A retrograde double fluorescent tracing study of the levator palpebrae superioris muscle in the cynomolgus monkey

In the cynomolgus monkey, motoneurons innervating the levator palpebrae superioris muscle form a nucleus within the oculomotor nuclei called the central caudal nucleus. After double fluorescent neuronal retrograde tracing experiments, using fast blue and diamidino yellow as tracers in the levator palpebrae superior muscles, labelled motoneurons (30%) were found in an unpaired central caudal nucleus. Approximately 2% of the labelled motoneurons were double-labelled. The labelled and double-labelled neurons were distributed randomly over the central caudal nucleus, lateralization of populations of levator motoneurons within this nucleus was not observed. The afferent innervation of the levator palpebrae superioris muscle was restricted to the ophthalmic branch area of the gasserian ganglion. Primary afferent labelled neurons were absent from the mesencephalic nucleus of the fifth nerve. Surprisingly, fast blue was also found in the ophthalmic branch area of the contralateral ganglion of Gasser, while diamidino yellow was present only ipsilaterally. About 1% of the afferent labelled neurons were double-labelled. The results reveal that in the cynomolgus monkey the central caudal nucleus is not only topographically but also functionally one nucleus. Afferent innervation of the levator palpebrae superioris muscle is probably bilaterally organized.