Primate dexterous hand movements are controlled by functionally distinct premotoneuronal systems

Dexterous hand movements are uniquely developed in primates and indispensable for their daily activities. Traditionally, they were thought to depend primarily on the evolutionarily "newer" direct corticomotoneuronal (CM) pathway. However, recent studies suggest that the "older" indirect corticospinal pathways, mediated by spinal premotor interneurons (PreM-INs), also contribute, highlighting the need to clarify their functional differences. Here, we recorded neuronal activity from PreM-INs and CM cells in macaques during a precision grip task to compare their roles in generating hand muscle activity. Our results show that PreM-INs exert stronger facilitation across a broader set of muscles, promoting synergistic coactivation, whereas CM cells provide more selective facilitation, enabling control of relatively individual muscles. Decomposition analysis further revealed that these systems correspond to different control modes-synergy-based and individual-based control-balancing stability and flexibility. These findings redefine our understanding of primate dexterous hand control as emerging from the cooperative integration of evolutionarily distinct premotoneuronal systems.