Brain Research, 537 (1990) 1-13
1
Elsevier BRES 16182
Research Reports
Phase-dependent modulation of dorsal root potentials evoked by peripheral nerve stimulation during fictive locomotion in the cat Jean-Pierre Gossard* and Serge Rossignol Centre de Recherche en Sciences Neurologiques, Facultd de Mddecine, Universit~ de Montreal, Montrdal, Que. (Canada)
(Accepted 24 July 1990) Key words: Dorsal root potential; Fictive locomotion; Presynaptic inhibition; Cat
To help elucidate the role of presynaptic mechanisms in the control of locomotor movements, the transmission of PAD pathways was investigated by recording dorsal root potentials (DRPs) evoked by electrical stimulation of cutaneous and muscle nerves of both hindlimbs at various phases of the fictive step cycle. Fictive locomotion occurred spontaneously in decorticate cats or by stimulating the mesencephalic locomotor region (MLR) as well as in low spinal cats injected with nialamide and L-DOPA. Evoked DRPs were superimposed on a fluctuating DRP accompanying the fictive locomotor rhythm (locomotor DRP) which typically consisted of two peaks of depolarization per cycle, the largest peak occurring during the flexor phase. The amplitude of evoked DRPs was substantially modulated throughout the locomotor cycle and followed a similar modulation pattern for all stimulated nerves whether ipsilateral (i-) or contralateral (co-). The amplitude of evoked DRPs decreased at the beginning of the flexor phase, dropped to a minimum later in the flexor phase and then increased during the extensor phase where it became maximum. Results were comparable in decorticate and spinal preparations and for L6 and 1-,7rootlets with cutaneous and muscle nerve stimulation. It is noteworthy that the modulation pattern for a given rootlet was similar for i- and co. stimulation, even though the bilateral locomotor DRPs fluctuate out-of-phase with each other, subjecting the stimulated fibres to opposite presynaptic polarization changes. This suggests that the modulation may depend more on the presynaptic mechanisms of the receiving fibres than on those of the stimulated fibres. These results demonstrate that the transmission in spinal pathways involved in primary afferent depolarization (PAD) is phasic.ally modulated by the activity in the spinal locomotor network. It is further suggested that the presynaptic inhibition associated with PAD evoked by movement-related sensory feedback during real locomotion could be modulated in a similar way. INTRODUCTION L o c o m o t o r movements are centrally generated in the spinal cord and partly regulated by segmental afferent feedback (see ref. 24). To be effective, the transmission of afferent pathways is phasically and tonically modulated by the central locomotor networks (see ref. 40). There is now some evidence suggesting that the excitability of primary afferent depolarization ( P A D ) pathways responsible for regulating presynaptic inhibition at the level of the primary afferents might also be modulated centrally during locomotion. Excitability testing with Wall's m e t h o d 44, dorsal root potentials (DRPs) and intra-axonal recordings of afferents all indicate that phasic presynaptic mechanisms are activated during fictive locomotion in decerebrate, decorticate and spinal cats injected with L - D O P A 1'2-4'11-13'15'21'22 as well as in walking preparations 11A4'45'46. Rhythmic presynaptic activities have also been observed during fictive scratching 4, mastication 3° and respiration 3s. Based on the concepts of P A D and presynaptic
inhibition 7"1s'28"31'32'42, these previous studies suggested that the spontaneous changes of polarization of primary afferent terminals accompanying locomotion could alter the afferent transmission in different phases of the locomotor cycle. They suggested that maximum presynaptic inhibition occurred during the flexor phase 1A3AS'21, or during the extensor phase 2-4, or sometimes in both phases 11-13'21'45'46. However, few of these studies investigated the actual changes in P A D evoked by sensory input. In one study, antidromic spikes, known as dorsal root reflexes ( D R R s ) , were elicited by muscle nerve stimulation and recorded in the medial gastrocnemius nerve with chronically implanted nerve cuff electrodes in intact and decerebrate cats walking on a treadmill 14. D R R s occurred in correlation with the flexor phase in both preparations suggesting that the evoked P A D , underlying the D R R , was larger during that phase. Partial results from another study 3 indicated that D R P s evoked by cutaneous and muscle nerve stimulation were reduced at the end of the extensor phase and at the beginning of the flexor phase during fictive locomotion in
* Present address: Department of Neurophysiology, Panum Insitute, University of Copenhagen, Copenhagen, Denmark. Correspondence: S. Rossignol, Centre de Recherche en Sciences Neurologiques, Facult6 de M6decine, Universit6 de Montr6al, C.P. 6128, Succ. A, Montr6al, (Qu6bec) Canada H3C 3J7. 0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
d e c o r t i c a t e cats. D R P r e c o r d i n g s in a w a k e rats walking o n a t r e a d m i l l 45'46 i n d i c a t e d that cyclic changes o f D R P a m p l i t u d e w e r e m o r e c o r r e l a t e d with the a f f e r e n t activity o f an a d j a c e n t d o r s a l r o o t l e t t h a n with the m o t o r e f f e r e n t activity ( e l e c t r o m y o g r a m s ) and that this c o r r e l a t i o n was s t r o n g e r d u r i n g stance t h a n d u r i n g swing. T h e b e h a v i o u r o f P A D p a t h w a y s d u r i n g l o c o m o t i o n is thus still u n c l e a r . F o r e x a m p l e , we d o not k n o w if the a f f e r e n t p o l a r i z a t i o n c h a n g e s a c c o m p a n y i n g the central l o c o m o t o r r h y t h m a n d P A D e v o k e d by p e r i p h e r a l input are g e n e r a t e d t h r o u g h t h e s a m e p a t h w a y s o r m e c h a nisms. It is also n o t k n o w n if t h e o b s e r v e d p r e s y n a p t i c changes
depend
on
stimulated
(giving) fibres o r on
r e c o r d e d ( r e c e i v i n g ) fibres, o r b o t h , n o r if t h e y are due to spinal a n d / o r s u p r a s p i n a l m e c h a n i s m s . In the p r e s e n t study, w e e x a m i n e t h e c h a n g e s in the transmission of P A D p a t h w a y s by r e c o r d i n g D R P s e v o k e d by p e r i p h e r a l n e r v e s t i m u l a t i o n d u r i n g fictive l o c o m o t i o n in d e c o r t i c a t e a n d spinal p a r a l y z e d cats. This will h e l p us to u n d e r s t a n d t h e r o l e of P A D p a t h w a y s in the central p r o g r a m m i n g of p r e s y n a p t i c m e c h a n i s m s r e l a t e d to l o c o m o t o r n e t w o r k s . A b r i e f d e s c r i p t i o n o f the results was g i v e n p r e v i o u s ly 23.
