| Abstract: | We have studied the effects of making large lesions of visual cortex on the synaptic organization of the dorsal lateral geniculate nucleus (LGN) in the cat. Visual cortex was removed at birth in one group of cats and during adulthood in a second group. Following survival periods of 6 months to 2 years, the organization of synapses made by afferents from the retina in the LGN was investigated quantitatively with the electron microscope. In single thin sections we determined the percentage of retinal axon terminals that made synapses in the LGN, the average number of synapses made by each retinal axon terminal, and the identity of each postsynaptic process. These measurements were made separately for retinogeniculate connections in the A and C laminae of the LGN. For comparison, similar sets of measurements were made in adult cats that had been reared normally. When single thin sections from the A or C laminae of the LGN in normal cats are examined, about 60% of the axon terminals from the retina are seen to make at least one synaptic contact. These contacts can be with dendrites or F profiles or both. On average, each retinogeniculate terminal makes approximately 1.4 synapses in the plane of a single section and contacts dendrites three times as often as F profiles. In the A laminae of the LGN in cats that received a visual cortex lesion at birth or in adulthood, the percentage of retinal terminals that make synapses is the same as in normal cats. Similarly, the average number of synaptic contacts made by each retinogeniculate terminal is not changed by a lesion of visual cortex. In contrast, the number of contacts made with dendrites is reduced markedly, by about 29% after a lesion at birth and 53% after a lesion as an adult. However, these reductions are offset by compensatory increases in the number of contacts made with F profiles, and thus the mean number of contacts made by each retinogeniculate terminal is stabilized at a normal value. In the C laminae of the LGN, retinogeniculate terminals also reapportion their synaptic contacts. In cats with a lesion during adulthood, the redistribution of synapses is compensatory, as in the A laminae. When a lesion is made at birth, however, the number of new retinal contacts made with F profiles exceeds the number of dendritic contacts that are lost. As a result, each retinogeniculate terminal makes about 26% more synapses, in total, than normal.(ABSTRACT TRUNCATED AT 400 WORDS) |
