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Department of Physiology, University of Alberta, Edmonton T6G 2H7, Canada
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ABSTRACT |
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 Abstract
 Introduction
Methods
Results
Discussion
References
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Whelan, P. J. and K. G. Pearson. Plasticity in reflex pathways controlling stepping in the cat. J. Neurophysiol. 78: 1643-1650, 1997. Previous studies have shown that stimulation of group `I' afferents from ankle extensor muscles can prolong the cycle period in decerebrate walking cats and that the magnitude of these effects can be altered after chronic axotomy of the lateral-gastrocnemius/soleus (LGS) nerve. The effectiveness of LGS group I afferents in prolonging the cycle period decreases after axotomy, whereas the effectiveness of the uncut medial-gastrocnemius (MG) group I afferents is increased. The objectives of this investigation were to establish the time course of these changes in effectiveness and to determine whether these changes persist after transection of the spinal cord. The effects of stimulating the LGS and/or MG group I afferents on the cycle period were examined in 22 walking decerebrate animals in which one LGS nerve had been cut for 2 to 31 days. The effectiveness of LGS group I afferents declined progressively in the postaxotomy period, beginning with significant decreases at 3 days and ending close to zero effectiveness at 31 days. Large increases in the effectiveness of MG group I afferents occurred 5 days after axotomy, but there was no progressive change from 5 to 31 days. To test whether these changes in effectiveness were localized to sites within the spinal cord, the cord was transected in some decerebrate animals and stepping induced by theadministration of L-DOPA L-3-4 dihydroxyphenylalanine (LDOPA) and Nialamide. The effects of stimulating the MG and/or the LGS group I afferents on the cycle period were reexamined. In all four animals tested, stimulating the axotomized LGS group I afferents had a reduced effectiveness during locomotor activity in both the decerebrate and spinal states, whereas the increased effectiveness of the MG group I afferents was retained after transection of the spinal cord in two of five animals. Different mechanisms may be responsible for the changes in strength of the LGS and MG group I afferent pathways that project onto the rhythm generating sites in the spinal cord. This possibility follows from our observations of a linear relationship between the time after axotomy and decreased effectiveness of LGS group I afferents but no significant relationship between time postaxotomy and increased effectiveness of MG group I afferents; no significant relationship between the decreased effectiveness of LGS group I afferents and the increased effectiveness of MG group I afferents; and, after spinalization, consistent (4/4 cases) preservation of decreased LGS effectiveness but frequent (3/5 cases) loss of increased MG effectiveness.
Motor performance is optimized by the interplay of commands from the brain and spinal cord, and sensory feedback from peripheral receptors. Optimization of motor output depends on a continual adjustment of motor systems to changing environmental conditions, to neuronal and biomechanical changes during development, and to injury. In this regard, many studies of motor adaptation and learning have explored modification of reflex transmission. These include studies on oculomotor reflexes in mammals and fish (Du Lac et al. 1995 All animals used in this study were cared for in accordance with the guidelines published by the American Physiological Society. The University of Alberta animal welfare committee approved the experimental procedures. Experiments were carried out in 22 adult cats of both sexes.
Chronic procedures
All 22 animals underwent the following minor surgical procedure. Under halothane anesthesia and aseptic conditions, the nerve supplying the LGS muscle of one hind leg was exposed and transected close to the muscle. The proximal nerve was tied with 6-0 silk to mark it for future identification. An antibiotic (Ayercillin, 1 ml) and, if necessary, an analgesic (Buprenorphine, 0.005-0.01 mg/kg) were administered for Acute procedures
Two acute preparations were used: premammillary decerebrate cats and spinal cats treated with L-3-4 dihydroxyphenylalanine (L-DOPA). Thirteen animals were studied only after decerebration, whereas 9 animals were studied after decerebration and later that day after transection of the spinal cord at the T12 level.
