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Development of locomotor activities in young chronic spinal rabbits
Neuroscience Letters, 3 (1976) 329--333 © Elsevier/North-Holland Biomedical Press
329
DEVELOPMENT OF LOCOMOTOR ACTIVITIES IN YOUNG CHRONIC SPINAL RABBITS
N.A. FAYEIN and D. VIALA Universit~ de Nancy I, Facult$ de M~decine A, Laboratoire de Physiologie (Directeur Pr. P. Arnould), F. 54019 Nancy Cedex and (D. V.) Universit~ Pierre et Marie Curie, Laboratoire de Neurophysiologie compar~e (Directeur Pr. P. Buser), F. 75230 Paris Cedex 05 (France) (Received November 2nd, 1976) (Accepted November 3rd, 1976)
SUMMARY
It was previously demonstrated that adult chronic spinal rabbits do not recover any spontaneous locomotor activity of the hindlimbs. Pharmacological stimuli may, nevertheless, evoke locomotor activities in the acute spinal animal; however, the pattern is alternate between both hindlimbs instead of being symmetrical (normal pattern in the intact animal). Present data indicate that young rabbits whose spinal cord was transected at birth can recover and spontaneously develop hindlimb locomotor movements over a period of one m o n t h after spinal section. These rhythmic activities are either synchronous or alternate between the two sides, or both. It is concluded that n o t only the alternate pattern but also a pure synchronous one can be generated in the lumbosacral cord in the y o u n g chronic spinal rabbit.
It is known from early observations that vertebrates including mammals may develop, to a certain extent, walking movements after transection of the spinal cord [ 2,4,6,7]. Complementary investigations have demonstrated t h a t locom o t o r activities recorded in the hindlimbs of spinal animals have a central origin. The locomotor r h y t h m is programmed by an intraspinal generator [1,2,5,10]. However, the lack of coordinated movements of progression in the adult chronic spinal rabbit [7] as well as the lack of any efferent rhythmic activities in the hindlimb nerves of the same curarized preparation have been generally reported [9]. But, after a pharmacological treatment associating a MAO inhibitor with DOPA applied to the acute spinal rabbit, central coordinated locomotor activities were obtained, with a predominant pattern of alternance of activities in the bilateral symmetrical nerves [ 10]. At times, synchronous activities could develop but they were never maintained over long periods, being rapidly replaced by alternance [8]. It was thus concluded
330 that the lumbosacral locomotor generator could not reproduce on its own the normal mode of rabbits' hopping progression, and that the synchronization of motor discharges in the bilateral symmetrical muscles required supraspinal levels and was very likely organized in the brain stem. In the present study, we investigated to what extent the development of locomotor activities and progression can be recovered in young chronic spinal rabbits whose spinal cord was transected a few hours after birth, and how that locomotor activity is organized. The behaviour of 18 chronic young rabbits was studied after spinal transection performed on the newborn animals between 24 and 48 h. The young rabbits were given back to the mother's care and left quiet for about two weeks. They were then submitted to daily nursing and passive mechanical mobilization of all joints of both hindlimbs in order to prevent progressive stiffness and muscular atrophy. Development of hopping movements was observed until the age of about 4--5 weeks with relatively good balance and postural adjustment during progression. The general state and m o t o r performance of the young animals were greatly dependent on a constant nursing and intensity of the training. But functional impairment generally appeared