The anatomy of geniculocortical connections in monocularly deprived cats

In monocularly deprived (MD) cats, many cells in the lateral geniculate nucleus (LGN) but few cells in the visual cortex respond to input from the deprived eye, suggesting that the connections to visual cortex from the deprived geniculate laminae may have been disrupted. It has been known for some time that the afferents representing the deprived eye terminate over a smaller percentage of layer IV than do those representing the experienced eye, but it is becoming increasingly clear that this alone cannot explain the inability of the deprived pathway to activate cortical cells. 2-Deoxyglucose studies of ocular dominance columns in MD cats have shown that the columns are often (a) restricted to layer IV, suggesting that intracortical connections may be disrupted, and (b) very faint, suggesting that MD alters the efficacy of the deprived pathway in addition to restricting its territory. Electron microscopy has shown that both deprived and experienced afferents end in terminals that contain mitochondria and round synaptic vesicles and that make asymmetric contacts with dendritic profiles. However, the terminals of deprived afferents differ from those of experienced afferents: they are 25% smaller, contain 33% fewer mitochondria, are more likely to make synapses that are presynaptically convex (and thus, perhaps, immature), make fewer perforated synapses, and synapse onto smaller spines. Further, the geniculocortical axons from deprived laminae appear to end in fewer synaptic terminals, than do those from the experienced laminae. The finding that the synaptic terminals of deprived afferents are both abnormal morphologically and fewer in number can help to explain the reduced effectiveness of the deprived eye in driving cortical cells but does not rule out additional effects such as suppression and loss of intracortical connectivity.