Brain Advance Access originally published online on June 30, 2004
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Brain, Vol. 127, No. 8, 1887-1898,
August 2004
© 2004 Guarantors of Brain
doi: 10.1093/brain/awh212
Cortico-motoneuronal excitation of three hand muscles determined by a novel penta-stimulation technique
Motor Cortex Laboratory, Department of Neurology, Johann Wolfgang Goethe-University of Frankfurt, Frankfurt am Main, Germany
Correspondence to: Ulf Ziemann, Motor Cortex Laboratory, Department of Neurology, Johann Wolfgang Goethe-University of Frankfurt, Schleusenweg 2–16, D-60528 Frankfurt am Main, Germany E-mail: u.ziemann{at}em.uni-frankfurt.de
Received January 9, 2003. Revised April 7, 2004. Accepted April 9, 2004.
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Summary |
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The cortico-motoneuronal system (CMS), i.e. the monosynaptic projection from primary motor cortex to motoneurons in lamina IX of the spinal cord is, among all mammals, best developed in humans. Increasing evidence suggests that the CMS is crucially important for skilled individuated finger movements. Little is known about to what extent the strength of the CMS differs between hand muscles. Here we measured CMS excitation to the first dorsal interosseus (FDI), abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles in healthy subjects by using a novel penta-stimulation technique (PST) and single motor unit (SMU) recordings. The PST is an extension of the triple-stimulation technique. It applies two additional supramaximal electrical stimuli at the wrist to the ‘peripheral nerve of no interest’ (in the case of the FDI and ADM the median nerve, in the case of the APB the ulnar nerve) to collide with the descending volleys in that nerve elicited by transcranial magnetic stimulation of motor cortex and electrical stimulation of Erb's point. This eliminates volume conduction from neighbouring muscles innervated by the nerve of no interest and, therefore, allows accurate determination of the PST response. The PST response was significantly larger in the FDI compared with the ADM and APB. This was validated by the SMU recordings, which showed a higher estimated amplitude of the mean compound excitatory postsynaptic potential in spinal motoneurons of the FDI than in those of the APB and ADM. Finally, as a possible functional correlate, the maximum rate of repetitive voluntary finger movements was higher for index finger abduction (prime mover, FDI) than for little finger abduction (prime mover, ADM) and thumb abduction (prime mover, APB), and individual differences in maximum rate between the different movements correlated with individual differences in the corresponding PST responses. In conclusion, PST is a valuable novel method for accurate quantification of CMS excitation. The findings strongly suggest that CMS excitation differs between hand muscles and that these differences directly link to capability differences in individuated finger movements.
Key Words: human cortico-motoneuronal system; intrinsic hand muscles; penta-stimulation technique; transcranial magnetic stimulation; single motor unit recording
Abbreviations: ADM = abductor digiti minimi muscle; AMT = active motor threshold; APB = abductor pollicis brevis muscle; cEPSP = compound excitatory post-synaptic potential; CMS = cortico-motoneuronal system; EMG = electromyogram; FDI = first dorsal interosseous muscle; ISI = inter-spike interval; M1 = primary motor cortex; MEP = motor evoked potential; MN = motor neuron; N1 = nerve of interest; N2 = nerve of no interest; PP = primary peak; PSTH = peri-stimulus time histogram; PST = penta-stimulation technique; RMT = resting motor threshold; SMU = single motor unit; TMS = transcranial magnetic stimulation; TST = triple-stimulation technique.
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Introduction |
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H. G. J. M. Kuypers provided systematic evidence that the cortico-motoneuronal system (CMS), i.e. the mono-synaptic projection from the primary motor cortex (M1) to spinal
-motoneurons (MNs) in the anterior horn (Rexed lamina IX), does not exist in lower mammals and is best developed in the great apes and humans (Kuypers, 1981
). Early investigators proposed already that the CMS is of crucial importance for dexterous individuated finger movements (Tower, 1940; Bernhard et al., 1953; Kuypers, 1962). This is supported by comparative anatomical work in mammals at different phylogenetic levels, which shows that the presence and degree of development of the CMS are correlated with digital dexterity (Heffner and Masterton, 1975, 1983; Bortoff and Strick, 1993). Further support comes from developmental studies in humans, which demonstrate parallel maturational profiles, continuing well into the second decade, of the myelination and axon diameter of fibres in the CMS (Paus et al., 1999), central motor conduction time when tested by transcranial magnetic stimulation (TMS) (Müller et al., 1991; Müller and Hömberg, 1992) and dexterity of fine finger coordination (Forssberg et al., 1991) and maximum rate of repetitive tapping movements (Müller and Hömberg, 1992). Lesion studies in monkeys show that the most conspicuous behavioural consequence of bilateral pyramidotomy in monkeys is a lasting loss of independent finger movements (Lawrence and Kuypers, 1968). Finally, single-unit recordings in monkey M1 indicate that a large fraction of cortico-spinal cells encode motor output parameters such as force (Cheney and Fetz, 1980), in particular small force increments (Evarts et al., 1983) and rate of change of force (Smith et al., 1975), and movement velocity and acceleration (Ashe and Georgopoulos, 1994; Schwartz and Moran, 2000; Mehring et al., 2003).
