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Propriospinal pathways in the ventral funicles of the cat spinal cord: their effects on lumbosacral motoneurones
502
Brain Research, 93 (1975) 502-506 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Propriospinal pathways in the ventral funicles of the cat spinal cord: their effects on lumbosacral motoneurones
D. A. VASILENKO A. A. Bogomoletz Institute of Physiology, Ukrainian Academy of Sciences, 252601 Kiev-24 (U.S.S.R.)
(Accepted April 28th, 1975)
In recent years propriospinal connexions have deserved increasing interest in view of their important role in intersegmental coordination and realization of supraspinal control. Short propriospinal pathways interconnecting several neighbouring segments are located both in the lateral and ventral funicles 9, while long ones connecting cervical and lumbosacral enlargements are concentrated in the ventral quadrants 2. Preliminary chronic sections resulting in degeneration of suprasegmental fibre systems give certain possibilities for obtaining selective activation of propriospinal pathways. Hitherto, short pathways in the dorsolateral funicle 5,6 and long pathways in more ventral parts of the same funicle a,4 were studied by this method. Investigation of propriospinal connexions in the ventral part of the spinal cord was begun recently 7 but it dealt only with short pathways and accurate distinction between effects from the ventrolateral and ventral funicles was not performed. This report is concerned with synaptic action evoked exclusively by stimulation of propriospinal axons in the ventral funicles; special attention being paid to comparison of effects from long and short pathways. Experiments were performed on 9 cats with chronic section of the ventral half of the spinal cord at the C4 or C5 level (Fig. 1H). The lesion was made under pentobarbital (Nembutal) anaesthesia 9 to 23 days before the acute experiment. Histological control indicated complete destruction of the ventral funicles in all cases while the extent of interruption of the lateral ones varied. Main experiments were also performed under Nembutal anaesthesia (initial dose 40 mg/kg); the animals were immobilized with gallamine triethiodide (Flaxedil) and artificially respirated. Records from lumbosacral motoneurones were made with conventional methods; microelectrodes were filled with 0.6 M K2SO4. Ventral funicles (VF) were stimulated with silver surface electrodes (interelectrode distance 3-4 ram), adjusted subdurally under the cord. Additional acute section of the cord preserving only VF bilaterally was performed (Fig. lI) in order to eliminate effects from other fibre systems. VF were stimulated usually at two levels: ThT-Thl0 (in different experiments) and La-L4 (8-10 mm cranial to the limiting lesion). Long propriospinal axons were excited in the first case
503
A_/'~'--'-'V - VI 2,5 F ~ 0
C i ~ i~~r~VF22,2 D 7,5
"~- ..........~.........1+5ms
!mv 1+5 ms
.....................l+Sms I 2my J
VFI~3
K
VF24,0
, ....
I ....
I ....
L
I
~
~
I
~
I
I
Z
I
I
S
I
Fig. 1. Synaptic potentials evoked in motoneurones by stimulation of the ventral funicles. Lower traces are intraceUular records; upper traces, the records from the ventrolateral surface of the spinal cord at the level of the rnicroelectrode insertion. The ventral funicles were stimulated at thoracic (VFz) and middle lumbar (VFD levels; intensity of stimulation is indicated in multiples of threshold for the surface volley. Records A-E show effects in a posterior biceps-semitendinosus motoneurone. F, G : show effects in a quadriceps motoneurone recorded in the same experiment. H: illustrates the extent of preliminary section. I: shows the extent of additional limiting section of the cord in this animal. J-M: are records from a peroneal motoneurone obtained in another experiment. The lowest traces in B, D and L are records of field potentials outside the ceils.
