Forms and spatial arrangement of neurons in the primary motor cortex of man.

The morphology and the spatial arrangement of neurons in the primary motor cortex (area 4) of the adult human brain have been investigated by the Golgi method. The human motor cortex displays a great variety of pyramidal and nonpyramidal cells, expressed as important differences in soma shape and size, and in the dimensions and the distribution of dendritic trees, which are described in detail. The neuronal composition of each layer and the white matter is analyzed. In sublayers III b and c, the somata and dendritic trees of pyramidal and nonpyramidal cells are grouped into columnar aggregations, 100-300 micron wide and separated by cell-sparse spaces of 50-100 micron width. Within the aggregations, the apical dendrites of pyramidal cells form several bundles. The dendrites of most nonpyramidal cells do not surpass the columnar width. Sections in different planes reveal that the columnar aggregations observed in sections perpendicular to the long axis of the precentral gyrus are actually the cross-sectioned representatives of elongate strips running parallel to the long axis of the gyrus. The dendrites and somata of pyramidal cells in layers III and V show a preferential orientation in the same direction, parallel to the main axis of the gyrus. In layers II, IV, and V, aggregations like those in layer III are not recognizable; however, in layer V, loose assemblages of neurons of all sizes group around the giant Betz cells. Layer VI has a radial appearance due to the radii of myelinated fibers entering and leaving the white matter. The vertical, columnar organization of layer III and the asymmetry of somata and dendritic trees are not observed consistently throughout area 4, but are more pronounced at the boundary of the precentral gyrus and the central sulcus.