Cortico-motoneuronal excitation of three hand muscles determined by a novel penta-stimulation technique

DOI: 10.1093/brain/awh212 Brain (2004), 127, 1887–1898

Cortico-motoneuronal excitation of three handmuscles determined by a novel penta-stimulationtechnique

Ulf Ziemann, Tihomir V. Ilic, Henrik Alle and Frank Meintzschel

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: [email protected]

Motor Cortex Laboratory, Department of Neurology,

Johann Wolfgang Goethe-University of Frankfurt, Frankfurt

am Main, Germany

SummaryThe cortico-motoneuronal system (CMS), i.e. the mono-

synaptic projection from primary motor cortex to moto-

neurons 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 skilledindividuated 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 sub-

jects 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 twoadditional 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, there-

fore, 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 estimatedamplitude 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 dif-ferent 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.

Keywords: 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.

Received January 9, 2003. Revised April 7, 2004. Accepted April 9, 2004. Advanced Access publication June 30, 2004

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IntroductionH. 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

a-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 investi-

gators 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 phylo-

genetic 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) (Muller et al., 1991; Muller and Homberg,

1992) and dexterity of fine finger coordination (Forssberget al.,

1991) and maximum rate of repetitive tapping movements

(Muller and Homberg, 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 accel-

eration (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 a-MNs of different

intrinsic hand muscles. The strength of the CMS may be estim-

ated in vivoby 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 intens-

ity 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 reduc-

tion 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 per-

form 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).

Methods

SubjectsThe study was divided into three experiments. Six subjects (mean

age 32.0 6 4.9 years; five male; five right-handed) participated in

experiment I, three subjects (33.0 6 3.5 years; all male; all right-

handed) in experiment II, and 16 subjects (32.8 6 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 IThis experiment used PST to quantify CMS excitation to three dif-

ferent 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 con-

tralateral wrist, and finally supramaximal electrical stimulation of the

brachial plexus at Erb’s point (Fig. 1). This links, through two colli-

sions 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 a-MNs

can be discharged by TMS (Magistris et al., 1998). TST was described

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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 (inner-

vated 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 þ V, where FDItest is the size of the true TST response gen-

erated in the FDI, and V is the volume-conducted response from

adjacent muscles. Accordingly, the TSTcontrol response is

FDImax þ 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 a-MNs activated by TMS can be estimated

as: P = TSTtest/TSTcontrol � 100% (Magistris et al., 1998).

This is the same as:

P = ðFDItest þ V=100 þ VÞ � 100%

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 þ 25/100 þ 25) � 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 con-

duction. We propose here a way to do this by extending TST to PST

(Fig. 2).

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 þ N2is followed by asecond supramaximal stimulus to N26 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 supramax-

imal 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 milli-

second (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

Fig. 1 Experimental protocol of the TST (Magistris et al., 1998).The CMS is simplified and consists of two cortico-motoneuronalcells each projecting to one FDI or APB spinal a-MN. The EMGtarget muscle is the FDI. (A) Measurement of the TSTtest

response. The first pulse is TMS to the hand area of M1contralateral to the target FDI. In this example, TMS activated50% of the CMS to the FDI and APB muscles (A1). The secondpulse is a supramaximal electrical pulse to the ulnar nerve (N1) atthe wrist, which collides with the descending volley in the ulnarnerve evoked by TMS (A2). The third pulse is supramaximalstimulation of the brachial plexus at Erb’s point. This results in asecond collision in those fibres of the ulnar nerve which had notbeen part of the first collision (A3). The TSTtest response consistsof a synchronized Erb response mediated by those fibres of theulnar nerve which were already activated by TMS (in this case,50% of the ulnar nerve fibres) (A4). In addition, a volume-conducted Erb response (V) from neighbouring muscles suppliedby the median nerve contributes to the TSTtest response (A4). Themagnitude of this effect is a priori not known. (B) Measurementof the TSTcontrol response. The first pulse is supramaximalstimulation of Erb’s point (B1). The second pulse is supramaximalstimulation of the ulnar nerve at the wrist, which collides with thedescending volley in the ulnar nerve from stimulation of Erb’spoint (B2). The third pulse is a second supramaximal stimulationat Erb’s point (B3), which results in the TSTcontrol response,consisting of the maximum Erb response in the FDI plus V (B4).

Cortico-motoneuronal excitation of the hand 1889



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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).