MATERIALS AND METHODS
Preparation The experiments were performed on 12 adult cats (2.0-4.7 kg). Under steroid anesthesia (Saffan 1 ml/kg (alphaxalone 9 mg/ml, alphadolone 3 mg/ml), a tracheotomy was performed and a cannula was inserted in the right jugular vein for administration of drugs and fluids. The right femoral or carotid artery was also cannulated to monitor arterial pressure which was maintained around 120 mm Hg by adjusting an inflow of 5% dextrose saline solution which was supplemented, when necessary, with norepinephrine bitartrate (Levophed 4 mg/liter). For 10 cats, the head was fixed in a rigid stereotaxic frame, an extensive craniotomy was performed and the cerebral cortex was completely ablated by suction33"35. One of these was later spinalized at T13 acutely. Two other cats were spinalized at T13 under barbiturate anesthesia (sodium pentobarbita145 mg/kg) a few days before the experiment to allow some recovery from spinal shock. On the day of the experiment, they were anaemically decerebrated under steroid anesthesia (Saffan 1 ml/kg) by ligating the common carotid arteries and the basilar artery just cranial to the branch point of the posterior inferior cerebellar arteries 36. In all cats, the nerves to sartorius (lateral and medial branches), vastus lateralis or semimembranosus muscles of the left hindlimb were dissected free and cut. Their proximal stumps were mounted on bipolar silver chloride recording electrodes (Electroneurogram (ENG): Fig. IF,E). Nerves and dorsal rootlets on the same side of the ENG recordings are designated as ipsilateral (i-); nerves and rootlets on the opposite side, as contralateral (co-). The ipsilateral superficial peroneal (iSP) (at the ankle level), ipsilateral tibialis anterior (iTA) and ipsilateral lateral gastroenemius-soleus (iLGS) nerves and the coSP and coTA nerves were exposed and enclosed in polymer cuff electrodes 29 for stimulation. The pelvis and lumbar spine were fixed in a frame and, following a laminectomy exposing L 5 to S 1 spinal segments, the dura and the pia were partially removed. L 6 and I_,7 dorsal rootlets (DRs) of both sides were cut and their proximal stumps mounted on bipolar silver chloride recording electrodes close to, but not touching, the spinal cord. Paraffin oil pools were formed by drawing up skin flaps surrounding the spinal
cord and the hindlimb nerves. The body temperature was kept near 38 °C by means of a radiant heat lamp. The animal was paralyzed with gallamine triethiodide injection (Flaxedil 20 mg/kg/h i.v.) and artificially respirated. Upon cessation of anaesthesia, fictive locomotion 34 sometimes occurred spontaneously in the decorticate preparations. If not. episodes of fictive locomotion were initiated by exteroceptive stimulation (tonic pinching) of various parts of the body or by trains of stimuli (200/~s pulses, 300 Hz, 150 ms) applied to a cutaneous nerve (e.g. SP) 25. In 2/9 decorticate cats, electrical stimulation (sinusoidal 60 Hz, 20-100 /~A) of the mesencephalic locomotor region (MLR" Horsley-Clarke coordinates P 2, L 4, H 0) with a low impedance tungsten electrode was necessary to initiate locomotor activity42. The spinal animals were injected with nialamide (50 mg/kg) and L-DOPA (80 mg/kg) 25 after surgery,
Stimulation and recordings Through polymer cuff electrodes, stimuli (single or a train of 4 square pulses, 200/~s, 300 Hz) were delivered to i- and co-hindlimb nerves (Fig. 1) in different parts of the locomotor cycle every 2 or 3 cycles (Fig. 2A). The threshold for nerve stimulation (T) was determined by the stimulus strength required to just evoke a deflection in the cord dorsum potential recording. Stimulus strengths ranged from 1 to 30 × T. ENGs and DRPs were recorded using AC-coupled amplifiers. The bandwidth used to record evoked DRPs and to eliminate the antidromic spiking activity was 0.1 or 0.3 Hz to 0.1 kHz. The ENGs were recorded with a larger bandwidth (100 Hz to 10 kHz). Data were recorded on a 7-channel FM tape recorder with a frequency response of 0-5000 Hz at a speed of 19 cm/s. The tapes were played back on an electrostatic printer (Gould ES-1000) and sections of data suited for analysis were digitized on a PDP 11/34 laboratory computer at 1 kHz. Interactive software 47 was used to detect and measure the characteristics of the recorded signals. The locomotor cycle was defined as the interval between the onset of two successive flexor ENG bursts.
iDRP
coDRP
%out ou; g,
MUSCULAR NERVE~RVECUTANEOUS MUSCULAR NERVE
Fig. 1. Experimental paradigm: ipsilateral (iDRP) and contralateral (coDRP) lumbar dorsal rootlets were cut and their proximal stumps recorded with bipolar Ag/AgCI electrodes. Electroneurographic activity (ENG) of flexor (F) and extensor (E) nerves were recorded with similar electrodes. Through electrodes inserted in polymer cuffszg, stimuli were delivered to i- and co-hindllmb cutaneous and muscle nerves in different parts of the locomotor cycle.
Phase plots were constructed from the integrated value of averaged DRP amplitude evoked by (2-25) stimuli in the different phases of the cycle. The integration was computed over an interval equal to the mean duration of the responses. Spontaneous activity in the control (non-stimulated) cycles corresponding to the phase of the stimuli in the stimulated cycle was subtracted from the evoked response. The ordinate of the phase plot was normalized to the largest response (100%). The ENG (rectified and integrated) and DRP signals of control cycles were averaged and normalized to 100%. Only sequences with steady cycle durations (.~ cycle 1232 + 160 ms) were chosen for this analysis so that the signals would not be significantly distorted by the normalizing procedure. In Figs. 3-7, the averaged signals were superimposed on the phase plot to facilitate viewing of temporal relationships. Because the stimulated cycles are often shortened by the stimulation and because the timing of stimuli is expressed as a phase of the control cycles, stimuli in the late phases of the cycle may appear absent. The spontaneous fluctuations of DRP accompanying the fictive locomotor rhythm (see ref. 13) is designated as 'locomotor DRP' or 'L-DRP' and the DRP evoked by peripheral stimulation, as 'evoked DRP' or 'E-DRP' throughout the paper. The minimum amplitude of E-DRP as well as the phases of the cycle of the minimum and maximum E-DRP amplitude were determined for each phase plot and averaged. Results were compared between L~ and l..7 dorsal rootlets recordings, i- and coDRs, cutaneous and muscle nerve stimulation (Tables I and II), spontaneous and MLR-induced fictive locomotion and analyzed with Student's t-tests. The phases of the cycle when the L-DRP had maximum and minimum ampfitude were determined for each phase plot and compared with those of maximum and minimum E-DRP, respectively, and analyzed with linear regressions.
RESULTS Locomotor modulation of evoked DRPs As previously reported (see ref. 13), L - D R P s on both sides of the spinal cord fluctuate at the rhythm of fictive locomotion. For example, Fig. 2 A shows two L - D R P s of iL 6 and iL 7 and one L - D R P of coL 7 recorded during spontaneous fictive locomotion. Note that the peaks of negativity ( > 5 0 g V ) in i D R P L 6 and i D R P L 7 occur mainly during the i- flexor phase (F-phase; Srtn bursts) and in c o D R P L 7 during the i- extensor phase (E-phase; VLn bursts). Since efferent activities alternate on both sides of the spinal cord during fictive locomotion 34, the peaks of negativity in i- and c o D R s indicate that maximum depolarization of afferents occurs during the F-phase on each side. Also note the antidromic firing in the 3 DRs. There are clear phasic antidromic bursts during the F-phase in the i D R P L 7 and probably two bursts per cycle in the i D R P L 6. Since antidromic firing could persist throughout the duration of an experiment (8 h in this example), an appropriate bandwidth was used to filter out the antidromic spiking in trials where the amplitude of E - D R P s was measured (see Materials and Methods)~ Also illustrated at the bottom of Fig. 2A, 3 trains of stimuli (4 pulses, 300 Hz, 10 x T) applied to the iSP nerve during the first third of the F-phase evoke responses superimposed on the 3 L-DRPs. The amplitude of E - D R P s given during the same phase of the cycle are
fairly constant, in shape and duration, from one ~yde to the other, and may reach 100/~V. In Fig. 2B, E - D R P s are shown at an expanded time base which reveals their characteristic long duration ( > 1 0 0 ms). Typically, the coE - D R P is of slightly smaller amplitude and has a longer time to peak 19. D R P responses to stimulation o f a cutaneous nerve Fig. 3 depicts representative examples of the results obtained when the P A D pathways were activated by cutaneous inputs. In each panel of the figure (A, B and C), the averaged signals of E N G s and L - D R P s are normalized with respect to time and are superimposed on the phase plot of the averaged amplitude of the E - D R P s
A
50 #V
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50 ,~V
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Srtn
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ah.
