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An autoradiographic analysis of the tecto-olivary projection in primates
Brain Research, 118 (1976) 245-257
245
© Elsevier/North-HollandBiomedicalPress, Amsterdam - Printed in The Netherlands
AN A U T O R A D I O G R A P H I C PROJECTION IN PRIMATES
ANALYSIS
OF
THE
TECTO-OLIVARY
A. FRANKFURTER, J. T. WEBER, G. J. ROYCE, N. L. STROMINGER and J. K. HARTING Department of Anatomy, University of Wisconsin, Madison, Wisc. 53706, and Department of Anatomy, Albany Medical College, Albany, iV. Y. 12208 (U.S.A.)
(Accepted April 13th, 1976)
SUMMARY Autoradiographic tracing methods were used to demonstrate a well-defined projection from the superior coUiculus to the inferior olivary complex in the monkey. This projection originates within the deep layers of the superior colliculus, descends within the contralateral tecto-spinal tract, and terminates within the caudal 1/3 of the medial accessory nucleus. The terminal field is restricted to a densely packed, darkly stained group of cells located in the most dorsal segment of subnucleus b. In one animal, another group of olivary afferents was identified. These fibers also descend within the contralateral tecto-spinal tract, and terminate within the dorsal cap of Kooy. While it was not possible to determine the origin of this projection, our data suggest that it arises within a region adjacent to the rostral pole of the superior colliculus. The present study further indicates that in the monkey relatively few axons which course within the classical tecto-spinal tract pass caudal to the medulla.
INTRODUCTION Anterograde axonal degeneration studies4,V,a0Aa,15-z0,27,28, most notably by Walberg 2%~s and Martin et al. 15, as well as the recent autoradiographic experiments of EdwardsS, 6, indicate that afferents from several different midbrain areas each project upon discrete segments of the inferior olivary complex. However, the results of fiber degeneration experiments, must be interpreted with caution because of the likelihood that fibers of passage have been interrupted, and also because silver impregnation methods occasionally fail to stain fine caliber axons. Consequently, the cells of origin and locus of termination of many descending fibers to the inferior olive have yet to be precisely identified. While the studies of Escobar and de Cardenas ~, Harting et al. 11, and Henkel
246 et al. 12 suggest that a portion of the midbrain-olivary projection arises from cells within the superior colliculus, other anatomical and electrophysiological data 1,1~.~s,~°, 28 do not support their findings. In view of these contradictory data, as well as the current interest in the origin of vision-related pathways influencing the inferior olive, this report describes convincing autoradiographic evidence for a prominent, and highly localized tecto-olivary projection in Saimiri sciureus and Macaca mulatta. METHODS Experiments were conducted on 3 rhesus monkeys and 4 squirrel monkeys. Injections of either tritiated proline or tritiated leucine were stereotaxically placed into the superior colliculi. Delivery of the tracer substance was accomplished with the aid of a specially designed microdrive apparatus which controlled a l0 #l Hamilton syringe equipped with a 26-gauge needle. The 3 rhesus monkeys were given unilateral injections (0.3/A) of [aH]leucine in concentrations of 30 ktCi/#l. Two squirrel monkeys (SM-210, SM-238) received bilateral injections (0.2-0.3 #l) of [aH]proline in concentrations varying between 20 and 33/~Ci//A. The other two squirrel monkeys (SM-194, SM-195) received unilateral injections (0.3 #1) of [aH]proline in concentrations of 33 #Ci//~l. The brains were fixed by perfusion with 10 % formalin 7 days postoperatively, placed in 30 ~o sucrose-formalin, and subsequently cut on a freezing microtome at 40 #m. Individual sections were mounted and coated with Kodak NTB-2 emulsion, stored in light-tight containers at 4 °C from 3 (RM-208, SM-238) to 6 (RM-200, RM-214, SM-194, SM-195, SM-210) weeks, developed, and lightly counterstained with cresyl violet. RESULTS Since it is seldom possible to specify with any degree of certainty the boundaries which separate regions where labeling is due to perikaryal uptake from regions where labeling is due to dendritic and axonal transport, line drawings are presented which illustrate the intensity and distribution of labeled protein at 3 collicular levels through the injection site for every experiment. Figs. 1 and 2 illustrate the injection sites, as well as the location of transported protein within the inferior olive in experiments RM-208, RM-214, and RM-200. A photomicrograph of the injection site in RM-214 is shown in Fig. 3A. In all 3 experiments it was possible to trace a densely labeled bundle of axons, traveling via what is commonly referred to as the tecto-spinal tract, or predorsal bundle, to a terminal field within the caudal 1/3 of the contralateral medial accessory olivary nucleus. At these levels of the inferior olive, heavy labeling was observed within and surrounding a densely packed, darkly stained group of neurons located in the most dorsal segment of subnucteus b (see Bowman and Sladek 2 and Martin et al. 15 for a detailed description of the morphology of the inferior olive). As shown in the line drawings at the bottom of Figs. 1 and 2, the majority of labeled axons.enter the inferior olivary complex between subnucleus c and the dorsal cap of Kooy. These
247
