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Corticospinal conduction studied with magnetic double stimulation in the intact human
Journal of the Neurological Sciences, 111 (1992) 180-188 © 1992 Elsevier Science Publishers B.V. All rights reserved 0022-510X/92/$05.00 JNS 03831
Corticospinal conduction studied with magnetic double stimulation in the intact human D e t l e f Claus a, M a r i a W e i s a, U w e J a h n k e a, A n d r e a s P l e w e b a n d C h r i s t o p h Brunh61zl ~ a Depa'tment of Neurology, University Erlangen-Niirnberg, D-8520 Erlangen, Germany, and b Department of Neurology, UniversityJena, 06900 Jena, Germany
(Received 2 December, 1991) (Revised, received 23 March, 1992) (Accepted 30 March, 1992) Key words: Central motor conduction; Double stimulation; Magnetic stimulation; Silent period
Summary A group of 8 healthy normal subjects (24-36 years old, mean age 29 years) were investigated. Transcranial magnetic double stimulation of the motor cortex was carried out at different interstimulus intervals. With both stimuli suprathreshold, an attenuation of the test response was found at interstimulus intervals of less than 200 msec (target relaxed or contracted). The manifestation of this attenuation correlated with central signs in 31 patients with multiple sclerosis. This phenomenon is (at least at longer intervals) probably not a result of the refractory spinal motoneuron pool, but of a supraspinal inhibitory mechanism or lack of corticospinal drive caused otherwise. At interstimulus intervals between 10 and 30 msec, the test response increases significantly (magnetic double stimulation 10% suprathreshold, target relaxed). This result is also seen with voluntary muscle contraction and with vibration applied to a relaxed target muscle. The facilitatory effect is probably caused by slowly conducted corticospinal volleys enabling summation, with descending impulses generated by the test stimulus. With the conditioning stimulus subthreshold and target muscle relaxed an intracortical inhibition of the test response could be confirmed at short interstimulus intervals.
Introduction Transcranial magnetic stimulation excites presynaptic neural structures in the motor cortex (Day et al. 1989a; Thompson et al. 1991) and generates descending corticospinal volleys of mainly indirect waves (Iwaves). In addition pyramidal axons may be directly stimulated and D-waves generated (Amassian et al. 1990; Berardelli et al. 1990). This method has yielded information on the functioning of the central motor system. Repetitive transcranial electrical stimulation was used to investigate stimulus summation at the spinal motoneuron pool (Inghilleri et al. 1990) as well as intracortical inhibition phenomena (Rothwell et al. 1991). The new technique of repetitive magnetic stimulation allows further insight into central excitatory and
Corresponding to: Prof. Dr. D. Claus, Department of Neurology, University of Erlangen-Niirnberg, Schwabachanlage 6, D-8520 Erlangen, Germany. Tel.: 09131/854444; Fax: 09131/854436.
inhibitory mechanisms. The aim of this study was to investigate interaction between paired transcranial magnetic stimuli under different conditions. Does the new method provide additional data about corti,, cospinal stimulus conduction?
Materials and methods
Eight healthy normal subjects participated in the experiments (5 females, 3 males, 24-36 years of age, mean age 29 years). They gave their informed consent and the experiments were approved by the local ethics committee. Normal results were collected for magnetic double stimulation in 21 healthy subjects (10 females, 11 males, age 20-47 years, mean age 28 years). A further group of 31 patients was investigated with the same method (16 females, 15 males, age 16-56 years, mean age 33 years). They all suffered from multiple sclerosis (MS). According to the criteria of Poser (Poser et al. 1983) 8 had definite, 10 probable, and 13 possible
181 MS. Abnormal CSF (elevated 7-globulin or presence of olignclonal bands) was found in 15/31, focal lesions in brain MRI in 15/20, delayed VEPs in 7/22, and extensor plantar responses in 6/31 cases. The patients were not disabled and only 10 had weakness or spasticity in either hand (14 sites). Two Magstim 200 magnetic stimulators were connected to a Bistim device (The Magstim Company, Whitland, Dyfed SA34 OHR, England), discharging into a circular high power coil (mean diameter 95 mm, 14 windings, Emax 108 V / m , maximum magnetic field 2.5 tesla, B° 34 kT/sec). The interstimulus interval of double stimuli could be assessed between 0 and 999 msec. According to our technical investigations, amplitude and shape of both stimuli were consistent at all interstimulus intervals down to a minimum of 1 msec, with a variability of 0.1 msec. At an interval of 0 msec (both magnetic stimulators discharging simultaneously), the amplitude varied by 13% around that of a single stimulus. The circular coil was placed fiat on the scalp with anticlockwise current direction in the coil and its center over the vertex. The coil position was then adjusted in order to generate maximum compound muscle responses (CMAP) in the right abductor digiti minimi muscle (ADM). Thereafter, the coil was fixed in this position for the duration of the experiment. The threshold for the excitation of the target muscle was determined independently for the first and second stimulus by increasing the intensity each time by 5% (given as a percentage of maximum output). The lowest intensity which gave three reproducible CMAPs (gain 0.2 m V / D ) was defined as the threshold (Claus 1990). The average threshold stimuli are 50% with ADM relaxed (relaxed threshold) and 39% with ADM contracted (contracted threshold). Responses were recorded from the right ADM by surface cup electrodes (7 mm diameter), amplified (gain 0.5-5 m V / D , bandpass 20 Hz to 5 kHz ( - 3 dB), sweep time 50 or 100 msec, sample rate at least 100 ~sec/point) and stored digitally for further measurements (Medelec MS20, Datamed program) (Claus et al. 1990). Electrodes were filled with jelly and adapted by the tendon belly method. Amplitude and area of the first negative potential component were measured. At least 3 trials were recorded for each condition and results were averaged. Further postprocessing with rectification or subtraction of traces was not carried out because of the variable shape of CMAPs. The compound muscle response following stimulus 1 (conditioning stimulus) we called CMAP1 and that after stimulus 2 (test stimulus) CMAP2. In cases of CMAPs not being clearly separated from one another (i.e., at A t < 5 msec), CMAP1 but not CMAP2 results were measured. The subject sat comfortably in an armchair with the right arm splinted. When the target muscle was re-
laxed, relaxation was monitored by EMG surface recording (gain 0.2 m V / D ) via a loudspeaker. When the target muscle was exerting 10% isometric contraction, muscle strength was recorded by a miniature load cell (Kyowa Electronic Instruments Co. Ltd., Tokyo, Japan) and shown to the subject on an oscilloscope. Statistical analysis was carried out using the programmes Statview (Abacus Concepts, Inc., Berkeley, CA 94704) and Systat (SYSTAT, Inc., Evanston, IL 60201-3793). The correlation between interstimulus intervals and results was investigated using Spearman's rank correlation test. These results were confirmed upon investigation of the significance of differences by administering the Wilcoxon test at certain intervals.
Experimental settings All experiments were performed in random order. Interstimulus intervals were randomly adjusted. (1) Muscle responses were recorded from the right ADM after peripheral supramaximal electrical double stimulation of the ulnar nerve at the wrist (square wave 0.2 msec). Amplitudes (baseline to peak) and areas of CMAPs were measured in order to assess the interstimulus interval at which CMAP2 decreased. (2) The attenuation of CMAPs after transcranial magnetic double stimulation was measured in the right ADM (10% contraction), magnetic stimuli both 10% suprathreshold (contracted threshold). Interstimulus intervals: 330, 200, 150, 120, 100, 90, 80, 70, 60, 50 reset. The duration of the silent period was measured after single transcranial stimulation (10% suprathreshold) with ADM contracted. (3) With ADM relaxed both magnetic stimuli were 10% above relaxed threshold, and the interstimulus intervals varied between 330 and 0 msec (330, 200, 150, 120, 100, 90, 80, 70, 60, 50, 40, 35, 30, 25, 20, 18, 16, 14, 12, 10, 5, 4, 3, 2, 0 msec). The experiment was also carried out with continuous vibration (sinusoidal stimulus 100 Hz, 0.1 nun) applied to the relaxed ADM (Vibrameter, SOMEDIC AB, Stockholm, Sweden). The same experiment was repeated with ADM voluntarily contracted (10%) in order to evaluate the influence of muscle contraction on ~ e results. (4) With ADM relaxed, the test stimulus was 10% above relaxed threshold; the conditioning stimulus, however, was 5% below relaxed threshold; interstimulus intervals varied between 30 and 0 msec (5 subjects). The experiment was carried out to investigate whether or not the strength of the conditioning stimulus has an influence on CMAP2 after the test stimulus. (5) To further investigate the direct influence of the conditioning stimulus on both the attenuation and the enlargement of CMAP2, its strength was altered. At an
182 interstimulus interval where CMAP2 was already attenuated by > 50% (double stimulus, both 10% above contracted threshold, At 50-90 msec), the strength of the conditioning stimulus was reduced each time by 5%. ADM was 10% contracted. A similar experiment was carried out on 3 subjects at an interstimulus interval of 25 msec and with both stimuli 10% above relaxed threshold, ADM relaxed. The conditioning stimulus was then attenuated each time by 5% down to zero. (6) In 21 normal subjects relative area and amplitude of CMAP2 were measured to both hands (42 sites) with ADM contracted after suprathreshold tran-
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scranial magnetic double stimulation (10% > contracted threshold) at an interval of 200 msec (normal results). The same technique was used in the 31 MS patients. In these patients CMCT was also investigated to both hands applying the usual technique as described elsewhere (Claus 1990).
