Spinocerebellar projections in the turtle. Observations on their origin and terminal organization.

Spinocerebellar projections were studied in the turtle, Pseudemys scripta elegans, by both anterograde (injections of 35S-methionine into various spinal segments) and retrograde (injections of horseradish peroxidase into the cerebellar cortex) tracing techniques. After unilateral HRP injections, labeled neurons were found on both sides of the spinal cord. The total number of retrogradely labeled spinocerebellar tract neurons was relatively small and neuronal aggregations were not observed. Most spinocerebellar tract neurons were found in the ventral horn, dorsal to the motoneuronal cell group. Some cells lay in the spinal intermediate zone and in the dorsal horn. The fibers mainly ascended in the lateral funiculus, some in the ventral and possibly some in the dorsal funiculi. Spinal projections terminated within the cerebellar granular layer. The spinocerebellar target area extended along the whole medio-lateral extent and covered about the rostral four fifths of the cerebellar plate. Individual target areas, separated from each other, as are the anterior and posterior spinocerebellar target areas of higher vertebrates, could not be identified. Similar to mammals, however, the terminal field in the turtle was not a homogenous one but consisted of zones of mossy fiber terminations of varying sizes and intensities.