Identification of cells of origin of tectopontine fibers in the cat superior colliculus: an experimental study with the horseradish peroxidase method.

The pontine projections from the superior colliculus in the cat have been studied by means of retrograde axonal transport of horseradish peroxidase (HRP). Following injections of HRP in the dorsolateral pontine nucleus, where the tectopontine fibers terminate, a fair number of labeled cells are found throughout the rostrocaudal extent of the ipsilateral superior colliculus. Relatively few of the labeled cells are of medium size (25-40 micron in diameter), more than 80% are small (10-25 micron), but no large cells are labeled. The cell bodies giving rise to tectopontine fibers are distributed in tectal layers deeper than the optic stratum (including this), with only a few in the deeper portion of the superficial gray layer. There are only few labelled cells in the relatively large lateral portion of the intermediate and deep gray layers were the largest neurons (more than 40 micron) are located. Most of these presumably belong to the tectoreticular and the tectospinal projections. The tectal neurons, distributed in various collicular layers, are supposed to receive different kinds of information from other parts of the central nervous system, e.g. from the retina, the cerebral cortex, the brain stem reticular formation, the spinal cord etc. The dorsolateral pontine nucleus appears to have a particular function in the integration of the input from the superior colliculus with those from other sources, especially from the inferior colliculus and the auditory cerebral cortex.