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The rubrospinal tract in a diprotodont marsupial (Trichosurus vulpecula)
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BRAIN RESEARCH
Short Communications
The rubrospinal tract in a diprotodont marsupial (Trichosurus vulpecula) The brush-tailed possum Trichosurus vulpecula is a diprotodont marsupial with good manipulative ability in its fore- and hindlimbs. Recent studies of the functional properties of the corticospinal tract in this animal have shown that non-pyramidal pathways may be of considerable importance in the initiation and execution of movements of the limbsS, s. Since the rubrospinal projection is prominent amongst such non-pyramidal descending pathways in other mammals, it was decided to investigate the significance of the rubrospinal tract in motor control in the brush-tailed possum. The present study demonstrates the course and termination of the rubrospinal tract in order to provide basic information for a subsequent physiological comparison of the corticospinal and rubrospinal systems. Electrolytic lesions were stereotactically placed in the right red nucleus of three possums. The lesion was made by an anodal current of 500/~A which was passed for 30 sec through a monopolar electrode. Each lesion was localised to a different part of the nucleus - - the rostral pole (RNL3), the caudal pole (RNL5) and a rather lateral area midway between these two poles (RNL2) (Fig. 1). In a control experiment (RNL6) the electrode was lowered as far as, but did not enter the red nucleus and no electrolytic lesion was made. After a survival period of l 3-15 days the animals were deeply anaesthetised and killed by aortic perfusion with normal saline followed by neutral formol-saline. Transverse sections 25 # m thick were cut with a freezing microtome from each spinal cord segment and at 0.25 m m intervals through the brain stem. Degenerating fibres were stained with the Fink-Heimer method z. A microscopic study of the silver impregnated sections was made and degenerating fibres and 'terminals' were identified according to the criteria of Fink and Heimer 2. Mapping of the degeneration and assess-
RNL2
RNL3
RNL5
Fig. 1. The lesion in each of 3 experiments is represented by a blackened area in the region of the right red nucleus. The dotted line ventral to the lesion in animal RNL2 represents the track of the electrode which was inadvertently lowered into the cerebral peduncle before final correct placement in the red nucleus. The slight damage due to puncture of the cerebral peduncle resulted in corticospinal tract degeneration. Labels indicate the red nucleus (RN), superior colliculus (SC), medial geniculate (MG), lateral geniculate (LG), and basilar pons (BP).
Brain Research, 41 (1972) 180-183
SHORT COMMUNICATIONS
181
B E
Fig. 2. To show the lesion in experiment RNL5 and the course and termination of the rubrospinal tract (see arrows). Rubrobulbar projections are not illustrated and the contralateral rubrospinal pathway only is shown (see text). The diagrams C, D and E represent transverse sections of levels C6, L2 and S1 respectively. Rexed's laminae are indicated on the right-hand side of the spinal cord diagrams and are numbered in diagram C. The enlargement of the spinal cord diagrams is about twice as great as for the brain stem outlines (A and B). The red nucleus (RN), the lesion (LES), superior colliculus (SC), medial geniculate (MG), vestibulocochlear nerve (8N), facial nucleus (VII), and the spinal trigeminal nucleus (V), are labelled.
