Histochemical and electromyographic analysis of craniomandibular muscles in the rhesus monkey, Macaca mulatta

The range of fiber composition was analyzed in four regions of the temporalis muscle of four normal rhesus monkeys. The middle region of the temporalis muscle contained significantly fewer type I fibers than the anterior region. Comparison of the same region between left and right temporalis muscles showed that a bilateral symmetry existed in the composition at the different comparable levels of the muscle except for the deep anterior temporalis. Biopsies were also taken from three muscles, the superficial anterior temporalis, superficial masseter, and the digastric muscles, of ten normal adult monkeys to determine the fiber composition. Both adult male and female monkeys demonstrated a low percentage of type I fibers in the anterior temporalis (less than 25%), and twice as many type I fibers in the masseter and digastric muscles (40 to 50%). The effect of applying a constant touch-pressure to the dorsal surface of the tongue using an acrylic wedge was studied subsequently in these ten animals. Behaviorally, the animals lowered their mandible more during the first 3 months of adapting to the tactile stimulus, and lowering of the mandible was accompanied by increased electromyographic activity (EMG) of the digastric muscle, but not of the anterior temporalis or masseter muscles. In fiber composition, only the anterior temporalis demonstrated a significant shift in percentage of type I fibers with a decrease during the first 3 months. In cross-sectional area, both the anterior temporalis and digastric muscles demonstrated a significant decrease for type I fibers during the 12 month period. Lowering the postural position of the mandible had more effect on the cross-sectional area of the individual fibers than on the metabolic activity of the muscles.