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Facilitation of responses to motor cortex stimulation Effects of isometric voluntary contraction.

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Facilitation of Responses to Motor
Cortex Stimulation: Effects of Isometric
Voluntary Contraction
A. Maertens de Noordhout, MD, PhD, J. L. Pepin, MD, P. Gerard, and P. J. Delwaide, MD, PhD
In 7 normal subjects we compared the facilitatory effect of isometric contraction of the tibialis anterior on the size of
electromyographic responses evoked in this muscle by electric stimuli applied over the cervical column and by electric
and magnetic percutaneous stimulation of the motor cortex. No significant difference was found between the degrees
of facilitation of the responses t o any of the stimuli. Using collision techniques, we also showed that the pyramidal fibers
activated by spinal and cortical stimuli are the same. Facilitation induced by isometric contraction (20% maximum) was
of similar or greater magnitude than that found with constant vibration of the tendon of the target muscle. In cases
where vibration and contraction had equal facilitatory effects, there was no further facilitation of the responses when
both conditions were applied together. These findings indicate that the facilitatory effect of isometric contraction of
the target muscle essentially originates at a spinal level rather than in the motor cortex.
Maertens de Noordhout A, Pepin JL, Gerard P, Delwaide PJ. Facilitation of responses to motor cortex
stimulation: effects of isometric voluntary contraction. Ann Neurol 1992;32:365-370
Transcranial stimulation of the motor cortex has gained
widespread popularity among neurophysiologists for
the assessment of conduction along central and peripheral motor pathways, particularly since the development of painless magnetic stimulators [ 11. Steady voluntary contraction of the target muscle exerts two main
effects on responses to cortical stimuli. First, this maneuver shortens response latencies E2-81, but not to a
similar extent when electric and magnetic shocks are
used 14, 57. The mechanisms of this latency shortening
have been discussed in detail elsewhere [8,9] and will
not be considered in this article. Isometric contraction
of the target muscle also increases the amplitude of
electromyographic (EMG) responses to both electric
and magnetic cortical stimuli {2-61. The origin of this
facilitatory effect of isometric contraction remains controversial. Some authors 13, 6, 77 ascribe it, at least in
part, to modifications of cortical excitability, while others [4, 5 , 81 support a segmental spinal mechanism.
Hess and colleagues {57 observed that the amount of
facilitation of responses evoked by electric or magnetic
stimuli was not identical, suggesting that this facilitation
might have multiple origins.
The aim of our study was to explore further the
mechanisms underlying this facilitation by comparing
the effect of isometric contraction of leg muscles on
EMG responses evoked by electric and magnetic corti-
cal stimuli, and by direct electric stimulation of the
long descending tracts in the cervical region. As cervical stimuli bypass pyramidal tract neurons (FTNs), responses cannot be influenced by excitability changes
occurring at the cortical level. Thus, comparing degrees
of facilitation of responses to spinal and cortical stimuli
allows us to determine whether voluntary contraction
modifies cortical or spinal excitability, or both. A prerequisite to any conclusion is that cortical and spinal
stimuli have to activate the same descending fibers. To
test this hypothesis, we tried to provoke collisions in
the pyramidal tract when paired cortical and spinal
stimuli were administered. We also compared the facilitatory effect of isometric contraction and of the tonic
vibration reflex (TVR)of the target muscle, which relies on segmental spinal mechanisms {lo, 111.
From the University Department of Neurology, HBpital de la Citadelle, Liirge, Belgium.
Address correspondence to Dr Maertens de Noordhout, University
Department of Neurology, HBpital de la Citadelle, Bd du 12" de
*Ooo Li'ge>
'gne
Received Dec 31, 1991, and in revised form Feb 27, 1992. Accepted
for publication Mar 13, 1992.
Subjects and Methods
Seven normal subjects (1 woman, 6 men; age range, 26-37
years) were studied. They gave informed consent and the
study was approved by the local ethical committee. The subjects were comfortably seated in a semireclined armchair,
with their eyes open.
EMG activity was recorded in the right tibialis anterior
(TA)muscle with I-cm-diameter,surface silver-silver chloride
(Ag-AgCI),self-adhesive electrodes.To be kept constant, the
level of voluntary contraction was continuously displayed on
Copyright 0 1992 by the American Neurological Association 365
a scope (Tektronix 5 103N, Beaverton, OR) placed in front
of the subject tested. Auditory feedback of the EMG activity
was also provided (Devices 40 10, Digitimer Ltd, Welwyn
Garden City, UK). The level of contraction was expressed
as a percentage of maximal muscle strength developed during
isometric foot dorsiflexion against a strain gauge (Grass
ETlOC, Quincy, MA) fixed under the foot and connected by
a steel wire to a nonextensible collar strapped around the
foot.