Data analysis
All data were recorded using a Vetter 4000A PCM recorder. Later, selected sequences were rectified, filtered (band-pass:10-100 Hz) and stored on computer disk using the Axotape data acquisition system (Axon Instruments). Data analyses were carried out using custom programs that could retrieve data from the Axotape files. The cycle periods before, during, and after the stimulus were calculated only during regular sequences of rhythmic locomotor activity. Each cycle period was calculated as the time between the occurrence of successive ST or IP bursts. All detection of the flexor bursts was made by manually tagging the onsets of the bursts using custom written software. A spreadsheet program (Microsoft Excel 5.0) was used to calculate the mean and standard deviation for these cycle periods and Student's t-tests detected significant differences between the conditions. The data were normalized according to the following equation to allow for comparisons between cats and between control and experimental nerves:
Time course of plastic changes after axotomy of the LGS nerve
Stimulation of the group I afferents in a chronically transected LGS nerve produces a smaller than normal increase in the cycle period, whereas the effects of stimulating group I afferents in the ipsilateral MG nerve on the cycle period are increased (Whelan et al. 1995b
Effects of spinalization on expression of plasticity
To establish whether one site mediating the changes in effectiveness is located within the spinal cord, the following protocol was used. In nine decerebrate animals for which there were significant changes in effectiveness for either the experimental MG or LGS nerves, the spinal cord was transected and locomotion was induced by applyingL-DOPA and Nialamide. Generally, the rhythm produced in the spinalized animals did not result in stepping movements that were as powerful as those obtained when the animal was walking in the decerebrate state. Even though the stepping was generally poor in the spinal state, the timing of the extensor and flexor EMG bursts in the VL and IP muscles were close to normal during periods of rhythmicity (Figs. 5, A and B, and 6, A and B).
Axotomy of the LGS nerve increases activity and weight
of MG
In three animals, the magnitude of EMG bursts in the MG muscle were monitored before and after axotomy of the LGS nerve. In all animals, a significant increase in the amplitude of the integrated MG EMG in the experimental limb occurred on the first day after axotomy (P < 0.05; data not shown). Figure 7 shows the average EMGs before (thin lines) and 3-7 days after axotomy (thick lines) for three animals. A significant increase in the amplitude of the EMG occurred ~100 ms after the onset of the MG burst for two cats (Fig. 7, A and B) and 200 ms for the third animal (Fig. 7C). The mean differences between the post- and preaxotomy MG EMGs were 56% for the cat in Fig. 7A, 14% for the cat in Fig. 7B, and 12% for the cat in Fig. 7C.
The data presented in this study support and extend a previous report that changes in LGS and MG group I pathways regulating stepping occur after axotomy of the LGS nerve (Whelan et al. 1995b Time course of plasticity
One of the aims of this investigation was to investigate the time course of the changes in the LGS and MG group I effectiveness after axotomy of the LGS nerve. The initial finding (Whelan et al. 1995b One site of plasticity is within the spinal cord
Because the oligosynaptic group I extensor pathway is open and functional in spinal animals (Conway et al. 1987 Functional relevance
The nervous system is endowed with an ability to reorganize and mold synaptic connections. Within this general context, the need for adaptive reflex modification has been recognized for many years (Bloedel et al. 1991
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
), head-orientating responses to sound in owls (Knudsen 1994), adaptive postural responses in humans (Horak and Diener 1994), classically conditioned cutaneous reflexes in cats (Rispal-Padel and Meftah 1992), and instrumental conditioning of the H reflex in monkeys and rats (Wolpaw and Carp 1993). Finally, a variety of procedures have revealed plasticity in monosynaptic reflex pathways in the spinal cord: transection of nerves (Eccles and McIntyre 1953; Gallego et al. 1979), tenotomy (Goldfarb and Muller 1971; Kozak and Westerman 1961), limb immobilization (Maier et al. 1972; Mayer et al. 1981), blocking afferent activity by tetrodotoxin (Gallego et al. 1979; Webb and Cope 1992), and instrumental conditioning (Carp and Wolpaw 1994). The counterintuitive conclusion from these studies on the monosynaptic reflex is that the absence of activity increases the strength of the monosynaptic reflex, whereas increased activity decreases the strength of the reflex (Mendell 1984).
