School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, Ontario, Canada.
Department of Systems Design Engineering, University of Waterloo, Ontario, Canada.
J Neurophysiol. 2019 Aug 1;122(2):823-832. doi: 10.1152/jn.00138.2019. Epub 2019 Jun 26.
Current understanding of human motor unit (MU) control and aging is mostly derived from hand and limb muscles that have spinal motor neuron innervations. The aim here was to characterize and test whether a muscle with a shared innervation supply from brainstem and spinal MU populations would demonstrate similar age-related adaptations as those reported for other muscles. In humans, the superior trapezius (ST) muscle acts to elevate and stabilize the scapula and has primary efferent supply from the spinal accessory nerve (cranial nerve XI) located in the brainstem. We compared electrophysiological properties obtained from intramuscular and surface recordings between 10 young (22-33 yr) and 10 old (77-88 yr) men at a range of voluntary isometric contraction intensities (from 15 to 100% of maximal efforts). The old group was 41% weaker with 43% lower MU discharge frequencies compared with the young (47.2 ± 9.6 Hz young and 26.7 ± 5.8 Hz old, < 0.05) during maximal efforts. There was no difference in MU number estimation between age groups (228 ± 105 young and 209 ± 89 old, = 0.33). Furthermore, there were no differences in needle detected near fiber (NF) stability parameters of jitter or jiggle. The old group had lower amplitude and smaller area of the stimulated compound muscle action potential and smaller NF MU potential area with higher NF counts. Thus, despite age-related ST weakness and lower MU discharge rates, there was minimal evidence of MU loss or compensatory reinnervation. The human superior trapezius (ST) has shared spinal and brainstem motor neuron innervation providing a unique model to explore the impact of aging on motor unit (MU) properties. Although the ST showed higher MU discharge rates compared with most spinally innervated muscles, voluntary strength and mean MU rates were lower in old compared with young at all contraction intensities. There was no age-related difference in MU number estimates with minimal electrophysiological evidence of collateral reinnervation.
目前对人类运动单位（MU）控制和衰老的理解主要来自手部和肢体肌肉，这些肌肉具有脊髓运动神经元支配。这里的目的是描述和测试具有来自脑干和脊髓 MU 群体共同支配供应的肌肉是否会表现出与其他肌肉报告的类似的与年龄相关的适应性。在人类中，上斜方肌（ST）肌肉的作用是提升和稳定肩胛骨，并且主要的传出供应来自位于脑干的副神经（颅神经 XI）。我们比较了 10 名年轻（22-33 岁）和 10 名老年（77-88 岁）男性在一系列自愿等长收缩强度（从 15%到 100%最大努力）下从肌内和表面记录获得的电生理特性。与年轻人相比，老年人的力量弱 41%，MU 放电频率低 43%（年轻人为 47.2±9.6 Hz，老年人为 26.7±5.8 Hz，<0.05），最大努力时。年龄组之间 MU 数量估计没有差异（年轻人为 228±105，老年人为 209±89，=0.33）。此外，在针检测到的近纤维（NF）抖动或摆动稳定性参数方面没有差异。老年人的刺激复合肌肉动作电位的幅度和面积较小，NF MU 电位面积较小，NF 计数较高。因此，尽管上斜方肌（ST）与年龄相关的虚弱和较低的 MU 放电率有关，但 MU 丧失或代偿性再支配的证据很少。人类上斜方肌（ST）具有共享的脊髓和脑干运动神经元支配，为探索衰老对运动单位（MU）特性的影响提供了一个独特的模型。尽管 ST 与大多数脊髓支配的肌肉相比具有更高的 MU 放电率，但在所有收缩强度下，老年人的自愿力量和平均 MU 率都低于年轻人。MU 数量估计没有与年龄相关的差异，电生理证据很少表明有侧支再支配。
