Bilateral asymmetries in the jaw-jerk reflex activity in man

In order to investigate whether there are bilateral differences in the sensitivity of the mandibular stretch (jaw-jerk) reflex between patients with a myogenous craniomandibular dysfunction (CMD) and healthy controls free from signs and symptoms of CMD, jaw-jerk reflexes were elicited under standardized conditions in two groups of 10 gender- and age-matched subjects. The reflexes were recorded bilaterally from the masseter and the anterior temporal muscles by means of bipolar surface electromyogram (EMG). Reflex amplitudes at a mandibular displacement of exactly 80 microns and at a background muscle activity of exactly 12% maximum voluntary contraction were determined from relations between reflex amplitude and jaw displacement. These were obtained at a visually controlled, constant clenching level. For both groups, comparisons were made between reflex amplitudes from the right- and the left-hand side. In CMD patients with predominantly unilateral jaw muscle pain, comparisons were also made between the pain and non-pain sides. Although significant side asymmetries were found in many individuals, no significant differences were found among bilateral asymmetries in reflex sensitivity between patients and controls. No influence of pain side on the asymmetries was found. It was concluded that neuromuscular factors do not cause significant bilateral differences in the sensitivity of the jaw-jerk reflex between patients with myogenous CMD and controls. In a control experiment, in which eight healthy control subjects participated, the influence of a possible asymmetry in jaw displacement on the reflex sensitivity was evaluated. To achieve this, reflexes were elicited not only by means of a bilaterally imposed mandibular load, but also by means of loading via a unilateral bite-fork, so that an equal, constant jaw displacement could be imposed successively on both sides of the mandible. As no significant differences were found in bilateral asymmetries in reflex sensitivity between unilateral and bilateral mandibular loading, the influence of a possible asymmetrical jaw displacement on side asymmetries in the jaw-jerk reflex sensitivity is negligible in our experimental model.