). Normally, stimulation of group I afferents in the lateral gastrocnemius and soleus (LGS) nerve is more effective in increasing the cycle period than stimulation of group I afferents in the medial gastrocnemius (MG) nerve. A few days after transection of the LGS nerve, however, the influence of group I afferents from the LGS and MG muscles is altered; the chronically sectioned LGS afferents become less effective, whereas the MG afferents become more effective (Whelan et al. 1995b).
METHODS
Abstract
Introduction
Methods
Results
Discussion
References
1 wk after surgery. Animals were allowed out of their cages on a daily basis after surgery. The cages were large enough (dimensions: 73 wide × 69 deep × 84 cm high) to permit the animal to move freely and to jump onto a ledge. After a period of 2-31 days, the acute surgical and experimental procedures were performed.
for details of cuffs). The threshold of the electrical stimulus to the extensor nerves (1 × T) was taken as the minimum voltage necessary to produce a visually detectable sciatic potential. The strength of the stimulus was expressed in multiples of this threshold level. Bipolar stainless steel recording electrodes (Cooner Wire, AS632) were sewn into the following muscles of both hind legs to record electromyographic (EMG) activity: MG (in the 3 cats that did not have the MG nerve cut), vastus lateralis (VL), semitendinosus (ST), and iliopsoas (IP). The wires from both the stimulating and the EMG electrodes were led subcutaneously to a multipolar connector on the back of the cat. After finishing this procedure, the animal was placed above a motorized treadmill. A sling under the abdomen aided in weight support and maintenance of lateral stability. The animal then was decerebrated by transecting the brain stem at a 50° angle from the anterior edge of the superior colliculus. The halothane anesthesia was discontinued at this time.
where b equals the stimulated cycle period, a represents the control cycle period, and c represents the time from the first flexor burst before the stimulated extensor burst to the offset of the stimulus train (see Fig. 1A for an illustration of the measured variables). The percentage effectiveness is a measure of how powerfully the stimulus could affect the step cycle. For example, if the stimulus was 100% effective, the next flexor burst would have been held off until the end of the stimulus train. By contrast, if the percentage effectiveness was 0%, the stimulus would have had no effect on the cycle period.
![Percentage effectiveness = [(<IT>b</IT> − <IT>a</IT>)/(<IT>c</IT> − <IT>a</IT>)] × 100](/content/vol78/issue3/fulltext/1643/img001.gif)
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FIG. 1.
Chronic axotomy of the lateral gastrocnemius and soleus (LGS) nerve increases the ability of group I afferents in the medial gastrocnemius (MG) to prolong the cycle period. A and B: rectified and filtered electromyographs (EMGs) from the hind leg muscles showing individual trials in the control (A) and experimental (B) legs (stimulus trains: 1,000-ms duration, 200 Hz, 2 × T). Note that in the control leg, stimulation of the MG nerve had only a small effect on the cycle period compared with the relatively large increase in the cycle period in the experimental leg (LGS cut 21 days). The parameters a, b and c used for quantifying the effectiveness of the stimulus trains are shown in A. C: bar graph showing the mean percent effectiveness of MG stimulation of the control ( 
) and experimental (
) legs for all animals tested. Error bars represent the standard deviation. * Significant difference between the 2 conditions (P < 0.05). Number on the abscissa below each bar indicates the duration of the axotomy. VL l and r, left and right vastus lateralis; IP l and r, left and right iliopsoas.
1 wk after axotomy of the ipsilateral LGS nerve. The procedure for recording EMG activity from intact walking animals has been described elsewhere (Hiebert et al. 1994). The magnitude of the EMG was calculated by integrating the rectified and filtered EMG during a 500-ms period. Usually, 20 steps were used to calculate the mean and standard deviation of the EMG.
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
). The first objective of the present study was to establish the time course of these changes by pooling data (10 animals) from this initial study with an additional group of animals. In total the effects of stimulating the MG nerve were measured in 17 of 20 cats and the LGS nerve in 18 of 20 cats (both measurements were made in 15 of 20 cats).
