Spinal and cortical premotor control of primate dexterous hand movements

Skilled hand movements are an evolutionary advance in primates. Phylogenetically distinct corticospinal pathways are involved in hand control: "newer" direct corticomotoneuronal (CM) pathways and "older" indirect corticospinal pathways mediated by spinal premotor interneurons (PreM-INs). However, the functional differences of these pathways remain unclear. Here we show that CM cells and PreM-INs have distinct physiological properties for the activation of hand muscles and provide different types of movement control signals while monkeys perform a precision grip task. Spike-triggered averaging of electromyographic activity indicated that PreM-INs coactivate a larger number of muscles, whereas CM cells more selectively control fewer muscles. The firing activity of PreM-INs was tightly correlated with their target muscle activity and had a greater contribution to generating hand muscle activity. In contrast, CM cell activity diverged temporally from the target muscle activity and had a smaller contribution to its generation. On the basis of these results, we hypothesize that PreM-INs produce gross muscle activity by activating synergistic muscles, whereas CM cells fine-tune target muscle activity. This idea was supported by dimensional reduction analyses of hand muscle activity, as PreM-IN activity was specifically correlated with lower dimensional control of muscle activity, and CM cell activity was correlated with higher dimensional control. These results indicate that the two pathways have distinct functions, synergistic control and fine tuning of hand muscle activity, both of which are essential for the development of dexterous hand movement in primates.