A unilateral section of the corticospinal tract at cervical level in primate does not lead to measurable cell loss in motor cortex.

The effects of a unilateral interruption of the dorsolateral funiculus at cervical level on the survival of neurons in the motor cortex were investigated in macaque monkeys. The lesion was made on the left side at the transition region between the 7(th) and 8(th) cervical segments, above the motoneurons controlling hand muscles. As a result, the homolateral hand became paretic, although an incomplete recovery of manual dexterity took place during 2 months post-lesion. A quantitative anatomical assessment of pyramidal neurons in layer V was performed in the hindlimb area of the primary motor cortex and in the supplementary motor area (SMA proper). The pyramidal neurons were visualized using the marker SMI-32 and thus included the subpopulation of corticospinal neurons. These quantitative data demonstrated that the vast majority of the axotomized corticospinal (CS) neurons did not degenerate. Rather, their somata shrank, compared to the opposite hemisphere or to intact monkeys. This conclusion is in contrast to some previous studies in monkeys that argued for a substantial degeneration of motor cortex neurons as a result of transection of the corticospinal tract; yet in agreement with others that concluded the survival of most CS neurons. The survival of the majority of CS axotomized neurons is also consistent with the observation of numerous CS axons 1 mm above the cervical hemisection.