5 days, and no significant increases were observed in two of the three animals in which the LGS nerve had been cut for 3 days.
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FIG. 2.
Chronic axotomy of the LGS nerve reduces the ability of group I afferents in the LGS nerve to prolong the cycle period. A and B: rectified and filtered EMG from hindlimb muscles showing individual trials in the control (A) and experimental (B) legs (stimulus trains: 1,000-ms duration; 200 Hz; 2 × T). Note that in the control leg, stimulation of the LGS nerve produced a large increase in the cycle period. In the experimental leg, in contrast, stimulation of the nerve using similar parameters only modestly increased the cycle period (LGS cut 21 days). C: bar graph showing the mean percent effectiveness of LGS stimulation of the control ( ) and experimental () legs for each individual experiment. Error bars represent the standard deviation. * Significant difference between the 2 conditions(P < 0.05). Numbers on the abscissa under each bar indicate the duration of the axotomy.
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FIG. 3.
Time course of changes in the effectiveness of MG and LGS group I afferents after chronic axotomy of the LGS nerve. A-D: scatterplots of the data shown in Figs. 1C and 2C with the abscissa indicating the number of days after axotomy and the ordinate indicating the effectiveness of the stimulus. Best fitted linear regression lines ( 
) are shown along with the regression coefficient and equation of the line for each graph. Each data point represents the mean effectiveness from an individual experiment.
) in Fig. 4].
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FIG. 4.
Changes in effectiveness of MG and LGS effectiveness are not correlated. Each point is the difference in percent effectiveness between experimental and control legs for MG and LGS in a single animal. Measurements were taken in animals from 2 to 31 days postaxotomy. , animals in which there was a large difference in effectiveness of 1 pair of nerves and only a small difference in the synergistic pair. A best fitted linear regression line () is shown along with its regression coefficient and the equation of the line.
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FIG. 5.
Spinalization can abolish differences in the effectiveness of the MG group I afferents. A and B: rectified and filtered EMG traces (knee extensor VL and hip flexor IP) from a stepping L-3-4 dihydroxyphenylalanine (L-DOPA) spinal cat (LGS nerve axotomized 21 days previously). A: control leg. Note that stimulation of the MG group I afferents modestly increased the cycle period ( 
, expected time of termination of VL activity in the absence of stimulation). B: Effects of stimulating the experimental MG nerve in the same animal during L-DOPA-induced stepping. In this animal, the effect on the cycle period was similar to A, i.e., the difference in effectiveness between control and experimental legs before spinalization (not shown) was not expressed following spinalization. C: bar graph summarizing the effects of stimulating the MG group I afferents in the experimental and control leg during decerebrate and spinal walking for each animal tested. Note that only 2 out of 5 animals showed a conservation of the changes in effectiveness after spinalization. * Significant difference between the 2 legs (P < 0.05). Error bars indicate the standard deviation. Numbers on the abscissa indicate the duration of the axotomy of the LGS nerve.
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FIG. 6.
Axotomy of the LGS nerve decreases the effectiveness of stimulating the LGS group I afferents in the decerebrate walking cat; this effect is preserved after spinalization. A and B: rectified and filtered EMG traces from a stepping L-DOPA spinal cat (data from the same animal as shown in Fig. 5). A: control leg. Stimulation of the LGS group I afferents produced a large increase in the cycle period. B: experimental leg. Stimulation of the LGS nerve had only a modest effect on the cycle period compared with A ( in A and B indicate the expected onset of the flexor burst in the absence of stimulation). C: bar graph summarizing the effects of stimulating the LGS group I afferents in the experimental and control legs during decerebrate and spinal walking for each animal tested. Note that in all animals, the changes in effectiveness observed during decerebration locomotion persisted after transection of the spinal cord. * Significant difference between the 2 legs (P < 0.05). Error bars indicate the standard deviation. Numbers on the abscissa indicate the duration of the axotomy of the LGS nerve.
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FIG. 7.