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Fig. 1. The line drawings at the top of the figure illustrate the distribution and density of silver grains, as plotted under low-power dark-field optics, within the superior colliculus of RM-208 (levels 284, 274, 264) and RM-214 (levels 160, 155, 145). The line drawings at the bottom of the figure show the location of transported label within the inferior olivary complex of RM-208 and RM-214. axons completely surround the dorsal extent of subnucleus b, apparently sending off collaterals which invade the cell group and encapsulate individual neurons, giving the terminal field a reticulated appearance. The distribution of label, as seen under low-power dark-field optics, results in a highly distinctive teardrop configuration (see Fig. 4A). It should be noted that in RM-214 and RM-200 (Figs. 1 and 2) sparse label was present within the corresponding region of the ipsilateral medial accessory nucleus. Since no labeled descending pathway could be followed on the ipsilateral side, it seems likely that this ipsilateral field of distribution represents terminals of axons
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Fig. 2. Line drawings of the superior colliculus and inferior olivary complex of RM-200 il[ustrating the injection site and the location of the transported label.
which cross the midline at the level of the inferior olive. The absence of silver grains over the ipsilateral subnucleus b in RM-208 (Fig. l) may be due to the shorter exposure period of the tissue in this particular case. Finally, the possibility of diffusional spread of labeled precursor into the central gray in RM-200 and RM-208 (Figs. 1 and 2) cannot be excluded. Despite this possibility, the patterns of label within the inferior olive in these two cases were quite similar to that observed in RM-214 in which the injection site was completely confined to the superior colliculus. The results of the 4 squirrel monkey experiments are shown in Figs. 5-8. Fig. 5 ($M238) illustrates a case in which both colliculi were injectxd. The injection of the left colliculus involved only the most superficial layers (see Fig. 3B). It was not possible to trace
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Fig. 4. Photomicrographs A and B show the distribution of silver grains over subnucleus b in RM-200 and SM-194 respectively (refer to Fig. 1, 130; and Fig. 7, 102). Photomicrograph C shows the distribution of silver grains over subnuclei b in SM-210 (refer to Fig. 8, 37), while D shows the distribution of silver grains over the dorsal cap of Kooy in SM-210 (refer to Fig. 8, 31). any descending pathways whatsoever from this particular injection; but, there were obvious ascending tecto-thalamic pathways which will be discussed in a future paper. In contrast, the injection of the right superior colliculus of SM-238 involved all layers, and possibly the most dorsal extent of the central gray (see Fig. 3B). The resultant pattern of silver grains over the inferior olive is strikingly similar to what has been described for RM-214 in which there was no involvement of the central gray. Specifically, there was dense label within the contralateral subnucleus b of the medial accessory nucleus, and very sparse label within this same region on the ipsilateral side. Figs. 6 (SM-194) and 7 (SM-195) illustrate the results of two experiments in which the injection sites covered extremely large areas of the colliculus, and possibly involved either the most dorsal regions of the central gray, or the pretectal region. However, as seen in the line drawings at the bottom of Figs. 6 and 7, the distribution of transported protein within the inferior olive is similar to all previously discussed cases. A photomicrograph of the terminal field in SM-194 is shown in Fig. 4B. The last experiment (SM-210) illustrates the results of an experiment in which large areas of both colliculi were injected (see Fig. 8). The injection on the right clearly involved numerous adjacent structures (e.g. the central gray, the pretectal
251
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f~M- 288 Fig. 5. The line drawings at the top of the figure illustrate the distribution and density of silver grains within the superior colliculi of SM-238. The injection in the left superior coUiculus involved only the superficial layers, whereas the injection in the right superior colliculus involved both the superficial and deeper layers. The line drawings at the bottom of the figure show the location of transported label within the inferior olivary complex of SM-238 resulting from the injection which involved the deeper layers (i.e., the right). No labeled tecto-olivary axons could be traced from the superficial injection site.
complex, the nucleus of the posterior commissure, and various thalamic nuclei). In comparison, the injection of the left superior colliculus did not involve any adjacent structures. The location of label within both medial accessory nuclei is also similar to all of the other cases described (see Figs. 8 and 4C). However, the distribution of label within the left inferior olive (i.e., contralateral to the massive injection) differs from the other experiments in that label was also observed in the dorsal cap of Kooy (see Figs. 8 and 4D). A separate descending pathway to this cell group could not be identified, and therefore it is concluded that axons which terminate in the dorsal cap of Kooy travel with the recto-spinal tract.
