Reflex sensitivity of human jaw-closing muscles during the silent period following a jaw jerk

Two successive force impulses were exerted upon the mandible in the opening direction with intervals of 20 to 120 ms. Electromyographic responses were recorded from the right temporal and masseter muscles and from the anterior belly of the right digastric muscle. Mandibular movement was recorded in the sagittal plane. The first-force impulse evoked a jaw jerk followed by a silent period. The jaw jerk following the second-force impulse was depressed when the interval between the force impulses was shorter than a certain value. The depression was total for at least 20 ms after the first jaw jerk. It is suggested that the silent period is the result of more than one inhibitory mechanism. From comparison of the resulting mandibular movement with movements resulting from single-force impulses, it is concluded that the jaw jerk may contribute to a small but significant extent to the stiffness of jaw-closing muscles.