Journal of Physiology (1991), 433, pp. 41-57 With 8 figures Printed in Great Britain

41

CHANGES IN THE RESPONSE TO MAGNETIC AND ELECTRICAL STIMULATION OF THE MOTOR CORTEX FOLLOWING MUSCLE STRETCH IN MAN

BY B. L. DAY*, H. RIESCHERt, A. STRUPPLERt, J. C. ROTHWELL* AND C. D. MARSDEN* From the *MRC Human Movement and Balance Unit, National Hospital, Queen Square, London WC1N 3BG and the tNeurologische Klinik und Poliklinik der Technischen Universitdt Miinchen, Mohlstrasse 28, 8000 Miinchen 80, Germany

(Received 3 August 1990) SUMMARY

1. The effect of muscle stretch on the EMG response from the stretched muscle to transcranial magnetic stimulation of the motor cortex was studied in eight subjects. Muscle stretch was produced by increasing the torque of a motor acting through a lever which was held at constant position by a flexion force of the index and middle fingers. EMG responses were recorded from fine-wire electrodes inserted into flexor digitorum profundus muscle in the forearm. They consisted of a spinal latency component and a long-latency component which could in some subjects be separated into an early and a late phase. 2. In four subjects, four intervals between the stretch and the cortical stimulus were explored using three intensities of cortical stimulation. At all three intensities, when the magnetic cortical stimulus was timed to produce an EMG response in the period of the later part of the long-latency stretch reflex the response was larger than when it was timed to produce a response in the period of the short-latency spinal reflex or when superimposed on the tonic muscle activity used to resist the standing torque of the motor. 3. When the intensity of magnetic cortical stimulation was reduced so that it was just below threshold to produce an EMG response in the short-latency reflex period or on the background tonic EMG activity, it still was capable of producing a response when superimposed on the long-latency stretch reflex. 4. In four subjects the time course of this effect was studied in more detail using only one intensity of magnetic cortical stimulus set to be just above threshold to produce a response in tonically active muscles. The time course of the facilitatory effect was similar to the time course of the later part of the long-latency stretch reflex. From these data it was not possible to determine whether the early part of the long-latency stretch reflex also was accompanied by the facilitatory effect since this component was present in only one of the four subjects. 5. The facilitatory effect persisted after the ulnar and median nerves were totally blocked at the wrist by injections of local anaesthetic. This suggests that inputs from muscle receptors of the stretched muscle contribute to the effect. MS 8012

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6. In a final series of experiments the facilitatory effect of muscle stretch on the response to magnetic cortical stimulation was compared with its effect on the response to electrical cortical stimulation. For this experiment the ring finger was used to resist the motor torque and EMG responses were recorded from surface electrodes placed over the ulnar flexor compartment of the forearm. As before, the EMG response to magnetic cortical stimulation was facilitated by an appropriately timed stretch. In contrast, with electrical cortical stimulation at intensities just above motor threshold for active muscles, the stretch stimulus was found to have no significant extra facilitatory effect on the size of the evoked muscle response. 7. These results are most readily explained by the stretch stimulus causing an increase in excitability of the motor cortex so that the corticospinal volleys produced by a transcranial magnetic cortical stimulus are larger than without the stretch. The absence of this facilitatory effect of stretch on the response to electrical stimulation may be explained by the different way that this stimulus activates the corticospinal neurones compared with the magnetic method. The time course of the facilitatory effect is compatible with the transcortical hypothesis of the long-latency stretch reflex. INTRODUCTION

Experiments on non-human primates have demonstrated that muscle stretch is a potent stimulus for changing neuronal activity in the motor cortex at short latency (Evarts, 1973; Evarts & Tanji, 1976; Tatton, Bawa, Bruce & Lee, 1978; Wolpaw, 1980; Cheney & Fetz, 1984). In man, it is not known whether the same pathway exists although the long-latency component of the human stretch reflex (LLSR) has been suggested to stem from activity in such a system, particularly for muscles controlling the hand (Marsden, Merton & Morton, 1972, 1973). According to this hypothesis, an increase in motor cortical activity following muscle stretch might be accompanied by a change in excitability of the motor cortex to a second test stimulus. The aim of the present experiments was to test this prediction. Both electrical and magnetic stimuli are capable of activating the human motor cortex through the scalp (Merton & Morton, 1980; Barker, Jalinous & Freeston, 1985) and can be used to test motor cortical excitability. In intact human subjects it is not yet possible to measure directly the response of the motor cortex to a scalp shock, but it is possible to measure the electromyographic (EMG) response evoked by the stimulus. The size of the EMG response reflects, among other things, the size of the descending volleys in the corticospinal tract. We have examined the effect of stretch upon the size of the EMG response in flexor digitorum profundus, recorded with wire electrodes, evoked by magnetic stimuli delivered via the scalp to the contralateral motor cortex. In addition, we have compared the effects of muscle stretch on the surface-recorded EMG response in the forearm flexors following electrical and magnetic cortical stimuli. Preliminary accounts of this work have been presented to the Physiological Society (Day, Riescher & Struppler, 1988b; Day, Marsden & Rothwell, 1989b). METHODS

