Institute of Neuroscience, Newcastle University, Medical School, Newcastle upon Tyne NE2 4HH, UK.
J Neurosci. 2011 Aug 3;31(31):11208-19. doi: 10.1523/JNEUROSCI.0257-11.2011.
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 (>500 μA, 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.
有力的实验证据表明，皮质脊髓束参与了对侧前肢的随意控制。虽然已经确定了皮质脊髓束向同侧脊髓回路的解剖投射，但对于其在控制同侧前肢中的潜在作用，人们的了解还不够充分。我们使用多种方法研究了来自对侧皮质脊髓束的、支配手部和前臂肌肉的运动神经元的传入。在三只麻醉猴子中，我们对 62 个运动神经元进行了细胞内记录，发现刺激对侧皮质脊髓束后没有单突触和仅有一个弱的多突触 EPSP。在清醒动物中，对初级运动皮层（M1）进行单刺激皮质内微刺激，未能在同侧肌肉中产生任何反应。对大多数皮质脊髓轴突在延髓锥体处进行强刺激（>500μA，单刺激），仅在比对侧更强的易化延迟更长的时间后，在同侧肌肉中产生弱的抑制。从 M1 神经放电（184 个细胞）的同侧肌肉活动中进行的尖峰触发平均，没有发现任何与单突触皮质运动神经元连接一致的尖峰后效应。我们还检查了 191 个 M1 神经元在同侧或对侧“精准握持抓握”运动期间的活动。许多细胞（67%）在同侧肢体运动试验期间调节其活动（与对侧试验的 90%相比），但这种活动的时间与对侧未运动手臂中的弱肌肉活动最佳相关。我们的结论是，在正常成年人中，来自对侧皮质脊髓束的任何传入同侧运动神经元的传入都很弱且间接，并且 M1 细胞放电的调制似乎主要与对侧肢体的控制有关。
