, 2007). For example, in Fig. 1A, the delay R1 was ∼40 ms and the Gaussian curve peaked at ∼60 ms, thus ∼100 ms after the previous motor unit discharge, i.e. a discharge rate of ∼10 Hz (Bawa & Lemon, 1993). The delay R1 was adjusted according to the
motor unit firing rate, so that TMS was delivered within the recovery phase of the after-hyperpolarization. Thus, when the computer triggered a single TMS pulse at Selleck PLX4032 delay R1, the effects on the membrane potential of the motoneuron are optimized, and peak(s) appeared in the PSTH (from an FDI unit) 20–35 ms after TMS (Fig. 1B). The(se) peak(s) reflect(s) the arrival of corticospinal input(s) at motoneuron level, and indicate(s) that the resulting corticospinal excitatory post-synaptic potential(s) [EPSP(s)] were sufficient to advance the ERK inhibitor discharge of the motoneuron, as compared with its firing rate during voluntary contraction, by shortening the after-hyperpolarisation duration (Fig. 1A and B). The peak in the PSTH is correlated to the ascending phase of the underlying EPSP at motoneuron level (Kirkwood & Sears, 1978; Ashby &
Zilm, 1982). Therefore, the TMS-induced peak in PSTH can be used to estimate the corticospinal EPSP produced at the motoneuron level. The hot spot for FDI and the RMT were determined at the beginning of the experiment. The intensity of both test and conditioning pulses influences the level of SICI (Chen et al., 1998; Sanger et al., 2001; Orth et al., 2003; Roshan et al., 2003; Garry & Thomson, 2009; Lackmy & Marchand-Pauvert, 2010). Therefore, the test pulse intensity was changed so as to evoke a peak in the PSTH of different size (normalized to the number of stimuli, see PSTH analysis below), reflecting corticospinal EPSPs of different size. It was necessary to adjust the intensity of the conditioning pulse to produce SICI without evoking a peak in the PSTH, to prevent possible for summation of corticospinal volleys (induced by the test and conditioning pulses) at the motoneuron level. As a consequence, the conditioning pulse could only be set to 0.6 RMT, an intensity at which TMS did not
produce a peak in the PSTH (Fig. 1C) but was sufficient to activate SICI (Fisher et al., 2002). At 0.65 RMT, a peak occurred in the PSTH of some motor units (see Results). A recording session consisted of sequential alternation (0.3 Hz) of isolated test and paired pulses (conditioning + test pulses with a 2-ms interval), to deliver as many test pulses (test peak) as paired pulses (conditioned peak). To avoid muscular fatigue (which can develop rapidly in FDI), 30–50 single and 30–50 paired pulses were delivered during each recording session; the session was stopped when the subjects developed fatigue or had difficulty in maintaining a steady motor unit discharge. Care was taken to ensure that the same motor unit was studied in each session, based on the shape of the potential, its firing rate, the hand position and the movement performed by the subject, and the peak latency in the PSTH.