1Laboratory of Neurobiology, National Institute for Physiological Sciences, Okazaki 444, Japan; and 2Department of Physiology, Faculty of Medicine, University of Montreal, Montreal, Quebec H3C 3J7, Canada
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ABSTRACT |
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Matsuyama, Kiyoji and
Trevor Drew.
Vestibulospinal and Reticulospinal Neuronal Activity During
Locomotion in the Intact Cat. II. Walking on an Inclined Plane.
J. Neurophysiol. 84: 2257-2276, 2000.
The
experiments described in this report were designed to determine the
contribution of vestibulospinal neurons (VSNs) in Deiters' nucleus and
of reticulospinal neurons (RSNs) in the medullary reticular formation
to the modifications of the walking pattern that are associated with
locomotion on an inclined plane. Neuronal discharge patterns were
recorded from 44 VSNs and 63 RSNs in cats trained to walk on a
treadmill whose orientation was varied from +20° (uphill) to 10°
(downhill), referred to as pitch tilt, and from 20° roll tilt left to
20° roll tilt right. During uphill locomotion, a majority of VSNs
(25/44) and rhythmically active RSNs (24/39) showed an increase in peak
discharge frequency, above that observed during locomotion on a level
surface. VSNs, unlike some of the RSNs, exhibited no major deviations
from the overall pattern of the activity recorded during level walking.
The relative increase in discharge frequency of the RSNs (on average,
31.8%) was slightly more than twice that observed in the VSNs (on
average, 14.4%), although the average absolute change in discharge
frequency was similar (18.2 Hz in VSNs and 21.6 Hz in RSNs). Changes in discharge frequency during roll tilt were generally more modest and
were more variable, than those observed during uphill locomotion as
were the relative changes in the different limb muscle electromyograms that we recorded. In general, discharge frequency in VSNs was more
frequently increased when the treadmill was rolled to the right (ear
down contralateral to the recording site) than when it was rolled to
the left. Most VSNs that showed significant linear relationships with
treadmill orientation in the roll plane increased their activity during
right roll and decreased activity during left roll. Discharge activity
in phasically modulated RSNs was also modified by roll tilt of the
treadmill. Modulation of activity in RSNs that discharged twice in each
step cycle was frequently reciprocal in that one burst of activity
would increase during left roll and the other during right roll. The
overall results indicate that each system contributes to the changes in
postural tone that are required to adapt the gait for modification on
an inclined surface. The characteristics of the discharge activity of
the VSNs suggest a role primarily in the overall control of the level
of electromyographic activity, while the characteristics of the RSNs
suggest an additional role in determining the relative level of
different muscles, particularly when the pattern is asymmetric.
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INTRODUCTION |
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In their natural
environment, animals are continually challenged by the nature of
the terrain over which they are walking and must be capable of
modifying their base rhythm and level of electromyographic (EMG)
activity to meet those challenges. One of those challenges is the need
to modify motor output to maintain equilibrium and speed when walking
on an inclined surface, either when walking up or downhill or when
walking across the pitch of a slope (referred to here as crosshill). In
the former case, the animal must produce symmetrical changes in
activity in both limbs of the same girdle, while in the latter case,
the changes are asymmetric. While there may be some spinal adaptive
mechanisms that could contribute to the modifications that are required
in these situations, the few data that are available show that cats with total transections of the spinal cord have little capacity to
modify their posture either during locomotion (Rossignol et al.
1999) or in response to perturbations during quiet standing (Pratt et al. 1994
). It seems likely, therefore, that
the modifications required in this circumstance are produced in
response to descending commands from supraspinal structures.
Among the structures likely to be involved in this type of
adaptation, most of the evidence points to the vestibulo- and
reticulospinal tracts (VST and RST, respectively) as playing an
important role. As detailed in the Introduction to the companion paper
(Matsuyama and Drew 2000), lesions that compromise these
brain stem pathways lead to a loss of muscle tonus, and preliminary
data from Brustein et al. (Brustein et al. 1994
;
Rossignol et al. 1999
) show that cats with lesions
restricted to the ventral spinal cord (in which the VST and RST are
found) have problems in adapting their gait to walk on an inclined
plane. In addition, the experiments of Orlovsky have shown that neurons
in both the vestibulospinal (Orlovsky 1972a
) and the
reticulospinal pathways (Orlovsky 1970
) increase their
discharge when the cat increases its motor output, while microstimulation of both pathways in the cat produces modification of
limb muscle activity during locomotion in a number of preparations (Degtyarenko et al. 1993
; Drew 1991
;
Drew and Rossignol 1984
; Gossard et al.
1996
; Leblond and Gossard 1997
; Orlovsky
1972b
; Perreault et al. 1994
; Russel and
Zajac 1979
). As Orlovsky (1972b)
has suggested,
such characteristics are compatible with a role for these pathways in
regulating the changes in the level of the EMG output that are
necessary to walk on an inclined surface (Carlson-Kuhta et al.
1998
; Smith et al. 1998
).
Certainly, limb muscle tonus and the discharge activity of both
vestibulo- and reticulospinal neurons (VSNs and RSNs, respectively) is
modulated by changes in the orientation of the head in the both the
pitch tilt (rotation around the transverse or coronal axis) and roll
tilt (rotation around the longitudinal axis) conditions, in
anesthetized, decerebrate and intact cats (Iwamoto et al.
1996; Manzoni et al. 1983
; Marchand et
al. 1987
; Peterson and Fukushima 1982
;
Schor and Miller 1981
, 1982
; Wilson et al.
1986
; see Wilson and Peterson 1981
for reference
to earlier studies) as well as in the lamprey (Deliagina et al.
