LAPPIN, A.K.*; NISHIKAWA, K.C.; PIEROTTI, D.J.; Northern Arizona University: How do toads accelerate their tongues at over 2000 m/s²?

During feeding, toads exhibit extreme tongue projection velocities (>350 cm/s) and accelerations (>2000 m/s²). This is accomplished via transmission of inertial forces from the rapidly opening mouth to the tongue. The rapid phase of mouth opening, during which the tongue is protracted, occurs in <10 ms and is powered by the paired mm. depressor mandibulae. Electromyography demonstrates that the mm. depressor mandibulae are pre-activated, sometimes for over 200 ms, before the onset of rapid mouth opening. We hypothesize that the mm. depressor mandibulae store elastic strain energy during isometric contraction against an opposing force, for example that generated via coactivation of the levator mandibulae muscles. At the onset of rapid mouth opening, the mm. depressor mandibulae appear to recover elastic strain energy stored during isometric pre-activation. To examine the behavior of the m. depressor mandibulae in vitro, we performed a series of load-clamp experiments in which the muscle was first stimulated isometrically and then allowed to shorten at a series of specific loads. The parameters of the load-clamp experiments were guided by the in vivo behavior of the muscle. Upon release, the shortening pattern of a pre-activated muscle is biphasic, with a brief fast phase followed by a longer slow phase. Our results suggest that the entire rapid phase of mouth opening, and thus tongue projection, is powered directly by the recovery of stored elastic strain energy from the mm. depressor mandibulae (fast phase in load clamp), rather than by active cross-bridge cycling (slow phase in load clamp). Furthermore, modulation of the duration of pre-activation appears control tongue projection velocity, acceleration, and distance in vivo.