Somatotopy of the corticospinal neurons in the rat.

By means of the retrograde transport of the horseradish peroxidase, injected into the intermediate zone and in the base of the dorsal horn of the rat's spinal cord, it was possible to observe cortical (layer Vb) labeled neurons in the primary somatosensory cortex. They were arranged in ranks with a somatotopical disposition: the neurons whose axons project to the caudalmost myelomeres were found closest to the midline, while the neurons projecting to a cervical level lay furthest away from the midline.