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Collateralization of the cervical corticospinal tract in the rat
NeuroscienceLetters, 105 (1989) 47-51 Elsevier Scientific Publishers Ireland Ltd.
47
NSL 06383
Collateralization of the cervical corticospinal tract in the rat A.A.M. Gribnau and P.J.W.C. Dederen Department of Anatomy and Embryology, Faculty of Medicine, Universityof Nijmegen, Nijmegen (The Netherlands) (Received 9 June 1989; Accepted 26 June 1989)
Key words. Corticospinal tract; Collateralization; Spinal cord; Motor control; Rat Anterograde staining with Phaseolusvulgaris leucoagglutinin (PHA-L) revealed the spinal arborization pattern of corticospinal tract (CST) fibers in the cervical enlargement of the rat. Within the confines of the pyramidal tract local nets of small fibers are present in addition to the rather large CST fibers with varicosities. CST termination is primarily located in lamina IV and extends into lamina V and VI. Extensive collateralizationof CST axons was found interconnecting neurons located both in different horizontal laminae and in subsequent spinal cord segments. This complex pattern of CST collateralization is suggested to add a co6rdinative role in motor control to this tract both through serial axo-dendritic contacts in the spinal gray and through axo-axonal contacts in the white as well as the gray matter. As in o t h e r r o d e n t s the fibers o f the c o r t i c o s p i n a l tract (CST) in the r a t arise f r o m the s e n s o r i m o t o r cortex, a n d after decussating, c o n t i n u e in the v e n t r a l m o s t p a r t o f the d o r s a l funiculus. In o r d e r to d i s c r i m i n a t e between the actual C S T fibers a n d their p a t h w a y , c o n t a i n i n g glial cells as well as b l o o d vessels, the latter are d e s i g n a t e d p y r a m i d a l tract (PT). F r o m earlier studies [1, 2, 4, 18] it b e c a m e a p p a r e n t t h a t rat C S T a x o n s p r i m a r i l y p r o j e c t to the c o n t r a l a t e r a l R e x e d l a m i n a e I I I - V I [12, 13] o f the spinal c o r d d o r s a l horn. M o r e recently, evidence was p r o v i d e d t h a t rat C S T fibers extend into the ventral h o r n l a m i n a e V I I a n d V I I I o f the cervical a n d l u m b a r enlargements. A l t h o u g h m o n o s y n a p t i c c o n t a c t s with m o t o n e u r o n s o f l a m i n a IX were questioned [7], with the f o r t h c o m i n g o f m o r e sensitive techniques the t e r m i n a t i o n p a t t e r n o f the rat C S T g r a d u a l l y a p p r o x i m a t e d that in cats a n d m o n k e y s [2, 6, 10, 11, 14]. U s i n g the k i d n e y b e a n lectin Phaseolus vulgar& leucoagglutinin ( P H A - L ) as an antero g r a d e tracer Casale et al. [5] even d e m o n s t r a t e d rat C S T p r o j e c t i o n s to the m o s t superficial l a m i n a e I a n d II o f the d o r s a l h o r n , c o r r o b o r a t i n g the c o n c u r r e n c y o f C S T t e r m i n a t i o n in m a m m a l i a n species. Despite the o b v i o u s relevance o f the c o m p a r t m e n t a l i z a t i o n o f C S T t e r m i n a t i o n into v a r i o u s h o r i z o n t a l l a m i n a e the l o n g i t u d i n a l p a t t e r n o f C S T c o l l a t e r a l i z a t i o n a n d
Correspondence: A.A.M. Gribnau, Department of Anatomy and Embryology, Faculty of Medicine, University of Nijmegen, P.O. Box 9101, 6500 H B Nijmegen, The Netherlands. 0304-3940/89/$ 03.50 © 1989 Elsevier Scientific Publishers Ireland Ltd.