Results
(1) When the peripheral nerve is stimulated electrically and supramaximally with double stimuli (4 subjects), then CMAP2 starts to attenuate at an interstim-
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interstimulus interval (ms) Fig. l. Amplitudes and areas of CMAPs recorded from the right ADM after supramaximalelectrical stimulation of the ulnar nerve at tfi~ wrist (4 subjects, mean of 3 trials each). A relevant attenuation of CMAP2 is seen at intervals less than 5 msec (0 msec = single stimulation).
183 ulus interval of 5 msec (mean area 84% and amplitude 90% of that of CMAP1). The attenuation of CMAP2 becomes relevant at an interstimulus interval of less than 5 msec (At 2 msec: CMAP2 area 22%, amplitude 23% of CMAP1). Therefore, the attenuation of CMAP2 after double stimulation of the motor cortex at At < 5 msec could be due to refractory neuromuscular transmission. No difference was established in the results between amplitudes and areas (Fig. 1). Thus, in the following paper only amplitudes will be discussed. The mean amplitude of CMAPs after peripheral nerve stimulation is 9.0 + 1.9 mV. After transcranial magnetic stimulation (10% > relaxed threshold) the mean amplitude in the relaxed ADM is 8% of that after peripheral nerve stimulation, thus indicating that only 8% of the motoneuron pool is excited. The amplitude of CMAPs recorded from the voluntarily contracted ADM after magnetic brain stimulation (10% > contracted threshold) is, however, 42% of that after peripheral stimuli. (2) After magnetic double stimulation of the motor cortex and with the ADM contracted, C AP2 attenuates at interstimulus intervals of less than 200 msec
5
(Spearman p < 0.001). The average value of the amplitude diminishes in comparison to that of CMAP1 (At 200 msec, 94%; 150 msec, 83%; 100 msec, 42%; 80 msec, 32%; 50 msec, 43%). When magnetic transcranial single stimuli are applied with the target muscle contracted, muscle activity is interrupted for an average of 192 + 68 msec (latency from CMAP to the beginning of voluntary muscle activity). This time is comparable with the interstimulus interval at which CMAP2 becomes attenuated. Due to the relatively large differences between interstimulus intervals from 100 to 330 msec, it was not possible to investigate the correlation between the silent period and the beginning of the attenuation of CMAP2. (3) CMAP2 after magnetic brain stimulation (both stimuli 10% > relaxed threshold) begins to attenuate at an interstimulus interval < 200 msec (P___<0.01), regardless of whether the ADM is relaxed or contracted (Fig. 2). This phenomenon is, however, more pronounced when the ADM is contracted. At interstimulus intervals of between 10 and 30 msec - ADM relaxed - CMAP2 enlarges significantly, and becomes even larger than CMAP1 ( P < 0.01). This
8 subjects, r. ADM relaxed, transcranial double stim., [ [ _ _ _ _ both 10% suprathresheld ]
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Fig. 2. Mean + SD of CMAP amplitudes recorded from the right ADM after magnetic double stimulation of the motor cortex. At interstimulus intervals < 200 msec CMAP2 starts to attenuate, no matter whether the target muscle is relaxed or contracted. At shorter intervals (10-30 msec) CMAP2 enlarges significantly.
184 A.J. (30 yrs.) Double sttm~ Cx, ! O~ suprathreshold r. ADH relaxed
A.J. (30 yrs.) Double stim. Cx, I subthr. (35~), 2 suprathr. (55R) r. ADM relaxed
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mYI !0 ms Fig. 3. Exampleof CMAPsrecordedafter magneticbrain stimulation with ADM relaxed. At interstimulus intervals of 20 and 30 msec CMAP2is significantlyenlarged. phenomenon is less pronounced when the ADM is contracted (Fig. 2, lower trace). In this case, however, CMAP2 reaches the amplitude of CMAP1 at interstimulus intervals of between 10 and 30 msec. Fig. 3 depicts an example of the enlargement of CMAP2 with the target muscle relaxed. Vibration applied to the relaxed target muscle does not cause any alteration to this result. At interstimulus intervals _~ 10 msec, CMAP2 is not always clearly separated from CMAP1. Therefore, it cannot be measured separately. (4) When the first stimulus is subthreshold (ADM relaxed), then the influence of interstimulus intervals on the muscle response after transcranial stimulation is completely different. In this case, the CMAP attenuates significantly at intervals of between 2 and 4 msec (P = 0.01) with a minimum at At 3 msec (attenuation to 25%). An example of this can be found in Fig. 4. At an interval of 0 msec (single stimulus), the amplitude recovers to approximately its reference value (110%). (5) The influence of the conditioning stimulus on CMAP2 after the test stimulus depends on various conditions such as interstimulus interval and muscle contraction. In order to determine whether its strength is significant for CMAP2, the conditioning stimulus was gradually reduced with ADM contracted and at an interstimulus interval where the second potential was
lOres Fig. 4. Example of the recording after cortical magnetic double stimulationwith a subthresholdconditioningstimulus.At short interstimulusintervalsCMAP2is significantlyattenuated. clearly attenuated. This attenuation ceased and CMAP2 enlarged when the first stimulus became subthreshold (Table 1). When the interstimulus interval is 25 msec, then the influence of the attenuation of the conditioning stimulus on the muscle response following a consistent test stimulus is completely different, At this interval, CMAP2 is enlarged (stimulus 1 and 2 10% > relaxed threshold, ADM relaxed). When the first stimulus becomes weaker, CMAP2 attenuates (3 subjects, Fig. 5). TABLE 1 CMAPs RECORDED FROM THE CONTRACTED ADM AFTIERMAGNETICDOUBLE STIMULATIONOF THE CORTEX The interstimulusintervalis such that CMAP2 is attenuated(50-90 msec). When the conditioningstimulus becomes weaker CMAPl attenuates and CMAP2 enlarges. This result proves an inhibitory influenceof the conditioningstimuluson CMAP2. Magneticstimulus 1 %± threshold AmplitudeCMAPl (mean+ SD) +10% +5% 0% -5%
3.0±1.2 1.5±1.1 1.4±0.9 0.4±0.3
8 subjects,ADM 10% contraction.
Stimulus2 Always10% > threshold CMAP2(mV) (mean± SD) 1.4±0.8 1.7±1~ 2.3±1~ 3.4±1.7
185 3 subjects, ADrl relaxed, double sum. (intervals 25 ms), stlm.2:1011 suprethreshol(I, (mean or 3 trtals each)
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Fig, 5. Magnetic double stimulation o f the motor corte~y;~,3 ~ubjects ( A D M relaxed). When the conditioning stimulus attenuates well below motor threshold C M A P 2 becomes smaller. This result proves a facilitatory influence o f the conditioning stimulus (even subthreshold) on the C M A P generated by the test stimulus.
Thus, despite the relatively long interstimulus interval of 25 msec, the conditioning stimulus is believed to have a facilitatory influence on the response succeeding the test stimulus. (6) In the normal group the area/amplitude of CMAP2 was at least 60% of CMAP1 (1 percentile; double stimulation, At 200 msec, target contracted). The CMCT was prolonged > 8 msec (normal results in Claus 1990) in 7/31 MS patients (8/62 sites). An abnormal attenuation of CMAP2 after double stimulation was seen in 10/31 patients (3/8 definite; 3/10 probable; 4/13 possible MS; 11/62 sites). There was a significant correlation between spasticity/centrai weakness in the hand and attenuation of CMAP2 ( P < 0.05). Despite a correlation with clinical signs of central motor disturbance, 7 of the hands with prolonged CMCT and 6 with attenuated CMAP2 were clinically unimpaired.
