Biomechanics and Neural Control of Ballistic Tongue Projection

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New insights in biology have often come from studies of animals that exhibit some extreme of performance. In such animals, principles of function common to all animals are exaggerated, and are therefore observed and studied more readily. Ballistic tongue projection in toads represents one such extreme of performance. During ballistic tongue projection, toad tongues can be projected from the mouth at accelerations exceeding 2000 m/s2. Previous and ongoing research in our laboratories has focused on elucidating the mechanisms by which the mouth opening muscles of toads produce the power required for this extremely rapid movement.During ballistic tongue projection, the mouth opening muscles of toads are activated for about 200 ms prior to rapid mouth opening. Inspired by this pattern of activation, the investigators developed a novel technique to quantify the in vivo force-velocity behavior of this muscle. The technique is similar to the well known quick-release or load-clamp techniques. The insertion of a muscle is attached to a dual servo motor force-lever, preserving the in vivo origin and line of action of the muscle. A stimulator is programmed to preactivate the muscle tetanically for a given time against a load that prevents the muscle from shortening. The force-lever is then released instantaneously to some smaller load that is varied systematically. The smaller the load, the greater the velocity of shortening.Preactivated isotonic force-velocity and power curves were constructed using data collected from these experiments, an innovation of this laboratory. The investigators have demonstrated that 200 ms preactivation increases the shortening velocity, and therefore the power output, of the mouth opening muscles of toads by about twenty-fold. They further showed that 200 ms preactivation increased the power output of a frog hind limb muscle by the same factor. These results suggest that enhancement of muscle power by isometric preactivation is likely a fundamental, though previously unknown, property of skeletal muscle.Both jaw and hind limb muscles shorten rapidly when clamped to smaller loads after 200 ms preactivation, and reach final velocity in a very short period of time (3-8 ms), which increases with increasing load. At the instant of release when the muscle is shortening rapidly, the force of shortening (F) is equal to mv/t, where m = the mass of that part of the muscle that is shortening plus the external load, v = the observed shortening velocity, and t = the time required to reach the observed velocity. The shorter the time required to reach a given velocity, the greater the force at the time of release. The present results demonstrate that the force of shortening increases substantially when muscles are preactivated compared to standard isotonic tests. The proposed studies will permit validation of this new technique, and quantification of the effects of preactivation on the force-velocity behavior of other muscles. This novel method has the potential to significantly impact the field of muscle physiology, and to inform our basic understanding of the mechanism of contraction in striated muscle.