1992a
,b
; Orlovsky et al. 1992
). However, there
is very little information as to how this afferent information may
modify the descending signal from these two structures during
locomotion on an inclined plane. Among the few pieces of data that are
available, there is a single report to show that the discharge activity
of vestibular neurons in the guinea pig is modified when body
orientation is changed during locomotion (Marlinsky
1992
). However, the work of Orlovsky and Pavlova
(1972)
suggests that the influence of vestibular afferents on
neurons in the lateral vestibular nucleus (LVN) in the decerebrate cat is diminished or abolished during locomotion, so that it is not certain
if the discharge of VSNs in the intact cat would show modification of
their frequency or pattern of discharge. With respect to the activity
of the reticulospinal neurons, we know of no information as to the
changes in activity that are to be observed in RSNs during locomotion
on an inclined surface.
Given this lack of information on the characteristics of these two
populations of neurons when the level of EMG activity has to be
modified, it is very difficult to determine what role these brain stem
structures play in adapting the level of EMG activity to the changing
environment and whether both structures play an equal role in
modulating muscle tone during locomotion or if one exerts a more potent
effect than the other. We therefore set out to record the discharge
characteristics of both populations of neurons from the same cats
during the same behavior. The results show that both groups probably
participate in the modification of muscle tonus but that each probably
has a distinct role to play in the adaptive process. A preliminary
report of this work has been published in abstract form
(Matsuyama and Drew 1996).
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METHODS |
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All details of animal training, surgical methods and the general
protocol are given in the companion paper (Matsuyama and Drew
2000). For the studies detailed in this report, the discharge activity of identified neurons was recorded during treadmill locomotion on a level surface and then with the treadmill pitch tilted at +20
(nose up), +10, and
10° (nose down). The treadmill was then rolled
to the left (
20 and
10°, ear down ipsilateral to the recording
site) and subsequently to the right (+20 and +10°). Sections of
locomotion on the level treadmill were interspersed with the locomotion
on an inclined plane. Data were recorded only when the required level
of inclination had been obtained; no recordings were made during the
dynamic changes in treadmill orientation. All data were recorded on a
14-channel Honeywell recorder.
Averaged displays of the cell discharge and unit activity for each
condition, together with raster displays of the data, were made as
described in the companion paper (Matsuyama and Drew
2000). To determine if cell discharge was significantly
different in any one of the test conditions from the discharge during
level treadmill walking, the two traces were normalized and averaged and then superimposed. Significant differences were defined as those in
which the average activity of the unit or EMG during the selected
condition deviated from the confidence limits of the standard error of
the mean of the control (level treadmill locomotion) activity at the
P < 0.01 level for 25 consecutive bins (see e.g., Figs. 1
and 3) (see also Drew 1993
).
Linear regressions were used to quantify the relationships between the
peak cell discharge (measured from the averages) and treadmill
inclination with tilt and roll being treated as separate conditions.
Because of the small number of points in these regressions (6), the
difficulty in accurately determining peak discharge frequency from the
averaged displays and the fact that only a single value was being used
to describe the activity during the whole sequence of locomotion in any
one condition, we set the significance level to 0.1 level for this
analysis. Although this level of significance is relatively low, we
preferred to increase the probability of Type I errors of inclusion
than of having too many Type II errors of exclusion. We would also
emphasize that the results obtained with this level of significance
agreed well with our subjective impression.
The phase of cell discharge was also measured from the averaged displays and was always calculated with respect to the onset of activity in the anterior head of the ipsilateral sartorius (iSrt). For the analysis of the phase of EMG activity, the phases were measured from the individual bursts of activity. Individual values were not used for any of the unit calculations because of the difficulty of determining the exact moment of onset or offset of activity in many of the neurons particularly for the VSNs.
In all of the text and figures that follow, ipsilateral and left are synonymous and refer to the side of the brain stem from which all of the unit recordings were made.
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RESULTS |
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The present paper reports on the changes in EMG activity and neuronal discharge of RSNs and VSNs during locomotion on a treadmill inclined at different orientations with respect to the horizontal plane. The neuronal database is the same as that used in the companion paper. As interpretation of neuronal discharge depends on an understanding of the changes in the locomotor pattern induced by the slopes, we will first, briefly, present the results of our analyses of the changes in EMG activity in these different conditions.
Electromyographic activity
As illustrated in Figs. 1 and 2, and summarized in Table 1, changing the orientation of the treadmill resulted in characteristic and reproducible changes in both the amplitude and pattern of EMG activity in most muscles in both cats.
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Such changes were most evident in the amplitude of the EMG activity, in
both flexors and extensors, when the pitch of the treadmill was
changed. As can be seen from inspection of Fig. 1A, there
was a significant increase in the amplitude of the EMG activity of all
the muscles during positive pitch (+20°). In addition, in the
hindlimb flexor, semitendinosus (St), there was invariably an extra
burst of activity that occurred during the stance phase of locomotion.
During negative pitch (10°, Fig. 1B), there was a small,
but significant decrease in the amplitude of the hindlimb flexor and
extensors. In the forelimb flexor, cleidobrachialis (ClB), there was a
significant decrease in the burst of activity that occurred during
swing, together with an additional period of activity during stance. A
similar additional period of activity was also occasionally observed in
the Srt (see e.g., Fig. 3B). All of these changes were
symmetrical on the ipsilateral and contralateral sides. As can be seen
from the data shown in Fig. 2A, there was a significant
relationship, over the studied range, between the slope of the
treadmill and the level of the EMG activity recorded from the flexors
and extensors of the fore- and hindlimbs. In all muscles, from both
cats, these relationships were positive and the significance level of
the regression was <0.05 in 16/20 cases (10 muscles from 2 cats: Table
1).
The changes in amplitude were smaller when the treadmill was rolled to
the left (ipsilateral) or to the right (20 and +20°, Figs. 1,
C and D, respectively, and 2B). In
general, the extensor muscles showed slightly increased amplitudes when
the treadmill was rolled in either direction, although these were
generally larger when the limb was loaded (i.e., muscles on the left,
ipsilateral side, when the treadmill was rolled to the left). In
contrast, the amplitude of the flexor muscles was slightly increased
when the treadmill was rolled away from the limb and decreased in the opposite direction, although the changes in flexor EMG activity were
also relatively small (see also Table 1). In addition, the second burst
of activity in the St, occurring at, or just before foot contact, was
frequently enhanced when the treadmill was rolled in either direction
(see Fig. 1, C and D). For some muscles, the relationship between EMG amplitude and treadmill roll was significantly linear, although the slopes were much smaller than those observed during tilts (Fig. 2B). This can be appreciated from the
fact that P < 0.05 in only 8/20 cases for the roll tilt
(Table 1).
