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Modular organization of excitatory and inhibitory reflex receptive fields elicited by electrical stimulation of the foot sole in man
Clinical Neurophysiology 111 (2000) 2160±2169
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Modular organization of excitatory and inhibitory re¯ex receptive ®elds elicited by electrical stimulation of the foot sole in man Finn A. Sonnenborg, Ole K. Andersen*, Lars Arendt-Nielsen Laboratory for Experimental Pain Research, Center for Sensory-Motor Interaction, Aalborg University, Fredrik Bajers Vej 7, D3, DK-9220 Aalborg, Denmark Accepted 8 September 2000
Abstract Objectives: The present study aimed to investigate how the inhibitory and excitatory re¯ex components of the human (polysynaptic) withdrawal re¯ex are organized depending on the stimulation site. The re¯exes were elicited during a voluntary pre-contraction (between 10 and 20% of maximum voluntary contraction) of two antagonistic muscles. Methods: Inhibitory and excitatory re¯ex receptive ®elds to tibialis anterior (TA) and soleus (SO) were mapped in 14 healthy subjects using randomized electrical stimulation at 16 sites of the foot sole. Low, non-painful (3£ perception threshold), and high, painful (1.5 £ pain threshold), stimulus intensities were used. Results: The inhibitory re¯ex receptive ®elds were organized in a highly functional manner supporting the action of the excitatory re¯ex. Together the two re¯exes result in an optimal withdrawal from the stimulus. Low stimulation intensity was found suf®cient to elicit the inhibitory re¯ex. High stimulation intensity caused a reversal of the inhibition to excitation in tibialis anterior. In soleus the inhibition was facilitated for stronger intensities. Conclusion: In conclusion, ®ndings in animals of a modular organization of inhibitory re¯exes are reproduced in humans. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Inhibition; Withdrawal re¯ex; Receptive ®eld; Nociception; Human; Flexor re¯ex
1. Introduction The early studies on the ¯exion re¯ex of the limb outlined excitatory re¯ex responses to avoid the stimulus but also inhibitory responses that further support the withdrawal of the limb (Sherrington, 1910). In the control of the withdrawal re¯ex, it has been demonstrated in animals that different afferent ®bre groups, both excitatory and inhibitory, converge onto common interneurones in the spinal a-motoneurone re¯ex pathways (see Lundberg, 1979; Schomburg, 1990; Burke, 1999; for reviews). At least 3 coexistent re¯ex control systems can be identi®ed for spinal motor inhibitory control: (1) a local muscle control system, involving muscle and tendon afferents (Schomburg, 1990); (2) a balance and perception system, which combines descending motor commands and cutaneous/proprioceptive input (Schomburg, 1990; Decchi et al., 1997; Zehr and Stein, 1999); and (3) a cutaneous system which activates single muscles or muscle groups to withdraw a skin area if the stimulus is potentially tissue damaging (Schouenborg et al., 1994). The spinal inte* Corresponding author. Fax: 145-9815-4008. E-mail address: [email protected] (O.K. Andersen).
gration of sensory input in motor control at a pre-motoneuronal level can be assumed to optimize the spinal movement control (Lundberg, 1979; Schomburg et al., 1999). Kugelberg et al. (1960) and Hagbarth and Finer (1963) observed indirect evidence that the re¯ex control of excitatory and inhibitory responses to electrical stimulation of the foot not always shared the same neural control as afferent input from deep structures. This lead to observations by Grimby (1963) who indirectly showed that the cutaneous re¯ex pathways convey information about stimulation site to the a-motoneurone probably through several interneurones. This ®ndings lead to the recent discoveries of a functional, modular organized withdrawal re¯ex in animals (Schouenborg et al., 1994). The presence of excitatory re¯ex receptive ®elds for individual muscles has recently been shown in humans (Andersen et al., 1999) supporting a functional, modular re¯ex organization in humans of the excitatory withdrawal re¯ex. The aim of the present study was to investigate if (1) the inhibitory re¯ex is organized in the same functional manner as the excitatory re¯ex in humans (Andersen et al., 1999) and as the inhibitory re¯ex in animals (Weng and Schouenborg, 1996), and (2) if this inhibition is depending on stimu-
1388-2457/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S13 88-2457(00)0047 2-7
CLINPH 2000537
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Fig. 1. Typical plot of EMG responses based on mean of 5 repetitions from one subject at one stimulation site (site 9). Responses from TA and SO at low and high stimulation intensities are shown. The dotted line shows the limits used to calculate the excitatory and inhibitory onset latencies. The grand mean onset latencies with standard deviation (mean ^ SD) of the ®rst and second response are shown below each plot. `X' marks onset of an excitatory response and `O' marks onset of an inhibitory response.
lation intensities ranging from non-painful to painful. In the present study a newly developed method (Andersen et al., 1999) for investigating the spatial re¯ex sensitivity was used to investigate the inhibitory re¯ex organization. 2. Methods and materials Fourteen subjects (4 females and 10 males, mean ages 23.3 ^ 3.1 years, mean height 178 ^ 8.8 cm) with no known neurological diseases participated in the study. Informed consent was obtained from all subjects and the Helsinki Declaration was respected. 2.1. Set-up The subjects were sitting in a chair supporting arm, back, neck, and left leg. The right foot was strapped into at pedal that prevented the subject from moving the foot. 2.2. Electromyogram The electromyogram (EMG) was recorded with Ag± AgCl surface electrodes placed parallel to the muscle ®bre direction over tibialis anterior (TA) and soleus (SOL) muscles. The recorded EMG-signals were ampli®ed 1000± 10 000 times, passband ®ltered (5±1000 Hz, 2nd order) and analogue to digital converted (2000 Hz, PCI-MIO 16E, National Instruments Corporation, Austin, TX, USA), displayed on computer screen, and stored on a computer
disk. The recording was started 150 ms before stimulus onset and lasted until 1000 ms after stimulus onset. 2.3. Electrical stimulation On the sole of the right foot, 16 electrodes (copper, diameter 0.8 cm) were mounted in a non-uniform grid (Fig. 1). To reduce the bias of differences in skin thickness, the subjects were instructed to grind off thick epidermal layers some days prior to the experiment. A common anode (10 £ 14 cm electrode, Pals, Axelgaard Ltd., Fallbrook, CA) was placed on the dorsum of the foot. This con®guration ensured that the stimulus was always perceived as coming from the sole of the foot. For each electrode position, the electrode was moved slightly in case the evoked sensation indicated direct nerve stimulation (i.e. the sensation radiating to the distal foot). A computer-controlled electrical relay and stimulator delivered a stimulus to one of the 16 electrodes in a randomized sequence. The computer chose the stimulation electrode and adjusted the intensity of the electrical stimulator (Neuromatic 2000 C stimulator, DISA, Skovlunde, Denmark) according to a pre-programmed randomized sequence. Thus, the subject could not predict the stimulus location. Each stimulus consisted of a constant current pulse train of 5 individual 2 ms pulses delivered at 200 Hz. This con®guration was used throughout the experiment. For each electrode position, the intensity of the stimulus was adjusted to
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ensure equal visual analogue scale (VAS) score independent of stimulation site. Two stimulus intensities were chosen. An intensity of 3£ the initial perception threshold (nonpainful) and an intensity of 1.5£ the initial pain threshold (painful) were selected for each electrode position. The inter-stimulus interval was between 20 and 30 s. 2.4. Sensory intensity rating The perceived intensity was rated on a 10 cm electronic continuous visual analogue scale. The scale was anchored by assigning `5.0' as the pain threshold, `0' as no sensation, and `10' as `most intense pain imaginable'. Each electrical stimulus was scored by the subject and stored on the computer. 2.5. Experimental procedure Before the experiment, the subject was asked to perform maximal voluntary contraction (MVC) of the soleus and tibial anterior muscles in two separate sessions. The measurement was repeated 3±4 times for both muscles. The RMS value (MVCRMS) in a 500 ms time window centred at the maximal peak of the EMG signals was calculated and the maximal MVCRMS of the 3±4 repetitions was used as reference point during the experiment. The perception threshold and pain threshold to each stimulation site were assessed through randomized ascending and descending stimulation intensities. Each threshold was assessed at least 3 times and the ®nal value was used as the threshold. A ®le containing a random sequence of site and associated stimulus intensity (low intensity (3£ perception threshold) or high intensity (1.5 £ pain threshold)) for electrical stimulation during the experiment was generated. The EMG signals from the investigated muscle were monitored on a custom made computer program (Labview, National Instruments Corporation, Austin, TX, USA) during the voluntary contraction. During re¯ex recordings, the computer program calculated the RMS value of the EMG muscle signal in real time in 150 ms non-overlapping windows. This signal was presented as a fraction of the MVCRMS and as feedback to the subject in order to maintain a contraction level between 10 and 20% of MVCRMS (Burne and Lippold, 1996). If the contraction level was within the range for at least 3 s, a stimulus was released. After each stimulus, the subject rated the sensory intensity on the electronic VAS. Each of the 16 stimulation sites was stimulated 5 times for both intensities. The study was divided into two measuring sessions. First recording during voluntary contractions of the TA muscle followed by a session of voluntary contraction of the SOL muscle. 2.6. Data analysis Initially each EMG sweep for TA and SOL was normalized to the background activity. As described by Andersen et al. (1999), spatial variation in the skin withdrawal re¯ex
can be visualized by two-dimensional interpolation (Kriging algorithm, non-uniformly spaced data-points, inverse distance method (Sandwell, 1987)) to describe the Re¯ex Receptive Fields (RRF) (Schouenborg and KalliomaÈki, 1990). This method both interpolates and extrapolates the data values to cover the entire plantar surface of the foot for a complete overview of the re¯ex sensitivity. In general, the extrapolation must be interpreted cautiously. The following features were calculated: root mean square (RMS) value of the normalized muscle EMG re¯ex responses, the time the EMG signal was inhibited, and ®nally the VAS to each stimulation. Inhibition of a voluntary contracted muscle was de®ned as 60% suppression of the EMG signal compared to the prestimulation contraction level (background level) (Fig. 1). The suppression should last longer than 15 ms to be accepted as an inhibition. If these two criteria were ful®lled, the inhibition onset time was determined as the time point where the EMG-signal ®rst was suppressed more than 60% of the background level. The inhibition offset time was calculated as the ®rst time the EMG suppression was less than 60% of the background level. If the EMG suppression was less than 60% background level with a duration of less than 10 ms, the crossing was not accepted as the offset but as a short lasting burst. The duration of the inhibition was subsequently calculated as the difference between inhibition onset and offset. The duration was calculated for every suppression phase in each sweep that met the inhibition criteria. For each sweep the total duration of inhibition was then calculated as the summation of all inhibitory periods detected in that sweep. An excitatory re¯ex response was de®ned as 100% facilitation of the EMG signal compared to the pre-stimulation contraction level (i.e. 2 times the background contraction level, see Fig. 1). The facilitation should last longer than 10 ms to be accepted as an excitatory component. If these two criteria were ful®lled, the excitation onset time was determined as the time point where the EMG-signal ®rst was facilitated more than 100% of the background level. Two different time intervals were chosen for re¯ex feature calculation. The limits of the analysis intervals were chosen based on an analysis of the inhibitory onset times, see results. A short re¯ex loop (SRL) covering 40± 150 ms after stimulation onset and a long re¯ex loop (LRL) covering 150±250 ms after stimulation onset were used. For calculation of inhibitory duration, the onset of the individual inhibitions was used to determine within which analysis interval the duration of inhibition should be included. A correlation analysis for comparison of interpolated maps of mean detection and pain thresholds was computed using a two-dimensional cross-correlation algorithm. 2.7. Statistics One-way repeated measure ANOVA was applied to the data when the data were normally distributed otherwise non-
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parametric Friedman Repeated Measures Analysis of Variance on Ranks was used. Student±Newman±Keuls (SNK) Test was used for post-hoc pairwise comparisons. The non-parametric two-sample paired test Wilcoxon Signed Rank test was used to evaluate paired observations. The results are presented as mean values and standard deviation.
