Abstract
The present study was undertaken to demonstrate the collateral branches of the corticospinal (CS) fibers in the upper cervical segments of the rhesus monkey spinal cord and to examine the ultrastructure of these collateral branches. A lesion was induced in the posterior limb of the left internal capsule where the CS tract fibers pass. This resulted in degeneration of the CS fibers of the ipsilateral medullary pyramid, contralateral and ipsilateral funiculi of the spinle cord. Examination of the number of degenerated CS fibers in the contralateral lateral funiculi of the second and fourth cervical segments (C 2 and C 4) at the light microscopic level, revealed that C 4 had more degenerated CS fibers than C 2. Such an increase in degenerated fibers of C 4 compared to C 2 was due mainly to an increase in the small-sized group of degenerated CS fibers. The increased degenerated CS fibers were not evenly distributed in the lateral funiculus of C 4, but condensed in a particular region of the lateral funiculus of C 4, i. e., (1) the centro-medial areas in all cases, and (2) the lateral areas in some cases. Ultramicroscopic observations revealed that the degenerated CS fibers had a collateral branch projecting from the node of Ranvier. It is concluded that the CS fibers have collateral branches, and the increased degenerated CS fibers observed after the lesion of the internal capsule were due partially to degeneration of such collateral branches.