48 termination, particularly in traversing subsequent spinal cord segments, might play a prominent role in motor control. Although prolonged postinjection survival times are required to accomplish longdistance anterograde PHA-L labelling, the complete filling of the axons as well as their terminations [8] suits the analysis of CST fiber arborization. In a pilot study using varying postinjection survival times after multiple iontophoretical PHA-L application into the sensorimotor cortex, a period of 21 days was appointed for complete CST labelling over its entire length throughout the spinal cord. Seven adult male Wistar rats, weighing approximately 200 g, were used in the present study. After deep anesthetization with pentobarbital (75 mg/kg, i.p.) they were placed in a stereotaxic apparatus. A 2.5% PHA-L (Vector Labs., U.S.A.) solution in 10 mM phosphate-buffered saline (pH 8.0) was injected iontophoretically through a glass micropipette using a positive pulsed 5 irA I)C current (7 s on, 7 s off) for 15 min. Seven injections encompassing the entire sensorimotor cortex of the left hemisphere were made per animal. Following PHA-L application, the animals were allowed to survive for 21 days, reanesthetized and perfused transcardially with physiological saline followed immediately by 250 ml of a solution consisting of 4% paraformaldehyde and 0.05% glutaraldehyde in 0.05 M phosphate buffer (PB), pH 7.4, and then by the same fixative containing 10% sucrose. After dissection of the brains and spinal cords the material was postfixed for 2 h in the same solution and then transferred into PB containing 10% sucrose at 4°C. Both brains and spinal cords were cut at 50/~m on a vibrating microtome (Microcut, BioRad) and the sections transferred into chilled 0.05 M Tris-buffered saline (TBS), pH 7.4. ]mmunohistochemical staining for PHA-L was performed using the avidin-biotinHRP complex (ABC, Vector Labs., U.S.A.). After rinsing 5 times in TBS, the sections were transferred into TBS containing 0.25% Triton X-100 and 0.1% bovine serum albumin (TBS-T). Subsequently, they were incubated overnight in TBS-T with the primary antibody (biotinylated anti-PHA-L, Vector Labs., U,S.A.) at a dilution of 1:2000 at room temperature. After thorough rinsing 2 times in TBS and TBS-T, respectively, the sections were incubated for 90 min in ABC in TBS-T. After again rinsing (twice in TBS and in 0.05 M TB (pH 7.4)), the sections were incubated for 10 min in freshly mixed DAB-Ni solution (0.05% diaminobenzidine tetrahydrochloride in 0.05 M TB, pH 7.6, containing 0.6% nickel-ammoniumsulfate and 0.01% hydrogen peroxide). After rinsing at least 3 times in 0.05 M TB, pH 7.6 the sections were mounted on gelatin-coated glass sides, air-dried overnight and then counterstained with Neutral red. Adjacent control sections were processed the same way in the absence of the primary antiserum, resulting in a negative PHA-L labelling. In all experiments, transverse sections of the brains were examined for the spread of PHA-L label at the individual injection sites (Fig. 1A). From each cervical enlargement a rostral (CFC2) and a caudal (C6-C7) portion were isolated, which then were sectioned either transversely and horizontally, respectively, or the reverse. The latter constellation applied to experiment 4, which is shown in Fig. 1. For the subdivision of the spinal gray into individual laminae we used the laminar scheme as developed
49
Fig. 1. Photomicrographs o f expt. 4. Brain (A) and cervical spinal cord segments C6-7 (B) were sectioned transversely, segments Ci-2 ( C - G ) horizontally. A: one o f the 7 injection sites; CC, corpus callosum. B a r = 0 . 4 m m . B: transverse section o f C6; PT, pyramidal tract; IV-VI, spinal cord laminae; cc, central canal. B a r = 0 . 2 m m . C: horizontal section o f C~_2; arrows indicate levels of D and F. B a r = 0 . 2 ram. D: magnification of C. B a r = 0 . 1 m m . E: labelled CST axon with local net within PT. Bar=0.01 ram. F: magnification of lamina V in C. B a r = 0.03 m m . G: horizontal section of Cl_2 located more ventrally than C. Bar = 0.05 m m .
