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Ultrastructural evidence for direct monosynaptic rubrospinal connections to motoneurons in Macaca mulatta
Neuroscience Letters, 95 (1988) 102-106 Elsevier Scientific Publishers Ireland Ltd.
102
NSL 05769
Ultrastructural evidence for direct monosynaptic rubrospinal connections to motoneurons in Macaca mulatta Diane Daly Ralston, Antonia M. Milroy and Gert Holstege Department of Anatomy, University of California San Francisco, San Francisco, CA 94143 (U.S.A.) (Received 4 July 1988; Revised version received 22 August 1988; Accepted 26 August 1988)
Key words: Red nucleus; Rubrospinal; Spinal cord; Motoneuron; Motor system; Fine structure; Primate The magnocellularis division of the red nucleus of the Macaea mulatta, a midbrain structure involved in processing motor information, is known by light microscopic analysis to project, via the rubrospinal tract, to the contralateral intermediate horn of the spinal cord. Physiological studies, however, provide additional evidence for direct monosynaptic connections to motoneurons subserving distal musculature. This electron microscopic study demonstrates, by analyzing the anterograde transport of 5% wheatgerm agglutinin horseradish peroxidase injected into the red nucleus, the presence of labeled terminals synapsing upon somata and proximal dendrites of motoneurons in the lateral portion of the ventral horn of the cervical enlargement of the spinal cord. We conclude that this anatomical evidence confirms the presence of direct monosynaptic connections to spinal motoneurons in the primate.
It has long been accepted that axons of the magnocellular neurons of the caudal division of the red nucleus (RNm) of the Macaca mulatta project, via the lateral funiculus, to the contralateral intermediate zone of the spinal cord [8, 11]. These projections were observed using light microscopical degeneration methods. Recently however, with the advent of more sensitive techniques that utilize axonal transport mechanisms combined with autoradiography or the histochemical demonstration of the transported wheatgerm agglutinin conjugated to horseradish peroxidase (WGAHRP) [10], light microscopic evidence has become available for a rubrospinal projection in the cat that includes the dorsolateral motoneuronal pool of the contralateral ventral horn at the level of C8 and T1 [5, 6, 9]. This projection in the cat suggests a direct monosynaptic connection from the RN to alpha motoneurons for the innervation of the distal musculature of the upper extremity. Physiological evidence for direct synaptic connections to motoneurons in cat and monkey has been demonstrated by several investigators [1, 14-16]. In addition, recordings from RNm neurons in awake monkeys performing specific motor tasks have shown activity in these Correspondence: D.D. Ralston, Research Specialist, Department of Anatomy, S-1334, University of California San Francisco, San Francisco, CA 94143, U.S.A. 0304-3940/88/$ 03.50 © 1988 Elsevier Scientific Publishers Ireland Ltd.
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neurons preceding, at the onset of and during movement, the RNm activity often being correlated with the direction and velocity of the movement [2, 3]. These studies suggested that there is monosynaptic RNm connectivity to the motoneurons subserving the muscles occupied with the task. In order to determine whether or not monosynaptic RNm-motoneuronal connections exist, we have undertaken an electron microscopical study of the fine structure of the rubrospinal tract terminations on motoneuronal cell groups utilizing the anterograde axonal transport of WGA-HRP. The light microscopic results are presented in the accompanying paper [7]. Two Macaca mulatta monkeys have been used in this study. The animals were housed and cared for pre- and postoperatively according to the N.I.H. guidelines and the Vivarium and Animal Care Committee requirements of the University of California. After fasting, the animals were sedated with ketamine hydrochloride (10 mg/kg) intramuscularly and anesthetized with nembutal 1:1 saline (28 mg/kg) intravenously. A craniotomy was performed and 1.5 or 0.3/A injections, respectively, of 5% W G A HRP were placed into the area of the RN on one side. After 3 or 4 days survival, the animals were reanesthetized and perfused intracardiaUy with phosphate-buffered saline followed by 2% paraformaldehyde and 2% glutaraldehyde at pH 7.4 at room temperature. 100 am serial vibratome sections of cervical, thoracic and lumbar spinal cord were reacted for the presence of H R P using a tetramethyl benzidine (TMB) protocol for electron microscopy [12] and appropriate blocks of sections of spinal cord revealing reaction product were prepared for electron microscopy using a slow osmication method for post fixation [4]. Sections of ventral horn, which contained reaction product, were then processed for electron microscopy using standard embedding techniques, sectioned on an LKB ultramicrotome, stained with uranyl acetate and Reynold's lead citrate and viewed with a JEOL 100 CXII. Light microscopical results of these experiments are reported in the preceding paper and have demonstrated the presence of label in the lateral motoneuron pools. 