Effects of voluntary contraction on descending volleys evoked by transcranial stimulation in conscious humans

Abstract

The spinal volleys evoked by single transcranial magnetic or electric stimulation over the cerebral motor cortex were recorded from a bipolar electrode inserted into the cervical epidural space of three conscious human subjects. These volleys were termed direct (D) and indirect (I) waves according to their latency. We measured the size and number of volleys elicited by magnetic stimulation at various intensities with subjects at rest and during 20 or 100 % maximum contraction of the contralateral first dorsal interosseous muscle (FDI). Surface EMG activity was also recorded. Electrical stimulation evoked a D‐wave volley. Magnetic stimulation at intensities up to about 15 % of stimulator output above threshold evoked only I‐waves. At higher intensities, a D‐wave could be seen in two of the three subjects. At all intensities tested, voluntary contraction increased the number and size of the I‐waves, particularly during maximum contractions. However, there was only a small effect on the threshold for evoking descending activity. Voluntary contraction produced large changes in the size of EMG responses recorded from FDI. Because the recorded epidural activity is destined for muscles other than the FDI, it is impossible to say to what extent increased activity contributes to voluntary facilitation of EMG responses. Indeed, our results suggest that the main factor responsible for enhancing EMG responses in the transition from rest to activity is likely to be increased excitability of spinal motoneurones, rather than increases in the corticospinal volley. The latter may be more important in producing EMG facilitation at different levels of voluntary contraction.

Age-related Differences in Pre- and Post-synaptic Motor Cortex Inhibition are Task Dependent

“…This interpretation has been supported by experimental evidence from epidural recordings recorded during relaxation of target muscles [45], although opposing results may be found in some subjects [50]. Nonetheless, the contribution of early and late I waves to the MEP are known to be altered when the target muscle is active [51], which may complicate the interpretation of the interactions between inhibitory circuits in active muscle. Furthermore, it is not known whether the composition of the descending volley is different in older adults, although recent evidence comparing MEP latencies between antero-posterior (preferential I3 wave activation) and posterior-anterior (preferential I1 wave activation) TMS showed similar recruitment of I waves in older adults [52].…”

A cortico-cortical mechanism mediating object-driven grasp in humans

“…Although it has long been known that TMS excites cortico-cortical inputs to motor cortex, our study has pinpointed significant facilitation at an ISI of 2.5 ms, which is most likely to reflect an augmented I 2 wave of corticospinal activity (16,17,25). Although TMS cannot define which cortico-cortical inputs are modulated during preparation to grasp, we know that the ventral premotor cortex is activated in both monkeys (5,26) and humans (27,28) before and during visually guided grasp and that projections from premotor to motor cortex can exert a particularly powerful modulation of the later I waves, including the I 2 (6).…”

“…Ten additional subjects participated (four males and six females, aged [19][20][21][22][23][24][25][26][27][28]. On each trial, one of two 200-ms acoustic tones instructed a subject to make either a little finger or index finger abduction cued by the TMS pulse and maintain the abducted posture for Ϸ1 s. TMS was delivered 1,200 ms (Ϯ10% jitter) after the start of the tone, and trials were separated by 8 s. A total of 96 trials were performed with combinations of the two movements and six TMS conditions repeated eight times in a pseudorandom order.…”

Cortical Excitability in Cryptogenic Localization‐Related Epilepsy: Interictal Transcranial Magnetic Stimulation Studies

“…The AT was similar in patients and controls. Activation of the target muscle implies excitability changes at the cortical and spinal level (30), and these changes might mask any influence the disease or the drugs might have on these structures. no significant difference in corticocortical inhibition and OUs studies led to the Same conclusions (1 4,15,16,21,22, …”