Activity coordination in the rabbit jaw muscles

Activation of the jaw muscles must be coordinated to ensure proper function of the masticatory system during a wide spectrum of behaviors. Objectives: The simultaneous variation in daily activity amplitude of four rabbit jaw muscles was studied to evaluate the coordination during different types and intensities of behavior. Methods: A radio-telemetric device was implanted in 15 rabbits for the continuous registration of the electromyograms of the jaw muscles during several weeks. Video surveillance provided information on the type of behavior accompanying the electromyograms. Per day and for each muscle, the amplitude of all processed samples was determined. Muscle activity level was expressed as a percentage of the day's peak activity. For each pair of simultaneous recorded muscles and for various behaviours, the coinciding activity amplitudes were counted using a two-dimensional histogram. This allows for the examination of all observed amplitude relationships between the two muscles and the frequency of their occurrence. Results: For various behaviors including chewing, the activities of the digastric and the jaw closing muscles were mutually exclusive, except at very low intensities (< 5%). This suggests little coactivation of these antagonistic muscles. In contrast, at all activity intensities the medial pterygoid activity level accompanied superficial masseter activity level. This indicates that these muscles are true synergists. Although the deep and superficial masseter showed synergistic behavior, a larger variety in amplitude combinations was present for these two masseter regions. Conclusion: The results clearly show that the digastric muscle is a true antagonist of all jaw closers for all daily behaviors. Despite a general pattern of coordination between the various jaw closers, the variability in their relationship suggests independent control.