Butler Carley L P, Sangari Sina, Chen Bing, Perez Monica A
Shirley Ryan AbilityLab, Chicago, IL, USA.
Department of Biomedical Engineering, Northwestern University, Evanston, IL, USA.
J Physiol. 2024 Dec;602(24):6909-6923. doi: 10.1113/JP285510. Epub 2024 Nov 6.
Most individuals with cervical spinal cord injury (SCI) show increased muscle weakness in the elbow extensor compared to elbow flexor muscles. Although this is a well-known functional deficit, the underlying neural mechanisms remain poorly understood. To address this question, we measured the suppression of voluntary electromyographic activity (svEMG; a measurement thought to reflect changes in intracortical inhibition) by applying low-intensity transcranial magnetic stimulation over the arm representation of the primary motor cortex during 10% of isometric maximal voluntary contraction (MVC) into elbow flexion or extension in individuals with and without chronic cervical SCI. We found that the svEMG latency and duration were not different between the biceps and triceps brachii in controls but prolonged in the triceps in individuals with SCI. The svEMG area was larger in the triceps compared to the biceps in both groups and further increased in SCI participants, suggesting a pronounced intracortical inhibitory input during elbow extension. A negative correlation was found between svEMG area and MVCs indicating that control and SCI participants with lower svEMG area had larger MVCs. The svEMG area was not different between 5% and 30% of MVC, making it less probable that differences in muscle strength between groups contributed to our results. These findings support the existence of strong inhibitory input to corticospinal projections controlling elbow extensor compared to flexor muscles, which is more pronounced after chronic cervical SCI. KEY POINTS: After cervical spinal cord injury (SCI), people often recover function in elbow flexor, but much less in elbow extensor muscles. The neural mechanisms contributing to this difference remain unknown. We measured the suppression of voluntary electromyographic activity (svEMG) elicited through low-intensity transcranial magnetic stimulation of the primary motor cortex (assumed to reflect changes in intracortical inhibition) in the biceps and triceps muscles in controls and individuals with cervical chronic incomplete SCI. We found increased svEMG area in the triceps compared to the biceps in controls and SCI participants, with this measurement being even more pronounced in the triceps after SCI. The svEMG area correlated with maximal voluntary contraction values in both groups, suggesting the people with lesser inhibition had larger motor output. Our results support the presence of strong cortical inhibitory input to corticospinal projections controlling elbow extensor compared to elbow flexors muscles after cervical SCI.
与肘部屈肌相比，大多数颈脊髓损伤（SCI）患者的肘伸肌肌力减弱。尽管这是一种众所周知的功能缺陷，但其潜在的神经机制仍知之甚少。为了解决这个问题，我们在有或没有慢性颈脊髓损伤的个体进行10%的等长最大自主收缩（MVC）使肘部屈曲或伸展时，通过在初级运动皮层的手臂代表区施加低强度经颅磁刺激，测量了自愿肌电图活动（svEMG；一种被认为反映皮质内抑制变化的测量方法）的抑制情况。我们发现，对照组肱二头肌和肱三头肌的svEMG潜伏期和持续时间没有差异，但脊髓损伤个体的肱三头肌svEMG潜伏期和持续时间延长。两组中肱三头肌的svEMG面积均大于肱二头肌，且脊髓损伤参与者的svEMG面积进一步增加，这表明在肘部伸展过程中存在明显的皮质内抑制性输入。在svEMG面积和MVC之间发现了负相关，这表明svEMG面积较小的对照组和脊髓损伤参与者具有较大的MVC。在5%至30%的MVC之间，svEMG面积没有差异，这使得两组之间肌肉力量的差异导致我们结果的可能性较小。这些发现支持了与屈肌相比，控制肘伸肌的皮质脊髓投射存在强烈抑制性输入，且在慢性颈脊髓损伤后更为明显。关键点：颈脊髓损伤（SCI）后，人们肘部屈肌的功能往往会恢复，但肘伸肌的恢复则少得多。造成这种差异的神经机制尚不清楚。我们测量了通过对对照组和慢性颈脊髓损伤不完全性个体的肱二头肌和肱三头肌进行低强度经颅磁刺激初级运动皮层（假定反映皮质内抑制变化）所引起的自愿肌电图活动（svEMG）的抑制情况。我们发现，对照组和脊髓损伤参与者中，肱三头肌的svEMG面积均大于肱二头肌，且脊髓损伤后肱三头肌的这一测量结果更为明显。两组中svEMG面积均与最大自主收缩值相关，这表明抑制作用较小的人运动输出较大。我们的结果支持了颈脊髓损伤后，与肘屈肌相比，控制肘伸肌的皮质脊髓投射存在强烈的皮质抑制性输入。
