Ounjian M, Roy R R, Eldred E, Garfinkel A, Payne J R, Armstrong A, Toga A W, Edgerton V R
Brain Research Institute, University of California, Los Angeles 90024.
J Neurobiol. 1991 Jul;22(5):547-59. doi: 10.1002/neu.480220510.
There is increasing evidence that the architectural design and arrangement of the fibers within a motor unit have important physiological and developmental ramifications. Limited data, however, are available to directly address this issue. In the present study the physiological properties of one motor unit in each of seven cat tibialis anterior (TA) muscles were determined. Each of these units then was repetitively stimulated to deplete the glycogen in all muscle fibers within the unit. Subsequently, the length, type of ending, and spatial distribution of fibers sampled from these physiologically and histochemically typed motor units were determined. Four fast fatigable (FF), one fast, fatigue resistant (FR), and two slow (S) motor units (MU) were studied. The samples consisted of all those glycogen-depleted fibers (9-27) contained within a single fascicle or a circumscribed area of each of the motor unit territories. The mean fiber lengths for the two slow motor units were 35.9 and 45.5 mm. The mean fiber lengths for the fast motor unit samples ranged from 8.8 to 48.5 mm. Some fibers of both the fast and slow units reached lengths of 58 mm. Most of the fibers in the slow units extended the entire distance between the proximal and distal musculotendinous planes, had relatively constant cross-sectional areas, and terminated at the tendon as blunt endings. In contrast, the majority of the fibers in the fast units terminated intrafascicularly at one end, and the cross-sectional area decreased progressively along their lengths, that is, showed a tapering pattern for a significant proportion of their lengths. Therefore, the force generated by units that end midfascicularly would appear to be transmitted to connective tissue elements and/or adjacent fibers. All fibers of a fast unit within a fascicle were located at approximately the same proximo-distal location. Thus, developmentally the selection of muscle fibers by a motoneuron would seem to be influenced by their spatial distribution. The architectural complexities of motor units also have clear implications for the mechanical interactions of active and inactive motor units. For example, the tension capabilities of a motor unit may be influenced not only by the spatial arrangement of its own fibers, but also by the level of activation of neighboring motor units.
越来越多的证据表明，运动单位内纤维的结构设计和排列具有重要的生理和发育影响。然而，直接解决这一问题的数据有限。在本研究中，测定了7只猫胫骨前肌（TA）中每块肌肉的一个运动单位的生理特性。然后对这些单位中的每一个进行重复刺激，以耗尽该单位内所有肌纤维中的糖原。随后，确定从这些经生理和组织化学分型的运动单位中取样的纤维的长度、末梢类型和空间分布。研究了4个快疲劳型（FF）、1个快抗疲劳型（FR）和2个慢型（S）运动单位（MU）。样本包括单个肌束或每个运动单位区域的限定区域内包含的所有那些糖原耗尽的纤维（9 - 27根）。两个慢运动单位的平均纤维长度分别为35.9毫米和45.5毫米。快运动单位样本的平均纤维长度在8.8至48.5毫米之间。快、慢单位的一些纤维长度达到58毫米。慢单位中的大多数纤维在近端和远端肌肉 - 肌腱平面之间延伸整个距离，具有相对恒定的横截面积，并在肌腱处呈钝端终止。相比之下，快单位中的大多数纤维在一端在肌束内终止，并且横截面积沿其长度逐渐减小，即在其大部分长度上呈现逐渐变细的模式。因此，在肌束中部终止的单位产生的力似乎会传递到结缔组织元件和/或相邻纤维。一个肌束内快单位的所有纤维位于大致相同的近端 - 远端位置。因此，从发育角度来看，运动神经元对肌纤维的选择似乎受其空间分布的影响。运动单位的结构复杂性对活跃和不活跃运动单位的机械相互作用也有明显影响。例如，一个运动单位的张力能力可能不仅受其自身纤维空间排列的影响，还受相邻运动单位的激活水平的影响。
