[Dorsal root afferent fiber termination in the spinal cord of the turtle Testudo horsfieldi and 3-dimensional reconstruction of the sensory-motoneuron connection].

HRP tracing methods and computer reconstruction were used to study the structural organization of sensory-motoneuron connections in the turtle. HRP was applied through suction electrodes to thin dorsal and ventral root filaments of superfused isolated lumbar spinal cord of the turtle Testudo horsefieldi. Single motoneurons were labeled iontophoretically with the use of intracellular glass microelectrodes. Labeled elements were examined by light microscopy. The Eutectic Neuron Tracing System and its associated program were used for 3-D reconstructions and morphometry. The distribution of dorsal root afferent fibers and their terminations were presented in a new scheme in which, beside the well known zones, new ones were shown in the Lissauer zone, motor nuclei, ventrolateral funiculus and in the contralateral medial gray matter (IV--V laminae). Unlike in the frog, the motoneuron dendritic field in the turtle was restricted to an ellipsoid space having a short axis in the rostro-caudal direction (300-500 microm). The dorsal root afferent fibers connected to motoneurons produced very short branches (50-70 microm) in a restricted rostro-caudal direction. One dorsal root fiber collateral had about 80 synapselike enlargements (approximately 10-fold fewer than in the frog). The putative sensory-motoneuron contacts were found on the I--VII order dendritic segments of the dorsal and ventromedial dendritic trees. It was shown that in the turtle only one first order collateral of the dorsal root fiber participated in the sensory-motoneuron connection with a small number (about 4) of putative contacts, which was also less than in the frog by a factor of 10. The simplicity of the synapse structure in the turtle is likely to be compensated through the higher efficiency of the signal transmission which is comparable to that in mammals.