Ascending projections of long descending propriospinal tract (LDPT) neurons.

Ascending projections of long descending propriospinal tract (LDPT) cells were investigated using the technique of double retrograde labeling. In rat, injection of one fluorescent dye was made into either the reticular formation or the cerebellum, and a second dye was injected into the lumbosacral enlargement (LSE). In cat, injections were made into the reticular formation and into the lumbosacral enlargement. Using fluorescence microscopy, observation of neurons in the cervical enlargement (CE) revealed single- and double-labeled cells which were either spinoreticular or spinocerebellar tract cells and/or LDPT cells. In both cat and rat, the location of double-labeled LDPT-spinoreticular cells were in the ventromedial spinal gray matter of the CE and were coextensive with single-labeled LDPT and spinoreticular cells. The locations of double-labeled LDPT-spinocerebellar cells in rat were in the ventromedial gray and were coextensive with single-labeled LDPT cells, but not with single-labeled spinocerebellar cells. The latter group was located in central lamina VII and medial laminae V and VI. Overall, the mean number of double-labeled cells was 40% of rat and 7% of cat LDPT cells, indicating projections to either the brainstem reticular formation or cerebellum as well as to the lumbosacral enlargement. Thus, a subpopulation of LDPT cells apparently also serves as a spinoreticular (SR) and spinocerebellar (SC) projection system.