ment of the extent of the lesion were assisted by identification of cell groupings on adjacent cresyl violet stained sections in all experiments. Prior to placement of the lesion in animal RNL5, repetitive stimulation of the red nucleus with the 500/,A current for periods of less than 0.25 sec elicited phasic flexion movements of the contralateral limbs. Since cortically evoked flick movements of the hindlimb are not mediated through the pyramidal tract in the brush-tailed possum 5, the above observations support the possibility that the red nucleus may play a significant role in m o t o r control in this animal. The course and termination o f the rubrospinal fibres in the animal R N L 5 are pictured in Fig. 2 and they were essentially the same in the other animals except R N L 2 where incidental damage to the cerebral peduncle resulted in corticospinal degeneration. Rubral efferents were widely distributed in the brain stem but they will not be described in this communication. After crossing in the ventral tegmental decussation (Fig. 2A) the fibres formed a number of bundles dorsal to the medial lemniscus. More caudally the fibres formed a compact wedge-shaped tract separated from the surface by the trapezoid body, lateral to the superior olive and ventral to the emerging fibres of the trigeminal nerve. The fibres travelled caudally ventromedial to the spinal trigeminal nucleus and lateral to the facial nucleus (Fig. 2B). Brain Research, 41 (1972) 180-183
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SHORT COMMUNICATIONS
In the spinal cord rubral efferents occupied a large area of the dorsolateral funiculus separated from the surface by the dorsal spinocerebellar fibres in cervical and thoracic regions (Fig. 2C). At spinal levels the rubrospinal tract comprised a mixed population of fine and large fibres. The small degenerating fibres averaged 2 /~m in diameter and the large fibres were 6/~m in diameter when measured with a micrometer eyepiece. Both types of fibres extended into coccygeal cord segments. The distribution of degenerating terminals was plotted with reference to the laminae of Rexed 9, which were mapped out in this species using cresyl violet stained sections. In cervical segments degenerating fibres entered the spinal grey matter at the junction of laminae 6 and 7 and were dist~'ibuted in a fan-shaped pattern throughout these laminae. Although most terminal degeneration was seen in lamina 6 and the dorsal part of lamina 7, some fibres also terminated in lamina 5, and in the mid-cervical region a few extended to the ventral part of lamina 7. Lamina 6 was not present in thoracic segments and terminals were restricted to laminae 5 and 7 in this part of the cord. A large number of fibres ended in the grey matter of the lumbosacral intumescence (Fig. 2D). In these lumbosacral segments terminal degeneration extended from the lower part of lamina 4 to the ventral region of lamina 7. There were some indications that the lesion in the rostral red nucleus (RNL3) resulted in a rather different pattern of degeneration in the lumbosacral region when compared with the results in RNL2 and RNL5: over 95 ~o of the degenerating fibres in the lumbosacral cord in RNL3 were 1-3 #m in diameter and the terminal degeneration was concentrated more dorsally in the grey matter. In contrast, large fibres about 6 #m in diameter constituted over 25 ~ of the degenerating rubrospinal fibres in animals RNL2 and RNL5. Almost all the fibres passing to spinal levels were contralateral to the lesion but a few ipsilateral fibres were present. It is possible that these may have actually arisen in the contralateral red nucleus and been interrupted after decussating. The course and termination of the rubrospinal tract in the brush-tailed possum were found to be very similar to those described in a recent study on a polyprotodont marsupial, Didelphis marsupialis virginiana 6. The observations reported here support the comments of Martin and Dom 6 regarding the uniform organization of the rubrospinal system in a variety of mammals that have been studied. The termination in the spinal grey matter of the rubrospinal tract differs significantly from that of the corticospinal tract in Trichosurus. The corticospinal terminals extend dorsally into lamina 3 and do not reach as far ventrally into lamina 77,8 as do those of the rubrospinal tract. A further point of distinction is the concentration of corticospinal degeneration in the medial parts of laminae 5 and 67,8 which contrasts with the predominantly lateral distribution in these laminae of rubrospinal fibres and terminals (Fig. 2C). Furthermore, the corticospinal tract terminates largely in segments C5-T1 and passes caudally only to T107,8, whereas the rubrospinal fibres were found to extend into coccygeal segments and to terminate heavily in both cervical and lumbosacral enlargements. The chief areas of spinal termination of rubrospinal fibres in the possum (laminae 6 and 7) have been shown in the cat to contain the last order interneurons that project to the large motoneurons of lamina 910. This anatomical evidence corre-
Brain Research, 41 (1972) 180-183
SHORT COMMUNICATIONS
183
lates with the physiological findings o f H o n g o et al. a,4 an d Baldissera et alA, that r u b r o s p i n a l fibres m a k e m o n o s y n a p t i c contacts with last o r d e r interneurons.