As well as the changes in amplitude, several of the muscles also showed changes in their phase of activity relative to the onset of the iSrt. For example, as can be seen in both Figs. 1 and 2, the EMG activity in the ipsilateral vastus lateralis (iVL) was relatively phase delayed during positive pitch (Fig. 1A), and phase advanced during negative pitch (Fig. 1B, see also Table 1). There were similar changes in the forelimb muscles that were most evident in cat RS12 [see iTriL(12) in Fig. 2C]. Significant linear relationships (P < 0.05) between phase and treadmill orientation were observed in 13/18 muscles in this condition. As for the changes in amplitude, the changes in relative phase were smaller when the treadmill was roll tilted than when the pitch was modified (Fig. 2D, Table 1). During the roll tilt, probabilities <0.05 were seen in only 4/18 muscles, including for the initial burst of activity in the iSt in both cats.
Vestibulospinal neurons
TYPE A VSNS.
When the cat walked on the inclined treadmill, many type A VSNs showed
small changes in the level and the relative phase of discharge activity
without, however, exhibiting any major change in their overall pattern
of activity. Figure 3 shows the effects of walking on an inclined plane on the discharge activity of a type A
VSN from cat RS13 that discharged with a double-burst
pattern similar to those documented in the companion paper
(Matsuyama and Drew 2000). As with most cells in
cat RS13, the discharge frequency of the cell was
markedly reduced at the end of the period of the iVL activity and
throughout the period of activity of the iSt. This relationship was
maintained at all changes of orientation of the treadmill in the
vertical plane (pitch: Fig. 3D), as well as in the
horizontal plane (not illustrated).
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TYPE B AND C VSNS. As for the type A VSNs, the basic pattern of activity in the type B VSNs also remained constant during the changes in treadmill orientation. Changes in the discharge frequency were generally modest although, as for the type A cells, when the treadmill was pitched at 20° many of the type B VSNs (7/13, 54%, see Table 2) showed a significant, although modest (see Fig. 5B), increase in the prominent peak of activity (classified as peak 2) that was observed in some of these neurons. None of the type B VSNs showed significant changes in the discharge frequency of peak 2 during negative pitch. The changes during roll tilts were variable, with 6/10 neurons increasing their discharge frequency when the treadmill was rolled left and 5/10 increasing their discharge frequency when the treadmill was rolled right.
In addition to the change in the prominent peak, there were also some minor, but relatively consistent, modifications of discharge frequency at other times in the step cycle, especially in cat RS13. During positive pitch, all type B VSNs showed a significant decrease in their discharge frequency during the time that the activity in the iSrt was increased, and another significant decrease in discharge activity, later in the cycle, corresponding to the time that the contralateral sartorius (coSrt) was active (see Fig. 6A). When the treadmill was pitched at
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CHANGES IN THE PHASE OF ACTIVITY. Overall, significant changes in phase in both type A and type B VSNs were observed mostly during pitch tilt of the treadmill and then mostly with respect to trough 1 and peak 2, although there were also changes in peak 1 in some type A VSNs. As in the examples illustrated in Figs. 3 and 4, most cells that showed a significant relationship between phase and treadmill orientation showed relative phase delays during positive pitch and relative advances during negative pitch. Altogether, 15/33 type A and B VSNs showed a significantly linear change in the phase of peak 2 during changes in treadmill pitch, whereas 11/20 type A VSNs showed a change in peak 1. As can be appreciated, the slopes for most of these cells were very similar in each condition, with the mean positive slope for each of the four measured points ranging from changes in phase of 0.003 to 0.004 phase/° in the tilt condition (equivalent to a change in phase of 0.09-0.12 over the 30° range of pitch examined) and from 0.002 to 0.003 in the roll condition (0.06-0.09 for the same 30°change). These values for the slope are very similar to those obtained for the EMGs (see Table 1).
Reticulospinal neurons
A total of 63 RSNs were recorded from the medullary reticular
formation (MRF) of the two cats during locomotion during at least three
conditions. As described in the companion publication (Matsuyama
and Drew 2000; see also Drew et al. 1986
, 1996
;
Perreault et al. 1993
), this population of RSNs had
diverse patterns of discharge during locomotion on the level treadmill
belt and included examples of what we have previously described as
EMG-related, locomotor-related, and unrelated neurons.
Many of the EMG-related RSNs changed their discharge frequency when the cat walked on the inclined treadmill. One example of such a RSN is shown in Fig. 7. This cell discharged in one discrete burst of activity during treadmill locomotion, coincident with the burst of activity in the iVL. During locomotion on the pitched treadmill, this RSN showed a clear and significant increase in the overall amplitude of its discharge activity when the treadmill was tilted up (Fig. 7A) and an equally clear and significant decrease in its activity when the treadmill was tilted down (Fig. 7B); these changes in discharge frequency paralleled the changes in the level of iVL activity. When the treadmill was rolled to the left (Fig. 7C), there was a slight decrease in the maximum discharge frequency of the cell together with a clear phase shift, and when it was rolled to the right (Fig. 7D) there was no change in the discharge pattern. Linear regressions of the averaged peak discharge against pitch (Fig. 7E) showed a positive, but nonsignificant relationship, between the cell discharge and the treadmill inclination. On the other hand, there was a significant relationship between the integrated unit discharge and the pitch of the treadmill. There was no relationship between either the peak or the integrated discharge and the degree of roll (Fig. 7F).