With the approval of the local ethical committee twelve subjects (six males, six females; age range 26-32 years), with no history of neurological disease, were studied. Two subjects participated

MOTOR CORTEX STIMULATION AND MUSCLE STRETCH 43 in pilot experiments in which stimuli were not randomized and their results, although in accordance with those described below, have not been included in this paper. Technical failures excluded two other subjects. The data presented therefore represent those collected from eight subjects. Subjects were seated, their forearms semipronated resting on a table in front of them with elbows flexed to approximately 135 deg (Fig. 1). Protruding 200 mm through the table were two levers, each connected to the output shaft of a torque motor (for details see Struppler, Riescher, Lorenzen, Groter, Schaller & Chen, 1986). The levers were free to travel independently forwards and backwards over a total excursion of 18 mm. Lever movement was transduced by a linear potentiometer connected directly to the lever. The position of the levers was displayed on a CRO to the subject who attempted to hold them in a central position using the index and middle fingers. Forward motion of the hand and arm was prevented by a vertical post which stood between the thumb and index finger. Although responses from only one arm were recorded, subjects found it more comfortable to sit symmetrically holding the two levers. The subject pulled on the lever at a point approximately 0 5 m from its centre of rotation by flexing the interphalangeal joints of the index and middle fingers with a force of around 5 % of maximum. The direction of the motor torque acted to extend these joints. Muscle stretch was achieved by increasing the motor torque from 0 5 to 1-5 N m for a period of 1 s. This increase in torque produced a stretch velocity at the fingertips of around 0-25 m/s. Subjects were instructed to not react to the stretch stimulus but to maintain a constant effort to keep the levers central. The electromyogram (EMG) of the stretched muscle was recorded from two fine-wire electrodes (3 mm exposed tip) inserted into flexor digitorum profundus (FDP) in one forearm. The EMG was amplified and bandpass filtered (30 Hz to 3 kHz) using a Nicolet Pathfinder II. The brain was stimulated using a magnetic stimulator (Novametrix Magstim 200) with a flat stimulating coil (9 cm mean diameter) centred over the vertex of the head. When studying muscles of the right arm the current in the stimulating coil flowed in an anticlockwise direction (viewed from above) so as to stimulate the left motor cortex. With this configuration, muscle responses are produced in muscles of the right arm and not the left provided low-intensity

stimuli near to motor threshold are used. Clockwise current was used when the left arm was studied. Initially the subject held the lever in its central position against the extending force offered by the standing torque of the motor. The lowest threshold scalp stimulating site was found by moving the coil in the anteroposterior direction on the head until the largest EMG response was obtained. The coil was then supported in a clamp and taped to the head of the subject in order to minimize relative motion between the head and coil. The motor threshold magnetic stimulus intensity was determined by the response in the averaged rectified EMG record constructed from sixteen trials at each stimulus intensity. This method was chosen since it was not possible to establish with certainty from an individual trial whether a small EMG response was present because of the high noise level produced by the voluntary contraction. The order of presentation of stimuli was randomized with equal probability of issuing either a stretch stimulus or a magnetic brain stimulus or both stimuli combined. When both stimuli were combined, four intervals between the stretch and the brain stimulus were intermixed randomly (the interval between trials varied from 4 to 6 s). A block was complete when each stimulus condition was applied at least sixteen times. In the first experiment four intervals between the stretch and brain stimulus were studied with three intensities of magnetic stimulation. The lowest intensity was set at around motor threshold and was subsequently increased in steps of 5 % maximum output of the stimulator. In the second experiment the magnetic stimulus intensity was set at around motor threshold throughout and each block of trials explored different intervals between stretch and brain stimuli. For each block, however, one of the intervals was set to produce a response to the magnetic brain stimulus coincident with the short-latency spinal stretch reflex (SLSR). In one subject the experiment was repeated after afferent input from cutaneous, joint and muscle receptors in the hand was abolished by anaesthetizing the median and ulnar nerves at the wrist using 10 ml of 1 % lignocaine injected near each nerve following electrical localization. EMGs, lever position and stimulus condition information were stored on-line using a Teac FM tape-recorder (frequency response: DC-3 kHz). These data were sorted, digitized and averaged offline.