1 Itm semithin sections of the ventrolateral portion of the ventral horn show label located in myelinated axons and adjacent to the somata of motoneurons and primary dendrites (Fig. 1). Thin sections demonstrate labeled myelinated axons of varying diameters entering the lateral portion of the ventral horn by way of the lateral funiculus. Electron microscopy confirms the presence of synaptic terminals which contain reaction product and which contact large neurons of the ventral horn. Most labeled synaptic profiles are large terminals containing rounded synaptic vesicles (Figs. 2 and 3) some of which also reveal occasional dense cored vesicles. These terminal types form asymmetric contacts with cell bodies, proximal dendrites or smaller dendritic profiles, some of which may be dendritic spines. Occasionally, other labeled profiles contain flattened or pleomorphic synaptic vesicles. They contact cell bodies and large diameter dendritic profiles and the postsynaptic membrane usually exhibits a subsynaptic density (Fig. 4). This ultrastructural study describes labeling of axon terminals in primate ventral horn following injections of W G A - H R P into RNm. We believe that the postsynaptic
105
neurons are motoneurons because of their very large size (50-75/tm in cell diameter) and their location within the ventral horn. Thus, we believe we have demonstrated the existence of a direct monosynaptic input to motoneurons in the lateral portion for the ventral horn of the spinal cord in the Macaca mulatta. Although the major rubral input to the spinal cord is to the intermediate zone, direct synaptic contacts with the motoneuronal somata and proximal dendrites infer a greater influence upon spinal motoneurons than previously thought. Labeled terminals which contain rounded synaptic vesicles and form asymmetric contacts with motoneuronal somata and primary dendrites constitute the predominant population of rubro-motoneuronal contacts in our study. In addition, occasional labeled terminals contain flattened or pleomorphic vesicles. The neurotransmitter species and their associated functions in each of these terminal types are as yet unknown. The RNm-motoneuronal contacts do not appear to demonstrate the same heterogeneity of terminal types or spatial distribution as do those of the cortico-motoneuronal system [13]. Given the relative sizes of the two descending systems and their appearance in phylogeny, it is suggested that the rubrospinal system acts in parallel with the corticospinal system but appears more homogeneous in its terminations. The present study, however, represents an observation of a limited portion of the motoneuronal receptive area and thus the extent of synaptic terminations onto secondary, tertiary or distal dendrites, which extend well into the intermediate zone of the spinal grey, is yet to be determined. We plan to extend our studies to examine the more distal regions of motoneuronal dendritic trees, to determine whether the rubrospinal projections terminate more widely over the motoneuronal surface, perhaps with a more heterogenous population of terminals, which would suggest a more complex interaction and a more diversified input to motoneurons. In conclusion, previous anatomical and physiological studies have suggested that there are monosynaptic inputs from the RNm to spinal motoneurons in both cat and monkey. This electron snicroscopic study is the first to provide morphological evidence for a direct, monosynaptic connection to large neurons in the motoneuron pool of the cervical spinal cord in the Macaca mulatta. Given the variation in the vesicle morphology within the labeled terminals, it is likely that different neurotrans-
Fig. 1. A 1 g m semithin section of the lateral portion o f the ventral horn of the cervical spinal cord (C8). W G A - H R P reaction product (open arrows) can be seen adjacent to large neurons as well as proximal dendrites. Lipofuscin granules can be seen within the neuronal cytoplasmic matrix. Bar = 20/an. Fig. 2. A round vesicle terminal (R) containing crystalline reaction product (closed arrow) forms an asymmetric synaptic contact (open arrow) with a primary dendrite (D). x 23,000. Bar = 1 pm. Fig. 3. A round vesicle terminal (R) containing crystalline reaction product (closed arrow) forms an asymmetrical contact (open arrow) with a neuronal s o m a (S), presumed to be that o f a motoneuron, x 22,000. Bar = 1 pm. Fig. 4. A terminal containing primarily flattened to pleomorphic vesicles (F) contacts the somata of a large neuron (open arrow) in the same region of the lateral ventral horn. The presence of crystalline reaction product in this terminal (closed arrows) infers the presence o f a diversified rubrospinal terminal population. x 28,000. Bar = I/~m.