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l
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I '100 ms'
Fig. 2. Locomotor DRPs and evoked DRPs. A: DRP fluctuations with antidromic discharges in 3 dorsal rootlets follow the rhythm of fictive locomotion (locomotor DRP (L-DRP)) evidenced by the alternating ENG activities of flexor Srtn, and extensor: VLn. Three trains of stimuli of iSP nerve (4 pulses, 10 x T) evoke dorsal root potentials (evoked DRP (E-DRP)) in i- and co-dorsal rootlets superimposed on locomotor DRPs. B: evoked DRPs at a faster time base.
A
B
STIM iSPn ( 4 x T )
STIM iSPn (30 x T)
R6216 Srtn
)~1~
C
STIM coSPn
DR6203
DR6206 N= ~32
Srtn Srtn
/1'
_' ; ~
X cycle = 728 ms
X cycle = 687 ms coTAn
VLn
VLn
HIP' '~ locomotor iDRPL6
locomotor iDRPL6
ocomotor iDRPL7
ocomotor
I
' I'
iDRPL6
locomotor coDRPL7 /~
t EVORKEsD
100°/o1
,o.o, ~
X
~ iL6 o----o ik7
1.0
200 ms ,o o o,or
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PHASE OF CYCLE
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100%
EVOKED DRPs
(10 xT)
\ ~ ,~--~iL6 o-- -o iL7
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P H A S E OF C Y C L E
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Fig. 3. Amplitude modulation of DRPs evoked by the stimulation of the cutaneous nerve SP during spontaneous fictive locomotion in a decorticate preparation. From top to bottom: averaged signals of integrated rectified ENGs, locomotor DRPs for a number of cycles (n) and the phase plot of averaged amplitude of evoked DRPs. A, iSP (single pulse, 4 x T); B, iSP (single pulse, 30 x T); C, coSP (4 pulses, 10 x T).
evoked by SP stimulation. Results of Fig. 3 were all recorded during spontaneous fictive locomotion in the same decorticate cat. In Fig. 3A, the phase plot shows that the amplitude of E-DRPs in the two iDRs elicited by a 4 x T stimulation decreases during the F-phase (Srtn activity), drops to a minimum towards the end of that phase and starts increasing to reach a maximum in the E-phase (VLn activity). The amplitude is still near its maximum value at the F - E transition. The pattern of modulation is quite similar for both iL 6 and iL 7 rootlets, as are the L-DRP patterns which, here, consist of one main wave of depolarization peaking at the end of the F-phase (3rd and 4th traces). Results presented in Fig 3B were obtained with a stronger iSP (30 x T) stimulation (same iL 6 DR; different iL7 DR). Again, the phase plot shows a comparable modulation pattern for the two DRs, i.e. reaching a minimum and a maximum amplitude during the F- and E-phase, respectively. In this particular example, the magnitude of modulation in iL6 is seen to be enhanced with stronger stimulation. This, however, was not consistently observed. Note that the period of E-phase in Fig. 3B is now indicated by the averaged ENG activity of coTAn. Results with different strengths of iSP
stimulation were obtained in 7 other cats with spontaneous fictive rhythm for 8 trials and in one cat with MLR-induced locomotor rhythm for one trial. Modulation characteristics were similar to those found with weak stimulation. In Fig. 3C, stimulation of coSPn with a short train of 4 stimuli (10 × T) elicits E-DRPs on both sides of the spinal cord (different iL 6 than in Fig. 3A,B). As in Fig. 2A, i- and co- L-DRPs are seen to vary in a reciprocal fashion (3rd and 4th traces). The trough in locomotor coDRPL 7 (4th trace) suggests that the terminals of the co-stimulated fibres (coSP) are repolarized during the F-phase relative to the i-fibres which are depolarized as suggested by the rising locomotor iDRPL 6. The phase plot shows that the amplitude modulation of E-DRPs in iL6 (empty diamonds) had a similar pattern to those described in Fig. 3A,B. This consistency in modulation patterns for a given rootlet level was found in all trials of i- and co- SP stimulation. On the other hand, the amplitude of E-DRPs in coL 7 (filled squares) increases to a maximum during the F-phase and then quickly decreases during the E-phase. Hence, the E-DRP modulation patterns in iL6 and c o l 7 are out-of-phase with each
other, as are the locomotor DRPs and ENGs. Comparable results were obtained with different strengths of coSP stimulation for two other trials recorded in this decorticate cat and in two other cats with MLR-induced locomotor rhythm for two trials. Results in Fig. 4 are taken from an experiment where episodes of fictive stepping alternated with episodes of tonic E N G activities. Here, E-DRPs could be evoked with the same stimulation during tonic activities of extensor (VLn) or flexor (Srtn) ENGs and during episodes of fictive locomotion. The locomotor iDRPL6 in Fig. 4A consists of two waves of depolarization of comparable amplitude during F- and E-phases separated by a trough of repolarization (3rd trace). The amplitude of E-DRPs elicited by an iSP stimulation (20 x T) during fictive stepping decreased during the F-phase and then increased at the beginning of the E-phase and reached a maximum towards the end of that phase. Note that the E-DRP modulation pattern is similar to previous examples (Fig. 3A,B) even though the pattern of L-DRP is quite different (see Discussion). The averaged E-DRP amplitude evoked with the same stimulation (20 x T) during an episode of tonic activity in VLn ENG with the Srtn E N G being silent, is shown on the left of the ordinate (filled diamond) of the phase plot. The amplitude of E - D R P equals the maximum value seen during the E-phase of fictive locomotion. Later on in the same experiment (Fig. 4B), similar conditions occurred and E-DRPs were elicited with a weaker stimulation of iSP (2 x T) during fictive stepping and during episodes of tonic VLn E N G activity, or of tonic Srtn E N G activity, while the antagonist muscle nerve was silent. The E - D R P amplitude obtained during extensor tonic activity (filled diamond on the left of the ordinate) was comparable to the maximum value obtained during the E-phase while the E - D R P amplitude during flexor tonic activity (filled
STIMiSPn (20 x "I')
A
B
8PA3802
san
STIM iSPn (2 x T) S.o
~1
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cycle = 1090 ms
X cycle = 1975 ms
VLn 200 ms
locomotor
400 ms
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/
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.
.
.
.
