The levator palpebrae superioris and orbicularis oculi are antagonistic muscles that function during movements of the eyelid. The levator also functions in conjunction with superior and inferior rectus muscles in coordinated eye/lid movements. The present study examined the innervation and morphology of these muscles in Cynomolgous monkeys (Macaca fascicularis) in order to provide a better understanding of the anatomical substrate for lid movements. Motoneurons innervating the levator and orbicularis muscles were identified and localized by retrograde transport of WGA/HRP and HRP. Retrogradely labelled levator motoneurons were distributed bilaterally throughout the caudal central division of the oculomotor nucleus. A few labelled cells were also present within the contralateral superior rectus division, possibly because of the spread of tracer at the injection site. The possibility that individual motoneurons collateralize to innervate the levator muscle bilaterally was tested by using double retrograde labelling techniques. Doubly labelled levator motoneurons could not be detected by suing a combination of tracers (HRP and Fast Blue). Motoneurons innervating the upper lid portion of the orbicularis oculi muscle were distributed within the dorsal subdivision of the ipsilateral facial motor nucleus, with a few neurons in the corresponding locus of the contralateral facial nucleus. Species differences in levator motoneuron distribution, particularly distinctions in lateral-eyed versus frontal-eyed mammals, are discussed in relation to the neural control of lid movements.
The levator palpebrae superioris contains three of the same ultrastructurally defined types of singly innervated muscle fiber found in the global layer of other extraocular muscles and an additional, unique slow-twitch fiber type. Moreover, the multiply innervated fiber types so characteristic of the other extraocular muscles are conspicuously absent from levator muscles. Unlike the rectus and oblique extraocular muscles, the levator lacks a layered distribution of fiber types. The morphological profiles of levator muscle fiber types are such that they generally do not respect traditional fiber classification schemes, but are consistent with a role for the levator in sustained elevation of the lid.
The orbicularis oculi muscle, by contrast, exhibited three distinct fiber types that resembled categories of skeletal muscle twitch fibers. One slow-twitch and two fast-twitch fiber types were noted. On the basis of oxidative enzyme profiles and mitochrondrial content, the majority of orbicularis oculi fibers would be fatigue-prone, an assessment consistent with their rapid onset/offset of action in blinks. Morphological differences between the levator palpebrae superioris and orbicularis oculi muscles reflect not only their dis-tinct functional roles in blinking and other lid movements, but also their diverse embryological origins.