Efferent projections of the neonatal superior colliculus: extraoculomotor-related brain stem structures.

The development of eye movements is a prolonged process which presumably involves the efferents of the superior colliculus. In the present study we sought to determine which, if any, of the colliculus efferents that influence eye movements in adult cats were present in neonatal kittens. The autoradiographic and orthograde horseradish peroxidase tracing methods were employed in kittens ranging from 6 h to 5 weeks of age and in adult cats. Surprisingly, most of the known projections from the superior colliculus which are believed to be involved in eye movements were already present in the youngest animals studied. These included projections to (a) the ventral central gray matter overlying the oculomotor nucleus, and (b) those portions of the pontine and medullary reticular formation which provide excitatory and inhibitory inputs to abducens neurons. Apparently, the pathways over which the superior colliculus influences eye movements are elaborated quite early in life. However, in the predorsal bundle and pontomedullary reticular areas the density of transported label was less in 1-day-old kittens than in older animals. Thus, anatomical as well as functional development of portions of this circuitry appear to require a significant period of postnatal maturation.