Discussion
Attenuation of the second muscle response After suprathreshold magnetic brain stimulation, CMAP2 attenuates when the interstimulus interval becomes shorter than 200 msec. The degree of attenuation is altered by muscle contraction but its duration is not (experiments 2 and 3; Fig. 2). After transcranial electrical stimulation complete recovery of the second muscle response was seen at an interval of 150 msec with the target relaxed and at 100 msec with the target contracted (Inghilleri et al. 1990). These intervals are slightly shorter than those following magnetic double stimulation. According to animal experiments by Phillips and Porter (1964) corticospinal neurons can follow a frequency of up to 200 Hz. In humans, the full recovery
cycle of D-waves after electrical brain stimulation is 3.5 msec ( > 280 Hz) (Inghilleri et al. 1989). Since D and I intervals are even shorter, corticospinal tracts are likely to have a shorter recovery cycle than cortical neurons. This recovery cycle is compatible with direct excitation of corticospinal neurons by electrical stimuli. After magnetic stimulation, it may last longer due to its mainly presynaptic site of action. Nevertheless, refractority of cortical or corticospinal neurons is unlikely to explain an attenuation of the second response at an interstimulus interval of up to 200 m~.c. Thus an electrical stimulus delivered 50 msec after a magnetic cortical stimulus was able to produce a CMAP (Day et al. 19891)). This result demonstrates that the spinal motoneuron pool and the corticospinal axons are no longer refractory after a delay of 50 msec. Inghilleri and coworkers found that after electrical stimulation, the greater the amplitude of the first muscle response the smaller that of the second (Inghilleri et al. 1990). They concluded that the first action potential causes the motoneuron pool to be partially refractory. After cortex stimulation CMAPs are generally smaller than after peripheral nerve stimulation, indicating that fewer motoneurons fire (experiment 1). Thus only 8% (relaxed) or 42% (contracted) of the motoneuron pool can be refractory after the conditioning stimulus. The second response enlarges when the conditioning stimulus becomes weaker (experiment 5, Table 1). Therefore, the conditioning stimulus is likely to exert an inhibitory influence at a supraspinal or spinal level. According to single unit recordings after magnetic brain stimulation (Boniface et al. 1991) motoneurons are ~ r a c t o r y for about 20 msec. At short interstimulus interval~, double firing of motoneurons is therefore relatively unlikely (Day et al. 198%). One may thus conclude that, at least at short intervals, the test stimulus excites preferentially spinal motoneurons which had not already been brought to firing threshold by the conditioning stimulus. Therefore, as opposed to peripheral nerve stimulation, no recovery cycle is investigated after transcranial double stimulation. Different explanations are discussed for the silent period after peripheral nerve stimulation. These include afterh~erpolarisation of spinal motoneurons, recurrent Renshaw inhibition, inhibition by flexor reflex afferents and Ia mediated disynaptic inhibition (Merton 1951; Granit 1955; Hultborn et al. 1979; Rothwell 1987; Kudina and Pantseva 1988). The silent period after transcranial stimulation can be seen without a CMAP (Marsden et al. 1983; Calancie et ai. 1987) and it is prolonged with increasing stimulus strength (Holmgren et al. 1990). The H-reflex is depressed only at the beginning of the silent period (Fuhr et al. 1991). It is therefore believed that at least in the late part of the silent period after cortical stimulation, a reduction in the excitability of the spinal motoneuron pool plays
186 only a minor role in determining the phenomenon. It is probably caused by lack of corticospinal drive. Intracortical inhibitory mechanisms may also play a role (Fuhr et ai. 1991). The duration of the silent period (experiment 2) and the interstimulus interval at which C AP2 is significantly attenuated are comparable with each other. This result would support the idea that a similar physiological mechanism is responsible for both the silent period and attenuation of CMAP2 after magnetic double stimuli. Spinal mechanisms may be partially responsible for the attenuation of the second response at short interstimulus intervals. They are however unlikely to act for as long as 200 msec. According to our results after peripheral nerve stimulation (Fig. 1), neuromuscular transmission cannot be responsible for an attenuation of CMAP2 after cortex double stimulation at relatively long interstimulus intervals. In 31 patients who had MS with only slight signs and symptoms, results after double stimulation were at least as sensitive as CMCT (experiment 6). A correlation was found between an attenuated CMAP2 and central signs in the investigated hand. Therefore, the attenuation of CMAP2 after magnetic double stimulation of the motor cortex may reflect disturbed corticospinal stimulus conduction or intracortical excitability. This result could also support the idea that a central mechanism is responsible for the attenuated second response after magnetic double stimulation at intervals shorter than 200 reset. The attenuated CMAP2 is, at least at interstimulus intervals longer than 50 msec, probably not due to refractority of the spinal motoneuron pool. Refractority of corticospinal neurons is also an unlikely cause. Supraspinal mechanisms seem to be responsible. The attenuation of CMAP2 is due tc lack of corticospinal drive, probably as a result of an intracortical inhibitory mechanism. Whether this intracortical phenomenon is presynaptic or postsynaptic cannot be conclusively decided upon since a similar attenuation has also been described after electrical transcranial stimulation (InghUleri et al. 1990). A corticospinal inhibitory input (Lemon et al. 1987) could also provide an explanation of the phenomenon.
Enlargement of the second muscle response The enlargement of CMAP2 after magnetic double stimulation at interstimulus intervals between 10 and 30 msec is significant. The facilitation is less pronounced when the target is contracted than when it is relaxed (experiment 3). This is probably due to the fact that more motoneurons were brought to firing threshold by the conditioning stimulus with the muscle contracted. As mentioned above the motoneurons remain refractory for some time and CMAP2 therefore becomes smaller with ADM contracted (Fig. 2).
However, there seems to exist a strong facilitatory influence of the conditioning stimulus on CMAP2. This is particu.larly obvious at interstimulus intervals between 10 and 30 msec (Fig. 3), which exceed the corticospinal volley duration of about 10 msec (Landgren et al. 1962; Phillips and Porter 1964; Clough et al. 1968; Amassian et al. 1990; Thompson et al. 1991). Due to these relatively long intervals, temporal summation of fast conducted I-waves generated by the conditioning stimulus and the test stimulus is unlikely at the motoneuron. This is in accordance with earlier results showing that summation between an afferent volley after mechanical muscle stimulation and a corticospinal volley lasts no longer than 14 msec (Claus et al. 1988). Furthermore, after double stimulation of the cervical cord, no relevant temporal summation is seen at the spinal motoneuron pool at interstimulus intervals of as long as 10 msec (Claus et al. 1991). After electrical transcranial stimulation a first short-latency facilitation of the H-reflex was followed by a second facilitation which lasted between 5 and 20 msec (Cowan et al. 1986). This was believed to be due to I-waves, activation of smaller corticospinal neurons or activity in spinal interneuronal networks. In human subjects transcranial stimuli above the motor threshold applied to the motor cortex are likely to effectively access fusi-motoneurons (Burke et al. 1990). This is thought to be mediated mainly by /3motoneurons, not ,y-motoneurons (Rothwell et al. 1990). Activation of muscle spindles by the conditioning stimulus could cause summation of primary spindle afferents with descending volleys generated by the test stimulus. This would however be extinguished by voluntary muscle contraction due to a-~,-coactivation. Furthermore, vibration applied to the relaxed ADM excites muscle spindles and augments the muscle response to cortical stimuli (Claus et al. 1988). The facilitatory effect of the test stimulus is much stronger than that of muscle vibration. The increase of the second response remains unaltered by vibration. The lack of influence of muscle vibration is an argument against the importance of spindle activation for the enlargement of CMAP2. The enlargement of CMAP2 at intervals between 10-30 msec is believed to be an effect of the conditioning stimulus since it disappears when this stimulus attenuates (Fig, 5). The facilitation is seen with a conditioning stimulus in the region of the motor threshold - or just below of it. The facilitatory influence of the conditioning stimulus is likely to be mediated by slow conducting corticospinal neurons (Lassek and Evans 1945). This idea is supported by the influence of stimulus intensity on the result, as demonstrated in Fig. 5. This strong facilitatory influence is able to Overcome the attenuation of the second response seen after paired transcranial shocks, as well as
187 the mechanisms responsible for H-reflex refractority (Fuhr et al. 1991). The lack of a facilitatory effect after electrical double stimuli at short intervals (Inghilleri et al. 1990) would support the hypothesis of an intracortical mechanism such as synchronization of pyramidal cells. This is, however, discordant to the late facilitation of the H-reflex by electrical cortical stimuli (Cowan et al. 1986). As opposed to electrical stimulation (Inghilleri et al. 1990) no significant enlargement of the CMAP was seen after magnetic stimulation with short intervals of 2-5 msec (both stimuli 10% > relaxed threshold, target relaxed). This is thought to be due to the fact that magnetic stimuli act preferentially on presynaptic structures in the motor cortex. Therefore, it is mainly I-waves and not D-waves which are generated with suprathreshold stimulation alone. Two distinct volleys are evoked only by paired electrical shocks at intervals of 1 msec or more (Inghilleri et al. 1989). The descending volleys may produce summation at motoneurons. This is probably not the case with mainly presynaptically acting magnetic stimuli (Day et al. 1989a). Thus, the lack of an enlargement of the CMAP at short interstimulus intervals could prove the presynaptic action of magnetic cortex stimuli. The slight enlargement of CMAP1 at an interstimulus interval of 0 msec (both magnetic stimulators discharging simultaneously, Fig. 2) could easily be a result of the variability of the magnetic stimulus mentioned above.
