Aging affects many motor functions, notably the spinal stretch reflexes and muscle spindle sensitivity. Spindle activation also depends on the elastic properties of the structures linked to the proprioceptive receptors. We have calculated a spindle efficacy index, SEI, for old rats. This index relates the spindle sensitivity, deduced from electroneurograms recording (ENG), to the passive stiffness of the muscle. Spindle sensitivity and passive incremental stiffness were calculated during ramp and hold stretches imposed on pseudo-isolated soleus muscles of control rats (aged 4 months, n=12) and old rats (aged 24 months, n=16). SEI were calculated for the dynamic and static phases of ramp (1-80 mm/s) and for hold (0.5-2mm) stretches imposed at two reference lengths: length threshold for spindle afferents discharges, L(n) (neurogram length) and slack length, L(s). The passive incremental stiffness was calculated from the peak and steady values of passive tension, measured under the stretch conditions usedfor the ENG recordings, and taking into account the muscle cross-sectional area. The pseudo-isolated soleus muscles were also stretched to establish the stress-strain relationship and to calculate muscle stiffness constant. The contralateral muscle was used to count muscle spindles and spindle fibers (ATPase staining) and immunostained to identify MyHC isoforms. L(n) and L(s) lengths were not significantly different in the control group, while L(n) was significantly greater than L(s) in old muscles. Under dynamic conditions, the SEI of old muscles was the same as in controls at L(s), but it was significantly lower than in controls at L(n) due to increased passive incremental stiffness under the stretch conditions used to analyze the ENG. Under static conditions, the SEI of old muscles was significantly lower than control values at all the stretch amplitudes and threshold lengths tested, due to increased passive incremental stiffness and decreased spindle sensitivity at L(s). The muscle stiffness constant values were greater in old muscles than in controls, confirming the changes in elastic properties under passive conditions due to aging. Aging also altered the intrafusal fibers: it increased the mean number of intrafusal fibers and the contents in the slow, neonatal and developmental isoforms intrafusal of MyHC have been modified. These structural modifications do not seem great enough to counteract the loss of the spindle sensitivity or the spindle efficacy under passive conditions and after the nerve was severed. However, they may help to maintain the spindle afferent message under natural conditions and under fusimotor control.
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机译：衰老会影响许多运动功能，特别是脊椎伸展反射和肌肉纺锤体敏感性。纺锤体的活化还取决于与本体感受受体相连的结构的弹性性质。我们已经计算出了老年大鼠的主轴功效指数SEI。该指数将心电图记录（ENG）推导出的主轴灵敏度与肌肉的被动僵硬度相关联。计算了对对照组大鼠（4个月，n = 12）和老龄大鼠（24月，n = 16）的假隔离比目鱼肌施加的斜度和保持拉伸期间的主轴敏感性和被动增量刚度。 SEI是针对两个参考长度施加的斜面的动态和静态相位（1-80 mm / s）和保持（0.5-2mm）拉伸的计算值：主轴传入放电的长度阈值，L（n）（神经图长度）和松弛长度L（s）。从被动张力的峰值和稳定值（在ENG记录所用的拉伸条件下测量）并考虑到肌肉的横截面积，来计算被动增量刚度。还拉伸伪隔离的比目鱼肌以建立应力-应变关系并计算肌肉刚度常数。对侧肌肉用于计数肌肉纺锤体和纺锤体纤维（ATPase染色）并进行免疫染色以鉴定MyHC亚型。对照组的L（n）和L（s）长度无明显差异，而旧肌肉中L（n）明显大于L（s）。在动态条件下，旧肌肉的SEI与L（s）处的对照组相同，但由于在分析ENG的拉伸条件下被动增加的刚度增加，因此其LEI明显低于L（n）处的对照组。在静态条件下，由于增加的被动增量刚度和L（s）的纺锤体敏感性降低，在所有测试的拉伸幅度和阈值长度下，旧肌肉的SEI均显着低于对照值。老肌肉的肌肉僵硬常数值比对照组大，证实了在被动条件下由于衰老而导致的弹性特性变化。衰老还改变了融合神经内纤维：它增加了融合神经内纤维的平均数量，MyHC融合神经内慢，新生儿和发育同工型中的含量也被改变。这些结构修饰似乎不足以抵消在被动条件下和神经切断后主轴敏感性或主轴功效的丧失。但是，它们可能有助于在自然条件下和融合运动控制下保持主轴传入消息。
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