106 m i t t e r s a r e i n v o l v e d in the a c t i v i t y o f the r u b r o s p i n a l tract. T h e s y n a p t i c i n p u t , o b s e r v e d to t e r m i n a t e o n s o m a t a a n d p r o x i m a l d e n d r i t e s , s u g g e s t s t h a t t h e red n u c l e us exerts a s u b s t a n t i a l d i r e c t i n f l u e n c e u p o n s p i n a l m o t o n e u r o n s . W e w o u l d like to t h a n k H . J . R a l s t o n , III, M . D . f o r his r e v i e w o f t h e m a n u s c r i p t a n d a c k n o w l e d g e the t e c h n i c a l a s s i s t a n c e o f M s . S. C a n c h o l a a n d M r . T. T o r r e s . S u p p o r t e d by N I H G r a n t N S 23347 a n d N A S A N C C 2 - 4 9 1 . 1 Cheney, P.D., Response of rubromotoneuronal cells identified by spike-triggered averaging of EMG activity in awake monkeys, Neurosci. Lett., 17 (1980) 137 142. 2 Gibson, A.R., Houk, J.C. and Kohlerman, N.J., Magnocellular red nucleus activity during different types of limb movement in the macaque monkey, J. Physiol. (Lond.), 358 (1985) 527--549. 3 Gibson, A.R., Houk, J.C. and Kohlerman, N.J., Relation between red nucleus discharge and movement parameters in trained macaque monkeys, J. Physiol. 358 (1985) 551--570. 4 Henry, M.A., Westrum, L.E. and Johnson, L.R., Enhanced ultrastructural visualization of the horseradish peroxidase-tetramethylbenzidine reaction product, J. Histochem. Cytochem., 33 (1985) 125t~ 1259. 5 Holstege, G., Anatomical evidence for an ipsilateral rubrospinal pathway and for direct rubrospinal projections to motoneurons in the cat, Neurosci. Lett., 74 (1987) 269 274. 6 Holstege, G. and Tan, J., Projections from the red nucleus and surrounding areas to the brainstem and spinal cord of the cat, Behav. Brain Res., 28 (1988) 33 57. 7 Holstege, G., Blok, B.F. and Ralston, D.D., Anatomical evidence for red nucleus projections to motoneuronal cell groups in the spinal cord of the monkey, Neurosci. Lett., 95 (1988) 97-101. 8 Kuypers, H.G.J.M., Fleming, W.R. and Farinholt, J.W., Subcorticospinal projections in the rhesus monkey. J. Comp. Neurol., 118 (1962) 107--137. 9 McCurdy, M.L., Hansma, D.I., Houk, J.C. and Gibson, A.R., Selective projections from the cat red nucleus to digit motor neurons, J. Comp. Neurol., 265 (1987) 367-379. I0 Mesulam, M.M., Tracing neuronal connections with horseradish peroxidase, In IBRO Handbook, Wiley, Chichester, 1982, p. 251. 11 Miller, R.A. and Strominger, N.L., Efferent connections of the red nucleus in the brainstem and spinal cord of the rhesus monkey, J. Comp. Neurol., 152 (1973) 327 346. 12 Olucha, F., Martinez-Garcia, F. and Lopez-Garcia., C. A new stabilizing-agent for the tetramethylbenzidine (TMB) reaction product in the histochemical detection of horseradish peroxidase (HRP), J. Neurosci. Methods, 13 (1985) 131 138. 13 Ralston, D.D. and Ralston III, H.J., The terminations of corticospinal tract axons in the macaque monkey, J. Comp. Neurol., 242 (1985) 325 337. 14 Shapovalov, A.I. and Karamjan, O.A., Short-latency interstitiospinal and rubrospinal synaptic influences on c~-motoneurons, Byull. Eksp. Biol. Med., 66 (1968) 1297-1300. 15 Shapovalov, A.I., Karamjan, O.A., Kurchavyi, G.G. and Repina, Z.A., Synaptic actions evoked from the red nucleus on the spinal ~-motoneurons in the rhesus monkey, Brain Res., 32 (1971) 32%348. 16 Shapovalov, A.I. and Kurchavyi, G.G., Effects of transmembrane polarization and TEA injections on monosynaptic actions from motor cortex, red nucleus and group Ia afferents on lumbar motoneurons in the monkey, Brain Res., 82 (1974) 49~7.
