Functional synaptic relations during the development of the retino-tectal projection in amphibians

Stimulation of the optic nerve elicited pronounced, monophasic potentials in the larval amphibian tectum. These potentials are identified as summed postsynaptic potentials generated by synchronous activation of optic nerve fibres. Functioning synaptic junctions are present in the rostrolateral portion of the tectum in early larval Xenopus. With development, the area on the tectum from which synaptic potentials are recorded expands caudally, until the whole tectal area can be synaptically activated just prior to metamorphic climax. These findings, taken in conjunction with other related studies, demonstrate that connexions between pre- and postsynaptic neurons in this system are not invariant but shift continuously throughout development.