Organization of corticospinal neurons in the cat.

The cells of origin of the corticospinal tract of the cat were identified using the retrograde horseradish peroxidase (HRP) labeling technique. Cortical neurons labeled from the spinal cord were confined to layer V and included large Betz cells, as well as many smaller neurons of this layer. Collections of 5-10 labeled neurons concentrated in areas of 300-500 micrometer diameter were observed, suggesting a columnar-type of organization of corticospinal neurons. Injections of HRP into different spinal segments were used to determine the somatotopic distribution of corticospinal neurons. Cortical neurons projecting to the cervical spinal enlargement were found in the lateral hemisphere, with more caudal spinal levels being represented successively more medial. There appeared to be little, if any, overlap in the distributions of neurons labeled from the cervical versus the lumbosacral spinal cord. Neurons projecting to the spinal enlargements were most abundant in the primary area 4, motor cortex (MI), but substantial populations of neurons were located in each of the subfields, areas 3a, 3b, 1 and 2, of the primary somatic sensory cortex (SI), plus area 2 pre-insularis of the second somatic sensory region (SII), and area 5 of the suprasylvian gyrus. This suggested, in view of the differences in inputs and response properties of neurons in these cortical regions, that the corticospinal projections from the different areas could represent multiple, independent functions in spinal cord sensory and motor control. The soma diameters of HRP-labeled corticospinal neurons varied widely, with a distinct, large-celled (Betz-type) and a small-celled population being present in the area 4 motor cortex. The largest labeled neurons of the somatic sensory cortical areas were intermediate in size. Three types of corticospinal neurons may exist, corresponding to the giant pyramidal (Betz) cells, the largest pyramidal neurons of the somatic sensory regions, and the abundant, smaller pyramidal cells which are found throughout the sensory and motor cortical fields.