Structure and function of retinal ganglion cells innervating the cat's geniculate wing: an in vitro study

We have examined in vitro the morphology and visual response properties of retinal ganglion cells innervating a component of the cat's lateral geniculate nucleus known as the geniculate wing (or retinorecipient zone of the pulvinar). Ganglion cells were first labeled in situ by retrograde transport of fluorescent microspheres from the geniculate wing. Labeled cells were injected intracellular with Lucifer yellow and biocytin in the isolated retina and visualized immunohistochemically. With one exception, stained cells appeared to belong to a single morphological class that corresponded closely to the epsilon cell of earlier descriptions (Leventhal et al., 1980; Rodieck and Watanabe, 1986). They had somas comparable in size to those of beta cells and large, sparse dendritic trees that ramified in the inner (ON) sublayer of the inner plexiform layer. Dendritic fields increased in size with eccentricity, but only within the central retina, and were among the largest so far reported for cat ganglion cells, exceeding those of alpha cells at most eccentricities. Dendritic profiles were typically elliptical with long axes pointing toward the area centralis. Axons were about as thick as those of beta cells and thicker than those of other varieties of non-alpha, non-beta ganglion cells. We recorded extracellularly from microsphere-labeled wing-projecting ganglion cells in a superfused, flattened eyecup preparation. All such cells exhibited sustained responses to standing contrast and had very large, concentric receptive fields with ON-centers and OFF-surrounds. Their response to gratings showed that they have relatively poor spatial resolution and a moderate amount of nonlinearity of spatial summation. These cells thus have many physiological response properties in common with ganglion cells previously termed "on-center tonic W-cells," "on-center sluggish sustained cells," and "Q-cells." These findings indicate that ganglion cells innervating the cat's geniculate wing form a structurally and functionally homogeneous class. Their large dendritic and receptive fields and low-pass spatial frequency tuning suggest that fine spatial resolution is not required for the execution of their functional role(s).