Control properties of single motor units

运动单位是由脊髓中的运动神经元组成的，它是轴突，它延伸到肌肉，以及它所支配的所有肌肉纤维。运动单位提供从中枢神经系统到骨骼肌的唯一输出，因此在运动的神经控制中起关键作用。运动单位是由来自大脑的下行自愿命令和来自感觉受体的反馈控制的。来自感觉感受器的冲动通过脊髓的反射通路激活运动单位。研究运动单位的反射控制的一种常见方法是向外周神经传递一个或两个电刺激脉冲，并记录反射反应（H反射），通过脊髓介导，返回到肌肉。这项技术提供了大量关于人类反射的信息。然而，在人类运动过程中，运动单位并不是沿着反射通路只接收一个或两个脉冲，而是在几秒钟内接收多个脉冲。运动单位如何对这种重复输入做出反应，在很大程度上尚未探索。此外，运动单位通常保持活跃，即使在刺激被关闭，但这些不自主收缩的机制仍然存在争议。本提案中描述的实验的短期目标是表征运动单位在不同频率、强度和不同水平的自主收缩下沿着沿着反射通路接收生理相关脉冲串时如何反应。长期目标是确定以下机制：1）重复刺激期间发生的反射幅度变化和2）刺激关闭后持续存在的不自主肌肉收缩。结果将确定运动单位如何响应时，他们收到的输入沿着反射路径，是更生理相关的比以前研究（即脉冲串，而不仅仅是一个或两个脉冲）。这一发现可能会为人类运动单位的神经控制带来新的思考方式。

A motor unit is composed of a motor neuron in the spinal cord, it's axon that travels out to the muscle, and all the muscle fibres it innervates. Motor units provide the only output from the central nervous system to skeletal muscle and thus play a key role in the neural control of movement. Motor units are controlled by descending voluntary commands from the brain and feedback from sensory receptors. Impulses from sensory receptors activate motor units via reflex pathways through the spinal cord. A common way to study the reflex control of motor units is to deliver one or two pulses of electrical stimulation to a peripheral nerve and record the reflex response (H-reflex), mediated through the spinal cord, that returns to the muscle. This technique has provided a wealth of information about human reflexes. During human movement, however, motor units do not receive only one or two impulses along reflex pathways but rather they receive multiple impulses over several seconds. How motor units respond to such repetitive inputs has gone largely unexplored. In addition, motor units often remain active even when the stimulation is turned off, but the mechanisms that underlie these involuntary contractions remains controversial. The short term goals for the experiments described in this proposal are to characterize how motor units respond when they receive physiologically relevant trains of impulses along reflex pathways at different frequencies, intensities and at different levels of voluntary contraction. The long term goals are to identify the mechanisms responsible for: 1) the changes in reflex amplitude that occur during repetitive stimulation and 2) the involuntary muscle contractions that persist after the stimulation has been turned off. The results will identify how motor units respond when they receive inputs along reflex pathways that are more physiologically relevant than have been studied previously (i.e. trains of impulses rather than just one or two impulses). The findings may lead to new way of thinking about the neural control of human motor units.