lntracortical inhibition At short interstimulus intervals a subthreshold conditioning stimulus causes attenuation of the muscle response (experiment 4, Fig. 4). This result was also described by Rothwell and collaborators (Rothwell et al. 1991). Subthreshold transcranial shocks could cause disynaptic corticospinal inhibition. The fact, however, that the attenuation of CMAP2 at short interstimulus intervals is not seen after electrical brain stimulation is a strong argument in favour of an intracortical inhibitory mechanism in the motor strip itself or in surrounding areas of the cortex (Rothwell et al. 1991) and against disynaptic corticospinal inhibition. This inhibition occu~ despite the fact that subthreshold electrical scalp stimuli can evoke descending volleys in relaxed subjects and cause facilitation of the H-reflex (Cowan et al. 1986). The phenomenon of intracorticai inhibition could well be of clinical diagnostic importance.
Conclusions The noninvasive technique of transcranial magnetic double stimulation provides new information about
inhibitory and facilitatory mechanisms in the central motor system. This has implications for the investigation of fatigue in central pareses as well as lack of intracortical inhibition in central hyperexcitability. Facilitation phenomena may offer information about the dispersion of corticospinal stimulus conduction in central motor disorders. It may also be of diagnostic use in movement disorders. Acknowledgement This work was supported by the Wilhelm Sander Foundation.
References Amassian, V.E., O.J. Quirk and M. Stewart (1990) A comparison of corticospinal activation by magnetic coil and electrical stimulation of monkey motor cortex, Electsoenceph. Clin. Neurophysiol., 77: 390-401. Bcrardelli, A., M. Inghilleri, G. Cruccu and M. Manfredi (1990) Descending volley after electrical and magnetic transcranial stimulation in man, Neurosci. l.~tt., 112: 54-58. Boniface, SJ., K.R. Mills and M. Schubert (1991) Responses of single spinal motoneurons to magnetic brain stimulation in healthy subjects and patients with multiple sclerosis, Brain, 114: 643-662. Burke, D., S.C. Gandevia and J.C. Rothwell (1990) Activation of presumed fusi-motoneurones by motor conical stimulation in human subjects, L Physiol., 425: 85P. Calancie, B., M. Nordin, U. Wallin and K.E. Hagbarth (1987) Motor-unit responses in human wrist flexor and extensor muscles to transcranial cortical stimulation, J. Neurophysiol., 58: 11681185. Claus, D. (1990) Central motor conduction: method and normal results, Muscle Nerve, 13: 1125-1132. Claus, D., K.R. Mills and N.M.F. Murray (1988) Facilitation of muscle responses to magnetic brain stimulation by mechanical stimuli in man, Exp. Brain Res., 71: 273-278. Claus, D., N.M.F. Murray, A. Spitzer and D. Fliigel (1990) The influence of stimulus type on the magnetic excitation of nerve structures, Electroenceph. Clin. Neurophysiol., 75: 342-349. Claus, D., M. Weis and A. Spitzer (1991) Motor potentials evoked in tibialis anterior by single and paired cervical stimuli in man, Neurosci. Lctt., 125: 198-200. Clough, J.F.M., D. Kernell and C.G. Phillips (1968) Monosyaaptic
excitation from the pyramidaltract and from primary spindle afferents to motoneurones of the baboon's hand and forearm, J. Physiol., 198: 145-166. Cowan, J.M.A., B.L. Day, C. Marsden and J.C. Rothwell (1986) The effect of percotaneous motor cortex stimulation on H reflexes in muscles of the arm and leg in intact man, J. Physiol., 377: 333-347. Day, B.L., D. Dressier, A. Maertens de Noorhout, C.D. Marsden, K. Nakashima, J.C. Rothwell and P.D. Thompson (1989a) Electric and magnetic stimulation of human motor cortex: surface EMG and single motor unit resoonses, J. Physiol., 412: 449-473. Day, B.L.; J.C. Rothwell, P.D. Thompson, A. Maertens de Noordbout, FL Nakashima, FL Shanon and C.D. Marsden (1989b) Delay in the execution of voluntary movement by electrical or magnetic brain stimulation in intact man, Brain, 112: 649-663. Fuhr, P., R. Agostino and M. Hallett (1991) Spinal motor neuron excitability during the silent period after cortical stimulation, Electroenceph. Clin. Neurophysiol., 81: 257-262. Granit, R. (1955) Receptors and Sensory Perception, Vail-Ballou Press, Binghamton, NY, Yale University Press, pp. 210-216.
188 Holmgren, H., L.E. Larsson and S. Pedersen (1990) Late muscular responses to transcranial cortical stimulation in man, Electroenceph. Clin. Neutophysiol., 75: 161-172. Hultboto, H., E. Piertot-Deseilligny and H. Wigstrom (1979) Recurrent inhibition and afterhyperpolarization following motoneutonal discharge in the cat, J. Physiol., 297: 253-266. Inghilleri, M., A. Berardelli, G. Ctoccu, A. Priori and M. ManfTedi (1989) Corticospinal potentials after transcranial stimulation in humans, J. Neutol. Neutosurg. Psychiat., 52: 970-974. Inghilleri, M., A. Berardelli, G. Crucu, A. Priori and M. Manfredi (1990) Motor potentials evoked by paired cortical stimuli, Electtoenceph. Clin. Neurophysiol., 77: 382-389. Kudina, L.P. and R.E. Pantseva (1988) Recurrent inhibition of firing motonentones in man, Electroenceph. Ciin. Neurophysiol., 69: 179-185. Landgren, S., C.G. Phillips and R. Porter (1962) Minimal synaptic actions of pyramidal impulses on some alpha motoneurones of the baboon's hand and forearm, J. Physiol., 161: 91-111. Lassek, A.M. and J.P. Evans (1945) The human pyramidal tract, J. Comp. Neutol., 83: 113-126. Lemon, R.N., R.B. Muir and G.W.H. Mantel (1987) The effects upon the activity of hand and forearm muscles of intracortical stimulation in the vicinity of corticomotor neurones in the conscions monkey, Exp. Brain Res., 66: 621-637. Marsden, C.D., P.A. Merton and H.B. Morton (1983) Direct electrical stimulation of the corticospinal pathways through the intact scalp in human subjects. In: Desmedt, J.E. (Ed.), Motor Control
Mechanisms in Health and Disease, 39, Raven Press, New York, NY, pp. 387-391. Merton, P.A. (1951) The silent period in a muscle of the human hand, J. Physiol., 114: 183-198. Phillips, C.G. and R. Porter (1964) The pyramidal projection to motoneutones of some muscle groups of the baboon's forelimb. In: Eccles, J.C. and Schad~, J.P. (Eds.), Physiology of Spinal Neurons, Progress in Brain Research, vol. 12, Elsevier, Amsterdam, pp. 222-242. Poser, C.M., D.W. Paty, L. Scheinberg, W.L McDonald, F.A. Davis, G.C. Ebers, K.P. Johnson, W.A. Sibley, D.H. Silberberg and W.W. Tourtellotte (1983) New diagnostic criteria for multiple sclerosis: guidelines for research protocols, Ann. Neutol., 13: 227-231. Rotbvlell, J.C. (1~87) C~nttol" of Human Voluntary Movement, Croom Helm, London, Sydney, pp. 105-157. Rothwell, J., A. Ferbert, M. Caramia, T. Kujirai, Yamagata, B. Day and P. Thompson (1991) lntracortical inh~itory circuits studied in humans, Neurology, 41 (Suppl. 1): 192. Rothwell, J.C., S.C. Gandevia and D. Burke (1990) Ac~vation of fusimotor neurones by motor cortical stimulation in human subjects, J. Physiol., 431: 743-756. Thompson, P.D., B.L. Day, H.A. Crockard, I. Calder, N.M.F. Murray, J.C. Rothwell and C.D. Marsden (1991) Intra-operative recording of motor tract potentials at the cervico-medullaryjunction following scalp electrical and magnetic stimulation of the motor cortex, J. Neutol. Nenrosurg. Psychiat., 54: 618-623.
