Different patterns of excitation and inhibition of the small hand and forearm muscles from magnetic brain stimulation in humans (original) (raw)
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Sensory modulation of voluntary and TMS-induced activation in hand muscles
Experimental Brain Research, 2008
Nociceptive suppression of tonic voluntary electromyographic (EMG) activity in human hand muscles (cutaneous silent period, CSP) is in its functional organization consistent with a spinal protective reXex. Motoneuronal excitability and its modulation may also be investigated by conditioned motor evoked potentials (MEPs). To date, eVects of exteroceptive stimuli on tonic EMG and on MEPs have been compared mainly using innocuous stimuli, while noxious stimuli have not been studied in great detail. In ten subjects, we recorded CSPs induced in volitionally activated Xexor pollicis brevis muscle (FPB) by noxious digit II (D2) stimulation, and in Wrst dorsal interosseous muscle (FDI) following noxious D2 and digit V (D5) stimulation. Then, transcranial magnetic stimulation (TMS) was used to evoke MEPs in the same hand muscles at rest-conditioned by equal noxious D2 or D5 stimulation and individually delayed-so that the MEPs occurred at times corresponding to immediately before, during, and immediately after the CSP in each subject. Immediately before the CSP, there was no signiWcant diVerence between nociceptive MEP modulation and tonic EMG modulation in any muscle-Wnger-combination. In the middle of the CSP, noxious Wnger stimulation exerted suppression of TMS-induced MEPs in all the three muscle-Wnger-combinations, but less so as compared to corresponding tonic EMG levels. After the CSP, MEPs remained suppressed when corresponding tonic EMG levels were signiWcantly enhanced. Notably, MEPs were also suppressed in cases when occurring at times corresponding to the excitatory long-loop reXex. Incomplete MEP suppression during the CSP may allow for an "emergency grip" even during noxious stimulation. MEP suppression outlasting the CSP is compatible with a "passive" re-synchronization of volitionally activated motor units rather than an "active" reXex involving recruitment of corticospinal motoneurons. The diVerences in tonic EMG and MEP modulation favors an eVect of noxious digital nerve stimulation on interneurons responsible for presynaptic inhibition rather than a postsynaptic inhibitory eVect on the motoneuron pool. The present Wndings caution against the use of nociceptive MEP modulation at rest to substitute for tonic EMG modulation as tested in CSP studies.
Effect of antagonistic voluntary contraction on motor responses in the forearm
Clinical Neurophysiology, 2000
We investigated the effects of voluntary contraction of agonist and antagonist muscles on motor evoked potentials (MEP) and on myoelectric activities in the target (agonist) muscle following transcranial magnetic stimulation (TMS). Methods: The left extensor carpi radialis (ECR) and¯exor carpi radialis (FCR) muscles were studied in 16 healthy subjects. H re¯exes, MEP induced by TMS, and background electromyographic (EMG) activity were recorded using surface electrodes at rest and during voluntary contraction of either agonist or antagonist muscles. Results: Voluntary contraction of antagonist muscles (at 10% of maximum contraction) enhanced the amplitudes of MEP for both muscles. The H re¯ex of the FCR muscle was inhibited by contraction (10% of maximum) of the ECR muscle. Background EMG activity did not differ between H-re¯ex trials and TMS trials. Enhancement of MEP amplitudes and background EMG activity during voluntary antagonist contraction was comparable in the two muscles. Appearance rate of MEP recorded by needle electrodes in response to subthreshold TMS was increased by antagonistic voluntary contraction. Conclusion: Facilitation occurs during voluntary contraction of antagonist muscles. Differences between the effects of voluntary contraction of the ECR muscle for the MEP and the H re¯ex of the FCR suggest that cortical facilitatory spread occurs between agonist and antagonist muscles.