School of Anatomy, University of New South Wales, Kensington, N.S.W. 2033 (Australia)
GARRY WARNER CHARLES R. R. WATSON
1 BALDISSERA,F., TEN BRUGGENCATE,G., AND LUNDBERG,A., Rubrospinal monosynaptic connexion with last-order interneurones of polysynaptic reflex paths, Brain Research, 27 (1971) 390-392. 2 FINK, R. P., AND HEIMER,L., TWOmethods for selective silver impregnation of degenerating axons and their synaptic endings in the central nervous system, Brain Research, 4 (1967) 369-374. 3 HONGO,R., JANKOWSKA,E., AND LUNDBERG,A., The rubrospinal tract. I. Effects on alpha-motoneurones innervating hindlimb muscles in cats, Exp. Brain Res., 7 (1969) 344-364. 4 HONGO,R., JANKOWSKA,E., AND LUNDBERG,A., The rubrospinal tract. II. Facilitation of interneuronal transmission in reflex paths to motoneurones, Exp. Brain Res., 7 (1969) 365-391. 5 HORE,J., AND PORTER,R,, The role of the pyramidal tract in the production of cortically evoked movements of the brush-tailed possum (Trichosurus vulpecula), Brain Research, 30 (1971) 232-234. 6 MARTIN,G. F., AND DOM, R., The rubrospinal tract of the opossum (Didelphis virginiana), J. comp. Neurol., 138 (1970) 19-30. 7 MARTIN,G. J., MEGIRIAN,D., AND ROEBUCK,A., The corticospinal tract of the marsupial phalanger (Trichosurus vulpecula), J. comp. Neurol., 139 (1970) 245-258. 8 REES, S., AND HORE, J., The motor cortex of the brush-tailed possum (Trichosurus vulpecula): motor representation, motor function and the pyramidal tract, Brain Research, 20 (1970) 439-451. 9 REXED,B., The cytoarchitectonic organization of the spinal cord in the cat, J. comp. Neurol., 95 (1951) 415-495. 10 STERLING,P., AND KUYPERS,H. G. J. M., Anatomical organization of the brachial spinal cord of the cat. III. The propriospinal connections, Brain Research, 7 (1968) 419-443. (Accepted February 23rd, 1972)
Brain Research, 41 (1972) 180-183
BRAIN RESEARCH
Short Communications
The rubrospinal tract in a diprotodont marsupial (Trichosurus vulpecula) The brush-tailed possum Trichosurus vulpecula is a diprotodont marsupial with good manipulative ability in its fore- and hindlimbs. Recent studies of the functional properties of the corticospinal tract in this animal have shown that non-pyramidal pathways may be of considerable importance in the initiation and execution of movements of the limbsS, s. Since the rubrospinal projection is prominent amongst such non-pyramidal descending pathways in other mammals, it was decided to investigate the significance of the rubrospinal tract in motor control in the brush-tailed possum. The present study demonstrates the course and termination of the rubrospinal tract in order to provide basic information for a subsequent physiological comparison of the corticospinal and rubrospinal systems. Electrolytic lesions were stereotactically placed in the right red nucleus of three possums. The lesion was made by an anodal current of 500/~A which was passed for 30 sec through a monopolar electrode. Each lesion was localised to a different part of the nucleus - - the rostral pole (RNL3), the caudal pole (RNL5) and a rather lateral area midway between these two poles (RNL2) (Fig. 1). In a control experiment (RNL6) the electrode was lowered as far as, but did not enter the red nucleus and no electrolytic lesion was made. After a survival period of l 3-15 days the animals were deeply anaesthetised and killed by aortic perfusion with normal saline followed by neutral formol-saline. Transverse sections 25 # m thick were cut with a freezing microtome from each spinal cord segment and at 0.25 m m intervals through the brain stem. Degenerating fibres were stained with the Fink-Heimer method z. A microscopic study of the silver impregnated sections was made and degenerating fibres and 'terminals' were identified according to the criteria of Fink and Heimer 2. Mapping of the degeneration and assess-
RNL2
RNL3
RNL5
Fig. 1. The lesion in each of 3 experiments is represented by a blackened area in the region of the right red nucleus. The dotted line ventral to the lesion in animal RNL2 represents the track of the electrode which was inadvertently lowered into the cerebral peduncle before final correct placement in the red nucleus. The slight damage due to puncture of the cerebral peduncle resulted in corticospinal tract degeneration. Labels indicate the red nucleus (RN), superior colliculus (SC), medial geniculate (MG), lateral geniculate (LG), and basilar pons (BP).
