MAGNETIC TRANSCRANIAL STIMULATION IN HEALTHY HUMANS - INFLUENCE ON THE BEHAVIOR OF UPPER-LIMB MOTOR UNITS

Aim of the study was to analyze the characteristics of motor action po tentials recruitment during magnetic trans-cranial stimulation (TCS) o f the brain. Coaxial needle recordings from hand and upper limb muscul ature, as well as surface electrodes were employed in 20 healthy contr ols during magnetic TCS with regular and figure-of-8 coil in different experimental protocols including: (a) simple reaction time paradigm d uring which TCS at subthreshold intensity for eliciting MEPs in relaxa tion was delivered at various intervals between the signal to move and the onset of the voluntary EMG burst; (b) suprathreshold TCS was rand omly delivered while the subject was voluntarily firing at a regular r ate one 'low' and/or 'high threshold' motor unit action potential (MUA P). The pre- and post-TCS MUAPs recruitment as well as their firing ra tes were compared; (c) recordings with two separate needles picking up individual MUAPs from the same or from two different muscles were obt ained in order to test 'synchrony' of MUAP's discharge before and afte r TCS; (d) the influence of the time-interval separating the last disc harged MUAP from TCS was evaluated. (e) differences between simultaneo us surface and depth recordings were examined. The following results w ere obtained. (a) The same low-amplitude MUAP which is first voluntari ly recruited at the onset of the EMG burst is the one initially fired by TCS in the pre-movement period. Latency shortenings and amplitude e nlargement of surface MEPs were observed with faster reaction times. S uch changes were coupled to the recruitment of high-threshold MUAPs be ing larger in amplitude and briefer in latency than the initial one. ( b) When using suprathreshold TCS, MEPs followed by silent periods were found. The SP was followed by a rebound acceleration of the MUAPs fir ing rate compared with pre-TCS levels. Besides rebound acceleration, n ew MUAPs of larger amplitude than the original (= pre-stimulus) ones w ere recruited beyond the voluntary control. This phenomenon-together w ith longer SPs- was progressively more pronounced with stronger stimul i. (c) TCS was affecting the 'synchrony' of MUAPs. (d) If the latency difference between the last pre-stimulus spike and the TCS was exceedi ng the half-cycle of the MUAP 'natural' firing, the SP was longer in d uration. (e) SPs not preceded by MEPs were clearly present in depth re cordings. Surface recordings mainly reflected the behavior of high-thr eshold and large MUAPs.