Optic terminals form axosomatic synapses with deep tectal neurons in Bufo marinus.

Previous studies have shown that a small proportion of the retinal fibres to the optic tectum terminates in the deep tectal cellular layers (layers 6 and 4) in Anura. Horseradish peroxidase was applied to the cut central end of the optic nerve of Bufo marinus to study the synaptic connections of these retinotectal afferents. Some of the retinotectal afferents arrive at the deep tectal layers and form synapses with dendrites (7% of all tectal inputs) and on the somata of the deep tectal cells (0.4% of all tectal inputs). These synapses may either serve as elements of the dimming detection system or could be the morphological substrates of a direct link between the visual and the descending motor system.