Bennett D J, Gorassini M, Prochazka A
Division of Neuroscience, University of Alberta, Edmonton, Canada.
Can J Physiol Pharmacol. 1994 May;72(5):525-34. doi: 10.1139/y94-076.
In three sets of experiments in nine normal subjects and a patient with a percutaneous wrist-stabilizing splint, we quantified the open-loop gain (OLG) of the stretch reflex acting about the elbow. The subjects exerted a steady mean flexing force and were instructed not to intervene (i.e., not to resist actively) when force or displacement perturbations were imposed on the forearm. The method was either to reconstruct transmission around the entire loop in a two-part experiment, or to use the attenuation of external perturbations in normal and electrically stimulated muscle to compute gain. Across all experiments, the mean magnitude of stretch reflex OLG was close to unity in the frequency range 1-2 Hz, and declined at higher frequencies, as required to ensure stability, given that the phase lag approached 180 degrees at 5 Hz. Inherent muscle stiffness was approximately equal to reflex stiffness. In functional terms, an OLG of 1 means that the yield caused by a force perturbation is approximately halved by reflex action (prevailing inherent muscle stiffness is doubled). Automatic scaling of reflex transmission at Ia/alpha-motoneuronal synapses ensures that the OLG remains close to unity as inherent stiffness increases. Trials in the patient with the wrist fixator gave similar results, indicating that the reflexes were proprioceptive ly mediated. In a fourth experiment in which the task was to catch a heavy ball, we compared the efficacy of inherent muscle stiffness and reflexes alone, with the subject's intentional reactions, which included predictive and voluntary components of response. The latter were far more effective in maintaining the position of the hand after the ball was caught than inherent and reflex stiffnesses alone. We conclude that stability requirements limit the extent to which stretch reflexes can augment inherent muscle stiffness. When inherent muscle stiffness is low, such as in our ball-catching task, the reflex stiffness is also low, and predictive and pre-programmed reactions predominate in load compensation, thus shifting the emphasis from automatic servo or equilibrium-point behaviour to higher order control.
在针对9名正常受试者和1名佩戴经皮手腕稳定夹板的患者进行的三组实验中，我们对作用于肘部的牵张反射的开环增益（OLG）进行了量化。受试者施加稳定的平均屈曲力，并被要求在前臂受到力或位移扰动时不进行干预（即不主动抵抗）。方法一是在一个两部分的实验中重建整个环路的传递，二是利用正常和电刺激肌肉中外力扰动的衰减来计算增益。在所有实验中，牵张反射OLG的平均幅值在1 - 2Hz频率范围内接近1，并在更高频率下下降，这是确保稳定性所必需的，因为在5Hz时相位滞后接近180度。固有肌肉刚度大约等于反射刚度。从功能角度来看，OLG为1意味着由力扰动引起的屈服通过反射作用大约减半（主要的固有肌肉刚度加倍）。Ia/α运动神经元突触处反射传递的自动缩放可确保随着固有刚度增加，OLG仍接近1。对佩戴手腕固定器的患者进行的试验得出了类似结果，表明反射是由本体感觉介导的。在第四个实验中，任务是接住一个重球，我们比较了仅固有肌肉刚度和反射的功效与受试者的有意反应，后者包括反应的预测性和自愿性成分。在接住球后，后者在保持手部位置方面比仅固有和反射刚度有效得多。我们得出结论，稳定性要求限制了牵张反射增强固有肌肉刚度的程度。当固有肌肉刚度较低时，如在我们的接球任务中，反射刚度也较低，并且预测性和预编程反应在负荷补偿中占主导地位，从而将重点从自动伺服或平衡点行为转移到更高阶控制。
