Efferent projections of the neonatal cat superior colliculus: facial and cerebellum-related brainstem structures.

The superior colliculus develops its influence over eye and pinna movements gradually during postnatal maturation. Because superior colliculus cells respond earlier in postnatal life to nonvisual than to visual cues, it seemed likely that efferents involved in pinna movements would develop earlier than those involved in eye movements. In the present study, we examined the projections of the superior colliculus to structures related to the cerebellum and facial nucleus believed to be involved in eye-head coordination and pinna movements. We did this by using the autoradiographic and horseradish peroxidase tracing techniques in 11 kittens, ranging in age from several hours to 14 days postnatal, and in seven adult cats. Even in the youngest animals studied, a dense projection was observed from the superior colliculus to each of the target structures examined. These included the parabigeminal nucleus, paralemniscal zone, dorsolateral pons, and inferior olive. Surprisingly, the only projection observed to undergo any postnatal maturational changes was the one to the paralemniscal zone (involved in the pinna-movement circuit of the superior colliculus), and the changes appeared as a reorganization of the terminal field rather than an increase in the density of transported label. Thus, no evidence was obtained to support our expectation that the superior colliculus efferents involved in orientation of the pinnae would develop earlier than those involved in visual orientation. Instead, each of the efferent projections of the superior colliculus examined in this study appears to be laid down prenatally and becomes adultlike long before functional maturity is reached. Presumably, then, the formation and elaboration of synaptic connections are the protracted postnatal processes that limit the functional properties of these neonatal efferent pathways.