title: Masseter length determines muscle spindle reflex excitability during jaw-closing movements.
abstract: BACKGROUND
  The masticatory muscles are considered to be important determinants of facial form, but little is known of the muscle spindle reflex characteristics and their relationship, if any, to face height. The aim of this study was to determine whether spindle reflexes, evoked by mechanical stimulation of an incisor and recorded on the masseter muscle, correlated with different facial patterns.
  
  
  METHODS
  Twenty-eight adult volunteers (16 women; ages, 19-38 years) underwent 2-N tap stimuli to their maxillary left central incisor during simulated mastication. Reflexes were recorded during local anesthesia of the stimulated tooth to eliminate the contribution from periodontal mechanoreceptors. Surface electromyograms of the reflex responses of the jaw muscles to these taps were recorded via bipolar electrodes on the masseter muscle and interpreted by using spike-triggered averaging of the surface electromyograms. Lateral cephalometric analysis was carried out with software (version 10.5, Dolphin, Los Angeles, Calif; and Mona Lisa, Canberra, Australia).
  
  
  RESULTS
  Two-newton tooth taps produced principally excitatory reflex responses beginning at 17 ms poststimulus. Correlation analysis showed a significant relationship between these muscle spindle reflexes and facial heights: specifically, shorter face heights were associated with stronger spindle reflexes. This correlation was strongest between the derived measure of masseter length and the spindle reflex strength during jaw closure (r = -0.49, P = 0.008).
  
  
  CONCLUSIONS
  These results suggest that a similar muscle spindle stimulus will generate a stronger reflex activation in the jaw muscles of patients with shorter faces compared with those with longer faces. This finding might help to explain the higher incidence of clenching or bruxism in those with short faces and also might, in the future, influence the design of orthodontic appliances and dental prostheses.
authors: Dental School University of Queensland, Australia.
authors: Shazia
authors: S
authors: Naser-Ud-Din
authors: Paul F
authors: PF
authors: Sowman
authors: Wayne J
authors: WJ
authors: Sampson
authors: Craig W
authors: CW
authors: Dreyer
authors: Kemal Sitki
authors: KS
authors: Türker
mesh_terms: D000328:Adult; D002508:Cephalometry; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology; D056966:Excitation Contraction Coupling / Q000502:physiology; D005260:Female; D006801:Humans; D007180:Incisor / Q000502:physiology; D007537:Isometric Contraction / Q000502:physiology; D008334:Mandible / Q000033:anatomy & histology / Q000502:physiology*; D008335:Mandibular Condyle / Q000033:anatomy & histology; D008406:Masseter Muscle / Q000033:anatomy & histology* / Q000502:physiology; D008437:Maxilla / Q000033:anatomy & histology; D009470:Muscle Spindles / Q000502:physiology*; D009129:Muscle Tonus / Q000502:physiology; D010812:Physical Stimulation; D016059:Range of Motion, Articular / Q000502:physiology; D012018:Reflex / Q000502:physiology*; D013997:Time Factors; D014716:Vertical Dimension; D014732:Vibration; D055815:Young Adult