after 4--5 weeks and the situation was rapidly degrading; the animals reached a plateau body weight and they could not stand any more. Experiments were performed on rabbits of 4--6 weeks old, whose hindlimbs still developed rhythmic and bilateral movements of large amplitude, both flexions and extensions involving several joints. For analysis of m o t o r activity, the activities of two antagonistic muscles of both right and left hindlimbs (a flexor, tibialis anterior and an extensor, gastrocnemius medialis) were recorded electromyographically. The h o m o n y m o u s muscles of the hindlimbs showed synchronous discharges at about 1/sec. during spontaneous flexion and extension movements (Fig. 1A and B). The characteristic pattern of normal progression for that species, i.e. jumping, was reflected by the EMG activity. But an alternate pattern could also be recorded and appeared through progressive desynchronization of symmetrical activities, generally, when the r h y t h m of the beats tended to increase frequency (Fig. 1C). The locomotor movements appeared spontaneously or could be induced by slight or prolonged pinching of the tail. In order to determine whether the synchronous pattern is organized or not at a central spinal level, the rhythmic afferences linked to the movement were suppressed by the injection of a curarizing drug (Flaxedil) and the animals were maintained under artificial ventilation. The rhythmic activities, recorded from the m o t o r nerves of the symmetrical flexors and extensors, showed the same bilateral organization as on the EMG recording, during locomotor phases mostly obtained by intense pressures on the tail; they rarely developed spontaneously (Fig. 2A). The frequency of the evoked rhythmic discharges could reach 1.6--2/sec. To ensure that the spinal section was complete and that no remaining connections with the cephalad cord might account for the present findings, a second section was further performed a few mm more caudal than the previous one. The locomotor patterns remained very
331
A LGM RGM
B LTA RTA
C LTA RTA
,
!s
Fig. 1. Electromyographic activities of locomotor movements in young chronic spinal rabbits. Spontaneous rhythmic discharges were recorded from left (LGM) and right (RGM) extensor muscles: gastrocnemius medialis (A)and from left (LTA) and right (RTA) flexor muscles: tibialis anterior (B). The discharge pattern showed good synchronization in both homonymous muscles activities for long sequences of locomotor movements. However, the central programme might switch from the synchronous to the alternate pattern of discharges in the symmetrical hindlimbs muscles (C).
similar after this second section although we noticed a slight acceleration of the discharge frequency, probably due to the mechanical excitation of the cord (Fig. 2B). From these findings it may be concluded that (1) young spinal rabbits recover m o t o r activities after spinal transection at birth, and are able to develop spontaneous walking movements when they grow up over one m o n t h of age; (2) as in the adult spinal rabbit, both alternate and synchronous bilateral
332
A LTAn LGMn
II ~ : lllil '~ . i ~U
~
I
~
_
~
I
I :
II _ill . II . i
RTAn RGMn
B LTA
n
LGMn RTA
n
RGMn
i
1S
a
Fig. 2. Lo co m o t o r nerve discharges in paralyzed young spinal rabbits. L o c o m o t o r activities were recorded bilaterally from a branch of the sciatic nerve innervating the gastrocnemius medialis (LGMn and RGMn), and from common peroneal nerve innervating the tibialis anterior (LTAn and RTAn). Motor activities were evoked through sustained pressure on the tail. A: predominant discharge pattern is bilaterally synchronous activities. B: this pattern remains unchanged after a second section of the spinal cord (although the discharge frequency is increased).