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On average the pain threshold was 6.6 ^ 2.3 times larger than the perception threshold, whereby the high stimulation intensity on average was 9.9 ^ 3.5 time larger than the perception threshold. The spatial variation in the detection and pain thresholds were strongly correlated (R 0:94). 3.2. Re¯ex response and analysis intervals
3. Results All 14 subjects were able to maintain the voluntary contraction level and inhibition and/or excitation periods could be elicited in all subjects by the electrical stimulation. The mean maximal voluntary contraction (MVC) of TA was 457 ^ 198 and SO 214 ^ 83 mV. 3.1. Stimulation intensities and sensory intensity scoring The mean perception threshold was 2.6 ^ 1.2 mA for the 16 stimulation sites. A signi®cant difference between the perception thresholds was found (Friedman, P , 0:01) with a lower perception threshold (SNK, P , 0:05) at the mid foot (site 11, 12, and 13) and toes (site 1 and 2) than the heel and distal forefoot (site 3, 4, 5, 6, 8, 15, and 16) (Fig. 2). The reported sensory ratings to the low stimulation intensities (3£ perception threshold) are shown in Fig. 2. A significant difference was found between the stimulation sites (Friedman, P , 0:01) with higher sensory ratings at two stimulation sites (site 8 and 16). A mean pain threshold of 16.0 ^ 7.3 mA was observed for all 16 stimulation sites (Fig. 2). A signi®cant difference between the pain thresholds was found (Friedman, P , 0:01) with higher pain thresholds (SNK, P , 0:05) at the heel (site 14, 15, and 16) and at the distal forefoot (site 3, 4, 5, and 6) than the mid foot (site 7, 8, 9, 10, 11, 12, and 13) and toes (site 1 and 2). The reported sensory ratings to the high stimulation intensities (1.5£ pain threshold) are shown in Fig. 2. A signi®cant difference was found in sensory ratings between the stimulation sites (Friedman, P , 0:01) with lower sensory ratings (SNK, P , 0:05) for sites 3, 6, and 7 compared with sites 1, 2, 8, 9, 13, and 16.
Fig. 2. The grand mean pain threshold, perception thresholds, and subjective intensity scores to the low intensity (3£ perception threshold) and high intensity (1.5£ pain thresholds) stimulation. The ®gure illustrates the spatially interpolated mean thresholds.
Low intensity and high intensity stimulation of the foot sole during voluntary muscle contraction resulted in different inhibitory and excitatory patterns for TA and SO muscles (Figs. 1 and 3). Generally the TA response consisted of one excitatory and one or two inhibitory responses (Figs. 1 and 3). Fig. 1 illustrates a typical response with the mean onset latencies listed in Table 1. A signi®cant difference in inhibitory onset latencies between the ®rst and second inhibitory periods was observed (Friedman, P , 0:05). Generally the SO response consisted of two inhibitory responses separated by one excitatory phase, though stimulation of the heel resulted in two excitatory phases, and two inhibitory responses (Figs. 1 and 3). Fig. 1 illustrates a typical response with the mean onset latencies listed in Table 1. Based on these different re¯ex components, two time intervals were chosen for the further analysis. (1) A short re¯ex loop (SRL) covering 40±150 ms after stimulation onset and (2) the long re¯ex loop (LRL) covering 150± 250 ms after stimulation onset as described in the Section 2. 3.3. TA inhibitory re¯ex response 3.3.1. Short re¯ex loop (40±150 ms) response to low intensity stimulation Low intensity electrical stimulation of the plantar foot side resulted in an inhibitory re¯ex response at the heel, digit 2 and distal lateral forefoot in the SRL interval (Fig. 4). Site 8 had a signi®cant larger response than the 7 sites expressing inhibition (site 2, 5, 6, 13, 14, 15, and 16, illustrated in Fig. 4, Friedman, P , 0:001). The duration of inhibition was largest on the forefoot (Fig. 4). The grand mean duration of inhibition was 39.8 ^ 19.3 ms. There was a signi®cant difference in the duration of inhibition (Friedman, P , 0:001), but no post-hoc differences were found, indicating a minimum difference between sites. 3.3.2. Short re¯ex loop (40±150 ms) response to high intensity stimulation No signi®cant inhibition was found in re¯ex size (RMS value, full interval) compared to baseline. The duration of inhibition was longer at the distal lateral foot and at the heel, complementary to the areas with excitatory re¯ex responses (Fig. 4). The grand mean duration of inhibition for all stimulation sites was 42.9 ^ 20.4 ms. A signi®cant difference was found between the duration of inhibition (Friedman, P , 0:001), but no post-hoc differences were found indicating a minimum difference between sites.
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Fig. 3. The grand mean EMG response from TA and SO to low intensity and high intensity stimulation for the 16 sites. The positions of the stimulation sites are shown in the centre of the ®gure. Each EMG response was normalized to the voluntary background activity. In the mean re¯ex response, there is a tendency for an early, excitatory re¯ex in TA for the low intensity stimulation, which is even more pronounced for the strong stimulus intensity. Further, a long latency inhibition is seen in the TA response for the strong stimulation intensity. For SOL inhibition seems be prevailing. However, please note the tendency for an early excitatory response for the posterior stimulation sites. For the statistically signi®cant changes, see Section 3.
3.3.3. Long re¯ex loop (150±250 ms) response to low intensity stimulation Inhibition of the muscle response was observed on all stimulation sites (Fig. 4). All sites were signi®cantly different in re¯ex response from the voluntary background activity (Wilcoxon, P , 0:05), while no difference was found between sites. No signi®cant difference between sites was observed in the duration of inhibition (Fig. 4). The grand mean duration of inhibition was 25.9 ^ 11.1 ms.
P , 0:05, even though inhibitory responses can be seen in the mean plots, see Fig. 3). A signi®cant difference in the re¯ex response was found between stimulation sites (Friedman, P , 0:001), but no post-hoc differences were found. No signi®cant differences between sites were observed in the duration of inhibition (Fig. 4). The grand mean duration of inhibition was 40.4 ^ 37.4 ms.
3.3.4. Long re¯ex loop (150±250 ms) response to high intensity stimulation High intensity stimulation resulted in inhibition at the medial and lateral sides (Fig. 4) (only signi®cant inhibited at site 5 compared to the background activity, Wilcoxon,
Excitatory responses were only elicited by high stimulation intensities in the short re¯ex loop of TA. At low intensity stimulation there was no signi®cant excitation found even though the mean plot may indicate this. The excitatory response was observed when the arch of the foot was stimu-
3.4. TA excitatory re¯ex response
F.A. Sonnenborg et al. / Clinical Neurophysiology 111 (2000) 2160±2169 Table 1 Grand mean onset latencies to excitatory and inhibitory responses elicit by low (3£ perception threshold) and high (1.5£ pain threshold) stimulus intensity a Muscle/ intensity
Event no.
Event
Onset latency (ms)
Number detected (out of 1120 sweeps)
TA (high)
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
Excitatory Inhibitory Excitatory Inhibitory Excitatory Inhibitory Excitatory Inhibitory Inhibitory Excitatory Inhibitory Excitatory Inhibitory Excitatory Excitatory Inhibitory
87.8 ^ 32.0 116.6 ^ 26.9 173.1 ^ 67.6 218.7 ^ 63.2 95.7 ^ 34.5 113.0 ^ 22.7 185.4 ^ 77.8 214.4 ^ 65.5 87.1 ^ 35.0 139.9 ^ 35.8 183.5 ^ 64.2 208.9 ^ 76.8 95.8 ^ 34.8 123.1 ^ 34.3 193.6 ^ 73.0 226.5 ^ 72.4
718 780 456 667 398 820 207 599 929 491 885 221 624 394 173 592
TA (low)
SO (high)
SO (low)
a
Event number illustrates the sequence in which the excitation/inhibition occurred. Event is the type of response detected. Onset latencies are presented in ms.
lated, see Fig. 4. Thus, a signi®cant re¯ex excitation was seen when the arch and medial side of the foot was stimulated at the high intensity, site 1, 3, 4, 7, 8, 9, 10, 11, 12, 13, and 14 (Wilcoxon, P , 0:05), illustrated on Fig. 4. A significant difference in re¯ex responses between the stimulation sites was found (Friedman, P , 0:001) with the heel (site 14, 15, and 16) and digit 2 having smaller responses than the mid foot (site 7, 8, 9, 11, and 12).