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NSL 05769
Ultrastructural evidence for direct monosynaptic rubrospinal connections to motoneurons in Macaca mulatta Diane Daly Ralston, Antonia M. Milroy and Gert Holstege Department of Anatomy, University of California San Francisco, San Francisco, CA 94143 (U.S.A.) (Received 4 July 1988; Revised version received 22 August 1988; Accepted 26 August 1988)
Key words: Red nucleus; Rubrospinal; Spinal cord; Motoneuron; Motor system; Fine structure; Primate The magnocellularis division of the red nucleus of the Macaea mulatta, a midbrain structure involved in processing motor information, is known by light microscopic analysis to project, via the rubrospinal tract, to the contralateral intermediate horn of the spinal cord. Physiological studies, however, provide additional evidence for direct monosynaptic connections to motoneurons subserving distal musculature. This electron microscopic study demonstrates, by analyzing the anterograde transport of 5% wheatgerm agglutinin horseradish peroxidase injected into the red nucleus, the presence of labeled terminals synapsing upon somata and proximal dendrites of motoneurons in the lateral portion of the ventral horn of the cervical enlargement of the spinal cord. We conclude that this anatomical evidence confirms the presence of direct monosynaptic connections to spinal motoneurons in the primate.
It has long been accepted that axons of the magnocellular neurons of the caudal division of the red nucleus (RNm) of the Macaca mulatta project, via the lateral funiculus, to the contralateral intermediate zone of the spinal cord [8, 11]. These projections were observed using light microscopical degeneration methods. Recently however, with the advent of more sensitive techniques that utilize axonal transport mechanisms combined with autoradiography or the histochemical demonstration of the transported wheatgerm agglutinin conjugated to horseradish peroxidase (WGAHRP) [10], light microscopic evidence has become available for a rubrospinal projection in the cat that includes the dorsolateral motoneuronal pool of the contralateral ventral horn at the level of C8 and T1 [5, 6, 9]. This projection in the cat suggests a direct monosynaptic connection from the RN to alpha motoneurons for the innervation of the distal musculature of the upper extremity. Physiological evidence for direct synaptic connections to motoneurons in cat and monkey has been demonstrated by several investigators [1, 14-16]. In addition, recordings from RNm neurons in awake monkeys performing specific motor tasks have shown activity in these Correspondence: D.D. Ralston, Research Specialist, Department of Anatomy, S-1334, University of California San Francisco, San Francisco, CA 94143, U.S.A. 0304-3940/88/$ 03.50 © 1988 Elsevier Scientific Publishers Ireland Ltd.
103
neurons preceding, at the onset of and during movement, the RNm activity often being correlated with the direction and velocity of the movement [2, 3]. These studies suggested that there is monosynaptic RNm connectivity to the motoneurons subserving the muscles occupied with the task. In order to determine whether or not monosynaptic RNm-motoneuronal connections exist, we have undertaken an electron microscopical study of the fine structure of the rubrospinal tract terminations on motoneuronal cell groups utilizing the anterograde axonal transport of WGA-HRP. The light microscopic results are presented in the accompanying paper [7]. Two Macaca mulatta monkeys have been used in this study. The animals were housed and cared for pre- and postoperatively according to the N.I.H. guidelines and the Vivarium and Animal Care Committee requirements of the University of California. After fasting, the animals were sedated with ketamine hydrochloride (10 mg/kg) intramuscularly and anesthetized with nembutal 1:1 saline (28 mg/kg) intravenously. A craniotomy was performed and 1.5 or 0.3/A injections, respectively, of 5% W G A HRP were placed into the area of the RN on one side. After 3 or 4 days survival, the animals were reanesthetized and perfused intracardiaUy with phosphate-buffered saline followed by 2% paraformaldehyde and 2% glutaraldehyde at pH 7.4 at room temperature. 100 am serial vibratome sections of cervical, thoracic and lumbar spinal cord were reacted for the presence of H R P using a tetramethyl benzidine (TMB) protocol for electron microscopy [12] and appropriate blocks of sections of spinal cord revealing reaction product were prepared for electron microscopy using a slow osmication method for post fixation [4]. Sections of ventral horn, which contained reaction product, were then processed for electron microscopy using standard embedding techniques, sectioned on an LKB ultramicrotome, stained with uranyl acetate and Reynold's lead citrate and viewed with a JEOL 100 CXII. Light microscopical results of these experiments are reported in the preceding paper and have demonstrated the presence of label in the lateral motoneuron pools. 