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0 PHASE OF CYCLE 70 0 PHASE OF CYCLE 1.0 Fig. 4. Amplitude of DRPs evoked by the stimulation of the SP nerve during tonic ENG activities and during spontaneous rictive locomotion in decorticate preparations. In A, the averaged amplitude of DRPs evoked by the stimulation of iSP (single pulse, 20 x T) during tonic extensor VLn ENG activity is shown on the left of the ordinate (filled diamond) and the amplitude modulation during the episode of fictive stepping is shown in the phase plot (empty diamonds). In B, the averaged amplitudes of DRPs evoked by a weaker iSP stimulation (single pulse, 2 x T) during tonic extensor VLn (filled diamond) or tonic flexor Srtn (rifled circle) ENG activities are shown on the left of the ordinate and the amplitude modulation during fictive stepping is shown in the phase plot (empty diamonds).
circle on the left of the ordinate) was just inferior to the minimum value seen during the F-phase. The amplitude of E - D R P did not fluctuate during tonic E N G activities and suggests there was no locomotion in on-recorded nerves as well.
TABLE I
Mean phase of the fictive step cycle (normalized step cycle = 1) when maximal (Max) and minimal (Min) E-DRP occurred for cutaneous (pooled iSP and coSP) and muscle (pooled iTA, co TA and iLGS) nerve stimulation, and minimum E-DRP amplitude expressed as a percentage of the largest value: mkdmax (%) Results from i- a n d co-stimulation were pooled together. All m e a n s have their standard deviations. N u m b e r of analyzed trials: No.; m e a n phase of the e n d of flexor activity (Flexor phase) serves as reference. See also Fig. 8A,B.
Stimulation
Level
No.
Phase of the cycle Max
M/n
Flexor phase
Amplitude (rain~max)
(%)
iSP a n d coSP
iL 6 iL 7 Total
15 10 25
0.85 + 0.22 0.84 + 0.21 0.85 + 0.22
0.31 + 0.07 0.31 + 0.10 0.31 + 0.08
0.44 + 0.09 0.50 + 0.05 0.46 + 0.08
41.2 + 15.3 32.7 + 18.7 37.8 + 17.2
iTA, coTA a n d iLOS
iL6 iL 7 Total
3 7 10
0.89 + 0.25 0.81 + 0.22 0.84 + 0.23
0.20 + 0.05 0.30 + 0.10 0.27 + 0.10
0.30 + 0.04 0.44 + 0.08 0.42 + 0.10
43.3 + 26.1 33.0 + 10.1 36.1 + 17.3
.&
STIM iTAn (2xT) ,~ hh,,v%, Srtn
STIMiTAn
B
DR 6211 N= 131
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cycle = 963 ms
C
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200 ms
2 cycle = 965 ms
200 ms
locomotor iDRPL7
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Fig. 5. Amplitude modulation of DRPs evoked by the stimulation of the flexor nerve TA during spontaneous fictive locomotion in decorticate preparations. Same display as in Fig. 3. A, iTA (4 pulses, 2 x T); B, iTA (4 pulses, 15 x T); C, coTA (4 pulses, 2 x T).
A
S T I M iGLn (2 x T)
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S T I M i G L n (30 x T)
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cycle = 91 7 ms X cycle = 772 ms
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locomotor iDRPL6
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locomotor ,.,:'¢~"' iDRPL7
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EVOKED
,
DRPs
~
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r
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PHASE OF CYCLE
110
PHASE OF CYCLE
1.0
Fig. 6. Amplitude modulation of DRPs evoked by the stimulation of the extensor nerve LGS during spontaneous (A) MLR-induced (B) fictive locomotion in decorticate preparations• Same display as in Fig. 3. A, iLGS (4 pulses, 2 x T); B, iLGS (4 pulses, 30 x T).
SPINAL CAT A
B
STIM iSPn (15xT) ~1~~
TAn
PA 7701
STIM iTAn (20xT) Srtn
[~kll,J
cycle = 1742 ms
X cycle = 3188 ms SmAn
~
VLn
800 ms
200 ms
~
locomotor iDRPL7
locomotor iDRPL6
100% -
100%-
EVOKED DRPs
EVOKED DRPs
0
DR 6402
PHASE OF CYCLE
i L7
1.0
0
PHASE OF CYCLE
1.0
Fig. 7. Amplitude modulation of DRPs evoked by the stimulation of the cutaneous nerve iSP and flexor nerve TA during fictive locomotion in spinal cats injected with nialamide and L-DOPA. Same display as in Fig. 3. A, iSP (single pulse, 15 x T); B, iTA (single pulse, 20 x T).
Table I lists various characteristics of amplitude modulation of E-DRPs elicited by cutaneous and muscle nerve stimulation: the number of trials obtained for iL 6 and iL 7 (No.) in different cats, the mean phase of the cycle for maximum (Max) and minimum (Min) E - D R P amplitude and the minimum amplitude expressed as percentage of the largest value. In the upper part of Table I, the results from iSP and coSP nerve stimulation were pooled together and averaged for iL 6 and iL 7 DRs since the modulation patterns were independent of the side of stimulation. The mean values show that, overall, the maximum E-DRP amplitude occurred in late extensor phase (mean 0.85 + 0.22 of the locomotor cycle) and that the minimum amplitude occurred during the F-phase (mean 0.31 + 0.08). Those values were comparable for iL6 and iL7 and were found to be not statistically different. The mean minimum amplitude was not statistically different for L 6 and L 7 DRs either (mean 37.8 + 17.2% of the largest value) although there were large standard deviations. D R P responses to stimulation o f muscle nerves
The amplitude modulation of E-DRPs elicited by the stimulation of the flexor nerve TA during spontaneous
fictive stepping is illustrated in the phase plots of Fig. 5. In Fig. 5A, the short train of stimuli (4 pulses, 2 x T) was strong enough to evoke E-DRPs on both sides during spontaneous fictive stepping. The phase plots shows that the modulation patterns of E - D R P amplitude of iL6 and coL 7 resemble those obtained with cutaneous stimuli in
TABLE II Mean phase of the fictive step cycle (normalized step cycle = 1) when the maximal (max) and minimal (min) E-DRP occurred for all stimulation recorded in ipsi- and contralateral L 6 and L 7 dorsal rootlets
Format as in Table I. Level
No.
Phase of the cycle Max
iL6 iL7 Total
21 17 38
coL6 coL7 Total
2 4 6
Flexor phase
Min
Amplitude (rain~max) (%)
0.86+0.21 0.28+0.08 0.41+0.09 40.3+16.8 0.83+0.21 0.31-+0.10 0.47+0.07 32.8+15.7 0.85+0.22 0.29_+0.09 0.43_+0.09 36.9+16.8 0.21+0.15 0.59+0.14 0.37+0.02 0.32+0.08 0.78+0.05 0.43+0.06 0.28+0.12 0.72+0.13 0.41+0.06
40.0+4.0 33.3_+11.6 35.5_+10.2
A
!