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Comparison of electrical with magnetic cortical stimulation Three subjects, including two of the authors, were studied. The experimental set-up differed slightly from that described above. The subject was seated with the right arm supinated and resting on a table. The arm was clamped to prevent movement. The output shaft of the torque Magnetic stimulus

Fig. 1. Experimental arrangement used for studying the effect of muscle stretch on the response to magnetic stimulation of the motor cortex. motor (Printed Motors type G9M4H) was horizontal and concentric with the proximal interphalangeal joint of the ring finger. The ring finger was flexed against a 15 mm cranked lever attached to the motor shaft which offered a constant extending torque. Muscle stretch was obtained by increasing the motor torque from 0 05 to 0X8 N m for 7 ms and then to 0X15 N m for a further 200 ms. Electrical stimulation of the brain was achieved using a Digitimer D180 stimulator via 9 mm diameter electrodes fixed to the scalp. The cathode was placed over the vertex of the head and the anode 7 cm lateral on a line joining the vertex to the left external auditory meatus. Magnetic stimulation of the brain was carried out as described above using a Novametrix Magstim 200 stimulator via a flat 9 cm (mean diameter) coil centred on the vertex of the head; when viewed from above current in the coil flowed in an anticlockwise direction. EMGs were recorded from 9 mm Ag-AgCl disc electrodes fixed to the skin of the forearm. The active electrode was placed approximately one-third of the distance from elbow to wrist over the ulnar flexor compartment of the forearm and the indifferent electrode over the medial epicondyle of the elbow. The position of the active electrode was adjusted so that it was optimally placed to pick up the maximum EMG activity when the subject flexed his ring finger against a small load. This EMG activity arose, presumably, from flexor digitorum profundus muscle although activity from other muscles, such as flexor digitorum sublimis and flexor carpi radialis, could be picked up by electrodes in this position. Therefore, for this experiment we use the general term of forearm flexor muscles to describe the source of the EMG responses that were recorded. EMGs were preamplified and bandpass filtered (80 Hz-2 5 kHz; Devices 3160), amplified (Devices 3120) and rectified. Stimuli were randomized so that the subject received with equal probability (n = 32 for each of the three conditions) either a stretch on its own or a stretch plus a magnetic cortical stimulus or

MOTOR CORTEX STIMULATION AND MUSCLE STRETCH a stretch

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plus an electrical cortical stimulus. Within one experiment, two intervals between stretch

onset and the cortical stimulus were intermixed. The intervals were such that the response to the

cortical stimulus appeared superimposed either on the SLSR or the LLSR. Since the magnetic stimulus produced a muscle response with a latency some 2 ms longer than that to an electrical stimulus (Day, Thompson, Dick, Nakashima & Marsden, 1987 b), the interval between stretch and a magnetic stimulus was set to be 2 ms shorter than the interval between stretch and an electrical stimulus. For each subject the experiment was repeated using three or four different intensities of cortical stimulation. Lever position and EMG data were digitized using a sampling frequency of 1 kHz and then averaged using a PDP12 computer.

Measurement The amplitude of the EMG response to brain stimulation was taken from the integrated rectified EMG. The integral was measured over a fixed interval of 14-30 ms following the brain stimulus. The presence of the stretch stimulus meant that the response to the brain stimulus was superimposed on a time-varying EMG level. This was taken into account by subtracting the integral of the EMG computed over the same period for the stretch stimulus on its own. Finally, the EMG difference was normalized by dividing by the integral of the background EMG (over a 16 ms period) obtained with the subject resisting the standing torque of the motor. RESULTS