252
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Fig. 6. Line drawings of the superior colliculus and inferior olivary complex of SM-194 illustrating the injection site and the location of the transported label.
Lastly, it is worth mentioning, that in all of the experiments, only a few scattered labeled axons were observed at levels of the neuroaxis caudal to the inferior olivary complex. DISCUSSION This study demonstrates a well-defined projection from the superior colliculus to the inferior olivary complex in the monkey. The finding that these tectofugal fibers terminate exclusively within a confined region of the medial accessory nucleus provides additional evidence for the degree to which afferents are topographically distributed within the inferior olive. While it was not possible to identify the laminar origin of this projection, transported protein was never observed within the inferior olive when the injections involved only the superficial layers of the superior cotlicutus
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Fig. 7. Line drawings of the superior colliculusand inferior olivary complex of SM-195 illustrating the injection site and location of the transported label.
(viz., stratum zonale, stratum griseum superficiale, and stratum opticum). Therefore, the cells of origin of this projection are located in a lamina, or laminae, ventral to the stratum opticum. Escobar and de Cardenas 7 have described a tecto-olivary pathway in the cat which is similar to the one described in this paper for the monkey. They indicate that this pathway terminates within nucleus beta (Brodal a) of the medial accessory olive. In contrast to their observation, our own data, as well as those of Edwards (personal communication) indicate that this pathway in the cat does not terminate within nucleus beta, but within a region of the medial accessory olive which is similar in position to subnucleus b in the monkey. While data regarding the organization of olivo-cerebellar pathways in the monkey are not available, physiological and anatomical evidence 1,a indicates that the principal
254
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Fig. 8. The line drawings at the top of the figure illustrate the distribution and density of silver grains within the superior colliculus of SM-210. The injection of the right superior colliculus involved adjacent thalamic and brain stem regions. The line drawings at the bottom of the figure show the location of transported label within the inferior olivary complex resulting from both injections.
cerebellar target of'subnucleus b' in the cat is the posterior vermis, This region o f the cat cerebellum is responsive to both auditory and visual stimulation, andthe distributions of the two types of evoked response are coextensiveS, 24. Moreover, deeerebration does not alter the evoked response to either type o f stimulation, suggestingthat the primary source ofteleceptive input into this region is subcortical in origin ~4. More specific evidence regarding the role of the vermis stems from the demonstration that in the monkey eye movements can be evoked by vermal stimulation, and the direction of these movements varies with electrode location ~. Thus, it may be that a motor map representing eye
255 movements is contained within the vermis. The response properties of neurons within the deep layers of the superior colliculus are in some respects quite similar to those found in the vermis, in as much as some cells can be driven by either auditory or visual stimulation, and others bimodally 9,25,29. Of more interest is the fact that the deep layers of the superior colliculus resemble the vermis in that eye movement direction is spatially encoded 21,23,z6. Our results thus suggest that two regions (i.e., the deep layers of the superior colliculus and the posterior vermis) which are known to be involved in the execution of eye movements are interconnected by way of the inferior olive. In view of the evidence that the inferior olive is thought to be the primary if not the exclusive source of climbing fibers in the cerebellum, and that the climbing fiber system has a profound effect on Purkinje cell activity, the pathway linking the tectum to the cerebellum is certainly of considerable functional importance. In addition to the main finding of this study, two other significant results were obtained. First, from a single experiment (SM-210), a crossed projection to the dorsal cap of Kooy was identified. The cells of origin of this pathway could not be determined, but it is clear that they are not located within the superior colliculus. Rather, the data suggest that this pathway arises from a cell group adjacent to the rostral pole of the superior colliculus. Apparently, this projection is different from the ipsilateral pathway to the dorsal cap of Kooy described by Mizuno et al. TM as arising from the pretectal complex. The pathway described herein appears to be the one defined electrophysiologically in the rabbit by