Magnetic cortical stimulation was achieved with a Magstim
200 stimulator (Magstim Ltd, Whitland-Dyfed, UK). The
center of the 12-cm-diameter coil was held 4 cm in front of
the vertex. The direction of current flowing in the coil seen
from above was counterclockwise. Electric anodal stimuli
were delivered by a Digitimer D180 stimulator, with a 100psec time constant. The stimulating anode was placed at the
vertex, and the cathode 7 cm in front, on the midline. For
electric stimulation of the pyramidal tract in the cervical region, the same electric stimulator was used. The stimulating
electrodes (same as for EMG recordings) were placed over
the spinous processes of the C6 and T 1 vertebrae, the caudal
one being negative.
The intensities of all three modalities of stimulation were
some 5 to 10% above the motor threshold for the fully
relaxed TA, and they were adjusted in order to produce
responses of similar amplitudes in each case. EMG responses
were amplified and rectified (Digitimer D150, Devices
4010), fed into a CED 1401 AID interface (Cambridge Electronic Design, Cambridge, UK), and then stored on floppy
disks (Copam PC 2863). As responses recorded from the
T A were sometimes polyphasic, it seemed more accurate to
measure their area instead of peak-to-peak amplitude of the
initial component.
Paired Cortical and Spinal Stimuii
In 3 of the subjects holding a slight (105%maximum) isometric contraction of the TA, magnetic cortical ( 4 0 - 6 0 s maximum) and electric cervical ( 8 0 4 0 % maximum) stimuli were
administered with interstimulus intervals of - 10 to + 10
msec. Actual T A responses to the paired stimuli were then
compared with those to each mode of stimulation taken separately and with their expected sum (electronically calculated).
For each experimental condition and interstimulus interval,
eight responses were averaged.
H-Refix Conditioned Sy Cervical Stimuli
In 1 subject, we also investigated whether cervical stimuli
could have activated some inhibitory descending tracts, being
responsible for the phenomenon we interpreted as collisions.
In this subject, it was easy to elicit an H-reflex in the TA.
We then measured the time course of modulation by cervical
stimuli of the H-reflex elicited in the relaxed TA, by applying
electric stimulation to the peroneal nerve at the head of the
fibula. The intensity of the cervical stimuli was below the
threshold for inducing motor responses in the relaxed TA,
but above the threshold if isometric T A contraction was performed. This intensity was chosen to ensure that the descending volley resulting from cervical stimuli induced no direct
depolarization of spinal motoneurons when the muscle was
relaxed.
Measurements of the Amount of Facilitation
In 7 subjects, 16 responses to each mode of stimulation were
collected, at rest and during isometric contraction of the TA.
The level of contraction was approximately 10% maximum.
The area under the curve of each single response was measured and then averaged. For each modality of stimulation,
the amount of facilitation was expressed as a ratio between
areas of EMG responses recorded during isometric contraction of the T A and those at rest. In 3 subjects, experiments
were conducted under several levels of voluntary contraction
( 5 , 10, and 20% maximum).
Comparison of the Facilitate y Eflect of Contraction
and Tonic Vibration R@ex
In 5 subjects, EMG responses to magnetic cortical stimuli
(5% above motor threshold at rest) were recorded in thenar
muscles under four experimental conditions: at rest (no contraction), under isometric (20% maximum) contraction, during elicitation of the TVR by constant vibration (100 Hz,
TMT 18, Heiwa Denshi) of the tendons of the thenar muscles, and during contraction plus vibration. For each experimental condition, 16 responses were collected.
Results
Paired Cortical and Spinal Stimuli
In 3 subjects tested, electric stimulation of the descending tracts in the cervical region produced a welldefined response in the TA, with a mean latency of
20.1 msec. The latency of responses to magnetic cortical stimuli was, on average, 26.4 msec. The shape of
the responses was similar with both types of stimuli,
although the responses to cervical stimuli were of
shorter duration. In the example illustrated (Fig lA),
when cortical stimuli were preceded by spinal ones
(4-msec interval), the early components of the TA responses to Cortical shocks were abolished. This was
observed in all 3 subjects, as long as spinal stimuli
preceded cortical ones by 0 to 5 msec, or if cortical
shocks preceded spinal ones by no more than 5 msec.
This phenomenon disappeared when the intensity of
the cortical stimuli was increased (> 60% maximum)
(Fig IB).
