Modulation of corticospinal influence over hand muscles during gripping tasks in man and monkey

Abstract

Transcranial magnetic brain stimulation (TMS) was used to investigate corticospinal influences during a task in which human subjects had to reach out and grasp and lift an object. TMS applied to the hand area of the motor cortex was delivered during eight different phases of the task. There was a striking phase-related modulation in the amplitude of the short-latency EMG responses elicited by TMS in six arm and hand muscles. Although several mechanisms probably contribute to this modulation, one result of their operation is a potentially greater influence of the cortex during particular phases of the task. Evidence is produced that one factor contributing to this modulation is a phase-related change in corticospinal excitability. The results are consistent with a strong excitatory corticospinal drive throughout the reach to brachioradialis and anterior deltoid, which contribute to hand transport, and to the extrinsic hand muscles, which orientate the hand and fingertips. In contrast, the intrinsic hand muscles appear to receive their strongest cortical input as the digits close around and first touch the object. TMS just before contact delayed the isometric parallel increase in load and grip forces necessary to lift the object. The particularly strong EMG and behavioral effects seen at touch may reflect a powerful interaction, at the cortical level, between cutaneous inputs signalling contact with the object and the effects of TMS. Central interactions between tactile afferent input and TMS were tested by delivering TMS at different times relative to the application of an unexpected load to an object held between the fingertips. The largest responses occurred when TMS was applied 60-80 ms after load onset. THe enhanced corticospinal influence that this represents probably contributes to the powerful, short-latency boosting in grip force observed when the object was suddenly subjected to an external load. Recording of corticospinal cells in the primary motor cortex of the awake monkey suggests that the phasic modulation observed with TMS may reflect the phasic-tonic pattern of corticomotoneuronal cell discharge during the task. Since the activation of corticospinal cells by low-intensity TMS is dependent upon their level of excitability, EMG responses evoked by TMS during the performance of skilled tasks in man may, in part, reflect changes in the excitability of these cells.