while activation of short propriospinal neurones with somata in upper lumbar segments occurred in the second one. Stimulation of the VF at Th level evoked a surface volley descending to lower lumbar segments with mean conduction velocity 65 m/sec. Synaptic potentials evoked by such stimulation were found in 69 motoneurones of 83 recorded ones. EPSPs (Fig. IA and B), IPSPs (Fig. IF) and mixed responses usually with an excitatory initial component were seen in different cells. On the whole an excitatory pattern of action dominated: initial hyperpolarizing potentials ('primary' IPSPs) were elicited only in 9 neurones; 'secondary' IPSPs in mixed responses usually were of low amplitude. Pronounced reciprocity in action on flexor and extensor motoneurones was not observed. Inhibitory effects were somewhat more frequent in motoneurones to quadriceps and flexor digitorum (FD) group, but small sampling does not show whether this is a systematical trend. On the contrary, clear differentiation was found in respect to threshold and intensity of action upon motoneurones to proximal (biceps, semitendinosus, semimembranosus, quadriceps) and distal muscles (FD, peroneal and tibial groups, gastrocnemius and soleus). Minimal PSPs were recorded
504
A
B
E
VF I 5,0
t
2~ C
.
D--I
VF 2 5,0 1,0
I F
G
| 2my
.............
VF 2 11,0
,J
1÷5ms
2my H
1'(3 VF 2 1,8
I
2"0 ,_
Q 1,6
2 m y .....................
3'0 J
ms VF2+ Q
i÷Sms
Fig. 2. Properties of synaptic actions evoked by VF stimulation. The same indications as in Fig. 1 are used. Records A-D show temporal facilitation of disynaptic EPSPs evoked in an anterior biceps-semimembranosus motoneurone by VF stimulation. E: illustrates the time course of such facilitation. Mean amplitude of the second EPSP in relation to the mean amplitude of the first one is plotted against the interval between two stimuli applied to the VF at the thoracic (opened circles) and lumbar (black circles) levels. Records F and G : show absence of significant temporal facilitation of monosynaptic EPSPs evoked in a flexor digitorum motoneurone by strong VF stimulation at the lumbar level. H ~ : illustrate facilitation of group I IPSP evoked in an anterior biceps-semimembranosus motoneurone by quadriceps (Q) stimulation when conditioning VF stimulation is applied.
in m o s t ' p r o x i m a l ' m o t o n e u r o n e s when a single stimulus was a p p l i e d to the V F with an intensity only 1.2-2.5 t h r e s h o l d for the surface volley (Fig. I A a n d F). O n l y 7 units o f 37 ( 1 8 . 9 ~ ) needed m o r e intensive stimulation. On the o t h e r h a n d stimuli greater t h a n 2.5 t h r e s h o l d were needed to evoke PSPs in 29/43 ' d i s t a l ' m o t o n e u r o n e s (67.5 ~ ) . The a m p l i t u d e o f PSPs in ' d i s t a l ' units was as a rule lower t h a n in ' p r o x i m a l ' ones. A l l b u t a few neurones in which even intensive V F s t i m u l a t i o n ( m o r e t h a n 8-10 times t h r e s h o l d for surface volley) failed to evoke PSP were m o t o n e u r o n e s to distal muscles. I n all m o t o n e u r o n e s PSPs could be considered only as p o l y s y n a p t i c (at least disynaptic) ones. M i n i m a l segmental delay (measured as the time f r o m p e a k o f positivity in the surface volley to the onset o f PSP) was 1.1-1.3 msec a n d in m a n y cells such delay reached 3.0-4.7 msec. There was no considerable difference between E P S P a n d I P S P latencies. N o n - m o n o s y n a p t i c i t y o f such PSPs was also confirmed by distinct t e m p o r a l facilitation during r e p e a t e d V F s t i m u l a t i o n (Fig. 2 A a n d B). Such facilitation was observed for 20-35 msec after a single V F stimulus (Fig. 2E) a n d was obvious in PSPs even with the shortest latencies. Properties described a b o v e a p p l y in large m e a s u r e to PSPs e v o k e d b y V F s t i m u l a t i o n at m i d d l e l u m b a r level (Fig. 1C-E, G, K , a n d Fig. 2C, D, E) if stimulus
505 intensity was moderate (up to 3-6 thresholds). These PSPs differed from PSPs evoked by Th stimulation by having somewhat higher amplitude and slope. Pattern of synaptic action in the recorded sample of units also differed to a some extent: 'primary' IPSPs were met more frequently (in 14 neurones out of 71) and a mixed pattern of action was more pronounced. Excitatory and inhibitory components of response in many cells could be differentiated in respect to their threshold (Fig. 1C-E). It must be mentioned that in 6 neurones initial effect of VF stimulation was a hyperpolarization in contrast to a depolarization when VF was stimulated at thoracic level. Further increase of VF stimulation intensity (more than 3-6 and up to 15-20 