B
MIN
MAX
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Elsevier BRES 16182
Research Reports
Phase-dependent modulation of dorsal root potentials evoked by peripheral nerve stimulation during fictive locomotion in the cat Jean-Pierre Gossard* and Serge Rossignol Centre de Recherche en Sciences Neurologiques, Facultd de Mddecine, Universit~ de Montreal, Montrdal, Que. (Canada)
(Accepted 24 July 1990) Key words: Dorsal root potential; Fictive locomotion; Presynaptic inhibition; Cat
To help elucidate the role of presynaptic mechanisms in the control of locomotor movements, the transmission of PAD pathways was investigated by recording dorsal root potentials (DRPs) evoked by electrical stimulation of cutaneous and muscle nerves of both hindlimbs at various phases of the fictive step cycle. Fictive locomotion occurred spontaneously in decorticate cats or by stimulating the mesencephalic locomotor region (MLR) as well as in low spinal cats injected with nialamide and L-DOPA. Evoked DRPs were superimposed on a fluctuating DRP accompanying the fictive locomotor rhythm (locomotor DRP) which typically consisted of two peaks of depolarization per cycle, the largest peak occurring during the flexor phase. The amplitude of evoked DRPs was substantially modulated throughout the locomotor cycle and followed a similar modulation pattern for all stimulated nerves whether ipsilateral (i-) or contralateral (co-). The amplitude of evoked DRPs decreased at the beginning of the flexor phase, dropped to a minimum later in the flexor phase and then increased during the extensor phase where it became maximum. Results were comparable in decorticate and spinal preparations and for L6 and 1-,7rootlets with cutaneous and muscle nerve stimulation. It is noteworthy that the modulation pattern for a given rootlet was similar for i- and co. stimulation, even though the bilateral locomotor DRPs fluctuate out-of-phase with each other, subjecting the stimulated fibres to opposite presynaptic polarization changes. This suggests that the modulation may depend more on the presynaptic mechanisms of the receiving fibres than on those of the stimulated fibres. These results demonstrate that the transmission in spinal pathways involved in primary afferent depolarization (PAD) is phasic.ally modulated by the activity in the spinal locomotor network. It is further suggested that the presynaptic inhibition associated with PAD evoked by movement-related sensory feedback during real locomotion could be modulated in a similar way. INTRODUCTION L o c o m o t o r movements are centrally generated in the spinal cord and partly regulated by segmental afferent feedback (see ref. 24). To be effective, the transmission of afferent pathways is phasically and tonically modulated by the central locomotor networks (see ref. 40). There is now some evidence suggesting that the excitability of primary afferent depolarization ( P A D ) pathways responsible for regulating presynaptic inhibition at the level of the primary afferents might also be modulated centrally during locomotion. Excitability testing with Wall's m e t h o d 44, dorsal root potentials (DRPs) and intra-axonal recordings of afferents all indicate that phasic presynaptic mechanisms are activated during fictive locomotion in decerebrate, decorticate and spinal cats injected with L - D O P A 1'2-4'11-13'15'21'22 as well as in walking preparations 11A4'45'46. Rhythmic presynaptic activities have also been observed during fictive scratching 4, mastication 3° and respiration 3s. Based on the concepts of P A D and presynaptic
inhibition 7"1s'28"31'32'42, these previous studies suggested that the spontaneous changes of polarization of primary afferent terminals accompanying locomotion could alter the afferent transmission in different phases of the locomotor cycle. They suggested that maximum presynaptic inhibition occurred during the flexor phase 1A3AS'21, or during the extensor phase 2-4, or sometimes in both phases 11-13'21'45'46. However, few of these studies investigated the actual changes in P A D evoked by sensory input. In one study, antidromic spikes, known as dorsal root reflexes ( D R R s ) , were elicited by muscle nerve stimulation and recorded in the medial gastrocnemius nerve with chronically implanted nerve cuff electrodes in intact and decerebrate cats walking on a treadmill 14. D R R s occurred in correlation with the flexor phase in both preparations suggesting that the evoked P A D , underlying the D R R , was larger during that phase. Partial results from another study 3 indicated that D R P s evoked by cutaneous and muscle nerve stimulation were reduced at the end of the extensor phase and at the beginning of the flexor phase during fictive locomotion in
* Present address: Department of Neurophysiology, Panum Insitute, University of Copenhagen, Copenhagen, Denmark. Correspondence: S. Rossignol, Centre de Recherche en Sciences Neurologiques, Facult6 de M6decine, Universit6 de Montr6al, C.P. 6128, Succ. A, Montr6al, (Qu6bec) Canada H3C 3J7. 0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
d e c o r t i c a t e cats. D R P r e c o r d i n g s in a w a k e rats walking o n a t r e a d m i l l 45'46 i n d i c a t e d that cyclic changes o f D R P a m p l i t u d e w e r e m o r e c o r r e l a t e d with the a f f e r e n t activity o f an a d j a c e n t d o r s a l r o o t l e t t h a n with the m o t o r e f f e r e n t activity ( e l e c t r o m y o g r a m s ) and that this c o r r e l a t i o n was s t r o n g e r d u r i n g stance t h a n d u r i n g swing. T h e b e h a v i o u r o f P A D p a t h w a y s d u r i n g l o c o m o t i o n is thus still u n c l e a r . F o r e x a m p l e , we d o not k n o w if the a f f e r e n t p o l a r i z a t i o n c h a n g e s a c c o m p a n y i n g the central l o c o m o t o r r h y t h m a n d P A D e v o k e d by p e r i p h e r a l input are g e n e r a t e d t h r o u g h t h e s a m e p a t h w a y s o r m e c h a nisms. It is also n o t k n o w n if t h e o b s e r v e d p r e s y n a p t i c changes
depend
on
stimulated
(giving) fibres o r on
r e c o r d e d ( r e c e i v i n g ) fibres, o r b o t h , n o r if t h e y are due to spinal a n d / o r s u p r a s p i n a l m e c h a n i s m s . In the p r e s e n t study, w e e x a m i n e t h e c h a n g e s in the transmission of P A D p a t h w a y s by r e c o r d i n g D R P s e v o k e d by p e r i p h e r a l n e r v e s t i m u l a t i o n d u r i n g fictive l o c o m o t i o n in d e c o r t i c a t e a n d spinal p a r a l y z e d cats. This will h e l p us to u n d e r s t a n d t h e r o l e of P A D p a t h w a y s in the central p r o g r a m m i n g of p r e s y n a p t i c m e c h a n i s m s r e l a t e d to l o c o m o t o r n e t w o r k s . A b r i e f d e s c r i p t i o n o f the results was g i v e n p r e v i o u s ly 23.
