Synaptic circuitry of physiologically identified W-cells in the cat's dorsal lateral geniculate nucleus

The cat's retinogeniculocortical system is comprised of at least 3 parallel pathways, the W-, X-, and Y-cell pathways. Prior studies, particularly at the level of the lateral geniculate nucleus, have focused on X- and Y-cells. In the present study, we describe the synaptic inputs for 2 geniculate W-cells from the parvocellular C-laminae after these neurons were physiologically identified and intracellularly labeled with HRP. For each of the W-cells, we examined electron micrographs taken from over 500 consecutive thin sections; we reconstructed the entire soma plus roughly 15% of the dendritic arbor and determined the pattern of synaptic inputs to these reconstructed regions of each neuron. In several ways, each W-cell exhibits a similar pattern of synaptic inputs. First, we estimate that each W-cell receives approximately 3000-4000 synaptic contacts, which occur most densely on dendrites 50-150 microns from each soma. Second, axosomatic contacts are extremely rare, and most derive from terminals with flattened or pleomorphic vesicles (F terminals). Third, terminals with round vesicles, large profiles, and pale mitochondria (RLP terminals), which are presumed to be retinal terminals, form only about 2-4% of all synapses onto these W-cells; these synapses occur on proximal dendrites. Fourth, F terminals, which provide roughly 15-20% of all synaptic input to these cells, occupy the same region of proximal dendritic arbor as do the RLP terminals. Fifth, and finally, terminals with round vesicles, small profiles, and dark mitochondria (RSD terminals) provide the majority of synapses along all portions of the dendritic arbor. Compared with geniculate X- and Y-cells of the A-laminae (Wilson et al., 1984), these W-cells are innervated by fewer synapses overall and, in particular, by dramatically fewer synapses from RLP (or retinal) terminals. This paucity of direct retinal input to geniculate W-cells might explain the remarkably poor responsiveness of these neurons to visual stimuli and to electrical activation of the optic chiasm.