threshold for surface volley) was tested in some motoneurones. Such increase at L level developed quite a different type of synaptic action in 16 cells of 22 tested. Shortlatency excitatory components occurred with segmental delay less than 1.0 msec. Such PSPs could be evoked primarily in those motoneurones where moderate VF stimulation was not effective (Fig. 1K, L and M). Because of a short segmental delay and absence of considerable temporal facilitation during repetitive VF stimulation (Fig. 2F and G) such EPSPs could be classified as monosynaptic. Usually they were accompanied by late components increasing the duration of synaptic action. Intensive VF stimulation at Th level failed to evoke monosynaptic PSPs. Interpretation of the data is complicated by the question: to what extent could the synaptic effects described above be considered exclusively a result of activation of propriospinal pathways descending in the VF? It can be supposed that part of these effects might be evoked by antidromic activation of ascending axons with collaterals to segmental neurones. Long ascending fibres in this funicle probably have no significant amount of collaterals projecting directly to the motoneurones, because only diand polysynaptic effects result from VF stimulation at the thoracic level. There is also indirect proof that the relative contribution of such collateral effects to recorded PSPs cannot be too high at all. It is difficult to believe that collaterals of ascending fibres exert selective actions upon 'proximal' and 'distal' motoneurones but such selectivity is quite obvious when the VF are stimulated. Activity of ascending fibres might be somewhat suppressed by their retrograde degeneration. PSP pattern suggests that long propriospinal fibres (and, probably, many short ones) have no monosynaptic connexions with lumbosacral motoneurones in spite of their termination in proximity to motor nuclei s. In this respect these fibre systems are in contrast to the pathways descending in the central part of the lateral funicle 3. At the same time PSPs evoked by VF stimulation have similar properties with the diand polysynaptic effects from the above mentioned 'lateral' pathways and probably are also mediated via certain lumbar interneurones. Ia interneurones are among them because Ia IPSPs following stimulation of the nerve to the antagonistic muscle (Fig. 2I) were facilitated after VF conditioning stimulation (Fig. 2J). VF also contain short propriospinal fibres terminating directly on motoneurones (both 'proximal' and 'distal'). Because of a high threshold for monosynaptic EPSPs it is possible to believe that such 'direct' fibres are thinner than fibres responsible for di- and polysynaptic excitatory and inhibitory action. A description of monosynaptic action from short 'ventral' propriospinal pathways was published recently7. It is
506 difficult to c o m p a r e these results with ours, since intensity o f V F s t i m u l a t i o n was m e a s u r e d there in values o f current b u t n o t in r e l a t i o n to the onset o f the volley in the VF. It seems however t h a t there is certain d i s a g r e e m e n t between o u r d a t a a n d the p r e v i o u s s t u d y ; m o n o s y n a p t i c EPSPs a n d I P S P s were described there as p r i m a r y effects in the m a j o r i t y o f m o t o n e u r o n e s . L o n g p r o p r i o s p i n a l p a t h w a y s in the V F constitute a substantial p a r t o f forel i m b - h i n d l i m b c o o r d i n a t i n g mechanisms, a l t h o u g h p r o b a b l y less d o m i n a n t t h a n long p a t h w a y s in the lateral funicle 3. Ventral p a t h w a y s o b v i o u s l y have special r e l a t i o n to r e g u l a t i o n o f p o s t u r a l r e a c t i o n s t h a t is reflected in higher intensity o f their a c t i o n u p o n m e c h a n i s m s governing p r o x i m a l muscle activities. M a n y short p r o p r i o s p i n a l fibres in the V F p r o b a b l y are axons o f i n t e r n e u r o n e s m e d i a t i n g different types o f reflex activities a n d r e g a r d e d before as ' s e g m e n t a l ' ones. Their p a r t i c i p a t i o n in t r a n s m i s s i o n o f activity f r o m ' m e d i a l ' descending tracts (reticulospinal a n d vestibulospinal) seems to be possible. A n a l o g o u s relay f u n c t i o n was f o u n d in m a n y ' s h o r t ' p r o p r i o s p i n a l interneurones in the lateral funicle a n d they t r a n s m i t activity p r e d o m i n a n t l y f r o m ' l a t e r a l ' descending pathways1,1°. The a u t h o r is grateful to Prof. P. G. K o s t y u k for helpful criticism and to Mrs. L i d i a M a n z h e l o for technical assistance.
