[The corticospinal tract and its role in motor control]

The corticospinal tract (CST) makes a direct connection to the motor neurons of the hand muscles in higher primates. The existence of such a "phylogenetically new" direct pathway has been regarded as the basis of highly developed fractionated finger movements in these species. In contrast, in lower mammalian species such as carnivores and rodents, the direct connection does not exist. Instead, the shortest pathway is disynaptic and mediated by propriospinal neurons (PNs) around the C3-C4 segments. Previously, the existence and role of the PN-mediated pathway in primates was not well established. We first demonstrated the existence of the pathway in macaque monkeys. Then, to assess its function, we made specific lesions of the CST at the C4/C5 level to interrupt the direct pathway. Surprisingly, the monkey recovered the fractionated finger movements within several weeks, while the hand dexterity did not recover after a lesion at C2. These results suggest that the PNs can take over the function of the direct pathway, and control precision grip. However, such findings indicate that the PNs can be involved in the recovery after CST lesion, but their function in the intact state is not clear. To clarify the normal function of the PNs, we recently developed a pathway-selective and reversible transmission blocking technique using a combination of two viral vectors that induce expression of enhanced tetanus neurotoxin (eTeNT) in the PNs during administration of doxycycline with the Tet-ON system. Induction of eTeNT expression resulted in an impairment in reaching and precision grip, suggesting a critical role of the PN-mediated indirect pathway in the control of finger dexterity.