Morphogenesis and physiogenesis of the retino-tectal connection in the chicken. II. the retino-tectal synapses

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Abstract
The preceding paper (Rager 1976) was concerned with the development of retinal ganglion cells and the maturation of photoreceptors. Here we discuss the formation of retino-tectal synapses as studied in terms of both morphology and physiology. The early response characteristics of the first and second postsynaptic tectal cells were also studied. The first retino-tectal synapses, sparse in number, were found with the electron microscope on the eleventh day of incubation. Strict criteria for their identification were employed. At the same stage postsynaptic potentials could be recorded for the first time. These potentials could be recognized as postsynaptic by the application of various techniques: combined ortho- and antidromic stimulation, variation of stimulus intensities, surface mapping, field potential profiles, and application of double shocks of varying intervals. From day twelve onward single unit recordings were also possible and confirmed these results. We were able to establish that morphological criteria for recognition of optic nerve synapses are also criteria for when they start to function. Tectal cells first respond with long-lasting repetitive discharges. Only a few days later the response burst has become shorter and is followed by post-excitatory inhibition. When retino-tectal synapses start to function the excitation is not confined to the superficial tectal cells, but reaches deeper neurons which are possibly tectal output cells. The results of this and the preceding paper (Rager 1976) indicate that the development of the retino-tectal connection in the chicken differs from that in the frog in several fundamental respects.