Aberrant pyramidal tract in the medial lemniscus of the human brainstem: normal distribution and pathological changes.

We examined the consistency of the so-called aberrant pyramidal tract (APT) in 150 consecutive autopsied human brains using the modified Bielschowsky stain for axons. We were able to identify the APT in all brains except for one with holoprosencephaly. The APT left the pyramidal tract within the crus cerebri and passed in the medial lemniscus of the pons through the upper medulla oblongata. In 13 of the 63 brains with cerebrovascular diseases, wallerian degeneration was found in the APT on the ipsilateral side of the cerebral lesions. Further, the APT showed depletion of small-sized fibers in 2 of the 5 brains with multiple-system atrophy. These findings confirmed that the APT is a normal descending fiber tract and a part of the pyramidal tract.