organization exist in the locomotor pattern of the hindlimbs but, unlike the adult spinal, the synchronous pattern can be maintained and may be the only pattern spontaneously observed during normal progression, and (3) the organization of both alternate and synchronous movements in the hindlimbs is generated by a central programme in the lumbosacral cord. Afferent control only helps spontaneous movements to develop and to be maintained. The spinal generator alone thus seems able to coordinate pluriarticular movements when the medullary pathways are disconnected from the higher nervous centers. But such a functional autonomous mechanism is impossible to express when the spinal transection is performed at or after the 15th day after birth. Later on, no recovery of locomotor activities can be obtained (unpublished results). It is well known that the CNS of newborn rabbits is particularly immature and 14 days represents the critical age to achieve complete myelinization of the supraspi.~al descending pathways [3]. The problem remains for consideration whether a central generator for both modalities of hindlimb movements pre-exists in the lumbosacral cord and just
333
expresses their intrinsic properties when n o t submitted to higher central control, or whether an arrangement of neuronal circuits m a y establish new connections towards a functional purpose in various experimental situations during the phase of maturation process when the spinal generator is n o t influenced by higher nervous control. REFERENCES 1 Brown, T.G., The intrinsic factors in the act of progression in the mammal, Proc. roy. Soc. B, 84 (1911) 308--319. 2 Grillner, S., Locomotion in vertebrates -- central mechanisms and reflex interaction, Physiol. Rev., 55 (1975) 247--304. 3 Langworthy, O.R., Relation of onset of decerebrate rigidity to the time of myelinisation of tracts in the brain-stem and spinal cord of young animals, Contr. Embryol. Carneg. lnstn, 89 (1926) 127--140. 4 Laughton, N.B., Studies on the nervous regulation of progression in mammals, Amer. J. Physiol., 70 (1924) 358--384. 5 Perret, C., Cabelguen, J.M. et Millanvoye, M., Caract~ristiques d'un rythme de type locomoteur chez le chat spinal aigu, J. Physiol. (Paris), 65 (1973) 473A. 6 Shurrager, P.S. and Dykman, R.A., Walking spinal carnivores, J. comp. physiol. Psychol., 44 (1951) 252--262. 7 Ten Cate, J., L o c o m o t o r y movements of the hindlimbs in rabbits after isolation of the lumbosacral cord, J. exp. Biol., 41 (1964) 359--362. 8 Valin, A. et Viala, D., Programmation bilat~rale des activit~s locomotrices spinales au niveau des membres post~rieurs du Lapin, J. Physiol. (Paris), 71 (1975) 167A. 9 Viala, D., Differences entre rythmes locomoteurs chez le Lapin spinal et ~ tronc c~r~bral intact, J. Physiol. (Paris), 61 (1969) 426--427. 10 Viala, D. et Buser, P., Modalit~s d'obtention de rythmes locomoteurs chez le Lapin spinal par traitements pharmacologiques (DOPA, 5-HTP, D-amphetamine), Brain Res., 35 (1971) 151--165.
329
DEVELOPMENT OF LOCOMOTOR ACTIVITIES IN YOUNG CHRONIC SPINAL RABBITS
N.A. FAYEIN and D. VIALA Universit~ de Nancy I, Facult$ de M~decine A, Laboratoire de Physiologie (Directeur Pr. P. Arnould), F. 54019 Nancy Cedex and (D. V.) Universit~ Pierre et Marie Curie, Laboratoire de Neurophysiologie compar~e (Directeur Pr. P. Buser), F. 75230 Paris Cedex 05 (France) (Received November 2nd, 1976) (Accepted November 3rd, 1976)
SUMMARY
It was previously demonstrated that adult chronic spinal rabbits do not recover any spontaneous locomotor activity of the hindlimbs. Pharmacological stimuli may, nevertheless, evoke locomotor activities in the acute spinal animal; however, the pattern is alternate between both hindlimbs instead of being symmetrical (normal pattern in the intact animal). Present data indicate that young rabbits whose spinal cord was transected at birth can recover and spontaneously develop hindlimb locomotor movements over a period of one m o n t h after spinal section. These rhythmic activities are either synchronous or alternate between the two sides, or both. It is concluded that n o t only the alternate pattern but also a pure synchronous one can be generated in the lumbosacral cord in the y o u n g chronic spinal rabbit.
