Origins and collateralization of corticospinal, corticopontine, corticorubral and corticostriatal tracts: a multiple retrograde fluorescent tracing study.

Cerebral cells of origin for the corticospinal (CST), corticopontine (CP), corticorubral (CR) and corticostriatal (CS) fibers in the rat were identified following the simultaneous retrograde transport of propidium iodide (PI), fast blue (FB), fluorogold (FG) and diamidino yellow (DY). PI was injected into the contralateral C4 spinal cord segment while FB, FG and DY were injected into the ipsilateral medial pontine nuclei, red nucleus and striatum, respectively. Labeled pyramidal neurons projecting corticospinal axons were contralateral to injection in lamina V and ranged in size from small to large. These CST neurons occupied two distinct cortical areas. The cortical neurons of origin for the corticopontine, corticorubral and corticostriatal fibers were ipsilateral to injections. Labeled neurons were localized in cortical lamina V for the corticopontine and corticorubral fibers while corticostriate neurons were located in laminae III, V and VI. The CP, CR and CS labeled cells occupied one large cortical area which topographically included parts of the medial (AGm) and lateral (AGl) agranular cortices and the primary (SI) somatosensory cortex. Considerable overlapping of the cortical neurons of origin for the four motor fiber systems was apparent. More than 98% of the labeled cells were single labeled while less than 2% were double labeled. No triple or quadruple labeled neurons were observed. Hence, morphological evidence is presented that cortical motor neurons project mainly individual, rather than collateral, axons to each of the four motor associated nuclei investigated in this study. However, only a few cortical neurons projected axons simultaneously to a maximum of two nuclei involved in the motor pathways.