Morphological changes of large layer V pyramidal neurons in cortical motor-related areas after spinal cord injury in macaque monkeys.

In primates, neurons giving rise to the corticospinal tract (CST) are distributed in several motor-related areas of the frontal lobe, such as the primary motor cortex (M1), the supplementary motor area (SMA), and the dorsal and ventral divisions of the premotor cortex (PMd, PMv). Recently, we have shown in macaque monkeys that the morphology of basal dendrites of CST neurons, i.e., large layer V pyramidal neurons, varies among the digit regions of the motor-related areas. Here, we investigated the alterations in basal dendrite morphology of CST neurons after spinal cord injury (SCI). In our monkey model, both the complexity and the spine density of basal dendrites were highly decreased throughout the areas. Notably, these events were less prominent for the PMd than for the M1, SMA, and PMv. In analyzing the density changes post-SCI of the filopodia-, thin-, stubby-, and mushroom-type spines, it was found that the density of filopodia-type spines was increased for all areas, whereas the other types of spines exhibited density decreases. Such spine density reductions were so limited for the PMd as compared to the other areas. The observed plastic changes of CST neurons may contribute to the recovery from impaired motor functions caused by SCI.