Direct projection of the corticospinal tract to the superficial laminae of the spinal cord in the rat.

The anterograde transport of both wheat germ agglutinin conjugated to horseradish peroxidase and the kidney bean lectin Phaseolus vulgaris leucoagglutinin was utilized to investigate the projection of primary sensorimotor corticospinal tract axons to the superficial laminae of the spinal dorsal horn in the rat. Both methods yielded qualitatively similar patterns of connectivity. Corticospinal tract axons were found to terminate within all laminae on the side contralateral to the injection site. Labeling was most dense within laminae III and IV and medial portions of laminae I, II, and V in the cervical and lumbar enlargements. Labeling in the ventral horn, though present, was relatively less dense. P. vulgaris leucoagglutinin-labeled axons within laminae I and II exhibited boutons en passant and terminaux; many of these axons also terminated or were collaterals of axons that terminated in deeper dorsal horn laminae. Results are discussed with reference to the somatotopic organization of the spinal cord and to a possible role for the cortex in the modulation of nociception within the spinal cord.