Force encoding in muscle spindles during stretch of passive muscle.
Force encoding in muscle spindles during stretch of passive muscle.
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DOI：
10.1371/journal.pcbi.1005767
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发表时间：
2017-09
影响因子：
4.3
通讯作者：
Ting LH
中科院分区：
文献类型：
--
作者：
Blum KP;Lamotte D'Incamps B;Zytnicki D;Ting LH
肌梭本体感受器在编码外部机械扰动对身体的影响方面发挥主要作用。在外部施加的被动（即电静止）肌肉拉伸过程中，肌梭的瞬时放电率（IFR）与拉伸特性（例如长度和速度）相关。然而，即使在被动肌肉中，也存在肌梭放电的历史依赖性瞬态，这些瞬态与肌肉长度和速度并不唯一相关，也不能由当前的肌梭模型重现。这些包括依赖于加速度的初始爆发、在肌肉等长时对拉伸速度的动态响应增加以及速率松弛，即当肌肉在拉伸后保持恒定长度时强直IFR的减少。我们收集了各种肌肉拉伸运动条件下的肌梭尖峰训练，包括峰值长度、速度和加速度的系统变化。我们证明了被动肌肉发射中的肌梭主要传入与肌肉力量相关的变量直接相关，而不是与长度相关的变量。整个肌肉肌腱力和力的一阶时间导数 (dF/dt) 的线性组合可预测被动肌肉拉伸（即延长）期间肌梭 Ia 传入神经中瞬时 IFR 的整个时间过程，包括初始爆发、对延长的动态响应以及延长后的速率松弛。与之前在对扰动的姿势响应中发现的加速度缩放类似，初始突发振幅与初始拉伸加速度或 dF/dt 的缩放效果相同，尽管后来的瞬态仅由 dF/dt 描述。延长开始时 dF/dt 的瞬时增加反映了由于跨桥动力学导致的肌肉短程刚度。我们的工作证明了肌肉跨桥动力学在历史依赖性肌梭IFR中的关键作用，在被动肌肉延长条件下，与对身体机械扰动的检测和感觉运动反应以及先前描述的肢体位置感知的历史依赖性相关。本体感觉信息对于运动至关重要，特别是在对身体外部扰动（例如推或碰撞）的感觉运动反应中，无论是保持肢体的姿势，还是在站立平衡控制期间。在这里，我们表明，被动肌肉在拉伸时阻力的快速增加可能会导致感觉信号增强，从而促进对身体突然机械扰动的检测和响应。我们的工作意义重大，因为肌梭放电的这些短暂增加之前尚未根据肌梭编码肌肉长度和速度变化的经典解释来解释。我们的工作表明，在许多情况下，肌肉力量感可以作为肌肉长度的良好指标，而且当我们的身体处于休息状态时，也会增加对扰动的感觉编码。进一步的工作可能会结合我们的发现来开发更准确的本体感受编码模型，该模型可以更好地预测我们对扰动的感觉、运动和知觉反应，以及它们如何受到影响感知和运动的神经系统疾病的影响。
Muscle spindle proprioceptive receptors play a primary role in encoding the effects of external mechanical perturbations to the body. During externally-imposed stretches of passive, i.e. electrically-quiescent, muscles, the instantaneous firing rates (IFRs) of muscle spindles are associated with characteristics of stretch such as length and velocity. However, even in passive muscle, there are history-dependent transients of muscle spindle firing that are not uniquely related to muscle length and velocity, nor reproduced by current muscle spindle models. These include acceleration-dependent initial bursts, increased dynamic response to stretch velocity if a muscle has been isometric, and rate relaxation, i.e., a decrease in tonic IFR when a muscle is held at a constant length after being stretched. We collected muscle spindle spike trains across a variety of muscle stretch kinematic conditions, including systematic changes in peak length, velocity, and acceleration. We demonstrate that muscle spindle primary afferents in passive muscle fire in direct relationship to muscle force-related variables, rather than length-related variables. Linear combinations of whole muscle-tendon force and the first time derivative of force (dF/dt) predict the entire time course of transient IFRs in muscle spindle Ia afferents during stretch (i.e., lengthening) of passive muscle, including the initial burst, the dynamic response to lengthening, and rate relaxation following lengthening. Similar to acceleration scaling found previously in postural responses to perturbations, initial burst amplitude scaled equally well to initial stretch acceleration or dF/dt, though later transients were only described by dF/dt. The transient increase in dF/dt at the onset of lengthening reflects muscle short-range stiffness due to cross-bridge dynamics. Our work demonstrates a critical role of muscle cross-bridge dynamics in history-dependent muscle spindle IFRs in passive muscle lengthening conditions relevant to the detection and sensorimotor response to mechanical perturbations to the body, and to previously-described history-dependence in perception of limb position. Proprioceptive sensory information is essential to movement, particularly in sensorimotor responses to external perturbations to the body–such as a push or bump–whether maintaining the posture of a limb, or during standing balance control. Here we show that rapid increase in resistive force of a passive muscle when stretched may cause enhanced sensory signals that facilitate the detection and response to sudden mechanical perturbations to the body. Our work is significant because these transient increases in muscle spindle firing have not been explained previously in terms of the classical explanation of muscle spindles encoding changes in muscle length and velocity. Our work suggests that a sense of muscle force may serve as a good proxy for muscle length in many conditions, but also increases sensory encoding of perturbations when our bodies are at rest. Further work may incorporate our findings to develop more accurate models of proprioceptive encoding that can better predict our sensory, motor, and perceptual responses to perturbation and how they are affected by neurological disorders that affect sensing and moving.
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