Propriospinal fibers interconnecting the spinal enlargements in some quadrupedal reptiles.

The cells of origin, course and site of termination of long propriospinal fibers interconnecting the intumescences have been studied with the aid of the horseradish peroxidase technique, as well as with anterograde degeneration techniques, in some quadrupedal reptiles (the lizards Tupinambis nigropunctatus and Varanus exanthematicus, and the turtles Testudo hermanni and Pseudemys scripta elegans). The anterograde degeneration findings suggest that long descending propriospinal fibers from the cervical intumescence are distributed bilaterally to the ventral gray of the lumbar enlargement, including the lateral motoneuron column. Long ascending fibers from the lumbar to the cervical intumescence are distributed, also bilaterally, to the ventromedial part of area VII--VIII. The cells of origin of long descending propriospinal fibers wre found in the medial part of area VII--VIII in the cervical intumescence, particularly contralateral to the injection side. The cells of origin of long ascending propriospinal fibers were found in the lumbar intumescence, also particularly contralateral to the injection side, in the ventromedial part of area VII--VIII. It seems likely that in the reptiles studied the long propriospinal fibers interconnecting the spinal enlargements are in large part organized as crossed connections. The demonstration of long propriospinal fibers in lizards and turtles--i.e. quadrupedal reptiles that move their limbs in a particular diagonal pattern--suggests that these pathways are of great importance for the coordination of forelimb and hindlimb movements.