Herrmann U, Flanders M
Neuroscience Graduate Program, University of Minnesota, Minneapolis, Minnesota 55455, USA.
J Neurosci. 1998 Oct 15;18(20):8402-16. doi: 10.1523/JNEUROSCI.18-20-08402.1998.
The directional activity of whole muscles has been shown to be broadly and often multimodally tuned, raising the question of how this tuning is subserved at the level of single motor units (SMUs). Previously defined rules of SMU activation would predict that units of the same muscle (or at least of the same neuromuscular compartment) are activated homogeneously with activity peaks in the same "best" direction(s). In the present study, the best directions of SMUs in human biceps (both heads) and deltoid (anterior, medial, and posterior portions) were determined by measuring the firing rate and threshold force of units for recruitment during isometric force ramps in many different directions. For all muscles studied, neighboring motor units could have significantly different best directions, suggesting that each muscle receives multiple directional commands. Furthermore, 17% of the units sampled clearly had a second-best direction, consistent with a convergence of different directional commands onto the same motoneuron. The best directions of the units changed gradually with location in the muscle. Best directions did not cluster into separate groups, thus, not supporting the existence of clearly distinguished neuromuscular compartments. Instead, the results reveal a more gradually distributed activation of the biceps and deltoid motoneuron pools. A model is proposed in which the central control mechanism optimizes the fulfillment of the continuously changing directional force requirements of a movement by gradually recruiting and derecruiting those units ideally suited for the production of the required force vector at any given time.
整块肌肉的定向活动已被证明具有广泛且常常是多模式的调谐，这就提出了一个问题，即这种调谐在单个运动单位（SMU）水平上是如何实现的。先前定义的SMU激活规则预测，同一肌肉（或至少同一神经肌肉隔室）的单位会以相同的“最佳”方向上的活动峰值均匀激活。在本研究中，通过测量在许多不同方向上等长力斜坡期间募集单位的放电率和阈值力，确定了人类肱二头肌（两个头）和三角肌（前部、中部和后部）中SMU的最佳方向。对于所有研究的肌肉，相邻的运动单位可能具有显著不同的最佳方向，这表明每块肌肉都接收多个定向指令。此外，所采样的单位中有17%明显有第二个最佳方向，这与不同的定向指令汇聚到同一个运动神经元上一致。单位的最佳方向随肌肉中的位置逐渐变化。最佳方向没有聚集成单独的组，因此，不支持明显区分的神经肌肉隔室的存在。相反，结果揭示了肱二头肌和三角肌运动神经元池的激活分布更为逐渐。提出了一个模型，其中中央控制机制通过逐渐募集和解除募集那些在任何给定时间最适合产生所需力向量的单位，来优化对运动中不断变化的定向力需求的满足。