50
by Brichta and Grant [3] for rat spinal cord as a guide. In contradistinction to the ipsilateral PT, the contralateral PT contained numerous labelled CST fibers (Fig. 1B, C). Horizontal sections revealed rather large labelled CST axons with varicosities and, in addition, local nets of small labelled fibers (Fig. I D, E). Unfortunately, the latter structures can be only poorly visualized because of focussation difficulties in photographing the rather thick vibratome sections at high magnification. This kind of labelling was never encountered in the corpus callosum (Fig. I A) which contains PHA-L-labelled fibers crossing towards the contralateral cortex. At medullary levels such nets were absent in the PT, whereas both at thoracic and at lumbar levels the PT exhibited similar labelled nets (preliminary observations). Conclusively, these local nets, which may account for at least part of the unmyelinated axons present in the rat CST as previously demonstrated [9], are characteristic for the spinal CST. CST termination is primarily located in lamina IV (Fig. IB). Beside, a reasonable amount of PHA-L labelling was found in laminae V and VI as can easily be deduced from horizontal sections (Fig. 1C, D). At higher magnification, the CST collateralization and termination pattern in all 3 laminae was found to consist of extensive arborizations of PHA-L-labelled fibers which eventually terminate upon one or more individual neurons (Fig. 1F, G). Although multiple collaterals of single axons must be substantiated by intracellular staining of individual axons, the present findings strongly suggest complex collateralization of CST axons interconnecting neurons located in both different horizontal laminae as well as in subsequent spinal cord segments. In cat and monkey substantial evidence was provided [16, 17] for terminations of collaterals of individual CST axons to various motoneurons, which in their turn project upon different limb muscles. In the rat, the presence of an extensive net of CST collaterals both in the pathway itself as well as among the interneuron pool in the spinal gray points to a very complex role of the CST in motor control. In addition to the serial axosomatic (axo-dendritic) contacts in the spinal gray, co6rdinated muscle activities may as well be mediated by axo-axonal contacts in both spinal cord compartments. We would like to thank Marion van de Coevering for typing the manuscript. I Antal, M., Termination areas of corticobulbar and corticospinal fibres in the rat, J. Hirnforsch., 25 (1984) 647 659. 2 Armand, J., The origin, course and terminations of corticospinal fibres in various mammals, Progr. Brain Res., 57 (1982) 329 360. 3 Brichta, A.M. and Grant, G., Cytoarchitectural organization of the spinal cord. In G. Paxinos (Ed.), The Rat Nervous System. Vol. 2, Hindbrain and Spinal Cord, Academic Press, New York, 1985, pp. 293 301. 4 Brown, L.T., Projections and termination of the corticospinal tract in rodents, Exp. Brain Res., 13 (1971) 432450. 5 Casale, E.J., Light, A.R. and Rustioni, A., Direct projection of the corticospinal tract to the superficial laminae of the spinal cord in the rat, J. Comp. Neurol., 278 (1988) 275 286. 6 Cheema, S.S., Rustioni, A. and Whitsel, B.L., Light and electron microscopic evidence lk~r a direct