Brain Research, 41 (1972) 180-183
SHORT COMMUNICATIONS
181
B E
Fig. 2. To show the lesion in experiment RNL5 and the course and termination of the rubrospinal tract (see arrows). Rubrobulbar projections are not illustrated and the contralateral rubrospinal pathway only is shown (see text). The diagrams C, D and E represent transverse sections of levels C6, L2 and S1 respectively. Rexed's laminae are indicated on the right-hand side of the spinal cord diagrams and are numbered in diagram C. The enlargement of the spinal cord diagrams is about twice as great as for the brain stem outlines (A and B). The red nucleus (RN), the lesion (LES), superior colliculus (SC), medial geniculate (MG), vestibulocochlear nerve (8N), facial nucleus (VII), and the spinal trigeminal nucleus (V), are labelled.
ment of the extent of the lesion were assisted by identification of cell groupings on adjacent cresyl violet stained sections in all experiments. Prior to placement of the lesion in animal RNL5, repetitive stimulation of the red nucleus with the 500/,A current for periods of less than 0.25 sec elicited phasic flexion movements of the contralateral limbs. Since cortically evoked flick movements of the hindlimb are not mediated through the pyramidal tract in the brush-tailed possum 5, the above observations support the possibility that the red nucleus may play a significant role in m o t o r control in this animal. The course and termination o f the rubrospinal fibres in the animal R N L 5 are pictured in Fig. 2 and they were essentially the same in the other animals except R N L 2 where incidental damage to the cerebral peduncle resulted in corticospinal degeneration. Rubral efferents were widely distributed in the brain stem but they will not be described in this communication. After crossing in the ventral tegmental decussation (Fig. 2A) the fibres formed a number of bundles dorsal to the medial lemniscus. More caudally the fibres formed a compact wedge-shaped tract separated from the surface by the trapezoid body, lateral to the superior olive and ventral to the emerging fibres of the trigeminal nerve. The fibres travelled caudally ventromedial to the spinal trigeminal nucleus and lateral to the facial nucleus (Fig. 2B). Brain Research, 41 (1972) 180-183
182
SHORT COMMUNICATIONS
In the spinal cord rubral efferents occupied a large area of the dorsolateral funiculus separated from the surface by the dorsal spinocerebellar fibres in cervical and thoracic regions (Fig. 2C). At spinal levels the rubrospinal tract comprised a mixed population of fine and large fibres. The small degenerating fibres averaged 2 /~m in diameter and the large fibres were 6/~m in diameter when measured with a micrometer eyepiece. Both types of fibres extended into coccygeal cord segments. The distribution of degenerating terminals was plotted with reference to the laminae of Rexed 9, which were mapped out in this species using cresyl violet stained sections. In cervical segments degenerating fibres entered the spinal grey matter at the junction of laminae 6 and 7 and were dist~'ibuted in a fan-shaped pattern throughout these laminae. Although most terminal degeneration was seen in lamina 6 and the dorsal part of lamina 7, some fibres also terminated in lamina 5, and in the mid-cervical region a few extended to the ventral part of lamina 7. Lamina 6 was not present in thoracic segments and terminals were restricted to laminae 5 and 7 in this part of the cord. A large number of fibres ended in the grey matter of the lumbosacral intumescence (Fig. 2D). In these lumbosacral segments terminal degeneration extended from the lower part of lamina 4 to the ventral region of lamina 7. There were some indications that the lesion in the rostral red nucleus (RNL3) resulted in a rather different pattern of degeneration in the lumbosacral region when compared with the results in RNL2 and RNL5: over 95 ~o of the degenerating fibres in the lumbosacral cord in RNL3 were 1-3 #m in diameter and the