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 þ N2

stimulation (Fig. 2B2) is equal to the minimal latency of the Erb

compound muscle action potential rounded down to the nearest milli-

second, 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 electro-

des 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-poster-

ior to medial-anterior, approximately perpendicular to the assumed

line of the central sulcus. This is the optimal orientation for a pre-

dominantly 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 mV 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 measure-

ments 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 mV/D) of the recording

Fig. 2 Experimental protocol of the PST. (A) Measurement of thePSTtest response. The CMS constitutes the same elements as inFig. 1. The first pulse is TMS to the hand area of M1 contralateralto the EMG target muscle (FDI). In this example, TMS activated50% of the CMS to the FDI and APB muscles (A1). Thesecond and third pulses are simultaneous supramaximal electricalstimuli to the ulnar nerve (N1) and the median nerve (N2) atthe wrist, which collide with the descending volleys in the ulnarnerve and median nerve elicited by TMS (A2). The fourth pulse isa second supramaximal stimulus to the median nerve (N2) at thewrist (A3). The fifth pulse is supramaximal electrical stimulationof Erb’s point. In the ulnar nerve, this results in a secondcollision in those fibres which had not been part of the firstcollision (A4). In the median nerve, stimulation of Erb’s pointleads to a collision with the antidromic pulse from the first mediannerve stimulus in those fibres which had not been part of the firstcollision, and to a collision with the antidromic pulse from thesecond median nerve stimulus in all remaining fibres (A4). As aresult, the PSTtest response consists purely of the Erb response inthe FDI, while a volume-conducted Erb response from neigh-bouring muscles supplied by the median nerve is eliminated bycollision (A5). (B) Measurement of the PSTcontrol response. Thefirst pulse is supramaximal stimulation of Erb’s point (B1). Thesecond and third pulses are simultaneous supramaximal electricalstimuli of the ulnar and median nerve at the wrist, which collidewith the descending volley in the ulnar and median nerve fromErb’s stimulation (B2). The fourth pulse is a second supramaximalpulse to the median nerve at the wrist (B3). The fifth pulse is asecond supramaximal stimulus at Erb’s point (B4), whichpropagates without collision along the ulnar nerve but iscompletely collided along the median nerve by the antidromicpulse from the second stimulus to this nerve (B4). The PSTcontrol

response consists of the maximum Erb response in the FDI withoutany volume-conducted response from neighbouring musclessupplied by the median nerve (B5). PST responses in the APB canbe measured in the same way, with the only exception that thefourth stimulus needs to be applied to the ulnar nerve.




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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 þ 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 supra-

maximal 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 ms

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 � 100%

as a function of TMS intensity and target muscle.

Experiment IISMU 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 a-MN by this CMS input can be quantified by estimating the

amplitude of the cEPSP, if certain assumptions are made about the

a-MN membrane trajectory (Ashby and Zilm, 1982). In cat a-MNs,

the rate of rise of the membrane trajectory is linear and varies inversely

with the inter-spike interval (ISI) when the a-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

a-MNs firing in the same frequency range, then the cEPSP amplitude

can be estimated by:

cEPSP½mV� = PP bin count � 10½mV�=no: of stimuli

ðcf: equation 3 in Ashby and Zilm, 1982Þ

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 a-MN but inver-

sely related to it (Jones and Bawa, 1995; Olivier et al., 1995). Under

these assumptions, the cEPSP amplitude is estimated by:

cEPSP ½mV� = PP bin count� ISI ½ms� � 0:1½mV=ms�=no:

of stimuli ðcf: equation 1 in Ashby and Zilm;1982Þ

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 a-MNs may occur

through non-monosynaptic pathways such as the cervical proprio-

spinal 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 hard-

ware 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

Fig. 3 PSTtest responses (black curves, upper traces in A–C)obtained at maximum TMS intensity (RMT þ 50% of maximumstimulator output) and PSTcontrol responses (grey curves, lowertraces in A–C) of the FDI (A), ADM (B) and APB (C) of onesubject. Each trace is the average of three trials. In A, arrowsindicate the timing of the five pulses in the PSTtest and PSTcontrol

responses. The timing of the pulses for determination of thePSTcontrol response differs from the experimental protocol inFig. 2B. This was due to hardware properties, which limited theinterval between the two Erb stimuli to a maximum of 9.9 ms. Thisdid not, however, result in a problem because the interval betweenthe second Erb stimulus and the second stimulus to the ‘nerve ofno interest’ (N2) clearly did not exceed the minimum latency ofthe Erb response minus the maximum M wave latency. As long asthis is not the case, complete collision between the descendingvolley from the second Erb stimulation and the antidromic actionpotentials from the second N2 stimulation occurs as indicated inFig. 2B4. The percentage value next to each of the PSTtest

responses denotes the PSTtest/PSTcontrol � 100% values. Note thatthis value was higher in FDI compared with ADM and APB. Barsindicate calibrations (time and voltage).




