Termination of supraspinal descending pathways in the spinal cord of the tegu lizard, Tupinambis nigropunctatus.

Descending fiber projections to the lizard spinal cord were studied using anterograde axonal degeneration. Following hemisection of the cord at the first spinal segment, degeneration was found in the white and gray matter as far down as the 31st (caudal) segment. Degenerating fibers in the white matter were confined to the ipsilateral side and were found in the medial longitudinal fasiculus and the outer half ot the lateral and ventral funiculi. Degeneration was more intense in the dorsolateral and ventromedial funiculi than in the ventrolateral funiculus. In the gray matter, REXED's criteria were applied to Nissl-stained material to delimit boundaries of ten laminae. Degeneration of suprospinal axons was most intense in the medial part of VII, dorsal and ventral commissures to ramify contralaterally in the medial part of VII, in VII, and in medial IX. No degeneration was present in the lateral part of the spinal gray on the contralateral side. In Golgi-stained material, dendrites of lateral IX cells were seen to extend into lamina VII, the dorsolateral part of VII, and the lateral funiculus. Thus, fibers of the ventromedial supraspinal pathway may make axodendritic contact with motoneurons of lateral IX as well as medial IX, ipsilaterally. In addition, there is a possibility of a crossed connection to contralateral motoneurons.