Brain Research, 133 (1977) 223-235
223
© Elsevier/North-HollandBiomedical Press
SOMATOTOPIC AND C O L U M N A R ORGANIZATION IN THE CORTICOTECTAL PROJECTION OF THE RAT SOMATIC SENSORY CORTEX
S. P.
WISE and E.
G.
JONES
Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Mo. 63110 (U.S.A.)
(Accepted December 30th, 1976)
SUMMARY Single injections of tritiated amino acids into the first somatic sensory area (SI) of the rat neocortex result in axoplasmically transported labeling of the stratum griseum intermediale and stratum griseum profundum of the ipsilateral superior colliculus. The terminal labeling in these layers takes the form of multiple, column-like patches. The SI projection is somatotopically organized with the face and head representations projecting to an extensive anterolateral part of the colliculus and the limb representations projecting to a restricted posterolateral part. Injections of horseradish peroxidase into the superior colliculus result in retrograde labeling of corticotectal cells in the superficial part of layer VB of SI and of the second somatic sensory area (SII).
INTRODUCTION A projection from the somatic sensory cortex to the superior colliculus has been reported in primates 3°, in the cat a5 and in the rat 32,35,43, but the details of the topographic pattern of this projection have not been studied in any species. Recent electrophysiological studies in the mouse12, ~3 and cat 52 have revealed that a well organized somatotopic representation within the deep layers of the superior colliculus is in "spatial register" with the exclusively visuotopic representation in the superficial layers. In the present study we have investigated the terminal distribution of the somatic sensory corticotectal fibers and the topographic and laminar organization of the cortical cells from which they arise. METHODS The experiments were performed on 18 albino rats of the Wistar strain. The autoradiographic material was prepared by injecting 12 adult rats with 0.1-0.6 #1 of
224 [ZH]proline and [aH]leucine in equal concentrations. Single or multiple injections of 50 #Ci/#l solutions were made into the SI cortex with a 1 #1 syringe or through glass micropipettes. After a survival of 24-48 h, the animals were perfused with 10~ buffered formalin. The brains were embedded in Paraplast and sectioned at 20 #m. A 1 in 10 series was mounted, coated with Kodak NTB-2 emulsion, exposed for 2 weeks at 4 °C, developed in Kodak D-19, fixed and stained through the emulsion with thionin 8. In 6 rats stereotaxic injections of 0.05 #1 of 25-50 ~ horseradish peroxidase (HRP; Sigma, type VI) were made into the lateral half of the superior colliculus through a 31-gauge needle attached to a 1 #1 syringe. Incidental corticocortical transport was prevented by excision of the part of the cortex traversed by the injection needle. After a 2-day survival the animals were perfused with 0.8-1.0~ paraformaldehyde and 1.25-2.5 ~ glutaraldehyde in 0.1 M phosphate buffer at pH 7.3. The brains were immediately removed, postfixed for 6-12 h, then allowed to sink in 30 sucrose in phosphate buffer. The brains were cut in the frontal plane on a freezing microtome at 50 #m, and a 1 in 5 series of sections incubated in 3,3'-diaminobenzidine tetrahydrochloride and 0.01 ~ hydrogen peroxide following the method of LaVail et al. 31. The sections were counterstained with 0.25 ~ thionin. Measurements of the areas and average diameters of labeled and unlabeled cells were made from camera lucida drawings of cell outlines using the computer system of Cowan and Wann 9. RESULTS Injections of tritiated amino acids were made in several parts of the first somatic sensory cortex (SI). The part of the body representation affected was assessed in several ways: (1) by reconstructing the extent of the injection site (Fig. 3) and comparing it with the somatotopic representation in SI as determined electrophysiologicallyr°,61,66; (2) by determining its relationship to the aggregations of layer IV granule cells61, each of which represents a separate body part; (3) by examining the terminal distribution of corticothalamic fibers in the thalamic ventrobasal complex and comparing this with the somatotopic map within the ventrobasal complex10,14. Eleven injections involving different parts of the SI cortex (see e.g. Fig. 4A) all lead to anterogradely transported terminal labeling that occupies a restricted part of the ipsilateral superior colliculus. The labeled corticotectal fibers reach the superior colliculus by ascending through the midbrain tegmentum from the cerebral peduncle. The terminal labeling in the colliculus often extends to the lateral boundary of the colliculus, but with the exception of the extreme anterior pole, never to the medial boundary. Not all layers of the superior colliculus are labeled to the same extent. The density is greatest over the stratum griseum intermediale and the horizontal spread of the label is always greatest in this layer. Significant, but much more restricted terminal labeling, is found over the underlying stratum griseum profundum. No labeling above background levels is found in the stratum opticum, the stratum griseum superficiale, or the stratum zonale after injection of SI (Figs. 1 and 2). By comparison, a large injection of tritiated amino acids which spread to involve the visual areas of the cortex, lead to heavy labeling over all three of these more superficial layers of the colliculus (Fig. 5).
