Differential sites of origin and collateralization of corticospinal neurons in the rat: a multiple fluorescent retrograde tracer study.

Cells of origin for corticospinal fibers in the rat were identified following retrograde transport of Fluoro-Gold (FG), Propidium iodide (PI), Fast blue (FB), and Diamidino yellow (DY) injected unilaterally into lumbar (FG), mid-thoracic (PI), cervical enlargement (FB), and cranial cervical (DY) spinal gray matter. Most labeled neurons were contralateral to injection in lamina V and ranged from small to very large. These cells occupied two distinct cortical regions: one rostral and the other larger and more caudal. Neurons of the rostral region projected axons solely to cervical spinal segments whereas neurons of the caudal region projected fibers to all spinal segments. Somatotopically, most neurons projecting to lumbar segments were most medial. More than 98% of all labeled cortical neurons contained only a single fluorescent tracer; however, within a single tissue section each of the 4 tracers could be found in these single labeled neurons. The few double labeled neurons contained only cervical (DY + FB) or thoracolumbar (PI + FG) tracers. No triple or quadruple labeled cells were seen. Hence morphological evidence is presented that corticospinal axons branch to terminate in more than one spinal region, but these collateral terminations are restricted to only a few adjacent spinal segments.