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response was signi®cantly inhibited when the distal forefoot was stimulated (site 1±6, 8, 9, 10, 12, and 13) compared to the background activity (Wilcoxon, P , 0:05) (Fig. 5). There was a signi®cant difference in re¯ex response between stimulation sites when comparing the muscle responses, with smaller responses at the distal forefoot than on the heel (Friedman, P , 0:001, sites 15 and 16 being signi®cantly larger than site 1, 2, 4, 5, 6, and 9, site 16 also larger than site 8, 10, 12, and 13). The duration of inhibition was short on the entire plantar side of the foot (compare the duration of inhibition between short re¯ex loop and long re¯ex loop in Fig. 4). The grand mean duration of inhibition was 34.3 ^ 18.0 ms. There was a signi®cant difference in the duration of inhibition (Friedman, P , 0:001), but no post-hoc differences were found. This indicated a minimum difference between sites. 3.6.2. short re¯ex loop (40±150 ms) response to high intensity stimulation Inhibition was observed when stimulating the forefoot (Wilcoxon, P , 0:05) (Fig. 5). A signi®cant difference in re¯ex responses between the stimulation sites was found (Friedman, P , 0:001) with stimulation at the heel (sites 12, 14, 15, and 16) evoking signi®cantly larger re¯ex responses than forefoot stimulation (site 1, 2, 3, 4, 5, 6, 8, and 9). The duration of inhibition was longer in the area
3.5. Low vs. strong stimulation intensities in the TA response When the low stimulation intensities from short re¯ex loop were compared to the long re¯ex loop responses independent of stimulation site, the short re¯ex loop had the longest duration of inhibition (39.8 ^ 19.3 ms vs. 25.9 ^ 11.1 ms) while the strongest inhibition was observed in the long re¯ex loop (24.5 ^ 30.4% vs. 216.0 ^ 23.5%) (Wilcoxon, P , 0:001). For the high intensity responses, the largest muscle response (20.4 ^ 44.8 vs. 1.46 ^ 38.9%) and the longest duration of inhibition (42.9 ^ 20.4 vs. 40.4 ^ 37.4 ms) were seen in the short re¯ex loop independent of the stimulation site (Wilcoxon, P , 0:001). 3.6. SO inhibitory re¯ex response 3.6.1. short re¯ex loop (40±150 ms) response to low intensity stimulation Low intensity stimulation resulted in an inhibitory re¯ex response, which covered the forefoot (Fig. 5). The muscle
Fig. 4. The re¯ex receptive ®elds for the inhibitory and excitatory muscle responses and inhibitory duration during TA voluntary contraction for low intensity and high intensity stimulation. The top row illustrates the muscles response analyzed in the short re¯ex loop (SRL) from 40 to 150 ms after stimulus onset and long re¯ex loop (LRL) from 150 to 250 ms. In the muscle response plots, the results of the statistical comparison with the background activity level are shown as: 4 for inhibition, 1 for excitation, and white circles as non-signi®cant difference (Wilcoxon, P , 0:05). The lower row illustrates the duration of inhibition.
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with inhibition (Fig. 5). A signi®cant difference was found between the duration of inhibition (Friedman, P , 0:001), especially the distal medial foot stimulation (site 2 and 3) had longer duration of inhibition, 68.1 ^ 41.4 ms, than stimulation on the heel (site 12, 14 and 15), 50.4 ^ 27.1 ms. The grand mean duration of inhibition was 60.4 ^ 35.1 ms. 3.6.3. Long re¯ex loop (150±250 ms) response to low intensity stimulation Inhibition of the muscle response was observed on all stimulation sites, but strong inhibition was seen for stimulation on the heel (Fig. 5). All sites, except site 2, were significantly inhibited compared to the voluntary background activity (Wilcoxon, P , 0:05). A signi®cant difference in re¯ex responses between the stimulation sites was found (Friedman, P , 0:001) with larger inhibition for stimulation on the heel (site 14, 15, and 16) than for stimulation on the distal forefoot (site 2, 4, 5, and 6). No signi®cant differences in the duration of inhibition were found between the sites, but there was a tendency to longer duration of inhibition for heel stimulation (Fig. 5). The grand mean duration of inhibition was 40.7 ^ 28.9 ms. 3.6.4. Long re¯ex loop (150±250 ms) response to high intensity stimulation High intensity stimulation resulted in inhibition at the
base of the heel (Fig. 5). Only stimulation at the base of the heel showed signi®cant inhibition compared to the background activity (site 14 and 15) (Wilcoxon, P , 0:05) (Fig. 5). A signi®cant difference in the re¯ex responses between the stimulation sites was found (Friedman, P , 0:001) with stronger inhibition on site 15, mean 222 ^ 51.2%, than site 1, 6, 8, 9, 10, 11, and 16, mean 0.9 ^ 51.4%. Stimulation of the heel and mid-foot evoked long duration of inhibition compared to the distal medial area. However, no signi®cant difference in the duration of inhibition was found between sites. The area with the longest duration of inhibition in long re¯ex loop was complementary to the areas with long duration of inhibition in the short re¯ex loop interval (Fig. 5). The grand mean duration of inhibition was 67.2 ^ 48.3 ms. When the low stimulation intensities from short re¯ex loop were compared to the long re¯ex loop responses independent of stimulation site, the long re¯ex loop had the largest inhibition (29.6 ^ 32 vs. 216.6 ^ 28%) and longest duration of inhibition (34.3 ^ 17.8 vs. 40.7 ^ 28.9 ms) (Wilcoxon, P , 0:001). On the other hand, for the high intensity responses, the short re¯ex loop had the largest inhibition (220.0 ^ 47.2 vs. 24.56 ^ 50.6%) while the long re¯ex loop had the longest duration of inhibition (60.4 ^ 35.1 vs. 67.2 ^ 48. 3 ms) (Wilcoxon, P , 0:001). 3.7. SO excitatory re¯ex response No signi®cant excitatory re¯exes were observed in SO at any stimulation intensity or analysis interval. 3.8. Low vs. high intensity stimulation For high intensity stimulation, TA excitatory responses were seen in the arch of the foot while statistically signi®cant inhibitory responses were found for low intensity stimulation surrounding this area (Fig. 4). The duration of TA inhibition was signi®cantly longer in the long re¯ex loop for high intensity stimulation compared to low stimulation intensity (40.4 ^ 37.4 vs. 25.9 ^ 11.1 ms), (Wilcoxon, P , 0:01). SO was stronger inhibited in the short re¯ex loop at high stimulation intensities, while mean inhibition decreased in the long re¯ex loop compared to the low stimulation intensity (Wilcoxon, P , 0:01). Duration of SO inhibition was signi®cantly longer in both intervals for high stimulation intensities compared to low stimulus intensities (SRL: 60.4 ^ 35.1 vs. 34.3 ^ 18 ms and LRL: 67.2 ^ 48.3 vs. 40.7 ^ 28.9 ms) (Wilcoxon, P , 0:01).
Fig. 5. The re¯ex receptive ®elds to the inhibitory and excitatory muscle responses and inhibitory duration during voluntary background activation of SO for low intensity and high intensity stimulation. The statistical comparison with the background activity level are shown as: 4 for inhibition, 1 for excitation and white circles as a non-signi®cant difference (Wilcoxon Signed Rank Test, P , 0:05). The lower row illustrates the duration of inhibition.