1 ANASTASIEVI(~,R., VASILENKO,D. A., KOSTYUKOV,A. I., AND PREOBRAZHENSKY,N. N., Reticulofugal activation of interneurones in the lateral region of the spinal grey matter in the cat, Neurophysiology (Kiev), 5 (1973) 525 536. 2 BARILARI, M. G., AND KUYPERS, H. G. J. M., Propriospinal fibres interconnecting the spinal enlargements in the cat, Brain Research, 14 (1969) 321-330. 3 JANKOVSKA,E., LUNDBERG,A., ROBERTS,W. J., AND STUART, D., A long propriospinal system with direct effect on motoneurones and on interneurones in the cat lumbosacral cord, Exp. Brain Res., 21 (1974) 169-194. 4 JANKOWSKA, E., LUNDBERG, A., AND STUART, D., Propriospinal control of last order interneurones of spinal reflex pathways in the cat, Brain Research, 53 (1973) 227-231. 5 KOSTYUK,P. G., VASILENKO,D. A., AND LANG, E., Propriospinal pathways in the dorsolateral funiculus and their effects on lumbosacral motoneuronal pools, Brain Research, 28 (1971) 233249. 6 KOSTYUK,P. G., VASILENKO,D. A., AND ZADOROZHNY,A. G., Responses of motoneurones of lumbar spinal cord evoked by activity from the propriospinal pathways, Neurophysiology (Kiev), 1 (1969) 5-14. 7 KOZHANOV,V. M., The propriospinal monosynaptic effects of the ventral descending pathways on the cat lumbar motoneurons, Seehenov Physiol. J. USSR, 60 (1974) 171-178. 8 RUSTIONI, A., KUYPERS, H. G. J. M., AND HOLSTEGE, G., Propriospinal projections from the ventral and lateral funiculi to the motoneurones in the lumbosacral cord of the cat, Brain Research, 24 (1971) 255-275. 9 STERLING,P., AND KUVPERS, H. G. J. M., Anatomical organization of the brachial spinal cord of the cat. III. The propriospinal connections, Brain Research, 7 (1968) 419-443. 10 VASILENKO,D. A., KosTvuI(ov, A. I., AND PILYAVSKY,A. I., Cortico- and rubrofugal activation of propriospinal interneurons sending axons into the dorsolateral funiculus of the cat spinal cord, Neurophysiology (Kiev), 4 (1972) 489-500.
Brain Research, 93 (1975) 502-506 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Propriospinal pathways in the ventral funicles of the cat spinal cord: their effects on lumbosacral motoneurones
