Zhen Ni

Physical practice induces excitability changes in human hand motor area during motor imagery

The present study was undertaken to investigate the effects of physical practice on excitability changes in human primary motor cortex (M1) during motor imagery (MI). Using different intensities of transcranial magnetic stimulation (TMS), we examined changes in the motor evoked potential (MEP) of the first dorsal interosseous (FDI) muscle with and without MI, and before and after physical practice. On comparing results for MEPs recorded before and after physical practice, the difference between the MEP amplitudes observed at rest and during MI only increased at higher TMS intensities. This finding indicates a physical practice-dependent increase of the higher threshold recruitment of corticospinal tract neurons (CTNs), consistent with synchronization for efficient movement, and provides evidence that neural mechanisms of MI depend not only on the type of movement but also on the extent of the motor adaptation (the physical practice). These present findings also show the benefit of MI and highlight beneficial neural mechanisms related to the activation of M1 during MI. In other words, MI may reflect functional changes of M1 that are similar to the changes observed after physical practice.

Excitability changes in human hand motor area induced by voluntary teeth clenching are dependent on muscle properties

To investigate whether the early effects of voluntary teeth clenching (VTC) among the first dorsal interosseous (FDI), abductor digiti minimi (ADM), and abductor pollicis brevis (APB) muscles are differently modulated depending on their muscle properties, we examined the responses of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation with selected current directions and by brainstem magnetic stimulation (BMS). Although MEP responses with anterior-medially current direction (preferentially elicited I1-waves) were facilitated in all three muscles, those responses with posterior-laterally current direction (preferentially elicited I3-waves) were different among FDI, ADM, and APB muscles. That is, MEP responses in FDI and APB muscles were significantly reduced, whereas those responses in ADM muscle were not significantly reduced. Further, inhibitory effects of VTC in FDI muscle were more potent than those in ADM or APB muscles. On the other hand, the responses to BMS were unchanged by VTC in all three muscles, suggesting that the modulations of MEP were attributed to the cortical origin. On the basis of our previous findings that the inhibitory connections in FDI muscle are more potent than those in ADM muscle (Takahashi et al. in Clin Neurophysiol 116:2757–2764, 2005), the cortical effects of VTC among three hand muscles are differently modulated, depending on muscle properties, presumably the extents of inhibitory connections to corticospinal tract neurons. Considering that the functional capacity in FDI muscle is higher than that in ADM or APB muscles, the cortical inhibitory effect of VTC might contribute to the sophisticated regulation of the motor outputs even during VTC.

Effects of motor imagery are dependent on motor strategies.

To investigate whether the facilitatory effects of motor imagery (MI) are dependent on motor strategies that vary with posture, we used transcranial magnetic stimulation to examine the effects of two forearm positions on motor-evoked potentials during an MI of index-finger abduction. MI-enhanced motor-evoked potentials of the first dorsal interosseous (prime mover) muscle in the forearm prone position were larger than those in the forearm neutral position. The opposite effects were seen in the extensor carpi radialis (synergist) muscle. These effects correspond to the different electromyography activities in the muscles when performing the actual movements in these two forearm positions. It is suggested that MI reflects different motor strategies in the contribution of agonist and synergist muscles towards a motor task.

Functional demanded excitability changes of human hand motor area

The present study was performed to examine if there are functional differences between the first dorsal interosseous (FDI) and the abductor digit minimi (ADM) muscles during different muscle contractions, namely dynamic and static contractions of the index and little finger abductions. It was also examined whether these functional differences occur at the cortical level. The motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and force curves, during the muscle contractions, were simultaneously recorded. Rest motor thresholds (RMTs) and active motor thresholds (AMTs), during dynamic and static contractions, were determined in the two muscles. In all trials, the background EMGs (B.EMGs) were kept at the same level in each muscle. Results showed that the target matching errors of dynamic contractions were statistically smaller in the FDI muscle than those in the ADM. In the FDI muscle, the AMT during dynamic contractions was significantly lower than during static ones and the MEPs elicited by TMS were larger during dynamic contractions than those during static ones. However, such results were not found in the ADM muscle. In order to investigate whether the differences were caused by the excitability changes that occurred in the cortical level, the responses elicited by subcortical stimulations were recorded using the same procedures as the experiment of TMS. Responses to subcortical stimulations during dynamic contractions were similar to those during static ones in either muscle. It is concluded that there are differences in the task-dependent MEP facilitations between the FDI and ADM muscles. And the differences are due to the functional demanded excitability changes accompanied by the cortical activation.