4. Discussion In the present study, electrical stimulation was used to map the cutaneous re¯ex receptive ®elds in healthy humans. The topographical maps of the excitatory and inhibitory zones for speci®c muscles were assessed by stimulation during voluntary contraction. The inhibition of voluntary
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muscle contraction was evaluated based on the size of the EMG suppression and the duration of the suppression while excitation was evaluated on EMG facilitation. Stimulation of speci®c skin areas was found to inhibit TA and SO. This indicated that re¯ex pathways encoding the spatial inhibitory information are present. In skin areas not belonging to the inhibitory zone, an excitatory re¯ex could be evoked in TA. A stimulus intensity dependency was found in TA as low stimulus intensity resulted in an inhibitory re¯ex whereas an excitatory re¯ex was observed for the high stimulus intensity. The organization and functionality of the inhibitory and excitatory re¯ex receptive ®elds will be discussed. 4.1. Which afferents are activated The present electrode con®guration (small cathodes on the plantar side and a common anode on the dorsal side) secured that the highest current density was within the epidermal layer underlying the cathode. However, direct activation of the intrinsic foot muscles cannot be avoided with absolute certainty, especially for the strong stimulus intensities. However, the overall sensation (pin-prick) is coming from the super®cial skin of the foot as reported by the volunteers and not from the location of the common anode on the dorsal side of the foot. Therefore, we do not believe the re¯ex itself is initiated by any proprioceptive input. Further, the topographical organization of the re¯ex receptive ®elds was related to the biomechanical function clearly suggesting that any proprioceptive input only had modulatory effect. The signi®cant difference between stimulation sites for the sensory ratings cannot be explained easily. Thus, the attempt to obtain even afferent in¯ow from all stimulation sites was not completely ful®lled, yet the pain threshold pattern did not follow the spatial variation in sensory ratings. This could indicate that the variation in sensory ratings probably originates from something else than the spatial variation in stimulation intensity. The 3 sites with signi®cant lower ratings were 11.8% smaller than the remaining 13 sites, i.e. lower afferent in¯ow. The strong correlation between the detection and pain thresholds suggests that the spatial variation in sensitivity is most likely re¯ecting skin characteristics rather than peripheral neural factors. The detection and pain thresholds were only determined with relaxed muscles although muscle contraction is known to affect cutaneous sensitivity to electrical stimuli (Chapman et al., 1987). One threshold determination was chosen (Pertovaara et al., 1992) primarily to avoid mental and physical fatigue since the procedure took almost 1 h for the 16 stimulation sites. However, the VAS ratings to the painful stimuli in the present study was similar to ratings during relaxed conditions (4.2 ^ 1.3 cm in the present study and 4.2 ^ 1.5 cm in Andersen et al. (1999)) suggesting
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minimal in¯uence on the supra-threshold sensory transmission (Chapman et al., 1987; Feine et al., 1990). Using microneurography (Burke et al., 1975) showed that electrical stimulation of the median and radial nerve at 5 times the detection threshold activated nociceptive ®bres in humans. In the present study, the high stimulation intensity was 9.9 ^ 3.5 times higher than the perception threshold, indicating that Ad afferents most likely were activated (Burke et al., 1975; Willer et al., 1978). The low intensity stimulation, 3 times the perception threshold, probably did not activate nociceptive afferents and the stimulus intensity was well below the pain threshold. 4.2. Main re¯ex results Both inhibition and excitation were observed in the re¯ex responses. The position of the excitatory re¯ex receptive ®elds on the foot sole is in agreement with earlier ®ndings (Grimby, 1963; Andersen et al., 1999) in relaxed muscles. Excitatory re¯ex receptive ®elds were found in TA for the high stimulation intensity in the short re¯ex loop interval in contrast to the inhibitory ®elds observed for low intensity stimulation. This indicates an intensity dependency for TA, with a reversal of the re¯ex response from inhibition to excitation for painful input compared to tactile input. Although the muscle response was facilitated at high stimulation intensities, the longest duration of inhibition was obtained at stimulation sites expressing no signi®cant change from background activity. This indicates that inhibitory responses were expressed by two independent parameters: the actual amplitude suppression and the duration of suppression. More powerful inhibitory re¯exes were observed in SO for high stimulation intensities compared to low intensities in the short re¯ex loop. Thus, the duration of inhibition was prolonged which indicated stronger inhibition. A clear spatial difference was found between the excitatory and inhibitory areas in both TA and SO. Inhibitory receptive ®elds were found in areas complementary to areas where excitatory re¯exes have previously been described (Andersen et al., 1999), and are in agreement with earlier ®ndings (Grimby, 1963; Kugelberg et al., 1960; Meinck et al., 1981; Andersen et al., 1999). In the long re¯ex loop, clear inhibition at low stimulation intensities was evoked from all sites in both muscles. 4.3. Stimulation method The technique of cutaneous stimulation during voluntary contraction to investigate inhibitory pathways has been used before (Kugelberg et al., 1960). However, the voluntary muscle contractions may potentially modulate the output since the voluntary drive alters the excitability of the motorneurone pool and hence may conceal the true inhibitory response. In rats, Weng and Schouenborg (1996) induced a tonic excitatory re¯ex by squeezing a small skin fold with a clip. This will not work in humans so a voluntary contraction is the only possibility. The inhibitory responses
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obtained do probably not originate from reciprocal inhibition. In human experimental studies, inhibitory re¯ex receptive ®elds have not been investigated as most studies have been performed in relaxed muscles (Andersen et al., 1999; Grimby, 1963) or at non-normalized muscle contraction levels (Decchi et al., 1997; Rossi and Decchi, 1994). Direct nerve stimulation has often been used to elicit the withdrawal re¯ex (Bathien and Bourdarias, 1972; Meinck et al., 1981; Ellrich and Treede, 1998). The present ®ndings indicated that spatially overlapping excitatory and inhibitory receptive ®elds may exist as observed in rats (Weng and Schouenborg, 1996). In studies using direct nerve stimulation, both receptive ®elds may possibly be activated. 4.4. Functional implication of the inhibitory input to the withdrawal re¯ex From the present ®ndings, inhibition of TA at the heel and inhibition of SO at the forefoot do extend the functional, modular organization of the spinal excitatory withdrawal re¯exes (Andersen et al., 1999). Hence, the muscles that may move the exposed skin area towards the stimulus are inhibited. Similar observations have been reported in humans (Kugelberg et al., 1960; Hagbarth, 1960; Meinck et al., 1981) and in animals (Siegler and Burrows, 1986; Hongo et al., 1990; Weng and Schouenborg, 1996). Weng and Schouenborg (1996) showed that in the rat inhibitory receptive ®elds to the excitatory withdrawal re¯ex in single muscles were located complementary to the excitatory re¯ex receptive ®elds with a small overlap. Plasticity of the re¯ex response elicited by a noxious stimulation seems to play an important role in the re¯ex control. In conscious humans the optimal re¯ex pattern can be adapted by repeating the stimuli 250 times (Hagbarth and Finer, 1963). This ®nding shows the possibility of relearning an optimal withdrawal (Holmberg and Schouenborg, 1996). In the standing position, load dependency, posture and balance also play an important role in the re¯ex organization (Decchi et al., 1997; Rossi and Decchi, 1994). The role of reciprocal inhibition cannot be excluded as a mechanism behind the speci®c inhibitory receptive ®elds (Kugelberg et al., 1960), although the antagonistic muscles were not signi®cantly activated by the stimulus in the present study. On the another hand, it seems unlikely that reciprocal inhibition is able to control the spatial variation in re¯ex inhibition on the foot sole. 4.5. Modular organization of the excitatory and inhibitory withdrawal re¯ex The present ®ndings of excitatory and inhibitory re¯exes during voluntary muscle contraction are in line with ®ndings of inhibitory receptive ®elds to the withdrawal re¯ex in the rat (Weng and Schouenborg, 1996). The latter observation extended the hypothesis of `modular' organization of the excitatory withdrawal re¯ex system (Schouenborg et al., 1994) also to include spatial information about inhibition
of the withdrawal re¯ex in the spinal rat. The present ®ndings show that the inhibitory re¯ex not only inhibits the excitatory withdrawal re¯ex (Weng and Schouenborg, 1996), but also inhibits tonic voluntary contraction of a muscle. Not only high threshold afferents activate the inhibitory re¯exes, but also low threshold afferents seem to play an important role. On the other hand, high stimulation intensities do activate excitatory withdrawal re¯exes that occasionally reverse the inhibitory re¯ex, which is seen in the TA excitatory response in the short re¯ex loop for the strong stimulus intensity in the present study. The re¯ex reversal can be explained by overlapping and competing excitatory and inhibitory receptive ®elds. The stimulus activates the inhibitory or excitatory re¯ex pathways with different gains resulting in the possibility of re¯ex reversal and thereby an optimal withdrawal. In conclusion, the present study demonstrates a method for mapping both excitatory and inhibitory re¯ex receptive ®elds during pre-contraction. Both inhibitory and excitatory re¯ex receptive ®elds of the tibialis anterior and soleus muscles were determined by electrical stimulation of the foot sole. The excitatory and inhibitory re¯ex receptive ®elds were organized in a highly functional manner optimizing the withdrawal of the exposed skin area. The overlapping of the excitatory and inhibitory receptive ®elds indicates a highly functional interaction between excitation and inhibition of the speci®c muscle taking stimulus intensity into consideration. Inhibitory receptive ®elds were identi®ed using stimulation intensities at 3 times the perception threshold indicating that non-nociceptive afferents are involved in the inhibitory response. At nociceptive intensities (1.5 times pain threshold) TA inhibition was reversed to an excitatory response while SO inhibition was facilitated. Acknowledgements This study was supported by the Danish National Research Foundation. References Andersen OK, Sonnenborg FA, Arendt-Nielsen L. Modular organization of human leg withdrawal re¯exes elicited by electrical stimulation of the foot sole. Muscle Nerve 1999;22:1520±1530. Bathien N, Bourdarias H. Lower limb cutaneous re¯exes in hemiplegia. Brain 1972;95:447±456. Burke D, Mackenzie RA, Skuse NF, Lethlean AK. Cutaneous afferent activity in median and radial nerve fascicles: a microelectrode study. J Neurol Neurosurg Psychiatry 1975;38:855±864. Burke RE. The use of state-dependent modulation of spinal re¯exes as a tool to investigate the organization of spinal interneurons. Exp Brain Res 1999;128:263±277. Burne JA, Lippold OC. Re¯ex inhibition following electrical stimulation over muscle tendons in man. Brain 1996;119:1107±1114. Chapman CE, Bushnell MC, Miron D, Duncan GH, Lund JP. Sensory perception during movement in man. Exp Brain Res 1987;68:516±524. Decchi B, Zalaf® A, Spidalieri R, Arrigucci U, Di TA, Rossi A. Spinal
F.A. Sonnenborg et al. / Clinical Neurophysiology 111 (2000) 2160±2169 re¯ex pattern to foot nociceptive stimulation in standing humans. Electroenceph clin Neurophysiol 1997;105:484±489. Ellrich J, Treede R-D. Convergence of nociceptive and non-nociceptive inputs onto spinal re¯ex pathways to the tibialis anterior muscle in humans. Acta Physiol Scand 1998;163:391±401. Feine JS, Chapman CE, Lund JP, Duncan GH, Bushnell MC. The perception of painful and non-painful stimuli during voluntary motor activity in man. Somatosens Mot Res 1990;7:113±124. Grimby L. Normal plantar response: Integration of ¯exor and extensor re¯ex components. J Neurol Neurosurg Psychiatry 1963;26:39±50. Hagbarth KE. Spinal withdrawal re¯exes in human lower limbs. J Neurol Neurosurg Psychiatry 1960;23:222±227. Hagbarth KE, Finer B. The plasticity of human withdrawal re¯exes to noxious skin stimuli in lower limbs. In: Moruzzi G, Fessard A, Jasper HH, editors. Brain mechanisms, progress in brain research, vol. 1, Amsterdam: Elsevier, 1963. pp. 65±78. Holmberg H, Schouenborg J. Postnatal development of the nociceptive withdrawal re¯exes in the rat: a behavioural and electromyographic study. J Physiol (Lond) 1996;493:239±252. Hongo T, Kudo N, Oguni E, Yoshida K. Spatial patterns of re¯ex evoked by pressure stimulation of the foot pads in cats. J Physiol (Lond) 1990;420:471±487. Kugelberg E, Eklund K, Grimby L. An electromyographic study of the nociceptive re¯exes of the lower limb Mechanism of the plantar responses. Brain 1960;83:394±410. Lundberg A. Multisensory control of spinal re¯ex pathways. In: Granit R, Pomeiano O, editors. Re¯ex control of posture & movement, progress in brain res, Amsterdam: Elsevier, 1979. pp. 11±28. Meinck HM, Piesiur-Strehlow B, Koehler W. Some principles of ¯exor re¯ex generation in human leg muscles. Electroenceph clin Neurophysiol 1981;52:140±150.