Figure 2 shows two examples of the stretch reflex of FDP on extending the index and middle fingers. In eight subjects the short-latency stretch reflex (SLSR) appeared with a latency of 24 + 3 ms (mean + S.D.) after stretch onset. The shortlatency event was followed by either one or two components (Fig. 2) in the averaged rectified EMG record. These longer latency events appeared at a latency of 35 + 3 and 52 + 5 ms respectively and may correspond to the early and late components of the long-latency stretch reflex (LLSR) described for the long flexor of the human thumb (Marsden, Merton, Morton, Adam & Hallett, 1978). In the present series all subjects had clear short-latency and late components of the long-latency components of the stretch reflex but the early component of the long-latency response was often inconspicuous or absent. For this reason we have concentrated on the short-latency and the late component of the long-latency stretch reflex. Figure 3 illustrates the effect of combining the stretch stimulus with the magnetic scalp stimulus using three intensities of stimulation. Two intervals between the stretch and magnetic stimuli are shown, along with the response to magnetic scalp stimulation alone (Fig. 3 C). One interstimulus interval produced a response to brain stimulation in the middle of the SLSR (Fig. 3A) and the other produced a response in the middle of the late component of the LLSR (Fig. 3B). The response to the brain stimulus appeared as extra activity superimposed on the control stretch reflex in the rectified EMG record. The size of the extra EMG response was related to the intensity of brain stimulation and the interval between the two stimuli. When superimposed on the SLSR the extra response due to the magnetic scalp shock was not consistently different to that produced by a brain stimulus on its own. However, when superimposed on the LLSR the extra EMG response was consistently much larger. When the intensity of brain stimulation was just sub-motor threshold (i.e. at 35% of the maximum output of the stimulator; motor threshold was obtained with stimuli between 35 and 40 % of stimulator output), an extra response was seen only when it was timed to appear in the LLSR.

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The amplitude of the response was quantified by computing the integral of the rectified EMG for a fixed period after the brain stimulus (equivalent to the first 16 ms of the shaded areas shown in Fig. 3, see Methods). This was a conservative measure since it took no account of the later components of the response to cortical and

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Fig. 2. Variability of the FDP stretch reflex between subjects. Shown are traces recorded from two subjects. The top trace of each pair is the average (n = 20) rectified EMG from wire electrodes in FDP muscle and the bottom trace is the displacement of the fingers following forcible extension of the index and middle fingers. The vertical line shows the time of onset of the stretch stimulus. Either two or three components of the stretch reflex could be distinguished (see text). In this and other figures the zero EMG level is given by the origin of the upstroke at the beginning of the EMG trace.

stretch stimuli which, when present (see Fig. 3, middle panel, 45% of maximum output), were seen only when superimposed on the LLSR. Although a potentially interesting phenomenon, the extra late responses were not observed in all subjects and shall not be discussed further in this paper. Figure 4 shows the mean amplitude of the EMG response taken from four subjects. At the bottom of the figure is shown the averaged stretch reflex (without brain stimulation) obtained from the same subjects. This allows direct comparison of the size of the response to combined brain and stretch stimulation compared with the underlying stretch-induced EMG activity upon which it was superimposed. A two-factor ANOVA on these data showed significant main effects of interstimulus interval (F = 109, P < 0O001) and intensity (F = 16-5, P < 0-001). The interval x intensity interaction term was not significant (F = 1-47, P > 005). It appeared that the stretch stimulus produced facilitation of the

MOTOR CORTEX STIMULATION AND MUSCLE STRETCH 47 response to a magnetic brain stimulus, but only at preferred times after application of the stretch. To determine which interstimulus intervals produced this facilitation, comparisons were made between the size of the EMG responses with and without a stretch for each of the four interstimulus intervals. At each interval the data

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100 0 100 Time (ms) Fig. 3. Effect of combining a stretch stimulus with a magnetic cortical stimulus in one subject. Each trace is the average of at least sixteen trials. The effect of three intensities of magnetic stimulation is shown (35, 40 and 45% of the maximum output of the stimulator) with two intervals between the stretch (S) stimulus and the magnetic (M) stimulus. In the left-hand panel (A) the magnetic cortical stimulus was timed to produce a response at the same time as the short-latency stretch reflex. In the middle panel (B) the magnetic stimulus was timed to produce a response in the long-latency stretch reflex. The right-hand panel (C) shows the effect of the magnetic cortical stimulus given on its own without a stretch while the subject resisted the standing torque of the motor. At the top is shown four traces superimposed which represent the position of the fingers obtained either without or with (at three intensities) a cortical stimulus. The cortical stimulus produced little extra displacement of the fingers except in B when the intensity of stimulation was 45 %. Under this condition the fingers were driven back almost to their pre-stretch position. The lower records show the rectified EMG from FDP muscle. In these records two traces have been superimposed obtained without and with a cortical stimulus at one intensity. The shaded areas indicate the extra EMG activity produced by the magnetic cortical stimulus. 0

obtained with the three intensities of magnetic stimulation were combined and statistical comparisons were made using Student's paired t test. Responses obtained with stretch at short interstimulus intervals (8 and 13 ms) were not significantly different to those produced by a cortical stimulus alone (P > 005), whereas with

