Retrograde tracing studies of subdivisions of the orbicularis oculi muscle in the rhesus monkey

Functionally and anatomically, the orbicularis oculi (OO) muscle can be subdivided in a pretarsal, a preseptal, and an orbital portion. In the rhesus monkey, fluorescent and neuronal retrograde tracing experiments were performed in the pretarsal or the orbital portion of the OO muscle, or both, using fast blue, diamidino yellow, and wheat germ agglutinin-horseradish peroxidase as tracers. The preseptal portion was not investigated because of close anatomical relationships to the other portions. It was found that motoneurons innervating the OO muscle are located exclusively within the intermediate subnucleus of the motor facial nucleus. The upper pretarsal motoneurons show a specific distribution in the dorso-rostral border area of the intermediate subnucleus, representing a dome-like organization, while lower pretarsal motoneurons are situated more ventrally in the adjacent area. The pretarsal motoneurons are all located dorsally in the rostral half and the upper part of the caudal half of the intermediate subnucleus. The upper pretarsal portion is subserved by about one third of the total intermediate motoneuron population. The size of the upper pretarsal motoneurons is similar to that of the motoneurons of the lower pretarsal portion of the OO muscle and falls, for the vast majority, into the large motoneuronal range. Motoneurons belonging to the upper and lower orbital portions are located ventrally and are more randomly distributed in the rostral half of the intermediate subnucleus. The size of orbital motoneurons varies from small to large. The large fraction of pretarsal motoneurons may reflect the specific function of the upper pretarsal portion during rapid and highly coordinated movements of the eyelids in different types of blinking.