Projections and termination of the corticospinal tract in rodents.

An examination of the precise mode of termination of the corticospinal system in the spinal cord of rodents has been conducted by use of light and electron microscope methods. This study confirms the position of the normal corticospinal t ract in rodents in the ventralmost portion of the dorsal column white matter. Three to four days following unilateral sensorimotor cortex ablation, Nauta-Gygax and Fink-Itcimcr silver methods reveal a dorsomedial projection of degenerating debris into the dorsal horn from the contralateral corticospinal tract. Although the silver methods do not show degeneration at survival times earlier than two days, the electron microscope shows degenerating axons and synaptie knobs as early as 24 hours following cortical lesion. The degenerating synaptic knobs are found only in the dorsal regions of the dorsal horn subjacent to the substantia gelatinosa. They usually make synaptic contact with several small to medium sized dendrites. These terminals do not appear to participate in axosomatic or axoaxonal synapses. No degeneration is seen in the ipsilateral corticospinal tract, the lateral white columns, or the ventral horn of the spinal cord.