Corticospinal and corticoreticulospinal projections have discrete but complementary roles in chronic motor behaviors after stroke.

Abstract

After corticospinal tract (CST) stroke, several motor deficits can emerge in the upper extremity (UE), including diminished muscle strength, motor control, and muscle individuation. Both the ipsilesional CST and contralesional corticoreticulospinal tract (CReST) innervate the paretic UE, but their relationship to motor behaviors after stroke remains uncertain. In this cross-sectional study of 15 chronic stroke and 28 healthy subjects, we examined two questions: whether the ipsilesional CST and contralesional CReST differentially relate to chronic motor behaviors in the paretic arm and hand, and whether the severity of motor deficits differ by proximal versus distal location. In the paretic biceps and first dorsal interosseous muscles, we therefore used transcranial magnetic stimulation to measure the projection strengths of the ipsilesional CST and contralesional CReST. We also used quantitative testing to measure strength, motor control, and muscle individuation in each muscle. Stroke subjects had comparable muscle strength to healthy subjects but poorer motor control and muscle individuation. In both muscles, stronger ipsilesional CST projections related to better motor control, whereas stronger contralesional CReST projections related to better muscle strength. Stronger CST projections related to better individuation in the biceps alone. The severity of motor control and individuation deficits was comparable in the arm and hand. These findings suggest that the ipsilesional CST and contralesional CReST have specialized but complementary roles in motor behaviors of the paretic arm and hand. They also suggest that deficits in motor control or muscle individuation are not segmentally biased, underscoring the functional reach and efficacy of the pathways.