Electron microscopic evidence for the existence of a corticonigral tract in the cat.

Young adult cats were subjected to unilateral ablation of the frontal and prefrontal cerebral cortex and were allowed to survive for 4 to 5 days. Routine electron microscopic technique was employed to examine orthograde degenerative changes in the ipsilateral and contralateral substantia nigra (Ni). A moderate number of degenerating synaptic boutons (d. s. b.) were observed in the ipsilateral Ni-pars comacta, and only very few d.s.b. were observed in the ipsilateral Ni-pars reticulata. The d. s. b. exhibited features attributed to the dark degeneration type, and might be referred to two main categories; "small-round-vesicle bouton" and "large-round-vesicle bouton" (Hajdu et al., 1973; Hassler et al. 1975). The d. s. b. performed asymmetrical synapses with dendrites of varying size, much more rarely -- with the neuronal perikarya, and extremely rarely -- with initial axonal segments. No degeneration was observed in the contralateral Ni.