The superior colliculus control of pinna movements in the cat: possible anatomical connections.

Possible anatomical pathways mediating superior colliculus control of pinna movements were determined in the cat using the orthograde autoradiographic tracing method and the retrograde horseradish peroxidase technique. This was done in the following manner. First, the division of the facial nucleus that innervates the pinna muscles was determined by injecting the pinna muscles with HRP and surveying the facial nucleus for retrogradely filled cells. Second, the brainstem regions that project the facial nucleus were identified using the horseradish peroxidase method. Third, the superior colliculus projections to these areas were studied using the autoradiographic tracing method. The results suggest that superior colliculus control of pinna movements is mediated entirely by indirect connections with the facial nucleus and that these connections occur mainly in a paralemniscal zone in the lateral midbrain. Of all the brainstem regions shown by the horseradish peroxidase experiments to project to the facial nucleus only this midbrain paralemniscal zone received a projection from the superior colliculus that was dense and overlapped precisely the region containing facial projecting neurons. Further autoradiographic tracing revealed that the facial nucleus was the primary brainstem target of this paralemniscal zone and that all paralemniscal fibers projecting to the facial nucleus ended in the subdivision that innervates the pinna muscles. Other paralemniscal efferents terminate in the opposite paralemniscal zone. The data suggest that other connections between the superior colliculus and the facial nucleus may occur in the cuneiform nucleus of the midbrain, the region around the oculomotor complex, and the reticular formation dorsal to the superior olive.