Windhorst U
Department of Clinical Neurosciences, University of Calgary, Faculty of Medicine, Alberta, Canada.
Neuroscience. 1994 Apr;59(3):713-27. doi: 10.1016/0306-4522(94)90189-9.
Static torque-angle relationships (invariant characteristics) as measured by Feldman [Feldman A. G. (1980) Neuroscience 5, 81-90] at the human elbow joint for constant descending excitatory drive have a monotonic convex shape determining joint angle-dependent stiffness. In contrast, for constant activation of elbow flexors, the torque increases, peaks and decreases again with increasing angle because of related moment arm alterations [Hasan Z. and Enoka R. M. (1985) Expl Brain Res. 59, 441-450]. Conversion of such constant-excitation torque-angle shapes into an invariant characteristic might result from action of the stretch reflex which adds excitation with increasing joint angle. To test whether a simple linear model of the stretch reflex could convert constant excitation torque-angle relationships into invariant characteristics, the following assumptions were made. (1) Muscle fibre length increases linearly with joint angle. (2) Reflex muscle excitation (electromyogram) is linearly related to muscle (fibre) length. With these assumptions, invariant characteristic shape cannot be derived from constant excitation torque-angle relationships because it would be sigmoid at low and nearly straight at large joint angles, whilst real flexor invariant characteristics are more convex at large than small angles. It is suggested that recurrent inhibition via Renshaw cells contributes to bend the invariant characteristics into their right shape. Renshaw cells show a nonlinear saturating dependence on motor axon input rate and amount of excitation, i.e. number of active axon collateral synapses. These relationships can contribute to shape motoneuron output so as to yield convex invariant characteristics. Whilst it is not quite clear whether the gain of recurrent inhibition from and to skeleto-motoneurons is high enough to co-determine the invariant characteristic shape significantly, recurrent inhibition of Ia inhibitory interneurons mediating reciprocal inhibition between antagonists is supposed to be quite strong and may influence joint stiffness by interacting with reciprocal inhibition. The arguments presented here extend those of Feldman and co-workers concerning the role of recurrent inhibition and in addition provide a possible explanation for the functional role of mutual inhibition between Renshaw cells. Together with reflex feedback, recurrent inhibition thus contributes to fine-regulate force output and joint stiffness. To account for this cooperation and to make another step towards a general theory of spinal cord circuits, major traits of a new concept are briefly outlined.
费尔德曼[费尔德曼 A.G.（1980 年）《神经科学》5 卷，81 - 90 页]在人类肘关节处测量的恒定下行兴奋性驱动下的静态扭矩 - 角度关系（不变特性）具有单调凸形，决定了与关节角度相关的刚度。相比之下，对于肘关节屈肌的恒定激活，由于相关力臂变化[哈桑 Z.和恩诺卡 R.M.（1985 年）《实验脑研究》59 卷，441 - 450 页]，扭矩随着角度增加先增大、达到峰值后又减小。将这种恒定兴奋扭矩 - 角度形状转换为不变特性可能是由于牵张反射的作用，牵张反射随着关节角度增加而增加兴奋性。为了测试牵张反射的简单线性模型是否能将恒定兴奋扭矩 - 角度关系转换为不变特性，做出了以下假设。（1）肌肉纤维长度随关节角度线性增加。（2）反射性肌肉兴奋（肌电图）与肌肉（纤维）长度线性相关。基于这些假设，无法从恒定兴奋扭矩 - 角度关系中得出不变特性形状，因为在小关节角度时它将呈 S 形，在大关节角度时几乎是直线形，而实际屈肌不变特性在大角度时比小角度时更凸。有人提出，通过闰绍细胞的回返抑制有助于将不变特性弯曲成其正确形状。闰绍细胞对运动轴突输入速率和兴奋量，即活跃轴突侧支突触数量，表现出非线性饱和依赖性。这些关系有助于塑造运动神经元输出，从而产生凸形不变特性。虽然尚不清楚从骨骼运动神经元到闰绍细胞以及从闰绍细胞到骨骼运动神经元的回返抑制增益是否足够高，足以显著共同决定不变特性形状，但介导拮抗肌之间交互抑制的 Ia 抑制性中间神经元的回返抑制被认为相当强，并且可能通过与交互抑制相互作用来影响关节刚度。这里提出的观点扩展了费尔德曼及其同事关于回返抑制作用的观点，此外还为闰绍细胞之间相互抑制的功能作用提供了一种可能的解释。因此，与反射反馈一起，回返抑制有助于精细调节力输出和关节刚度。为了解释这种协同作用，并朝着脊髓回路的一般理论再迈进一步，简要概述了一个新概念的主要特征。
