School of Kinesiology, The University of British Columbia, Vancouver, British Columbia, Canada.
J Neurophysiol. 2013 Aug;110(4):899-906. doi: 10.1152/jn.00065.2013. Epub 2013 May 29.
Standing balance is often threatened in everyday life. These threats typically involve scenarios in which either the likelihood or the consequence of falling is higher than normal. When cats are placed in these scenarios they respond by increasing the sensitivity of muscle spindles imbedded in the leg muscles, presumably to increase balance-relevant afferent information available to the nervous system. At present, it is unknown whether humans also respond to such postural threats by altering muscle spindle sensitivity. Here we present two studies that probed the effects of postural threat on spinal stretch reflexes. In study 1 we manipulated the threat associated with an increased consequence of a fall by having subjects stand at the edge of an elevated surface (3.2 m). In study 2 we manipulated the threat by increasing the likelihood of a fall by occasionally tilting the support surface on which subjects stood. In both scenarios we used Hoffmann (H) and tendon stretch (T) reflexes to probe the spinal stretch reflex circuit of the soleus muscle. We observed increased T-reflex amplitudes and unchanged H-reflex amplitudes in both threat scenarios. These results suggest that the synaptic state of the spinal stretch reflex is unaffected by postural threat and that therefore the muscle spindles activated in the T-reflexes must be more sensitive in the threatening conditions. We propose that this increase in sensitivity may function to satisfy the conflicting needs to restrict movement with threat, while maintaining a certain amount of sensory information related to postural control.
站立平衡在日常生活中经常受到威胁。这些威胁通常涉及跌倒的可能性或后果高于正常水平的情况。当猫处于这些情况下时，它们会通过增加嵌入腿部肌肉中的肌梭的敏感性来做出反应，大概是为了增加可供神经系统使用的与平衡相关的传入信息。目前，尚不清楚人类是否也会通过改变肌梭敏感性来应对这种姿势威胁。在这里，我们提出了两项研究，探讨了姿势威胁对脊髓牵张反射的影响。在研究 1 中，我们通过让受试者站在高架表面（3.2 米）的边缘来操纵与跌倒后果增加相关的威胁。在研究 2 中，我们通过偶尔倾斜受试者站立的支撑表面来增加跌倒的可能性来操纵威胁。在这两种情况下，我们都使用 Hoffmann（H）和肌腱伸展（T）反射来探测比目鱼肌的脊髓牵张反射回路。我们观察到在两种威胁情况下 T 反射幅度增加而 H 反射幅度不变。这些结果表明，姿势威胁不会影响脊髓牵张反射的突触状态，因此在威胁条件下激活的肌梭必须更加敏感。我们提出，这种敏感性的增加可能有助于满足在有威胁时限制运动的冲突需求，同时保持与姿势控制相关的一定量的感觉信息。
