Physiological tremor in human jaw-muscle system.

Small, quasi-rhythmical tremor of the jaw occurs at rest and during voluntary movements. In peripheral limbs, tremor consists of a component due to mechanical resonance properties of the system, and a neurogenic component mediated by a central pacemaker or neural loops. The present study attempted to determine if these components were present in jaw tremor measured with a position transducer held lightly between the incisors. When weights were suspended from the mandible, the tremor frequency was unaltered; sharp taps delivered to the jaw did not elicit any kind of damped oscillations of the system. These findings indicated the absence of mechanical resonance in the system. No correlation of the tremor signal with the electrocardiogram was found. However, a strong correlation was found between the rectified electromyographic signals recorded over the masseter muscles and the tremor signal, where the electromyographic signal preceded jaw movement by 20-30 msec. Frequency-domain analysis also showed positive peaks in a majority of coherence functions between electromyographic and tremor signals. These results suggested a strong neurogenic component of the tremor.