Central projections of selected spinal dorsal roots in anuran amphibians

Abstract

Abstract The total central distribution of single brachial or lumbar dorsal roots has been assessed in toad ( Bufo marinus ) and in frog ( Rana catesbeiana ). Two methods were employed to examine the afferent fibers. The uranyl nitrate modification of the Nauta method (Nauta, '66) was used to study degenerating elements resulting from single root transection. A radioautographic tracing technique was also employed to examine the distribution of essentially normal afferent fibers. This latter method involved local injection of small amounts of L‐leucine‐H 3 into a single ganglion of otherwise intact nerves and subsequent determination of location of the incorporated label in the CNS. The distribution was ascertained by counting silver grains microscopically in radioautographs of the CNS for standard sized areas. The results obtained by either method essentially were identical. All dorsal root projections are distributed unilaterally within the CNS. Fibers passed into a dorsolateral bundle and into the dorsal column. Collaterals from the dorsal column passed into the dorsal and intermediate gray matter, but no primary afferent fibers distributed within the ventral horn. Both brachial and lumbar roots distributed segmentally over approximately five spinal segments. Root fibers coursed rostrally in a somatotopically organized dorsal column. This pathway ultimately passed into the cerebellum. Some evidence was obtained in support of a dorsal column‐medial lemniscal relay in anuran amphibians.