In intact walking animals, the amplitude of the MG EMG bursts during stance increased after axotomy of the LGS nerve. A-C: averages of rectified and filtered EMGs (n = 20) recorded from the MG muscle of 3 animals as they walked bipedally on a treadmill (0.4 m/s). Thin traces show the average amplitudes of MG EMG before the LGS nerve was cut. Thick traces show the amplitude of the MG EMG on the day of the terminal experiment.
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FIG. 8.
Changes in the weights of the MG and LG muscles after axotomy of the LGS nerve. A and B: scatterplots showing the percentage change in the wet weight of each muscle after axotomy of the LGS nerve. Each data point represents an individual animal. Best fitting linear regression lines with regression coefficients are shown along with their regression coefficients and equations. Weights of the MG and LG muscles in the experimental leg increased and decreased respectively with time after axotomy of the LGS nerve.
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
). The results of this investigation show that decreases in the effectiveness of the LGS group I afferents controlling cycle period are not correlated with increases in the effectiveness of MG group I afferents. In addition, changes in effectiveness often were conserved after transection of the spinal cord, thus indicating one site of plasticity lies within the spinal cord.
), that spatial summation between the flexor reflex afferent (FRA) system and the group I oligosynaptic pathway (Gossard et al. 1994), and that stimulation of group I extensor afferents resets and entrains the locomotor rhythm, thus demonstrating that these afferents access the rhythm generating circuitry of the spinal cord (Conway et al. 1987; Pearson and Collins 1993; Pearson et al. 1992). Although we did not directly measure changes in transmission through the oligosynaptic pathway, we did measure influences on stepping that have been attributed to transmission in this pathway (Whelan et al. 1995a). Future experiments using intracellular recording techniques (Gossard et al. 1994; McCrea et al. 1995) will test the hypothesis that transmission in group I oligosynaptic pathways is altered after axotomy of the LGS nerve.
) that the onset of the changes in the effectiveness of both the MG and LGS group I pathways could occur within 3 days was confirmed. In addition, two differences in the expression of the plasticity in the LGS and MG group I pathways were found. First, there was no correlation between the decrease in the effectiveness of the LGS pathway and the increase in effectiveness of the MG pathway (Fig. 4). This was especially evident during the first week where, in three of five animals, there were large changes in one pathway without significant changes in the other pathway. Second, the effectiveness of the LGS group I pathway progressively declined over time, whereas the effectiveness of the MG group I pathway did not increase linearly (Fig. 3). These two results suggest that different mechanisms may underlie the plasticity in the MG and the LGS group I pathways. Different mechanisms also have been postulated to account for the opposite changes in homonymous monosynaptic reflex strength that occur after axotomy of the nerve and after interventions that leave the nerve intact but abolish afferent conduction (Gallego et al. 1979; Webb and Cope 1992). It has been suggested that the reduction in the monosynaptic reflex that occurs after axotomy may be due to pathological changes in the afferents, such as retraction of the axon terminals (Mendell 1984). Given that axotomy of the LGS group I afferents reduces the effectiveness of the presumptive oligosynaptic pathway affecting stepping and the homonymous monosynaptic pathway, it is conceivable that similar mechanisms underlie both processes. However, one difference between our findings and those on the monosynaptic group Ia pathway is the time course for the reduction in the reflex strength may be slower in the monosynaptic pathway after axotomy of LGS afferents. The earliest reported reduction in the amplitude of the homonymous monosynaptic excitatory postsynaptic potential (EPSP) is 7 days after axotomy of the extensor nerve (Gallego et al. 1979), whereas the results presented here show considerable declines in the LGS group I pathway beginning as early as 3 days.