terminal degeneration was concentrated more dorsally in the grey matter. In contrast, large fibres about 6 #m in diameter constituted over 25 ~ of the degenerating rubrospinal fibres in animals RNL2 and RNL5. Almost all the fibres passing to spinal levels were contralateral to the lesion but a few ipsilateral fibres were present. It is possible that these may have actually arisen in the contralateral red nucleus and been interrupted after decussating. The course and termination of the rubrospinal tract in the brush-tailed possum were found to be very similar to those described in a recent study on a polyprotodont marsupial, Didelphis marsupialis virginiana 6. The observations reported here support the comments of Martin and Dom 6 regarding the uniform organization of the rubrospinal system in a variety of mammals that have been studied. The termination in the spinal grey matter of the rubrospinal tract differs significantly from that of the corticospinal tract in Trichosurus. The corticospinal terminals extend dorsally into lamina 3 and do not reach as far ventrally into lamina 77,8 as do those of the rubrospinal tract. A further point of distinction is the concentration of corticospinal degeneration in the medial parts of laminae 5 and 67,8 which contrasts with the predominantly lateral distribution in these laminae of rubrospinal fibres and terminals (Fig. 2C). Furthermore, the corticospinal tract terminates largely in segments C5-T1 and passes caudally only to T107,8, whereas the rubrospinal fibres were found to extend into coccygeal segments and to terminate heavily in both cervical and lumbosacral enlargements. The chief areas of spinal termination of rubrospinal fibres in the possum (laminae 6 and 7) have been shown in the cat to contain the last order interneurons that project to the large motoneurons of lamina 910. This anatomical evidence corre-
Brain Research, 41 (1972) 180-183
SHORT COMMUNICATIONS
183
lates with the physiological findings o f H o n g o et al. a,4 an d Baldissera et alA, that r u b r o s p i n a l fibres m a k e m o n o s y n a p t i c contacts with last o r d e r interneurons.
School of Anatomy, University of New South Wales, Kensington, N.S.W. 2033 (Australia)
GARRY WARNER CHARLES R. R. WATSON
1 BALDISSERA,F., TEN BRUGGENCATE,G., AND LUNDBERG,A., Rubrospinal monosynaptic connexion with last-order interneurones of polysynaptic reflex paths, Brain Research, 27 (1971) 390-392. 2 FINK, R. P., AND HEIMER,L., TWOmethods for selective silver impregnation of degenerating axons and their synaptic endings in the central nervous system, Brain Research, 4 (1967) 369-374. 3 HONGO,R., JANKOWSKA,E., AND LUNDBERG,A., The rubrospinal tract. I. Effects on alpha-motoneurones innervating hindlimb muscles in cats, Exp. Brain Res., 7 (1969) 344-364. 4 HONGO,R., JANKOWSKA,E., AND LUNDBERG,A., The rubrospinal tract. II. Facilitation of interneuronal transmission in reflex paths to motoneurones, Exp. Brain Res., 7 (1969) 365-391. 5 HORE,J., AND PORTER,R,, The role of the pyramidal tract in the production of cortically evoked movements of the brush-tailed possum (Trichosurus vulpecula), Brain Research, 30 (1971) 232-234. 6 MARTIN,G. F., AND DOM, R., The rubrospinal tract of the opossum (Didelphis virginiana), J. comp. Neurol., 138 (1970) 19-30. 7 MARTIN,G. J., MEGIRIAN,D., AND ROEBUCK,A., The corticospinal tract of the marsupial phalanger (Trichosurus vulpecula), J. comp. Neurol., 139 (1970) 245-258. 8 REES, S., AND HORE, J., The motor cortex of the brush-tailed possum (Trichosurus vulpecula): motor representation, motor function and the pyramidal tract, Brain Research, 20 (1970) 439-451. 9 REXED,B., The cytoarchitectonic organization of the spinal cord in the cat, J. comp. Neurol., 95 (1951) 415-495. 10 STERLING,P., AND KUYPERS,H. G. J. M., Anatomical organization of the brachial spinal cord of the cat. III. The propriospinal connections, Brain Research, 7 (1968) 419-443. (Accepted February 23rd, 1972)
Brain Research, 41 (1972) 180-183