Department of Neurophysiology, National Institute of Neuroscience, Kodaira, 187-8502 Tokyo, Japan.
School of Biomedical Engineering, Science and Health Systems, Drexel University, Philadelphia, PA 19104.
Proc Natl Acad Sci U S A. 2020 Nov 3;117(44):27655-27666. doi: 10.1073/pnas.1919253117. Epub 2020 Oct 15.
Modular organization of the spinal motor system is thought to reduce the cognitive complexity of simultaneously controlling the large number of muscles and joints in the human body. Although modular organization has been confirmed in the hindlimb control system of several animal species, it has yet to be established in the forelimb motor system or in primates. Expanding upon experiments originally performed in the frog lumbar spinal cord, we examined whether costimulation of two sites in the macaque monkey cervical spinal cord results in motor activity that is a simple linear sum of the responses evoked by stimulating each site individually. Similar to previous observations in the frog and rodent hindlimb, our analysis revealed that in most cases (77% of all pairs) the directions of the force fields elicited by costimulation were highly similar to those predicted by the simple linear sum of those elicited by stimulating each site individually. A comparable simple summation of electromyography (EMG) output, especially in the proximal muscles, suggested that this linear summation of force field direction was produced by a spinal neural mechanism whereby the forelimb motor output recruited by costimulation was also summed linearly. We further found that the force field magnitudes exhibited supralinear (amplified) summation, which was also observed in the EMG output of distal forelimb muscles, implying a novel feature of primate forelimb control. Overall, our observations support the idea that complex movements in the primate forelimb control system are made possible by flexibly combined spinal motor modules.
脊髓运动系统的模块化组织被认为可以降低同时控制人体大量肌肉和关节的认知复杂性。尽管模块化组织已经在几种动物的后肢控制系统中得到证实，但它尚未在灵长类动物的前肢运动系统中得到证实。在最初在青蛙腰脊髓中进行的实验的基础上，我们研究了刺激猕猴颈椎脊髓的两个部位是否会导致运动活动，该运动活动是单独刺激每个部位所引起的反应的简单线性总和。与之前在青蛙和啮齿动物后肢中的观察结果相似，我们的分析表明，在大多数情况下（所有对的 77%），由共刺激引起的力场的方向与单独刺激每个部位所引起的力场的方向非常相似。类似的肌电图（EMG）输出的简单总和，尤其是在近端肌肉中，表明这种力场方向的线性总和是由脊髓神经机制产生的，其中共刺激募集的前肢运动输出也呈线性总和。我们还发现力场幅度表现出超线性（放大）总和，这在远端前肢肌肉的 EMG 输出中也观察到，这暗示了灵长类前肢控制的一个新特征。总体而言，我们的观察结果支持这样的观点，即灵长类前肢控制系统中的复杂运动是通过灵活组合的脊髓运动模块来实现的。