H-Refex Conditioned by Cervical Stimuli
In 1 subject tested, cervical stimuli produced marked
facilitation of an appropriately timed H-reflex in the
TA, although they did not evoke any direct response in
relaxed spinal motoneurons innervating the TA. Peak
facilitation (13 1%) was observed when peroneal nerve
stimulation preceded the cervical shock by 8 msec (Fig
2A). Facilitation lasted for 4 msec with the stimulus
intensities used here. No inhibitory effect of cervical
shocks on the H-reflex evoked in the TA could be
disclosed at any of the explored interstimulus intervals.
Measurements of the Amount of Facilitation
In 7 subjects, responses were evoked in the fully relaxed TA by magnetic and electric cortical stimuli and
366 Annals of Neurology Vol 32 No 3 September 1992
STIMULUS
TYPE
T - / / . v - - .
A . SPINAL
y - b p ,
B. CORTICAL
A + B(act ual)
10.3mv
AcB (expected)
I
A
-14
-10
-8
-6
-2
-4
0
m
ondtlodw-ast h
-
e
A
-r
B: CORTICAL
"
A + B(act ual)
c-(e
,-rvcial
A+B (expected)
B
by electric spinal ones. However, in the latter condition, the stimulus intensity needed to evoke a response
was usually close to the maximal output of the stimulator (750 V). For this reason, only slightly suprathreshold (40-60% maximum) cortical stimuli were used.
Areas of the EMG responses recorded in the relaxed
TA after magnetic cortical, electric cortical, and electric
spinal stimuli were 412
86 nanovoltsecond (nVs),
388 k 91 nVs, and 355 65 nVs, respectively. During isometric contraction (10% maximum) of the TA,
these values were 5.07 2 1.07 microvoltsecond (pVs),
5.32 ? 1.43 pVs, and 4.72 k 1.19 pVs. Mean area
ratios between responses recorded during contraction
and at rest were thus 12.3 2 2.7 with magnetic cortical
stimuli, 13.7
3.9 with electric cortical shocks, and
13.3 + 3.5 with electric spinal shocks (Fig 3A). These
ratios were not significantly different from each other
(Wilcoxon). One example of the facilitatory effect of
*
*
reflex
stimulation
-+h
both (interval: Ems)
B
Fig 1 . (A) Electromyographic responses, recorded in the right
tibialis anterior (TA) muscle of a normal subject, to electric
stimuli (500 V ) applied over the cervical spinal column (upper
trace), to magnetic cortical stimuli (1 tesla) (second trace), and
to cortical stimuli preceded by spinal ones with a 4-msec interval
(third trace). Each trace represents the average of eight uesponses. The bottom trace is the theoretical sum of the responses
to cortical and spinal stimuli, calculated electronically. On the
third trace, the early components of the responses to cortical stimuli are abolished. This suggests that collisions occurred in the
corticospinal pathways, between the antidromic volley consecutive
to spinal stimuli and the orthodromic one resulting from cortical
shocks (see text for details). (B) Same as in A , but the intensities of the spinal (750 V ) and cortical (1.3 tesla) stimuli are
increased. In this condition, collisions are no longer evident, as
shown by the similar morphology of the third and fourth traces.
*
-12
F ig 2. (A)Modulation of amplitude of the tibialis anterior
(TA)H-refex in 1 subject conditioned by electric stimulation
over the cervical spine, The intensity of the cervical stimuli was
sufficient t o activate long descending tracts, but too weak t o depolarize spinal motoneurons in the absence of voluntary contraction of the TA. Each point is the average of eight conditioned
responses. The H-refex is clearly facilitated when a peroneal
nerve stimulus is applied 2 to 8 msec before the cervical shocks
were applied. Facilitation is maximal for a test-conditioning interval of 8 msec, which corresponds to simultaneous arrival o f
both l a dnd descending inputs at the motoneurons. No inhibition of H-refex is observed at any of the explored timings.
(B) Illustration of the H-refex modulation at an 8-msec testconditioning interval. Upper trace: H-refex alone; middle trace:
absence of TA direct response t o cervical stimulation; lower trace:
H-refex facilitated when conditioned by cervical stimuli. Each
trace is the average of eight responses. Horizontal bar = 20
msec; vertical bar = 1 mV.
isometric Contraction on the EMG responses of the
TA to the three modes of stimulation is given in Figure
3B. In 3 individuals, modifications of the level of voluntary contraction ( 5 , 10, and 20% maximum) did not
provoke any significant difference of facilitation between responses to the three modalities of stimulation
(unpaired t). However, facilitation wsls greater with
higher levels of contraction (Fig 4).
