ventrally in VPLc and dorsally within ventroposterior inferior n. (VPI) only after injections of the forelimb. A more scattered, dorsal distribution of labeling was seen in the rest of VPLc from injections involving more proximal parts of the body representation in SII. Studies of the interconnections between the thalamus and second somatosensory cortex (SII) in primates have produced conflicting results. According to one set of observations, the ventrobasal complex (as defined by its cutaneous inputs and connections with SI 7) is reciprocally interconnected with SII1,s. Recently, however, Friedman and colleagues 4 suggested that the predominant source of thalamocortical projections to the spinal parts of SII may be the ventroposterior inferior nucleus and not the ventroposterior lateral nucleus. A similar argument regarding VPI has also been made with respect to the projections to SII in the raccoon 5. In contrast, the ventrobasal complex in the cat has reciprocal, precisely topographic connections with SII 2. Consequently, it seemed necessary to review the source of these connections in primates. A primary goal of this re-examination was to determine whether a topographical organization exists in these connections similar to the arrangement that characterizes the distribution of projections to SII in the cat. In these experiments anterogradely transported tritiated amino acids were injected into the cortex of the lateral sulcus following physiological mapping of SII in order to label corticothalamic fibers. This procedure was preferred over
retrograde labeling of thalamocortical cells with H R P in order to avoid possible problems from injecting fibers of passage in the white matter underlying SII and because it was felt that, given the reciprocity between thalamocortical and corticothalamic connections, the responsible thalamic nuclei would be more readily visualized. These experiments were conducted on 7 Macaca fascicularis monkeys that were anesthetized with an initial intraperitoneal injection of 25 mg/kg sodium pentobarbital; this was supplemented as needed to maintain an areflexive status by 5 mg intravenous injections throughout all procedures. The animals were held in a stereotaxic device and the exposed lateral sulcus was explored with glass-coated tungsten or platinum-iridium electrodes for evoked responses to electrical stimulation of the hand or foot. When an active site was located, manually applied tactile stimuli were used to determine the location of the most effective cutaneous receptive field for the cluster of cells in the vicinity of the electrode tract. In these experiments, injections consisted of a cocktail of equal parts of tritiated proline and leucine. The amino acids were dried in a vacuum evaporator, reconstituted in saline to 50/~Ci for each for a total of 100
Correspondence: H. Burton, Dept. of Anatomy and Neurobiology, Washington University School of Medicine, 4566 Scott Avenue, St. Louis, MO 63110, U.S.A. 0006-8993/84/$03.00 (~) 1984 Elsevier Science Publishers B.V.
368
2 mm M31
~
(
face M25
insula
iteral sulcus ( i n t e r i o r lip)
M26
Fig. 1. Distribution of injected tritiated amino acids within the cortex of the lateral sulcus in five hemispheres. The rows are arranged to show the most posteriorly placed single injection of 100 nl in the top row (M31), two 100-nl injections into the fundal region that was centered near the posterior extent of the insula (M22) in the second row, four injections of 400-600 nl each spaced at 1-mm AP intervals along the upper bank of the lateral sulcus posterior to the insula (M24) in the middle row, two injections of 300 nl each spaced 1.5 mm apart along the lateral sulcus that covered the outer face (M25), and one injection of 450 nl that was mostly confined to the inner face of the parietal operculum (M26) in the bottom two rows, respectively. In each row the indicated boundaries of the injection sites were obtained from sections viewed with bright-field illumination on a projection microscope at a magnification of x 15. The AP extent and exposure duration of these injections was as follows: M31, 1.8 mm after 9 weeks; M22, 3.4 mm after 8 weeks; M24, 4 mm after 6 weeks; M25, 3.2 mm after 10.5 weeks; M26, 1.6 mrn after 8 weeks. Inset on lower right shows an expanded and flattened view of the lateral sulcus6 with a diagrammatic indication of the location of the injections taken from planimetric reconstructions in the different cases and a generalized summary of the location of SII in this sulcus.
