Cells of origin of crossed and uncrossed corticospinal fibers in the cat: a quantitative horseradish peroxidase study.

The cortical distribution of the cells of origin of the dorsolateral and the ventral corticospinal tracts were studied in cat. This was done by making subtotal spinal transections, which in different experiments spared different portions of one ventral or one lateral funiculus at C5--C7. One week later horseradish peroxidase (HRP) injections were made one segment caudal to the lesion and the cortical distribution of the HRP labeled neurons was studied. Thus, it was found that the dorsolateral corticospinal tract at C5--C7 is composed of crossed and uncrossed fibers in a ratio of about 10 : 1, while the ventral corticospinal tract, which contains much fewer cortical fibers, is composed and uncrossed fibers in a ratio of approximately 1 : 1. Further, the primary motor cortex (area 4) was found to contribute fibers to both the crossed and the uncrossed dorsolateral corticospinal tract as well as to both the crossed and the uncrossed ventral corticospinal tract. The primary somatosensory cortex (area 3a, 3b, 1--2, 5a, 5b) as well as the secondary somatosensory cortex (area 2 pre-insularis), on the other hand, were found to contribute fibers mainly to the crossed dorsolateral tract. Area 4 was found to display a further organization, such that it contains a medial and a lateral part, both of which contribute mainly fibers to the crossed dorsolateral tract, while the remainder of area 4 contributes fibers to the crossed and uncrossed dorsolateral as well as to the crossed and uncrossed ventral tracts.