Abstract
The corticospinal tract contributes to the control of finger muscles during precision and
power grip. The involvement of different sets of cortical interneuronal circuits during
these distinct grasping behaviors remains unknown. To examine this question in humans we
used noninvasive transcranial magnetic stimulation (TMS) over the hand representation of
the primary motor cortex to elicit motor evoked potentials (MEPs) in an intrinsic finger
muscle during index finger abduction (control task), precision grip, and power grip. The
TMS coil was oriented to induce currents in the brain in the latero-medial (LM),
posterior–anterior (PA), and anterior–posterior (AP) direction to preferentially activate
corticospinal axons directly and early and late synaptic inputs to corticospinal neurons,
respectively. We found that AP–LM MEP latency differences were consistently longer during
power grip compared with index finger abduction and precision grip, while PA–LM
differences remained similar across tasks. Short-interval intracortical facilitation,
targeting AP but not PA inputs, increased during power grip compared with other tasks. Our
novel findings suggest that cortical structures activated by PA and AP stimuli are
differentially active during precision and power grip. We propose that a preferential
recruitment of late synaptic inputs to corticospinal neurons may be achieved when humans
perform a power grip.