Three trials of transcranial magnetic stimulation (TMS) during the maximum voluntary muscle contraction (MVC) were repeated at 15-minute intervals for 1 hour to examine the effects on motor evoked potentials (MEPs) in the digital muscles and pinching muscle force before and after 4 high-intensity TMSs (test 1 condition) or sham TMS (test 2 condition) with MVC. Under the placebo condition, real TMS with MVC was administered only before and 1 hour after the sham TMS with MVC. Magnetic stimulation at the foramen magnum level (FMS) with MVC was performed by the same protocol as that for the test 2 condition. As a result, MEP sizes in the digital muscles significantly increased after TMS with MVC under test conditions compared with the placebo conditions (P < 0.05). Pinching muscle force was significantly larger 45 minutes and 1 hour after TMS with MVC under the test conditions than under the placebo condition (P < 0.05). FMS significantly decreased MEP amplitudes 60 minutes after ...
Facilitation of muscle responses to magnetic brain stimulation by mechanical stimuli in man
Experimental Brain Research, 1988
Transcranial magnetic brain stimuh were applied to 9 normal subjects and compound muscle action potentials were recorded from the right abductor digiti minimi with surface electrodes. Vibration of 120 Hz, 0.6 mm peak to peak amplitude, applied to the muscle tendon enhanced its responses to magnetic brain stimuli. This facilitation corresponds to the tonic vibration reflex. Inhibition of muscle responses was not seen with vibration. Thus it is likely that the known inhibition of stretch reflexes by vibration is purely presynaptic. Small rectangular mechanical stimuli (rise time 200 ram/s, amplitude I ram) applied to ADM elicited short and long loop reflex responses. When brain stimuli were given 7-16 ms after the muscle tap, muscle responses were enhanced. It is argued that this is a result of the summation of the effects of Ia afferent impulses and descending pyramidal volleys at the alpha motoneurones. A separate late facilitation corresponding with the arrival of muscle afferent inputs to the sensori-motor cortex was not seen.
Experimental Brain Research, 2000
The aim of this study was to determine whether prolonged, repetitive mixed nerve stimulation (duty cycle 1 s, 500 ms on–500 ms off, 10 Hz) of the ulnar nerve leads to a change in excitability of primary motor cortex in normal human subjects. Motor-evoked potentials (MEPs) generated in three intrinsic hand muscles [abductor digiti minimi (ADM), first dorsal interosseous (FDI) and abductor pollicis brevis (APB)] by focal transcranial magnetic stimulation were recorded during complete relaxation before and after a period of prolonged repetitive ulnar nerve stimulation at the wrist. Transcranial magnetic stimuli were applied at seven scalp sites separated by 1 cm: the optimal scalp site for eliciting MEPs in the target muscle (FDI), three sites medial to the optimal site and three sites lateral to the optimal stimulation site. The area of the MEPs evoked in the ulnar- (FDI, ADM) but not the median-innervated (APB) muscles was increased after prolonged ulnar nerve stimulation. Centre of gravity measures demonstrated that there was no significant difference in the distribution of cortical excitability after the peripheral stimulation. F-wave responses in the intrinsic hand muscles were not altered after prolonged ulnar nerve stimulation, suggesting that the changes in MEP areas were not the result of stimulus-induced increases in the excitability of spinal motoneurones. Control experiments employing transcranial electric stimulation provided no evidence for a spinal origin for the excitability changes. These results demonstrate that in normal human subjects the excitability of the cortical projection to hand muscles can be altered in a manner determined by the peripheral stimulus applied.