Effects of intermanual transfer induced by repetitive precision grip on input-output properties of untrained contralateral limb muscles

Although there were many reports relating to intermanual transfer of behavioral motor tasks in humans, it is still not well-known whether the transfer phenomenon between the trained and untrained hand is accompanied by corresponding changes in motor system. In the present study we applied transcranial magnetic stimulation to investigate the practice effects of unilateral fingertip precision grip on corticospinal excitability, regarding both the trained and untrained hand muscles. The results showed that after practice fingertip grip force became steady and safety margin dramatically decreased not only in the trained hand, but also in the untrained hand. Regarding MEP and background EMG (B.EMG) activities, the regression slope of MEP/B.EMG ratio in the first dorsal interosseous (FDI) muscle became significantly steeper after practice in both hands, but in the thenar (TH) muscle there were no clear modulations. These results indicated that through practice qualitative or functional changes of corticospinal systems related to the reorganization for a fingertip precision grip prominently reflect only on FDI muscle which plays a dominant role in the task. More importantly, such effects were simultaneously seen in the untrained hand correspondent to the trained hand, i.e., changes of input–output property in M1 occur not only in the trained hand, but also in the untrained hand. Based on the present results, we suggest that training-induced neural adaptations of the central nervous system may include improvement of its predicting fingertip grip force for self-lifting of the object in the untrained hand.

Motor strategies and excitability changes of human hand motor area are dependent on different voluntary drives.

The present study examined whether there were different voluntary drives between intended and non‐intended muscle contractions. In experiment 1, during intended and non‐intended muscle contractions, electromyograms (EMGs) were recorded from the first dorsal interosseous (FDI) and extensor carpi radialis (ECR) muscles when force levels were varied from 10% to 50% maximal voluntary contraction (MVC) in 10% MVC steps. In experiment 2, using transcranial magnetic stimulation, motor‐evoked potentials (MEPs) were recorded from the FDI muscle when EMGs were varied from 10% to 40% EMGmax (EMG activities during MVC) in 10% EMGmax steps during intended and non‐intended muscle contractions. In experiment 3, at 10% MVC force level MEPs were recorded before and after practice. The results showed that, in the FDI muscle, EMGs during intended muscle contractions were larger than those during non‐intended ones at higher force levels (30–50% MVC). In the ECR muscle, reverse results were observed. At comparable EMG levels of the FDI muscle MEPs were the same during intended and non‐intended muscle contractions. After practice, MEPs during intended muscle contraction became larger than those during non‐intended at 10% MVC force level, while EMGs were the same between two muscle contractions. It is concluded that motor strategies and excitability changes of hand motor area are different during intended and non‐intended muscle contractions, and these differences are due to the different voluntary drives of intended and non‐intended. The present findings may contribute to the understanding of rehabilitation for patients suffering from damages of the central motor system.

Remote effects of voluntary teeth clenching on excitability changes of the human hand motor area

The aim of the present study was to investigate effects of voluntary teeth clenching (VTC) on motor evoked potentials (MEPs) from the first dorsal interosseous (FDI) muscle to transcranial magnetic stimulation (TMS) by different oriented currents (anterior-medially (AM), posterior-laterally (PL)) of the human motor cortex. In active FDI, VTC enhanced MEP responses induced by AM directed current but reduced these responses induced by PL. In relaxed FDI, VTC enhanced MEP responses by AM but had no significant effects on those by PL. Thus, the results suggest that any components produced by AM directed current were enhanced, whereas those by PL directed currents were not affected or reduced. The present evidence indicates that I-waves recorded at the same latency were not completely the same between those produced by AM and PL directed currents. Because VTC had no influence on responses to brainstem electrical stimulation (BES) or F-waves just after the onset of teeth clenching [T. Furubayashi, K. Sugawara, T. Kasai, A. Hayashi, R. Hanajima, Y. Shiio, N.K. Iwara, Y. Ugawa, Remote effects of self-paced teeth clenching on the excitability of hand motor area, Exp. Brain Res., 148 (2003) 261-265], these modulatory effects on MEPs to both AM and PL directed currents must be due to changes of the motor cortical excitability. Thus, we conclude that VTC affects the motor cortical circuits activated by PL and AM directed currents differentially; it facilitates the one and inhibits the other. This is the first demonstration of opposite effects of the same maneuver on MEP responses elicited by AM and PL directed currents.