Observations on the afferent and efferent connections of the avian isthmo-optic nucleus.

The efferent and afferent connections of the avian isthmo-optic nucleus (ION) were studied using light microscopic techniques. Injections of [3H]proline into the nucleus resulted in labeling of centrifugal endings in the retina at the junction of the inner plexiform layer and inner nuclear layer, but produced no other transported label to any thalamic or mesencephalic nucleus. The origin of the tectal afferents to the ION was demonstrated by means of injections of [3H]proline into the most superficial layers of the optic tectum and by stereotaxic injections of horseradish peroxidase into the ION. The tectal efferent cell bodies were located in lamin h of the optic tectum and at the junction of laminae h and i.