51 corticospinal projection to superficial laminae of the dorsal horn in cats and monkeys, J. Comp. Neurol., 225 (1984) 276-290. 7 Gemma, M., Perego, G.B., Pizzini, G. and Tredici, G., Distribution of the corticospinal fibers in the cervical and lumbar enlargements of the rat spinal cord, J. Hirnfosch., 28 (1987) 457-462. 8 Gerfen, C.R. and Sawchenko, P.E., An anterograde neuroanatomical tracing method that shows the detailed morphology of neurons, their axons and terminals: immunohistochemical localization of an axonally transported plant lectin, Phaseolus vulgaris leucoagglutinin (PHA-L), Brain Res., 290 (1984) 219-238. 9 Joosten, E.A.J. and Gribnau, A.A.M., Unmyelinated corticospinal axons in adult rat pyramidal tract. An electron microscopic tracer study, Brain Res., 459 (1988) 173-177. 10 Nyberg-Hansen, R. and Brodal, A., Sites of termination of corticospinal fibers in the cat. An experimental study with silver impregnation methods, J. Comp. Neurol., 120 (1963) 369-391. 11 Ralston, D.D. and Ralston, H.J., The terminations of corticospinal tract axons in the macaque monkey, J. Comp. Neurol., 242 (1985) 325-337. 12 Rexed, B., The cytoarchitectonic organization of the spinal cord in the cat, J. Comp. Neurol., 96 (1952) 415-495. 13 Rexed, B., A cytoarchitectonic atlas of the spinal cord in the cat, J. Comp. Neurol., 100 (1954) 297-379. 14 Scheibel, M.E. and Scheibel, A.B., Terminal axonal patterns in cat spinal cord. I. The lateral corticospinal tract, Brain Res., 2 (1966) 333-350. 15 Scheibel, M.E. and Scheibel, A.B., Terminal axonal patterns in cat spinal cord. II. The dorsal horn, Brain Res., 9 (1968) 32-58. 16 Shinoda, Y., Yamaguchi, T. and Futami, T., Multiple axon collaterals of single corticospinal axons in the cat spinal cord, J. Neurophysiol., 55 (1986) 425-448. 17 Shinoda, Y., Yokota, J.-I. and Futami, T., Divergent projection of individual corticospinal axons to motoneurons of multiple muscles in the monkey, Neurosci. Lett., 23 (1981) 7-12. 18 Wen, C.-Y., Tseng, T.M., Chen, W.-P. and Shieh, J.-Y., An experimental neuroanatomical study of the corticospinal system of the albino rat, J. Formosan Med. Assoc., 74 (1975) 419-441.
47
NSL 06383
Collateralization of the cervical corticospinal tract in the rat A.A.M. Gribnau and P.J.W.C. Dederen Department of Anatomy and Embryology, Faculty of Medicine, Universityof Nijmegen, Nijmegen (The Netherlands) (Received 9 June 1989; Accepted 26 June 1989)
Key words. Corticospinal tract; Collateralization; Spinal cord; Motor control; Rat Anterograde staining with Phaseolusvulgaris leucoagglutinin (PHA-L) revealed the spinal arborization pattern of corticospinal tract (CST) fibers in the cervical enlargement of the rat. Within the confines of the pyramidal tract local nets of small fibers are present in addition to the rather large CST fibers with varicosities. CST termination is primarily located in lamina IV and extends into lamina V and VI. Extensive collateralizationof CST axons was found interconnecting neurons located both in different horizontal laminae and in subsequent spinal cord segments. This complex pattern of CST collateralization is suggested to add a co6rdinative role in motor control to this tract both through serial axo-dendritic contacts in the spinal gray and through axo-axonal contacts in the white as well as the gray matter. As in o t h e r r o d e n t s the fibers o f the c o r t i c o s p i n a l tract (CST) in the r a t arise f r o m the s e n s o r i m o t o r cortex, a n d after decussating, c o n t i n u e in the v e n t r a l m o s t p a r t o f the d o r s a l funiculus. In o r d e r to d i s c r i m i n a t e between the actual C S T fibers a n d their p a t h w a y , c o n t a i n i n g glial cells as well as b l o o d vessels, the latter are d e s i g n a t e d p y r a m i d a l tract (PT). F r o m earlier studies [1, 2, 4, 18] it b e c a m e a p p a r e n t t h a t rat C S T a x o n s p r i m a r i l y p r o j e c t to the c o n t r a l a t e r a l R e x e d l a m i n a e I I I - V I [12, 13] o f the spinal