Abstract
ObjectiveTo 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. MethodsMotor 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 120μs 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. ResultsThe S–D time constant with PA was shorter than for AP stimulation (230.9±97.2 vs. 294.2±90.9μs; p<0.001). These values were similar to those calculated after stimulation of ulnar nerve (197±47μs). MEP latencies to AP, but not PA stimulation were affected by pulse width, showing longer latencies following short duration stimuli. ConclusionPA 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. SignificanceMore selective stimulation of neural elements may be achieved by manipulating pulse width and orientation.
Highlights
Peterchev and colleagues developed a new device in which it is possible to change the duration of the Transcranial magnetic stimulation (TMS) output over a limited range. They calculated values for activation of motor cortex in healthy volunteers (Peterchev et al, 2013) that was in agreement with an earlier estimate of Barker and colleagues who used a different technological approach (Barker et al, 1991). Both were similar to the S–D time constant for magnetic stimulation of peripheral nerve estimated by Barker et al (1991), and compatible with the idea that at threshold, TMS of motor cortex likely activates relatively large myelinated axons in brain
The third aim was to compare these estimates of central motor S–D time constants with those measured using the same TMS techniques on peripheral motor axons in the ulnar nerve, in order to illustrate that measurements in the cortex reflect the membrane properties of large myelinated axons
The relationship between the motor threshold and the pulse width is usually described by a strength– duration curve parameterized by time constant and rheobase
Summary
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 ls 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. MEP latencies to AP, but not PA stimulation were affected by pulse width, showing longer latencies following short duration stimuli.
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