MATERIALS AND METHODS
Preparation The experiments were performed on 12 adult cats (2.0-4.7 kg). Under steroid anesthesia (Saffan 1 ml/kg (alphaxalone 9 mg/ml, alphadolone 3 mg/ml), a tracheotomy was performed and a cannula was inserted in the right jugular vein for administration of drugs and fluids. The right femoral or carotid artery was also cannulated to monitor arterial pressure which was maintained around 120 mm Hg by adjusting an inflow of 5% dextrose saline solution which was supplemented, when necessary, with norepinephrine bitartrate (Levophed 4 mg/liter). For 10 cats, the head was fixed in a rigid stereotaxic frame, an extensive craniotomy was performed and the cerebral cortex was completely ablated by suction33"35. One of these was later spinalized at T13 acutely. Two other cats were spinalized at T13 under barbiturate anesthesia (sodium pentobarbita145 mg/kg) a few days before the experiment to allow some recovery from spinal shock. On the day of the experiment, they were anaemically decerebrated under steroid anesthesia (Saffan 1 ml/kg) by ligating the common carotid arteries and the basilar artery just cranial to the branch point of the posterior inferior cerebellar arteries 36. In all cats, the nerves to sartorius (lateral and medial branches), vastus lateralis or semimembranosus muscles of the left hindlimb were dissected free and cut. Their proximal stumps were mounted on bipolar silver chloride recording electrodes (Electroneurogram (ENG): Fig. IF,E). Nerves and dorsal rootlets on the same side of the ENG recordings are designated as ipsilateral (i-); nerves and rootlets on the opposite side, as contralateral (co-). The ipsilateral superficial peroneal (iSP) (at the ankle level), ipsilateral tibialis anterior (iTA) and ipsilateral lateral gastroenemius-soleus (iLGS) nerves and the coSP and coTA nerves were exposed and enclosed in polymer cuff electrodes 29 for stimulation. The pelvis and lumbar spine were fixed in a frame and, following a laminectomy exposing L 5 to S 1 spinal segments, the dura and the pia were partially removed. L 6 and I_,7 dorsal rootlets (DRs) of both sides were cut and their proximal stumps mounted on bipolar silver chloride recording electrodes close to, but not touching, the spinal cord. Paraffin oil pools were formed by drawing up skin flaps surrounding the spinal
cord and the hindlimb nerves. The body temperature was kept near 38 °C by means of a radiant heat lamp. The animal was paralyzed with gallamine triethiodide injection (Flaxedil 20 mg/kg/h i.v.) and artificially respirated. Upon cessation of anaesthesia, fictive locomotion 34 sometimes occurred spontaneously in the decorticate preparations. If not. episodes of fictive locomotion were initiated by exteroceptive stimulation (tonic pinching) of various parts of the body or by trains of stimuli (200/~s pulses, 300 Hz, 150 ms) applied to a cutaneous nerve (e.g. SP) 25. In 2/9 decorticate cats, electrical stimulation (sinusoidal 60 Hz, 20-100 /~A) of the mesencephalic locomotor region (MLR" Horsley-Clarke coordinates P 2, L 4, H 0) with a low impedance tungsten electrode was necessary to initiate locomotor activity42. The spinal animals were injected with nialamide (50 mg/kg) and L-DOPA (80 mg/kg) 25 after surgery,
Stimulation and recordings Through polymer cuff electrodes, stimuli (single or a train of 4 square pulses, 200/~s, 300 Hz) were delivered to i- and co-hindlimb nerves (Fig. 1) in different parts of the locomotor cycle every 2 or 3 cycles (Fig. 2A). The threshold for nerve stimulation (T) was determined by the stimulus strength required to just evoke a deflection in the cord dorsum potential recording. Stimulus strengths ranged from 1 to 30 × T. ENGs and DRPs were recorded using AC-coupled amplifiers. The bandwidth used to record evoked DRPs and to eliminate the antidromic spiking activity was 0.1 or 0.3 Hz to 0.1 kHz. The ENGs were recorded with a larger bandwidth (100 Hz to 10 kHz). Data were recorded on a 7-channel FM tape recorder with a frequency response of 0-5000 Hz at a speed of 19 cm/s. The tapes were played back on an electrostatic printer (Gould ES-1000) and sections of data suited for analysis were digitized on a PDP 11/34 laboratory computer at 1 kHz. Interactive software 47 was used to detect and measure the characteristics of the recorded signals. The locomotor cycle was defined as the interval between the onset of two successive flexor ENG bursts.
iDRP
coDRP
%out ou; g,
MUSCULAR NERVE~RVECUTANEOUS MUSCULAR NERVE
Fig. 1. Experimental paradigm: ipsilateral (iDRP) and contralateral (coDRP) lumbar dorsal rootlets were cut and their proximal stumps recorded with bipolar Ag/AgCI electrodes. Electroneurographic activity (ENG) of flexor (F) and extensor (E) nerves were recorded with similar electrodes. Through electrodes inserted in polymer cuffszg, stimuli were delivered to i- and co-hindllmb cutaneous and muscle nerves in different parts of the locomotor cycle.
Phase plots were constructed from the integrated value of averaged DRP amplitude evoked by (2-25) stimuli in the different phases of the cycle. The integration was computed over an interval equal to the mean duration of the responses. Spontaneous activity in the control (non-stimulated) cycles corresponding to the phase of the stimuli in the stimulated cycle was subtracted from the evoked response. The ordinate of the phase plot was normalized to the largest response (100%). The ENG (rectified and integrated) and DRP signals of control cycles were averaged and normalized to 100%. Only sequences with steady cycle durations (.~ cycle 1232 + 160 ms) were chosen for this analysis so that the signals would not be significantly distorted by the normalizing procedure. In Figs. 3-7, the averaged signals were superimposed on the phase plot to facilitate viewing of temporal relationships. Because the stimulated cycles are often shortened by the stimulation and because the timing of stimuli is expressed as a phase of the control cycles, stimuli in the late phases of the cycle may appear absent. The spontaneous fluctuations of DRP accompanying the fictive locomotor rhythm (see ref. 13) is designated as 'locomotor DRP' or 'L-DRP' and the DRP evoked by peripheral stimulation, as 'evoked DRP' or 'E-DRP' throughout the paper. The minimum amplitude of E-DRP as well as the phases of the cycle of the minimum and maximum E-DRP amplitude were determined for each phase plot and averaged. Results were compared between L~ and l..7 dorsal rootlets recordings, i- and coDRs, cutaneous and muscle nerve stimulation (Tables I and II), spontaneous and MLR-induced fictive locomotion and analyzed with Student's t-tests. The phases of the cycle when the L-DRP had maximum and minimum ampfitude were determined for each phase plot and compared with those of maximum and minimum E-DRP, respectively, and analyzed with linear regressions.
RESULTS Locomotor modulation of evoked DRPs As previously reported (see ref. 13), L - D R P s on both sides of the spinal cord fluctuate at the rhythm of fictive locomotion. For example, Fig. 2 A shows two L - D R P s of iL 6 and iL 7 and one L - D R P of coL 7 recorded during spontaneous fictive locomotion. Note that the peaks of negativity ( > 5 0 g V ) in i D R P L 6 and i D R P L 7 occur mainly during the i- flexor phase (F-phase; Srtn bursts) and in c o D R P L 7 during the i- extensor phase (E-phase; VLn bursts). Since efferent activities alternate on both sides of the spinal cord during fictive locomotion 34, the peaks of negativity in i- and c o D R s indicate that maximum depolarization of afferents occurs during the F-phase on each side. Also note the antidromic firing in the 3 DRs. There are clear phasic antidromic bursts during the F-phase in the i D R P L 7 and probably two bursts per cycle in the i D R P L 6. Since antidromic firing could persist throughout the duration of an experiment (8 h in this example), an appropriate bandwidth was used to filter out the antidromic spiking in trials where the amplitude of E - D R P s was measured (see Materials and Methods)~ Also illustrated at the bottom of Fig. 2A, 3 trains of stimuli (4 pulses, 300 Hz, 10 x T) applied to the iSP nerve during the first third of the F-phase evoke responses superimposed on the 3 L-DRPs. The amplitude of E - D R P s given during the same phase of the cycle are
fairly constant, in shape and duration, from one ~yde to the other, and may reach 100/~V. In Fig. 2B, E - D R P s are shown at an expanded time base which reveals their characteristic long duration ( > 1 0 0 ms). Typically, the coE - D R P is of slightly smaller amplitude and has a longer time to peak 19. D R P responses to stimulation o f a cutaneous nerve Fig. 3 depicts representative examples of the results obtained when the P A D pathways were activated by cutaneous inputs. In each panel of the figure (A, B and C), the averaged signals of E N G s and L - D R P s are normalized with respect to time and are superimposed on the phase plot of the averaged amplitude of the E - D R P s
A
50 #V
ioRP~
] 5 0 #V
iDRPL 7
50 ,~V
coDRPL 7
Srtn
,aL
ah.
