Background: To date most investigations of human masticatory reflexes have been performed under static mandibular conditions. It is not known whether muscle spindle reflexes would be modulated during simulated chewing.
Methods: In 28 consenting informed adult volunteers with no neuromuscular or dental problems, we delivered 1 and 2 N taps to the upper central incisor each time the mandible went through individualized jaw separation (~ 14-16mm) during both jaw opening and closing. Responses of the jaw muscles to these taps were examined by assembling spike triggered averages of the masseteric electromyogram (EMG) around the time of the stimulus. To examine the possibility of reflex modulation by jaw movement, static tests were also performed using EMG feedback. EMG during static trials was matched to that which occurred at the stimulus delivery time of both the opening and closing phases of the dynamic trial.
Results: We have found that stimulation of muscle spindles does elicit a significant excitatory reflex during chewing. There is a significant reduction of this reflex excitation during the jaw-closing phase while during the jaw-opening phase, no significant modulation was evident.
Conclusion: While these findings differ from those of several limb studies where muscle spindle-evoked reflexes are shown to be significantly down-regulated during lengthening movements of the homonymous muscle, similar findings have been documented during static studies on muscles of mastication.