Fig. 1. Darkfield (A) and brightfield (B) photomicrographs from the same section and at the same magnification showing 3 patches of terminal labeling (large arrows) mainly in the stratum griseum intermediale of the superior colliculus following a single injection of tritiated amino acids in the face area of the ipsilateral SI cortex (see Fig. 3C). A small amount of labeling also appears in the stratum griseum profundum deep to the most lateral patch. Small arrows indicate same blood vessels. Thionin counterstain. × 60. Abbreviations used in this and following figures : A, stratum album intermediale; AQ, cerebral aqueduct; CG, central gray matter; CI, internal capsule; CP, caudate-putamen; cp, cerebral peduncle; GP, globus pallidus; 1, stratum griseum intermediale; IC, inferior colliculus; MG, medial geniculate complex; O, stratum opticum; P, stratum griseum profundum; PC, posterior commissure; R, red nucleus; S, stratum griseum superficiale; I-VI, layers of cerebral cortex.
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Fig. 2. Projection drawings of frontal sections at 200/~m intervals showing (stipple) the patch-like pattern of terminal labeling in the superior colliculus following an injection of tritiated amino acids in the face area of the ipsilateral SI.
After all injections in the somatic sensory cortex, the terminal labeling within the appropriate layers of the colliculus takes the form of relatively discrete patches extending through the thickness of these layers and each measuring approximately 200 #m in width (Figs. 1, 2 and 5). The patches are separated from each other by smaller gaps which contain little or no label. The number of patches varies from 1 to 4 depending on the size of the injection and the part of SI injected. Some of the patches seem to be aligned in long (up to 1 ram) anteroposteriorly oriented strips. Multiple patches are seen after single injections (Figs. 1, 3B, C, D and 5). Injections of [3H]amino acids in different parts of the somatotopic representation in SI lead to labeling in different and generally non-overlapping parts of the superior colliculus, provided that the injection sites do not overlap. Injections in the face and head area (Figs. 2A and C, and 1) lead to labeling of the anterior and lateral
227 parts of the colliculus. The label after injection of these areas often extends across much of the mediolateral extent of the extreme anterior pole of the colliculus, but, posteriorly, it becomes restricted to the lateral two-thirds. The anteroposterior extent of the SI face and head projection is large and these fibers and terminals occupy approximately 6 2 ~ of the horizontal extent of the stratum griseum intermediale (about 3 sq. ram) of the colliculus. Injections of the hindlimb area of $1 (Fig. 3D) result in a small area of labeling in the extreme posterolateral part of the colliculus. Injections of the forelimb area (Fig. 3B) result in labeling of a small posterolateral part of the colliculus between the face and hindlimb projection area and with little apparent overlap. Injections of H R P into the superior colliculus (Figs. 7 and 4B) heavily involved all layers of the colliculus, but inevitably spread to nearby sites, such as the external nucleus of the inferior colliculus and the underlying parts of the mesencephalic reticular formation. However, there was no involvement by the injection site of the cerebral peduncle, the diencephalon, the periaqueductal gray matter or the pons. Retrogradely labeled cells after such injections are found in a number of ipsilateral cortical areas including the first and second somatic sensory areas. In SI and SII, the retrogradely labeled corticotectal cells are large pyramidal cells and in SI, where 60 were measured 9, the somal diameters range from 25.6 to 13.6 #m (~ = 19.5; S.D., 2.6 ante riot t
Fig. 3. Surface reconstructions (A-D) of the distribution of terminal labeling (lines) in the superior colliculi of 4 representative animals in which injections of isotope (left, dorsal and lateral view) were made in the ipsilateral SI cortex. Injection sites, as judged by cytoarchitectonic criteria in the individual brains, were all confined to SI.
228
Fig. 4. A: photomicrograph from an autoradiograph showing anterior part of injection site in SI cortex of brain shown in Fig. 3A. Thionin counterstain. × 22. B: photomicrograph of large injections of HRP in superior colliculi of same animal. Injection on right extends into midbrain reticular formation. Thionin counterstain, x 10.