c o r d d o r s a l horn. M o r e recently, evidence was p r o v i d e d t h a t rat C S T fibers extend into the ventral h o r n l a m i n a e V I I a n d V I I I o f the cervical a n d l u m b a r enlargements. A l t h o u g h m o n o s y n a p t i c c o n t a c t s with m o t o n e u r o n s o f l a m i n a IX were questioned [7], with the f o r t h c o m i n g o f m o r e sensitive techniques the t e r m i n a t i o n p a t t e r n o f the rat C S T g r a d u a l l y a p p r o x i m a t e d that in cats a n d m o n k e y s [2, 6, 10, 11, 14]. U s i n g the k i d n e y b e a n lectin Phaseolus vulgar& leucoagglutinin ( P H A - L ) as an antero g r a d e tracer Casale et al. [5] even d e m o n s t r a t e d rat C S T p r o j e c t i o n s to the m o s t superficial l a m i n a e I a n d II o f the d o r s a l h o r n , c o r r o b o r a t i n g the c o n c u r r e n c y o f C S T t e r m i n a t i o n in m a m m a l i a n species. Despite the o b v i o u s relevance o f the c o m p a r t m e n t a l i z a t i o n o f C S T t e r m i n a t i o n into v a r i o u s h o r i z o n t a l l a m i n a e the l o n g i t u d i n a l p a t t e r n o f C S T c o l l a t e r a l i z a t i o n a n d
Correspondence: A.A.M. Gribnau, Department of Anatomy and Embryology, Faculty of Medicine, University of Nijmegen, P.O. Box 9101, 6500 H B Nijmegen, The Netherlands. 0304-3940/89/$ 03.50 © 1989 Elsevier Scientific Publishers Ireland Ltd.
48 termination, particularly in traversing subsequent spinal cord segments, might play a prominent role in motor control. Although prolonged postinjection survival times are required to accomplish longdistance anterograde PHA-L labelling, the complete filling of the axons as well as their terminations [8] suits the analysis of CST fiber arborization. In a pilot study using varying postinjection survival times after multiple iontophoretical PHA-L application into the sensorimotor cortex, a period of 21 days was appointed for complete CST labelling over its entire length throughout the spinal cord. Seven adult male Wistar rats, weighing approximately 200 g, were used in the present study. After deep anesthetization with pentobarbital (75 mg/kg, i.p.) they were placed in a stereotaxic apparatus. A 2.5% PHA-L (Vector Labs., U.S.A.) solution in 10 mM phosphate-buffered saline (pH 8.0) was injected iontophoretically through a glass micropipette using a positive pulsed 5 irA I)C current (7 s on, 7 s off) for 15 min. Seven injections encompassing the entire sensorimotor cortex of the left hemisphere were made per animal. Following PHA-L application, the animals were allowed to survive for 21 days, reanesthetized and perfused transcardially with physiological saline followed immediately by 250 ml of a solution consisting of 4% paraformaldehyde and 0.05% glutaraldehyde in 0.05 M phosphate buffer (PB), pH 7.4, and then by the same fixative containing 10% sucrose. After dissection of the brains and spinal cords the material was postfixed for 2 h in the same solution and then transferred into PB containing 10% sucrose at 4°C. Both brains and spinal cords were cut at 50/~m on a vibrating microtome (Microcut, BioRad) and the sections transferred into chilled 0.05 M Tris-buffered saline (TBS), pH 7.4. ]mmunohistochemical staining for PHA-L was performed using the avidin-biotinHRP complex (ABC, Vector Labs., U.S.A.). After rinsing 5 times in TBS, the sections were transferred into TBS containing 0.25% Triton X-100 and 0.1% bovine serum albumin (TBS-T). Subsequently, they were incubated overnight in TBS-T with the primary antibody (biotinylated anti-PHA-L, Vector Labs., U,S.A.) at a dilution of 1:2000 at room