Abstract :
[en] Objective: To compare the strength-duration (S-D) time constants of motor cortex structures activated by current pulses oriented posterior-anterior (PA) or anterior-posterior (AP) across the central sulcus.
Methods: Motor threshold and input–output curve, along with motor evoked potential (MEP) latencies, of first dorsal interosseus were determined at pulse widths of 30, 60, and 120 μs using a controllable pulse parameter (cTMS) device, with the coil oriented PA or AP. These were used to estimate the S-D time constant and we compared with data for responses evoked by cTMS of the ulnar nerve at the elbow.
Results: The S-D time constant with PA was shorter than for AP stimulation (230.9 ± 97.2 vs. 294.2 ± 90.9 us; p<0.001). These values were similar to those calculated after stimulation of ulnar nerve (197 ± 47us). MEP latencies to AP, but not PA stimulation were affected by pulse width, showing longer latencies following short duration stimuli.
Conclusion: PA and AP stimuli appear to activate the axons of neurons with different time constants. Short duration AP pulses are more selective than longer pulses in recruiting longer latency corticospinal output.
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