Different views of the location and organization of corticospinal (CS) neurons in the rat have resulted from fiber degeneration studies, electrophysiologically derived somatotopic maps, studies of CS axonal branching, and functional alterations after ablation of parts of the motor-sensory cortex. To locate more precisely CS neurons that projected to different levels of the spinal cord, and to determine whether or not they were arranged somatotopically, we labeled them with horseradish peroxidase (HRP) from their cut spinal axons. These labeled neurons were arranged in a major caudal band about 4.5 mm long and 3 mm wide corresponding principally to areas 3, 4, and 6, and a minor rostral band in the anterior part of area 10. The caudal band also corresponded to electrophysiologically derived somatotopic hindlimb and forelimb motor areas. The most significant finding was that CS neurons labeled from the lumbar spinal cord and from cervical levels were intermixed generally throughout the caudal band, showing virtually no somatotopic anatomic arrangement. The rostral band, which corresponded to mouth parts in somatotopic maps, was an unexpected finding. Calculations based on estimates of the numbers of CS axons at different cord levels indicated that about 10,000 CS neurons in each cortex projected as far as the rostral cervical cord, and about 2000 continued as far as lumbar levels. The HRP method as used was capable of labeling a majority of the calculated numbers of CS neurons, but showed a large variance in the total numbers of neurons labeled. The distributions of the neurons within the domains of the bands was consistent regardless of the numbers labeled.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22865/1/0000427.pd