It is known from early observations that vertebrates including mammals may develop, to a certain extent, walking movements after transection of the spinal cord [ 2,4,6,7]. Complementary investigations have demonstrated t h a t locom o t o r activities recorded in the hindlimbs of spinal animals have a central origin. The locomotor r h y t h m is programmed by an intraspinal generator [1,2,5,10]. However, the lack of coordinated movements of progression in the adult chronic spinal rabbit [7] as well as the lack of any efferent rhythmic activities in the hindlimb nerves of the same curarized preparation have been generally reported [9]. But, after a pharmacological treatment associating a MAO inhibitor with DOPA applied to the acute spinal rabbit, central coordinated locomotor activities were obtained, with a predominant pattern of alternance of activities in the bilateral symmetrical nerves [ 10]. At times, synchronous activities could develop but they were never maintained over long periods, being rapidly replaced by alternance [8]. It was thus concluded
330 that the lumbosacral locomotor generator could not reproduce on its own the normal mode of rabbits' hopping progression, and that the synchronization of motor discharges in the bilateral symmetrical muscles required supraspinal levels and was very likely organized in the brain stem. In the present study, we investigated to what extent the development of locomotor activities and progression can be recovered in young chronic spinal rabbits whose spinal cord was transected a few hours after birth, and how that locomotor activity is organized. The behaviour of 18 chronic young rabbits was studied after spinal transection performed on the newborn animals between 24 and 48 h. The young rabbits were given back to the mother's care and left quiet for about two weeks. They were then submitted to daily nursing and passive mechanical mobilization of all joints of both hindlimbs in order to prevent progressive stiffness and muscular atrophy. Development of hopping movements was observed until the age of about 4--5 weeks with relatively good balance and postural adjustment during progression. The general state and m o t o r performance of the young animals were greatly dependent on a constant nursing and intensity of the training. But functional impairment generally appeared after 4--5 weeks and the situation was rapidly degrading; the animals reached a plateau body weight and they could not stand any more. Experiments were performed on rabbits of 4--6 weeks old, whose hindlimbs still developed rhythmic and bilateral movements of large amplitude, both flexions and extensions involving several joints. For analysis of m o t o r activity, the activities of two antagonistic muscles of both right and left hindlimbs (a flexor, tibialis anterior and an extensor, gastrocnemius medialis) were recorded electromyographically. The h o m o n y m o u s muscles of the hindlimbs showed synchronous discharges at about 1/sec. during spontaneous flexion and extension movements (Fig. 1A and B). The characteristic pattern of normal progression for that species, i.e. jumping, was reflected by the EMG activity. But an alternate pattern could also be recorded and appeared through progressive desynchronization of symmetrical activities, generally, when the r h y t h m of the beats tended to increase frequency (Fig. 1C). The locomotor movements appeared spontaneously or could be induced by slight or prolonged pinching of the tail. In order to determine whether the synchronous pattern is organized or not at a central spinal level, the rhythmic afferences linked to the movement were suppressed by the injection of a curarizing drug (Flaxedil) and the animals were maintained under artificial ventilation. The rhythmic activities, recorded from the m o t o r nerves of the symmetrical flexors and extensors, showed the same bilateral organization as on the EMG recording, during locomotor phases mostly obtained by intense pressures on the tail; they rarely developed spontaneously (Fig. 2A). The frequency of the evoked rhythmic discharges could reach 1.6--2/sec. To ensure that the spinal section was complete and that no remaining connections with the cephalad cord might account for the present findings, a second section was further performed a few mm more caudal than the previous one. The locomotor patterns remained very
331
A LGM RGM
B LTA RTA
C LTA RTA
,
!s
Fig. 1. Electromyographic activities of locomotor movements in young chronic spinal rabbits. Spontaneous rhythmic discharges were recorded from left (LGM) and right (RGM) extensor muscles: gastrocnemius medialis (A)and from left (LTA) and right (RTA) flexor muscles: tibialis anterior (B). The discharge pattern showed good synchronization in both homonymous muscles activities for long sequences of locomotor movements. However, the central programme might switch from the synchronous to the alternate pattern of discharges in the symmetrical hindlimbs muscles (C).
