Peters S E
Department of Biology, University of North Carolina, Charlotte 28223.
J Morphol. 1994 Aug;221(2):121-31. doi: 10.1002/jmor.1052210202.
A study of the mechanical properties of the twitch motor units in the ankle extensor muscles of bullfrogs was undertaken to expand our view of the diversity of motor unit properties among vertebrates. Two muscles were chosen that represent a wide range of extensor function: the plantaris longus (PL) is a large muscle providing most of the force for ankle extension in hopping and swimming, and the tibialis posticus (TP) is relatively small and may act as an ankle stabilizer or be primarily postural in function. Both muscles have highly fatigable motor units, but also some (especially in TP) low or non-fatigable ones. Mean tetanic tensions of motor units in both muscles are relatively large as compared with those of mammals but are especially large in PL. No clear correlations were found between contraction times and either motor unit tetanic tensions or fatigability, nor did the motor units fall into clearly defined types based on any functional parameters. Overall contraction and relaxation times are slow compared with those of mammals and are somewhat slow compared to those of other frogs; unlike results from earlier studies, the large units of PL are slower than the small units of TP. This results in PL units reaching fused tetani at lower stimulus frequencies. The twitch/tetanus and force/frequency ratios in PL motor units are much larger than those of TP, giving PL units greater relative forces at lower stimulus rates. These results are discussed in the context of motor unit function.
为了拓展我们对脊椎动物运动单位特性多样性的认识，对牛蛙踝关节伸肌中抽搐运动单位的力学特性进行了一项研究。选择了两块代表广泛伸肌功能的肌肉：跖肌（PL）是一块大肌肉，在跳跃和游泳时为踝关节伸展提供大部分力量；胫骨后肌（TP）相对较小，可能起到踝关节稳定器的作用，或者主要起姿势维持功能。这两块肌肉都有高度易疲劳的运动单位，但也有一些（特别是在TP中）低疲劳或不易疲劳的运动单位。与哺乳动物相比，这两块肌肉中运动单位的平均强直张力相对较大，但在PL中尤其大。未发现收缩时间与运动单位强直张力或疲劳性之间存在明显相关性，也没有根据任何功能参数将运动单位明确分为不同类型。与哺乳动物相比，整体收缩和舒张时间较慢，与其他青蛙相比也有些慢；与早期研究结果不同的是，PL的大运动单位比TP的小运动单位慢。这导致PL运动单位在较低刺激频率下达到融合强直。PL运动单位的抽搐/强直和力/频率比值比TP大得多，使得PL运动单位在较低刺激速率下具有更大的相对力量。在运动单位功能的背景下对这些结果进行了讨论。
