Abstract
Neuroimaging combined with transcranial magnetic stimulation (TMS) to primary motor cortex (M1) is an emerging technique that can examine motor-system functionality through evoked activity. However, because sensory afferents from twitching muscles are widely represented in motor areas the amount of evoked activity directly resulting from TMS remains unclear. We delivered suprathreshold TMS to left M1 or gave electrical right median nerve stimulation (MNS) in 18 healthy volunteers while simultaneously conducting functional magnetic resonance imaging and monitoring with electromyography (EMG). We examined in detail the localization of TMS-, muscle afferent- and superficial afferent-induced activity in M1 subdivisions. Muscle afferent- and TMS-evoked activity occurred mainly in rostral M1, while superficial afferents generated a slightly different activation distribution. In 12 participants who yielded quantifiable EMG, differences in brain activity ascribed to differences in movement-size were adjusted using integrated information from the EMGs. Sensory components only explained 10–20% of the suprathreshold TMS-induced activity, indicating that locally and remotely evoked activity in motor areas mostly resulted from the recruitment of neural and synaptic activity. The present study appears to justify the use of fMRI combined with suprathreshold TMS to M1 for evoked motor network imaging.
Highlights
For online control of movement, the status of muscles controlling the target effector needs to be properly monitored and integrated with a motor plan
For participants who required stimulation above 100% of the machine output to reach the 120% resting motor threshold (RMT) stimulation, the maximum machine output of the transcranial magnetic stimulation (TMS) stimulator was reset to 110% of the default machine output, which was within the safety guidelines of the manufacturer and its distributor in Japan (Magstim and Miyuki Giken)
EMG data from six subjects were excluded from the integral EMG value (iEMG) analysis in the motor-median nerve stimulation (MNS) condition because artifacts substantially overlapped with compound muscle action potential (CMAP)
Summary
For online control of movement, the status of muscles controlling the target effector needs to be properly monitored and integrated with a motor plan. Somatosensory afferents from the effector carry critical information for this somatosensory-motor integration. Such integration likely occurs at multiple levels in the central nervous system, from the spinal cord up through cortical motor areas. Few human neuroimaging studies have addressed the issue of motor-area activity ascribed to sensory afferents. This omission likely results from difficulty controlling the effects of sensory afferents during voluntary movement accompanying a complicated temporal pattern of muscle activity
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
