Department of Integrative Physiology, University of Colorado, Boulder, Colorado, USA.
J Neurophysiol. 2011 Jul;106(1):265-73. doi: 10.1152/jn.00954.2010. Epub 2011 May 4.
Presynaptic modulation of Ia afferents converging onto the motor neuron pool of the extensor carpi radialis (ECR) was compared during contractions (20% of maximal force) sustained to failure as subjects controlled either the angular position of the wrist while supporting an inertial load (position task) or exerted an equivalent force against a rigid restraint (force task). Test Hoffmann (H) reflexes were evoked in the ECR by stimulating the radial nerve above the elbow. Conditioned H reflexes were obtained by stimulating either the median nerve above the elbow or at the wrist (palmar branch) to assess presynaptic inhibition of homonymous (D1 inhibition) and heteronymous Ia afferents (heteronymous Ia facilitation), respectively. The position task was briefer than the force task (P = 0.001), although the maximal voluntary force and electromyograph for ECR declined similarly at failure for both tasks. Changes in the amplitude of the conditioned H reflex were positively correlated between the two conditioning methods (P = 0.02) and differed between the two tasks (P < 0.05). The amplitude of the conditioned H reflex during the position task first increased (129 ± 20.5% of the initial value, P < 0.001) before returning to its initial value (P = 0.22), whereas it increased progressively during the force task to reach 122 ± 17.4% of the initial value at failure (P < 0.001). Moreover, changes in conditioned H reflexes were associated with the time to task failure and force fluctuations. The results suggest a task- and time-dependent modulation of presynaptic inhibition of Ia afferents during fatiguing contractions.
在受试者控制手腕角度（位置任务）或对刚性约束施加等效力（力量任务）以维持失败的 20%最大力收缩期间，比较了作用于伸腕肌（ECR）运动神经元池的 Ia 传入纤维的突触前调制。通过刺激肘上方的桡神经在 ECR 中引发测试性 Hoffmann（H）反射。通过刺激肘上方或腕部（掌支）的正中神经获得条件性 H 反射，以分别评估同相（D1 抑制）和异相 Ia 传入纤维的突触前抑制（异相 Ia 易化）。位置任务比力量任务更短（P = 0.001），尽管两种任务的最大自主力和 ECR 肌电图在失败时都相似地下降。两种条件方法之间条件性 H 反射幅度的变化呈正相关（P = 0.02），并且在两种任务之间存在差异（P <0.05）。在位置任务中，条件性 H 反射的幅度首先增加（初始值的 129 ± 20.5%，P <0.001），然后恢复到初始值（P = 0.22），而在力量任务中则逐渐增加，达到初始值的 122 ± 17.4%在失败时（P <0.001）。此外，条件性 H 反射的变化与任务失败时间和力波动有关。结果表明，在疲劳收缩期间，Ia 传入纤维的突触前抑制存在任务和时间依赖性调节。