Despite this substantial evidence for a crucial role for the CMS in finger movements, little is known about to what extent the strength of the CMS differs between -MNs of different intrinsic hand muscles. The strength of the CMS may be estimated in vivo by the size of the compound excitatory postsynaptic potential (cEPSP) in single motor unit (SMU) recordings (Maertens de Noordhout et al., 1999; Mills, 2002), and by the amplitude of the motor evoked potential (MEP) in the surface electromyogram (EMG). Both measures are elicited by synchronous excitation of the CMS, for instance by TMS applied to motor cortex. CMS excitation at a given TMS intensity depends on the intrinsic excitability of the activated neural elements, and the density of CMS fibres at the site of stimulation (Hallett et al., 1999). From the measures of cEPSP or MEP, it is not possible to disentangle to what extent these two effects contribute. Therefore, throughout this paper, the term CMS excitation implicates both CMS excitability and CMS fibre density.
Considerable differences in CMS excitation may be expected between intrinsic hand muscles because they participate to various extents in functionally important finger movements, such as the pincer precision grip where the first dorsal interosseous (FDI) muscle and the abductor pollicis brevis (APB) muscle are agonists, while the abductor digiti minimi (ADM) muscle is much less involved (Maier and Hepp-Reymond, 1995). CMS excitation measured by cEPSP amplitude in SMU recordings seems maximal for intrinsic hand and forearm finger extensor muscles and significantly less for forearm finger flexor and proximal arm muscles (Palmer and Ashby, 1992; Maertens de Noordhout et al., 1999). A systematic comparison between intrinsic hand muscles was, however, not done. One disadvantage of SMU recordings is that many SMUs need to be studied to obtain an approximate picture of the whole system. A less tedious way to assess CMS excitation is through MEP amplitude elicited by TMS. Accurate quantification of CMS excitation is, however, precluded due to chronodispersion of the cortico-spinal volley, resulting in phase cancellation and, in turn, significant reduction of MEP amplitude. This problem was solved by the triple-stimulation technique (TST) (Magistris et al., 1998). TST was described for the anatomically isolated ADM but not for other intrinsic hand muscles, such as the FDI or APB, in which the TST response may be contaminated to a considerable extent by volume-conducted responses from neighbouring muscles. In order to measure CMS excitation to various intrinsic hand muscles (FDI, ADM and APB) without the problem of volume conduction, we introduce here, as an extension of the TST, a novel penta-stimulation technique (PST). In addition, we perform SMU recordings to validate the PST data. Finally, as a possible functional correlate of CMS excitation, we test the maximum rate of those repetitive voluntary individuated finger movements in which these muscles are prime movers (index finger abduction for FDI, little finger abduction for ADM, and thumb abduction for APB).
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Methods |
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Subjects
The study was divided into three experiments. Six subjects (mean age 32.0 ± 4.9 years; five male; five right-handed) participated in experiment I, three subjects (33.0 ± 3.5 years; all male; all right-handed) in experiment II, and 16 subjects (32.8 ± 4.4 years; 12 male; 15 right-handed) in experiment III. All subjects gave written informed consent. The study conformed to the Declaration of Helsinki and was approved by the Ethics Committee of The Hospital of Johann Wolfgang Goethe-University of Frankfurt am Main, Germany.
Experiment I
This experiment used PST to quantify CMS excitation to three different hand muscles: FDI, ADM and APB. PST is an extension of the TST (Magistris et al., 1998). TST applies TMS to the M1 hand area, followed by supramaximal peripheral nerve stimulation at the contralateral wrist, and finally supramaximal electrical stimulation of the brachial plexus at Erb's point (Fig. 1). This links, through two collisions along the peripheral nerve, central to peripheral conduction and eliminates the problem of desynchronization of the MEP (Fig. 1). This technique revealed that, in healthy subjects, all or nearly all -MNs can be discharged by TMS (Magistris et al., 1998). TST was described for the ADM, but not other hand muscles (Magistris et al., 1998, 1999). In contrast to the anatomically isolated ADM, the TST response of other hand muscles such as the FDI or APB may be contaminated to a significant extent by volume conduction from closely adjacent muscles supplied by a different nerve. For instance, if the FDI (innervated by the ulnar nerve) were the TST target muscle, then volume-conducted responses from thenar muscles innervated by the median nerve may contribute to the TST response (Fig. 1A4 and B4). The size of this contamination is a priori unknown and depends on factors such as the individual anatomy (i.e. distance of the electrical dipole sources), the exact location of the EMG electrodes, the conduction properties of the connective tissue and the relative size of the dipole sources in the neighbouring muscles (Dumitru and DeLisa, 1991). Volume conduction between even relatively distant muscles, such as the forearm flexors and extensors, may easily exceed 20% (Reynolds and Ashby, 1999). In the TST protocol, volume conduction is only partially controlled for by the TSTcontrol response (Fig. 1B). Suppose that the FDI is the TST target muscle. Then the TSTtest response is FDItest + V, where FDItest is the size of the true TST response generated in the FDI, and V is the volume-conducted response from adjacent muscles. Accordingly, the TSTcontrol response is FDImax + V, where FDImax can be set to 100% because this response is produced by supramaximal stimulation of Erb's point. Finally, the proportion (P) of -MNs activated by TMS can be estimated as: P = TSTtest/TSTcontrol x 100% (Magistris et al., 1998).