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Pertovaara A, Kemppainen P, LeppaÈnen H. Lowered cutaneous sensitivity to non-painful electrical stimulation during isometric exercise in humans. Exp Brain Res 1992;89:447±452. Rossi A, Decchi B. Flexibility of lower limb re¯ex responses to painful cutaneous stimulation in standing humans: evidence of load-dependent modulation. J Physiol (Lond) 1994;481:521±532. Sandwell DT. Biharmonic spline interpolation of GEOS-3 and SEASAT altimeter data. Geophys Res Lett 1987;14:139±142. Schomburg ED. Spinal sensorimotor system and their supraspinal control. Neurosci Res 1990;7:265±340. Schomburg ED, Steffens H, Kniffki KD. Contribution of group III and IV muscle afferents to multisensorial spinal motor control in cats. Neurosci Res 1999;33:195±206. Schouenborg J, KalliomaÈki J. Functional organization of the nociceptive withdrawal re¯exes I. Activation of hindlimb muscles in the rat. Exp Brain Res 1990;83:67±78. Schouenborg J, Weng H-R, Holmberg H. Modular organization of spinal re¯exes: a new hypothesis. NIPS 1994;9:261±265. Sherrington CS. Flexion-re¯ex of the limb, crossed extension-re¯ex and re¯ex stepping and standing. J Physiol (Lond) 1910:28±121. Siegler MV, Burrows M. Receptive ®elds of motor neurons underlying local tactile re¯exes in the locust. J Neurosci 1986;6:507±513. Weng H-R, Schouenborg J. Cutaneous inhibitory receptive ®elds of withdrawal re¯exes in the decerebrate spinal rat. J Physiol (Lond) 1996;493:253±265. Willer JC, Boureau F, Albe-Fessard D. Role of large diameter cutaneous afferents in transmission of nociceptive messages: electrophysiological study in man. Brain Res 1978;152:358±364. Zehr EP, Stein RB. What functions do re¯exes serve during human locomotion? Prog Neurobiol 1999;58:185±205.
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Modular organization of excitatory and inhibitory re¯ex receptive ®elds elicited by electrical stimulation of the foot sole in man Finn A. Sonnenborg, Ole K. Andersen*, Lars Arendt-Nielsen Laboratory for Experimental Pain Research, Center for Sensory-Motor Interaction, Aalborg University, Fredrik Bajers Vej 7, D3, DK-9220 Aalborg, Denmark Accepted 8 September 2000
Abstract Objectives: The present study aimed to investigate how the inhibitory and excitatory re¯ex components of the human (polysynaptic) withdrawal re¯ex are organized depending on the stimulation site. The re¯exes were elicited during a voluntary pre-contraction (between 10 and 20% of maximum voluntary contraction) of two antagonistic muscles. Methods: Inhibitory and excitatory re¯ex receptive ®elds to tibialis anterior (TA) and soleus (SO) were mapped in 14 healthy subjects using randomized electrical stimulation at 16 sites of the foot sole. Low, non-painful (3£ perception threshold), and high, painful (1.5 £ pain threshold), stimulus intensities were used. Results: The inhibitory re¯ex receptive ®elds were organized in a highly functional manner supporting the action of the excitatory re¯ex. Together the two re¯exes result in an optimal withdrawal from the stimulus. Low stimulation intensity was found suf®cient to elicit the inhibitory re¯ex. High stimulation intensity caused a reversal of the inhibition to excitation in tibialis anterior. In soleus the inhibition was facilitated for stronger intensities. Conclusion: In conclusion, ®ndings in animals of a modular organization of inhibitory re¯exes are reproduced in humans. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Inhibition; Withdrawal re¯ex; Receptive ®eld; Nociception; Human; Flexor re¯ex
1. Introduction The early studies on the ¯exion re¯ex of the limb outlined excitatory re¯ex responses to avoid the stimulus but also inhibitory responses that further support the withdrawal of the limb (Sherrington, 1910). In the control of the withdrawal re¯ex, it has been demonstrated in animals that different afferent ®bre groups, both excitatory and inhibitory, converge onto common interneurones in the spinal a-motoneurone re¯ex pathways (see Lundberg, 1979; Schomburg, 1990; Burke, 1999; for reviews). At least 3 coexistent re¯ex control systems can be identi®ed for spinal motor inhibitory control: (1) a local muscle control system, involving muscle and tendon afferents (Schomburg, 1990); (2) a balance and perception system, which combines descending motor commands and cutaneous/proprioceptive input (Schomburg, 1990; Decchi et al., 1997; Zehr and Stein, 1999); and (3) a cutaneous system which activates single muscles or muscle groups to withdraw a skin area if the stimulus is potentially tissue damaging (Schouenborg et al., 1994). The spinal inte* Corresponding author. Fax: 145-9815-4008. E-mail address: [email protected] (O.K. Andersen).
gration of sensory input in motor control at a pre-motoneuronal level can be assumed to optimize the spinal movement control (Lundberg, 1979; Schomburg et al., 1999). Kugelberg et al. (1960) and Hagbarth and Finer (1963) observed indirect evidence that the re¯ex control of excitatory and inhibitory responses to electrical stimulation of the foot not always shared the same neural control as afferent input from deep structures. This lead to observations by Grimby (1963) who indirectly showed that the cutaneous re¯ex pathways convey information about stimulation site to the a-motoneurone probably through several interneurones. This ®ndings lead to the recent discoveries of a functional, modular organized withdrawal re¯ex in animals (Schouenborg et al., 1994). The presence of excitatory re¯ex receptive ®elds for individual muscles has recently been shown in humans (Andersen et al., 1999) supporting a functional, modular re¯ex organization in humans of the excitatory withdrawal re¯ex. The aim of the present study was to investigate if (1) the inhibitory re¯ex is organized in the same functional manner as the excitatory re¯ex in humans (Andersen et al., 1999) and as the inhibitory re¯ex in animals (Weng and Schouenborg, 1996), and (2) if this inhibition is depending on stimu-
1388-2457/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S13 88-2457(00)0047 2-7
CLINPH 2000537
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Fig. 1. Typical plot of EMG responses based on mean of 5 repetitions from one subject at one stimulation site (site 9). Responses from TA and SO at low and high stimulation intensities are shown. The dotted line shows the limits used to calculate the excitatory and inhibitory onset latencies. The grand mean onset latencies with standard deviation (mean ^ SD) of the ®rst and second response are shown below each plot. `X' marks onset of an excitatory response and `O' marks onset of an inhibitory response.
lation intensities ranging from non-painful to painful. In the present study a newly developed method (Andersen et al., 1999) for investigating the spatial re¯ex sensitivity was used to investigate the inhibitory re¯ex organization. 2. Methods and materials Fourteen subjects (4 females and 10 males, mean ages 23.3 ^ 3.1 years, mean height 178 ^ 8.8 cm) with no known neurological diseases participated in the study. Informed consent was obtained from all subjects and the Helsinki Declaration was respected. 2.1. Set-up The subjects were sitting in a chair supporting arm, back, neck, and left leg. The right foot was strapped into at pedal that prevented the subject from moving the foot. 2.2. Electromyogram The electromyogram (EMG) was recorded with Ag± AgCl surface electrodes placed parallel to the muscle ®bre direction over tibialis anterior (TA) and soleus (SOL) muscles. The recorded EMG-signals were ampli®ed 1000± 10 000 times, passband ®ltered (5±1000 Hz, 2nd order) and analogue to digital converted (2000 Hz, PCI-MIO 16E, National Instruments Corporation, Austin, TX, USA), displayed on computer screen, and stored on a computer
disk. The recording was started 150 ms before stimulus onset and lasted until 1000 ms after stimulus onset. 2.3. Electrical stimulation On the sole of the right foot, 16 electrodes (copper, diameter 0.8 cm) were mounted in a non-uniform grid (Fig. 1). To reduce the bias of differences in skin thickness, the subjects were instructed to grind off thick epidermal layers some days prior to the experiment. A common anode (10 £ 14 cm electrode, Pals, Axelgaard Ltd., Fallbrook, CA) was placed on the dorsum of the foot. This con®guration ensured that the stimulus was always perceived as coming from the sole of the foot. For each electrode position, the electrode was moved slightly in case the evoked sensation indicated direct nerve stimulation (i.e. the sensation radiating to the distal foot). A computer-controlled electrical relay and stimulator delivered a stimulus to one of the 16 electrodes in a randomized sequence. The computer chose the stimulation electrode and adjusted the intensity of the electrical stimulator (Neuromatic 2000 C stimulator, DISA, Skovlunde, Denmark) according to a pre-programmed randomized sequence. Thus, the subject could not predict the stimulus location. Each stimulus consisted of a constant current pulse train of 5 individual 2 ms pulses delivered at 200 Hz. This con®guration was used throughout the experiment. For each electrode position, the intensity of the stimulus was adjusted to
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ensure equal visual analogue scale (VAS) score independent of stimulation site. Two stimulus intensities were chosen. An intensity of 3£ the initial perception threshold (nonpainful) and an intensity of 1.5£ the initial pain threshold (painful) were selected for each electrode position. The inter-stimulus interval was between 20 and 30 s. 2.4. Sensory intensity rating The perceived intensity was rated on a 10 cm electronic continuous visual analogue scale. The scale was anchored by assigning `5.0' as the pain threshold, `0' as no sensation, and `10' as `most intense pain imaginable'. Each electrical stimulus was scored by the subject and stored on the computer. 2.5. Experimental procedure Before the experiment, the subject was asked to perform maximal voluntary contraction (MVC) of the soleus and tibial anterior muscles in two separate sessions. The measurement was repeated 3±4 times for both muscles. The RMS value (MVCRMS) in a 500 ms time window centred at the maximal peak of the EMG signals was calculated and the maximal MVCRMS of the 3±4 repetitions was used as reference point during the experiment. The perception threshold and pain threshold to each stimulation site were assessed through randomized ascending and descending stimulation intensities. Each threshold was assessed at least 3 times and the ®nal value was used as the threshold. A ®le containing a random sequence of site and associated stimulus intensity (low intensity (3£ perception threshold) or high intensity (1.5 £ pain threshold)) for electrical stimulation during the experiment was generated. The EMG signals from the investigated muscle were monitored on a custom made computer program (Labview, National Instruments Corporation, Austin, TX, USA) during the voluntary contraction. During re¯ex recordings, the computer program calculated the RMS value of the EMG muscle signal in real time in 150 ms non-overlapping windows. This signal was presented as a fraction of the MVCRMS and as feedback to the subject in order to maintain a contraction level between 10 and 20% of MVCRMS (Burne and Lippold, 1996). If the contraction level was within the range for at least 3 s, a stimulus was released. After each stimulus, the subject rated the sensory intensity on the electronic VAS. Each of the 16 stimulation sites was stimulated 5 times for both intensities. The study was divided into two measuring sessions. First recording during voluntary contractions of the TA muscle followed by a session of voluntary contraction of the SOL muscle. 2.6. Data analysis Initially each EMG sweep for TA and SOL was normalized to the background activity. As described by Andersen et al. (1999), spatial variation in the skin withdrawal re¯ex
