Impaired arpeggio movement in skilled reaching by rubrospinal tract lesions in the rat: a behavioral/anatomical fractionation.

Spinal cord injury damaging the rubrospinal tract (RST) interferes with skilled forelimb movement, but identification of the precise role of the RST in this behavior is impeded by the difficulty of surgically isolating the RST from other pathways running within the lateral funiculus (LF). The present study used a skilled reaching task and a behavioral/anatomical dissection method to identify the contribution of the RST to skilled forelimb movement. Rats were trained on the skilled reaching task and subjected to lesions of the LF. Based on histological evaluation, the animals were assigned to large, medium, or small LF lesion size groups. End point and arm/hand/digit movements were subsequently identified for each group. Success was impaired in all groups, but the impairment was not related to lesion size. Frame-by-frame qualitative analysis of the video recordings revealed that large LF lesions abolished the elements of digits close, digits open, arpeggio, grasp, supination 2, and release. Medium LF lesions interfered with a subset of the movement elements that were shown to be affected by the large LF lesions, namely arpeggio and grasp. Only the arpeggio movement was compromised after small LF lesions. The results show that not only does the LF contribute to skilled reaching, but because the RST was likely to have been damaged in all lesion groups, the RST is more involved in hand rotation than in digit use. The results are discussed in relation to the fiber tracts that are likely to be damaged in the different LF lesion groups.