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This is the same as:
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Suppose that FDItest is 0% (e.g. when TMS is subthreshold) and V = 25% of FDImax, then P would be entirely volume conducted and amounts to P = (0 + 25/100 + 25) x 100% = 20%. If FDItest were 50 or 100% and V = 25%, then P would be 60 or 100%, respectively. If FDItest were 50% and V = 50%, then P would be 66.7%. It follows that the relative contribution of the volume-conducted response to P increases with V and decreases with FDItest. One way to account for the problem of volume conduction is to determine the size of V experimentally by relating the amplitude of the maximum volume-conducted response to the maximum M wave (Dumitru and DeLisa, 1991). However, a better way would be elimination of volume conduction. We propose here a way to do this by extending TST to PST (Fig. 2).
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The first two pulses are the same as in the TST protocol: TMS (Fig. 2A1) is followed by supramaximal electrical stimulation at the wrist of the nerve of interest (N1, ulnar nerve in the case of the FDI) (Fig. 2A2). The inter-stimulus interval is the minimal MEP latency rounded down to the nearest millisecond, minus the maximum M wave latency rounded up to the nearest millisecond (Magistris et al., 1998
). The action potentials descending from M1 stimulation collide with the antidromic action potentials elicited in N1 at the wrist and up the arm (Fig. 2A2). At the same time as N1 stimulation, another supramaximal electrical stimulus is applied at the wrist to the ‘nerve of no interest’ (N2, median nerve in the case of the FDI) (Fig. 2A2). This stimulus results in collision with the descending action potentials from M1 stimulation in N2 at the wrist and up the arm (Fig. 2A2). Stimulation of N1 + N2 is followed by a second supramaximal stimulus to N2 6 ms later (Fig. 2A3). This delay respects the relative refractory period of arm nerves, which normally is <5 ms (Kimura, 2001). This allows a second supramaximal excitation of N2. The fifth pulse is supramaximal electrical stimulation of the brachial plexus at Erb's point (Fig. 2A4). The inter-stimulus interval between N1 stimulation and Erb's point stimulation is equal to the minimal latency of the Erb compound muscle action potential rounded down to the nearest millisecond, minus the maximum M wave latency rounded up to the nearest millisecond (Magistris et al., 1998). In N1, this results in a second collision close to Erb's point with those antidromic action potentials elicited by N1 stimulation which were not eliminated in the first collision with the descending action potentials from M1 stimulation (Fig. 2A4). The remaining fibres lead to an Erb response in the EMG target muscle, which will be referred to as the PSTtest response (Fig. 2A5). In N2, part of the Erb response collides with antidromic action potentials from the first N2 stimulus, which in turn were not collided by the descending action potentials from M1 stimulation (Fig. 2A4). The second N2 stimulus is necessary to collide with those actions potentials from Erb stimulation which did not collide with the first N2 stimulus (cf. Fig. 2A4). As a result, no Erb response occurs in muscles innervated by N2 (Fig. 2A5). This eliminates the problem of volume conduction from neighbouring muscles innervated by N2. The volume-conducted response produced by the second N2 stimulation does not interfere with the PSTtest response due to sufficiently different timing (cf. Fig. 3A).
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The PSTcontrol response is generated by a similar sequence of five pulses, but the first pulse (TMS of M1) is substituted by supramaximal stimulation of Erb's point (Fig. 2B1). Also, the inter-stimulus intervals change. The interval between this Erb stimulus and N1 + N2 stimulation (Fig. 2B2) is equal to the minimal latency of the Erb compound muscle action potential rounded down to the nearest millisecond, minus the maximum M wave latency rounded up to the nearest millisecond (Magistris et al., 1998
). In N1 and N2, this results in complete collision with the descending action potentials from Erb stimulation close to the wrist (Fig. 2B2). The second N2 stimulus is applied 6 ms later (for rationale, see above) (Fig. 2B3). Finally, the second supramaximal Erb pulse is given 9.9 ms after the first Erb pulse (Fig. 2B4). This interval was pre-set by hardware limitations (see below, and Fig. 3A). In N1, this leads to a maximal Erb response in the EMG target muscle, referred to as the PSTcontrol response (Fig. 2B5). In N2, the action potentials from the second Erb stimulation are completely collided with the antidromic action potentials from the second N2 stimulation (Fig. 2B4). As a consequence, the PSTcontrol response is not contaminated by volume-conducted responses from muscles innervated by N2 (Fig. 2B5). Typically, the PSTcontrol response is slightly smaller than the compound muscle action potential elicited by single-pulse supramaximal Erb stimulation. This is caused by a ‘back-response’, which is an antidromic ephaptic muscle to nerve response elicited by supramaximal peripheral nerve stimulation (Magistris et al., 1998). When present, the ‘back-response’ collides with part of the second Erb response, resulting in diminution of the PSTcontrol response. As this phenomenon occurs the same way in the PSTtest response, it should cancel out when normalizing PSTtest to PSTcontrol.
