The microscopic changes are those that have already been described. The degenerations result from lesions in the brain or cord, and when in the cord are described as either ascending or descending, according to the tracts affected.

Descending degeneration is the usual result of disease of the cerebral cortex and internal capsule. It involves the motor tracts and the anterior and lateral pyramidal columns.

If the lesion is unilateral and above the decussation, the anterior tract on the same side and the lateral pyramidal tract on the opposite side will be affected.

Ascending degeneration usually follows transverse myelitis and involves the sensory tract, the posterior columns below, and the columns of Goll above.