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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 volun-

tary 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 mV) 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 sti-

mulation of the nerve of interest at the wrist (cathode proximal, con-

stant current square wave pulses, duration 1 ms). Initially, stimulus

intensity was set to the M wave threshold in the surface EMG. Sub-

sequently, 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 con-

structed. 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 a-MN cEPSP (in mV) elicited by

TMS and Ia stimulation as a function of target muscle.

Experiment IIIThis experiment explored the maximum rate of repetitive voluntary

finger movements of the dominant hand as a possible functional cor-

relate 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 analysisIn 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 esti-

mated cEPSP amplitude (dependent measure) was calculated sepa-

rately 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 rela-

tionships between CMS excitation and maximum voluntary move-

ment 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 (depen-

dent 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).

Table 1A SMU characteristics to CMS input

Muscle No. ofSMUs

AMT(% MSO)

TMSintensitya

Inter-spikeinterval (ms)

PP latency(ms)

PP duration(ms)

Estimated cEPSPamplitude (mV)

FDI 10 40.9 6 9.5 1.02 6 0.08 132.1 6 28.0 26.8 6 2.1 3.5 6 1.0* 4.5 6 1.3*, #

ADM 11 43.2 6 8.9 1.02 6 0.08 148.2 6 16.8§ 26.6 6 2.2 5.0 6 1.1* 2.8 6 0.7*

APB 10 41.0 6 7.5 1.04 6 0.05 110.2 6 16.9§ 25.4 6 1.5 5.2 6 2.2 2.4 6 1.3#

Intensity normalized to AMT; *,#,§indicate significant differences (P < 0.016).

Table 1B SMU characteristics to Ia afferent input

Muscle No. ofSMUs

Sensory perceptionthreshold (mA)

Stimulusintensitya

Inter-spikeinterval (ms)

PP latency(ms)

PP duration(ms)

Estimated cEPSPamplitude (mV)

FDI 10 2.3 6 0.6 1.6 6 0.5 133.9 6 29.7 35.7 6 2.7 2.3 6 0.8 0.9 6 0.8#

ADM 11 2.4 6 1.1 1.5 6 0.5 145.1 6 23.8§ 34.9 6 3.1 2.2 6 0.8 1.3 6 1.2APB 10 3.2 6 1.1 1.1 6 0.3 112.5 6 28.9§ 33.6 6 1.5 2.4 6 0.9 2.7 6 1.6#

Intensity normalized to sensory perception threshold; #,§indicate significant differences (P < 0.016).




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ResultsExperiment IPST was well tolerated, although all subjects found supramax-

imal electrical stimulation of Erb’s point painful.

PSTtest responses at maximum TMS intensity (RMT þ50%) 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 þ N2 stimulation. Mean RMT was not different

between muscles [F(2,5) = 0.20, P = 0.82; FDI, 39.8 6

7.4%; ADM, 40.3 6 8.5%; APB, 39.7 6 7.2%], and therefore

did not account for the differences between muscles in the

PSTtest/PSTcontrol intensity curve.

An estimation of the size of the volume-conducted response

in the target muscle was possible by comparing the response

elicitedbystimulationofN2withthemaximumMwaveelicited

by N1 stimulation. This analysis revealed that N2/N1 � 100%

was far larger in the APB (52.5618.7%) than in the FDI (7.16

2.6%) and ADM (4.2 6 1.0%). An ANOVA of conventional

single-pulse MEP amplitude normalized to the maximum M

wavedidnotshowaneffectofmuscle[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 chrono-

dispersion of the cortico-spinal volley.

Experiment IIIn the PSTHs of representative SMU recordings (Fig. 5), the

estimated a-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 mus-

cles. 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 6 1.3 mV) than in the ADM (2.8 6

0.7 mV,P= 0.019) and APB (2.461.3 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).

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 6 2.4 mV (FDI),

4.1 6 1.0 mV (ADM) and 2.9 6 1.6 mV (APB). The

Fig. 4 (A) Mean PSTtest/PSTcontrol intensity curves (n = 6) for the relaxed FDI (black circles), ADM (white squares) and APB (greytriangles). TMS intensity (x-axis) is given as RMT (set to zero) plus the percentage of maximum stimulator output (%MSO). Note that thecurve of the FDI runs above those of the ADM and APB. (B) MEP intensity curves as elicited by single-pulse TMS. MEP amplitude isrelated to the maximum M wave (y-axis). Otherwise conventions and arrangement are the same as in A. Note the lack of differencesbetween muscles.