similar after this second section although we noticed a slight acceleration of the discharge frequency, probably due to the mechanical excitation of the cord (Fig. 2B). From these findings it may be concluded that (1) young spinal rabbits recover m o t o r activities after spinal transection at birth, and are able to develop spontaneous walking movements when they grow up over one m o n t h of age; (2) as in the adult spinal rabbit, both alternate and synchronous bilateral
332
A LTAn LGMn
II ~ : lllil '~ . i ~U
~
I
~
_
~
I
I :
II _ill . II . i
RTAn RGMn
B LTA
n
LGMn RTA
n
RGMn
i
1S
a
Fig. 2. Lo co m o t o r nerve discharges in paralyzed young spinal rabbits. L o c o m o t o r activities were recorded bilaterally from a branch of the sciatic nerve innervating the gastrocnemius medialis (LGMn and RGMn), and from common peroneal nerve innervating the tibialis anterior (LTAn and RTAn). Motor activities were evoked through sustained pressure on the tail. A: predominant discharge pattern is bilaterally synchronous activities. B: this pattern remains unchanged after a second section of the spinal cord (although the discharge frequency is increased).
organization exist in the locomotor pattern of the hindlimbs but, unlike the adult spinal, the synchronous pattern can be maintained and may be the only pattern spontaneously observed during normal progression, and (3) the organization of both alternate and synchronous movements in the hindlimbs is generated by a central programme in the lumbosacral cord. Afferent control only helps spontaneous movements to develop and to be maintained. The spinal generator alone thus seems able to coordinate pluriarticular movements when the medullary pathways are disconnected from the higher nervous centers. But such a functional autonomous mechanism is impossible to express when the spinal transection is performed at or after the 15th day after birth. Later on, no recovery of locomotor activities can be obtained (unpublished results). It is well known that the CNS of newborn rabbits is particularly immature and 14 days represents the critical age to achieve complete myelinization of the supraspi.~al descending pathways [3]. The problem remains for consideration whether a central generator for both modalities of hindlimb movements pre-exists in the lumbosacral cord and just
333
expresses their intrinsic properties when n o t submitted to higher central control, or whether an arrangement of neuronal circuits m a y establish new connections towards a functional purpose in various experimental situations during the phase of maturation process when the spinal generator is n o t influenced by higher nervous control. REFERENCES 1 Brown, T.G., The intrinsic factors in the act of progression in the mammal, Proc. roy. Soc. B, 84 (1911) 308--319. 2 Grillner, S., Locomotion in vertebrates -- central mechanisms and reflex interaction, Physiol. Rev., 55 (1975) 247--304. 3 Langworthy, O.R., Relation of onset of decerebrate rigidity to the time of myelinisation of tracts in the brain-stem and spinal cord of young animals, Contr. Embryol. Carneg. lnstn, 89 (1926) 127--140. 4 Laughton, N.B., Studies on the nervous regulation of progression in mammals, Amer. J. Physiol., 70 (1924) 358--384. 5 Perret, C., Cabelguen, J.M. et Millanvoye, M., Caract~ristiques d'un rythme de type locomoteur chez le chat spinal aigu, J. Physiol. (Paris), 65 (1973) 473A. 6 Shurrager, P.S. and Dykman, R.A., Walking spinal carnivores, J. comp. physiol. Psychol., 44 (1951) 252--262. 7 Ten Cate, J., L o c o m o t o r y movements of the hindlimbs in rabbits after isolation of the lumbosacral cord, J. exp. Biol., 41 (1964) 359--362. 8 Valin, A. et Viala, D., Programmation bilat~rale des activit~s locomotrices spinales au niveau des membres post~rieurs du Lapin, J. Physiol. (Paris), 71 (1975) 167A. 9 Viala, D., Differences entre rythmes locomoteurs chez le Lapin spinal et ~ tronc c~r~bral intact, J. Physiol. (Paris), 61 (1969) 426--427. 10 Viala, D. et Buser, P., Modalit~s d'obtention de rythmes locomoteurs chez le Lapin spinal par traitements pharmacologiques (DOPA, 5-HTP, D-amphetamine), Brain Res., 35 (1971) 151--165.