Corticospinal conduction studied with magnetic double stimulation in the intact human D e t l e f Claus a, M a r i a W e i s a, U w e J a h n k e a, A n d r e a s P l e w e b a n d C h r i s t o p h Brunh61zl ~ a Depa'tment of Neurology, University Erlangen-Niirnberg, D-8520 Erlangen, Germany, and b Department of Neurology, UniversityJena, 06900 Jena, Germany
(Received 2 December, 1991) (Revised, received 23 March, 1992) (Accepted 30 March, 1992) Key words: Central motor conduction; Double stimulation; Magnetic stimulation; Silent period
Summary A group of 8 healthy normal subjects (24-36 years old, mean age 29 years) were investigated. Transcranial magnetic double stimulation of the motor cortex was carried out at different interstimulus intervals. With both stimuli suprathreshold, an attenuation of the test response was found at interstimulus intervals of less than 200 msec (target relaxed or contracted). The manifestation of this attenuation correlated with central signs in 31 patients with multiple sclerosis. This phenomenon is (at least at longer intervals) probably not a result of the refractory spinal motoneuron pool, but of a supraspinal inhibitory mechanism or lack of corticospinal drive caused otherwise. At interstimulus intervals between 10 and 30 msec, the test response increases significantly (magnetic double stimulation 10% suprathreshold, target relaxed). This result is also seen with voluntary muscle contraction and with vibration applied to a relaxed target muscle. The facilitatory effect is probably caused by slowly conducted corticospinal volleys enabling summation, with descending impulses generated by the test stimulus. With the conditioning stimulus subthreshold and target muscle relaxed an intracortical inhibition of the test response could be confirmed at short interstimulus intervals.
Introduction Transcranial magnetic stimulation excites presynaptic neural structures in the motor cortex (Day et al. 1989a; Thompson et al. 1991) and generates descending corticospinal volleys of mainly indirect waves (Iwaves). In addition pyramidal axons may be directly stimulated and D-waves generated (Amassian et al. 1990; Berardelli et al. 1990). This method has yielded information on the functioning of the central motor system. Repetitive transcranial electrical stimulation was used to investigate stimulus summation at the spinal motoneuron pool (Inghilleri et al. 1990) as well as intracortical inhibition phenomena (Rothwell et al. 1991). The new technique of repetitive magnetic stimulation allows further insight into central excitatory and
Corresponding to: Prof. Dr. D. Claus, Department of Neurology, University of Erlangen-Niirnberg, Schwabachanlage 6, D-8520 Erlangen, Germany. Tel.: 09131/854444; Fax: 09131/854436.
inhibitory mechanisms. The aim of this study was to investigate interaction between paired transcranial magnetic stimuli under different conditions. Does the new method provide additional data about corti,, cospinal stimulus conduction?
Materials and methods
Eight healthy normal subjects participated in the experiments (5 females, 3 males, 24-36 years of age, mean age 29 years). They gave their informed consent and the experiments were approved by the local ethics committee. Normal results were collected for magnetic double stimulation in 21 healthy subjects (10 females, 11 males, age 20-47 years, mean age 28 years). A further group of 31 patients was investigated with the same method (16 females, 15 males, age 16-56 years, mean age 33 years). They all suffered from multiple sclerosis (MS). According to the criteria of Poser (Poser et al. 1983) 8 had definite, 10 probable, and 13 possible
181 MS. Abnormal CSF (elevated 7-globulin or presence of olignclonal bands) was found in 15/31, focal lesions in brain MRI in 15/20, delayed VEPs in 7/22, and extensor plantar responses in 6/31 cases. The patients were not disabled and only 10 had weakness or spasticity in either hand (14 sites). Two Magstim 200 magnetic stimulators were connected to a Bistim device (The Magstim Company, Whitland, Dyfed SA34 OHR, England), discharging into a circular high power coil (mean diameter 95 mm, 14 windings, Emax 108 V / m , maximum magnetic field 2.5 tesla, B° 34 kT/sec). The interstimulus interval of double stimuli could be assessed between 0 and 999 msec. According to our technical investigations, amplitude and shape of both stimuli were consistent at all interstimulus intervals down to a minimum of 1 msec, with a variability of 0.1 msec. At an interval of 0 msec (both magnetic stimulators discharging simultaneously), the amplitude varied by 13% around that of a single stimulus. The circular coil was placed fiat on the scalp with anticlockwise current direction in the coil and its center over the vertex. The coil position was then adjusted in order to generate maximum compound muscle responses (CMAP) in the right abductor digiti minimi muscle (ADM). Thereafter, the coil was fixed in this position for the duration of the experiment. The threshold for the excitation of the target muscle was determined independently for the first and second stimulus by increasing the intensity each time by 5% (given as a percentage of maximum output). The lowest intensity which gave three reproducible CMAPs (gain 0.2 m V / D ) was defined as the threshold (Claus 1990). The average threshold stimuli are 50% with ADM relaxed (relaxed threshold) and 39% with ADM contracted (contracted threshold). Responses were recorded from the right ADM by surface cup electrodes (7 mm diameter), amplified (gain 0.5-5 m V / D , bandpass 20 Hz to 5 kHz ( - 3 dB), sweep time 50 or 100 msec, sample rate at least 100 ~sec/point) and stored digitally for further measurements (Medelec MS20, Datamed program) (Claus et al. 1990). Electrodes were filled with jelly and adapted by the tendon belly method. Amplitude and area of the first negative potential component were measured. At least 3 trials were recorded for each condition and results were averaged. Further postprocessing with rectification or subtraction of traces was not carried out because of the variable shape of CMAPs. The compound muscle response following stimulus 1 (conditioning stimulus) we called CMAP1 and that after stimulus 2 (test stimulus) CMAP2. In cases of CMAPs not being clearly separated from one another (i.e., at A t < 5 msec), CMAP1 but not CMAP2 results were measured. The subject sat comfortably in an armchair with the right arm splinted. When the target muscle was re-
laxed, relaxation was monitored by EMG surface recording (gain 0.2 m V / D ) via a loudspeaker. When the target muscle was exerting 10% isometric contraction, muscle strength was recorded by a miniature load cell (Kyowa Electronic Instruments Co. Ltd., Tokyo, Japan) and shown to the subject on an oscilloscope. Statistical analysis was carried out using the programmes Statview (Abacus Concepts, Inc., Berkeley, CA 94704) and Systat (SYSTAT, Inc., Evanston, IL 60201-3793). The correlation between interstimulus intervals and results was investigated using Spearman's rank correlation test. These results were confirmed upon investigation of the significance of differences by administering the Wilcoxon test at certain intervals.
Experimental settings All experiments were performed in random order. Interstimulus intervals were randomly adjusted. (1) Muscle responses were recorded from the right ADM after peripheral supramaximal electrical double stimulation of the ulnar nerve at the wrist (square wave 0.2 msec). Amplitudes (baseline to peak) and areas of CMAPs were measured in order to assess the interstimulus interval at which CMAP2 decreased. (2) The attenuation of CMAPs after transcranial magnetic double stimulation was measured in the right ADM (10% contraction), magnetic stimuli both 10% suprathreshold (contracted threshold). Interstimulus intervals: 330, 200, 150, 120, 100, 90, 80, 70, 60, 50 reset. The duration of the silent period was measured after single transcranial stimulation (10% suprathreshold) with ADM contracted. (3) With ADM relaxed both magnetic stimuli were 10% above relaxed threshold, and the interstimulus intervals varied between 330 and 0 msec (330, 200, 150, 120, 100, 90, 80, 70, 60, 50, 40, 35, 30, 25, 20, 18, 16, 14, 12, 10, 5, 4, 3, 2, 0 msec). The experiment was also carried out with continuous vibration (sinusoidal stimulus 100 Hz, 0.1 nun) applied to the relaxed ADM (Vibrameter, SOMEDIC AB, Stockholm, Sweden). The same experiment was repeated with ADM voluntarily contracted (10%) in order to evaluate the influence of muscle contraction on ~ e results. (4) With ADM relaxed, the test stimulus was 10% above relaxed threshold; the conditioning stimulus, however, was 5% below relaxed threshold; interstimulus intervals varied between 30 and 0 msec (5 subjects). The experiment was carried out to investigate whether or not the strength of the conditioning stimulus has an influence on CMAP2 after the test stimulus. (5) To further investigate the direct influence of the conditioning stimulus on both the attenuation and the enlargement of CMAP2, its strength was altered. At an
182 interstimulus interval where CMAP2 was already attenuated by > 50% (double stimulus, both 10% above contracted threshold, At 50-90 msec), the strength of the conditioning stimulus was reduced each time by 5%. ADM was 10% contracted. A similar experiment was carried out on 3 subjects at an interstimulus interval of 25 msec and with both stimuli 10% above relaxed threshold, ADM relaxed. The conditioning stimulus was then attenuated each time by 5% down to zero. (6) In 21 normal subjects relative area and amplitude of CMAP2 were measured to both hands (42 sites) with ADM contracted after suprathreshold tran-
I
scranial magnetic double stimulation (10% > contracted threshold) at an interval of 200 msec (normal results). The same technique was used in the 31 MS patients. In these patients CMCT was also investigated to both hands applying the usual technique as described elsewhere (Claus 1990).