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~
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,~
~
,,,,.-
~
l
STIM
I
I
I ls
B
~
,i 5 0 #,V
] so .v
50/zV
I '100 ms'
Fig. 2. Locomotor DRPs and evoked DRPs. A: DRP fluctuations with antidromic discharges in 3 dorsal rootlets follow the rhythm of fictive locomotion (locomotor DRP (L-DRP)) evidenced by the alternating ENG activities of flexor Srtn, and extensor: VLn. Three trains of stimuli of iSP nerve (4 pulses, 10 x T) evoke dorsal root potentials (evoked DRP (E-DRP)) in i- and co-dorsal rootlets superimposed on locomotor DRPs. B: evoked DRPs at a faster time base.
A
B
STIM iSPn ( 4 x T )
STIM iSPn (30 x T)
R6216 Srtn
)~1~
C
STIM coSPn
DR6203
DR6206 N= ~32
Srtn Srtn
/1'
_' ; ~
X cycle = 728 ms
X cycle = 687 ms coTAn
VLn
VLn
HIP' '~ locomotor iDRPL6
locomotor iDRPL6
ocomotor iDRPL7
ocomotor
I
' I'
iDRPL6
locomotor coDRPL7 /~
t EVORKEsD
100°/o1
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~ iL6 o----o ik7
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(10 xT)
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P H A S E OF C Y C L E
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P H A S E OF CYCLE
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Fig. 3. Amplitude modulation of DRPs evoked by the stimulation of the cutaneous nerve SP during spontaneous fictive locomotion in a decorticate preparation. From top to bottom: averaged signals of integrated rectified ENGs, locomotor DRPs for a number of cycles (n) and the phase plot of averaged amplitude of evoked DRPs. A, iSP (single pulse, 4 x T); B, iSP (single pulse, 30 x T); C, coSP (4 pulses, 10 x T).
evoked by SP stimulation. Results of Fig. 3 were all recorded during spontaneous fictive locomotion in the same decorticate cat. In Fig. 3A, the phase plot shows that the amplitude of E-DRPs in the two iDRs elicited by a 4 x T stimulation decreases during the F-phase (Srtn activity), drops to a minimum towards the end of that phase and starts increasing to reach a maximum in the E-phase (VLn activity). The amplitude is still near its maximum value at the F - E transition. The pattern of modulation is quite similar for both iL 6 and iL 7 rootlets, as are the L-DRP patterns which, here, consist of one main wave of depolarization peaking at the end of the F-phase (3rd and 4th traces). Results presented in Fig 3B were obtained with a stronger iSP (30 x T) stimulation (same iL 6 DR; different iL7 DR). Again, the phase plot shows a comparable modulation pattern for the two DRs, i.e. reaching a minimum and a maximum amplitude during the F- and E-phase, respectively. In this particular example, the magnitude of modulation in iL6 is seen to be enhanced with stronger stimulation. This, however, was not consistently observed. Note that the period of E-phase in Fig. 3B is now indicated by the averaged ENG activity of coTAn. Results with different strengths of iSP
stimulation were obtained in 7 other cats with spontaneous fictive rhythm for 8 trials and in one cat with MLR-induced locomotor rhythm for one trial. Modulation characteristics were similar to those found with weak stimulation. In Fig. 3C, stimulation of coSPn with a short train of 4 stimuli (10 × T) elicits E-DRPs on both sides of the spinal cord (different iL 6 than in Fig. 3A,B). As in Fig. 2A, i- and co- L-DRPs are seen to vary in a reciprocal fashion (3rd and 4th traces). The trough in locomotor coDRPL 7 (4th trace) suggests that the terminals of the co-stimulated fibres (coSP) are repolarized during the F-phase relative to the i-fibres which are depolarized as suggested by the rising locomotor iDRPL 6. The phase plot shows that the amplitude modulation of E-DRPs in iL6 (empty diamonds) had a similar pattern to those described in Fig. 3A,B. This consistency in modulation patterns for a given rootlet level was found in all trials of i- and co- SP stimulation. On the other hand, the amplitude of E-DRPs in coL 7 (filled squares) increases to a maximum during the F-phase and then quickly decreases during the E-phase. Hence, the E-DRP modulation patterns in iL6 and c o l 7 are out-of-phase with each
other, as are the locomotor DRPs and ENGs. Comparable results were obtained with different strengths of coSP stimulation for two other trials recorded in this decorticate cat and in two other cats with MLR-induced locomotor rhythm for two trials. Results in Fig. 4 are taken from an experiment where episodes of fictive stepping alternated with episodes of tonic E N G activities. Here, E-DRPs could be evoked with the same stimulation during tonic activities of extensor (VLn) or flexor (Srtn) ENGs and during episodes of fictive locomotion. The locomotor iDRPL6 in Fig. 4A consists of two waves of depolarization of comparable amplitude during F- and E-phases separated by a trough of repolarization (3rd trace). The amplitude of E-DRPs elicited by an iSP stimulation (20 x T) during fictive stepping decreased during the F-phase and then increased at the beginning of the E-phase and reached a maximum towards the end of that phase. Note that the E-DRP modulation pattern is similar to previous examples (Fig. 3A,B) even though the pattern of L-DRP is quite different (see Discussion). The averaged E-DRP amplitude evoked with the same stimulation (20 x T) during an episode of tonic activity in VLn ENG with the Srtn E N G being silent, is shown on the left of the ordinate (filled diamond) of the phase plot. The amplitude of E - D R P equals the maximum value seen during the E-phase of fictive locomotion. Later on in the same experiment (Fig. 4B), similar conditions occurred and E-DRPs were elicited with a weaker stimulation of iSP (2 x T) during fictive stepping and during episodes of tonic VLn E N G activity, or of tonic Srtn E N G activity, while the antagonist muscle nerve was silent. The E - D R P amplitude obtained during extensor tonic activity (filled diamond on the left of the ordinate) was comparable to the maximum value obtained during the E-phase while the E - D R P amplitude during flexor tonic activity (filled
STIMiSPn (20 x "I')
A
B
8PA3802
san
STIM iSPn (2 x T) S.o
~1
PA3808
cycle = 1090 ms
X cycle = 1975 ms
VLn 200 ms
locomotor
400 ms
locomotor iDRPL6
iDRPL6
~00~o.1 t EVOKED
4
/
DRPs 1
t
DRPs
.
.
.
.
.
]
,
,
,
0 PHASE OF CYCLE 70 0 PHASE OF CYCLE 1.0 Fig. 4. Amplitude of DRPs evoked by the stimulation of the SP nerve during tonic ENG activities and during spontaneous rictive locomotion in decorticate preparations. In A, the averaged amplitude of DRPs evoked by the stimulation of iSP (single pulse, 20 x T) during tonic extensor VLn ENG activity is shown on the left of the ordinate (filled diamond) and the amplitude modulation during the episode of fictive stepping is shown in the phase plot (empty diamonds). In B, the averaged amplitudes of DRPs evoked by a weaker iSP stimulation (single pulse, 2 x T) during tonic extensor VLn (filled diamond) or tonic flexor Srtn (rifled circle) ENG activities are shown on the left of the ordinate and the amplitude modulation during fictive stepping is shown in the phase plot (empty diamonds).
circle on the left of the ordinate) was just inferior to the minimum value seen during the F-phase. The amplitude of E - D R P did not fluctuate during tonic E N G activities and suggests there was no locomotion in on-recorded nerves as well.
