Cells of origin, course, and termination patterns of the ventral, uncrossed component of the mature rat corticospinal tract.

The cells of origin, the course, and termination patterns of the ventral, uncrossed component of the rat corticospinal tract (CST) was investigated by using retrograde and anterograde tracing methods. Anterograde tracing with biotin dextran-amine (BDA) revealed the position and detailed morphology of CST fibers in the spinal cord. Cross sections on spinal levels C4, T8, and L4 showed labeled fibers in the ipsilateral ventral funiculus on all levels. Although ipsilateral ventral CST fibers run close to the midline in the cervical cord, they were found to disperse more in the ventromedial funiculus at lower spinal levels. To study the termination patterns of the ipsilateral ventral projection, a dorsal spinal cord hemisection was performed at level T8, severing the crossed dorsomedial and dorsolateral components but leaving ipsilateral ventral running fibers intact. These fibers were observed to have sometimes several collaterals with terminal arbors extending into different spinal segments, innervating mostly laminae III-VI. Structures closely resembling synaptic boutons were identified in these arbors. By retrograde tracing in animals with dorsal spinal cord hemisection, we found labeled cells equally distributed throughout the spinally projecting cortical areas corresponding to the level of tracer injection. Labeled cells were found in layer V. The diameter of the labeled cells was not significantly different from other spinally projecting cortical neurons. In summary, a neuroanatomically complete ipsilateral, ventral corticospinal projection down to low spinal levels was found. The large extension of the terminal arborizations in intermediate laminae of the spinal cord suggests a modulatory role of this CST component.