Results
(1) When the peripheral nerve is stimulated electrically and supramaximally with double stimuli (4 subjects), then CMAP2 starts to attenuate at an interstim-
El. double slim. Ulnar nerve
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interstimulus interval (ms) Fig. l. Amplitudes and areas of CMAPs recorded from the right ADM after supramaximalelectrical stimulation of the ulnar nerve at tfi~ wrist (4 subjects, mean of 3 trials each). A relevant attenuation of CMAP2 is seen at intervals less than 5 msec (0 msec = single stimulation).
183 ulus interval of 5 msec (mean area 84% and amplitude 90% of that of CMAP1). The attenuation of CMAP2 becomes relevant at an interstimulus interval of less than 5 msec (At 2 msec: CMAP2 area 22%, amplitude 23% of CMAP1). Therefore, the attenuation of CMAP2 after double stimulation of the motor cortex at At < 5 msec could be due to refractory neuromuscular transmission. No difference was established in the results between amplitudes and areas (Fig. 1). Thus, in the following paper only amplitudes will be discussed. The mean amplitude of CMAPs after peripheral nerve stimulation is 9.0 + 1.9 mV. After transcranial magnetic stimulation (10% > relaxed threshold) the mean amplitude in the relaxed ADM is 8% of that after peripheral nerve stimulation, thus indicating that only 8% of the motoneuron pool is excited. The amplitude of CMAPs recorded from the voluntarily contracted ADM after magnetic brain stimulation (10% > contracted threshold) is, however, 42% of that after peripheral stimuli. (2) After magnetic double stimulation of the motor cortex and with the ADM contracted, C AP2 attenuates at interstimulus intervals of less than 200 msec
5
(Spearman p < 0.001). The average value of the amplitude diminishes in comparison to that of CMAP1 (At 200 msec, 94%; 150 msec, 83%; 100 msec, 42%; 80 msec, 32%; 50 msec, 43%). When magnetic transcranial single stimuli are applied with the target muscle contracted, muscle activity is interrupted for an average of 192 + 68 msec (latency from CMAP to the beginning of voluntary muscle activity). This time is comparable with the interstimulus interval at which CMAP2 becomes attenuated. Due to the relatively large differences between interstimulus intervals from 100 to 330 msec, it was not possible to investigate the correlation between the silent period and the beginning of the attenuation of CMAP2. (3) CMAP2 after magnetic brain stimulation (both stimuli 10% > relaxed threshold) begins to attenuate at an interstimulus interval < 200 msec (P___<0.01), regardless of whether the ADM is relaxed or contracted (Fig. 2). This phenomenon is, however, more pronounced when the ADM is contracted. At interstimulus intervals of between 10 and 30 msec - ADM relaxed - CMAP2 enlarges significantly, and becomes even larger than CMAP1 ( P < 0.01). This
8 subjects, r. ADM relaxed, transcranial double stim., [ [ _ _ _ _ both 10% suprathresheld ]
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Fig. 2. Mean + SD of CMAP amplitudes recorded from the right ADM after magnetic double stimulation of the motor cortex. At interstimulus intervals < 200 msec CMAP2 starts to attenuate, no matter whether the target muscle is relaxed or contracted. At shorter intervals (10-30 msec) CMAP2 enlarges significantly.
184 A.J. (30 yrs.) Double sttm~ Cx, ! O~ suprathreshold r. ADH relaxed
A.J. (30 yrs.) Double stim. Cx, I subthr. (35~), 2 suprathr. (55R) r. ADM relaxed
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mYI !0 ms Fig. 3. Exampleof CMAPsrecordedafter magneticbrain stimulation with ADM relaxed. At interstimulus intervals of 20 and 30 msec CMAP2is significantlyenlarged. phenomenon is less pronounced when the ADM is contracted (Fig. 2, lower trace). In this case, however, CMAP2 reaches the amplitude of CMAP1 at interstimulus intervals of between 10 and 30 msec. Fig. 3 depicts an example of the enlargement of CMAP2 with the target muscle relaxed. Vibration applied to the relaxed target muscle does not cause any alteration to this result. At interstimulus intervals _~ 10 msec, CMAP2 is not always clearly separated from CMAP1. Therefore, it cannot be measured separately. (4) When the first stimulus is subthreshold (ADM relaxed), then the influence of interstimulus intervals on the muscle response after transcranial stimulation is completely different. In this case, the CMAP attenuates significantly at intervals of between 2 and 4 msec (P = 0.01) with a minimum at At 3 msec (attenuation to 25%). An example of this can be found in Fig. 4. At an interval of 0 msec (single stimulus), the amplitude recovers to approximately its reference value (110%). (5) The influence of the conditioning stimulus on CMAP2 after the test stimulus depends on various conditions such as interstimulus interval and muscle contraction. In order to determine whether its strength is significant for CMAP2, the conditioning stimulus was gradually reduced with ADM contracted and at an interstimulus interval where the second potential was
lOres Fig. 4. Example of the recording after cortical magnetic double stimulationwith a subthresholdconditioningstimulus.At short interstimulusintervalsCMAP2is significantlyattenuated. clearly attenuated. This attenuation ceased and CMAP2 enlarged when the first stimulus became subthreshold (Table 1). When the interstimulus interval is 25 msec, then the influence of the attenuation of the conditioning stimulus on the muscle response following a consistent test stimulus is completely different, At this interval, CMAP2 is enlarged (stimulus 1 and 2 10% > relaxed threshold, ADM relaxed). When the first stimulus becomes weaker, CMAP2 attenuates (3 subjects, Fig. 5). TABLE 1 CMAPs RECORDED FROM THE CONTRACTED ADM AFTIERMAGNETICDOUBLE STIMULATIONOF THE CORTEX The interstimulusintervalis such that CMAP2 is attenuated(50-90 msec). When the conditioningstimulus becomes weaker CMAPl attenuates and CMAP2 enlarges. This result proves an inhibitory influenceof the conditioningstimuluson CMAP2. Magneticstimulus 1 %± threshold AmplitudeCMAPl (mean+ SD) +10% +5% 0% -5%
3.0±1.2 1.5±1.1 1.4±0.9 0.4±0.3
8 subjects,ADM 10% contraction.
Stimulus2 Always10% > threshold CMAP2(mV) (mean± SD) 1.4±0.8 1.7±1~ 2.3±1~ 3.4±1.7
185 3 subjects, ADrl relaxed, double sum. (intervals 25 ms), stlm.2:1011 suprethreshol(I, (mean or 3 trtals each)
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Fig, 5. Magnetic double stimulation o f the motor corte~y;~,3 ~ubjects ( A D M relaxed). When the conditioning stimulus attenuates well below motor threshold C M A P 2 becomes smaller. This result proves a facilitatory influence o f the conditioning stimulus (even subthreshold) on the C M A P generated by the test stimulus.
Thus, despite the relatively long interstimulus interval of 25 msec, the conditioning stimulus is believed to have a facilitatory influence on the response succeeding the test stimulus. (6) In the normal group the area/amplitude of CMAP2 was at least 60% of CMAP1 (1 percentile; double stimulation, At 200 msec, target contracted). The CMCT was prolonged > 8 msec (normal results in Claus 1990) in 7/31 MS patients (8/62 sites). An abnormal attenuation of CMAP2 after double stimulation was seen in 10/31 patients (3/8 definite; 3/10 probable; 4/13 possible MS; 11/62 sites). There was a significant correlation between spasticity/centrai weakness in the hand and attenuation of CMAP2 ( P < 0.05). Despite a correlation with clinical signs of central motor disturbance, 7 of the hands with prolonged CMCT and 6 with attenuated CMAP2 were clinically unimpaired.