Maekawa and Simpson la as arising from the nucleus of the optic tract and terminating within the contralateral medial accessory nucleus. If this assumption is correct, then our experiment has identified an olivary cell group by which visual input is conveyed to the vestibular cerebellum (Maekawa and Simpson were able to elicit climbing fiber responses in the flocculus and nodulus by stimulating the nucleus of the optic tract). Second, we have demonstrated that in the monkey all of the crossed descending tectofugal axons course caudally within the tecto-spinal tract, or predorsal bundle, and that there is a significant reduction in the number of fibers comprising this pathway at progressively more caudal levels of the neuroaxis. In fact, relatively few labeled axons were observed to pass caudal to the inferior olive. A similar observation in the tree shrew was reported by Hatting et al. 11. Thus, in contrast to its name, it seems that the tecto-spinal tract is comprised primarily of axons which terminate within mesencephalic, pontine, and medullary cell groups. LIST OF ABBREVIATIONS a
= subnucleus a, medial accessory nucleus, inferior olive APt = pretectal area AS -- aqueduct of Sylvius (cerebral aqueduct) b = subnucleus b, medial accessory nucleus, inferior olive BC = brachium conjunctivum (superior cerebellar peduncle)
GC GL GM IV k LC LL LLD NIV
= = = = = = = = =
central gray lateral geniculate nucleus medial geniculate nucleus fourth ventricle dorsal cap of Kooy locus coeruleus lateral lemniscus nucleus of the lateral lemniseus trochlear nerve
256 c
- subnucleus c, medial accessory nucleus, inferior olive Col = inferior colliculus CoS - superior colliculus DAO -- dorsal accessory nucleus, inferior olive DG = dorsal tegmental nucleus of Gudden FLM = medial longitudinal fasciculus
NIil NIV PG Pul PuL PuM PY
.... oculomotor nucleus trochlear nucleus - parabigeminal nucleus - inferior pulvinar nucleus lateral pulvinar nucleus -- medial pulvinar nucleus =- pyramid
ACKNOWLEDGEMENTS This investigation was supported by G r a n t s EY01277 a n d BMS-75-00466 to J.K.H. a n d NS 12208 to N.L.S. Dr. F r a n k f u r t e r is a Postdoctoral Fellow i n the D e p a r t m e n t of A n a t o m y , U n i v e r s i t y of Wisconsin, M a d i s o n a n d is supported by N I H T r a i n i n g G r a n t 5-T01-GM00723. We appreciate the assistance of Mrs. B o n n i e Bade a n d we t h a n k R. W. Guillery for his helpful c o m m e n t s o n the manuscript.
REFERENCES 1 Armstrong, D. M., Functional significance of connections of the inferior olive, PhysioL Rev., 54 (1974) 358-417. 2 Bowman, J. B. and Sladek, J. R., Jr., Morphology of the inferior olivary complex of the rhesus monkey ( Macaca mulatta) , J. comp. NeuroL, 152 (1974) 299-316. 3 Brodal, A., In J. Jansen and A. Brodal (Eds.), Aspects of Cerebellar Amatomy, Johan Grundt Tanum, Oslo, 1954. 4 Courville, J. and Otabe, S., The rubro-olivary projection in the macaque: an experimental study with silver impregnation methods, J. comp. NeuroL, 158 (1974) 479-491. 5 Edwards, S. B., The ascending and descending projections of the red nucleus in the cat: an experimental study using an autoradiographic tracing method, Brain Research, 48 (1972) 45-63. 6 Edwards, S. B., Autoradiographic studies of the projections of the midbrain reticular formation: descending projections of nucleus cuneiformis, J. comp. NeuroL, 161 (1975) 341-358. 7 Escobar, A. and deCardenas, Ma., de La Luz J., On the connections between the superior colliculus and the inferior olivary nucleus. An experimental study in the cat, BoL De Estud. Med. BioL, 25 (1968) 281-290. 8 Fadiga, E. and Pupilli, G. C., Teleceptive components of the cerebellar function, PhysioL Rev., 44 (1964) 432-486. 9 Gordon, B., Receptive fields in deep layers of cat superior colliculus, J. NeurophysioL, 36 (1973) 157-178. 10 Hamilton, B. L. and Skultety, F. M., Efferent connections of the periaqueductal gray matter in the cat, J. comp. NeuroL, 139 (1970) 105-I 14. 11 Harting, J. K., Hall, W. C., Diamond, I. T. and Martin, G. F., Anterograde degeneration study of the superior colliculus in Tupaia glis: evidence for a subdivision between superficial and deep layers, J. comp. NeuroL, 148 (1973) 361-386. 12 Henkel, C. K., Linauts, M. and Martin, G. F., The origin of the annulo-olivary tract with notes on other mesencephalo-olivary pathways. A study by the horseradish peroxidase method, Brain Research, 100 (1975) 145-150. 13 Mabuchi, M. and Knsama, T., Mesodiencephalic projections to the inferior olive and vestibular and perihypoglossal nuclei, Brain Research, 17 (1970) 133-136. 14 Maekawa, K. and Simpson, J. I., Climbing fiber activation of Purkinje cells in the flocculus by impulses transferred through the visual pathway, Brain Research, 139 (1972) 245-251. 15 Martin, G. F., Dom, R., King, J. S., Robards, M. and Watson C. R. R., The inferior olivary nucleus of the opossum (Didelphis marsupialis virginiana), its organization and connections, d. comp. NeuroL, 160 (1975) 507-534.
