Postnatal development of the corticofugal projection from striate cortex to lateral geniculate nucleus in kittens

In kittens ranging in age from the eighth postnatal day to twentieth week, 288 striate cortex cells were identified as projecting to the dorsal lateral geniculate nucleus (LGN) on the basis of antidromic activation from LGN and of histological localization of cortical layer VI. These cells (C-G cells) were classified as complex, simple, or visually unresponsive, as in the adult. During the second-third postnatal weeks, most of the C-G cells were visually unresponsive. They often showed a marked step in the negative stroke of spikes evoked by single shock stimulation. When paired shocks with short intervals were applied, the second spike fractionated at the step to reveal the A and B components. This tendency was not observed at 8-9 weeks or later. At the second week, all the visually responsive cells were complex, while simple cells were recorded at 3 weeks or later. Until the fourth week, LGN-latencies of complex cells were very long (11-35 ms), although they were the shortest group at each age. Adult-like latencies were first observed at the fifth week in complex cells and at the eighth-ninth weeks in simple cells. The axonal conduction velocities of complex cells attained to the values suggesting myelination at the fifth week, while those of simple cells much later. These results suggest that complex C-G cells may mature prior to simple cells. By comparison with previous results it is also suggested that myelination of the retino-geniculo-cortico-geniculate projections may occur in a retinofugal order.