Evidence for reticulospinal contributions to coordinated finger movements in humans.

The reticulospinal tract was recently shown to have synaptic connections to the intrinsic muscles of the fingers in nonhuman primates, indicating it may contribute to hand function long thought to be controlled exclusively through corticospinal pathways. Our objective was to obtain evidence supporting the hypothesis that these same anatomical connections exist in humans. startReact, an involuntary release of a planned movement via the startle reflex, provides a noninvasive means to examine the reticulospinal tract in humans. We found that startReact was triggered during coordinated grasp but not individuated finger movements. This result suggests that the reticulospinal tract does have connections to the intrinsic muscles of the fingers in humans but its functional role is limited to coordinated movement of the whole hand. These results do not diminish the well-established role of corticospinal pathways in the control of hand movement. Indeed, they cement the significance of corticospinal pathways in individuated finger movement control. Still, these results point to an updated and expanded view of distal hand control where reticulospinal and corticospinal pathways work in parallel to generate a large repertoire of diverse, coordinated movement in the hand. Finally, the presence of reticulospinal pathways to the muscles of the hand makes this pathway an attractive therapeutic target for clinical populations where the corticospinal tract is absent or injured.