Morgan D L, Claflin D R, Julian F J
Department of Electrical and Computer Systems Engineering, Monash University, Clayton, Victoria, Australia.
Biophys J. 1990 May;57(5):1001-7. doi: 10.1016/S0006-3495(90)82619-9.
Experiments were undertaken to determine the contribution of passive tension to total tension during rapid shortening in a stimulated muscle fiber. Results were obtained by applying shortening movements at constant velocities slightly less than Vu (the velocity of unloaded shortening) to intact twitch fibers isolated from the frog (Rana temporaria). The tension maintained by unstimulated fibers during such shortening movements ("dynamic passive tension") from moderately long lengths was greater than zero but much less than the passive tension measured under static conditions ("static passive tension") at the same lengths. Fibers maximally activated by electrical stimulation and then shortened at the same velocity over the same range of average sarcomere lengths maintained tension that was greater than zero but less than the dynamic passive tension. For average sarcomere lengths up to approximately 3.1 microns, the dynamic passive tension appeared to be substantially abolished by activation. The onset of the apparent disappearance of dynamic passive tension was studied by initiating the stimulation and the shortening movement simultaneously. The resulting tension response exhibited a latency relaxation that was increased in amplitude compared with the isometric case, followed by a brief tension rise, giving way to a steady tension level equal to that expected if stimulation had been initiated well before the release. These changes are qualitatively explained in terms of the establishment of a steady state distribution of deformations of attached cross-bridges.
开展了实验以确定在受刺激的肌纤维快速缩短过程中被动张力对总张力的贡献。通过以略低于Vu（无负荷缩短速度）的恒定速度对从青蛙（林蛙）分离出的完整单收缩纤维施加缩短运动来获得结果。在这种缩短运动期间（“动态被动张力”），从中等长度开始，未受刺激的纤维所维持的张力大于零，但远小于在相同长度下静态条件下测量的被动张力（“静态被动张力”）。通过电刺激最大程度激活的纤维，然后在相同的平均肌节长度范围内以相同速度缩短，所维持的张力大于零但小于动态被动张力。对于平均肌节长度达约3.1微米的情况，动态被动张力似乎因激活而基本消除。通过同时启动刺激和缩短运动来研究动态被动张力明显消失的起始情况。所产生的张力响应表现出潜伏期松弛，与等长情况相比，其幅度有所增加，随后是短暂的张力上升，接着是稳定的张力水平，该水平等于如果在释放之前很早就开始刺激时预期的水平。这些变化根据附着的横桥变形的稳态分布的建立从定性角度进行了解释。