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Several RSNs discharged twice in each step cycle and were modulated in a similar manner to the example illustrated in Fig. 8. In this example, and in several of the other neurons of this type (see Fig. 9), there was sometimes a marked change in the pattern of activity of the cell. During level treadmill locomotion, this neuron was characterized by a large burst of activity that overlapped, and covaried, with the period of activity in the coSrt (peak 2) and another, indistinct period of activity that occurred during the time that the iSrt was active (peak 1). During positive pitch, both peaks of activity increased (Fig. 8A), and during negative pitch, both peaks decreased to the extent that the discharge was almost absent (Fig. 8B). When the treadmill was rolled to the left (Fig. 8C), there was a slight increase in peak 1 and a clear decrease in the amplitude of peak 2. When the treadmill was rolled to the right (Fig. 8D), peak 1 decreased in amplitude and peak 2 showed an increase. As shown by the linear regressions of Fig. 8E, there were significant, linear relationships between the discharge frequency of each peak and the degree of pitch of the treadmill. Peak 1 bore no relationship to treadmill roll, whereas peak 2 did (Fig. 8F). Qualitatively similar types of relationships were seen for other cells that showed two periods of activity.
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Changes in the pattern of activity with changes in the orientation of
the treadmill were observed in many of the RSNs. Figure 9 illustrates four examples showing
changes of pattern at different orientations of the treadmill both with
respect to the pattern observed during level locomotion and with
respect to the pattern observed at different orientations. During
locomotion on the level treadmill, the RSN illustrated in Fig.
9A, for example, increased its discharge frequency in phase
with the period of activity with the iSrt and stayed active until the
end of the period of activity of the coSrt. During uphill locomotion
(+20), there was a clear differentiation of this increased period of
activity into two separate bursts; one of these bursts occurred during
the time of ipsilateral swing of the hindlimb and the other during
contralateral hindlimb swing. When the treadmill was rolled left
(20L), there was a marked increase in the duration of the first burst
of activity and a slight decrease in the second. During locomotion with
the treadmill rolled right (+20R), there was little change from the control situation. Figure 9B illustrates a similar cell that
also discharged with two periods of increased activity during level treadmill locomotion. As for the previous example, this RSN showed increased activity in the two periods of activity during uphill locomotion and a clear asymmetry in the two bursts when the treadmill was rolled left. In this example, however, there was an increase in the
second period of activity when the treadmill was rolled right, as for
the cell in Fig. 8. Similar changes in the discharge frequency and
pattern are illustrated in Fig. 9C for a neuron whose
activity, during uphill locomotion, covaried with the period of
activity in the iClB and coClB. In this neuron, however, only one
period of increased activity was observed during level locomotion; the
second period of increased activity only becoming evident when the
orientation of the treadmill was modified.
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Changes in pattern were also sometimes observed in neurons whose period of discharge activity covaried with the period of activity of extensor muscles. During locomotion on the level treadmill, the discharge frequency of the RSN illustrated in Fig. 9D covaried with the activity of the ipsilateral lateral head of the triceps brachii (iTriL). During uphill locomotion, this period of activity was increased, and there was a distinct peak of activity that occurred at about the same time as the period of activity in the coClB. During locomotion with the treadmill rolled to the left, there was a decrease in the period of activity with the minima occurring at about the same time as the coClB. In addition, there was the appearance of a completely new period of activity that covaried with the period of activity in the iClB. During rolls to the right, the cell behaved similarly to the condition when the cat walked uphill.
Overall, the majority of single- (9/13) and double-burst (11/12) EMG-related RSNs showed a significant increase in discharge frequency when the cat walked on the treadmill pitch-tilted at 20° (Table 3). During downhill locomotion, most of the single-burst RSNs (9/13), but fewer of the double-burst neurons (3/8), showed a significant decrease in activity. Only one cell of each type showed an increase in activity during downhill locomotion. As for the VSNs, the changes in discharge frequency during treadmill roll were more variable, especially during rolls to the left. Thus of the single-burst RSNs, 3/9 increased their discharge during rolls to the left and 4/9 decreased their discharge; in contrast, during treadmill roll to the right, 5/9 were increased but none decreased. For the double-burst neurons, during rolls to the left, the initial peak was increased in 7/12 RSNs while the second peak was decreased in 5/10. When the treadmill was rolled to the right, there were increases in 4/9 RSNs of both peak 1 and peak 2.
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Most of the locomotor-related and unrelated neurons that were recorded in these situations differed from the EMG-related RSNs in that their discharge frequency was mostly unchanged by any of the treadmill manipulations (Table 3). Interestingly, the antidromically activated neurons that were recorded but that did not discharge at all during level locomotion (silent) also did not discharge when the treadmill was pitched at 20°. Although such silent neurons were not recorded during roll tilts, these neurons seemed to be unresponsive both during passive manipulation and during overt motor activity.
Linear relationships between cell discharge and treadmill orientation
The relationship between peak discharge frequency and treadmill
inclination for all of the VSNs and RSNs that showed statistically significant relationships is illustrated in Fig.
10. Comparing first the level of
discharge frequency of VSNs (Fig. 10A) and RSNs (Fig.
10D) during treadmill pitch, it can be seen that the peak discharge of the VSNs was substantially higher than that in the population of RSNs throughout the range of inclinations that were studied. However, analysis of the incremental change in peak discharge rate suggests that the two populations are similar. Overall for those
VSNs that showed a positive increase in discharge rate when the
treadmill was tilted up, there was an average increase of 0.91 Hz/°
or an increase of 27.3 Hz from 10 to +20°. For the similar
population of RSNs, the average increase in discharge rate was 1.08 Hz/°, which corresponds to an overall increase in discharge rate of
32.4 Hz from
10 to +20°. Thus the two populations of cells operate
in different ranges but have similar responses to changes in treadmill
pitch.
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Nevertheless inspection of the percentage change in discharge frequency shows that the two populations are far from identical. As can be seen from Fig. 10, B and E, the VSNs show a relatively small percentage change in discharge frequency while the RSNs show a relatively large percentage change. Indeed comparison of the mean positive slopes for the two populations of cells showed that the RSNs showed approximately twice the rate of increase (1.59%/°) as did the VSNs (0.74%/°). In other words, compared with level locomotion the average RSN shows a relative increase in discharge frequency of 31.8% when the cat walks up a slope at 20° while the average VSN increases by only 14.8%.