#m). In layer V as a whole, the mean somal diameter of all cells (153 labeled and unlabeled cells were measured) is 14.5 # m ; S.D., 3.0 #m. In SI and SII, the labeled somata are found exclusively in the deep, magnocellular part of layer V (layer VB). In this sublayer, mean somal diameter of 53 labeled and unlabeled cells equals 16.0 # m ; S. D., 3.9 #m). The labeled somata are not distributed homogeneously throughout layer VB but are found to be concentrated instead in the most superficial part of this sublayer (Fig. 6). After injections which involved most of the colliculus, the labeled cells are found throughout SI and SII and are concentrated in the areas of SI containing the dense aggregations of layer IV granule cells which receive input from the trigeminal system and including the "barrel" regions (see Fig. 7 and ref. 61). The retrogradely labeled neuronal somata are clustered in groups of 5-30; the smallest of these clusters measures 250 # m in the mediolateral dimension and 1 m m in anteroposterior extent, but larger clusters measuring up to 2.5 m m in mediolateral extent are also found. The clusters can be seen to fuse with and separate from each other at variable intervals. In the trigeminal representation, the smaller clusters do not appear to be specifically related to individual barrels.
229
Fig. 5. Darkfield (A) and brightfield (B) photomicrographs showing extensive patches of terminal labeling in the stratum griseum intermediale resulting from a large injection of isotope in ipsilateral SI and adjoining areas. Additional labeling in lateral part of overlying stratum griseum superficiale results from extension of injection posteriorly into a portion of the visual cortex. Thionin counterstain. ": 40. DISCUSSION These experiments indicate that there is al well organized somatotopic pattern in the somatic sensory corticotectal projection. The s o m a t o t o p y o f the corticotectal projection agrees precisely with the somatotopic map f o u n d in the mouse in the single unit study of Dr~ger and Hubel la. The projection from the somatic sensory cortex is more extensive than previously recognized la. It clearly does not provide the sole
230
Fig. 6. Darkfield and brightfield photomicrographs at same magnificationshowing retrograde labeling of clusters of pyramidal cells confined to layer VB of the SI cortex following an injection of HRP in the ipsilateral superior colliculus. Thionin counterstain. × 50. somatic sensory input to the superior colliculus since the ascending somatic sensory paths, particularly the spinotectal, also terminate in the deep layers2,3,37, 39,54. However, the precision of the somatotopic map is clearly respected, if not determined, by the descending corticotectal system. The laminar distribution of the somatic sensory corticotectal projection to the strata grisea intermediale and profundum is also in agreement with published data on cellular responses in the superior colliculus since only cells in the deeper layers respond to somatic stimulation in the mouselZ, 13 and other species17,4s,51, 52. The influence of the somatic sensory cortex upon the receptive field properties of cells in the superior colliculus has not been determined. In the cat, however, the receptive field properties of the visually responsive cells in the superficial layers are profoundly influenced by the visual cortex~,4v,51, 6z. After ablation of the visual cortex in the cat (though not apparently in some other species 49,53) the cells lose direction selectivity and can no longer be driven by the ipsilateral eye. The corticotectal cells of the visual cortex, moreover, have the same restricted laminar distribution in the cortex as their counterparts in the somatic sensory areas16,zl,36,as, ~9. By analogy, therefore, the somatic sensory cortex might play some comparable role in determining the response properties of cells in the deeper layers of the superior colliculus. However, no single unit studies involving ablation of the somatic sensory areas or destruction of other somatic sensory inputs to the colliculus have yet been carried out. In view of the relatively large number of corticotectal cells in certain parts of SI, the influence of this cortex upon the deep layers may be as profound as that of
231 the visual cortex upon the superficial layers. The large size, homogeneity and very discrete laminar distribution of the SI corticotectal neurons should also make them particularly accessible to physiological studies. In the mouse and cat the visual map in the superficial layers has been found to be
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Fig. ?. Projection drawings of frontal sections (above) showing (dots) distribution of retrogradely labeled corticotecta] cells in SI and adjacent cortical areas following an injection of HRP (black) in the superior colliculus. Section 46 is through middle of injection site. Lower part of figure shows surface reconstruction of positions of labeled cells on oblique dorsolateral view of hemisphere. Figurine (after W¢lker 61) indicates approximate position of SI cortex but is not intended to be an accurate representation of position or size of S[ in this brain.
