Biomechanics and Neural Control of Ballistic Tongue Projection

生物学上的新见解往往来自对表现出某种极端表现的动物的研究。在这种动物中，所有动物共有的功能原理被夸大了，因此更容易观察和研究。蟾蜍的弹道舌头突起就是这样一种极端的表现。在弹道舌头投射过程中，以超过2000m/s2的速度从口中投射出蟾蜍舌头。我们实验室以前和正在进行的研究都集中在阐明蟾蜍的张嘴肌肉产生这种极快运动所需的力量的机制。在弹道舌头投射过程中，蟾蜍的张嘴肌肉在快速张嘴之前被激活约200毫秒。在这种激活模式的启发下，研究人员开发了一种新的技术来量化这块肌肉在体内的力-速度行为。该技术类似于众所周知的快速释放或负载钳位技术。肌肉的插入连接到一个双伺服马达力杆上，保留了肌肉在体内的起始点和作用线。刺激器被编程为在给定的时间内强直地预激活肌肉，以对抗防止肌肉缩短的负荷。然后，力杆被瞬间释放到某个较小的载荷，该载荷是系统变化的。载荷越小，缩短的速度越大。利用这些实验收集的数据构建了预激活等渗力-速度和功率曲线，这是本实验室的创新之处。研究人员已经证明，200毫秒的预激活会使蟾蜍张嘴肌肉的缩短速度增加约20倍，从而使功率输出增加约20倍。他们进一步表明，200毫秒的预激活以同样的因素增加了青蛙后肢肌肉的功率输出。这些结果表明，通过等长预激活来增强肌肉力量可能是骨骼肌的一种基本特性，尽管以前还不知道。在预激活200ms后，当夹住较小的负荷时，颌肌和后肢肌肉都会迅速缩短，并在很短的时间内(3-8ms)达到最终速度，这一速度随着负荷的增加而增加。在肌肉迅速缩短的释放瞬间，缩短的力(F)等于mV/t，其中m=肌肉缩短部分的质量加上外部载荷，v=观察到的缩短速度，t=达到观察到的速度所需的时间。达到给定速度所需的时间越短，释放时的力就越大。目前的结果表明，与标准的等张测试相比，当肌肉被预激活时，缩短力显著增加。拟议中的研究将允许验证这一新技术，并量化预激活对其他肌肉的力-速度行为的影响。这种新的方法有可能对肌肉生理学领域产生重大影响，并有助于我们对横纹肌收缩机制的基本理解。

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.