Grinnell A D, Trussell L O
J Physiol. 1983 May;338:221-41. doi: 10.1113/jphysiol.1983.sp014670.
A wide range of motor unit sizes exists in each frog sartorius, with values of tetanus and twitch tensions extending over 62- and 400-fold ranges, respectively. These differences primarily represent differences in the number, rather than the size, of muscle fibres in each motor unit. Tetanus-to-twitch tension ratios varied markedly between different motor units, ranging from 1.5 to over 190. Because directly stimulated muscle fibres have tetanus/twitch ratios no larger than 3, it is concluded that high tetanus/twitch ratios arise from recruitment of muscle fibres during the tetanus which respond with only subthreshold depolarization during a twitch. Small motor units, as judged either by their twitch or tetanus tension, were associated with higher tetanus/twitch ratios, suggesting that the average safety margin of a motor unit increases with the motor unit's size. Indeed, when the tetanus/twitch ratio of a motor unit is used to determine the fraction of fibres in the unit with subthreshold neuromuscular junctions, it is observed that there is a direct linear relationship between the size of the motor unit and its over-all efficacy of synaptic transmission. Measurements of the effects of changes in calcium concentration on motor unit twitch tension confirmed the last conclusion. Furthermore, this analysis revealed that large motor units, although having a wide range of transmission safety margins, are largely comprised of junctions of uniformly high safety margin. In motor units of smaller size, synaptic strengths become more evenly distributed over a wide range of values. Small motor units had consistently longer twitch rise-times than did larger units. This decrease in rise-time with motor unit size paralleled the decrease in tetanus/twitch ratios, raising questions about the regulation of muscle fibre contraction kinetics.
每只青蛙的缝匠肌中存在多种不同大小的运动单位，其强直收缩张力和单收缩张力的值分别在62倍和400倍的范围内变化。这些差异主要体现在每个运动单位中肌纤维的数量不同，而非大小不同。不同运动单位之间的强直收缩与单收缩张力之比差异显著，范围从1.5到超过190。由于直接刺激的肌纤维的强直收缩/单收缩比值不超过3，因此可以得出结论，高强直收缩/单收缩比值是由于在强直收缩期间募集了那些在单收缩时仅产生阈下去极化反应的肌纤维。通过单收缩或强直收缩张力判断的小运动单位，其强直收缩/单收缩比值更高，这表明运动单位的平均安全系数随运动单位大小的增加而增加。实际上，当用运动单位的强直收缩/单收缩比值来确定该单位中具有阈下神经肌肉接头的纤维比例时，可以观察到运动单位的大小与其突触传递的总体效能之间存在直接的线性关系。钙浓度变化对运动单位单收缩张力影响的测量结果证实了这一结论。此外，该分析还表明，大运动单位虽然具有广泛的传递安全系数范围，但主要由安全系数一致较高的接头组成。在较小的运动单位中，突触强度在很宽的值范围内分布得更加均匀。小运动单位的单收缩上升时间始终比大运动单位的长。随着运动单位大小的增加，上升时间的这种减少与强直收缩/单收缩比值的降低平行，这引发了关于肌纤维收缩动力学调节的问题。