257 16 Mettler, F. A., The tegmento-olivary and central tegmental fasciculi, J. comp. Neurol., 80 (1944) 149-175. 17 Miller, R. A. and Strominger, N. L., Efferent connections of the red nucleus to the brainstem and spinal cord of the rhesus monkey, J. comp. Neurol., 152 (1973) 327-346. 18 Mizuno, N., Mochizuki, K., Akimoto, C. and Matsushimi, R., Pretectai projections to the inferior olive in the rabbit, Exp. Neurol., 39 (1973) 498-506. 19 Ogawa, T., The tractus tegmenti medialis and its connection with the inferior olive in the cat, J. comp. Neurol., 70 (1939) 181-190. 20 Rasmussen, A. T., Tractus tecto-spinalis in the cat, J. comp. Neurol., 63 (1936) 501-525. 21 Robinson, D. A., Eye movements evoked by collicular stimulation in the alert monkey, Vision Res., 12 (1972) 1795-1808. 22 Ron, S. and Robinson, D.A., Eye movements evoked by cerebellar stimulation in the alert monkey, J. Neurophysiol., 36 (1973) 1004-1022. 23 Schiller, P. H. and Koerner, F., Discharge characteristics of single units in superior colliculus of the alert rhesus monkey, J. Neurophysiol., 34 (1971) 920-936. 24 Snider, R. S. and Stowell, A., Receiving areas of the tactile, auditory, and visual systems in the cerebellum, J. Neurophysiol., 7 (1944) 331-357. 25 Stein, B. E. and Arigbede, M. O., Unimodal and multimodal response properties of neurons in the cat's superior colliculus, Exp. Neurol., 36 (1972) 179-196. 26 Straschill, M. and Rieger, P., Eye movements evoked by focal stimulation of the cat's superior colliculus, Brain Research, 59 (1973) 211-227. 27 Walberg, F., Further studies on the descending connections to the inferior olive: reticulo-olivary fibers: an experimental study in the cat, J. comp. Neurol., 114 (1960) 79-87. 28 Walberg, F., Descending connections from the mesencephalon to the inferior olive: an experimental study in the cat, Exp. Brain Res., 21 (1974) 145-156. 29 Updyke, B.V., Characteristics of unit responses in superior colliculus of the cebus monkey, J. NeurophysioL, 37 (1974) 896-909.
245
© Elsevier/North-HollandBiomedicalPress, Amsterdam - Printed in The Netherlands
AN A U T O R A D I O G R A P H I C PROJECTION IN PRIMATES
ANALYSIS
OF
THE
TECTO-OLIVARY
A. FRANKFURTER, J. T. WEBER, G. J. ROYCE, N. L. STROMINGER and J. K. HARTING Department of Anatomy, University of Wisconsin, Madison, Wisc. 53706, and Department of Anatomy, Albany Medical College, Albany, iV. Y. 12208 (U.S.A.)
(Accepted April 13th, 1976)
SUMMARY Autoradiographic tracing methods were used to demonstrate a well-defined projection from the superior coUiculus to the inferior olivary complex in the monkey. This projection originates within the deep layers of the superior colliculus, descends within the contralateral tecto-spinal tract, and terminates within the caudal 1/3 of the medial accessory nucleus. The terminal field is restricted to a densely packed, darkly stained group of cells located in the most dorsal segment of subnucleus b. In one animal, another group of olivary afferents was identified. These fibers also descend within the contralateral tecto-spinal tract, and terminate within the dorsal cap of Kooy. While it was not possible to determine the origin of this projection, our data suggest that it arises within a region adjacent to the rostral pole of the superior colliculus. The present study further indicates that in the monkey relatively few axons which course within the classical tecto-spinal tract pass caudal to the medulla.