1 Itm semithin sections of the ventrolateral portion of the ventral horn show label located in myelinated axons and adjacent to the somata of motoneurons and primary dendrites (Fig. 1). Thin sections demonstrate labeled myelinated axons of varying diameters entering the lateral portion of the ventral horn by way of the lateral funiculus. Electron microscopy confirms the presence of synaptic terminals which contain reaction product and which contact large neurons of the ventral horn. Most labeled synaptic profiles are large terminals containing rounded synaptic vesicles (Figs. 2 and 3) some of which also reveal occasional dense cored vesicles. These terminal types form asymmetric contacts with cell bodies, proximal dendrites or smaller dendritic profiles, some of which may be dendritic spines. Occasionally, other labeled profiles contain flattened or pleomorphic synaptic vesicles. They contact cell bodies and large diameter dendritic profiles and the postsynaptic membrane usually exhibits a subsynaptic density (Fig. 4). This ultrastructural study describes labeling of axon terminals in primate ventral horn following injections of W G A - H R P into RNm. We believe that the postsynaptic
105
neurons are motoneurons because of their very large size (50-75/tm in cell diameter) and their location within the ventral horn. Thus, we believe we have demonstrated the existence of a direct monosynaptic input to motoneurons in the lateral portion for the ventral horn of the spinal cord in the Macaca mulatta. Although the major rubral input to the spinal cord is to the intermediate zone, direct synaptic contacts with the motoneuronal somata and proximal dendrites infer a greater influence upon spinal motoneurons than previously thought. Labeled terminals which contain rounded synaptic vesicles and form asymmetric contacts with motoneuronal somata and primary dendrites constitute the predominant population of rubro-motoneuronal contacts in our study. In addition, occasional labeled terminals contain flattened or pleomorphic vesicles. The neurotransmitter species and their associated functions in each of these terminal types are as yet unknown. The RNm-motoneuronal contacts do not appear to demonstrate the same heterogeneity of terminal types or spatial distribution as do those of the cortico-motoneuronal system [13]. Given the relative sizes of the two descending systems and their appearance in phylogeny, it is suggested that the rubrospinal system acts in parallel with the corticospinal system but appears more homogeneous in its terminations. The present study, however, represents an observation of a limited portion of the motoneuronal receptive area and thus the extent of synaptic terminations onto secondary, tertiary or distal dendrites, which extend well into the intermediate zone of the spinal grey, is yet to be determined. We plan to extend our studies to examine the more distal regions of motoneuronal dendritic trees, to determine whether the rubrospinal projections terminate more widely over the motoneuronal surface, perhaps with a more heterogenous population of terminals, which would suggest a more complex interaction and a more diversified input to motoneurons. In conclusion, previous anatomical and physiological studies have suggested that there are monosynaptic inputs from the RNm to spinal motoneurons in both cat and monkey. This electron snicroscopic study is the first to provide morphological evidence for a direct, monosynaptic connection to large neurons in the motoneuron pool of the cervical spinal cord in the Macaca mulatta. Given the variation in the vesicle morphology within the labeled terminals, it is likely that different neurotrans-
Fig. 1. A 1 g m semithin section of the lateral portion o f the ventral horn of the cervical spinal cord (C8). W G A - H R P reaction product (open arrows) can be seen adjacent to large neurons as well as proximal dendrites. Lipofuscin granules can be seen within the neuronal cytoplasmic matrix. Bar = 20/an. Fig. 2. A round vesicle terminal (R) containing crystalline reaction product (closed arrow) forms an asymmetric synaptic contact (open arrow) with a primary dendrite (D). x 23,000. Bar = 1 pm. Fig. 3. A round vesicle terminal (R) containing crystalline reaction product (closed arrow) forms an asymmetrical contact (open arrow) with a neuronal s o m a (S), presumed to be that o f a motoneuron, x 22,000. Bar = 1 pm. Fig. 4. A terminal containing primarily flattened to pleomorphic vesicles (F) contacts the somata of a large neuron (open arrow) in the same region of the lateral ventral horn. The presence of crystalline reaction product in this terminal (closed arrows) infers the presence o f a diversified rubrospinal terminal population. x 28,000. Bar = I/~m.