temperature. After thorough rinsing 2 times in TBS and TBS-T, respectively, the sections were incubated for 90 min in ABC in TBS-T. After again rinsing (twice in TBS and in 0.05 M TB (pH 7.4)), the sections were incubated for 10 min in freshly mixed DAB-Ni solution (0.05% diaminobenzidine tetrahydrochloride in 0.05 M TB, pH 7.6, containing 0.6% nickel-ammoniumsulfate and 0.01% hydrogen peroxide). After rinsing at least 3 times in 0.05 M TB, pH 7.6 the sections were mounted on gelatin-coated glass sides, air-dried overnight and then counterstained with Neutral red. Adjacent control sections were processed the same way in the absence of the primary antiserum, resulting in a negative PHA-L labelling. In all experiments, transverse sections of the brains were examined for the spread of PHA-L label at the individual injection sites (Fig. 1A). From each cervical enlargement a rostral (CFC2) and a caudal (C6-C7) portion were isolated, which then were sectioned either transversely and horizontally, respectively, or the reverse. The latter constellation applied to experiment 4, which is shown in Fig. 1. For the subdivision of the spinal gray into individual laminae we used the laminar scheme as developed
49
Fig. 1. Photomicrographs o f expt. 4. Brain (A) and cervical spinal cord segments C6-7 (B) were sectioned transversely, segments Ci-2 ( C - G ) horizontally. A: one o f the 7 injection sites; CC, corpus callosum. B a r = 0 . 4 m m . B: transverse section o f C6; PT, pyramidal tract; IV-VI, spinal cord laminae; cc, central canal. B a r = 0 . 2 m m . C: horizontal section o f C~_2; arrows indicate levels of D and F. B a r = 0 . 2 ram. D: magnification of C. B a r = 0 . 1 m m . E: labelled CST axon with local net within PT. Bar=0.01 ram. F: magnification of lamina V in C. B a r = 0.03 m m . G: horizontal section of Cl_2 located more ventrally than C. Bar = 0.05 m m .
50
by Brichta and Grant [3] for rat spinal cord as a guide. In contradistinction to the ipsilateral PT, the contralateral PT contained numerous labelled CST fibers (Fig. 1B, C). Horizontal sections revealed rather large labelled CST axons with varicosities and, in addition, local nets of small labelled fibers (Fig. I D, E). Unfortunately, the latter structures can be only poorly visualized because of focussation difficulties in photographing the rather thick vibratome sections at high magnification. This kind of labelling was never encountered in the corpus callosum (Fig. I A) which contains PHA-L-labelled fibers crossing towards the contralateral cortex. At medullary levels such nets were absent in the PT, whereas both at thoracic and at lumbar levels the PT exhibited similar labelled nets (preliminary observations). Conclusively, these local nets, which may account for at least part of the unmyelinated axons present in the rat CST as previously demonstrated [9], are characteristic for the spinal CST. CST termination is primarily located in lamina IV (Fig. IB). Beside, a reasonable amount of PHA-L labelling was found in laminae V and VI as can easily be deduced from horizontal sections (Fig. 1C, D). At higher magnification, the CST collateralization and termination pattern in all 3 laminae was found to consist of extensive arborizations of PHA-L-labelled fibers which eventually terminate upon one or more individual neurons (Fig. 1F, G). Although multiple collaterals of single axons must be substantiated by intracellular staining of individual axons, the present findings strongly suggest complex collateralization of CST axons interconnecting neurons located in both different horizontal laminae as well as in subsequent spinal cord segments. In cat and monkey substantial