There are contralateral and less studied ipsilateral (i), indirect cortical descending projections to motoneurons (MNs). We compared ipsilateral cortical descending influences on MNs of wrist flexors by applying transcranial magnetic stimulation (TMS) over the right primary motor cortex at actively maintained flexion and extension wrist positions in uni- and bimanual tasks in right-handed participants (n = 23). The iTMS response includes a short latency (~ 25 ms) motor evoked potential (iMEP), a silent period (iSP) and a long latency (~ 60 ms) facilitation called rebound (iRB). We also investigated whether the interaction between the two hands while holding an object in a bimanual task involves ipsilateral cortical descending influences. In the unimanual task, iTMS responses in the right wrist flexors were unaffected by changes in wrist position. In the bimanual task with an object, iMEPs in the right wrist flexors were larger when the ipsilateral wrist was in flexion compared to extension. Without the object, only iRB were larger when the ipsilateral wrist was extended. Thus, ipsilateral cortical descending influences on MNs were modulated only in bimanual tasks and depended on how the two hands interacted. It is concluded that the left and right cortices cooperate in bimanual tasks involving holding an object with both hands, with possible involvement of oligo- and poly-synaptic, as well as transcallosal projections to MNs. The possible involvement of spinal and transcortical stretch and cutaneous reflexes in bimanual tasks when holding an object is discussed in the context of the well-established notion that indirect, referent control underlies motor actions.

有对侧和较少研究的同侧 (i)、运动神经元 (MN) 的间接皮质下降投射。我们通过在右手参与者的单手和双手任务中主动保持屈曲和伸展手腕位置时，在右侧初级运动皮层上应用经颅磁刺激（TMS），比较同侧皮质下降对腕屈肌 MN 的影响（n = 23 ）  。iTMS 反应包括短潜伏期（约 25 毫秒）运动诱发电位 (iMEP)、静默期 (iSP) 和称为回弹 (iRB) 的长潜伏期（约 60 毫秒）促进。我们还研究了在双手任务中握住物体时两只手之间的相互作用是否涉及同侧皮质下降的影响。在单手任务中，右腕屈肌的 iTMS 反应不受手腕位置变化的影响。在双手操作物体的任务中，与伸展时相比，当同侧手腕屈曲时，右腕屈肌中的 iMEP 更大。在没有物体的情况下，当同侧手腕伸展时，只有 iRB 更大。因此，同侧皮质下降对 MN 的影响仅在双手任务中受到调节，并且取决于两只手如何相互作用。结论是，左右皮质在双手任务中合作，包括用双手握住物体，可能涉及寡突触和多突触，以及对 MN 的经胼胝体投射。在间接、参照控制是运动动作的基础这一公认概念的背景下，讨论了握住物体时双手任务中可能涉及脊柱和经皮层拉伸以及皮肤反射的情况。