Bodine S C, Roy R R, Eldred E, Edgerton V R
J Neurophysiol. 1987 Jun;57(6):1730-45. doi: 10.1152/jn.1987.57.6.1730.
In 11 tibialis anterior muscles of the cat, a single motor unit was characterized physiologically and subsequently depleted of its glycogen through repetitive stimulation of an isolated ventral root filament. Muscle cross sections were stained for glycogen using a periodic acid-Schiff reaction, and single-fiber optical densities were determined to identify those fibers belonging to the stimulated motor unit. Innervation ratios were determined by counting the total number of muscle fibers in a motor unit in sections taken through several levels of the muscle. The average innervation ratios for the fast, fatigueable (FF) and fast, fatigue-resistant (FR) units were similar. However, the slow units (S) contained 61% fewer fibers than the fast units (FF and FR). Muscle fibers belonging to S and FR units were similar in cross-sectional area, whereas fibers belonging to FF units were significantly larger than fibers belonging to either S or FR units. Additionally, muscle fibers innervated by a single motoneuron varied by two- to eightfold in cross-sectional area. Specific tensions, based on total cross-sectional area determined by summing the areas of all muscle fibers of each unit, showed a modest difference between fast and slow units, the means being 23.5 and 17.2 N X cm-2, respectively. Variations in maximum tension among units could be explained principally by innervation ratio, although fiber cross-sectional area and specific tension did contribute to differences between unit types.
在11只猫的胫前肌中，对单个运动单位进行了生理学特征分析，随后通过重复刺激分离的腹根细丝耗尽其糖原。使用过碘酸希夫反应对肌肉横截面进行糖原染色，并测定单纤维光密度以识别属于受刺激运动单位的那些纤维。通过计算在肌肉不同水平处获取的切片中运动单位内肌肉纤维的总数来确定神经支配比。快速、易疲劳（FF）和快速、抗疲劳（FR）单位的平均神经支配比相似。然而，慢肌单位（S）所含的纤维比快肌单位（FF和FR）少61%。属于S和FR单位的肌肉纤维横截面积相似，而属于FF单位的纤维明显大于属于S或FR单位的纤维。此外，由单个运动神经元支配的肌肉纤维横截面积相差2至8倍。基于通过将每个单位所有肌肉纤维的面积相加确定的总横截面积计算的比张力显示，快肌单位和慢肌单位之间存在适度差异，平均值分别为23.5和17.2 N×cm-2。单位间最大张力的变化主要可以由神经支配比来解释，尽管纤维横截面积和比张力确实导致了单位类型之间的差异。