can be visualized by two-dimensional interpolation (Kriging algorithm, non-uniformly spaced data-points, inverse distance method (Sandwell, 1987)) to describe the Re¯ex Receptive Fields (RRF) (Schouenborg and KalliomaÈki, 1990). This method both interpolates and extrapolates the data values to cover the entire plantar surface of the foot for a complete overview of the re¯ex sensitivity. In general, the extrapolation must be interpreted cautiously. The following features were calculated: root mean square (RMS) value of the normalized muscle EMG re¯ex responses, the time the EMG signal was inhibited, and ®nally the VAS to each stimulation. Inhibition of a voluntary contracted muscle was de®ned as 60% suppression of the EMG signal compared to the prestimulation contraction level (background level) (Fig. 1). The suppression should last longer than 15 ms to be accepted as an inhibition. If these two criteria were ful®lled, the inhibition onset time was determined as the time point where the EMG-signal ®rst was suppressed more than 60% of the background level. The inhibition offset time was calculated as the ®rst time the EMG suppression was less than 60% of the background level. If the EMG suppression was less than 60% background level with a duration of less than 10 ms, the crossing was not accepted as the offset but as a short lasting burst. The duration of the inhibition was subsequently calculated as the difference between inhibition onset and offset. The duration was calculated for every suppression phase in each sweep that met the inhibition criteria. For each sweep the total duration of inhibition was then calculated as the summation of all inhibitory periods detected in that sweep. An excitatory re¯ex response was de®ned as 100% facilitation of the EMG signal compared to the pre-stimulation contraction level (i.e. 2 times the background contraction level, see Fig. 1). The facilitation should last longer than 10 ms to be accepted as an excitatory component. If these two criteria were ful®lled, the excitation onset time was determined as the time point where the EMG-signal ®rst was facilitated more than 100% of the background level. Two different time intervals were chosen for re¯ex feature calculation. The limits of the analysis intervals were chosen based on an analysis of the inhibitory onset times, see results. A short re¯ex loop (SRL) covering 40± 150 ms after stimulation onset and a long re¯ex loop (LRL) covering 150±250 ms after stimulation onset were used. For calculation of inhibitory duration, the onset of the individual inhibitions was used to determine within which analysis interval the duration of inhibition should be included. A correlation analysis for comparison of interpolated maps of mean detection and pain thresholds was computed using a two-dimensional cross-correlation algorithm. 2.7. Statistics One-way repeated measure ANOVA was applied to the data when the data were normally distributed otherwise non-
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parametric Friedman Repeated Measures Analysis of Variance on Ranks was used. Student±Newman±Keuls (SNK) Test was used for post-hoc pairwise comparisons. The non-parametric two-sample paired test Wilcoxon Signed Rank test was used to evaluate paired observations. The results are presented as mean values and standard deviation.
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On average the pain threshold was 6.6 ^ 2.3 times larger than the perception threshold, whereby the high stimulation intensity on average was 9.9 ^ 3.5 time larger than the perception threshold. The spatial variation in the detection and pain thresholds were strongly correlated (R 0:94). 3.2. Re¯ex response and analysis intervals
3. Results All 14 subjects were able to maintain the voluntary contraction level and inhibition and/or excitation periods could be elicited in all subjects by the electrical stimulation. The mean maximal voluntary contraction (MVC) of TA was 457 ^ 198 and SO 214 ^ 83 mV. 3.1. Stimulation intensities and sensory intensity scoring The mean perception threshold was 2.6 ^ 1.2 mA for the 16 stimulation sites. A signi®cant difference between the perception thresholds was found (Friedman, P , 0:01) with a lower perception threshold (SNK, P , 0:05) at the mid foot (site 11, 12, and 13) and toes (site 1 and 2) than the heel and distal forefoot (site 3, 4, 5, 6, 8, 15, and 16) (Fig. 2). The reported sensory ratings to the low stimulation intensities (3£ perception threshold) are shown in Fig. 2. A significant difference was found between the stimulation sites (Friedman, P , 0:01) with higher sensory ratings at two stimulation sites (site 8 and 16). A mean pain threshold of 16.0 ^ 7.3 mA was observed for all 16 stimulation sites (Fig. 2). A signi®cant difference between the pain thresholds was found (Friedman, P , 0:01) with higher pain thresholds (SNK, P , 0:05) at the heel (site 14, 15, and 16) and at the distal forefoot (site 3, 4, 5, and 6) than the mid foot (site 7, 8, 9, 10, 11, 12, and 13) and toes (site 1 and 2). The reported sensory ratings to the high stimulation intensities (1.5£ pain threshold) are shown in Fig. 2. A signi®cant difference was found in sensory ratings between the stimulation sites (Friedman, P , 0:01) with lower sensory ratings (SNK, P , 0:05) for sites 3, 6, and 7 compared with sites 1, 2, 8, 9, 13, and 16.
Fig. 2. The grand mean pain threshold, perception thresholds, and subjective intensity scores to the low intensity (3£ perception threshold) and high intensity (1.5£ pain thresholds) stimulation. The ®gure illustrates the spatially interpolated mean thresholds.
Low intensity and high intensity stimulation of the foot sole during voluntary muscle contraction resulted in different inhibitory and excitatory patterns for TA and SO muscles (Figs. 1 and 3). Generally the TA response consisted of one excitatory and one or two inhibitory responses (Figs. 1 and 3). Fig. 1 illustrates a typical response with the mean onset latencies listed in Table 1. A signi®cant difference in inhibitory onset latencies between the ®rst and second inhibitory periods was observed (Friedman, P , 0:05). Generally the SO response consisted of two inhibitory responses separated by one excitatory phase, though stimulation of the heel resulted in two excitatory phases, and two inhibitory responses (Figs. 1 and 3). Fig. 1 illustrates a typical response with the mean onset latencies listed in Table 1. Based on these different re¯ex components, two time intervals were chosen for the further analysis. (1) A short re¯ex loop (SRL) covering 40±150 ms after stimulation onset and (2) the long re¯ex loop (LRL) covering 150± 250 ms after stimulation onset as described in the Section 2. 3.3. TA inhibitory re¯ex response 3.3.1. Short re¯ex loop (40±150 ms) response to low intensity stimulation Low intensity electrical stimulation of the plantar foot side resulted in an inhibitory re¯ex response at the heel, digit 2 and distal lateral forefoot in the SRL interval (Fig. 4). Site 8 had a signi®cant larger response than the 7 sites expressing inhibition (site 2, 5, 6, 13, 14, 15, and 16, illustrated in Fig. 4, Friedman, P , 0:001). The duration of inhibition was largest on the forefoot (Fig. 4). The grand mean duration of inhibition was 39.8 ^ 19.3 ms. There was a signi®cant difference in the duration of inhibition (Friedman, P , 0:001), but no post-hoc differences were found, indicating a minimum difference between sites. 3.3.2. Short re¯ex loop (40±150 ms) response to high intensity stimulation No signi®cant inhibition was found in re¯ex size (RMS value, full interval) compared to baseline. The duration of inhibition was longer at the distal lateral foot and at the heel, complementary to the areas with excitatory re¯ex responses (Fig. 4). The grand mean duration of inhibition for all stimulation sites was 42.9 ^ 20.4 ms. A signi®cant difference was found between the duration of inhibition (Friedman, P , 0:001), but no post-hoc differences were found indicating a minimum difference between sites.
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Fig. 3. The grand mean EMG response from TA and SO to low intensity and high intensity stimulation for the 16 sites. The positions of the stimulation sites are shown in the centre of the ®gure. Each EMG response was normalized to the voluntary background activity. In the mean re¯ex response, there is a tendency for an early, excitatory re¯ex in TA for the low intensity stimulation, which is even more pronounced for the strong stimulus intensity. Further, a long latency inhibition is seen in the TA response for the strong stimulation intensity. For SOL inhibition seems be prevailing. However, please note the tendency for an early excitatory response for the posterior stimulation sites. For the statistically signi®cant changes, see Section 3.
3.3.3. Long re¯ex loop (150±250 ms) response to low intensity stimulation Inhibition of the muscle response was observed on all stimulation sites (Fig. 4). All sites were signi®cantly different in re¯ex response from the voluntary background activity (Wilcoxon, P , 0:05), while no difference was found between sites. No signi®cant difference between sites was observed in the duration of inhibition (Fig. 4). The grand mean duration of inhibition was 25.9 ^ 11.1 ms.
P , 0:05, even though inhibitory responses can be seen in the mean plots, see Fig. 3). A signi®cant difference in the re¯ex response was found between stimulation sites (Friedman, P , 0:001), but no post-hoc differences were found. No signi®cant differences between sites were observed in the duration of inhibition (Fig. 4). The grand mean duration of inhibition was 40.4 ^ 37.4 ms.
