Structural correlates of functionally distinct X-cells in the lateral geniculate nucleus of the cat

In the companion paper (Humphrey and Weller, '88), we demonstrated 2 physiologically different groups of X-cells (XL and XN) in the A-laminae of the cat lateral geniculate nucleus. In order to investigate their possible morphological correlates, we iontophoresed horseradish peroxidase intracellularly into physiologically identified XL- and XN-cells and examined their light microscopic appearance. The 11 HRP-labeled XL-cells constituted the smallest relay neurons in the A-laminae, and were similar morphologically. All had small somata (mean soma size = 236 micron2), very thin (less than 1.0 micron) axons, few primary dendrites, and narrow, sinuous distal dendrites, which usually formed trees that were oriented perpendicular to laminar borders. The dendrites could be smooth or display beadlike varicosities, hairlike appendages, and/or occasional complex stalked appendages, but their most consistent feature was numerous clusters of grapelike dendritic appendages located at or near dendritic branch points. The 14 labeled XN-cells were structurally more heterogeneous, and they included relay neurons and interneurons. Eight of 11 XN-relay cells differed markedly from the XL-cells. These XN-cells were multipolar neurons with medium to large somata (mean soma size = 365 micron2), small to medium-size axons (1.0-2.0 micron), numerous primary dendrites, and straight distal dendrites that formed radially symmetric trees. The dendrites of the cells were largely smooth, except for occasional spines and/or hairs, and they were devoid of grapelike and other complex appendages. The three other XN-relay neurons had morphologies either similar to XL-cells or intermediate between XL-cells and more simple, multipolar XN-relay cells, but two of these cells had larger somata and axons than most XL-cells. Finally, three XN-cells were intrageniculate interneurons, which possessed small somata (mean soma size = 174 micron2), fine sinuous dendrites covered with beadlike varicosities on stalked appendages, and no obvious axon. These results reveal that, despite minor overlap, there are marked structural differences between XL- and XN-cells. Among the relay cells, these differences relate to soma and axon diameter, dendritic orientation, and the presence or absence of grapelike dendritic appendages. Our finding that interneurons were strongly excited at short latencies by spot onset supports the hypothesis (Mastronarde, '87a; Humphrey and Weller, '88) that such interneurons provide the major inhibitory input to XL-cells, and that this input is important in generating the spot-induced early dips in XL-cell discharge.