BALANCING FUNCTION OF THE MASTICATORY MUSCLES DURING INCISAL BITING IN 2 MURID RODENTS, APODEMUS-SPECIOSUS AND CLETHRIONOMYS-RUFOCANUS

The functional significance of masticatory muscle direction was estima ted using a mechanical model in two murid rodents: the Japanese field mouse (Apodemus speciosus) and the gray red-backed vole (Clethrionomys rufocanus). Theoretical analyses of the data suggest that a balancing mechanism among the muscle forces occurs during incisal power stroke. The activation of the large deep masseter in both murids results in m arked tensile separation of two hemimandibles at the flexible mandibul ar symphysis. Activation of the internal pterygoid decreases this larg e tensile force at the symphysis more efficiently than other muscles. The lines of action of the deep masseter and internal pterygoid are al igned to produce such a balancing function in both species studied her e. The resultant force generated by the deep masseter on both sides is opposite in direction to the reaction force at the lower incisor tip. Therefore, the large deep masseter forms an effective mandibular supp ort mechanism when the reaction forces during biting push the mandible downward. Because of the area of insertion and the line of action, th e posterior temporalis appears to have an important role in stabilizin g the position of the mandibular condyle in the glenoid fossa during i ncisal biting. (C) 1998 Wiley-Liss, Inc.