D. A. VASILENKO A. A. Bogomoletz Institute of Physiology, Ukrainian Academy of Sciences, 252601 Kiev-24 (U.S.S.R.)
(Accepted April 28th, 1975)
In recent years propriospinal connexions have deserved increasing interest in view of their important role in intersegmental coordination and realization of supraspinal control. Short propriospinal pathways interconnecting several neighbouring segments are located both in the lateral and ventral funicles 9, while long ones connecting cervical and lumbosacral enlargements are concentrated in the ventral quadrants 2. Preliminary chronic sections resulting in degeneration of suprasegmental fibre systems give certain possibilities for obtaining selective activation of propriospinal pathways. Hitherto, short pathways in the dorsolateral funicle 5,6 and long pathways in more ventral parts of the same funicle a,4 were studied by this method. Investigation of propriospinal connexions in the ventral part of the spinal cord was begun recently 7 but it dealt only with short pathways and accurate distinction between effects from the ventrolateral and ventral funicles was not performed. This report is concerned with synaptic action evoked exclusively by stimulation of propriospinal axons in the ventral funicles; special attention being paid to comparison of effects from long and short pathways. Experiments were performed on 9 cats with chronic section of the ventral half of the spinal cord at the C4 or C5 level (Fig. 1H). The lesion was made under pentobarbital (Nembutal) anaesthesia 9 to 23 days before the acute experiment. Histological control indicated complete destruction of the ventral funicles in all cases while the extent of interruption of the lateral ones varied. Main experiments were also performed under Nembutal anaesthesia (initial dose 40 mg/kg); the animals were immobilized with gallamine triethiodide (Flaxedil) and artificially respirated. Records from lumbosacral motoneurones were made with conventional methods; microelectrodes were filled with 0.6 M K2SO4. Ventral funicles (VF) were stimulated with silver surface electrodes (interelectrode distance 3-4 ram), adjusted subdurally under the cord. Additional acute section of the cord preserving only VF bilaterally was performed (Fig. lI) in order to eliminate effects from other fibre systems. VF were stimulated usually at two levels: ThT-Thl0 (in different experiments) and La-L4 (8-10 mm cranial to the limiting lesion). Long propriospinal axons were excited in the first case
503
A_/'~'--'-'V - VI 2,5 F ~ 0
C i ~ i~~r~VF22,2 D 7,5
"~- ..........~.........1+5ms
!mv 1+5 ms
.....................l+Sms I 2my J
VFI~3
K
VF24,0
, ....
I ....
I ....
L
I
~
~
I
~
I
I
Z
I
I
S
I
Fig. 1. Synaptic potentials evoked in motoneurones by stimulation of the ventral funicles. Lower traces are intraceUular records; upper traces, the records from the ventrolateral surface of the spinal cord at the level of the rnicroelectrode insertion. The ventral funicles were stimulated at thoracic (VFz) and middle lumbar (VFD levels; intensity of stimulation is indicated in multiples of threshold for the surface volley. Records A-E show effects in a posterior biceps-semitendinosus motoneurone. F, G : show effects in a quadriceps motoneurone recorded in the same experiment. H: illustrates the extent of preliminary section. I: shows the extent of additional limiting section of the cord in this animal. J-M: are records from a peroneal motoneurone obtained in another experiment. The lowest traces in B, D and L are records of field potentials outside the ceils.
while activation of short propriospinal neurones with somata in upper lumbar segments occurred in the second one. Stimulation of the VF at Th level evoked a surface volley descending to lower lumbar segments with mean conduction velocity 65 m/sec. Synaptic potentials evoked by such stimulation were found in 69 motoneurones of 83 recorded ones. EPSPs (Fig. IA and B), IPSPs (Fig. IF) and mixed responses usually with an excitatory initial component were seen in different cells. On the whole an excitatory pattern of action dominated: initial hyperpolarizing potentials ('primary' IPSPs) were elicited only in 9 neurones; 'secondary' IPSPs in mixed responses usually were of low amplitude. Pronounced reciprocity in action on flexor and extensor motoneurones was not observed. Inhibitory effects were somewhat more frequent in motoneurones to quadriceps and flexor digitorum (FD) group, but small sampling does not show whether this is a systematical trend. On the contrary, clear differentiation was found in respect to threshold and intensity of action upon motoneurones to proximal (biceps, semitendinosus, semimembranosus, quadriceps) and distal muscles (FD, peroneal and tibial groups, gastrocnemius and soleus). Minimal PSPs were recorded
504
A
B
E
VF I 5,0
t
2~ C
.
D--I
VF 2 5,0 1,0
I F
G
| 2my
.............
