Masticatory muscle activity during rhythmic jaw movements in the anaesthetized guinea-pig

Albino guinea-pigs anaesthetized with ketamine were observed to produce spontaneous rhythmic jaw movements which consisted of a slight opening of the jaw (∼-4 mm) followed by a return to the original position in which the molar teeth were separated by approx. 1 mm. When a wooden stick was placed between the molar teeth cyclic chewing movements with alternating clenching and releasing of the stick could be evoked. Electromyographic (EMG) activity was recorded from the digastric, masseter and medial pterygoid (M.PT.) muscles with fine-wire electrodes. Each spontaneous jaw movement began with jaw opening accompanied by a burst of digastric EMG activity of 90 ms in duration. During this behaviour low level EMG activity was sometimes observed in the masseter muscle. The M.PT. muscle showed high levels of EMG activity occurring simultaneously with the digastric activity. During stick-chewing an 85 ms duration burst of high-level masseter-EMG activity appeared, beginning approx. 33 ms after cessation of the digastric activity. In contrast, the M.PT. muscle showed high levels of EMG activity which occurred simultaneously with the masseter activity. The results of this study indicate a diverse and complex role for the M.PT. muscle.