The retinal projection to the thalamus in the cat: a quantitative investigation and a comparison with the retinotectal pathway.

The projection of cat retinal ganglion cells to the thalamus was examined using the method of retrograde axonal transport of horseradish peroxidase (HRP). After the injection site was determined physiologically, HRP was applied by one of three methods: iontophoretic injection of minimal amounts, single pressure injections and multiple pressure injections. Iontophoretic injections into single laminae of the dorsal part of the lateral geniculate nucleus (LGNd) revealed that laminae A and A1 receive almost exclusively axon terminals from alpha and beta cells. Single pressure injections elucidated the retinotopic organization of the LGNd. Multiple injections lead to HRP uptake in the whole LGNd including parts of adjacent thalamic nuclei and revealed that at least 77% of all retinal ganglion cells project to the thalamus. This pathway is made up of all alpha cells, all beta cells and almost half of the gamma cells. The thalamus receives its visual input predominantly from the ipsilateral temporal and the contralateral nasal retina; some alpha cells were also labeled in the contralateral temporal retina. The shape of the decussation line was analyzed and its width was found to be proportional to the average ganglion cell spacing along the dorsoventral axis of the retina. From a comparison of the retinothalamic and retinotectal pathways, an estimate of the number of cells with bifurcating axons could be given. The axons of all alpha cells, 10% of the beta cells, and every second gamma cell bifurcate; this amounts to 30% of the retinal ganglion cells.