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 patie nts with a myogenous craniomandibular dysfunction (CMD) and healthy co ntrols free from signs and symptoms of CMD, jaw-jerk reflexes were eli cited under standardized conditions in two groups of 10 gender- and ag e-matched subjects. The reflexes were recorded bilaterally from the ma sseter and the anterior temporal muscles by means of bipolar surface e lectromyogram (EMG). Reflex amplitudes at a mandibular displacement of exactly 80 mum and at a background muscle activity of exactly 12% max imum voluntary contraction were determined from relations between refl ex amplitude and jaw displacement. These were obtained at a visually c ontrolled, 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, compa risons were also made between the pain and non-pain sides. Although si gnificant side asymmetries were found in many individuals, no signific ant differences were found among bilateral asymmetries in reflex sensi tivity 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 particip ated, the influence of a possible asymmetry in jaw displacement on the reflex sensitivity was evaluated. To achieve this, reflexes were elic ited not only by means of a bilaterally imposed mandibular load, but a lso 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 ma ndibular loading, the influence of a possible asymmetrical jaw displac ement on side asymmetries in the jaw-jerk reflex sensitivity is neglig ible in our experimental model.