Excitability changes in human forearm corticospinal projections and spinal reflex pathways during rhythmic voluntary movement of the opposite limb

Abstract

Rhythmic movements brought about by the contraction of muscles on one side of the body give rise to phase-locked changes in the excitability of the homologous motor pathways of the opposite limb. Such crossed facilitation should favour patterns of bimanual coordination in which homologous muscles are engaged simultaneously, and disrupt those in which the muscles are activated in an alternating fashion. In order to examine these issues, we obtained responses to transcranial magnetic stimulation (TMS), to stimulation of the cervicomedullary junction (cervicomedullary-evoked potentials, CMEPs), to peripheral nerve stimulation (H-reflexes and f-waves), and elicited stretch reflexes in the relaxed right flexor carpi radialis (FCR) muscle during rhythmic (2 Hz) flexion and extension movements of the opposite (left) wrist. The potentials evoked by TMS in right FCR were potentiated during the phases of movement in which the left FCR was most strongly engaged. In contrast, CMEPs were unaffected by the movements of the opposite limb. These results suggest that there was systematic variation of the excitability of the motor cortex ipsilateral to the moving limb. H-reflexes and stretch reflexes recorded in right FCR were modulated in phase with the activation of left FCR. As the f-waves did not vary in corresponding fashion, it appears that the phasic modulation of the H-reflex was mediated by presynaptic inhibition of Ia afferents. The observation that both H-reflexes and f-waves were depressed markedly during movements of the opposite indicates that there may also have been postsynaptic inhibition or disfacilitation of the largest motor units. Our findings indicate that the patterned modulation of excitability in motor pathways that occurs during rhythmic movements of the opposite limb is mediated primarily by interhemispheric interactions between cortical motor areas.

Preconditioning tDCS facilitates subsequent tDCS effect on skill acquisition in older adults

“…MEP amplitude was determined as the peak-to-peak amplitude following TMS. Trials in which root mean squared (RMS) EMG exceeded 10 µV (Carson et al 2004) during the 40 ms immediately preceding the TMS pulse were discarded.…”

Functional Responses in the Human Spinal Cord during Willed Motor Actions: Evidence for Side- and Rate-Dependent Activity

“…It has been recently shown that in sitting volunteers performing voluntary foot (Baldissera et al, 2002) or contralateral wrist (Carson et al, 2004) flexions/ extensions, the excitability of corticospinal pathways targeting the resting wrist muscles is modulated in phase with movements, whereas the H-reflex and f-waves are depressed markedly during contraction of homologous muscles of the contralateral forearm (Hortobágyi et al, 2003;Carson et al, 2004). Moreover, in standing subjects, overt electromyographic activity in forearm muscles has been described to accompany movements of the contralateral forearm (Slijper and Latash, 2000) or ipsilateral foot (Baldissera and Esposti, 2005).…”

Emotion and motor preparation: A transcranial magnetic stimulation study of corticospinal motor tract excitability

“…Larger MEPs reflect greater CST excitability. MEPs were discarded if the root-mean square (RMS) of the EMG exceeded 15 V (Carson et al, 2004) during the 70 msec immediately preceding the TMS pulse. No more than three trials were discarded per image condition per participant.…”