Subjects were seated in a comfortable reclining chair. PSTtest response intensity curves were measured in the FDI, ADM and APB of the dominant hand separately in three different sessions, with the order of muscle pseudo-randomized and balanced across subjects. The EMG was recorded with surface Ag–AgCl cup electrodes in a belly-tendon montage. The raw EMG was amplified and 0.02–2 kHz band-pass filtered (Counterpoint Electromyograph®, Dantec Electronics, Skovlunde, Denmark), passed through a CED micro 1401 laboratory interface (Cambridge Electronic Design, Cambridge, UK) for digitization (sampling rate, 4 kHz) and then fed into a Pentium PC for on-line display and off-line analysis, using customized data collection and conditional averaging software (Spike 2® for Windows, Version 3.05, Cambridge Electronic Design). Focal TMS was applied over the hand area of the dominant M1 through a figure-of-eight coil (outer diameter of each wing, 9 cm; peak magnetic field, 
1.5 T) connected to a MAGSTIM 200 magnetic stimulator (Magstim, Whitland, UK) with a monophasic current waveform. The stimulating coil was placed flat on the scalp with the handle pointing backwards and rotated 45° away from the midline. Thus, the current induced in the brain was directed from lateral-posterior to medial-anterior, approximately perpendicular to the assumed line of the central sulcus. This is the optimal orientation for a predominantly trans-synaptic activation of the CMS (Kaneko et al., 1996). The optimal coil position for activating the EMG target muscle was determined as the site where TMS produced consistently the largest MEP at slightly suprathreshold stimulus intensity. This site was marked on the scalp with a pen in order to ensure constant coil placement throughout the experiment. The resting motor threshold (RMT) was determined in the relaxed target muscle to the nearest 1% of maximum stimulator output. RMT was defined as the minimum stimulus intensity which elicited an MEP >50 µV in at least five of 10 consecutive trials (Rossini et al., 1999). RMT will be reported as a percentage of the maximum stimulator output. The PST measurements were performed whilst the target muscle was relaxed. Complete voluntary relaxation was monitored by continuous audiovisual feedback of the raw EMG at a high gain (50 µV/D) of the recording device. Trials contaminated by EMG activity were discarded from the analysis. TMS intensity was set electronically through the remote port of the magnetic stimulator, and was varied in 5% steps of maximum stimulator output from RMT – 10% to RMT + 50% (i.e. 13 different intensities). Supramaximal electrical stimulation of the peripheral nerves of the dominant arm at the wrist was applied through bipolar electrodes (cathode distal), using constant current square wave pulses (duration, 0.2 ms). The Counterpoint Electromyograph® was used for N1 stimulation, and a Nicolet stimulator (MEDIAN, Nicolet EME GmbH, 63801 Kleinostheim, Germany) for N2 stimulation. For supramaximal electrical stimulation of Erb's point of the dominant arm, a Digitimer D185 multipulse stimulator (Digitimer Ltd, Welwyn Garden City, Hertfordshire, UK) with a square wave pulse of 50 µs duration was used (maximum output 1.000 V). Gold cup stimulating electrodes were placed over Erb's point (cathode) and the acromion (anode).
The following conditions were tested in pseudo-randomized order: (i) TMS alone (13 intensities) to obtain the conventional single-pulse MEP intensity curve; (ii) N1 alone to obtain the maximum M wave as a reference for the MEP intensity curve; (iii) double N2 alone (inter-stimulus interval, 6 ms) to quantify the volume-conducted response in the target muscle; and (iv) PSTtest response (13 TMS intensities). Conditions were presented in pseudo-randomized order. Each condition was repeated three times, resulting in a total of 84 trials. The mean inter-trial interval was 10 s, with a random interval variation of 25% to reduce anticipation of the next trial. The PSTcontrol response (three repeats) was tested immediately after the PSTtest measurements. The primary measure of experiment I was PSTtest/PSTcontrol x 100% as a function of TMS intensity and target muscle.
