Moo Eng Kuan, Herzog Walter
Human Performance Laboratory, Faculty of Kinesiology, University of Calgary, Calgary, AB, Canada.
Front Physiol. 2020 May 12;11:448. doi: 10.3389/fphys.2020.00448. eCollection 2020.
The seemingly uniform striation pattern of skeletal muscles, quantified in terms of sarcomere lengths (SLs), is inherently non-uniform across all hierarchical levels. The SL non-uniformity theory has been used to explain the force creep in isometric contractions, force depression following shortening of activated muscle, and residual force enhancement following lengthening of activated muscle. Our understanding of sarcomere contraction dynamics has been derived primarily from experiments using regular bright-field light microscopy or laser diffraction techniques to measure striation/diffraction patterns in isolated muscle fibers or myofibrils. However, the collagenous extracellular matrices present around the muscle fibers, as well as the complex architecture in the whole muscles may lead to different contraction dynamics of sarcomeres than seen in the studies. Here, we used multi-photon excitation microscopy to visualize individual sarcomeres in intact muscle tendon units (MTUs) of mouse tibialis anterior (TA), and quantified the temporal changes of SL distribution as a function of SLs in relaxed and maximally activated muscles for quasi-steady state, fixed-end isometric conditions. The corresponding muscle forces were simultaneously measured using a force transducer. We found that SL non-uniformity, quantified by the coefficient of variation (CV) of SLs, decreased at a rate of 1.9-3.1%/s in the activated muscles, but remained constant in the relaxed muscles. The force loss during the quasi-steady state likely did not play a role in the decrease of SL non-uniformity, as similar force losses were found in the activated and relaxed muscles, but the CV of SLs in the relaxed muscles underwent negligible change over time. We conclude that sarcomeres in the mid-belly of maximally contracting whole muscles constantly re-organize their lengths into a more uniform pattern over time. The molecular mechanisms accounting for SL non-uniformity appear to differ in active and passive muscles, and need further elucidation, as do the functional implications of the SL non-uniformity.
骨骼肌看似均匀的条纹模式，以肌节长度（SLs）来量化，在所有层次水平上本质上是不均匀的。SL不均匀性理论已被用于解释等长收缩中的力蠕变、激活肌肉缩短后的力降低以及激活肌肉延长后的残余力增强。我们对肌节收缩动力学的理解主要来自于使用常规明场光学显微镜或激光衍射技术来测量分离的肌纤维或肌原纤维中的条纹/衍射模式的实验。然而，肌肉纤维周围存在的胶原细胞外基质以及整个肌肉中的复杂结构可能导致肌节的收缩动力学与这些研究中观察到的不同。在这里，我们使用多光子激发显微镜来可视化小鼠胫骨前肌（TA）完整肌肉肌腱单元（MTUs）中的单个肌节，并在准稳态、固定端等长条件下，量化了松弛和最大激活肌肉中SL分布随SLs的时间变化。使用力传感器同时测量相应的肌肉力。我们发现，以SLs的变异系数（CV）量化的SL不均匀性在激活肌肉中以1.9 - 3.1%/秒的速率降低，但在松弛肌肉中保持不变。准稳态期间的力损失可能在SL不均匀性的降低中不起作用，因为在激活和松弛肌肉中发现了相似的力损失，但松弛肌肉中SLs的CV随时间的变化可忽略不计。我们得出结论，最大收缩的整块肌肉中部的肌节会随着时间不断地将其长度重新组织成更均匀的模式。解释SL不均匀性的分子机制在主动和被动肌肉中似乎有所不同，需要进一步阐明，SL不均匀性的功能意义也是如此。
