Meeting Abstract
Studies of limb muscles often conclude that stretch and recoil of series elastic elements act to decouple joint movement from contraction of muscle. However, this phenomenon, often associated with a functional increase in movement speed, has rarely been documented for feeding systems. In lungfishes and salamanders, the muscles responsible for jaw depression and adduction attach to the mandible on opposite sides of the jaw joint, and may potentially antagonize each other as they control jaw movement. Previous electromyography studies have revealed temporal decoupling between activation of jaw muscles and their expected action upon the mandible in both taxa. Such delays suggest the presence of a compliant element within the system. Recently developed biplanar high-speed x-ray approaches now permit analyses of distinct contractile and elastic components within this system. We used fluoromicrometry and XROMM to measure the relationship between changes in length of active muscle fascicles and whole muscle-tendon units (MTUs)with respect to chewing gape cycles in Axolotls. We predicted finding evidence of decoupling of jaw movement from contraction of muscle fibers, either as fiber shortening or isometry during MTU lengthening (or isometry). In support of this prediction, our data showed durations of muscle fiber shortening averaging 0.27 ± 0.15s during MTU lengthening or isometry, suggesting the action of a series elastic element. Jaw closing speeds averaged 7.8 ± 1.6 cm s-1, or up to 8.4 jaw adductor resting lengths s-1 (a conservative estimate, calculated using the length of the MTU as reference length). Such speeds are in the upper range of what sarcomeres can achieve and thus suggest that elastic recoil enables faster jaw movements, which in turn may help minimize risks of losing struggling prey.