Comparison of the Facilitatory Effect of Contraction
and Tonic Vibration
In 5 subjects, vibration of the tendons of the thenar
muscles facilitated the responses to magnetic cortical
stimulation recorded in these muscles. In 3 of them,
the facilitation induced by vibration was prominent and
Maertens de Noordhout et al: Facilitation to Cortical Stimulation 367
~
area
conthest
ratio
2o
15
1
area ratio
cont./rest
t
"1
r
*
electric
electric
magnetlc
spinal
cortical
cortical
T
15
10
10
5
5
0
electric
spinal
electric
cortical
maqnetic
cortical
0
stimulus type
A
I
magnetic
cortical
cortical
electric
stimulus type
area ratio
Fig 4. lnjuence of the level of voluntary isometric contraction of
the facilitation o f the electromyographic responses, recorded in the
tibialis anterior (TA) of 3 normal subjects, to electric cervical
and electric and magnetic cortical stimuli. With background
contraction of 5 , 10, and 20% of the maximal strength of the
muscle, the facilitation ratios are not significantly dzferent,
whatever the type of stimulation. Error bars = I standard deviation.
11
,
B
Fig 3. (A)Mean sudace ratios of the electromyographic responses, recorded in the tibialis anterior (TA) o f 7 normal subjects, to electric cervical and electric and magnetic cortical stimuli. during steady isometric contraction of the target muscle
(10% maximum) and at rest. The facilitation ratios of the responses to all three modalities of stimulation by voluntary contraction are not signrjicantly different from each other. Error
bars = 1 standard deviation. (B) Example of the actual T A responses recorded in 1 subject, as reported in A. Stimulus intensities were matched in order t o produce responses of similar size in
each case. Responses were rectified and then averaged, and their
area was measured. Each trace shows the average of 16 responses.
equal to that observed during isometric (20% maximum) contraction (Fig 5A). In these subjects, when
vibration and contraction were applied simultaneously,
there was no additional facilitation of the responses. In
the other 2 subjects, vibration induced less facilitation
of the responses than did voluntary contraction (Fig
5B). When both conditions were applied together, facilitation was less than the expected sum.
Discussion
In the first set of experiments, we showed that when
a slightly supraliminal cortical stimulus is preceded or
followed by a cervical shock with a maximum interval
of 5 msec, the early components of the response to
cortical stimulation are abolished. This finding suggests
that a collision phenomenon rakes place in the pyrami-
A
B
Fig 5 . (A) Comparison ofthe facilitatoyy pffects of voluntary
isometric contraction (second trace from top), tonic vibration
(third trace) of the tendon. and both conditions applied together
(bottom trace) on electromyographic responses of the thenar
muscles of 1 subject. The first trace shows the mean response recorded at rest. Each trace is the average of 16 responses. In this
subject, the facilitatory effects of contraction and vibration were
equal, and there was no summation of these efferts when both
conditions were applied simultaneously. (B) Same as in A, but
in this subject, facilitation induced by tonic vibration was less
than that occurring during isometric contraction. When both
maneuvers were used together, the responses showed some extra
facilitation. However, it was less than the expected sum of facilitations caused by both conditions taken separately.
dal tract, between the orthodromic volley evoked by
the cortical stimulus and the antidromic one resulting
from the spinal stimulus. Such collisions are easily
evoked in monkeys' pyramidal tract C123. Another
explanation would be that spinal motoneurons are
reached by the second volley during their refractory
period. This is unlikely, since the aforementioned phenomenon was observed for various interstimulus inter-
368 Annals of Neurology Vol 32 No 3 September 1992
vals. For example, when cervical stimuli precede cortical ones by 4 msec, the motoneurons are reached by
the descending volleys consecutive to the two stimuli
with an interval of the order of 9 to 10 msec. This is
far longer than the refractory period of motoneurons,
which does not exceed 3 msec C8]. Also, the fact that
cervical spinal stimuli facilitate rather than inhibit an
appropriately timed H-reflex elicited in the T A rules
out direct activation of inhibitory descending tracts by
spinal stimuli at the intensities used here. H-reflex
facilitation by cervical stimuli was maximal for a testconditioning interval of 8 msec. This timing corresponds to simultaneous arrival at the spinal motoneurons of the volley elicited in Ia fibers by peroneal nerve
stimulation and descending input from a cervical shock.
As this facilitatory effect lasted for several milliseconds,
one might expect that some spinal interneurons were
also facilitated by the descending input.
Thus, the existence of collisions implies that orthodromic and antidromic volleys produced by spinal and
cortical stimuli must be propagated along the same fibers, at least when weak cortical stimuli are used. In
our experiments, late components of responses to cortical stimulation were not annihilated by the collisions.