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interstimulus intervals of 34 and 44 ms the responses were significantly larger (P < 0 05 and P < 001 respectively). Thus, for all intensities of magnetic stimulation the response was larger when it was timed to coincide with the LLSR than when superimposed on the SLSR. 16 14

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Fig. 4. Pooled data from four subjects showing the mean (±+S.E.M.) EMG response size to the magnetic cortical stimulus at three intensities (low, medium and high) when combined with a stretch stimulus using four interstimulus intervals. The response size is expressed as a fraction of background activity. Low, medium and high intensities represent increments of 5 % of the maximum output of the stimulator with the lowest intensity set approximately at motor threshold to produce an EMG response in active muscles. The points on the far left of the graph (control) indicate the size of the response to the magnetic cortical stimulus alone obtained with the subject resisting the standing torque

of the motor without a stretch. The bottom trace is the average EMG response from the same four subjects following a stretch stimulus given at a time 0 ms. The bottom trace has been aligned with the top graph so that the size of the EMG response can be compared with the underlying EMG level upon which it was superimposed.

In the second experiment the complete time course of this facilitation was sought. For this experiment the magnetic stimulus was set at about motor threshold intensity and ten intervals between the stretch and magnetic stimuli were studied which probed the entire stretch reflex period. The mean response size taken from four

MOTOR CORTEX STIMULATION AND MUSCLE STRETCH 49 subjects is shown in Fig. 5. For comparison, the average isolated stretch reflex from the same four subjects is shown below the curve on the same time scale. The time course of facilitation of the response to a magnetic stimulus to the scalp appeared to correspond to the late component of the LLSR. However, it should be emphasized

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Fig. 5. Pooled data from four subjects showing the mean (± S.E.M.) EMG response size to the magnetic cortical stimulus at one intensity (around motor threshold) when combined with a stretch stimulus using ten interstimulus intervals. The response size is expressed as a fraction of background EMG activity. Although the intensity used was similar to the 'low' intensity of Fig. 4, the response sizes were not identical probably because different subjects were studied with different relative short- and long-latency stretch reflex sizes. The bottom trace is the average EMG response from the four subjects following a stretch stimulus given at time 0 ms. The short-latency stretch reflex (SLSR) and the long-latency stretch reflex (LLSR) are indicated below the trace. The bottom trace has been aligned with the top graph so that the size of the EMG response can be compared with the underlying EMG level upon which it was superimposed.

that only one of these four subjects had a clear early component of the LLSR and so it is difficult to comment on this component of the LLSR. One of the subjects who contributed to the data of Fig. 5 was studied in more detail. In this subject the SLSR and the late component of the LLSR were well

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separated (since the early component of the LLSR was absent) and of about equal size, as shown in Fig. 6. Figure 7A shows the response in this subject to a magnetic stimulus set at just above motor threshold intensity and given at different times relative to stretch onset. The brain stimulus produced a very small EMG response Pre 5 mm

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when superimposed on either the background muscle activity or the short-latency component of the stretch reflex. Only when it was superimposed on the long-latency component was the response facilitated. The facilitatory effect of the stretch stimulus was studied in the same subject following a complete anaesthetic block of the median and ulnar nerves at the wrist. The effectiveness of the block was assessed clinically and was judged to be complete when the intrinsic hand muscles were totally paralysed and skin sensation (touch and pin-prick) was absent. The purpose of the double block was to remove stretch-induced afferent input arising from cutaneous, joint and muscle receptors in the hand, sparing the forearm. Figure 6 shows that the short- and long-latency components of the stretch reflex were differentially affected by the block. The SLSR remained unchanged while the LLSR was reduced to about half its original size. During the block (Fig. 7B) the same intensity of brain stimulation was judged to be sub-motor threshold since EMG responses were no longer visible when superimposed on the SLSR (most likely, this was the result of minor alterations in the position of the magnetic coil relative to the scalp, although an effect of the hand block itself cannot be excluded). Extra EMG responses due to brain stimulation, however, were still present at two intervals when the response was timed to coincide with the LLSR, although the response was smaller than before the