3.3.4. Long re¯ex loop (150±250 ms) response to high intensity stimulation High intensity stimulation resulted in inhibition at the medial and lateral sides (Fig. 4) (only signi®cant inhibited at site 5 compared to the background activity, Wilcoxon,
Excitatory responses were only elicited by high stimulation intensities in the short re¯ex loop of TA. At low intensity stimulation there was no signi®cant excitation found even though the mean plot may indicate this. The excitatory response was observed when the arch of the foot was stimu-
3.4. TA excitatory re¯ex response
F.A. Sonnenborg et al. / Clinical Neurophysiology 111 (2000) 2160±2169 Table 1 Grand mean onset latencies to excitatory and inhibitory responses elicit by low (3£ perception threshold) and high (1.5£ pain threshold) stimulus intensity a Muscle/ intensity
Event no.
Event
Onset latency (ms)
Number detected (out of 1120 sweeps)
TA (high)
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
Excitatory Inhibitory Excitatory Inhibitory Excitatory Inhibitory Excitatory Inhibitory Inhibitory Excitatory Inhibitory Excitatory Inhibitory Excitatory Excitatory Inhibitory
87.8 ^ 32.0 116.6 ^ 26.9 173.1 ^ 67.6 218.7 ^ 63.2 95.7 ^ 34.5 113.0 ^ 22.7 185.4 ^ 77.8 214.4 ^ 65.5 87.1 ^ 35.0 139.9 ^ 35.8 183.5 ^ 64.2 208.9 ^ 76.8 95.8 ^ 34.8 123.1 ^ 34.3 193.6 ^ 73.0 226.5 ^ 72.4
718 780 456 667 398 820 207 599 929 491 885 221 624 394 173 592
TA (low)
SO (high)
SO (low)
a
Event number illustrates the sequence in which the excitation/inhibition occurred. Event is the type of response detected. Onset latencies are presented in ms.
lated, see Fig. 4. Thus, a signi®cant re¯ex excitation was seen when the arch and medial side of the foot was stimulated at the high intensity, site 1, 3, 4, 7, 8, 9, 10, 11, 12, 13, and 14 (Wilcoxon, P , 0:05), illustrated on Fig. 4. A significant difference in re¯ex responses between the stimulation sites was found (Friedman, P , 0:001) with the heel (site 14, 15, and 16) and digit 2 having smaller responses than the mid foot (site 7, 8, 9, 11, and 12).
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response was signi®cantly inhibited when the distal forefoot was stimulated (site 1±6, 8, 9, 10, 12, and 13) compared to the background activity (Wilcoxon, P , 0:05) (Fig. 5). There was a signi®cant difference in re¯ex response between stimulation sites when comparing the muscle responses, with smaller responses at the distal forefoot than on the heel (Friedman, P , 0:001, sites 15 and 16 being signi®cantly larger than site 1, 2, 4, 5, 6, and 9, site 16 also larger than site 8, 10, 12, and 13). The duration of inhibition was short on the entire plantar side of the foot (compare the duration of inhibition between short re¯ex loop and long re¯ex loop in Fig. 4). The grand mean duration of inhibition was 34.3 ^ 18.0 ms. There was a signi®cant difference in the duration of inhibition (Friedman, P , 0:001), but no post-hoc differences were found. This indicated a minimum difference between sites. 3.6.2. short re¯ex loop (40±150 ms) response to high intensity stimulation Inhibition was observed when stimulating the forefoot (Wilcoxon, P , 0:05) (Fig. 5). A signi®cant difference in re¯ex responses between the stimulation sites was found (Friedman, P , 0:001) with stimulation at the heel (sites 12, 14, 15, and 16) evoking signi®cantly larger re¯ex responses than forefoot stimulation (site 1, 2, 3, 4, 5, 6, 8, and 9). The duration of inhibition was longer in the area
3.5. Low vs. strong stimulation intensities in the TA response When the low stimulation intensities from short re¯ex loop were compared to the long re¯ex loop responses independent of stimulation site, the short re¯ex loop had the longest duration of inhibition (39.8 ^ 19.3 ms vs. 25.9 ^ 11.1 ms) while the strongest inhibition was observed in the long re¯ex loop (24.5 ^ 30.4% vs. 216.0 ^ 23.5%) (Wilcoxon, P , 0:001). For the high intensity responses, the largest muscle response (20.4 ^ 44.8 vs. 1.46 ^ 38.9%) and the longest duration of inhibition (42.9 ^ 20.4 vs. 40.4 ^ 37.4 ms) were seen in the short re¯ex loop independent of the stimulation site (Wilcoxon, P , 0:001). 3.6. SO inhibitory re¯ex response 3.6.1. short re¯ex loop (40±150 ms) response to low intensity stimulation Low intensity stimulation resulted in an inhibitory re¯ex response, which covered the forefoot (Fig. 5). The muscle
Fig. 4. The re¯ex receptive ®elds for the inhibitory and excitatory muscle responses and inhibitory duration during TA voluntary contraction for low intensity and high intensity stimulation. The top row illustrates the muscles response analyzed in the short re¯ex loop (SRL) from 40 to 150 ms after stimulus onset and long re¯ex loop (LRL) from 150 to 250 ms. In the muscle response plots, the results of the statistical comparison with the background activity level are shown as: 4 for inhibition, 1 for excitation, and white circles as non-signi®cant difference (Wilcoxon, P , 0:05). The lower row illustrates the duration of inhibition.
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with inhibition (Fig. 5). A signi®cant difference was found between the duration of inhibition (Friedman, P , 0:001), especially the distal medial foot stimulation (site 2 and 3) had longer duration of inhibition, 68.1 ^ 41.4 ms, than stimulation on the heel (site 12, 14 and 15), 50.4 ^ 27.1 ms. The grand mean duration of inhibition was 60.4 ^ 35.1 ms. 3.6.3. Long re¯ex loop (150±250 ms) response to low intensity stimulation Inhibition of the muscle response was observed on all stimulation sites, but strong inhibition was seen for stimulation on the heel (Fig. 5). All sites, except site 2, were significantly inhibited compared to the voluntary background activity (Wilcoxon, P , 0:05). A signi®cant difference in re¯ex responses between the stimulation sites was found (Friedman, P , 0:001) with larger inhibition for stimulation on the heel (site 14, 15, and 16) than for stimulation on the distal forefoot (site 2, 4, 5, and 6). No signi®cant differences in the duration of inhibition were found between the sites, but there was a tendency to longer duration of inhibition for heel stimulation (Fig. 5). The grand mean duration of inhibition was 40.7 ^ 28.9 ms. 3.6.4. Long re¯ex loop (150±250 ms) response to high intensity stimulation High intensity stimulation resulted in inhibition at the
base of the heel (Fig. 5). Only stimulation at the base of the heel showed signi®cant inhibition compared to the background activity (site 14 and 15) (Wilcoxon, P , 0:05) (Fig. 5). A signi®cant difference in the re¯ex responses between the stimulation sites was found (Friedman, P , 0:001) with stronger inhibition on site 15, mean 222 ^ 51.2%, than site 1, 6, 8, 9, 10, 11, and 16, mean 0.9 ^ 51.4%. Stimulation of the heel and mid-foot evoked long duration of inhibition compared to the distal medial area. However, no signi®cant difference in the duration of inhibition was found between sites. The area with the longest duration of inhibition in long re¯ex loop was complementary to the areas with long duration of inhibition in the short re¯ex loop interval (Fig. 5). The grand mean duration of inhibition was 67.2 ^ 48.3 ms. When the low stimulation intensities from short re¯ex loop were compared to the long re¯ex loop responses independent of stimulation site, the long re¯ex loop had the largest inhibition (29.6 ^ 32 vs. 216.6 ^ 28%) and longest duration of inhibition (34.3 ^ 17.8 vs. 40.7 ^ 28.9 ms) (Wilcoxon, P , 0:001). On the other hand, for the high intensity responses, the short re¯ex loop had the largest inhibition (220.0 ^ 47.2 vs. 24.56 ^ 50.6%) while the long re¯ex loop had the longest duration of inhibition (60.4 ^ 35.1 vs. 67.2 ^ 48. 3 ms) (Wilcoxon, P , 0:001). 3.7. SO excitatory re¯ex response No signi®cant excitatory re¯exes were observed in SO at any stimulation intensity or analysis interval. 3.8. Low vs. high intensity stimulation For high intensity stimulation, TA excitatory responses were seen in the arch of the foot while statistically signi®cant inhibitory responses were found for low intensity stimulation surrounding this area (Fig. 4). The duration of TA inhibition was signi®cantly longer in the long re¯ex loop for high intensity stimulation compared to low stimulation intensity (40.4 ^ 37.4 vs. 25.9 ^ 11.1 ms), (Wilcoxon, P , 0:01). SO was stronger inhibited in the short re¯ex loop at high stimulation intensities, while mean inhibition decreased in the long re¯ex loop compared to the low stimulation intensity (Wilcoxon, P , 0:01). Duration of SO inhibition was signi®cantly longer in both intervals for high stimulation intensities compared to low stimulus intensities (SRL: 60.4 ^ 35.1 vs. 34.3 ^ 18 ms and LRL: 67.2 ^ 48.3 vs. 40.7 ^ 28.9 ms) (Wilcoxon, P , 0:01).
Fig. 5. The re¯ex receptive ®elds to the inhibitory and excitatory muscle responses and inhibitory duration during voluntary background activation of SO for low intensity and high intensity stimulation. The statistical comparison with the background activity level are shown as: 4 for inhibition, 1 for excitation and white circles as a non-signi®cant difference (Wilcoxon Signed Rank Test, P , 0:05). The lower row illustrates the duration of inhibition.