~Ci//A, l o a d e d i n t o m i c r o p i p e t t e s w i t h tip o p e n i n g s o f
a n i m a l s w e r e r e t u r n e d to t h e i r c a g e s for p o s t - o p e r a -
25-50/~m, and injected hydraulically into the vicinity
tive s u r v i v a l s of o n e w e e k in o r d e r to p e r m i t m a x i -
of t h e f o r e l i m b or t h e h i n d l i m b r e p r e s e n t a t i o n in SII.
m u m a c c u m u l a t i o n o f r a d i o a c t i v i t y in t h e t h a l a m u s
F o l l o w i n g single o r m u l t i p l e i n j e c t i o n s (Fig. 1), t h e
f r o m slow a × o p l a s m i c t r a n s p o r t . S u b s e q u e n t p r o c e -
369 dures involving perfusion of the brains, tissue prepaYations for autoradiography and data collations of the distribution of radioactivity on the autoradiograms have been documented previously1.6. The locations of the injection sites for five of the cases are presented in Fig. 1. In these experiments the injected tracers covered a substantial portion of the parietal operculum starting from the posterior end of the insula. In two cases the injections spread posterior from the forelimb representations on the outer face of the parietal operculum to involve substantially the buried part of area 7 that invades the. lateral sulcus behind SII (M24 and M25); part of the face area of SII was also covered by the injection in M25. In one case (M26) only the distal forelimb digit area on the inner face of the parietal operculum was heavily labeled; in two cases (M22 and M31) the fundal regions were covered by the injections. In M31 the injection primarily affected cortex lying posterior to the foot and tail representations in SII. In this case, however, the tracers spread rostrally along the fundus of the lateral sulcus to just touch on the hindlimb representation. In M22, the injection was placed near the posterior end of the insula, along the superior limiting sulcus, where portions of the trunk and leg representations in SII are located 11. The distribution of anterograde labeling on selected coronal sections through the thalamus from each of these cases has been illustrated in Fig. 2. The results indicated that autoradiographically labeled processes were located in several ventral posterior (VP) nuclei including ventroposterior lateral pars caudalis (VPLc), ventroposterior medial (VPM) and ventroposterior inferior (VPI). In addition, labeling was also seen in central lateral (CL), posterior nucleus (Po), anterior pulvinar nucleus (Pla) and the dorsomedial part of medial geniculate (MG) in some of the experiments. Comparisons between the five cases on the distribution of labeling in the ventroposterior nuclei showed that the region with the densest patches of silver grains differed. In M31, labeling was located furthest lateral in VPLc, while in M25 and M26, it was most intense in more medial parts of VPLc. In M26, in which the injection was particularly delimited within the distal digit zone on the inner face of the parietal operculum, the labeling was located furthest medial along the lateral margin of VPM (Fig. 2Q, R). In these cases involving the forelimb, a
triangular zone of intense labeling extended down to the ventralmost margins of the ventroposterior nuclei to a portion of the thalamus that includes the VPI region (Fig. 2M, N, Q, R). The cells in this region are somewhat smaller than those normally conspicuous in VPLc. However, additional labeling was seen in each of these cases further dorsal in the region of VPLc that has larger cells (Fig. 2 M - O , Q, R). The topography of these projections can be seen by comparing the location of the dense zones of labeling in M24-M26. In M24 the labeling in VPLc was found dorsal, lateral and posterior to VPI (Fig. 21, J); it was, therefore, lateral to the labeling in M25 and M26. The injection in M24, which was medial and posterior to the injections in the other forelimb cases, mostly involved the proximal parts of the arm and hand areas of SII. In contrast, the injection in M25 was placed medial and posterior to that in M26 and so probably involved more proximal parts of the digit and hand representation9,11. The labeling in M25 overlapped with that seen in M26, but did not extend as far medial in VPLc. The injection in M25 also extended further anterior on the outer face of the parietal operculum to the face region of SII; consequently, labeling was noted in the posterior parts of VPM (Fig. 2P). A comparison between the distributions of labeling in M31 vs M22 also indicated different topographies in VPLc. In M31 the labeling in VPLc was particularly confined to the ventrolateral edge of the nucleus (Fig. 2A-C), but in M22 labeling was distributed more medially (Fig. 2 F - H ) , especially in its most posterior extent (Fig. 2H). This portion of VPLc is known to be concerned with the trunk and leg representations of the body, and these are the regions that were probably represented in the vicinity of the injection site in M22. Dense labeling was seen in Po in those cases (M31, M24) in which the injection invaded the retroinsular cortex. Additional labeling in this nucleus in other cases (M26, Fig. 2T) may indicate that SII also sends some projections to Po. However, in some of these experiments the posterior border of VPM was not well demarcated on the thionine-stained sections and some of the labeling situated close to the lateral edge of CM (Fig. 2K, S) could be interpreted as label in the trigeminal relay. In several cases some labeling was distributed
~y~i!