Experiment II
SMU recordings were performed to validate the PST findings in experiment I. TMS at around active motor threshold (AMT) intensity increases the firing probability of a voluntarily activated SMU some 20–30 ms after the TMS pulse, the primary peak (PP) in the peri-stimulus time histogram (PSTH) (for review, Mills, 2002). Excitation of the -MN by this CMS input can be quantified by estimating the amplitude of the cEPSP, if certain assumptions are made about the -MN membrane trajectory (Ashby and Zilm, 1982). In cat -MNs, the rate of rise of the membrane trajectory is linear and varies inversely with the inter-spike interval (ISI) when the -MN fires in the low frequency range (ISI 100 ms) while the distance from the deepest part of the membrane trajectory to threshold (the ‘scoop’) is constant at 10 mV (Schwindt and Calvin, 1972). If this is applied to human -MNs firing in the same frequency range, then the cEPSP amplitude can be estimated by:
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However, alternative evidence exists that the rate of rise of the membrane trajectory is constant so that the PP bin count is no longer independent of the mean ISI of the voluntarily firing -MN but inversely related to it (Jones and Bawa, 1995; Olivier et al., 1995). Under these assumptions, the cEPSP amplitude is estimated by:
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Finally, the PP elicited by TMS usually consists of several subpeaks caused by multiple discharges of the CMS (Amassian et al., 1987; Di Lazzaro et al., 2004). Excitation of upper limb -MNs may occur through non-monosynaptic pathways such as the cervical propriospinal pre-MNs (Pierrot-Deseilligny, 1996). Therefore, it is difficult to argue that the subpeaks of the PP, except the first subpeak, originate exclusively from monosynaptic excitation. To exclude contamination of the cEPSP by non-monosynaptic excitation, an additional analysis was run (according to equation 3 in Ashby and Zilm, 1982), which was limited to the first subpeak in the PP.
A total of 31 SMUs (FDI, 10; ADM, 11; APB, 10) from three subjects were studied. SMUs were recorded with a thin (0.3 mm diameter) concentric needle electrode (Toennies, Hoechberg, Germany) with a narrow recording field (0.019 mm2). Processing of the EMG signal followed the same steps, and used the same hardware and software as in experiment I (see above). Subjects were instructed to discharge the SMU regularly at a rate of 5–10 Hz during slight voluntary contraction of usually <5% of maximum force. Audiovisual feedback of the EMG signal was provided. Only the SMU recruited at lowest force threshold was recorded. For PSTH construction, a Schmitt-trigger was implemented into the software (Spike2®). The triggered SMU was displayed at an expanded time scale on a hold-screen to allow for a meticulous monitoring of the SMU waveform throughout the experiment. In the case of slightest doubt about the identity of an SMU, or in the case of multiple units, the recording was cancelled and the data discarded from the analysis. Excitation of each SMU was tested by CMS and muscle spindle afferent (Ia) input delivered at random delay with respect to voluntary SMU discharge. Ia input was tested as a control to explore the specificity of any differences between muscles in cEPSP amplitude elicited by CMS input. Focal single-pulse TMS (same apparatus as in experiment I) was used to excite the CMS. Initially, stimulus intensity was set to AMT, i.e. the minimum intensity required to produce a small MEP (>100 µV) in the surface EMG, measured in the average of five consecutive trials during slight muscle contraction at 5–10% of maximum force. Ia input was elicited using bipolar electrical stimulation of the nerve of interest at the wrist (cathode proximal, constant current square wave pulses, duration 1 ms). Initially, stimulus intensity was set to the M wave threshold in the surface EMG. Subsequently, TMS and/or Ia stimulus intensity was reduced in some instances to prevent multiple-unit firing, or increased to drive the indexed SMU (Table 1). TMS and Ia stimulation were applied in random sequence in the same SMU recording. At least 60 stimuli were obtained for each mode of stimulation. For analysis of the PP, conditional PSTHs with a bin resolution of 0.25 ms were constructed. The analysis time of each sweep included 100 ms before and after the stimulus. Identification of small changes in SMU firing probability was facilitated by cumulative sum (CUSUM) analysis (Ellaway, 1978). PP latency was defined as the delay after the stimulus when a consistent rise in the CUSUM (bin count – mean bin count) occurred. To accept a PP, its onset had to fall within 20–31 ms after TMS, and within 25–40 ms after Ia stimulation (Mills, 2002). PP duration was measured from rise to onset of a consistent fall of the CUSUM function. The primary measure of experiment II was the estimated mean amplitude of the -MN cEPSP (in mV) elicited by TMS and Ia stimulation as a function of target muscle.
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Results were very similar if mean cEPSP amplitudes elicited by CMS input were analysed according to equation 1 in Ashby and Zilm (1982)
(see Methods): 6.0 ± 2.4 mV (FDI), 4.1 ± 1.0 mV (ADM) and 2.9 ± 1.6 mV (APB). The ANOVA revealed a significant effect of muscle [F(2,28) = 8.32, P = 0.0013], and post hoc t tests showed that the mean cEPSP of the FDI was larger than those of the ADM (P = 0.016) and APB (P = 0.0003). Finally, restriction of the analysis to the first subpeak of the PP (see Methods) confirmed the results found with analyses of the whole PP. The mean duration of the first subpeak in the PSTH was sufficiently short in all muscles (FDI, 1.06 ± 0.22 ms; ADM, 1.18 ± 0.39 ms; APB, 1.18 ± 0.34 ms) to largely exclude a contribution from non-monosynaptic input. The ANOVA showed a significant effect of muscle [F(2,27) = 5.69, P = 0.0087]. This was explained in the post hoc tests by larger mean cEPSP in the FDI (1.74 ± 0.51 mV) than in ADM (1.15 ± 0.45 mV, P = 0.013) and APB (1.02 ± 0.46 mV, P = 0.0032).
