Abstract
The central projections of different groups of cat retinal ganglion cells were studied following small iontophoretic injections of horseradish peroxidase (HRP) into physiologically characterized sites. Analysis was restricted to labeled cells in the upper periphery of the nasal retina, contralateral to the injection site. Injections were made to the A lamina and C lamina of the dorsal lateral geniculate nucleus (LGNd‐A,C), the geniculate wing (LGNd‐W), the ventral lateral geniculate nucleus (LGNv), the pretectum (PT), and the superior colliculus (SC).
The dendritic fields of alpha, beta, and epsilon cells were well labeled by the procedures we employed. A group, termed “g~1~,” had somal sizes within the range of the smaller beta and epsilon cells, but dendritic morphologies distinct from either class. The g~1~ group may consist of a number of types, but our material provided no basis for further distinguishing them. Many cells were observed that had smaller somas; all had thin axons, and few had dendritic fields that labeled to any significant extent. We were not able to further distinguish these cells, and refer to this group, which may include a number of types, as “g~2~” cells.
From the peripheral nasal retina, alpha cells project to LGNd‐A, LGNd‐C, PT, and SC. Beta cells project to LGNd‐A, LGNd‐C, and PT. Epsilon and g~1~ cells project to the LGNd‐C, LGNd‐W, LGNv, PT, and SC.
We determined the total spatial density of cells in the region of the retina analyzed, using a Nissl‐stained preparation. We then estimated the relative fraction of cells in each of the above groupings by injecting HRP throughout a cross section of the optic tract. Multiplying this relative fraction by the total spatial density gave an estimate of the spatial density of each of these groupings. From the spatial density of cells labeled from the injection site; we were able to estimate the fraction of cells of each retinal grouping that project to each of the zones investigated.
By these calculations, almost all alpha cells from the upper nasal retina project to LGNd‐A and LGNd‐C; most project to SC, and about a third to PT. Beta cells, by contrast, project almost exclusively to LGNd‐A, with about 10% going to LGNd‐C, and about 1% to the PT. The great majority of epsilon cells, if not all, project to LGNd‐W, and up to half of this population also project to the other zones noted above. The remaining groups show little indication of branching, and separate to project, in varying proportions, to the terminal zones noted above.