232 in topographic register with that of other modalities (including auditory as well as somatic sensory maps) in the deeper layers 12,13,52. In the deeper layers are mapped those parts of the body or those points in auditory space which lie near the part of the visual field mapped superficial to them in the visual layers. The role of the deep layers of the colliculus in orientation of the fixation point toward the part of visual space mapped onto the immediately overlying layers 45,4s,55 suggests that the overlapping of the topographic maps is important in the orientation of the animal to somatic, auditory and visual stimuli in its environment. In view of the recent elucidation of principles of columnar organization of othe~ laminated structures such as the cerebral cortex, the finding of a similar organization in the terminal field of the somatic sensory corticotectal projection is of interest. The corticotectal fibers from SI terminate in the deep layers of the colliculus in discrete patches about 200 /~m in width. These terminal patches are similar to patches of terminal labeling seen in the stratum griseum superficiale of the ipsilateral colliculus TM 20,zz and in the nucleus of the optic tract 5 of both sides after injections of tritiated amino acids involving the retina of the cat. There is some evidence in the present work that the corticotectal projection from the visual cortex to the stratum griseum superficiale also forms disjunctive terminal patches. Registration of retinotectal patches, visual corticotectal patches and somatic sensory corticotectal patches has not yet been demonstrated but such an organization, if present, would be a suitable anatomical substrate for the precise registration of the visual and somatic sensory maps. The cells of origin of the SI corticotectal fibers seem also to be arranged in discrete clusters which, in a sense, can be interpreted as conforming to a column-like organization. It is not possible to be certain of the exact dimensions of the smallest of the corticotectal clusters since in our material serial sections were not routinely prepared, but these findings are consistent with current concepts that groupings of cortical cells may be organized in output columns biased preferentially to make certain types of corticofugal or corticocortical connection 4,v,2s,41,56,64. The columnar organization of the somatic sensory and other sensory areas of the cortex is determined in the first instance by the thalamic input 1,za-z5,34,4°,44,59. In view of this and the marked vertical organization of intracortical connections linking the thalamic recipient cells to other cells in the cortex 26,56, the groupings of corticotectal cells in layer V of SI probably receive indirectly, inputs from specific groupings of receptors and from specific parts of the body surface. This modular organization could then, in turn, be imposed upon the superior colliculus by the layer VB cells. There is also the possibility for more direct thalamic inputs to the corticotectal cells, for thalamic afferents terminating in the SI cortex of the rat 63 and other species 2v terminate not only in the internal granular layer (layer IV) but also in a discrete secondary band at the junction of layers VB and VI. A similar deep secondary zone of thalamic terminals is present in the visual cortex of the rat 42, mouse 11, squirrel 46 and cat ~. In the visual cortex of the cat there is some evidence for monosynaptic activation of corticotectal cells by thalamic afferents 5°. The SI corticotectal cells clearly have anatomical characteristics similar to those of the visual corticotectal cells and, therefore, they may also have similar types of synaptic relationship.
233 The r e t r o g r a d e l y labeled cells in the cortex after injections o f the superior colliculus a p p e a r to be labeled as the result o f t e r m i n a l u p t a k e a n d n o t b y t r a n s p o r t in d a m a g e d fibers o f passage 19,z9. N o m a j o r corticofugal fiber tracts pass within the region o f the injection site a n d there is a differential s u b l a m i n a r o r g a n i z a t i o n o f the corticotectal cells when c o m p a r e d to corticofugal cells projecting to o t h e r subcortical loci 65. The d e t e r m i n a t i o n t h a t the injection was effective in p r o v i d i n g sufficient H R P for t e r m i n a l u p t a k e was m a d e b y e x a m i n a t i o n of subcortical nuclei, such as the z o n a incerta 16 a n d the inferior colliculus which project heavily to the superior colliculus. The z o n a incerta a n d o t h e r a p p r o p r i a t e nuclei were heavily p o p u l a t e d with retrogradely labeled ceils after the H R P injection. Finally, no e n d o g e n o u s peroxidase activity has been f o u n d in the n e o c o r t e x 5s,64. ACKNOWLEDGEMENTS This research was s u p p o r t e d b y G r a n t s N S 10526 a n d E Y 00092 f r o m the N a t i o n a l Institutes o f Health, U n i t e d States Public H e a l t h Service. W e t h a n k Ms. Bertha M c C l u r e for technical assistance.
