Retinal ganglion cells projecting to the rabbit accessory optic system.

Recent evidence from extracellular recording studies indicates that the medial terminal nucleus (MTN) of the rabbit accessory optic system receives inputs from a particular functional class of retinal ganglion cells--specifically, the on-type direction-selective cells. These ganglion cells have been selectively labeled by the retrograde transport of horseradish peroxidase (HRP) injected into the MTN. The number of labeled cells, their distribution over the retina, and their soma areas were determined. In one animal in which the HRP injection completely filled the nucleus, two thousand ganglion cells were labeled. This number agrees with previous estimates of the number of retinal axons terminating in the MTN. Unlike results in avians, none of the ganglion cells was displaced--i.e., they were not Dogiel cells. The density of labeled cells was highest in the visual streak and, overall, the distribution of labeled cells corresponded to the physiologically determined distribution of on-type direction-selective cells. Cells labeled by the HRP injection were among the 20% largest cells in the retina. This result, in conjunction with conclusions from other studies, leads to the prediction that on-type direction-selective cells can be characterized morphologically as cells with large cell bodies and a very extensive dendritic spread in which the dendrites ramify in the vitreal sublamina of the inner plexiform layer.