The pathway controlling the pupillary light reflex in urodeles

Although the photomechanical response of the iris in amphibians has been characterized, the central pathway of reflex control is unknown. We investigated this pathway by electrophysiological and anatomical techniques in urodeles. Sustained responding neurons in the pretectum were localized and characterized. Three neuron types were found and compared with tonic neurons in mammals. They were driven by the contralateral retina and their dendrites arborized in the pretectal neuropil. The localization of cell bodies resulted in the identification of a physiological nucleus comparable with the nucleus praetectalis olivaris. It was shown that the tonic neurons project ipsi- and contralaterally to the oculomotor region, where axons terminate in deep layers of the stratum album. Using horseradish peroxidase transport the ciliary ganglion in urodeles was identified. It occurs in ontogenesis parallel to the first detectable pupillary light reflex corresponding to the onset of metamorphosis.