TABLE I
Mean phase of the fictive step cycle (normalized step cycle = 1) when maximal (Max) and minimal (Min) E-DRP occurred for cutaneous (pooled iSP and coSP) and muscle (pooled iTA, co TA and iLGS) nerve stimulation, and minimum E-DRP amplitude expressed as a percentage of the largest value: mkdmax (%) Results from i- a n d co-stimulation were pooled together. All m e a n s have their standard deviations. N u m b e r of analyzed trials: No.; m e a n phase of the e n d of flexor activity (Flexor phase) serves as reference. See also Fig. 8A,B.
Stimulation
Level
No.
Phase of the cycle Max
M/n
Flexor phase
Amplitude (rain~max)
(%)
iSP a n d coSP
iL 6 iL 7 Total
15 10 25
0.85 + 0.22 0.84 + 0.21 0.85 + 0.22
0.31 + 0.07 0.31 + 0.10 0.31 + 0.08
0.44 + 0.09 0.50 + 0.05 0.46 + 0.08
41.2 + 15.3 32.7 + 18.7 37.8 + 17.2
iTA, coTA a n d iLOS
iL6 iL 7 Total
3 7 10
0.89 + 0.25 0.81 + 0.22 0.84 + 0.23
0.20 + 0.05 0.30 + 0.10 0.27 + 0.10
0.30 + 0.04 0.44 + 0.08 0.42 + 0.10
43.3 + 26.1 33.0 + 10.1 36.1 + 17.3
.&
STIM iTAn (2xT) ,~ hh,,v%, Srtn
STIMiTAn
B
DR 6211 N= 131
'~
Srtn
I,~l /L~ ~,,J~,l~J,
cycle = 963 ms
C
STIM coTAn
DR6212 N ':--!5 Srtn
/
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200 ms
2 cycle = 965 ms
200 ms
locomotor iDRPL7
t2 x F~
DR 6102 N -12
2 cycle = 1394 ms VLn
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VLn
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locomotor M ~ j / J ' ~ ~ iDRPL7
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,
,
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EVOKED DRPs
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0
, ~
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EVOKED DRPs
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locomotor
-
/
200 ms
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~,
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10
',
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"'%/~'--~" coDRPL
l
'B
• ~coL7
PHASE OF CYCLE
EVOKE° i /
iL7
PHASE OF CYCLE ~ 0
0
PHASE OF CYCLE
10
Fig. 5. Amplitude modulation of DRPs evoked by the stimulation of the flexor nerve TA during spontaneous fictive locomotion in decorticate preparations. Same display as in Fig. 3. A, iTA (4 pulses, 2 x T); B, iTA (4 pulses, 15 x T); C, coTA (4 pulses, 2 x T).
A
S T I M iGLn (2 x T)
[3
,~!~M~,rl
S T I M i G L n (30 x T)
Dn 6210
~ll
bR 630S
cycle = 91 7 ms X cycle = 772 ms
VLn
VLn 200 ms
locomotor iDRPL6
!
/ x ~ ' ~
,' 200 ms ~ ~ f - "
.~,
locomotor i DRPL7
locomotor ,.,:'¢~"' iDRPL7
100%~ ~~°y~,~. ,,' " DRPs
~, -.~,
,,#
100%]r
iL7
I "o
EVOKED
~
/
EVOKED
,
DRPs
~
"
""
', "
r
~// •0
c~- o IL7
PHASE OF CYCLE
110
PHASE OF CYCLE
1.0
Fig. 6. Amplitude modulation of DRPs evoked by the stimulation of the extensor nerve LGS during spontaneous (A) MLR-induced (B) fictive locomotion in decorticate preparations• Same display as in Fig. 3. A, iLGS (4 pulses, 2 x T); B, iLGS (4 pulses, 30 x T).
SPINAL CAT A
B
STIM iSPn (15xT) ~1~~
TAn
PA 7701
STIM iTAn (20xT) Srtn
[~kll,J
cycle = 1742 ms
X cycle = 3188 ms SmAn
~
VLn
800 ms
200 ms
~
locomotor iDRPL7
locomotor iDRPL6
100% -
100%-
EVOKED DRPs
EVOKED DRPs
0
DR 6402
PHASE OF CYCLE
i L7
1.0
0
PHASE OF CYCLE
1.0
Fig. 7. Amplitude modulation of DRPs evoked by the stimulation of the cutaneous nerve iSP and flexor nerve TA during fictive locomotion in spinal cats injected with nialamide and L-DOPA. Same display as in Fig. 3. A, iSP (single pulse, 15 x T); B, iTA (single pulse, 20 x T).
Table I lists various characteristics of amplitude modulation of E-DRPs elicited by cutaneous and muscle nerve stimulation: the number of trials obtained for iL 6 and iL 7 (No.) in different cats, the mean phase of the cycle for maximum (Max) and minimum (Min) E - D R P amplitude and the minimum amplitude expressed as percentage of the largest value. In the upper part of Table I, the results from iSP and coSP nerve stimulation were pooled together and averaged for iL 6 and iL 7 DRs since the modulation patterns were independent of the side of stimulation. The mean values show that, overall, the maximum E-DRP amplitude occurred in late extensor phase (mean 0.85 + 0.22 of the locomotor cycle) and that the minimum amplitude occurred during the F-phase (mean 0.31 + 0.08). Those values were comparable for iL6 and iL7 and were found to be not statistically different. The mean minimum amplitude was not statistically different for L 6 and L 7 DRs either (mean 37.8 + 17.2% of the largest value) although there were large standard deviations. D R P responses to stimulation o f muscle nerves
The amplitude modulation of E-DRPs elicited by the stimulation of the flexor nerve TA during spontaneous
fictive stepping is illustrated in the phase plots of Fig. 5. In Fig. 5A, the short train of stimuli (4 pulses, 2 x T) was strong enough to evoke E-DRPs on both sides during spontaneous fictive stepping. The phase plots shows that the modulation patterns of E - D R P amplitude of iL6 and coL 7 resemble those obtained with cutaneous stimuli in
TABLE II Mean phase of the fictive step cycle (normalized step cycle = 1) when the maximal (max) and minimal (min) E-DRP occurred for all stimulation recorded in ipsi- and contralateral L 6 and L 7 dorsal rootlets
Format as in Table I. Level
No.
Phase of the cycle Max
iL6 iL7 Total
21 17 38
coL6 coL7 Total
2 4 6
Flexor phase
Min
Amplitude (rain~max) (%)
0.86+0.21 0.28+0.08 0.41+0.09 40.3+16.8 0.83+0.21 0.31-+0.10 0.47+0.07 32.8+15.7 0.85+0.22 0.29_+0.09 0.43_+0.09 36.9+16.8 0.21+0.15 0.59+0.14 0.37+0.02 0.32+0.08 0.78+0.05 0.43+0.06 0.28+0.12 0.72+0.13 0.41+0.06
40.0+4.0 33.3_+11.6 35.5_+10.2
A
!
B
MIN
MAX
Ot
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