An experimental study of the termination of the lateral geniculo–cortical pathway in the cat and monkey

The thalamic projection to the visual cortex has been studied in the cat and monkey by experimental light and electron microscopic techniques. After large lesions of the lateral geniculate nucleus degeneration is confined to the ipsilateral hemisphere. In the cat it is found in areas 17, 18 and 19 and in the lateral suprasylvian area, terminal degeneration occurring predominantly in layer IV, with less in layers I, III and V ; fibre degeneration crossing layers VI and V towards layer IV is coarser in area 18 than elsewhere. Some fine horizontal degenerating fibres are seen in layer I. In the monkey terminal degeneration is restricted to area 17; again degenerating fibres ascend to layer IV where there is dense fragmentation, but in contrast to the cat there is also a second, less dense, but distinct, band in layer Illb. A little fine, horizontal fibre degeneration is present in layer I and there is slight terminal degeneration in this site and in layer V. Electron microscopy shows that degenerating terminals are recognizable in the visual cortex at several stages according to survival period, but that most stages can exist simultaneously in any one site, and that all are associated with asymmetrical membrane thickenings. Mapping of electron microscopic sections confirms the laminar pattern seen with the light microscope. In area 17 of the cat and monkey and in area 19 of the cat over 80% of degenerating terminals end on dendritic spines, the rest making synaptic contact mainly with dendritic shafts, and very few with the soma of stellate cells, but in area 18 some 10 % are related to stellate cell bodies. In layer IV of all areas degenerating terminals tend to occur in clusters which are separated by approximately 100μm. Where degenerating thalamic afferents end on cell somata or varicose dendrites almost all are identifiable as derived from stellate cells. Although it is difficult to identify positively the parent dendrites bearing the spines which receive the majority of the thalamo-cortical afferents, it is suggested that some, at least, of them may also originate from stellate cells.