,~
~
~
371 within the anterior pulvinar nucleus (Pla). The most extensive example of this was seen in M24 in which the injection was centered along the posterior portion of SII in the vicinity of the arm and hand region. In this case the tracer clearly spread posterior and medial (Fig. 1) into area 7b as defined previously 6. Traces of labeling in Pla in the other experiments can also be attributed to area 7b since in M24 and M31 some of area 7b was also injected. In M26 the micropipettes passed through area 7b to reach SII and some leakage was seen in the superificial cortex at the point where the penetrations were made. The primary finding of these experiments is that projections from SII to the ventroposterior nuclei are topographically organized. This result confirms similar observations in the cat in which topographically organized thalamocortical and corticothalamic connections were demonstrated 2. It is likely that the thalamocortical connections in the primate are also topographically arranged from the same nuclei 1. The densest projections were traced from the forelimb portions of SII to the most ventral and posterior aspects of VPLc. In this location at more rostral levels, many peri-cellular aggregations of silver grains occupied the dorsolateral segment of a paler, small-celled area that has previously been designated VPI in the monkey (see review in ref. 1). Consequently, the present observations are consistent with reports that SII receives thalamic connections from this nucleus inasmuch as corticothalamic and thalamocortical connections are reciprocal 4,5. Several factors, however, urge some precaution in
concluding that VPI provides the primary relay to SII. First, the present experiments have shown that labeling occurred in other parts of VPLc in cases involving the forelimb area of SII and also in cases where VPI was not labeled at all. The locations of the labeling in these different cases varied according to the segments of the SII body representation affected by the injections. In addition, in a previous study, a similar localization of labeled cells was seen in the middle of VPLc following an injection of H R P into the hand and arm area of SII as was shown here for M24 and M253. The projections to the more central parts of VPLc are more scattered than those to the more ventral, distal limb areas of the thalamus. A similar scattering of SII's connections has been described in the cat10. Consequently, in cases of small injections that were nearly confined to the distal limb representation, it would be likely that only the most ventral portions of VPLc and VPI contained label as in M26 of the present study. Projections to the remainder of VPLc can be seen when other components of the SII representation are covered by the injected tracers. We greatly appreciated Dr. A. D. Craig's participation in some of the initial recordings from SII, Dr. E. G. Jones' review of some of the labeling distributions in the thalamus, and M. Kopf's technical assistance. We are also grateful to R. Freund for his photographic assistance and J. Hoffmann for typing. This study was supported by funds from N I H Grant NS 09809.
Fig. 2. Plots of the distribution of labeling in the thalami of the cases whose injection sites have been shown in Fig. 1. Corresponding rows in the two figures have been matched according to the cases: A-D, M31 (autoradiograms exposed for 9 weeks); E-H, M22 (exposure 6 weeks); I-L, M24 (exposure 6 weeks); M-P, M25 (exposure 7 weeks); Q-T, M26 (exposure 11 weeks). The labeling was plotted with the aid of dark-field illumination and camera lucida. The most anterior sections are shown on the left for each row. Distances between adjacent sections were as follows: 400/~m, A-D, F-H, J-L, M-O, S-T; 600/am, E-F; 800/lm, A-B, O-P, Q-S; 1000/am, I-J. Sections in A - D were plotted at × 30; those in E-T were plotted at × 15. Abbreviations: CL, central lateral n.; CM, centrum median n.; mc, magnoceUular n. of MG; MG, medial geniculate n,; Pla, anterior pulvinar n.; Pli, inferior pulvinar n. ; Plm, medial pulvinar n.; Po, posterior n. ; VMb, ventromedial pars basalis n.; VPI, ventroposterior inferior n.; VPLc, ventroposterior lateral pars caudalis n. ; VPM, ventroposterior medial n.
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