Rosant C, Nagel M D, Pérot C
UMR-CNRS 6600 Biomécanique et Génie Biomédical, Université de Technologie de Compiègne, BP 20529, F-60205 Compiègne, France.
Exp Neurol. 2006 Jul;200(1):191-9. doi: 10.1016/j.expneurol.2006.02.003. Epub 2006 Apr 19.
Spindle discharges are affected by muscle unloading, and changes in passive stiffness of the muscle-tendon unit may contribute to the changes in spindle solicitation. To test this hypothesis, we determined the spindle sensitivity from electroneurograms of the soleus nerve, and, concomitantly, we measured the incremental passive muscle tension. Both measurements were done from ramp and hold stretches imposed to the soleus muscle after the Achilles tendon was severed. The ratio between the spindle sensitivity and the passive stiffness gave a "spindle efficacy index" (SEI). The experiments were conducted on control rats (C, n = 12) and on rats that had undergone hindlimb unloading (HU, n = 12) for 21 days. The muscle threshold lengths for electroneurogram to discharge (neurogram length, Ln) and for detecting passive tension (slack length, Ls) were determined, and, when these lengths differed, the stretches were imposed at these two initial lengths. The contralateral muscles were used to count muscle spindles and spindle fibers (ATPase staining) and to identify MyHC isoforms by immunostaining. Ln and Ls values were identical for the C muscles, while after HU, Ln was significantly shorter than Ls, which indicated that spindle afferents were more sensitive since they discharged before any passive tension was developed by the soleus muscle. At Ln, spindle sensitivity and passive stiffness did not differ for C and HU muscles. Consequently, when calculated at this relatively short initial muscle length, the SEI was maintained (or even slightly increased) after HU. This held under dynamic conditions (ramp phase of the stretch) and under static conditions (hold phase of the stretch). At Ls, the dynamic and static incremental stiffness values increased significantly after HU. Under dynamic conditions, the spindle sensitivity also increased after HU but to a less degree than incremental stiffness, which led to a significant decrease in SEI. Under static conditions, the spindle sensitivity presented a high increase, and, consequently, SEI was not modified. These functional changes were associated with structural adaptations: HU did not alter the total number of muscle spindles, but the number of spindles containing three nuclear chain fibers increased significantly. The main change in intrafusal MyHC content concerned the slow type I MyHC isoform. In conclusion, after a period of muscle unloading, the spindle discharges were maintained or even enhanced in several experimental conditions. This may be due to a better transmission of the external stretch to muscle spindles through stiffer elastic structures but also to own muscle spindle adaptations which reinforce the spindle sensitivity, notably under static conditions.
肌梭放电受肌肉卸载的影响，而肌腱单元被动僵硬度的变化可能导致肌梭激发的改变。为验证这一假设，我们通过比目鱼肌神经的肌电图确定肌梭敏感性，同时测量被动肌肉张力增量。这两项测量均在跟腱切断后对比目鱼肌进行斜坡拉伸和保持拉伸时完成。肌梭敏感性与被动僵硬度的比值得出一个“肌梭效能指数”（SEI）。实验在对照大鼠（C组，n = 12）和经历21天 hindlimb unloading（HU组，n = 12）的大鼠上进行。确定了肌电图放电的肌肉阈值长度（神经图长度，Ln）和检测被动张力的长度（松弛长度，Ls），当这两个长度不同时，在这两个初始长度施加拉伸。对侧肌肉用于计数肌梭和梭内纤维（ATP酶染色），并通过免疫染色鉴定肌球蛋白重链（MyHC）亚型。C组肌肉的Ln和Ls值相同，而HU组后，Ln显著短于Ls，这表明肌梭传入纤维更敏感，因为它们在比目鱼肌产生任何被动张力之前就放电了。在Ln时，C组和HU组肌肉的肌梭敏感性和被动僵硬度无差异。因此，在这个相对较短的初始肌肉长度计算时，HU组后SEI得以维持（甚至略有增加）。这在动态条件下（拉伸的斜坡阶段）和静态条件下（拉伸的保持阶段）均成立。在Ls时，HU组后动态和静态增量僵硬度值显著增加。在动态条件下，HU组后肌梭敏感性也增加，但程度小于增量僵硬度，这导致SEI显著降低。在静态条件下，肌梭敏感性大幅增加，因此SEI未改变。这些功能变化与结构适应性相关：HU组未改变肌梭总数，但含有三根核链纤维的肌梭数量显著增加。梭内MyHC含量的主要变化涉及慢I型MyHC亚型。总之，经过一段时间的肌肉卸载后，在几种实验条件下肌梭放电得以维持甚至增强。这可能是由于通过更硬的弹性结构，外部拉伸能更好地传递至肌梭，也可能是由于肌梭自身的适应性变化增强了肌梭敏感性，特别是在静态条件下。
