Muscle fiber types of human orbicularis oculi muscle

The aim of this study was to elucidate the muscle type of the preseptal, pretarsal, and ciliary parts of the orbicularis oculi muscle in humans using immunostaining. The eyelids of 5 Korean adult cadavers were used (3 male and 2 female cadavers; age range, 50-85 years). A 1:1000 mouse monoclonal anti-skeletal myosin antibody solution (fast, M4276; Sigma, St Louis, MO) was used for immunostaining. On sagittal sections, preseptal, upper pretarsal, midpretarsal, lower pretarsal, and ciliary (muscle of Riolan) parts were selected, and 0.38 × 0.038-mm rectangular areas (0.1444 mm) were photographed. The number and size of the muscle fibers in each part of the orbicularis oculi muscle were evaluated by the image analyzer program and calculated per unit area (1 mm).On the whole, fast fibers (mean, 87.8% ± 3.7%; range, 85.6%-91.7%) occupied a significantly larger portion of the muscle (P = 0.000 [t-test]) than nonfast fibers (mean, 12.2% ± 3.7%; range, 8.3%-14.4%). Among the 3 areas (preseptal, pretarsal, and ciliary parts), the ciliary part had a significantly (P = 0.019 [Scheffé]) higher portion (91.7%) of fast fibers than the pretarsal part (86.6%). The diameter of the fast fibers (mean, 17.7 ± 2.6 μm) was significantly greater (P = 0.000 [t-test]) than the nonfast fibers (mean, 13.0 ± 2.1 μm).Our results showed that the eyelid has a higher proportion of fast muscle fibers than the mouth (pars peripheralis, 73% fast fibers; and pars marginalis, 66% fast fibers). Thus, closing of the eyelids is faster than closing of the mouth; however, the duration or power associated with closing of the mouth is stronger than closing of the eyelids.