Lack of Evidence for Direct Corticospinal Contributions to Control of the Ipsilateral Forelimb in Monkey

Abstract

Strong experimental evidence implicates the corticospinal tract in voluntary control of the contralateral forelimb. Its potential role in controlling the ipsilateral forelimb is less well understood, although anatomical projections to ipsilateral spinal circuits are identified. We investigated inputs to motoneurons innervating hand and forearm muscles from the ipsilateral corticospinal tract using multiple methods. Intracellular recordings from 62 motoneurons in three anesthetized monkeys revealed no monosynaptic and only one weak oligosynaptic EPSP after stimulation of the ipsilateral corticospinal tract. Single stimulus intracortical microstimulation of the primary motor cortex (M1) in awake animals failed to produce any responses in ipsilateral muscles. Strong stimulation (Ͼ500 A, single stimulus) of the majority of corticospinal axons at the medullary pyramids revealed only weak suppressions in ipsilateral muscles at longer latencies than the robust facilitations seen contralaterally. Spike-triggered averaging of ipsilateral muscle activity from M1 neural discharge (184 cells) did not reveal any postspike effects consistent with monosynaptic corticomotoneuronal connections. We also examined the activity of 191 M1 neurons during ipsilateral or contralateral "reach to precision grip" movements. Many cells (67%) modulated their activity during ipsilateral limb movement trials (compared with 90% with contralateral trials), but the timing of this activity was best correlated with weak muscle activity in the contralateral nonmoving arm. We conclude that, in normal adults, any inputs to forelimb motoneurons from the ipsilateral corticospinal tract are weak and indirect and that modulation of M1 cell firing seems to be related primarily to control of the contralateral limb.

EEG signatures of arm isometric exertions in preparation, planning and execution

“…This is in good agreement with invasive studies on primates and informs of similar neural processing in humans. The observed laterality of ERPs is also compatible with the view of relatively direct role of contralateral motor cortex (Soteropoulos et al, 2011) in execution of tasks and participation of both ipsilateral and contralateral hemispheres in preparation and planning (Schaal et al, 2004). These findings suggest that PM/SMA is active in early preparation; PC activity is added for direction-dependent processing; and M1 activity is mostly related to generation of descending neural motor commands.…”

Preclinical and Clinical Evidence on Ipsilateral Corticospinal Projections: Implication for Motor Recovery

“…However, a significant ipsilateral response was predominantly recorded in proximal or truncal muscles upon stimulation of SMA [44, 45], and, occasionally, M1 [43]. In vivo cortical stimulation studies performed in rhesus [24] and marmoset monkeys [42] showed that ipsilateral M1 projections are not monosynaptic. Although the lack of monosynaptic connection is not unanticipated given that the majority of contralateral fibers also do not monosynaptically connect to motor neurons, the presence of polysynaptic connections does not rule out the involvement of the contralateral hemisphere or contralateral brainstem nuclei upon stimulation of ipsilateral cortical regions.…”

Interlimb neural coupling: Implications for poststroke hemiparesis

“…Neuroanatomically, one cerebral hemisphere is responsible for motor function of the contralateral body side; however, some fibers play a key role in ipsilateral movements [76]. In poststroke subjects, motor impairment may also arise on the ipsilesional or same side of the body but not as severe as on the contralesional body side [77,78].…”

“…Sur le plan neuro-anatomique, un hémisphère cérébral est chargé de la fonction motrice du côté controlatéral du corps ; cependant, certaines fibres ont un rôle clé dans les mouvements ipsilatéraux [76]. Chez les patients post-AVC, la déficience motrice peut aussi survenir sur le côté ipsilésionnel du corps mais elle n'est pas aussi sévère que celle du côté contralé-sionnel [77,78].…”