106 m i t t e r s a r e i n v o l v e d in the a c t i v i t y o f the r u b r o s p i n a l tract. T h e s y n a p t i c i n p u t , o b s e r v e d to t e r m i n a t e o n s o m a t a a n d p r o x i m a l d e n d r i t e s , s u g g e s t s t h a t t h e red n u c l e us exerts a s u b s t a n t i a l d i r e c t i n f l u e n c e u p o n s p i n a l m o t o n e u r o n s . W e w o u l d like to t h a n k H . J . R a l s t o n , III, M . D . f o r his r e v i e w o f t h e m a n u s c r i p t a n d a c k n o w l e d g e the t e c h n i c a l a s s i s t a n c e o f M s . S. C a n c h o l a a n d M r . T. T o r r e s . S u p p o r t e d by N I H G r a n t N S 23347 a n d N A S A N C C 2 - 4 9 1 . 1 Cheney, P.D., Response of rubromotoneuronal cells identified by spike-triggered averaging of EMG activity in awake monkeys, Neurosci. Lett., 17 (1980) 137 142. 2 Gibson, A.R., Houk, J.C. and Kohlerman, N.J., Magnocellular red nucleus activity during different types of limb movement in the macaque monkey, J. Physiol. (Lond.), 358 (1985) 527--549. 3 Gibson, A.R., Houk, J.C. and Kohlerman, N.J., Relation between red nucleus discharge and movement parameters in trained macaque monkeys, J. Physiol. 358 (1985) 551--570. 4 Henry, M.A., Westrum, L.E. and Johnson, L.R., Enhanced ultrastructural visualization of the horseradish peroxidase-tetramethylbenzidine reaction product, J. Histochem. Cytochem., 33 (1985) 125t~ 1259. 5 Holstege, G., Anatomical evidence for an ipsilateral rubrospinal pathway and for direct rubrospinal projections to motoneurons in the cat, Neurosci. Lett., 74 (1987) 269 274. 6 Holstege, G. and Tan, J., Projections from the red nucleus and surrounding areas to the brainstem and spinal cord of the cat, Behav. Brain Res., 28 (1988) 33 57. 7 Holstege, G., Blok, B.F. and Ralston, D.D., Anatomical evidence for red nucleus projections to motoneuronal cell groups in the spinal cord of the monkey, Neurosci. Lett., 95 (1988) 97-101. 8 Kuypers, H.G.J.M., Fleming, W.R. and Farinholt, J.W., Subcorticospinal projections in the rhesus monkey. J. Comp. Neurol., 118 (1962) 107--137. 9 McCurdy, M.L., Hansma, D.I., Houk, J.C. and Gibson, A.R., Selective projections from the cat red nucleus to digit motor neurons, J. Comp. Neurol., 265 (1987) 367-379. I0 Mesulam, M.M., Tracing neuronal connections with horseradish peroxidase, In IBRO Handbook, Wiley, Chichester, 1982, p. 251. 11 Miller, R.A. and Strominger, N.L., Efferent connections of the red nucleus in the brainstem and spinal cord of the rhesus monkey, J. Comp. Neurol., 152 (1973) 327 346. 12 Olucha, F., Martinez-Garcia, F. and Lopez-Garcia., C. A new stabilizing-agent for the tetramethylbenzidine (TMB) reaction product in the histochemical detection of horseradish peroxidase (HRP), J. Neurosci. Methods, 13 (1985) 131 138. 13 Ralston, D.D. and Ralston III, H.J., The terminations of corticospinal tract axons in the macaque monkey, J. Comp. Neurol., 242 (1985) 325 337. 14 Shapovalov, A.I. and Karamjan, O.A., Short-latency interstitiospinal and rubrospinal synaptic influences on c~-motoneurons, Byull. Eksp. Biol. Med., 66 (1968) 1297-1300. 15 Shapovalov, A.I., Karamjan, O.A., Kurchavyi, G.G. and Repina, Z.A., Synaptic actions evoked from the red nucleus on the spinal ~-motoneurons in the rhesus monkey, Brain Res., 32 (1971) 32%348. 16 Shapovalov, A.I. and Kurchavyi, G.G., Effects of transmembrane polarization and TEA injections on monosynaptic actions from motor cortex, red nucleus and group Ia afferents on lumbar motoneurons in the monkey, Brain Res., 82 (1974) 49~7.