The changes in discharge frequency during roll were generally smaller than those observed during changes in treadmill pitch, especially for the RSNs. Using a similar method of calculation for the roll as was used in the preceding text, the average VSN changes its discharge frequency by 19 Hz for a 30° change in treadmill orientation (0.64 Hz/°), while the average RSN changes by a only slightly smaller value of 13.5 Hz (0.45 Hz/°). In terms of percentage changes (see Fig. 10, C and E), the VSNs showed a change of 0.63%/° (or a change of 18.9% over 30°), while the RSNs changed by 0.93%/° (or a change of 27.9% over the same range).
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DISCUSSION |
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The results presented in this paper detail the changes in neuronal discharge pattern and rate of spinal projecting neurons in the two structures, the LVN and the pontomedullary reticular formation (PMRF), that are most likely implicated in adjusting the level of EMG activity when a cat changes its overall posture during locomotion. The data show that neurons in both structures show increases in the rate of discharge when an animal walks up an inclined plane and corresponding decreases when it walks down; discharge rate is also, in general, increased by contralateral side down roll tilts, although the responses are more variable. However, the pattern of discharge and the modifications of pattern were quite different in the two structures suggesting that each has a distinct role to play in these adaptive processes.
EMG correlates of walking on an inclined plane
As recently detailed by Smith's group (Carlson-Kuhta et
al. 1998; Smith and Carlson-Kuhta 1995
;
Smith et al. 1998
), characteristic changes were observed
in the level of EMG activity in both the flexor and extensor muscles of
the hindlimbs during uphill and downhill walking. In agreement with the
data presented by Smith, we also observed increased activity in both
the extensor and flexor muscles that we recorded during uphill
locomotion, together with a change in the pattern of the St muscle to a
double-burst pattern of activity. As discussed in Carlson-Kuhta
et al. (1998)
, these modifications in extensor EMG pattern are
undoubtedly adaptations to the change in body posture. The increases in
flexor muscle activity can be explained by the more crouched posture of
the cat during uphill walking and by the increased flexion of all joints of the cat during this activity (Carlson-Kuhta et al.
1998
). Our results during downhill walking for the hindlimb
muscles are also in agreement with the data presented by Smith
et al. (1998)
in that activity in the knee extensor, VL, was
decreased, while activity in the St was slightly increased. In
addition, as documented by Smith et al. (1998)
for the
hip flexor, iliopsoas, we also observed an additional period of
activity in stance in a hip flexor, the anterior head of the sartorius,
during downhill walking (see Fig. 3B), although in our
experiments this additional burst was facultative. This difference may
be explained by the relatively small angle of downhill tilt used in our
studies (
10°). For example, inspection of Fig. 10 in Smith
et al. (1998)
suggests that the double burst in iliopsoas was
equally poorly developed at a downward pitch of 10° in their
experiments. It is equally possible that the slight difference in speed
may have made a difference; average cycle durations in the studies of
Smith et al. (1998)
were in the range of 500-800 ms,
while in our study they were in the order of 1,000 ms.
In our studies we also recorded the activity of a single flexor (ClB)
and extensor (TriL) muscle of the forelimb. The EMGs of these muscles
showed similar changes to those documented in the hindlimb muscles
during both uphill and downhill walking: i.e., extensor and flexor
muscle EMG activity was increased during uphill walking and extensor
muscle activity was decreased during downhill walking. Moreover, the
cleidobrachialis showed a pronounced burst of activity in stance during
the downhill walking, even at 10° (see Fig. 1C), similar
to that observed in the hip flexors. This double burst may reflect that
these forelimb muscles are also absorbing power during the stance phase
in this condition, as suggested by Smith et al. (1998)
for the iliopsoas muscle. Indeed, comparison of the amplitude of the
activity during stance in the ClB and the Srt muscles in our studies
suggests that these forelimb muscles may have a more important role
than the hindlimbs in braking the animal during downhill walking.
The changes in locomotion also induced relative changes in the phase of
onset of the activity of the other muscles, and particularly the fore-
and hindlimb extensors, TriL and VL (see Table 1), that were related to
the changes in the duration of the flexor muscle period of activity
that can be seen in Fig. 1 and that were detailed in the papers by
Carlson-Kuhta et al. (1998) and Smith et al.
(1998)
. These changes in the relative phase of the muscles were
parallelled by changes in the relative phase of many of the cells (see
e.g., Fig. 3).
As might be expected, during locomotion with the treadmill rolled to one side or the other, we observed asymmetric changes in the activity of muscles of a single girdle, although in general the modifications in EMG activity were of smaller magnitude and more variable than those observed during uphill and downhill walking. The more robust of the modifications was an increase in the level of extensor muscle activity on the side to which the treadmill was rolled. This increase in activity would presumably help to offset the expected increase in loading on these limbs produced by the change in body orientation. The fact that increases in EMG activity were sometimes seen on the opposite side to which the treadmill was rolled (see e.g., Fig. 3) suggests, however, that symmetrical changes in limb musculature EMG activity may sometimes be required even during behavioral changes that favor asymmetry. The flexor muscles showed the inverse pattern, being significantly greater when the treadmill was rolled away from the respective limb. The increased activity in the flexor muscles may, at least in part, be explained by the fact that swing duration was decreased on the side to which the treadmill was rolled and increased on the opposite side (see Fig. 1). This increase in flexor EMG activity in swing is presumably required to lift the leg higher to avoid hitting the treadmill belt, in a similar manner to when the cat walks uphill. Changes in the phase of the activity were mostly nonsignificant and small as were the changes in the phase of the neuronal activity.
Overall, these EMG data show that the adaptations to changes in treadmill pitch were more robust than those observed when the treadmill orientation was changed in the roll plane at least with respect to the level and phase of the activity in the major flexor and extensor muscles.
Discharge activity of VSNs
PITCH TILT.