VF 2 11,0
,J
1÷5ms
2my H
1'(3 VF 2 1,8
I
2"0 ,_
Q 1,6
2 m y .....................
3'0 J
ms VF2+ Q
i÷Sms
Fig. 2. Properties of synaptic actions evoked by VF stimulation. The same indications as in Fig. 1 are used. Records A-D show temporal facilitation of disynaptic EPSPs evoked in an anterior biceps-semimembranosus motoneurone by VF stimulation. E: illustrates the time course of such facilitation. Mean amplitude of the second EPSP in relation to the mean amplitude of the first one is plotted against the interval between two stimuli applied to the VF at the thoracic (opened circles) and lumbar (black circles) levels. Records F and G : show absence of significant temporal facilitation of monosynaptic EPSPs evoked in a flexor digitorum motoneurone by strong VF stimulation at the lumbar level. H ~ : illustrate facilitation of group I IPSP evoked in an anterior biceps-semimembranosus motoneurone by quadriceps (Q) stimulation when conditioning VF stimulation is applied.
in m o s t ' p r o x i m a l ' m o t o n e u r o n e s when a single stimulus was a p p l i e d to the V F with an intensity only 1.2-2.5 t h r e s h o l d for the surface volley (Fig. I A a n d F). O n l y 7 units o f 37 ( 1 8 . 9 ~ ) needed m o r e intensive stimulation. On the o t h e r h a n d stimuli greater t h a n 2.5 t h r e s h o l d were needed to evoke PSPs in 29/43 ' d i s t a l ' m o t o n e u r o n e s (67.5 ~ ) . The a m p l i t u d e o f PSPs in ' d i s t a l ' units was as a rule lower t h a n in ' p r o x i m a l ' ones. A l l b u t a few neurones in which even intensive V F s t i m u l a t i o n ( m o r e t h a n 8-10 times t h r e s h o l d for surface volley) failed to evoke PSP were m o t o n e u r o n e s to distal muscles. I n all m o t o n e u r o n e s PSPs could be considered only as p o l y s y n a p t i c (at least disynaptic) ones. M i n i m a l segmental delay (measured as the time f r o m p e a k o f positivity in the surface volley to the onset o f PSP) was 1.1-1.3 msec a n d in m a n y cells such delay reached 3.0-4.7 msec. There was no considerable difference between E P S P a n d I P S P latencies. N o n - m o n o s y n a p t i c i t y o f such PSPs was also confirmed by distinct t e m p o r a l facilitation during r e p e a t e d V F s t i m u l a t i o n (Fig. 2 A a n d B). Such facilitation was observed for 20-35 msec after a single V F stimulus (Fig. 2E) a n d was obvious in PSPs even with the shortest latencies. Properties described a b o v e a p p l y in large m e a s u r e to PSPs e v o k e d b y V F s t i m u l a t i o n at m i d d l e l u m b a r level (Fig. 1C-E, G, K , a n d Fig. 2C, D, E) if stimulus
505 intensity was moderate (up to 3-6 thresholds). These PSPs differed from PSPs evoked by Th stimulation by having somewhat higher amplitude and slope. Pattern of synaptic action in the recorded sample of units also differed to a some extent: 'primary' IPSPs were met more frequently (in 14 neurones out of 71) and a mixed pattern of action was more pronounced. Excitatory and inhibitory components of response in many cells could be differentiated in respect to their threshold (Fig. 1C-E). It must be mentioned that in 6 neurones initial effect of VF stimulation was a hyperpolarization in contrast to a depolarization when VF was stimulated at thoracic level. Further increase of VF stimulation intensity (more than 3-6 and up to 15-20 threshold for surface volley) was tested in some motoneurones. Such increase at L level developed quite a different type of synaptic action in 16 cells of 22 tested. Shortlatency excitatory components occurred with segmental delay less than 1.0 msec. Such PSPs could be evoked primarily in those motoneurones where moderate VF stimulation was not effective (Fig. 1K, L and M). Because of a short segmental delay and absence of considerable temporal facilitation during repetitive VF stimulation (Fig. 2F and G) such EPSPs could be classified as monosynaptic. Usually they were accompanied by late components increasing the duration of synaptic action. Intensive VF stimulation at Th