Discussion
Attenuation of the second muscle response After suprathreshold magnetic brain stimulation, CMAP2 attenuates when the interstimulus interval becomes shorter than 200 msec. The degree of attenuation is altered by muscle contraction but its duration is not (experiments 2 and 3; Fig. 2). After transcranial electrical stimulation complete recovery of the second muscle response was seen at an interval of 150 msec with the target relaxed and at 100 msec with the target contracted (Inghilleri et al. 1990). These intervals are slightly shorter than those following magnetic double stimulation. According to animal experiments by Phillips and Porter (1964) corticospinal neurons can follow a frequency of up to 200 Hz. In humans, the full recovery
cycle of D-waves after electrical brain stimulation is 3.5 msec ( > 280 Hz) (Inghilleri et al. 1989). Since D and I intervals are even shorter, corticospinal tracts are likely to have a shorter recovery cycle than cortical neurons. This recovery cycle is compatible with direct excitation of corticospinal neurons by electrical stimuli. After magnetic stimulation, it may last longer due to its mainly presynaptic site of action. Nevertheless, refractority of cortical or corticospinal neurons is unlikely to explain an attenuation of the second response at an interstimulus interval of up to 200 m~.c. Thus an electrical stimulus delivered 50 msec after a magnetic cortical stimulus was able to produce a CMAP (Day et al. 19891)). This result demonstrates that the spinal motoneuron pool and the corticospinal axons are no longer refractory after a delay of 50 msec. Inghilleri and coworkers found that after electrical stimulation, the greater the amplitude of the first muscle response the smaller that of the second (Inghilleri et al. 1990). They concluded that the first action potential causes the motoneuron pool to be partially refractory. After cortex stimulation CMAPs are generally smaller than after peripheral nerve stimulation, indicating that fewer motoneurons fire (experiment 1). Thus only 8% (relaxed) or 42% (contracted) of the motoneuron pool can be refractory after the conditioning stimulus. The second response enlarges when the conditioning stimulus becomes weaker (experiment 5, Table 1). Therefore, the conditioning stimulus is likely to exert an inhibitory influence at a supraspinal or spinal level. According to single unit recordings after magnetic brain stimulation (Boniface et al. 1991) motoneurons are ~ r a c t o r y for about 20 msec. At short interstimulus interval~, double firing of motoneurons is therefore relatively unlikely (Day et al. 198%). One may thus conclude that, at least at short intervals, the test stimulus excites preferentially spinal motoneurons which had not already been brought to firing threshold by the conditioning stimulus. Therefore, as opposed to peripheral nerve stimulation, no recovery cycle is investigated after transcranial double stimulation. Different explanations are discussed for the silent period after peripheral nerve stimulation. These include afterh~erpolarisation of spinal motoneurons, recurrent Renshaw inhibition, inhibition by flexor reflex afferents and Ia mediated disynaptic inhibition (Merton 1951; Granit 1955; Hultborn et al. 1979; Rothwell 1987; Kudina and Pantseva 1988). The silent period after transcranial stimulation can be seen without a CMAP (Marsden et al. 1983; Calancie et ai. 1987) and it is prolonged with increasing stimulus strength (Holmgren et al. 1990). The H-reflex is depressed only at the beginning of the silent period (Fuhr et al. 1991). It is therefore believed that at least in the late part of the silent period after cortical stimulation, a reduction in the excitability of the spinal motoneuron pool plays
186 only a minor role in determining the phenomenon. It is probably caused by lack of corticospinal drive. Intracortical inhibitory mechanisms may also play a role (Fuhr et ai. 1991). The duration of the silent period (experiment 2) and the interstimulus interval at which C AP2 is significantly attenuated are comparable with each other. This result would support the idea that a similar physiological mechanism is responsible for both the silent period and attenuation of CMAP2 after magnetic double stimuli. Spinal mechanisms may be partially responsible for the attenuation of the second response at short interstimulus intervals. They are however unlikely to act for as long as 200 msec. According to our results after peripheral nerve stimulation (Fig. 1), neuromuscular transmission cannot be responsible for an attenuation of CMAP2 after cortex double stimulation at relatively long interstimulus intervals. In 31 patients who had MS with only slight signs and symptoms, results after double stimulation were at least as sensitive as CMCT (experiment 6). A correlation was found between an attenuated CMAP2 and central signs in the investigated hand. Therefore, the attenuation of CMAP2 after magnetic double stimulation of the motor cortex may reflect disturbed corticospinal stimulus conduction or intracortical excitability. This result could also support the idea that a central mechanism is responsible for the attenuated second response after magnetic double stimulation at intervals shorter than 200 reset. The attenuated CMAP2 is, at least at interstimulus intervals longer than 50 msec, probably not due to refractority of the spinal motoneuron pool. Refractority of corticospinal neurons is also an unlikely cause. Supraspinal mechanisms seem to be responsible. The attenuation of CMAP2 is due tc lack of corticospinal drive, probably as a result of an intracortical inhibitory mechanism. Whether this intracortical phenomenon is presynaptic or postsynaptic cannot be conclusively decided upon since a similar attenuation has also been described after electrical transcranial stimulation (InghUleri et al. 1990). A corticospinal inhibitory input (Lemon et al. 1987) could also provide an explanation of the phenomenon.
Enlargement of the second muscle response The enlargement of CMAP2 after magnetic double stimulation at interstimulus intervals between 10 and 30 msec is significant. The facilitation is less pronounced when the target is contracted than when it is relaxed (experiment 3). This is probably due to the fact that more motoneurons were brought to firing threshold by the conditioning stimulus with the muscle contracted. As mentioned above the motoneurons remain refractory for some time and CMAP2 therefore becomes smaller with ADM contracted (Fig. 2).
However, there seems to exist a strong facilitatory influence of the conditioning stimulus on CMAP2. This is particu.larly obvious at interstimulus intervals between 10 and 30 msec (Fig. 3), which exceed the corticospinal volley duration of about 10 msec (Landgren et al. 1962; Phillips and Porter 1964; Clough et al. 1968; Amassian et al. 1990; Thompson et al. 1991). Due to these relatively long intervals, temporal summation of fast conducted I-waves generated by the conditioning stimulus and the test stimulus is unlikely at the motoneuron. This is in accordance with earlier results showing that summation between an afferent volley after mechanical muscle stimulation and a corticospinal volley lasts no longer than 14 msec (Claus et al. 1988). Furthermore, after double stimulation of the cervical cord, no relevant temporal summation is seen at the spinal motoneuron pool at interstimulus intervals of as long as 10 msec (Claus et al. 1991). After electrical transcranial stimulation a first short-latency facilitation of the H-reflex was followed by a second facilitation which lasted between 5 and 20 msec (Cowan et al. 1986). This was believed to be due to I-waves, activation of smaller corticospinal neurons or activity in spinal interneuronal networks. In human subjects transcranial stimuli above the motor threshold applied to the motor cortex are likely to effectively access fusi-motoneurons (Burke et al. 1990). This is thought to be mediated mainly by /3motoneurons, not ,y-motoneurons (Rothwell et al. 1990). Activation of muscle spindles by the conditioning stimulus could cause summation of primary spindle afferents with descending volleys generated by the test stimulus. This would however be extinguished by voluntary muscle contraction due to a-~,-coactivation. Furthermore, vibration applied to the relaxed ADM excites muscle spindles and augments the muscle response to cortical stimuli (Claus et al. 1988). The facilitatory effect of the test stimulus is much stronger than that of muscle vibration. The increase of the second response remains unaltered by vibration. The lack of influence of muscle vibration is an argument against the importance of spindle activation for the enlargement of CMAP2. The enlargement of CMAP2 at intervals between 10-30 msec is believed to be an effect of the conditioning stimulus since it disappears when this stimulus attenuates (Fig, 5). The facilitation is seen with a conditioning stimulus in the region of the motor threshold - or just below of it. The facilitatory influence of the conditioning stimulus is likely to be mediated by slow conducting corticospinal neurons (Lassek and Evans 1945). This idea is supported by the influence of stimulus intensity on the result, as demonstrated in Fig. 5. This strong facilitatory influence is able to Overcome the attenuation of the second response seen after paired transcranial shocks, as well as
187 the mechanisms responsible for H-reflex refractority (Fuhr et al. 1991). The lack of a facilitatory effect after electrical double stimuli at short intervals (Inghilleri et al. 1990) would support the hypothesis of an intracortical mechanism such as synchronization of pyramidal cells. This is, however, discordant to the late facilitation of the H-reflex by electrical cortical stimuli (Cowan et al. 1986). As opposed to electrical stimulation (Inghilleri et al. 1990) no significant enlargement of the CMAP was seen after magnetic stimulation with short intervals of 2-5 msec (both stimuli 10% > relaxed threshold, target relaxed). This is thought to be due to the fact that magnetic stimuli act preferentially on presynaptic structures in the motor cortex. Therefore, it is mainly I-waves and not D-waves which are generated with suprathreshold stimulation alone. Two distinct volleys are evoked only by paired electrical shocks at intervals of 1 msec or more (Inghilleri et al. 1989). The descending volleys may produce summation at motoneurons. This is probably not the case with mainly presynaptically acting magnetic stimuli (Day et al. 1989a). Thus, the lack of an enlargement of the CMAP at short interstimulus intervals could prove the presynaptic action of magnetic cortex stimuli. The slight enlargement of CMAP1 at an interstimulus interval of 0 msec (both magnetic stimulators discharging simultaneously, Fig. 2) could easily be a result of the variability of the magnetic stimulus mentioned above.
