The ramifications and terminals of optic fibres in layers 2 and 3 of the avian optic tectum: a golgi and light and electron microscopic anterograde tracer study.

The ramification patterns and terminals of optic fibres in layers 2 and 3 of the optic tectum were studied in Golgi-stained and immunolabelled preparations made from the brains of chicks and pigeons. The different neuronal structures of layers 2 and 3 were also examined. In Golgi preparations, two types of optic fibre were found both in chick and pigeon tectum according to their thickness and terminal branching patterns. The same types of optic fibres were also found to be present in the anterograde tracer experiments after iontophoresis of biotinylated dextran amine into the optic nerve. The varicose terminals of thin fibres mostly terminated on terminal dendritic sections of radiate and pyramidal-like neurons, contacting them on their apical dendrites. The medium-thick fibre terminals in layer 2 mainly established synapses with horizontally extending dendrites, which may therefore be contacts with inhibitory local circuit neurons. The medium-thick optic fibre bushy-like arborisation in layer 3 established synapses with larger dendrites and also stem dendrites. Their terminals formed groups with different dendritic profiles, some of which were partly covered by glial processes, and/or were located among converging dendrites. The presence of these glomerular-like synapses in layer 3 proves that the optic terminals in layer 3 also take part in the transmission of optic impulses to the nucleus rotundus. Copyright 2003 S. Karger AG, Basel