INTRODUCTION Anterograde axonal degeneration studies4,V,a0Aa,15-z0,27,28, most notably by Walberg 2%~s and Martin et al. 15, as well as the recent autoradiographic experiments of EdwardsS, 6, indicate that afferents from several different midbrain areas each project upon discrete segments of the inferior olivary complex. However, the results of fiber degeneration experiments, must be interpreted with caution because of the likelihood that fibers of passage have been interrupted, and also because silver impregnation methods occasionally fail to stain fine caliber axons. Consequently, the cells of origin and locus of termination of many descending fibers to the inferior olive have yet to be precisely identified. While the studies of Escobar and de Cardenas ~, Harting et al. 11, and Henkel
246 et al. 12 suggest that a portion of the midbrain-olivary projection arises from cells within the superior colliculus, other anatomical and electrophysiological data 1,1~.~s,~°, 28 do not support their findings. In view of these contradictory data, as well as the current interest in the origin of vision-related pathways influencing the inferior olive, this report describes convincing autoradiographic evidence for a prominent, and highly localized tecto-olivary projection in Saimiri sciureus and Macaca mulatta. METHODS Experiments were conducted on 3 rhesus monkeys and 4 squirrel monkeys. Injections of either tritiated proline or tritiated leucine were stereotaxically placed into the superior colliculi. Delivery of the tracer substance was accomplished with the aid of a specially designed microdrive apparatus which controlled a l0 #l Hamilton syringe equipped with a 26-gauge needle. The 3 rhesus monkeys were given unilateral injections (0.3/A) of [aH]leucine in concentrations of 30 ktCi/#l. Two squirrel monkeys (SM-210, SM-238) received bilateral injections (0.2-0.3 #l) of [aH]proline in concentrations varying between 20 and 33/~Ci//A. The other two squirrel monkeys (SM-194, SM-195) received unilateral injections (0.3 #1) of [aH]proline in concentrations of 33 #Ci//~l. The brains were fixed by perfusion with 10 % formalin 7 days postoperatively, placed in 30 ~o sucrose-formalin, and subsequently cut on a freezing microtome at 40 #m. Individual sections were mounted and coated with Kodak NTB-2 emulsion, stored in light-tight containers at 4 °C from 3 (RM-208, SM-238) to 6 (RM-200, RM-214, SM-194, SM-195, SM-210) weeks, developed, and lightly counterstained with cresyl violet. RESULTS Since it is seldom possible to specify with any degree of certainty the boundaries which separate regions where labeling is due to perikaryal uptake from regions where labeling is due to dendritic and axonal transport, line drawings are presented which illustrate the intensity and distribution of labeled protein at 3 collicular levels through the injection site for every experiment. Figs. 1 and 2 illustrate the injection sites, as well as the location of transported protein within the inferior olive in experiments RM-208, RM-214, and RM-200. A photomicrograph of the injection site in RM-214 is shown in Fig. 3A. In all 3 experiments it was possible to trace a densely labeled bundle of axons, traveling via what is commonly referred to as the tecto-spinal tract, or predorsal bundle, to a terminal field within the caudal 1/3 of the contralateral medial accessory olivary nucleus. At these levels of the inferior olive, heavy labeling was observed within and surrounding a densely packed, darkly stained group of neurons located in the most dorsal segment of subnucteus b (see Bowman and Sladek 2 and Martin et al. 15 for a detailed description of the morphology of the inferior olive). As shown in the line drawings at the bottom of Figs. 1 and 2, the majority of labeled axons.enter the inferior olivary complex between subnucleus c and the dorsal cap of Kooy. These
247
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Fig. 1. The line drawings at the top of the figure illustrate the distribution and density of silver grains, as plotted under low-power dark-field optics, within the superior colliculus of RM-208 (levels 284, 274, 264) and RM-214 (levels 160, 155, 145). The line drawings at the bottom of the figure show the location of transported label within the inferior olivary complex of RM-208 and RM-214. axons completely surround the dorsal extent of subnucleus b, apparently sending off collaterals which invade the cell group and encapsulate individual neurons, giving the terminal field a reticulated appearance. The distribution of label, as seen under low-power dark-field optics, results in a highly distinctive teardrop configuration (see Fig. 4A). It should be noted that in RM-214 and RM-200 (Figs. 1 and 2) sparse label was present within the corresponding region of the ipsilateral medial accessory nucleus. Since no labeled descending pathway could be followed on the ipsilateral side, it seems likely that this ipsilateral field of distribution represents terminals of axons
248
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Fig. 2. Line drawings of the superior colliculus and inferior olivary complex of RM-200 il[ustrating the injection site and the location of the transported label.
which cross the midline at the level of the inferior olive. The absence of silver grains over the ipsilateral subnucleus b in RM-208 (Fig. l) may be due to the shorter exposure period of the tissue in this particular case. Finally, the possibility of diffusional spread of labeled precursor into the central gray in RM-200 and RM-208 (Figs. 1 and 2) cannot be excluded. Despite this possibility, the patterns of label within the inferior olive in these two cases were quite similar to that observed in RM-214 in which the injection site was completely confined to the superior colliculus. The results of the 4 squirrel monkey experiments are shown in Figs. 5-8. Fig. 5 ($M238) illustrates a case in which both colliculi were injectxd. The injection of the left colliculus involved only the most superficial layers (see Fig. 3B). It was not possible to trace
F:
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~
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)hotomicrographs (A) and (B) show the injection sites in Macaca No. 214 and Saimiri No. 238, respectively.
