Humans with cervical spinal cord injury (SCI) often recover voluntary control of elbow flexors and, to a much lesser extent, elbow extensor muscles. The neural mechanisms underlying this asymmetrical recovery remain unknown. Anatomical and physiological evidence in animals and humans indicates that corticospinal and reticulospinal pathways differentially control elbow flexor and extensor motoneurons; therefore, it is possible that reorganization in these pathways contributes to the asymmetrical recovery of elbow muscles after SCI. To test this hypothesis, we examined motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation over the arm representation of the primary motor cortex, maximal voluntary contractions, the StartReact response (a shortening in reaction time evoked by a startling stimulus), and the effect of an acoustic startle cue on MEPs elicited by cervicomedullary stimulation (CMEPs) on biceps and triceps brachii in males and females with and without chronic cervical incomplete SCI. We found that SCI participants showed similar MEPs and maximal voluntary contractions in biceps but smaller responses in triceps compared with controls, suggesting reduced corticospinal inputs to elbow extensors. The StartReact and CMEP facilitation was larger in biceps but similar to controls in triceps, suggesting enhanced reticulospinal inputs to elbow flexors. These findings support the hypothesis that the recovery of biceps after cervical SCI results, at least in part, from increased reticulospinal inputs and that the lack of these extra inputs combined with the loss of corticospinal drive contribute to the pronounced weakness found in triceps.

SIGNIFICANCE STATEMENT Although a number of individuals with cervical incomplete spinal cord injury show limited functional recovery of elbow extensors compared with elbow flexor muscles, to date, the neural mechanisms underlying this asymmetrical recovery remain unknown. Here, we provide for the first time evidence for increased reticulospinal inputs to biceps but not triceps brachii and loss of corticospinal drive to triceps brachii in humans with tetraplegia. We propose that this reorganization in descending control contributes to the asymmetrical recovery between elbow flexor and extensor muscles after cervical spinal cord injury.

患有颈脊髓损伤（SCI）的人通常可以恢复对肘屈肌的自愿控制，在较小程度上还可以恢复肘伸肌。这种不对称恢复的神经机制仍然未知。动物和人类的解剖学和生理学证据表明，皮质脊髓和网状脊髓途径差异控制肘屈肌和伸肌神经元。因此，这些途径中的重组可能会导致SCI后肘肌的不对称恢复。为了检验这一假设，我们研究了经颅磁刺激在主要运动皮层的手臂代表处产生的运动诱发电位（MEP），最大的自主收缩，StartReact响应（通过惊人的刺激引起的反应时间的缩短），以及在患有和不患有慢性宫颈不完整SCI的男性和女性中，子宫颈刺激（CMEP）对子宫颈刺激引起的MEP的影响都受到惊吓提示的影响。我们发现，与对照组相比，SCI参与者在二头肌中表现出相似的MEP和最大自愿收缩，但在三头肌中的反应较小，这表明皮质脊髓向肘伸肌的输入减少。在二头肌中，StartReact和CMEP的促进作用较大，但在三头肌中与对照相似，表明网状脊髓输入肘屈肌的能力增强。这些发现支持以下假设：颈椎脊髓损伤后肱二头肌的恢复至少部分是由于网状脊髓输入增加而导致的，而缺乏这些额外输入加上皮质脊髓驱动的丧失导致了三头肌的明显无力。

重要声明尽管与颈屈肌相比，许多颈椎不全性脊髓损伤患者的肘伸肌功能恢复有限，但迄今为止，这种不对称恢复的神经机制仍然未知。在这里，我们首次为四肢瘫痪的人提供了向二头肌而不是肱三头肌增加网状脊髓输入的证据，以及对肱三头肌的皮质脊髓驱动的损失。我们提出，这种降序控制的重组有助于颈脊髓损伤后肘部屈肌和伸肌之间的不对称恢复。