MOTOR CORTEX STIMULATION AND MUSCLE STRETCH

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Fig. 7. Records from one subject (same as for Fig. 6) showing the effect of muscle stretch the response to magnetic cortical stimulation before and after complete anaesthetic block of the median and ulnar nerves at the wrist. Each trace represents the average (n > 16 ) EMG response to stretch (applied at t = 0) superimposed with the response to stretch plus a magnetic cortical stimulus at different interstimulus intervals indicated by the numbers on the left of the figure. The extra EMG response to the cortical stimulus is shown by the shaded areas in the traces. The left-hand panel (A) shows the effect obtained before anaesthesia using a stimulus intensity just above motor threshold (25% of the maximum output of the stimulator). Under this condition the cortical stimulus produced a very small response when timed to arrive on the short-latency stretch reflex (delay of 8 ms). The response size was much larger when superimposed on the long-latency stretch reflex (delays of 38, 48, 53 and 58 ms). With longer interstimulus intervals the response size returned to control values. The middle panel (B) shows the effect using the same intensity of cortical stimulation immediately following block of median and ulnar nerves. Under this condition responses were only visible with delays of 48 and 58 ms. The traces illustrated in the right-hand panel (C) were obtained with a slightly higher intensity of cortical stimulation (30 % of maximum output) while the nerves were still anaesthetized. For the four interstimulus intervals studied the effect was similar to that obtained before the block. on

block (intervals of 48 and 58 ms). Figure 7 C shows also that while the nerves were still anaesthetized, increasing the stimulus intensity by 5 % produced extra EMG responses on top of the LLSR similar to those seen prior to the block, at least for the three intervals studied (38, 48 and 58 ms). In addition, the extra EMG response on top of the SLSR (8 ms interval) was similar to that before the block.

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Comparison of electrical with magnetic stimulation Figure 8 shows an example of how muscle stretch could differentially affect the size of the EMG response to a cortical stimulus depending upon whether the electrical or magnetic stimulator was used. In this example, the size of the EMG response to

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Fig. 8. Raw data from one subject contrasting the effect of muscle stretch (S) on the response to magnetic (M) cortical stimulation (top traces) with that to electrical (E) cortical stimulation (bottom traces). Superimposed are the average (n = 32) surface rectified EMG response from finger flexor muscles in the forearm to stretch alone (dotted lines) together with the response to stretch plus a cortical stimulus (continuous line). In this experiment the increase in motor torque was such as to displace the fingertip about 17 mm with a peak velocity of around 05 m/s. The shaded areas indicate the extra EMG response produced by the cortical stimulus. The EMG response to the cortical stimulus was timed to appear either on the short-latency stretch reflex (left) or the long-latency reflex (right). The intensities of the magnetic (38 % of maximum output of the stimulator) and the electrical stimulation (26 % of maximum output of the stimulator) were adjusted to produce approxim#tely equal response sizes when superimposed on the short-latency stretch reflex.

cortical stimulation was the same for both magnetic and electrical stimuli when the response was timed to superimpose on the SLSR. When the response was timed to coincide with the late component of the LLSR the response to magnetic stimulation was larger than that to electrical stimulation. The same result was obtained when the mean data from the eleven individual experiments were pooled. The size of the extra EMG response to cortical stimulation when superimposed on the SLSR was the same for both modes of stimulation (mean size + S.E.M. expressed as a fraction of background EMG; electrical 3-3 + 05; magnetic: 3-2 + 0-6; P > 005). When superimposed on the LLSR the size of response to magnetic stimulation was significantly

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larger (electrical: 4-8 + 1 1; magnetic: 9 9 + 1-7; P < 0 02). Although the size of the response to electrical stimulation was larger when superimposed on the LLSR than on the SLSR, the effect was not statistically significant (P > 0 05). DISCUSSION

