Morphological characteristics of tectal neurons of layer I projecting to the nucleus geniculatus lateralis ventralis in chick

Some visual information is sent to the nucleus geniculatus lateralis ventralis (GLv) via the cells in layer I (I cells) of the tectum in birds and is used for color vision, papillary reflex, and kineoptic functions. To reveal the morphological features of 'I cells' projecting to the GLv, they were retrogradely labeled with DiI (1,1'-dioctadecyl-3, 3, 3', 3'-tetramethylindo-carbocyanine perchlorate) in chicks. Two different types of neurons, 'spear dendritic I cells' and 'forked dendritic I cells' were identified. The former had small spindlelike soma and an apical dendrite extending to the tectal surface, and the latter had somewhat larger triangular or polygonal soma and plural ascending dendrites. Most of the labeled dendritic endings bifurcated horizontally in layer F, and showed the ending patterns similar to the terminals of optic nerve fibers.