cEPSP amplitude increases with stimulus intensity (Bawa and Lemon, 1993) and decreases with -MN size (Henneman et al., 1965; Awiszus and Feistner, 1994). AMT, TMS intensity normalized to AMT (Table 1A), sensory perception threshold and Ia stimulus intensity normalized to sensory perception threshold (Table 1B) were not different between muscles. Therefore, stimulus intensity did not explain the observed differences between muscles in mean cEPSP amplitude. -MN size correlates inversely with axon conduction velocity. Since the peripheral conduction distance is very similar for the three hand muscles, differences in -MN size can be estimated from differences in PP latency. In the present SMU sample, no such differences were found for PP latencies elicited by TMS (Table 1A), or Ia afferent stimulation (Table 1B). This suggests that -MN size did not explain the observed differences between muscles in mean cEPSP amplitude.
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Experiment III
This experiment explored the maximum rate of repetitive voluntary finger movements of the dominant hand as a possible functional correlate of the differences in CMS excitation found in experiments I and II. Subjects were seated in a chair with the forearm resting on a plate. The elbow was flexed at 90° and the upper arm adducted. Elbow and wrist were firmly fixed by tape. Subjects were instructed to perform repetitive finger movements at the highest possible rate over a period of 20 s. Three individuated finger movements were tested: index finger abductions (prime mover, FDI), little finger abductions (prime mover, ADM) and thumb abductions (prime mover, APB). The forearm was pronated for index and little finger abductions, and semi-pronated for thumb abductions. The order of movements was randomized and balanced across subjects. Muscle activity was recorded with bipolar surface EMG, using the same settings as in experiment I. The EMG of the prime mover was played back to the subjects via a loudspeaker. The primary measure of experiment III was the movement rate (in Hz) of the three individuated finger movements. It was determined by marking the onset of each EMG burst of the prime mover using customized data collection software (Spike 2®). The movement rate was then calculated as the reciprocal value of the mean EMG burst interval (in seconds).
Statistical analysis
In experiment I, a repeated measures ANOVA (analysis of variance) model of PSTtest/PSTcontrol (dependent measure) was used to analyse the within-subject effects of muscle (three levels) and stimulus intensity (13 levels). In experiment II, a factorial ANOVA of estimated cEPSP amplitude (dependent measure) was calculated separately for the two modes of stimulation (TMS and Ia stimulation) to analyse the effect of muscle (three levels). In experiment III, a repeated measure ANOVA of maximum movement rate (dependent measure) was used to test the within-subject effect of movement type (three levels). Whenever appropriate, post hoc comparisons were conducted by two-tailed t tests corrected for multiple comparisons by the Bonferroni–Dunn method. In order to evaluate possible relationships between CMS excitation and maximum voluntary movement rate, linear regressions were calculated for individual muscle ratios (FDI/ADM, FDI/APB and ADM/APB) of PSTtest/PSTcontrol at maximum TMS intensity (independent measure) versus individual ratios of the maximum rate of the corresponding movements (dependent measure) in those six subjects who participated in experiment I and III. In all tests, statistical significance was assumed if P < 0.05. Statistics were run with StatView® for Windows, Version 5.0.1 (SAS Institute Inc., Cary, NC).
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Experiment I
PST was well tolerated, although all subjects found supramaximal electrical stimulation of Erb's point painful.
PSTtest responses at maximum TMS intensity (RMT + 50%) and PSTcontrol responses are shown in the three hand muscles of one subject in Fig. 3A–C. The ANOVA of PSTtest/PSTcontrol revealed significant effects of muscle [F(2,5) = 6.45, P = 0.016], intensity [F(12,5) = 11.86, P < 0.0001] and the interaction between muscle and intensity [F(24,5) = 2.02, P = 0.0072]. The post hoc comparisons showed that the PSTtest/PSTcontrol intensity curve of the FDI was significantly above those of the ADM (P = 0.011) and APB (P = 0.012), while those of ADM and APB were not different from each other (P = 0.96) (Fig. 4A). With subthreshold TMS intensities, the PSTtest response was absent in all muscles (Fig. 4A), indicating complete collision of the Erb response by N1 + N2 stimulation. Mean RMT was not different between muscles [F(2,5) = 0.20, P = 0.82; FDI, 39.8 ± 7.4%; ADM, 40.3 ± 8.5%; APB, 39.7 ± 7.2%], and therefore did not account for the differences between muscles in the PSTtest/PSTcontrol intensity curve.
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An estimation of the size of the volume-conducted response in the target muscle was possible by comparing the response elicited by stimulation of N2 with the maximum M wave elicited by N1 stimulation. This analysis revealed that N2/N1 x 100% was far larger in the APB (52.5 ± 18.7%) than in the FDI (7.1 ± 2.6%) and ADM (4.2 ± 1.0%). An ANOVA of conventional single-pulse MEP amplitude normalized to the maximum M wave did not show an effect of muscle [F(2,5) = 0.92, P = 0.43] or the interaction between muscle and intensity [F(24,5) = 0.76, P = 0.78] (Fig. 4B). This illustrates that conventional MEP intensity curves cannot be used to determine CMS excitation because, in contrast to the PST responses (Fig. 4A), two effects are not controlled for: volume conduction from neighbouring muscles innervated by the nerve of no interest, and chronodispersion of the cortico-spinal volley.