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© Elsevier/North-HollandBiomedical Press
SOMATOTOPIC AND C O L U M N A R ORGANIZATION IN THE CORTICOTECTAL PROJECTION OF THE RAT SOMATIC SENSORY CORTEX
S. P.
WISE and E.
G.
JONES
Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Mo. 63110 (U.S.A.)
(Accepted December 30th, 1976)
SUMMARY Single injections of tritiated amino acids into the first somatic sensory area (SI) of the rat neocortex result in axoplasmically transported labeling of the stratum griseum intermediale and stratum griseum profundum of the ipsilateral superior colliculus. The terminal labeling in these layers takes the form of multiple, column-like patches. The SI projection is somatotopically organized with the face and head representations projecting to an extensive anterolateral part of the colliculus and the limb representations projecting to a restricted posterolateral part. Injections of horseradish peroxidase into the superior colliculus result in retrograde labeling of corticotectal cells in the superficial part of layer VB of SI and of the second somatic sensory area (SII).
INTRODUCTION A projection from the somatic sensory cortex to the superior colliculus has been reported in primates 3°, in the cat a5 and in the rat 32,35,43, but the details of the topographic pattern of this projection have not been studied in any species. Recent electrophysiological studies in the mouse12, ~3 and cat 52 have revealed that a well organized somatotopic representation within the deep layers of the superior colliculus is in "spatial register" with the exclusively visuotopic representation in the superficial layers. In the present study we have investigated the terminal distribution of the somatic sensory corticotectal fibers and the topographic and laminar organization of the cortical cells from which they arise. METHODS The experiments were performed on 18 albino rats of the Wistar strain. The autoradiographic material was prepared by injecting 12 adult rats with 0.1-0.6 #1 of
224 [ZH]proline and [aH]leucine in equal concentrations. Single or multiple injections of 50 #Ci/#l solutions were made into the SI cortex with a 1 #1 syringe or through glass micropipettes. After a survival of 24-48 h, the animals were perfused with 10~ buffered formalin. The brains were embedded in Paraplast and sectioned at 20 #m. A 1 in 10 series was mounted, coated with Kodak NTB-2 emulsion, exposed for 2 weeks at 4 °C, developed in Kodak D-19, fixed and stained through the emulsion with thionin 8. In 6 rats stereotaxic injections of 0.05 #1 of 25-50 ~ horseradish peroxidase (HRP; Sigma, type VI) were made into the lateral half of the superior colliculus through a 31-gauge needle attached to a 1 #1 syringe. Incidental corticocortical transport was prevented by excision of the part of the cortex traversed by the injection needle. After a 2-day survival the animals were perfused with 0.8-1.0~ paraformaldehyde and 1.25-2.5 ~ glutaraldehyde in 0.1 M phosphate buffer at pH 7.3. The brains were immediately removed, postfixed for 6-12 h, then allowed to sink in 30 sucrose in phosphate buffer. The brains were cut in the frontal plane on a freezing microtome at 50 #m, and a 1 in 5 series of sections incubated in 3,3'-diaminobenzidine tetrahydrochloride and 0.01 ~ hydrogen peroxide following the method of LaVail et al. 31. The sections were counterstained with 0.25 ~ thionin. Measurements of the areas and average diameters of labeled and unlabeled cells were made from camera lucida drawings of cell outlines using the computer system of Cowan and Wann 9. RESULTS Injections of tritiated amino acids were made in several parts of the first somatic sensory cortex (SI). The part of the body representation affected was assessed in several ways: (1) by reconstructing the extent of the injection site (Fig. 3) and comparing it with the somatotopic representation in SI as determined electrophysiologicallyr°,61,66; (2) by determining its relationship to the aggregations of layer IV granule cells61, each of which represents a separate body part; (3) by examining the terminal distribution of corticothalamic fibers in the thalamic ventrobasal complex and comparing this with the somatotopic map within the ventrobasal complex10,14. Eleven injections involving different parts of the SI cortex (see e.g. Fig. 4A) all lead to anterogradely transported terminal labeling that occupies a restricted part of the ipsilateral superior colliculus. The labeled corticotectal fibers reach the superior colliculus by ascending through the midbrain tegmentum from the cerebral peduncle. The terminal labeling in the colliculus often extends to the lateral boundary of the colliculus, but with the exception of the extreme anterior pole, never to the medial boundary. Not all layers of the superior colliculus are labeled to the same extent. The density is greatest over the stratum griseum intermediale and the horizontal spread of the label is always greatest in this layer. Significant, but much more restricted terminal labeling, is found over the underlying stratum griseum profundum. No labeling above background levels is found in the stratum opticum, the stratum griseum superficiale, or the stratum zonale after injection of SI (Figs. 1 and 2). By comparison, a large injection of tritiated amino acids which spread to involve the visual areas of the cortex, lead to heavy labeling over all three of these more superficial layers of the colliculus (Fig. 5).
