Morphology of rabbit retinal ganglion cells projecting to the medial terminal nucleus of the accessory optic system

Rabbit retinal ganglion cells were retrogradely labeled following injection of rhodamine‐labeled microspheres into the medial terminal nucleus. The small fraction of rhodamine‐labeled neurons reached their peak concentration within the visual streak and then decreased with increasing eccentricity until none were encountered in the far periphery. The same rabbits also received injections of the fluorescent tracer Fast Blue into the superior colliculus. No double‐labeled neurons were observed, i.e., ganglion cells projecting to the medial terminal nucleus (MTN) had no axon collaterals to the superior colliculus. In fixed retinae rhodamine‐labeled ganglion cells were intracellularly injected with the fluorescent dye Lucifer Yellow to reveal their full dendritic arborization. MTN‐projecting cells had mediumsized to large somata with thin and frequently branched dendrites. The large dendritic trees had a distinct morphology and were predominatly unistratified in a narrow band that presumably corresponded to the electrophysiologically determined on‐sublamina of the inner plexiform layer. Dendritic field sizes were inversely related to ganglion cell density, thus providing an eccentricity‐independent, constant dendritic coverage factor. Approximately five to six dendritic fields from neighboring cells cover every point of the retina. Published reports claim that the physiological class of on‐direction‐selective ganglion cells provides the sole retinal input to the MTN in the rabbit. In this context morphological features of MTN‐projecting cells and their presumed functional correlation with on‐direction‐selective ganglion cells are discussed.