4. Discussion In the present study, electrical stimulation was used to map the cutaneous re¯ex receptive ®elds in healthy humans. The topographical maps of the excitatory and inhibitory zones for speci®c muscles were assessed by stimulation during voluntary contraction. The inhibition of voluntary
F.A. Sonnenborg et al. / Clinical Neurophysiology 111 (2000) 2160±2169
muscle contraction was evaluated based on the size of the EMG suppression and the duration of the suppression while excitation was evaluated on EMG facilitation. Stimulation of speci®c skin areas was found to inhibit TA and SO. This indicated that re¯ex pathways encoding the spatial inhibitory information are present. In skin areas not belonging to the inhibitory zone, an excitatory re¯ex could be evoked in TA. A stimulus intensity dependency was found in TA as low stimulus intensity resulted in an inhibitory re¯ex whereas an excitatory re¯ex was observed for the high stimulus intensity. The organization and functionality of the inhibitory and excitatory re¯ex receptive ®elds will be discussed. 4.1. Which afferents are activated The present electrode con®guration (small cathodes on the plantar side and a common anode on the dorsal side) secured that the highest current density was within the epidermal layer underlying the cathode. However, direct activation of the intrinsic foot muscles cannot be avoided with absolute certainty, especially for the strong stimulus intensities. However, the overall sensation (pin-prick) is coming from the super®cial skin of the foot as reported by the volunteers and not from the location of the common anode on the dorsal side of the foot. Therefore, we do not believe the re¯ex itself is initiated by any proprioceptive input. Further, the topographical organization of the re¯ex receptive ®elds was related to the biomechanical function clearly suggesting that any proprioceptive input only had modulatory effect. The signi®cant difference between stimulation sites for the sensory ratings cannot be explained easily. Thus, the attempt to obtain even afferent in¯ow from all stimulation sites was not completely ful®lled, yet the pain threshold pattern did not follow the spatial variation in sensory ratings. This could indicate that the variation in sensory ratings probably originates from something else than the spatial variation in stimulation intensity. The 3 sites with signi®cant lower ratings were 11.8% smaller than the remaining 13 sites, i.e. lower afferent in¯ow. The strong correlation between the detection and pain thresholds suggests that the spatial variation in sensitivity is most likely re¯ecting skin characteristics rather than peripheral neural factors. The detection and pain thresholds were only determined with relaxed muscles although muscle contraction is known to affect cutaneous sensitivity to electrical stimuli (Chapman et al., 1987). One threshold determination was chosen (Pertovaara et al., 1992) primarily to avoid mental and physical fatigue since the procedure took almost 1 h for the 16 stimulation sites. However, the VAS ratings to the painful stimuli in the present study was similar to ratings during relaxed conditions (4.2 ^ 1.3 cm in the present study and 4.2 ^ 1.5 cm in Andersen et al. (1999)) suggesting
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minimal in¯uence on the supra-threshold sensory transmission (Chapman et al., 1987; Feine et al., 1990). Using microneurography (Burke et al., 1975) showed that electrical stimulation of the median and radial nerve at 5 times the detection threshold activated nociceptive ®bres in humans. In the present study, the high stimulation intensity was 9.9 ^ 3.5 times higher than the perception threshold, indicating that Ad afferents most likely were activated (Burke et al., 1975; Willer et al., 1978). The low intensity stimulation, 3 times the perception threshold, probably did not activate nociceptive afferents and the stimulus intensity was well below the pain threshold. 4.2. Main re¯ex results Both inhibition and excitation were observed in the re¯ex responses. The position of the excitatory re¯ex receptive ®elds on the foot sole is in agreement with earlier ®ndings (Grimby, 1963; Andersen et al., 1999) in relaxed muscles. Excitatory re¯ex receptive ®elds were found in TA for the high stimulation intensity in the short re¯ex loop interval in contrast to the inhibitory ®elds observed for low intensity stimulation. This indicates an intensity dependency for TA, with a reversal of the re¯ex response from inhibition to excitation for painful input compared to tactile input. Although the muscle response was facilitated at high stimulation intensities, the longest duration of inhibition was obtained at stimulation sites expressing no signi®cant change from background activity. This indicates that inhibitory responses were expressed by two independent parameters: the actual amplitude suppression and the duration of suppression. More powerful inhibitory re¯exes were observed in SO for high stimulation intensities compared to low intensities in the short re¯ex loop. Thus, the duration of inhibition was prolonged which indicated stronger inhibition. A clear spatial difference was found between the excitatory and inhibitory areas in both TA and SO. Inhibitory receptive ®elds were found in areas complementary to areas where excitatory re¯exes have previously been described (Andersen et al., 1999), and are in agreement with earlier ®ndings (Grimby, 1963; Kugelberg et al., 1960; Meinck et al., 1981; Andersen et al., 1999). In the long re¯ex loop, clear inhibition at low stimulation intensities was evoked from all sites in both muscles. 4.3. Stimulation method The technique of cutaneous stimulation during voluntary contraction to investigate inhibitory pathways has been used before (Kugelberg et al., 1960). However, the voluntary muscle contractions may potentially modulate the output since the voluntary drive alters the excitability of the motorneurone pool and hence may conceal the true inhibitory response. In rats, Weng and Schouenborg (1996) induced a tonic excitatory re¯ex by squeezing a small skin fold with a clip. This will not work in humans so a voluntary contraction is the only possibility. The inhibitory responses
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obtained do probably not originate from reciprocal inhibition. In human experimental studies, inhibitory re¯ex receptive ®elds have not been investigated as most studies have been performed in relaxed muscles (Andersen et al., 1999; Grimby, 1963) or at non-normalized muscle contraction levels (Decchi et al., 1997; Rossi and Decchi, 1994). Direct nerve stimulation has often been used to elicit the withdrawal re¯ex (Bathien and Bourdarias, 1972; Meinck et al., 1981; Ellrich and Treede, 1998). The present ®ndings indicated that spatially overlapping excitatory and inhibitory receptive ®elds may exist as observed in rats (Weng and Schouenborg, 1996). In studies using direct nerve stimulation, both receptive ®elds may possibly be activated. 4.4. Functional implication of the inhibitory input to the withdrawal re¯ex From the present ®ndings, inhibition of TA at the heel and inhibition of SO at the forefoot do extend the functional, modular organization of the spinal excitatory withdrawal re¯exes (Andersen et al., 1999). Hence, the muscles that may move the exposed skin area towards the stimulus are inhibited. Similar observations have been reported in humans (Kugelberg et al., 1960; Hagbarth, 1960; Meinck et al., 1981) and in animals (Siegler and Burrows, 1986; Hongo et al., 1990; Weng and Schouenborg, 1996). Weng and Schouenborg (1996) showed that in the rat inhibitory receptive ®elds to the excitatory withdrawal re¯ex in single muscles were located complementary to the excitatory re¯ex receptive ®elds with a small overlap. Plasticity of the re¯ex response elicited by a noxious stimulation seems to play an important role in the re¯ex control. In conscious humans the optimal re¯ex pattern can be adapted by repeating the stimuli 250 times (Hagbarth and Finer, 1963). This ®nding shows the possibility of relearning an optimal withdrawal (Holmberg and Schouenborg, 1996). In the standing position, load dependency, posture and balance also play an important role in the re¯ex organization (Decchi et al., 1997; Rossi and Decchi, 1994). The role of reciprocal inhibition cannot be excluded as a mechanism behind the speci®c inhibitory receptive ®elds (Kugelberg et al., 1960), although the antagonistic muscles were not signi®cantly activated by the stimulus in the present study. On the another hand, it seems unlikely that reciprocal inhibition is able to control the spatial variation in re¯ex inhibition on the foot sole. 4.5. Modular organization of the excitatory and inhibitory withdrawal re¯ex The present ®ndings of excitatory and inhibitory re¯exes during voluntary muscle contraction are in line with ®ndings of inhibitory receptive ®elds to the withdrawal re¯ex in the rat (Weng and Schouenborg, 1996). The latter observation extended the hypothesis of `modular' organization of the excitatory withdrawal re¯ex system (Schouenborg et al., 1994) also to include spatial information about inhibition
of the withdrawal re¯ex in the spinal rat. The present ®ndings show that the inhibitory re¯ex not only inhibits the excitatory withdrawal re¯ex (Weng and Schouenborg, 1996), but also inhibits tonic voluntary contraction of a muscle. Not only high threshold afferents activate the inhibitory re¯exes, but also low threshold afferents seem to play an important role. On the other hand, high stimulation intensities do activate excitatory withdrawal re¯exes that occasionally reverse the inhibitory re¯ex, which is seen in the TA excitatory response in the short re¯ex loop for the strong stimulus intensity in the present study. The re¯ex reversal can be explained by overlapping and competing excitatory and inhibitory receptive ®elds. The stimulus activates the inhibitory or excitatory re¯ex pathways with different gains resulting in the possibility of re¯ex reversal and thereby an optimal withdrawal. In conclusion, the present study demonstrates a method for mapping both excitatory and inhibitory re¯ex receptive ®elds during pre-contraction. Both inhibitory and excitatory re¯ex receptive ®elds of the tibialis anterior and soleus muscles were determined by electrical stimulation of the foot sole. The excitatory and inhibitory re¯ex receptive ®elds were organized in a highly functional manner optimizing the withdrawal of the exposed skin area. The overlapping of the excitatory and inhibitory receptive ®elds indicates a highly functional interaction between excitation and inhibition of the speci®c muscle taking stimulus intensity into consideration. Inhibitory receptive ®elds were identi®ed using stimulation intensities at 3 times the perception threshold indicating that non-nociceptive afferents are involved in the inhibitory response. At nociceptive intensities (1.5 times pain threshold) TA inhibition was reversed to an excitatory response while SO inhibition was facilitated. Acknowledgements This study was supported by the Danish National Research Foundation. References Andersen OK, Sonnenborg FA, Arendt-Nielsen L. Modular organization of human leg withdrawal re¯exes elicited by electrical stimulation of the foot sole. Muscle Nerve 1999;22:1520±1530. Bathien N, Bourdarias H. Lower limb cutaneous re¯exes in hemiplegia. Brain 1972;95:447±456. Burke D, Mackenzie RA, Skuse NF, Lethlean AK. Cutaneous afferent activity in median and radial nerve fascicles: a microelectrode study. J Neurol Neurosurg Psychiatry 1975;38:855±864. Burke RE. The use of state-dependent modulation of spinal re¯exes as a tool to investigate the organization of spinal interneurons. Exp Brain Res 1999;128:263±277. Burne JA, Lippold OC. Re¯ex inhibition following electrical stimulation over muscle tendons in man. Brain 1996;119:1107±1114. Chapman CE, Bushnell MC, Miron D, Duncan GH, Lund JP. Sensory perception during movement in man. Exp Brain Res 1987;68:516±524. Decchi B, Zalaf® A, Spidalieri R, Arrigucci U, Di TA, Rossi A. Spinal
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