Fig. 1. Darkfield (A) and brightfield (B) photomicrographs from the same section and at the same magnification showing 3 patches of terminal labeling (large arrows) mainly in the stratum griseum intermediale of the superior colliculus following a single injection of tritiated amino acids in the face area of the ipsilateral SI cortex (see Fig. 3C). A small amount of labeling also appears in the stratum griseum profundum deep to the most lateral patch. Small arrows indicate same blood vessels. Thionin counterstain. × 60. Abbreviations used in this and following figures : A, stratum album intermediale; AQ, cerebral aqueduct; CG, central gray matter; CI, internal capsule; CP, caudate-putamen; cp, cerebral peduncle; GP, globus pallidus; 1, stratum griseum intermediale; IC, inferior colliculus; MG, medial geniculate complex; O, stratum opticum; P, stratum griseum profundum; PC, posterior commissure; R, red nucleus; S, stratum griseum superficiale; I-VI, layers of cerebral cortex.
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Fig. 2. Projection drawings of frontal sections at 200/~m intervals showing (stipple) the patch-like pattern of terminal labeling in the superior colliculus following an injection of tritiated amino acids in the face area of the ipsilateral SI.
After all injections in the somatic sensory cortex, the terminal labeling within the appropriate layers of the colliculus takes the form of relatively discrete patches extending through the thickness of these layers and each measuring approximately 200 #m in width (Figs. 1, 2 and 5). The patches are separated from each other by smaller gaps which contain little or no label. The number of patches varies from 1 to 4 depending on the size of the injection and the part of SI injected. Some of the patches seem to be aligned in long (up to 1 ram) anteroposteriorly oriented strips. Multiple patches are seen after single injections (Figs. 1, 3B, C, D and 5). Injections of [3H]amino acids in different parts of the somatotopic representation in SI lead to labeling in different and generally non-overlapping parts of the superior colliculus, provided that the injection sites do not overlap. Injections in the face and head area (Figs. 2A and C, and 1) lead to labeling of the anterior and lateral
227 parts of the colliculus. The label after injection of these areas often extends across much of the mediolateral extent of the extreme anterior pole of the colliculus, but, posteriorly, it becomes restricted to the lateral two-thirds. The anteroposterior extent of the SI face and head projection is large and these fibers and terminals occupy approximately 6 2 ~ of the horizontal extent of the stratum griseum intermediale (about 3 sq. ram) of the colliculus. Injections of the hindlimb area of $1 (Fig. 3D) result in a small area of labeling in the extreme posterolateral part of the colliculus. Injections of the forelimb area (Fig. 3B) result in labeling of a small posterolateral part of the colliculus between the face and hindlimb projection area and with little apparent overlap. Injections of H R P into the superior colliculus (Figs. 7 and 4B) heavily involved all layers of the colliculus, but inevitably spread to nearby sites, such as the external nucleus of the inferior colliculus and the underlying parts of the mesencephalic reticular formation. However, there was no involvement by the injection site of the cerebral peduncle, the diencephalon, the periaqueductal gray matter or the pons. Retrogradely labeled cells after such injections are found in a number of ipsilateral cortical areas including the first and second somatic sensory areas. In SI and SII, the retrogradely labeled corticotectal cells are large pyramidal cells and in SI, where 60 were measured 9, the somal diameters range from 25.6 to 13.6 #m (~ = 19.5; S.D., 2.6 ante riot t
Fig. 3. Surface reconstructions (A-D) of the distribution of terminal labeling (lines) in the superior colliculi of 4 representative animals in which injections of isotope (left, dorsal and lateral view) were made in the ipsilateral SI cortex. Injection sites, as judged by cytoarchitectonic criteria in the individual brains, were all confined to SI.
228
Fig. 4. A: photomicrograph from an autoradiograph showing anterior part of injection site in SI cortex of brain shown in Fig. 3A. Thionin counterstain. × 22. B: photomicrograph of large injections of HRP in superior colliculi of same animal. Injection on right extends into midbrain reticular formation. Thionin counterstain, x 10.