The present experiments have investigated the stretch reflex of the long flexors of the fingers, although in the second series of experiments, when surface electrodes were used, electromyographic activity from other forearm flexor muscles may have contributed to the responses. This muscle was chosen because it has well-developed short-latency and long-latency stretch reflex components which are approximately equal in size, unlike the long flexor of the thumb in which the short-latency stretch reflex is usually very weak. However, for the thumb, the long-latency stretch reflex may be separated into two components (Marsden et al. 1978) and there is some evidence to suggest that these two components are controlled by different mechanisms (Marsden, Rothwell & Traub, 1980). The long flexor of the fingers also may show two distinct components of the long-latency stretch reflex although in the present series the first of these components often was inconspicuous. For this reason we shall restrict our discussion to the later second of these components and leave the question open as to whether the earlier first component may use a transcortical circuit. The results have shown that EMG responses to magnetic stimulation of the brain were larger if they were elicited during the long-latency stretch reflex than they were if elicited in the short-latency stretch reflex even when the size of the short- and longlatency reflex components were approximately equal. Furthermore, the intensity of magnetic stimulation could be adjusted so that even though it was below threshold for eliciting any EMG response in the short-latency reflex period, the same stimulus could produce a clear extra response if given within the long-latency period. These effects were not seen when low-intensity electrical stimulation of the brain was used. Our explanation for these findings is as follows. The short-latency component of the stretch reflex is mediated solely by spinal circuitry, whereas the long-latency component has a contribution from a transcortical pathway. Activity in the latter pathway may increase the excitability of the motor cortex to externally applied magnetic stimuli such that when a magnetic shock is given at an appropriate time, the descending volley which it evokes in the corticospinal tract is larger than if given at other times. Thus, even if the EMG levels are comparable in the short- and the long-latency components of the stretch reflex, the larger descending volley evoked by a magnetic stimulus in the long-latency period will produce a larger muscle twitch than if the same stimulus was used to evoke an EMG response in the short-latency period. If electrical stimulation is used, this difference is less clear because, at the relatively low intensities of stimulation used in the present experiments, electrical stimulation activates corticospinal pathways in a different manner to magnetic stimulation (Day et al. 1987b; Day, Dressler, Maertens de Noordhout, Marsden, Nakashima, Rothwell & Thompson, 1989 a). Day et al. suggested that with stimulus intensities just above motor threshold, electrical stimulation of the motor cortex through the scalp activates pyramidal tract neurones directly whereas magnetic

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stimulation (with the coil centred on the vertex) tends to activate the pyramidal tract cells indirectly via synaptic connections. This hypothesis has received support from direct recording of descending corticospinal volleys following the two forms of stimulation in the monkey (Amassian, Quirk & Stewart, 1987). However, more recent experiments on the macaque monkey have not supported this hypothesis (Edgley, Eyre, Lemon & Miller, 1990). In their experiments, Edgley et al. (1990) showed that both electrical and magnetic stimulation activated corticospinal neurones directly. They suggested that results from human experiments could be explained by the electrical stimulus activating the corticospinal neurones directly at a deep level whereas the magnetic stimulus activates the corticospinal neurones directly, but more superficially, at the initial segment. For the purposes of the present paper it does not matter which of these hypotheses is correct since the response to a magnetic stimulus will be readily influenced by levels of cortical excitability irrespective of whether the cortical output cells are stimulated transynaptically or directly at their initial segment. In contrast, if the electrical method stimulates the axons of pyramidal tract neurones at internodal regions, the response to electrical cortical stimulation will be relatively insensitive to the level of excitability of the motor cortex. Although this explanation may be satisfactory, we lack clear evidence on one crucial point. That is, that the descending motor volley evoked by magnetic stimulation is larger when it is timed to produce responses in the period of the longlatency stretch reflex than it is in the short-latency period. In conscious man it is not possible to monitor this volley directly and we have to rely on measurements of the size of the evoked muscle twitch. Unfortunately, the size of the latter depends not only on the size of the descending volley, but also on the level of spinal omotoneurone excitability. So can our results be accounted for by changes in spinal cord excitability without invoking any motor cortical effects at all? To a first approximation, in active muscles the excitability of the spinal motoneurones is related to the amount of muscle activity. Since there were occasions when the facilitatory effect was present when the long-latency component of the stretch reflex was equal to, or even smaller than, the short-latency component, changes in spinal motoneurone excitability could not easily explain the results. However, although the sizes of the short- and the long-latency EMG components of the stretch reflex may be equal, the size or excitability of the subliminal fringe of undischarged motoneurones may differ. If the excitability of the subliminal fringe is larger in the long-latency period than in the short-latency period, then the response to magnetic stimulation would be larger in the former than the latter. There are two reasons for supposing this cannot account for all of the present results. First, there were occasions on which the intensity of magnetic stimulation could be reduced such that no response was evident in the short-latency period, whereas clear responses were produced in the long-latency period. Only if there were no subliminal fringe at all available in the short-latency period would this be explicable by the theory. Second, it would be difficult to account for the difference in results between magnetic and electric stimulation. Both forms of stimulation appear to activate the same rapidly conducting descending corticospinal pathways (Day et al. 1989a), and probably recruit spinal motoneurones in the same order (Gandevia & Rothwell, 1987; Hess,