level failed to evoke monosynaptic PSPs. Interpretation of the data is complicated by the question: to what extent could the synaptic effects described above be considered exclusively a result of activation of propriospinal pathways descending in the VF? It can be supposed that part of these effects might be evoked by antidromic activation of ascending axons with collaterals to segmental neurones. Long ascending fibres in this funicle probably have no significant amount of collaterals projecting directly to the motoneurones, because only diand polysynaptic effects result from VF stimulation at the thoracic level. There is also indirect proof that the relative contribution of such collateral effects to recorded PSPs cannot be too high at all. It is difficult to believe that collaterals of ascending fibres exert selective actions upon 'proximal' and 'distal' motoneurones but such selectivity is quite obvious when the VF are stimulated. Activity of ascending fibres might be somewhat suppressed by their retrograde degeneration. PSP pattern suggests that long propriospinal fibres (and, probably, many short ones) have no monosynaptic connexions with lumbosacral motoneurones in spite of their termination in proximity to motor nuclei s. In this respect these fibre systems are in contrast to the pathways descending in the central part of the lateral funicle 3. At the same time PSPs evoked by VF stimulation have similar properties with the diand polysynaptic effects from the above mentioned 'lateral' pathways and probably are also mediated via certain lumbar interneurones. Ia interneurones are among them because Ia IPSPs following stimulation of the nerve to the antagonistic muscle (Fig. 2I) were facilitated after VF conditioning stimulation (Fig. 2J). VF also contain short propriospinal fibres terminating directly on motoneurones (both 'proximal' and 'distal'). Because of a high threshold for monosynaptic EPSPs it is possible to believe that such 'direct' fibres are thinner than fibres responsible for di- and polysynaptic excitatory and inhibitory action. A description of monosynaptic action from short 'ventral' propriospinal pathways was published recently7. It is
506 difficult to c o m p a r e these results with ours, since intensity o f V F s t i m u l a t i o n was m e a s u r e d there in values o f current b u t n o t in r e l a t i o n to the onset o f the volley in the VF. It seems however t h a t there is certain d i s a g r e e m e n t between o u r d a t a a n d the p r e v i o u s s t u d y ; m o n o s y n a p t i c EPSPs a n d I P S P s were described there as p r i m a r y effects in the m a j o r i t y o f m o t o n e u r o n e s . L o n g p r o p r i o s p i n a l p a t h w a y s in the V F constitute a substantial p a r t o f forel i m b - h i n d l i m b c o o r d i n a t i n g mechanisms, a l t h o u g h p r o b a b l y less d o m i n a n t t h a n long p a t h w a y s in the lateral funicle 3. Ventral p a t h w a y s o b v i o u s l y have special r e l a t i o n to r e g u l a t i o n o f p o s t u r a l r e a c t i o n s t h a t is reflected in higher intensity o f their a c t i o n u p o n m e c h a n i s m s governing p r o x i m a l muscle activities. M a n y short p r o p r i o s p i n a l fibres in the V F p r o b a b l y are axons o f i n t e r n e u r o n e s m e d i a t i n g different types o f reflex activities a n d r e g a r d e d before as ' s e g m e n t a l ' ones. Their p a r t i c i p a t i o n in t r a n s m i s s i o n o f activity f r o m ' m e d i a l ' descending tracts (reticulospinal a n d vestibulospinal) seems to be possible. A n a l o g o u s relay f u n c t i o n was f o u n d in m a n y ' s h o r t ' p r o p r i o s p i n a l interneurones in the lateral funicle a n d they t r a n s m i t activity p r e d o m i n a n t l y f r o m ' l a t e r a l ' descending pathways1,1°. The a u t h o r is grateful to Prof. P. G. K o s t y u k for helpful criticism and to Mrs. L i d i a M a n z h e l o for technical assistance.
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