Patterns of fiber degeneration in the dorsal lateral geniculate nucleus of the cat following lesions in the visual cortex

Golgi preparations show a group of relatively fine axons ending in the dorsal lateral geniculate nucleus. These, the type I axons, are characterized by many short collateral terminal branches and form a well oriented plexus within the lateral geniculate nucleus. Individual axons distribute terminals to more than one geniculate lamina and some can be seen to come from the region of the internal capsule. Four or more days after damage to the visual cortex, the Nauta method shows relatively fine degenerating fibers that approach the lateral geniculate nucleus from a rostral direction. Some pass straight into lamina a, While others enter the three major laminae via the central interlaminar nucleus. The appearance of this degeneration suggests that the type I axons are cortico-genciulate, as does the sequence of the diencephalic degenerative changes. Thus, relatively coarse corticofugal fibers to the lateral thalamus and tectum show heavy degeneration within three days. The fine axons in the lateral geniculate nucleus show slight, early degeneration after three days, extensive degeneration after four days. They are possibly collaterals of the coarser corticofugal fibers. A second group of coarse fibers which runs laterally from the lateral geniculate nucleus shows early degeneration after four days but does not show consistent and clear degeneration until five or more days. Since retrograde cell changes are already recognizable after four days the third group is regarded as geniculo-cortical.