During uphill walking, most VSNs showed an increase in their discharge
frequency as might be expected if, as discussed in the companion paper
(Matsuyama and Drew 2000), they contribute to the
production of the extensor muscle tonus. Indeed in general, there were
parallel increases in the discharge frequency of both peaks 1 and 2 of
the type A, double-peak VSNs and of the amplitude of the extensor
muscle EMG activity (see Figs. 3 and 4) when the treadmill was pitch
tilted at 20°. There were also parallel changes in the phase of
activity in the type A VSN discharge patterns and the extensor muscle
EMGs with respect to the reference muscle, the iSrt. For example,
inspection of Fig. 4 reveals the clear phase delay of both the iVL and
peak 2, with respect to iSrt, when the cat walked on the treadmill
pitch tilted at 20°. This relationship is equally evident from the
raster display of Fig. 3D where there was a constant
relationship between the end of the period of activity in peak 2 and
the offset of activity in the iVL, despite the changes in the phase of
activity of both cell and EMG with respect to the onset of the iSrt
(Fig. 3, A and B). Overall these data
support the view that increased activity in the discharge frequency of
these type A VSNs contributes to the increased level of activity
observed in the extensor EMGs during locomotion on an inclined plane.
Although only a few type C VSNs were recorded, the results obtained
from those cells whose discharge covaried with the extensor muscles
suggests a similar relationship with EMG amplitude. However, because of
the small number and the heterogeneity of the population, this small
group of VSNs will not be discussed further.
ROLL TILT. During side down and side up roll tilts, the changes in the peak discharge frequency and the phase of the cell discharge with changes in treadmill orientation were both more modest and more variable as were both the level and phase of the EMG activity. The interpretation of these responses is further complicated by the fact that the two peaks of activity in the type A VSNs were sometimes modulated differentially by the left and right rolls. Considering first peak 2, which we suggest contributes to the activity in the iVL, the results showed that in some cells, the discharge frequency in this peak was increased in one direction and in others in the opposite (Table 2). Although it is possible that this may be related to the variability in the level of EMG activity in the iVL during roll tilts, direct comparison of the change in amplitude of the iVL in those VSNs in which peak 2 discharge frequency was modified revealed no direct comparison between the two measures (not illustrated). Alternatively, the increased discharge frequency may be indicative of a more integrative signal, reflecting the relative level of activity in both extensors and flexors, possible in both hindlimbs, as suggested for the activity of the type B neurons in the preceding text.
Changes in the discharge frequency of peak 1 differed from those in peak 2 in that there was more frequently decreased activity during rolls to the ipsilateral side and increased activity during rolls to the right. Again these results are compatible with our suggestion that the activity of this peak may contribute to the level of EMG activity in the contralateral limbs and particularly the contralateral forelimb, which shows a similar change in EMG activity (Table 1). Type B VSNs also tended to show increased discharge frequencies during both left and right roll; this again would be in agreement with our previous suggestion (Matsuyama and Drew 2000SOURCE OF THE MODULATION.
The source of the signal that is responsible for the modulation of
these VSNs is difficult to determine in these intact animals, which are
unrestrained and in which the EMG activity of the muscles is
dynamically and rhythmically modulated. One obvious source of input to
these VSNs is input from the labyrinths which project both mono- and
polysynaptically to VSNs in Deiters neurons (Ito et al.
1969; Peterson 1970
; Shinoda et al.
1994
; Walberg et al. 1958
; Wilson et al.
1967
). Indeed there is a wealth of information, mainly from
experiments in decerebrate cats, showing that both the modifications in
the level of limb muscle EMG and in the activity of VSNs are compatible
with the type of roll and pitch tilts used in this study. For example,
roll tilt of a decerebrate animal generally leads to an increase in the
level of EMG activity of the ipsilateral (ear down) extensors and a
decrease of activity in the contralateral ones (Ezure and Wilson
1984
; Kasper et al. 1988a
,b
;
Schor and Miller 1981
; Wilson et al.
1986
). Similarly recordings from neurons (including VSNs) in
Deiters' nucleus in the decerebrate cat show that static or
low-frequency sinusoidal roll tilt of an animal may lead to a variety
of different patterns of activity in VSNs similar to those found in the
current study (Boyle and Pompeiano 1979
; Iwamoto
et al. 1996
; Marchand et al. 1987
; Schor
and Miller 1982
). Thus adopting the nomenclature of Duensing and Schaefer (1959
; detailed in Peterson
1970
),
VSNs increase their activity during rolls to the
ipsilateral side and decrease their activity during roll to the
contralateral side,
VSNs show the reciprocal pattern of activity,
and
VSNs increase their activity in both conditions. As
Peterson (1970)
has shown, all three types of VSN are to
be found throughout Deiters' nucleus. However, while Peterson
(1970)
suggested that cells projecting to lumbar regions of the
spinal cord would discharge to roll tilts in either or both directions
(
,
, and
types), as in this study, Marchand et al.
(1987)
have suggested that most lumbar-projecting VSNs are of
the
type. Thus it is not certain whether the modifications of
discharge activity in VSNs with roll tilt that are detailed in this
study are compatible, or not, with the findings in the more tightly
controlled conditions in the decerebrate cat. Studies in which the
responses of VSNs in Deiters' nucleus to pitch tilt have been examined
are less frequent, but the available information suggests that the
sensitivity of these neurons to pitch tilt is normally less than that
to roll tilt (Iwamoto et al. 1996
; Kasper et al.
1988a
). This is opposite to the sensitivity that we observed during locomotion in these intact cats.
