Morphological characterization of slow and fast pyramidal tract cells in the cat.

In adult cats the morphology of slow and fast pyramidal tract (Pt) neurons was studied following intracellular HRP injections and Golgi impregnation. Both types of neurons are pyramidal cells and their soma are all located in the fifth layer of the motor area. As a rule, fast Pt neurons have large somata and their basal and apical dendrites occupy a larger territory in the tangential plane. In layer I, terminal apical dendrites of fast Pt neurons are smooth and divide poorly while those of slow Pt neurons bear a moderate amount of spines and branch profusely. Midway between the pia and layer V, in the third layer, the apical shafts of both types of Pt cells run upward with little branching. These shafts are more numerous in fast Pt cells (7 to 16) and they are almost devoid of spines. Those of slow Pt cells in layer III number between 5 and 9 and are densely covered with spines. Oblique and horizontal branches of slow and fast Pt neurons extend in layer V and some of them invade the lower part of layer III. It is suggested that this zone corresponds to a true fourth layer in the motor area. In both types of cells oblique and lateral branches bear numerous spines. Within the basal dendritic territory of Pt cells, one has to distinguish two dendritic systems: a short and a long one. The former spreads downward obliquely and appears to remain within layer V. The latter is made up of long descending vertical (antiapical) and oblique dendrites (tap root). While both types of cells may have long antiapical dendrites that run down radially to the lower part of layer VI, tap root dendrites which expand laterally below the cell body for considerable distances are a distinctive feature of fast Pt neurons. Though basal dendrites of all Pt cells bear spines, their number, distribution and shape are very variable in fast Pt cells.