MOTOR CORTEX STIMULATION AND MUSCLE STRETCH 55 Mills & Murray, 1987). If this is so then no theory based on subliminal fringe excitability can account for the results that we have obtained. One other possible explanation is that the SLSR may use a different pool of motoneurones to that used by the LLSR and, furthermore, the descending input from the brain stimulus may project only onto those motoneurones employed by the LLSR. Apart from the difficulties this theory would encounter to explain the differences between the effect of stretch on responses to electrical and magnetic stimulation, the work of Calancie & Bawa (1984) showed that the SLSR and the LLSR do not derive from functionally separate motoneurone pools since they share common motor units. In addition, it is unlikely that descending input from a cortical stimulus does not project onto motoneurones employed by the SLSR since it has been shown that in relaxed muscles an H reflex (the electrical analogue of the SLSR) can be facilitated by a cortical stimulus at an intensity which does not produce a muscle response on its own (Cowan, Day, Marsden & Rothwell, 1986). The results, therefore, are difficult to explain at the level of the spinal motoneurones. Indeed, if our hypothesis to explain the differences between electrical and magnetic stimulation is correct then the results are equally difficult to explain at any level downstream of the motor cortex. Even so, we cannot yet exclude the remote possibility that descending volleys produced by magnetic and electrical cortical stimulation have differential effects on some spinal interneurones. Should the same interneurones be used exclusively in the generation of the LLSR then our results could be explained by summation at this interneuronal site in the spinal cord. At first sight our results do not appear to agree with those from others who claimed to have produced cortical facilitation by a peripheral stimulus on the response to electrical cortical stimulation. Cullen, Merton & Walker (1986) showed that an electrical cortical stimulus which was subthreshold to produce a direct EMG response in the relaxed flexor pollicis longus muscle could, when appropriately timed, augment the late EMG response (latency 37 ms) produced by striking the thumb pad with a hammer. It is possible that this facilitation took place in the spinal cord. Even an electrical stimulus subthreshold to produce a direct EMG response in relaxed muscles can produce a descending volley in the corticospinal tract which is capable of augmenting the response to a peripheral stimulus by summation at the spinal motoneurones (Cowan et al. 1986). Bergamasco, Bergamini, Cantello & Troni (1986) used a train of two to four electrical stimuli delivered to the median nerve at the wrist to augment the thenar muscle EMG response to an electrical cortical stimulus. They suggested that this facilitation had probably not taken place in the spinal cord since F-wave size was not increased over the same period. Certainly their time course of facilitation (evident with interstimulus intervals of 40-80 ms) was very similar to ours obtained with magnetic cortical stimulation. The apparent discrepancy with our results obtained with electrical stimulation may be explained by the differences in intensity of cortical stimulation that were employed. In our experiments the electrical cortical stimulus intensity was set to be just above motor threshold for active muscles whereas the stimulus intensity of Bergamasco et al. was set similarly but for relaxed muscles. This probably means that our electrical stimulus produced only direct activation of corticospinal neurones (see above) whereas that of Bergamasco et al. produced indirect as well as direct activation in order to discharge

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the relaxed muscle (Day et al. 1987 a). Under this condition their electrical stimulus may have been more akin to the magnetic stimulus used in the present experiments. Signals from muscle spindles probably form the major input to the transcortical stretch reflex circuit. The results of the anaesthetization experiment in the present series are in agreement with this idea. Block of both ulnar and median nerves of the wrist should have eliminated afferent input from joint and cutaneous receptors in the hand following the torque motor perturbation, whilst leaving afferents from the muscle receptors in the forearm unaffected. The motor cortex appeared to increase its excitability following stretch in the usual way during anaesthetization of the hand, indicating that muscle receptors provide an important excitatory input to the motor cortex. Indeed, Day et al. (1988a) failed to find cortical facilitation of the response to magnetic cortical stimulation by electric stimulation of the digital nerves of the index and middle fingers. Instead, these preliminary experiments indicated that the cutaneous stimulus produced a small, short-lasting inhibitory action on the motor cortex. In conclusion, the present results provide evidence that the excitability of the motor cortex is increased for a period after the onset of muscle stretch. When the time delay for nervous conduction is taken into account, the period of increased motor cortex excitability corresponds closely to the period occupied by the longlatency stretch reflex (at least its later component). Our data, therefore, are compatible with the long-loop transcortical hypothesis of the long-latency stretch reflex. We should like to thank Dr M. Jahnke for inserting the intramuscular wire electrodes and Nora Pahlke for preparing the illustrations. Support for this work was provided by the Medical Research Council of Great Britain and by the Guarantors of Brain. REFERENCES

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