The morphology of motoneurons and dorsal root fibers in the frog's spinal cord.

Ventral and dorsal roots of the frog's spinal cord were filled with cobaltous chloride, and the resulting cobaltous sulfide precipitate, following treatment with H2S-buffer solutions, was intensified with physical developers. A ventromedial and a dorsolateral motoneuron group could be discerned in the ventral horn. The ventromedial, motoneurons gave origin to a strong dendrite crossing to the contralateral side. In the dendritic arborization pattern of the dorsolateral motoneurons a dorsomedial, a dorsal and a lateral dendritic array were distinguished. They were regarded as representing three different input channels to the motoneurons. Intramedullary branching of motor axons and recurrent axon collaterals were never observed. The dorsal root could be divided into a medial and lateral division carrying small and large caliber fibers, respectively. The end-branches of the small caliber fibers were seen to terminate in the substantia gelatinosa. Fine collaterals of the large caliber fibers also terminated in the substantia gelatinosa; coarser collaterals penetrated deeper and terminated in a triangular-shaped area in the base of the dorsal horn and in the intermediate gray matter. From this area a tail was followed into the ventral horn and several synapses were seen on the proximal dendrites and on the somata of motoneurons. A few dorsal root fibers could be seen crossing to the contralateral side.