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ANOVArevealedasignificanteffectofmuscle [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 6

0.22 ms; ADM, 1.18 6 0.39 ms; APB, 1.18 6 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 6 0.51 mV) than in

ADM (1.15 6 0.45 mV, P = 0.013) and APB (1.02 6 0.46

mV, P = 0.0032).

cEPSP amplitude increases with stimulus intensity (Bawa

and Lemon, 1993) and decreases with a-MN size (Henneman

et al., 1965; Awiszus and Feistner, 1994). AMT, TMS intensity

normalized to AMT (Table1A), 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 dif-

ferences between muscles in mean cEPSP amplitude. a-MN

size correlates inversely with axon conduction velocity. Since

the peripheral conduction distance is very similar for the three

hand muscles, differences ina-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 thata-

MN size did not explain the observed differences between

muscles in mean cEPSP amplitude.

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 a-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 a-MNs indicates that CMS input depolarizes these

neurons faster to threshold than ADM a-MNs. This supports

the view further that CMS excitation to a-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 IIIThe ANOVA of the maximum individuated finger movement

rate revealed a highly significant effect of movement type

Fig. 5 PSTHs of representative SMUs recorded from FDI (A), ADM (B) and APB (C). Black bins and left y-axes indicate the firingprobability calculated as bin count/no. of stimuli for TMS (upper diagrams) and Ia afferent stimulation (lower diagrams) in the same SMUs.Bin resolution is 0.25 ms. The time of TMS and Ia afferent stimulation is at 0 ms. Grey lines and right y-axes indicate the cumulative sum(CUSUM) function of bin count minus mean bin count. The estimated amplitude of the a-MN cEPSP elicited by the two modes ofstimulation was calculated from the bin count in the primary peak of each SMU (for details, see Methods) and is given in the left uppercorner of each diagram. Note that the cEPSP elicited by TMS was largest in the SMU of the FDI and lower in those of the ADM and APB,while the cEPSP elicited by Ia afferent stimulation was smallest in the SMU of the FDI and larger in those of the ADM and APB.




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[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 6 1.2 Hz) compared with little

finger abduction (3.6 6 1.0 Hz, P < 0.0001) and thumb abduc-

tion (4.0 6 1.2 Hz, P = 0.0056), while maximum movement

rates of little finger and thumb were not different from

each other.

Correlation between PST response andmaximum movement rateRegression analysis revealed that the individual muscle ratios

of the PSTtest/PSTcontrol response at maximum TMS intensity

correlated linearly with the individual ratios of the correspond-

ing 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).

DiscussionThe 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—PSTThe 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 descend-

ing activity in that nerve, and hence to eliminate the problem of

volume-conducted responses from neighbouring muscles sup-

plied by that nerve (cf. Fig. 2). Due to its anatomically isolated

location, volume-conducted responses are usually not a prob-

lem 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 þ 52.5/100 þ 52.5) � 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 þ 52.5/100 þ 52.5) � 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 a-MNs of a target

muscle requires facilitation manoeuvres such as voluntary

Fig. 6 Mean estimated a-MN cEPSP amplitudes elicited by TMSand Ia afferent input as revealed by SMU recordings in the FDI(black bars, n = 10), ADM (grey bars, n = 11) and APB (whitebars, n = 10). Error bars are 1 SEM. Asterisks indicate significantdifferences (P < 0.016).

Fig. 7 Maximum rate of three different individuated voluntaryfinger movements. Grey circles and lines show individual data,black circles and error bars are the means 6 1 SEM. Note thatmaximum movement rate was significantly higher for index fingerabduction than for little finger abduction or thumb abduction.




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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 poster-

ior 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 ele-

ments 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 exclu-

sively 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 stimu-

lation (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 onea-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

theCMSitself isnotdirectly relevant.Themainpoint is that, ata

given intensity, TMS discharges more a-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 recordingsA potential limitation of the PST response is that it does not

control volume conduction from neighbouring muscles innerv-

ated 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 rateIn 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;

Fig. 8 Correlations between individual muscle ratio of PSTtest/PSTcontrol at maximum TMS intensity (x-axis) and individual ratio ofthe corresponding maximum movement rates (MR, y-axis). All six subjects who participated in experiment I and in experiment III areincluded in the analysis. (A) FDI/ADM versus index finger abduction/little finger abduction; (B) FDI/APB versus index/thumb abduction;(C) ADM/APB versus little/thumb. Regression lines are plotted, and R2 and P values are indicated in the upper left corner of eachdiagram. Note significant correlations in A and B.




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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 (Jancke 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.

AcknowledgementsT.V.I. was a fellow of the Alexander von Humboldt

Foundation.

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