Trigeminotectal projections in cats and the pathway of extraocular muscle proprioception

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Abstract
Projections from the sensory trigeminal nuclei to the superior colliculus (SC) in the cat were studied by means of retrograde axonal transport of horseradish peroxidase (HRP). After injections of a small amount (0.2–0.5 μl) of HRP suspension into various parts of the SC. labeled cells were found in the contralateral sensory trigeminal nuclei. Many trigeminotectal neurons occurred in the most rostral part of the oral nucleus, clustered in its ventromedial part. Some labeled cells were also found in the ventral part of the principal sensory nucleus as well as in the interpolar and caudal nuclei. Many trigeminotectal neurons in the rostral part of the oral nucleus project to the deep layers of the entire colliculus with a slight preponderance to its lateral part, and there is some indication of rostrocaudal correspondence within the trigeminotectal projection. It thus appears that neurons in the principal sensory nucleus send more fibers to the rostral part of the SC while those in the caudal and interpolar nuclei give off more axons terminating in the caudal SC. The possible role of the trigeminotectal projection is discussed with a comment on the pathway of the extraocular muscle proprioception.