Department of Neurorehabilitation, Zurzachcare, Bad Zurzach, Switzerland.
Department of Physiology, University of Athens, Athens, Greece.
J Neurophysiol. 2022 Jan 1;127(1):161-172. doi: 10.1152/jn.00297.2021. Epub 2021 Dec 15.
Normal subjects can completely eliminate resistance upon imposed head-on-trunk rotations when they are asked to relax. It is not, however, clear how neck reflexes to stretch can be voluntarily suppressed. Reflexive responses might be modified by adjusting the gain of the reflex loop through descending control. Theoretically, necessary corrections upon interfering disturbances during coordinated motor performance requiring the interplay of relaxation/activation may be missing if muscle relaxation is taking place exclusively by this mechanism. It has been alternatively proposed that sensory information from the periphery may be allowed to "neutralize" neck reflexes if it is fed back with opposite sign to the structures driving the reflexes. Six healthy subjects were asked to relax while subjected to head-on-trunk rotations generated by a head motor. After any initial resistance had completely subsided, the head was unexpectedly exposed to "ramp-and-hold" perturbations of up to 2° amplitude and 0.7 s duration. Resistance to stretch consistently reappeared thereupon, suggesting that stretch reflex gain had not been set to zero during the previously achieved complete relaxation. Resistance to perturbations under these circumstances was compared with the forces generated when the same ramp-and-hold displacements were delivered unpredictably to the head held stationary. A quantitative model of neck proprioceptive reflexes suppression has been thus constructed. Gain scheduling or "motor set" cannot sufficiently account for the voluntary reflex suppression during slow passive head rotations. Instead, we propose as underlying mechanism, the "neutralization" of the controlling servo by means of continuous feedback tracking displacement and force signals from the periphery. Head stabilizing neck reflexes can be voluntarily suppressed or activated depending on the task at hand. By applying brief perturbations unexpectedly, both during passive head-on-trunk movements and at rest, we investigated the mechanism of voluntary suppression of resistance to stretch. A physiologically plausible, neuromechanical model of voluntary/reflexive interactions was constructed favoring feedback over reflex gain adjustments. Accordingly, muscle relaxation during imposed head movements is based on sensory feedback similarly to muscle contractions during purposeful movements.
正常受试者在放松时，当被要求头对头躯干旋转时，可以完全消除阻力。然而，目前尚不清楚如何主动抑制颈部伸展反射。通过调节反射环路的增益，可以通过下行控制来改变反射反应。从理论上讲，如果肌肉放松完全通过这种机制发生，则在需要放松/激活相互作用的协调运动表现中，对干扰干扰进行必要的校正可能会丢失。有人还提出，如果将来自外围的感觉信息反馈到驱动反射的结构上，则可以用相反的符号“中和”颈部反射。要求 6 名健康受试者在头部电机产生的头对头躯干旋转时放松。在最初的阻力完全消失后，头部突然受到高达 2°幅度和 0.7 s 持续时间的“斜坡和保持”扰动。拉伸阻力随之再次出现，表明在先前达到的完全放松期间，拉伸反射增益未设置为零。在这些情况下，与当以不可预测的方式将相同的斜坡和保持位移传递到头保持静止时产生的力相比，拉伸阻力的变化。因此构建了颈本体感受反射抑制的定量模型。增益调度或“运动设定”不能充分说明在缓慢被动头部旋转过程中自愿反射抑制。相反，我们提出作为潜在机制，通过连续反馈跟踪来自外围的位移和力信号来“中和”控制伺服。可以根据手头的任务自愿抑制或激活头部稳定颈部反射。通过意外施加短暂的扰动，无论是在被动头对头躯干运动期间还是在休息期间，我们研究了自愿抑制伸展阻力的机制。构建了一种自愿/反射相互作用的生理上合理的神经机械模型，该模型有利于反馈而不是反射增益调整。因此，在强制头部运动期间的肌肉放松与有目的运动期间的肌肉收缩相似，基于感觉反馈。
