The emergence of a discretely distributed pattern of corticospinal projection neurons.

The distribution of cortical neurons which project to the spinal cord was investigated in the developing and adult rat using the retrograde transport of horseradish peroxidase (HRP). Following injections of HRP into the cervical spinal cord of a postnatal day 4 rat, retrogradely labeled neurons are located in layer Vb throughout the neocortex. Gradually, over the first two postnatal weeks, labeled cells can no longer be found laterally or caudally in the neocortex. By the end of the second postnatal week, cortical neurons which project to the spinal cord are located in essentially their adult positions. These consist of: (1) a dorsomedial band of cells in layer V of frontal cortex, (2) a dorsomedial band in layer Vb of posterior frontal/anterior parietal cortex, and (3) a small group of cells located laterally in an area corresponding to SII. This change in the regional distribution of cortical neurons projecting to the spinal cord is interpreted as an economically adaptive way for the cortex to interconnect highly organized sensory and motor systems.