Discharge activity of RSNs
As discussed in the companion paper (Matsuyama and Drew
2000), the discharge patterns of the RSNs during level walking
were quite different from those observed in the VSNs. In particular, those RSNs that we defined as EMG-related discharged in discrete phasic
bursts of activity that were temporally related to the appearance and
duration of the bursts of activity in selected fore- or hindlimb,
flexor or extensor muscles. These cells, particularly those that
discharged with a single burst of activity and whose activity covaried
with the EMG of extensor muscles, showed relatively simple
modifications in activity that were directly related to the level of
EMG (see e.g., Fig. 7). As might be expected on the basis of the close
link between neuronal activity and EMG activity in these neurons, there
was a strongly significant increase in discharge activity during uphill
walking (Table 3). Thus these neurons could well contribute to the
increase in activity in the different extensor muscles in this
condition. The significant increases in discharge activity in a large
proportion of the double-burst neurons during uphill walking is also
compatible with our suggestion that these neurons contribute to
controlling the level of activity in flexor muscles (see
Matsuyama and Drew 2000
), which are likewise increased
during uphill walking.
Given the good correlation between the frequency of neuronal discharge
and the level of EMG activity during uphill locomotion, supporting the
view of an increased excitatory drive to the motoneurons in this
condition, one would expect a similar close correlation between
neuronal discharge frequency and the level of EMG activity during
crosshill walking. However, as for the VSNs, the changes in both
discharge frequency and activity pattern were more variable during
crosshill locomotion even for the single-burst neurons whose activity
was best correlated to the activity of ipsilateral extensor muscles.
Indeed, all seven of the RSNs that discharged once in each cycle and
whose discharge activity covaried with the period of activity of either
the iVL or the iTriL showed greater activity during rolls to the right
(contralateral side) than to the left (e.g., Fig. 7); in 4/7 cases, the
slopes of this activity showed a significant linear relationship with
treadmill orientation. This is the opposite of what one would expect
given, that on average, activity in the ipsilateral extensors was
increased during left rolls (Figs. 1 and 2 and Table 1). A similar
discrepancy between the result expected on the basis of the temporal
correlations and the activity during uphill walking was found for many
of the double-burst neurons. We have suggested that the discharge
frequency of most double-burst neurons covaries with the period of
activity of the ipsilateral and contralateral flexor muscles (Table 1 in Matsuyama and Drew 2000). Given the changes in EMG
activity that we documented during roll tilt (Figs. 1 and 2 and Table
1), one would expect the initial burst of activity (active in phase
with the ipsilateral flexors) to increase during right rolls and the reciprocal pattern of activity to occur with the second burst of
activity. In fact, as for the RSNs related to extensor muscle activity,
such was not the case. Rather, in most double-burst neurons the second
burst showed increased activity during right roll with significant
linear relationships between cell discharge activity and treadmill
orientation in 6/9 RSNs. Activity in the first peak was more variable,
showing increased activity during left roll (4/9 RSNs, see Fig. 8) or
increased activity during right roll, 4/9 cases. Thus in the majority
of cases, the pattern of activity, as for the single-burst neurons, was
the inverse of that expected on the basis of the changes in the level
of EMG activity.
The reasons for this apparent discrepancy are not clear. It is unlikely
that these RSNs are acting through disinhibition (see e.g.,
Manzoni et al. 1983) as most of the RSNs showed positive relationships between cell discharge and EMG activity during uphill locomotion. Moreover, both the present (our unpublished observations) and our previous studies (Drew and Rossignol 1990a
,b
)
have shown that microstimulation in these regions of the MRF, in the
intact cat at rest, evoked predominantly excitatory effects in the EMGs recorded from both flexor and extensor limb muscles. Indeed these previous studies showed that in the intact, awake cat, microstimulation in the same regions as examined in the present study evoked
facilitatory responses from 92% of stimulated loci (Drew and
Rossignol 1990b
).
Why then is there not a simple relationship between discharge frequency
and the level of EMG activity during roll tilt as there seems to be
during pitch tilt? One possible explanation comes from a consideration
of some of the characteristics of the functional organization of the
reticulospinal system, particularly as it pertains to its distributed
output and the extent to which it is affected by afferent input. For
example, it has been well documented that individual axons of the
reticulospinal system may branch to innervate different levels of the
spinal cord (Peterson et al. 1975) and may innervate
multiple segments at either cervical or lumbar levels, both
ipsilaterally and contralaterally (Matsuyama et al. 1988
, 1997
,
1999
). This is consistent with the results from our previous
studies (Drew 1991
; Drew and Rossignol 1984
, 1990a
,b
) showing that microstimulation of small regions of the MRF may influence the activity of several muscles in multiple limbs.
Therefore it is probable that even those RSNs whose discharge covaries
with the period of activity of only one or two EMGs may well influence
other muscles in other limbs. Thus it is possible that the discharge
pattern observed during level treadmill locomotion may reflect only the
dominant relationship of a given RSN with the limb musculature and that
other parts of this relationship become evident only when the pattern
of motor activity is modified. This is particularly likely when the
treadmill is rolled and asymmetric changes in EMG occur in the
ipsilateral and contralateral limbs. Inspection of Fig. 9 supports this
suggestion given that a number of RSNs show clear changes in the
pattern of activity during changes in treadmill orientation. Thus the
fact that the changes in discharge frequency during roll tilt are
incompatible with the expectations based on discharge activity during
level treadmill locomotion may simply reflect that the integrative
effect on the ensemble of muscles affected by a given RSN is also
different from that required during level or uphill treadmill
locomotion when the activity is bilaterally symmetric.
It is also possible that the discharge patterns observed in these RSNs
are heavily influenced by the afferent input that they receive
particularly from neck afferents. Both our own studies (Drew et
al. 1986, 1996
) and those of others (e.g., Siegel and Tomaszewski 1983
) have shown that a majority of RSNs, including those with modulated discharge patterns during locomotion, receive afferent input from the entire body and are heavily influenced by
movements of the head that will activate both vestibular and neck
afferents. If the RSNs are equally influenced by these inputs during
locomotion, then it is possible that changes in treadmill orientation
might strongly influence neuronal discharge activity. In this respect,
it is interesting that in both the decerebrate cat (Bolton et
al. 1992
; Manzoni et al. 1983
) and in the
lamprey (Deliagana et al. 1992a
,b
), RSNs are intensely
activated by roll tilt to the contralateral side. In addition,
complementary input from neck afferents, which will be activated as the
cat rotates the neck on the body to bring the head close to the normal
position, will also tend to activate RSNs in the manner found in this
study (Srivastava et al. 1984
). Thus roll tilting the
body to the right would result in a leftward rotation of the neck (chin
to the right), relative to the body, which would tend to augment
activity in RSNs in the left PMRF (side-down neurons in the study of
Srivastava et al. 1984
). In fact, it is possible that
the neck input might provide the overriding stimulus responsible for
the modification of the modulation of these neurons as our own
unpublished observations suggest that the rhythmical modulation in RSNs
are, indeed, modified when the cat makes voluntary movements of its head.