lntracortical inhibition At short interstimulus intervals a subthreshold conditioning stimulus causes attenuation of the muscle response (experiment 4, Fig. 4). This result was also described by Rothwell and collaborators (Rothwell et al. 1991). Subthreshold transcranial shocks could cause disynaptic corticospinal inhibition. The fact, however, that the attenuation of CMAP2 at short interstimulus intervals is not seen after electrical brain stimulation is a strong argument in favour of an intracortical inhibitory mechanism in the motor strip itself or in surrounding areas of the cortex (Rothwell et al. 1991) and against disynaptic corticospinal inhibition. This inhibition occu~ despite the fact that subthreshold electrical scalp stimuli can evoke descending volleys in relaxed subjects and cause facilitation of the H-reflex (Cowan et al. 1986). The phenomenon of intracorticai inhibition could well be of clinical diagnostic importance.
Conclusions The noninvasive technique of transcranial magnetic double stimulation provides new information about
inhibitory and facilitatory mechanisms in the central motor system. This has implications for the investigation of fatigue in central pareses as well as lack of intracortical inhibition in central hyperexcitability. Facilitation phenomena may offer information about the dispersion of corticospinal stimulus conduction in central motor disorders. It may also be of diagnostic use in movement disorders. Acknowledgement This work was supported by the Wilhelm Sander Foundation.
References Amassian, V.E., O.J. Quirk and M. Stewart (1990) A comparison of corticospinal activation by magnetic coil and electrical stimulation of monkey motor cortex, Electsoenceph. Clin. Neurophysiol., 77: 390-401. Bcrardelli, A., M. Inghilleri, G. Cruccu and M. Manfredi (1990) Descending volley after electrical and magnetic transcranial stimulation in man, Neurosci. l.~tt., 112: 54-58. Boniface, SJ., K.R. Mills and M. Schubert (1991) Responses of single spinal motoneurons to magnetic brain stimulation in healthy subjects and patients with multiple sclerosis, Brain, 114: 643-662. Burke, D., S.C. Gandevia and J.C. Rothwell (1990) Activation of presumed fusi-motoneurones by motor conical stimulation in human subjects, L Physiol., 425: 85P. Calancie, B., M. Nordin, U. Wallin and K.E. Hagbarth (1987) Motor-unit responses in human wrist flexor and extensor muscles to transcranial cortical stimulation, J. Neurophysiol., 58: 11681185. Claus, D. (1990) Central motor conduction: method and normal results, Muscle Nerve, 13: 1125-1132. Claus, D., K.R. Mills and N.M.F. Murray (1988) Facilitation of muscle responses to magnetic brain stimulation by mechanical stimuli in man, Exp. Brain Res., 71: 273-278. Claus, D., N.M.F. Murray, A. Spitzer and D. Fliigel (1990) The influence of stimulus type on the magnetic excitation of nerve structures, Electroenceph. Clin. Neurophysiol., 75: 342-349. Claus, D., M. Weis and A. Spitzer (1991) Motor potentials evoked in tibialis anterior by single and paired cervical stimuli in man, Neurosci. Lctt., 125: 198-200. Clough, J.F.M., D. Kernell and C.G. Phillips (1968) Monosyaaptic
excitation from the pyramidaltract and from primary spindle afferents to motoneurones of the baboon's hand and forearm, J. Physiol., 198: 145-166. Cowan, J.M.A., B.L. Day, C. Marsden and J.C. Rothwell (1986) The effect of percotaneous motor cortex stimulation on H reflexes in muscles of the arm and leg in intact man, J. Physiol., 377: 333-347. Day, B.L., D. Dressier, A. Maertens de Noorhout, C.D. Marsden, K. Nakashima, J.C. Rothwell and P.D. Thompson (1989a) Electric and magnetic stimulation of human motor cortex: surface EMG and single motor unit resoonses, J. Physiol., 412: 449-473. Day, B.L.; J.C. Rothwell, P.D. Thompson, A. Maertens de Noordbout, FL Nakashima, FL Shanon and C.D. Marsden (1989b) Delay in the execution of voluntary movement by electrical or magnetic brain stimulation in intact man, Brain, 112: 649-663. Fuhr, P., R. Agostino and M. Hallett (1991) Spinal motor neuron excitability during the silent period after cortical stimulation, Electroenceph. Clin. Neurophysiol., 81: 257-262. Granit, R. (1955) Receptors and Sensory Perception, Vail-Ballou Press, Binghamton, NY, Yale University Press, pp. 210-216.
188 Holmgren, H., L.E. Larsson and S. Pedersen (1990) Late muscular responses to transcranial cortical stimulation in man, Electroenceph. Clin. Neutophysiol., 75: 161-172. Hultboto, H., E. Piertot-Deseilligny and H. Wigstrom (1979) Recurrent inhibition and afterhyperpolarization following motoneutonal discharge in the cat, J. Physiol., 297: 253-266. Inghilleri, M., A. Berardelli, G. Ctoccu, A. Priori and M. ManfTedi (1989) Corticospinal potentials after transcranial stimulation in humans, J. Neutol. Neutosurg. Psychiat., 52: 970-974. Inghilleri, M., A. Berardelli, G. Crucu, A. Priori and M. Manfredi (1990) Motor potentials evoked by paired cortical stimuli, Electtoenceph. Clin. Neurophysiol., 77: 382-389. Kudina, L.P. and R.E. Pantseva (1988) Recurrent inhibition of firing motonentones in man, Electroenceph. Ciin. Neurophysiol., 69: 179-185. Landgren, S., C.G. Phillips and R. Porter (1962) Minimal synaptic actions of pyramidal impulses on some alpha motoneurones of the baboon's hand and forearm, J. Physiol., 161: 91-111. Lassek, A.M. and J.P. Evans (1945) The human pyramidal tract, J. Comp. Neutol., 83: 113-126. Lemon, R.N., R.B. Muir and G.W.H. Mantel (1987) The effects upon the activity of hand and forearm muscles of intracortical stimulation in the vicinity of corticomotor neurones in the conscions monkey, Exp. Brain Res., 66: 621-637. Marsden, C.D., P.A. Merton and H.B. Morton (1983) Direct electrical stimulation of the corticospinal pathways through the intact scalp in human subjects. In: Desmedt, J.E. (Ed.), Motor Control
Mechanisms in Health and Disease, 39, Raven Press, New York, NY, pp. 387-391. Merton, P.A. (1951) The silent period in a muscle of the human hand, J. Physiol., 114: 183-198. Phillips, C.G. and R. Porter (1964) The pyramidal projection to motoneutones of some muscle groups of the baboon's forelimb. In: Eccles, J.C. and Schad~, J.P. (Eds.), Physiology of Spinal Neurons, Progress in Brain Research, vol. 12, Elsevier, Amsterdam, pp. 222-242. Poser, C.M., D.W. Paty, L. Scheinberg, W.L McDonald, F.A. Davis, G.C. Ebers, K.P. Johnson, W.A. Sibley, D.H. Silberberg and W.W. Tourtellotte (1983) New diagnostic criteria for multiple sclerosis: guidelines for research protocols, Ann. Neutol., 13: 227-231. Rotbvlell, J.C. (1~87) C~nttol" of Human Voluntary Movement, Croom Helm, London, Sydney, pp. 105-157. Rothwell, J., A. Ferbert, M. Caramia, T. Kujirai, Yamagata, B. Day and P. Thompson (1991) lntracortical inh~itory circuits studied in humans, Neurology, 41 (Suppl. 1): 192. Rothwell, J.C., S.C. Gandevia and D. Burke (1990) Ac~vation of fusimotor neurones by motor cortical stimulation in human subjects, J. Physiol., 431: 743-756. Thompson, P.D., B.L. Day, H.A. Crockard, I. Calder, N.M.F. Murray, J.C. Rothwell and C.D. Marsden (1991) Intra-operative recording of motor tract potentials at the cervico-medullaryjunction following scalp electrical and magnetic stimulation of the motor cortex, J. Neutol. Nenrosurg. Psychiat., 54: 618-623.