evidence was provided [16, 17] for terminations of collaterals of individual CST axons to various motoneurons, which in their turn project upon different limb muscles. In the rat, the presence of an extensive net of CST collaterals both in the pathway itself as well as among the interneuron pool in the spinal gray points to a very complex role of the CST in motor control. In addition to the serial axosomatic (axo-dendritic) contacts in the spinal gray, co6rdinated muscle activities may as well be mediated by axo-axonal contacts in both spinal cord compartments. We would like to thank Marion van de Coevering for typing the manuscript. I Antal, M., Termination areas of corticobulbar and corticospinal fibres in the rat, J. Hirnforsch., 25 (1984) 647 659. 2 Armand, J., The origin, course and terminations of corticospinal fibres in various mammals, Progr. Brain Res., 57 (1982) 329 360. 3 Brichta, A.M. and Grant, G., Cytoarchitectural organization of the spinal cord. In G. Paxinos (Ed.), The Rat Nervous System. Vol. 2, Hindbrain and Spinal Cord, Academic Press, New York, 1985, pp. 293 301. 4 Brown, L.T., Projections and termination of the corticospinal tract in rodents, Exp. Brain Res., 13 (1971) 432450. 5 Casale, E.J., Light, A.R. and Rustioni, A., Direct projection of the corticospinal tract to the superficial laminae of the spinal cord in the rat, J. Comp. Neurol., 278 (1988) 275 286. 6 Cheema, S.S., Rustioni, A. and Whitsel, B.L., Light and electron microscopic evidence lk~r a direct
51 corticospinal projection to superficial laminae of the dorsal horn in cats and monkeys, J. Comp. Neurol., 225 (1984) 276-290. 7 Gemma, M., Perego, G.B., Pizzini, G. and Tredici, G., Distribution of the corticospinal fibers in the cervical and lumbar enlargements of the rat spinal cord, J. Hirnfosch., 28 (1987) 457-462. 8 Gerfen, C.R. and Sawchenko, P.E., An anterograde neuroanatomical tracing method that shows the detailed morphology of neurons, their axons and terminals: immunohistochemical localization of an axonally transported plant lectin, Phaseolus vulgaris leucoagglutinin (PHA-L), Brain Res., 290 (1984) 219-238. 9 Joosten, E.A.J. and Gribnau, A.A.M., Unmyelinated corticospinal axons in adult rat pyramidal tract. An electron microscopic tracer study, Brain Res., 459 (1988) 173-177. 10 Nyberg-Hansen, R. and Brodal, A., Sites of termination of corticospinal fibers in the cat. An experimental study with silver impregnation methods, J. Comp. Neurol., 120 (1963) 369-391. 11 Ralston, D.D. and Ralston, H.J., The terminations of corticospinal tract axons in the macaque monkey, J. Comp. Neurol., 242 (1985) 325-337. 12 Rexed, B., The cytoarchitectonic organization of the spinal cord in the cat, J. Comp. Neurol., 96 (1952) 415-495. 13 Rexed, B., A cytoarchitectonic atlas of the spinal cord in the cat, J. Comp. Neurol., 100 (1954) 297-379. 14 Scheibel, M.E. and Scheibel, A.B., Terminal axonal patterns in cat spinal cord. I. The lateral corticospinal tract, Brain Res., 2 (1966) 333-350. 15 Scheibel, M.E. and Scheibel, A.B., Terminal axonal patterns in cat spinal cord. II. The dorsal horn, Brain Res., 9 (1968) 32-58. 16 Shinoda, Y., Yamaguchi, T. and Futami, T., Multiple axon collaterals of single corticospinal axons in the cat spinal cord, J. Neurophysiol., 55 (1986) 425-448. 17 Shinoda, Y., Yokota, J.-I. and Futami, T., Divergent projection of individual corticospinal axons to motoneurons of multiple muscles in the monkey, Neurosci. Lett., 23 (1981) 7-12. 18 Wen, C.-Y., Tseng, T.M., Chen, W.-P. and Shieh, J.-Y., An experimental neuroanatomical study of the corticospinal system of the albino rat, J. Formosan Med. Assoc., 74 (1975) 419-441.