) presumably increases feedback from group Ia afferents from the MG muscle during early stance. Because both Ia and Ib afferents contribute to the oligosynaptic pathway regulating cycle period (Guertin et al. 1995), an attractive possibility is that increased activity in MG group I afferents is one factor leading to strengthening of the pathway. This could be tested in future experiments by establishing whether reducing feedback from MG group I afferents during the recovery period after LGS axotomy (by tenotomy of the MG muscle, for example) has any influence on the changes in effectiveness of the MG group I afferents. If an increase in feedback from MG group I afferents is necessary for plasticity to develop, then it will be a distinctly different phenomenon than in the monosynaptic group Ia pathway where a decrease in activity leads to an increase in EPSP amplitude (Webb and Cope 1992).
; Gossard et al. 1994; Pearson et al. 1992), one locus for the plasticity may be within the circuitry of the spinal cord. Alternatively, or in addition, another locus could lie in the brain stem or cerebellum. To explore this issue, we spinalized some of the decerebrate animals in which we observed positive signs of plasticity in both the MG and the LGS group I pathway. In two of five animals, the increase in MG effectiveness persisted after spinalization (Fig. 5), and in four of four animals, a significant reduction in the LGS effectiveness was retained in the spinal state (Fig. 6). These observations demonstrate that one site for plasticity of the group I pathways regulating stepping is in the spinal cord. The conservation of plastic changes in the LGS pathway after spinalization is consistent with the idea that changes in this pathway are a direct result of axotomy. On the other hand, the abolition of differences between control and experimental MG pathways after spinalization in three of five animals indicates a role for supraspinal structures in the expression of plastic changes in the MG pathway. The simplest explanation is that transmission in modifiable spinal pathways is regulated by tonic signals from the brain stem. Considerable evidence exists for brain stem regulation of spinal reflex pathways (Baldissera et al. 1981), and Gossard et al. (1994) have shown that transmission in extensor group I oligosynaptic pathways is expressed gradually after stimulation of the mesencephalic locomotor region (MLR) in the brain stem. Also, preliminary observations from our laboratory indicate that stimulation of the MLR can alter the effectiveness of extensor group I stimulation in increasing the cycle period in decerebrate walking cats (Whelan 1996). Another, more speculative, possibility is that plasticity occurs in long-loop reflex pathways from group I afferents to supraspinal structures that contribute to regulating the cycle period. Long-loop reflex pathways are known to contribute to reflex responses evoked by perturbations of the limbs in humans (Thilmann et al. 1991), but they have not yet been shown to contribute to reflexes regulating stepping. If these pathways do exist, and are modifiable, it would support the growing evidence that adaptive modification of motor systems depends of plastic changes at multiple sites (Bloedel et al. 1991; Raymond et al. 1996; Wolpaw and Carp 1993).
; Ito 1976). During development, after injury, and also during the learning of new tasks, adaptation of reflexes must occur if the motor output is to remain optimized. For example, the gain of the vestibulo-ocular reflex can be altered if the visual field is altered by wearing prisms for a number of days. This adaptation ensures that eye gaze remains stable even though the visual field is distorted (Raymond et al. 1996).
) and decerebrate cat (Guertin et al. 1995; Whelan et al. 1995a). Thus after axotomy, the increased effectiveness of the MG group I pathway may compensate for the lack of timing cues from the LGS group I afferents. Moreover, the general absence of changes in the MG pathway for a few days immediately after axotomy is appropriate because this ensures that recalibration of reflex strength occurs only after a relatively permanent change in the use of the MG muscle. Examples of situations where such changes may be induced under normal conditions include increases in weight during development and increases in muscular strength with training.
; Ito 1976)
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ACKNOWLEDGEMENTS |
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We thank Dr. K. Fouad for valuable comments on the manuscript and R. Gramlich for technical assistance.
This work was supported by grants from the Medical Research Council of Canada and the Alberta Heritage Foundation for Medical Research.
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FOOTNOTES |
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Present address of P. J. Whelan, Laboratory of Neural Control, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Building 49, Room 3A50, Bethesda, MD 20892-4455.
Address for reprint requests: K. G. Pearson, Dept. of Physiology, University of Alberta, Edmonton T6G 2H7, Canada.
Received 12 February 1997; accepted in final form 22 May 1997.
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REFERENCES |
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Abstract
Introduction
Methods
Results
Discussion
References
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