This is easily explained by the known phenomenon of
multiple descending volleys evoked in the pyramidal
tract by single cortical shocks C4, 8, 12-14}. The first
of these volleys collided with the antidromic one, leaving the later action potentials intact. When the intensity
of cortical stimuli was increased, the collision phenomenon was no longer evident. This might indicate that
strong cortical stimuli are able to recruit some PTNs
not excited by cervical shocks.
The fact that slightly supraliminal cortical and cervical stimuli activate the same descending fibers allows
comparison of the facilitatory effect of voluntary isometric contraction on responses to each of these stimulation modalities. In 7 normal subjects, there was no
statistically significant difference between mean facilitation ratios obtained with the various stimulation modalities. This held true for several levels of voluntary
contraction in 3 subjects. The small amplitude of the
maximal responses to cervical electric shocks prevented us from using stronger cortical stimuli, as we
wished to obtain responses of similar size with all types
of stimulations. Cervical stimuli excite long descending
tracts distally to the cell bodies of PTNs. For this reason, changes in the level of excitability of these neurons cannot modify responses to cervical stimuli. As
facilitation of responses to electric and magnetic cortical stimuli was not different from that to cervical
shocks, it strongly suggests that the facilitatory effect of
isometric contraction of the target muscle takes place at
the level of spinal motoneurons and not in the motor
cortex. This mechanism was already put forward by
Day and colleagues 143. We did not observe a greater
facilitation of the responses to magnetic cortical stimuli
compared to electric ones, as reported by Hess and
colleagues [S}. However, these authors appear to have
measured the peak amplitude of the first negative component of the responses instead of the area of the entire EMG potentials. The mechanisms of activation of
PTNs by electric and magnetic cortical stimuli are
somewhat different C8, 91. Thus the shape of the
evoked responses is not strictly the same with both
modes of stimulation. The responses are often more
polyphasic with electric stimuli 15, 8}. For this reason,
adequate comparison of the responses to different
modes of stimulation requires measurements of the
area of the whole potentials instead of peak amplitude
of the initial component.
In subjects who showed a similar facilitatory effect
of voluntary contraction of the target muscle and TVR,
there was no further facilitation when both conditions
were applied together. This indicates that the same
pathways are involved in both cases. This is yet another
argument favoring a spinal origin of the facilitation produced by isometric contraction of the target muscle, as
it is well known that the TVR uses a segmental spinal
pathway [lo, 11, 151. However, in some cases, vibration had less facilitatory effect than contraction did. In
these subjects, applying both conditions together did
induce some extra facilitation, but it was never equal
to or greater than the algebraic sum of their separate
facilitatory effects.
The fact that facilitation of the responses to cortical
stimuli by isometric contraction takes place at the level
of spinal motoneurons does not rule out excitability
changes of PTNs or cortical interneurons under certain
conditions, particularly at and just before the initiation
of movement. In monkeys, the firing rate of some
PTNs may be increased up to 100 msec prior to the
movement [lb]. If spinal motoneurons are relatively
inexcitable, this increased activity of FTNs may not be
followed by muscle activity. However, if cortical stimuli are applied during this period, the facilitation then
becomes evident [7, 177. In humans, the amount of
voluntary facilitation of the responses in a given muscle
can be task dependent 1181.
Besides the artificial condition of cortical stimulation, our findings might help in understanding the
physiology of the cortical drive of isometric muscle
contraction. In monkeys, many PTNs show phasic
bursts of activity corresponding to the buildup of force,
while these FTNs fire tonically in a steady-state condition while a steady force is exerted 1191. Thus, corrical
activity does not show large fluctuations during isometric muscle activity. However, a small proportion of
PTNs (13%) tend to discharge more often when the
contraction is maintained for long periods of time,
probably to compensate some loss of activity in phasic
spinal motoneurons 1201. Such an increase in cortical
Maertens de Noordhout et al: Facilitation to Cortical Stimulation 369
output is probably too slow to be disclosed by our
rather gross experimental paradigm.
In conclusion, this study demonstrated that the facilitatory effect of steady isometric contraction of the target muscle on EMG responses to electric and magnetic
cortical stimuli takes place at the level of spinal motoneurons, and not in the motor cortex, at least when
slightly suprathreshold cortical stimuli are used. For
strong brain stimuli, no conclusion can be drawn on
the basis of this work, as collisions were no longer
evident. For paraclinical use of cortical stimulation, this
finding implies that changes of the activation threshold
of a given muscle do not automatically reflect cortical
excitability modifications. Instead, they are more likely
to originate at a spinal level.
8.
9.
10.
11.
12.
13.
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