PurposeThe purpose of this study was to determine whether proprioceptive nerves are present in Müller's muscle.MethodsThis was a prospective cohort study in which histologic and immunofluorescence analyses of excised Müller's muscle specimens were performed. Twenty fresh Müller's muscle's specimens from patients undergoing posterior approach ptosis surgery in one center between 2017 and 2018 were evaluated by histologic and immunofluorescent analysis. Axonal types were determined by measuring axon diameter in methylene blue stained plastic sections and by immunofluorescence of frozen sections.ResultsWe identified large (greater than 10 microns) and small myelinated fibers in the Müller's muscle, with 6.4% of these fibers being large. Immunofluorescent labeling with choline acetyltransferase showed no evidence of skeletal motor axons in the samples, indicating large axons are likely to be sensory and proprioceptive. In addition, we identified C-fibers using double labeling with peripherin and neural cell adhesion molecules.ConclusionsOverall, large myelinated sensory fibers are present in the Müller's muscle, likely serving proprioceptive innervation. This suggests that proprioception signals from Müller's muscle may have a role in eyelid spatial positioning and retracting, in addition to visual deprivation. This finding sheds new light on our understanding of this complex mechanism.