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.
.
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250
Fig. 4. Photomicrographs A and B show the distribution of silver grains over subnucleus b in RM-200 and SM-194 respectively (refer to Fig. 1, 130; and Fig. 7, 102). Photomicrograph C shows the distribution of silver grains over subnuclei b in SM-210 (refer to Fig. 8, 37), while D shows the distribution of silver grains over the dorsal cap of Kooy in SM-210 (refer to Fig. 8, 31). any descending pathways whatsoever from this particular injection; but, there were obvious ascending tecto-thalamic pathways which will be discussed in a future paper. In contrast, the injection of the right superior colliculus of SM-238 involved all layers, and possibly the most dorsal extent of the central gray (see Fig. 3B). The resultant pattern of silver grains over the inferior olive is strikingly similar to what has been described for RM-214 in which there was no involvement of the central gray. Specifically, there was dense label within the contralateral subnucleus b of the medial accessory nucleus, and very sparse label within this same region on the ipsilateral side. Figs. 6 (SM-194) and 7 (SM-195) illustrate the results of two experiments in which the injection sites covered extremely large areas of the colliculus, and possibly involved either the most dorsal regions of the central gray, or the pretectal region. However, as seen in the line drawings at the bottom of Figs. 6 and 7, the distribution of transported protein within the inferior olive is similar to all previously discussed cases. A photomicrograph of the terminal field in SM-194 is shown in Fig. 4B. The last experiment (SM-210) illustrates the results of an experiment in which large areas of both colliculi were injected (see Fig. 8). The injection on the right clearly involved numerous adjacent structures (e.g. the central gray, the pretectal
251
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f~M- 288 Fig. 5. The line drawings at the top of the figure illustrate the distribution and density of silver grains within the superior colliculi of SM-238. The injection in the left superior coUiculus involved only the superficial layers, whereas the injection in the right superior colliculus involved both the superficial and deeper layers. The line drawings at the bottom of the figure show the location of transported label within the inferior olivary complex of SM-238 resulting from the injection which involved the deeper layers (i.e., the right). No labeled tecto-olivary axons could be traced from the superficial injection site.
complex, the nucleus of the posterior commissure, and various thalamic nuclei). In comparison, the injection of the left superior colliculus did not involve any adjacent structures. The location of label within both medial accessory nuclei is also similar to all of the other cases described (see Figs. 8 and 4C). However, the distribution of label within the left inferior olive (i.e., contralateral to the massive injection) differs from the other experiments in that label was also observed in the dorsal cap of Kooy (see Figs. 8 and 4D). A separate descending pathway to this cell group could not be identified, and therefore it is concluded that axons which terminate in the dorsal cap of Kooy travel with the recto-spinal tract.
252
147
127
ROSTRAt
o
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SM-I~
Fig. 6. Line drawings of the superior colliculus and inferior olivary complex of SM-194 illustrating the injection site and the location of the transported label.
Lastly, it is worth mentioning, that in all of the experiments, only a few scattered labeled axons were observed at levels of the neuroaxis caudal to the inferior olivary complex. DISCUSSION This study demonstrates a well-defined projection from the superior colliculus to the inferior olivary complex in the monkey. The finding that these tectofugal fibers terminate exclusively within a confined region of the medial accessory nucleus provides additional evidence for the degree to which afferents are topographically distributed within the inferior olive. While it was not possible to identify the laminar origin of this projection, transported protein was never observed within the inferior olive when the injections involved only the superficial layers of the superior cotlicutus
253
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o
147
b~5
o
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Fig. 7. Line drawings of the superior colliculusand inferior olivary complex of SM-195 illustrating the injection site and location of the transported label.
(viz., stratum zonale, stratum griseum superficiale, and stratum opticum). Therefore, the cells of origin of this projection are located in a lamina, or laminae, ventral to the stratum opticum. Escobar and de Cardenas 7 have described a tecto-olivary pathway in the cat which is similar to the one described in this paper for the monkey. They indicate that this pathway terminates within nucleus beta (Brodal a) of the medial accessory olive. In contrast to their observation, our own data, as well as those of Edwards (personal communication) indicate that this pathway in the cat does not terminate within nucleus beta, but within a region of the medial accessory olive which is similar in position to subnucleus b in the monkey. While data regarding the organization of olivo-cerebellar pathways in the monkey are not available, physiological and anatomical evidence 1,a indicates that the principal
254
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/
36
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Fig. 8. The line drawings at the top of the figure illustrate the distribution and density of silver grains within the superior colliculus of SM-210. The injection of the right superior colliculus involved adjacent thalamic and brain stem regions. The line drawings at the bottom of the figure show the location of transported label within the inferior olivary complex resulting from both injections.
