Origin of the pyramidal tract determined with horseradish peroxidase

The origin of the axons contained in the pyramidal tract (PT) of the cat was established using retrograde transport of horseradish peroxidase (HRP). A complete section was made through a PT at the level of the medulla oblongata and HRP was applied to the sectioned axons. Cat brains were cut in frontal and sagittal planes and HRP-labeled cells were plotted in outlines of the brain sections. The entire cortical region containing PT cells was divided into 8 subregions and the percent of PT cells was determined in each. Surface cortex, subregions 1, 3 and 8, contained only 30--40% of PT cells; the majority resided in deep sulcal cortex, in subregions 2, 4, 5, 6 and 7. Subregion 1 (containing 6--12% of PT cells) extends rostral to the cruciate sulcus; subregion 3 (15--22%) extends from the cruciate sulcus caudally to the ansate sulcus; subregion 8 (7--8%) covers cortex laterally adjacent to subregion 3. The hidden banks of the cruciate sulcus contained the greatest concentration of PT cells, 28--34% in the dorsal bank (subregion 5) and 15--20% in the ventral bank (subregion 4). The coronal sulcus contained only 2--5% of PT cells in its dorsal bank (subregion 6) and 1--4% in its ventral bank (subregion 7). The presylvian sulcus contained 8--12% of all PT cells in its lateral bank (subregion 2). This new cortical area is not yet considered part of 'PT cortex'. Qualitative limitations of this study are discussed.