#m). In layer V as a whole, the mean somal diameter of all cells (153 labeled and unlabeled cells were measured) is 14.5 # m ; S.D., 3.0 #m. In SI and SII, the labeled somata are found exclusively in the deep, magnocellular part of layer V (layer VB). In this sublayer, mean somal diameter of 53 labeled and unlabeled cells equals 16.0 # m ; S. D., 3.9 #m). The labeled somata are not distributed homogeneously throughout layer VB but are found to be concentrated instead in the most superficial part of this sublayer (Fig. 6). After injections which involved most of the colliculus, the labeled cells are found throughout SI and SII and are concentrated in the areas of SI containing the dense aggregations of layer IV granule cells which receive input from the trigeminal system and including the "barrel" regions (see Fig. 7 and ref. 61). The retrogradely labeled neuronal somata are clustered in groups of 5-30; the smallest of these clusters measures 250 # m in the mediolateral dimension and 1 m m in anteroposterior extent, but larger clusters measuring up to 2.5 m m in mediolateral extent are also found. The clusters can be seen to fuse with and separate from each other at variable intervals. In the trigeminal representation, the smaller clusters do not appear to be specifically related to individual barrels.
229
Fig. 5. Darkfield (A) and brightfield (B) photomicrographs showing extensive patches of terminal labeling in the stratum griseum intermediale resulting from a large injection of isotope in ipsilateral SI and adjoining areas. Additional labeling in lateral part of overlying stratum griseum superficiale results from extension of injection posteriorly into a portion of the visual cortex. Thionin counterstain. ": 40. DISCUSSION These experiments indicate that there is al well organized somatotopic pattern in the somatic sensory corticotectal projection. The s o m a t o t o p y o f the corticotectal projection agrees precisely with the somatotopic map f o u n d in the mouse in the single unit study of Dr~ger and Hubel la. The projection from the somatic sensory cortex is more extensive than previously recognized la. It clearly does not provide the sole
230
Fig. 6. Darkfield and brightfield photomicrographs at same magnificationshowing retrograde labeling of clusters of pyramidal cells confined to layer VB of the SI cortex following an injection of HRP in the ipsilateral superior colliculus. Thionin counterstain. × 50. somatic sensory input to the superior colliculus since the ascending somatic sensory paths, particularly the spinotectal, also terminate in the deep layers2,3,37, 39,54. However, the precision of the somatotopic map is clearly respected, if not determined, by the descending corticotectal system. The laminar distribution of the somatic sensory corticotectal projection to the strata grisea intermediale and profundum is also in agreement with published data on cellular responses in the superior colliculus since only cells in the deeper layers respond to somatic stimulation in the mouselZ, 13 and other species17,4s,51, 52. The influence of the somatic sensory cortex upon the receptive field properties of cells in the superior colliculus has not been determined. In the cat, however, the receptive field properties of the visually responsive cells in the superficial layers are profoundly influenced by the visual cortex~,4v,51, 6z. After ablation of the visual cortex in the cat (though not apparently in some other species 49,53) the cells lose direction selectivity and can no longer be driven by the ipsilateral eye. The corticotectal cells of the visual cortex, moreover, have the same restricted laminar distribution in the cortex as their counterparts in the somatic sensory areas16,zl,36,as, ~9. By analogy, therefore, the somatic sensory cortex might play some comparable role in determining the response properties of cells in the deeper layers of the superior colliculus. However, no single unit studies involving ablation of the somatic sensory areas or destruction of other somatic sensory inputs to the colliculus have yet been carried out. In view of the relatively large number of corticotectal cells in certain parts of SI, the influence of this cortex upon the deep layers may be as profound as that of
231 the visual cortex upon the superficial layers. The large size, homogeneity and very discrete laminar distribution of the SI corticotectal neurons should also make them particularly accessible to physiological studies. In the mouse and cat the visual map in the superficial layers has been found to be
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Fig. ?. Projection drawings of frontal sections (above) showing (dots) distribution of retrogradely labeled corticotecta] cells in SI and adjacent cortical areas following an injection of HRP (black) in the superior colliculus. Section 46 is through middle of injection site. Lower part of figure shows surface reconstruction of positions of labeled cells on oblique dorsolateral view of hemisphere. Figurine (after W¢lker 61) indicates approximate position of SI cortex but is not intended to be an accurate representation of position or size of S[ in this brain.