Experiment II
In the PSTHs of representative SMU recordings (Fig. 5), the estimated -MN cEPSP elicited by TMS was larger in the FDI than in the ADM and APB, while the cEPSP elicited by Ia stimulation was smaller in the FDI than in the other two muscles. The ANOVA of the mean cEPSP amplitude elicited by TMS (calculated according to equation 3 in Ashby and Zilm, 1982) revealed a significant effect of muscle [F(2,28) = 9.64, P = 0.0007]. In the post hoc tests, the mean cEPSP amplitude was higher in the FDI (4.5 ± 1.3 mV) than in the ADM (2.8 ± 0.7 mV, P = 0.019) and APB (2.4 ± 1.3 mV, P = 0.0003) (Fig. 6, Table 1A). The ANOVA of the mean cEPSP amplitude elicited by Ia stimulation also demonstrated a significant effect of muscle [F(2,28) = 5.73, P = 0.0082]. This was explained by lower mean cEPSP amplitude in the FDI than APB (P = 0.0031) (Fig. 6, Table 1B).
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The mean duration of the PP elicited by TMS was shorter in the FDI compared with the ADM (P = 0.0158) (Table 1A). PP duration equals the rise time of the underlying
-MN cEPSP (Ashby and Zilm, 1982). The cEPSP builds up by temporal summation of multiple cortico-spinal discharges separated by 1.5 ms (Mills, 2002, cf. also Fig. 5). The short cEPSP rise time of FDI -MNs indicates that CMS input depolarizes these neurons faster to threshold than ADM -MNs. This supports the view further that CMS excitation to -MNs of the FDI is more powerful than to those of the ADM. This difference is specific to CMS input because it was not found with Ia input (Table 1B).
Experiment III
The ANOVA of the maximum individuated finger movement rate revealed a highly significant effect of movement type [F(2,14) = 13.83, P < 0.0001] (Fig. 7). Post hoc t tests showed that this was explained by a higher maximum movement rate of index finger abduction (4.5 ± 1.2 Hz) compared with little finger abduction (3.6 ± 1.0 Hz, P < 0.0001) and thumb abduction (4.0 ± 1.2 Hz, P = 0.0056), while maximum movement rates of little finger and thumb were not different from each other.
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Correlation between PST response and maximum movement rate
Regression analysis revealed that the individual muscle ratios of the PSTtest/PSTcontrol response at maximum TMS intensity correlated linearly with the individual ratios of the corresponding maximum movement rates for FDI/ADM versus index finger abduction/little finger abduction (Fig. 8A), and FDI/APB versus index finger abduction/thumb abduction (Fig. 8B), while ADM/APB versus little finger abduction/thumb abduction did not correlate (Fig. 8C).
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Discussion |
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TopSummaryIntroductionMethodsResultsDiscussionReferences |
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The major finding of this study is that CMS excitation differs between intrinsic hand muscles. It was stronger in the FDI than in the ADM or APB. CMS excitation was determined by a novel PST, which eliminates volume-conducted responses from neighbouring muscles as a potential source of substantial overestimation of the evoked response. SMU recordings validated the PST data. Additional behavioural experiments showed that individual muscle ratios of the PST response correlated with individual ratios of the maximum rate of the corresponding finger movements. This strongly supports the view that, in humans, CMS excitation tightly links to the functional capacity of individuated finger movements.
Experiment I—PST
The PST is an extension of the TST (Magistris et al., 1998, 1999). By adding two supramaximal electrical pulses to the ‘nerve of no interest’, it is possible to collide with all descending activity in that nerve, and hence to eliminate the problem of volume-conducted responses from neighbouring muscles supplied by that nerve (cf. Fig. 2). Due to its anatomically isolated location, volume-conducted responses are usually not a problem for the ADM, where the TST was originally described (Magistris et al., 1998, 1999). The situation is different for the thenar where muscles supplied by the ulnar and median nerve are located in close proximity. In the present study, the mean volume-conducted response in the APB was 52.5% of its maximum M wave. If TMS failed completely to activate the CMS, e.g. due to subthreshold stimulus intensity, or due to CMS lesion, TST applied to the APB would result in a large mean error of (0 + 52.5/100 + 52.5) x 100% = 34.4% (cf. Methods). Even if TMS activated 90% of the CMS, the mean error would still be 3.4%, i.e. the TSTtest/TSTcontrol response would be (90 + 52.5/100 + 52.5) x 100% = 93.4%. According to the currently accepted lower limit of the normative TSTtest/TSTcontrol response in the ADM of 93% (Magistris et al., 1998, 1999), that error could potentially obscure a partial central motor conduction failure. We found mean PSTtest/PSTcontrol values that were considerably less than 93% even with maximum TMS intensity (Fig. 4A) because all measurements were performed during voluntary muscle relaxation. Activation of all or nearly all -MNs of a target muscle requires facilitation manoeuvres such as voluntary contraction of the target muscle in most subjects (Magistris et al., 1998).
