title: Stretching of the Mueller muscle results in involuntary contraction of the levator muscle.
abstract: OBJECTIVE
  Since the levator muscle involuntarily and tonically contracts against the weight and elastic resistance of the upper eyelid to maintain an adequate visual field, a mechanoreceptor such as a muscle spindle or a periodontal mechanoreceptor is thought to be essential for its functioning. It was surmised that the Mueller muscle might act as a serial kind of muscle spindle of the levator muscle.
  
  
  METHODS
  The response of the bilateral levator muscles evoked by stretching the Mueller muscle of each eyelid of 87 patients with dermatochalasis or aponeurotic blepharoptosis was electromyographically and photographically recorded.
  
  
  RESULTS
  Stretching of the unilateral Mueller muscle evoked contraction of the ipsilateral levator muscle in 18 and of the bilateral levator muscle in 69 of the 87 patients.
  
  
  CONCLUSIONS
  The Mueller muscle can be thought of as a large, serial kind of muscle spindle, so that stretching by voluntary phasic contraction of the levator muscle for initial eye opening may evoke an afferent impulse to the mesencephalic trigeminal nucleus. Subsequently, this nucleus may stimulate the central caudal nucleus of the oculomotor nuclear complex, leading to involuntary tonic contraction of the ipsilateral or bilateral levator muscles, in the form of a continuous stretch reflex, to maintain an adequate visual field.
authors.affiliation: Department of Plastic and Reconstructive Surgery, Shinshu University School of Medicine, Matsumoto, Japan. kmatsuo@hsp.md.shinshu-u.ac.jp
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Matsuo
mesh_terms: D000328:Adult; D019882:Blepharoplasty / Q000379:methods*; D001763:Blepharoptosis / Q000503:physiopathology / Q000601:surgery*; D003483:Cutis Laxa / Q000503:physiopathology / Q000601:surgery*; D004576:Electromyography; D005143:Eyelids / Q000503:physiopathology / Q000601:surgery*; D005260:Female; D006801:Humans; D008297:Male; D008465:Mechanoreceptors; D009119:Muscle Contraction / Q000502:physiology*; D009470:Muscle Spindles / Q000502:physiology; D009475:Neurons, Afferent / Q000502:physiology; D009801:Oculomotor Muscles / Q000294:innervation / Q000503:physiopathology / Q000601:surgery*; D010781:Photography