Blouin Jean-Sébastien, Walsh Lee D, Nickolls Peter, Gandevia Simon C
Prince of Wales Medical Research Institute, University of New South Wales, Sydney 2031, Australia.
J Appl Physiol (1985). 2009 Feb;106(2):370-7. doi: 10.1152/japplphysiol.90939.2008. Epub 2008 Nov 13.
Control of posture and movement requires control of the output from motoneurons. Motoneurons of human lower limb muscles exhibit sustained, submaximal activity to high-frequency electrical trains, which has been hypothesized to be partly triggered by monosynaptic Ia afferents. The possibility to trigger such behavior in upper limb motoneurons and the potential unique role of Ia afferents to trigger such behavior remain unclear. Subjects (n = 9) received high-frequency trains of electrical stimuli over biceps brachii and flexor pollicis longus (FPL). We chose to study the FPL muscle because it has weak monosynaptic Ia afferent connectivity and it is involved in fine motor control of the thumb. Two types of stimulus trains (100-Hz bursts and triangular ramps) were tested at five intensities below painful levels. All subjects exhibited enhanced torque in biceps and FPL muscles after both types of high-frequency train. Torques also persisted after stimulation, particularly for the highest stimulus intensity. To separate the evoked torques that resulted from a peripheral mechanism (e.g., muscle potentiation) and that which resulted from a central origin, we studied FPL responses to high-frequency trains after complete combined nerve blocks of the median and radial nerves (n = 2). During the blocks, high-frequency trains over the FPL did not yield torque enhancements or persisting torques. These results suggest that enhanced contractions of central origin can be elicited in motoneurons innervating the upper limb, despite weak monosynaptic Ia connections for FPL. Their presence in a recently evolved human muscle (FPL) indicates that these enhanced contractions may have a broad role in controlling tonic postural outputs of hand muscles and that they may be available even for fine motor activities involving the thumb.
姿势和运动的控制需要对运动神经元的输出进行控制。人类下肢肌肉的运动神经元对高频电刺激串表现出持续的、次最大活动，据推测这部分是由单突触Ia传入纤维触发的。在上肢运动神经元中触发这种行为的可能性以及Ia传入纤维触发这种行为的潜在独特作用仍不清楚。9名受试者的肱二头肌和拇长屈肌（FPL）接受了高频电刺激串。我们选择研究FPL肌肉，因为它的单突触Ia传入连接较弱，且参与拇指的精细运动控制。在低于疼痛水平的五种强度下测试了两种类型的刺激串（100Hz脉冲和三角波斜坡）。两种高频刺激串后，所有受试者的肱二头肌和FPL肌肉扭矩均增强。刺激后扭矩也持续存在，尤其是在最高刺激强度时。为了区分由外周机制（如肌肉增强）引起的诱发扭矩和由中枢起源引起的诱发扭矩，我们研究了正中神经和桡神经完全联合神经阻滞（n = 2）后FPL对高频刺激串的反应。在阻滞期间，FPL上的高频刺激串未产生扭矩增强或持续扭矩。这些结果表明，尽管FPL的单突触Ia连接较弱，但在上肢运动神经元中仍可引发中枢起源的增强收缩。它们在最近进化的人类肌肉（FPL）中的存在表明，这些增强收缩可能在控制手部肌肉的紧张姿势输出中具有广泛作用，并且即使对于涉及拇指的精细运动活动也可能存在。
