Changes in motor-evoked potential latency during grasping after tetraplegia

The corticospinal pathway contributes to the control of grasping in intact humans. After spinal cord injury (SCI), there is an extensive reorganization in the corticospinal pathway; however, its contribution to the control of grasping after the injury remains poorly understood. We addressed this question by using transcranial magnetic stimulation (TMS) over the hand representation of the motor cortex to elicit motor-evoked potentials (MEPs) in an intrinsic finger muscle during precision grip and power grip with the TMS coil oriented to induce currents in the brain in the latero-medial (LM) direction to activate corticospinal axons directly and in the posterior-anterior (PA) and anterior-posterior (AP) directions to activate the axon indirectly through synaptic inputs in humans with and without cervical incomplete SCI. We found prolonged MEP latencies in all coil orientations in both tasks in SCI compared with control subjects. The latencies of MEPs elicited by AP relative to LM stimuli were consistently longer during power compared with precision grip in controls and SCI subjects. In contrast, PA relative to LM MEP latencies were similar between tasks across groups. Central conduction time of AP MEPs was prolonged during power compared with precision grip in controls and SCI participants. Our results support evidence indicating that inputs activated by AP and PA currents are engaged to a different extent during fine and gross grasping in humans with and without SCI. NEW & NOTEWORTHY The mechanisms contributing to the control of hand function in humans with spinal cord injury (SCI) remain poorly understood. Here, we demonstrate for the first time that the latency of corticospinal responses elicited by transcranial magnetic stimulation anterior-posterior induced currents, relative to latero-medial currents, was prolonged during power compared with precision grip in humans with and without SCI. Gross grasping might represent a stragegy to engage networks activated by anterior-posterior currents after SCI.