Morphology of physiologically identified W-cells in the C laminae of the cat's lateral geniculate nucleus.

The projections from the retina through the lateral geniculate nucleus to the visual cortex in the cat are comprised of three parallel, independent pathways. These three pathways involve, respectively, W-, X-, and Y-cells in the retina and the lateral geniculate nucleus. In this study, we investigated the morphology of those neurons in the C laminae of the lateral geniculate nucleus that receive input from retinal W-cells. Micropipettes filled with horseradish peroxidase (HRP) were used to record the responses of single geniculate neurons identified as W-, X-, or Y-cells. Five W-cells in the C laminae then were stained intracellularly by impalement of the cell with the microelectrode followed by iontophoresis of HRP. Each of these neurons has a small to medium size soma and fine dendrites oriented roughly parallel to the plane of the geniculate laminar borders. These geniculate W-cells differ among themselves mainly in the detail of their dendritic processes. Three of these cells have virtually no dendritic appendages, while the other two have complex appendages not previously reported among neurons in the C laminae. Also, the degree of horizontal orientation of the dendrites differs considerably among these cells. Overall, the morphology of these W-cells is generally consistent with the class 4 cell briefly described by Guillery (Guillery, R. W. (1966) J. Comp. Neurol. 128: 21-50), and consequently, our data indicate that class 4 neurons are the morphological correlate of neurons physiologically classified as W-cells. Since greater morphological detail is presented by these HRP-filled cells than by previous Golgi-impregnated preparations, we can describe more completely the morphology of class 4 cells.