THE MORPHOLOGY OF COLLICULAR AND RETINAL AXONS ENDING ON SMALL RELAY (W-LIKE) CELLS OF THE PRIMATE LATERAL GENICULATE-NUCLEUS

The lateral geniculate nucleus (LGN) of every primate examined contain s a set of small relay cells in addition to separate sets of magnocell ular and parvocellular relay cells. These small cells receive a direct retinal projection, and an indirect retinal projection via the superi or colliculus (SC). Receptive-field analyses of the small LGN cells in the bush baby, a lorisiform primate, indicate that this cell class is composed of subclasses, similar in physiology to cat W cells. In an e ffort to identify some of these subclasses, we have examined the morph ological features of retinal and collicular axonal arbors that end on small W-like cells in the LGN of the bush baby, Galago crassicaudatus. Small cells in this species are found in a prominent pair of koniocel lular (K) layers as well as the interlaminar zones (ILZs). Retinal arb ors were examined by bulk iontophoretic injection of horseradish perox idase into the optic tract. Collicular arbors were filled via iontopho retic injection of biocytin into the superficial layers of the SC. For ty-eight axon arbors were completely reconstructed and quantitatively evaluated. Our findings show that retinal and collicular axon terminal s differ in morphology on the basis of a number of criteria. Our analy ses also suggest that retinal axons may have a stronger influence on K cells and collicular axons have a stronger influence on ILZ cells. Th e ramifications of these findings are provocative since these small LG N cells are known to project directly to the cytochrome-oxidase (CO) b lobs within striate cortex. This relationship suggests that CO blob ce lls receive complex visual input not only from magnocellular and parvo cellular LGN cells, but also from small cell pathways that are differe ntially influenced by retinal and collicular cells.