Brain Research, 1995
Aim of the study was to analyze the characteristics of motor action potentials recruitment during magnetic trans-cranial stimulation (KS) of the brain. Coaxial needle recordings from hand and upper limb musculature, as well as surface electrodes were employed in 20 healthy controls during magnetic TCS with regular and figure-of-8 coil in different experimental protocols including: (a) simple reaction time paradigm during which TCS at subthreshold intensity for eliciting EPs in relaxation was delivered at various intervals between the signal to move and the onset of the voluntary EMG burst; (b) prathreshold TCS was randomly delivered while the subject was voluntarily firing at a regular rate one 'low' and/or 'high thre$!gold' motor unit action potcl**!~*I (MUAP), The pre-and post-TCS MWAPs recruitment as well as their firing rates were comphncd; (c) recordings with two separate needles picking up iradiuidrtal MUM% from the same or from two different muscles were obtained in order to test ,:+r?chrony" df MUAP's discharge before and after TCS; (d) the influence of the time-interval separating the last discharged MUAP ~~~~~ KS was evaluated, (e) differences between simultaneous surface and depth recordings were examined. The fdhing re~:dts wt+vc obtained. (a) The same low-amplitude MUAP which is first voluntarily rccruitcd at the onset of the E s thr NZ i:&ially fired by TCS in the prc-movcmcnt period. Latency shortenings and :$?nliQr& cn!XPrrri'nt of surface were uhsvr\~,~I with faster reaction times. Such changes wcrc coupled to the recruitment 01 hrgh-threshold MUAPs being in amplitude LN! briefer in latency than the initial one, (b) When usin suprathreshold TCS, MEPs followed by silent s were ft~nd, l'b s SP was followed by a rebound acceleration of the M APs firing rate compared with pre-TCS Icvcls.
Brain, 2004
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.
Neuroscience Letters, 2001
The purpose of this study was to characterize the neuromuscular control during shortening (SHO) and lengthening (LEN) contractions by investigating the input-output (I/O) property in the corticospinal tract. To this end, the relation between various stimulus intensities applied via transcranial magnetic stimulation and the size of motor evoked potentials was investigated in six healthy subjects during elbow flexion and extension. The measured I/O property demonstrates a sigmoidal shape, and is characterized by a plateau value, maximum slope and threshold. The results demonstrated that both the plateau value and maximum slope were significantly lower during LEN contraction than during SHO contraction (P , 0:05), whereas the threshold was found not to be significantly different. These results suggest that both the maximum excitation level and the gain of the corticospinal tract are reduced during LEN contractions.
Clinical Neurophysiology, 2001
Background: Reliable recording of motor evoked potentials (MEPs) of the masseter muscle by transcranial magnetic stimulation (TMS) has proved more difficult than from facial or intrinsic hand muscles. Up to now it was unclear whether this difficulty was due to methodological and/or anatomical reasons. Methods: The mechanism of pyramidal cell activation in masseter MEPs was investigated by using magnetic and electric transcranial stimulation. Analysing the effect of magnetic coil positioning and orientation over the scalp, and scrutinizing the masseter recording technique to avoid compound motor action potential (CMAP) contamination from facial muscles, an optimized method of masseter MEPs was developed. Results: In particular, an antero-lateral inducing current orientation in the stimulating coil, approximately paralleling the central sulcus, proved clearly more effective for the masseter muscles than the postero-lateral orientation (P ¼ 0:005) found optimal for intrinsic hand muscles. The thus evoked masseter MEPs by transcranial magnetic stimulation (TMS) were found to be identical in shape, amplitude and latency as those evoked by transcranial electric stimulation (TES), evidencing a direct rather than trans-synaptic activation of the pyramidal cells. Conclusions: We conclude that in TMS evoked MEPs of masseter muscles, the direct stimulation of the pyramidal tract is more easily achieved than the trans-synaptic activation, which is in contrast to the intrinsic hand muscles. We hypothesize that the presynaptic projections to pyramidal cells of the masticatory muscles are less abundant than in hand muscles, and are therefore less accessible to trans-synaptic stimulation.
Neuroscience Letters, 1993
Transcranial magnetic stimulation of the motor cortex produces a motor-evoked potential (MEP) in the electromyogram followed by a silent period (SP) which is thought to be due to cortical inhibition. In this study, the topography and size of the cortical areas from which an MEP and SP are evoked in the abductor pollicis brevis muscle (APB) of the hand were compared. The SP area was found to be large, encompassing and surrounding the MEP area. These findings infer the existence of an inhibitory surround limiting the excitatory area for APB in the human motor cortex.