cerebellar target of'subnucleus b' in the cat is the posterior vermis, This region o f the cat cerebellum is responsive to both auditory and visual stimulation, andthe distributions of the two types of evoked response are coextensiveS, 24. Moreover, deeerebration does not alter the evoked response to either type o f stimulation, suggestingthat the primary source ofteleceptive input into this region is subcortical in origin ~4. More specific evidence regarding the role of the vermis stems from the demonstration that in the monkey eye movements can be evoked by vermal stimulation, and the direction of these movements varies with electrode location ~. Thus, it may be that a motor map representing eye
255 movements is contained within the vermis. The response properties of neurons within the deep layers of the superior colliculus are in some respects quite similar to those found in the vermis, in as much as some cells can be driven by either auditory or visual stimulation, and others bimodally 9,25,29. Of more interest is the fact that the deep layers of the superior colliculus resemble the vermis in that eye movement direction is spatially encoded 21,23,z6. Our results thus suggest that two regions (i.e., the deep layers of the superior colliculus and the posterior vermis) which are known to be involved in the execution of eye movements are interconnected by way of the inferior olive. In view of the evidence that the inferior olive is thought to be the primary if not the exclusive source of climbing fibers in the cerebellum, and that the climbing fiber system has a profound effect on Purkinje cell activity, the pathway linking the tectum to the cerebellum is certainly of considerable functional importance. In addition to the main finding of this study, two other significant results were obtained. First, from a single experiment (SM-210), a crossed projection to the dorsal cap of Kooy was identified. The cells of origin of this pathway could not be determined, but it is clear that they are not located within the superior colliculus. Rather, the data suggest that this pathway arises from a cell group adjacent to the rostral pole of the superior colliculus. Apparently, this projection is different from the ipsilateral pathway to the dorsal cap of Kooy described by Mizuno et al. TM as arising from the pretectal complex. The pathway described herein appears to be the one defined electrophysiologically in the rabbit by Maekawa and Simpson la as arising from the nucleus of the optic tract and terminating within the contralateral medial accessory nucleus. If this assumption is correct, then our experiment has identified an olivary cell group by which visual input is conveyed to the vestibular cerebellum (Maekawa and Simpson were able to elicit climbing fiber responses in the flocculus and nodulus by stimulating the nucleus of the optic tract). Second, we have demonstrated that in the monkey all of the crossed descending tectofugal axons course caudally within the tecto-spinal tract, or predorsal bundle, and that there is a significant reduction in the number of fibers comprising this pathway at progressively more caudal levels of the neuroaxis. In fact, relatively few labeled axons were observed to pass caudal to the inferior olive. A similar observation in the tree shrew was reported by Hatting et al. 11. Thus, in contrast to its name, it seems that the tecto-spinal tract is comprised primarily of axons which terminate within mesencephalic, pontine, and medullary cell groups. LIST OF ABBREVIATIONS a
= subnucleus a, medial accessory nucleus, inferior olive APt = pretectal area AS -- aqueduct of Sylvius (cerebral aqueduct) b = subnucleus b, medial accessory nucleus, inferior olive BC = brachium conjunctivum (superior cerebellar peduncle)
GC GL GM IV k LC LL LLD NIV
= = = = = = = = =
central gray lateral geniculate nucleus medial geniculate nucleus fourth ventricle dorsal cap of Kooy locus coeruleus lateral lemniscus nucleus of the lateral lemniseus trochlear nerve
256 c
- subnucleus c, medial accessory nucleus, inferior olive Col = inferior colliculus CoS - superior colliculus DAO -- dorsal accessory nucleus, inferior olive DG = dorsal tegmental nucleus of Gudden FLM = medial longitudinal fasciculus
NIil NIV PG Pul PuL PuM PY
.... oculomotor nucleus trochlear nucleus - parabigeminal nucleus - inferior pulvinar nucleus lateral pulvinar nucleus -- medial pulvinar nucleus =- pyramid
ACKNOWLEDGEMENTS This investigation was supported by G r a n t s EY01277 a n d BMS-75-00466 to J.K.H. a n d NS 12208 to N.L.S. Dr. F r a n k f u r t e r is a Postdoctoral Fellow i n the D e p a r t m e n t of A n a t o m y , U n i v e r s i t y of Wisconsin, M a d i s o n a n d is supported by N I H T r a i n i n g G r a n t 5-T01-GM00723. We appreciate the assistance of Mrs. B o n n i e Bade a n d we t h a n k R. W. Guillery for his helpful c o m m e n t s o n the manuscript.
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