These results and discussion also imply that the putative correlations
that are made between unit activity and EMG activity are highly
dependent on the postural and locomotor behavior of the animal. The
apparent fixed covariation between unit and EMG observed during level
treadmill walking changes during locomotion on an inclined plane
according to the orientation and, therefore, might also be different
during other types of behavior. The actual pattern of activity that is
observed in these RSNs may, therefore, reflect faithfully neither the
input nor the output but rather a combination of the two. As we have
previously argued (Drew 1991; Drew et al.
1986
), the modified discharge would be reorganized into an
appropriate functional pattern by the state and level of activity of
interneuronal pathways in the spinal cord.
It is also important to note that none of the RSNs (or VSNs) that we
recorded showed major changes in pattern of the type that we observed
in some of the EMGs. As discussed in the preceding sections, St changed
from a single clear burst of activity preceding swing during level
locomotion to a double burst of activity during uphill locomotion with
the additional burst occurring during stance. Similarly, both ClB and
Srt discharged in a double burst during downhill walking, again with
the additional burst occurring during stance. However, none of the RSNs
that we recorded that showed temporal links to the activity of the
flexor muscles during level walking showed changes of this type during
the pitch tilts. Although this may simply reflect a sampling bias, we
feel that this is an unlikely explanation as the characteristics of the
population of cells that we recorded in the present study agrees well
with those of the populations described in our previous publications in
both intact (Drew et al. 1986, 1996
) and fictively
walking (Perreault et al. 1993
) cats as well as those of
a much larger database from other, unpublished experiments. An
alternative explanation is that these changes in intralimb timing might
be controlled by other supraspinal structures, such as the motor cortex
or the red nucleus. Our results from lesion experiments (Jiang
and Drew 1996
) and from neuronal recordings (Drew
1993
; Widajewicz et al. 1994
; S. Lavoie
and T. Drew, unpublished observations) suggest that both of these
structures play a role in regulating or selecting patterns of muscular
activity during voluntary gait modifications, and it is possible that
they might play a similar role during these locomotor tasks. Although
this might seem in contradiction to the findings that neurons in motor
cortex show little, if any, modification of their discharge when
walking uphill (Armstrong and Drew 1984
;
Beloozerova and Sirota 1993
), both of these studies on
the motor cortex examined neuronal discharge at a maximum inclination of 10°. Given the relatively weak modifications in neuronal
discharge, even in these brain stem neurons, that were produced at this
inclination, it is quite possible that cortical cells may provide
supplementary input at greater degrees of treadmill inclination.
Comparative and functional considerations
It is probable that the changes in activity that we observed in
these two populations of brain stem neurons provide the major descending signal responsible for modifying the level of EMG activity during uphill or crosshill walking in the intact animal. As such, the
results support the view of Orlovsky (1972b), based on
his experiments in decerebrate cats, that one of the major roles of the
brain stem pathways is to regulate the level of EMG activity in tasks
such as the present one and in conditions, in general, where increased
power is required such as when walking uphill or at a faster speed. The
detailed analysis made in the present report, however, taken together
with the arguments made in the companion paper, allows us to go further
than this and to suggest that each of these two pathways has
complementary but different specific roles to play in adjusting posture
during different locomotor tasks.
As suggested in the companion paper (Matsuyama and Drew
2000), VSNs seem to provide a signal that regulates the level
of EMG activity in all four limbs and that is sensitive to the pattern of the interlimb coordination. The data in the present report suggest
that during locomotion on an inclined plane, there is a shift in the
amplitude of the descending signal with no, or little, major change in
pattern. Thus the major role of the VSNs might be to adjust the gain of
the system. In the case of the RSNs, however, although there is also a
change in the level of the descending signal, there is also a change in
the pattern of the discharge, depending on the inclination of the
treadmill. In this case, the signal does not appear to be simply
adjusting gain but may also play a role in more subtly determining the
relative level of activity in different muscles, particularly when the pattern is asymmetric. In addition, although it is probable that the
final expression of the descending signal from both systems is
determined by the state and excitability of the interneuronal systems
within the spinal cord on which they impinge, it is interesting to note
that the two descending pathways exert their effects on motoneurons
through two, largely separate interneuronal systems (Gossard et
al. 1996
), thus providing another substrate for independent control.
Overall these results show that both of the brain stem systems that we studied may contribute to the modifications of the level of EMG activity that are observed during these locomotor tasks that require an adaptation of the posture of the cat. For both systems, we suggest that the signal descending to the spinal cord from any one neuron may simultaneously influence the activity of a number of muscles, probably in more than one limb and probably involving both flexors and extensors. The characteristics of the signal provided by the vestibulospinal system would suggest that it provides a more generalized overall bias to the level of EMG activity and especially to the extensor muscles. On the other hand, the more variable and specific nature of the discharge patterns observed in neurons of the reticulospinal system, particularly when taken together with the changes in pattern observed in some RSNs, suggests that these cells might have a more specific role to play in coordinating the level of activity in groups of muscles involved in the production of different postural patterns.
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FOOTNOTES |
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Address for reprint requests: T. Drew, Dept. of Physiology, Faculty of Medicine, University of Montreal, PO Box 6128, Station "centre-ville," Montreal, Quebec H3C 3J7, Canada (E-mail: drewt{at}ere.umontreal.ca).
Received 3 December 1999; accepted in final form 19 July 2000.
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