Sensorimotor Integration During Grasping Is Mediated by Distinct M1 Circuits.

Motor control relies on the dynamic interplay between excitatory and inhibitory influences shaping sensorimotor integration during hand movements. In this study, we investigated short-latency afferent inhibition (SAI)-a neurophysiological marker of sensorimotor integration-during different isometric grasping behaviours (precision vs. power grip). We applied transcranial magnetic stimulation (TMS) with different coil orientations [antero-posterior (AP) vs. postero-anterior (PA)] to engage distinct neuronal populations within the primary motor cortex (M1). We found increased SAI in the AP direction during grasp execution and enhanced corticospinal excitability for precision grip when tested with AP stimulation. These findings provide evidence that distinct cortical circuits within M1 are differentially engaged during different hand configurations. Notably, we observe no grip-specific modulation of SAI, which may reflect less topographically precise distribution of thalamocortical afferents-along with their lower temporal resolution, potentially shaped by cholinergic modulation. In addition, the execution of a grasping action engages a complex parieto-frontal network, with the ventral premotor cortex (PMv) playing a crucial role in motor planning by transforming object-related visual properties into motor plans. PMv may play a more prominent role than primary somatosensory cortex in distinguishing between precision and power grips, as recent findings suggest a preferential involvement of superficial M1 populations-targeted by PMv input-during precision grip execution. Future studies should investigate SAI dynamics across different phases (i.e. preparation vs. execution) of naturalistic prehension.