One critical issue in interpreting the present findings is that TMS with the induced current in the brain directed from posterior to anterior activates the CMS largely trans-synaptically rather than directly (Kaneko et al., 1996; Sakai et al., 1997; Di Lazzaro et al., 2004). Therefore, PST and TST are not tests of CMS excitation sensu strictu but include those neural elements in the cortex activated by TMS and their synapses with the CMS. It is possible that the observed differences in CMS excitation between muscles rely predominantly or even exclusively on differences in these neural elements in M1. Several pieces of evidence support an important role for the neural network in M1 in determining CMS excitation. For instance, MEP amplitudes of the FDI were facilitated more by a pincer grip than simple index finger abduction, when elicited by TMS (Flament et al., 1993). This task difference was significantly less when MEPs were elicited by transcranial electrical stimulation (Flament et al., 1993), which activates the CMS to some extent directly and thus bypasses neural elements in M1 (Di Lazzaro et al., 2004). Some direct evidence for differences in CMS excitation sensu strictu between intrinsic hand muscles also exists. Electrical stimulation of the monkey pyramidal tract evoked the largest EPSP in one -MN of the second dorsal interosseous muscle compared with other intrinsic hand muscles (Lemon, 1990). For the purposes of the present study, this distinction between the neural network in M1 and the CMS itself is not directly relevant. The main point is that, at a given intensity, TMS discharges more -MNs of the FDI compared with the ADM or APB. This may be a consequence of a more excitable neural network in M1, a more excitable CMS, a higher density of intracortical or CMS fibres at the site of stimulation, or any combination of these.
One may argue that the PST is not needed because TST, when limited to the ADM, is appropriate to test CMS excitation and integrity. At least two situations illustrate that this is not always the case. (i) This study demonstrates that, in healthy subjects, CMS excitation is not the same for different hand muscles. Therefore, information about CMS excitation to the ADM does not generalize to CMS excitation of the hand. (ii) Neurological disease may affect the CMS to intrinsic hand muscles differently. For instance, patients with amyotrophic lateral sclerosis may show predominant CMS degeneration to thenar muscles while the CMS to hypothenar muscles is not or less affected (Weber et al., 2000). Therefore, the PST may be the method of choice whenever there is interest in extending the assessment of the CMS to intrinsic hand muscles other than the ADM, although its usefulness in patients remains to be proven in future studies.
Experiment II—SMU recordings
A potential limitation of the PST response is that it does not control volume conduction from neighbouring muscles innervated by the ‘nerve of interest’. PST separates ulnar and median nerve innervations but not each muscle innervated by the nerve of interest. Therefore, it was rather important to show that the SMU data validate the PST findings. SMU recordings are an established means to estimate cEPSP amplitude (Ashby and Zilm, 1982; Mills, 2002). The SMU data also showed that the observed differences between muscles in cEPSP amplitude were specific to CMS input because they were not observed with Ia input.
The estimated amplitude of the cEPSP elicited by TMS was not compared previously between intrinsic hand muscles. The present findings corroborate one earlier study where cEPSPs were elicited by anodal transcranial electrical stimulation, and where it was found that the mean cEPSP amplitude was slightly larger in the FDI than in the ADM (Maertens de Noordhout et al., 1999).
Experiment III—maximum movement rate
In humans, neuronal activity in M1 correlates with movement rate as indicated by measurements of regional cerebral blood flow and changes in BOLD signal (Blinkenberg et al., 1996; Rao et al., 1996; Sadato et al., 1996; Jenkins et al., 1997; Kastrup et al., 2002). Furthermore, low-frequency repetitive TMS in its virtual lesion mode, when applied over M1, results in significant slowing of fastest finger tapping (Jäncke et al., 2004). These data suggest that M1 is responsible for fastest repetitive movements but do not bear on the question of how this activity is translated from cortex to muscle.
The present study provides such a link because it shows, for the first time in different intrinsic hand muscles, that CMS excitation tightly correlated to functional capacity when defined by maximal finger movement rate (cf. Figs 7 and 8). Although correlations do not imply a causal relationship, a multitude of evidence from comparative anatomical, developmental and lesion studies (see Introduction) further strengthens the idea of causality between CMS excitation and hand function. Another argument in favour of causality between CMS excitation and motor function is as follows: TMS and voluntary muscle contraction activate the same fibres of the CMS (Hess and Mills, 1986; Bawa and Lemon, 1993). Voluntary facilitation of the CMS can be measured by an increase in MEP amplitude, which was interpreted to reflect the degree to which the CMS is engaged by volitional activity (Semmler and Nordstrom, 1998). Vice versa, excitation of the CMS (by TMS) under resting conditions should then reflect the accessibility of this system to voluntary activation.
In conclusion, CMS excitation, as measured by the novel PST and validated by SMU recordings, differs between intrinsic muscles of the human hand. This CMS excitation directly links to functional capacity such as the maximal rate of individuated finger movements.
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Acknowledgements |
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T.V.I. was a fellow of the Alexander von Humboldt Foundation.
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