Department of Physical Medicine and Rehabilitation, Center for the Neural Basis of Cognition, Systems Neuroscience Institute, University of Pittsburgh, Pittsburgh, PA, USA.
J Neurophysiol. 2012 May;107(10):2901-11. doi: 10.1152/jn.00850.2011. Epub 2012 Feb 22.
In uninjured humans, it is well established that voluntary contraction of muscles on one side of the body can facilitate transmission in the contralateral corticospinal pathway. This crossed facilitatory effect may favor interlimb coordination and motor performance. Whether this aspect of corticospinal function is preserved after chronic spinal cord injury (SCI) is unknown. Here, using transcranial magnetic stimulation, we show in patients with chronic cervical SCI (C(5)-C(8)) that the size of motor evoked potentials (MEPs) in a resting intrinsic hand muscle remained unchanged during increasing levels of voluntary contraction with a contralateral distal or proximal arm muscle. In contrast, MEP size in a resting hand muscle was increased during the same motor tasks in healthy control subjects. The magnitude of voluntary electromyography was negatively correlated with MEP size after chronic cervical SCI and positively correlated in healthy control subjects. To examine the mechanisms contributing to MEP crossed facilitation we examined short-interval intracortical inhibition (SICI), interhemispheric inhibition (IHI), and motoneuronal behavior by testing F waves and cervicomedullary MEPs (CMEPs). During strong voluntary contractions SICI was unchanged after cervical SCI and decreased in healthy control subjects compared with rest. F-wave amplitude and persistence and CMEP size remained unchanged after cervical SCI and increased in healthy control subjects compared with rest. In addition, during strong voluntary contractions IHI was unchanged in cervical SCI compared with rest. Our results indicate that GABAergic intracortical circuits, interhemispheric glutamatergic projections between motor cortices, and excitability of index finger motoneurons are neural mechanisms underlying, at least in part, the lack of crossed corticospinal facilitation observed after SCI. Our data point to the spinal motoneurons as a critical site for modulating corticospinal transmission after chronic cervical SCI.
在未受伤的人类中，人们已经充分认识到，身体一侧的肌肉的自愿收缩可以促进对侧皮质脊髓束的传递。这种交叉促进效应可能有利于肢体间的协调和运动表现。在慢性脊髓损伤（SCI）后，皮质脊髓功能的这一方面是否得到保留尚不清楚。在这里，我们使用经颅磁刺激，在患有慢性颈段 SCI（C5-C8）的患者中显示，在使用对侧远端或近端手臂肌肉进行逐渐增加的自愿收缩期间，静息内在手部肌肉中的运动诱发电位（MEP）的大小保持不变。相比之下，在健康对照组中，在相同的运动任务中，静息手部肌肉中的 MEP 大小会增加。静息颈段 SCI 后，MEP 大小与自愿肌电图的幅度呈负相关，而在健康对照组中呈正相关。为了检查导致 MEP 交叉促进的机制，我们通过测试 F 波和颈髓 MEPs（CMEPs）检查了短程皮质内抑制（SICI）、半球间抑制（IHI）和运动神经元行为。在强烈的自愿收缩期间，颈段 SCI 后的 SICI 保持不变，而与休息相比，健康对照组中的 SICI 下降。F 波幅度和持续时间以及 CMEP 大小在颈段 SCI 后保持不变，而在健康对照组中与休息相比则增加。此外，在强烈的自愿收缩期间，颈段 SCI 中的 IHI 与休息时相比保持不变。我们的结果表明，GABA 能皮质内回路、皮质之间的兴奋性谷氨酸能投射以及食指运动神经元的兴奋性是至少部分解释 SCI 后观察到的交叉皮质脊髓促进作用缺失的神经机制。我们的数据表明，脊髓运动神经元是调节慢性颈段 SCI 后皮质脊髓传递的关键部位。
