Muscle Contraction Group, School of Physiology, Pharmacology & Neuroscience, University of Bristol, Bristol, UK.
Faculty of Medicine, Imperial College London, London, UK.
J Gen Physiol. 2021 Oct 4;153(10). doi: 10.1085/jgp.202012859. Epub 2021 Aug 4.
X-ray diffraction studies of muscle have provided a wealth of information on muscle structure and physiology, and the meridian of the diffraction pattern is particularly informative. Reconditi et al. (2014. J. Physiol.https://doi.org/10.1113/jphysiol.2013.267849) performed superb experiments on changes to the M3 meridional peak as a function of sarcomere length (SL). They found that the M3 intensity dropped almost linearly as sarcomere length increased at least to about SL = 3.0 µm, and that it followed the same track as tension, pointing toward zero at the end of overlap at ∼3.6 µm. They concluded that, just as tension could only be generated by overlapped myosin heads, so ordered myosin heads contributing to the M3 intensity could only occur in the overlap region of the A-band, and that nonoverlapped heads must be highly disordered. Here we show that this conclusion is not consistent with x-ray diffraction theory; it would not explain their observations. We discuss one possible reason for the change in M3 intensity with increasing sarcomere length in terms of increasing axial misalignment of the myosin filaments that at longer sarcomere lengths is limited by the elastic stretching of the M-band and titin.
X 射线衍射研究为肌肉结构和生理学提供了丰富的信息，而衍射图谱的子午线尤其具有信息量。Reconditi 等人（2014. J. Physiol.https://doi.org/10.1113/jphysiol.2013.267849）对肌节长度（SL）变化对 M3 子午线峰的影响进行了出色的实验研究。他们发现，M3 强度随着肌节长度的增加几乎呈线性下降，至少在 SL = 3.0 µm 左右，并且与张力的轨迹相同，在约 3.6 µm 的重叠末端指向零。他们得出结论，正如张力只能由重叠的肌球蛋白头部产生一样，因此对 M3 强度有贡献的有序肌球蛋白头部只能出现在 A 带的重叠区域，而不重叠的头部必须高度无序。在这里，我们表明这个结论与 X 射线衍射理论不一致；它不能解释他们的观察结果。我们根据肌球蛋白丝的轴向不对准程度的增加，讨论了 M3 强度随肌节长度增加而变化的一个可能原因，在较长的肌节长度下，这种不对准程度受到 M 带和titin 的弹性拉伸的限制。