232 in topographic register with that of other modalities (including auditory as well as somatic sensory maps) in the deeper layers 12,13,52. In the deeper layers are mapped those parts of the body or those points in auditory space which lie near the part of the visual field mapped superficial to them in the visual layers. The role of the deep layers of the colliculus in orientation of the fixation point toward the part of visual space mapped onto the immediately overlying layers 45,4s,55 suggests that the overlapping of the topographic maps is important in the orientation of the animal to somatic, auditory and visual stimuli in its environment. In view of the recent elucidation of principles of columnar organization of othe~ laminated structures such as the cerebral cortex, the finding of a similar organization in the terminal field of the somatic sensory corticotectal projection is of interest. The corticotectal fibers from SI terminate in the deep layers of the colliculus in discrete patches about 200 /~m in width. These terminal patches are similar to patches of terminal labeling seen in the stratum griseum superficiale of the ipsilateral colliculus TM 20,zz and in the nucleus of the optic tract 5 of both sides after injections of tritiated amino acids involving the retina of the cat. There is some evidence in the present work that the corticotectal projection from the visual cortex to the stratum griseum superficiale also forms disjunctive terminal patches. Registration of retinotectal patches, visual corticotectal patches and somatic sensory corticotectal patches has not yet been demonstrated but such an organization, if present, would be a suitable anatomical substrate for the precise registration of the visual and somatic sensory maps. The cells of origin of the SI corticotectal fibers seem also to be arranged in discrete clusters which, in a sense, can be interpreted as conforming to a column-like organization. It is not possible to be certain of the exact dimensions of the smallest of the corticotectal clusters since in our material serial sections were not routinely prepared, but these findings are consistent with current concepts that groupings of cortical cells may be organized in output columns biased preferentially to make certain types of corticofugal or corticocortical connection 4,v,2s,41,56,64. The columnar organization of the somatic sensory and other sensory areas of the cortex is determined in the first instance by the thalamic input 1,za-z5,34,4°,44,59. In view of this and the marked vertical organization of intracortical connections linking the thalamic recipient cells to other cells in the cortex 26,56, the groupings of corticotectal cells in layer V of SI probably receive indirectly, inputs from specific groupings of receptors and from specific parts of the body surface. This modular organization could then, in turn, be imposed upon the superior colliculus by the layer VB cells. There is also the possibility for more direct thalamic inputs to the corticotectal cells, for thalamic afferents terminating in the SI cortex of the rat 63 and other species 2v terminate not only in the internal granular layer (layer IV) but also in a discrete secondary band at the junction of layers VB and VI. A similar deep secondary zone of thalamic terminals is present in the visual cortex of the rat 42, mouse 11, squirrel 46 and cat ~. In the visual cortex of the cat there is some evidence for monosynaptic activation of corticotectal cells by thalamic afferents 5°. The SI corticotectal cells clearly have anatomical characteristics similar to those of the visual corticotectal cells and, therefore, they may also have similar types of synaptic relationship.
233 The r e t r o g r a d e l y labeled cells in the cortex after injections o f the superior colliculus a p p e a r to be labeled as the result o f t e r m i n a l u p t a k e a n d n o t b y t r a n s p o r t in d a m a g e d fibers o f passage 19,z9. N o m a j o r corticofugal fiber tracts pass within the region o f the injection site a n d there is a differential s u b l a m i n a r o r g a n i z a t i o n o f the corticotectal cells when c o m p a r e d to corticofugal cells projecting to o t h e r subcortical loci 65. The d e t e r m i n a t i o n t h a t the injection was effective in p r o v i d i n g sufficient H R P for t e r m i n a l u p t a k e was m a d e b y e x a m i n a t i o n of subcortical nuclei, such as the z o n a incerta 16 a n d the inferior colliculus which project heavily to the superior colliculus. The z o n a incerta a n d o t h e r a p p r o p r i a t e nuclei were heavily p o p u l a t e d with retrogradely labeled ceils after the H R P injection. Finally, no e n d o g e n o u s peroxidase activity has been f o u n d in the n e o c o r t e x 5s,64. ACKNOWLEDGEMENTS This research was s u p p o r t e d b y G r a n t s N S 10526 a n d E Y 00092 f r o m the N a t i o n a l Institutes o f Health, U n i t e d States Public H e a l t h Service. W e t h a n k Ms. Bertha M c C l u r e for technical assistance.
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