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中大量肌肉和关节的认知复杂性。尽管模块化组织已在几种动物的后肢控制系统中得到确认，但尚未在前肢运动系统或灵长类动物中建立。扩展最初在青蛙腰椎中进行的实验，我们检查了猕猴颈脊髓中两个部位的共同刺激是否导致运动活动，这是通过单独刺激每个部位而引起的反应的简单线性总和。类似于先前在青蛙和啮齿动物后肢中的观察，我们的分析表明，在大多数情况下（所有对中的77％），通过共同刺激引起的力场方向与通过单独刺激每个部位引起的力场的简单线性求和所预测的力场方向非常相似。肌电图（EMG）输出的可比较简单求和，特别是在近端肌肉中，表明力场方向的这种线性求和是由脊髓神经机制产生的，而通过共刺激募集的前肢运动输出也被线性求和。我们进一步发现，力场大小表现出超线性（放大）求和，这在远侧前肢肌肉的EMG输出中也观察到，这意味着灵长类动物前肢控制的新特征。总体，