Celichowski J, Pogrzebna M, Raikova R T
Department of Neurobiology, University School of Physical Education, Poznan, Poland.
Arch Ital Biol. 2005 Feb;143(1):51-63.
The course of unfused tetani with the sag effect in fast motor units of rat medial gastrocnemius was studied. The analysis of the course of successive contractions within these tetani showed that the high peak force at the beginning of tetanus before the sag resulted from temporary, very efficient sum mation of contractions at this phase, both in FF (fast fatigable) and FR (fast resistant to fatigue) units. The process of summation developed in spite of parallel shortening of the contraction and relaxation. The peak of tetanus force was visible on the average at the 2nd contraction in FF units and at the 5th contraction in FR units. After the tetanus peak the process of the efficient summation was completed and the force decreased what was visible as a sag. In the following part of the tetanus, mainly in FF units, the potentiation occurred and the force of successive contractions increased. The rise of force was visible in spite of shortening of the contraction time and was due to prolongation of the relaxation in this part of the tetanus. These observations indicated that the processes of the summation of successive contractions before sag and during the potentiation underwent different mechanisms discussed in this paper. Considerable release of Ca2+ ions from the sarcoplasmic reticulum was proposed as a possible mechanism responsible for a very efficient summation at the beginning of the tetanus whereas phosphorylation of regulatory light chain of myosin (RLC) in muscle fibers was considered as the reason of potentiation. Moreover, the present analysis revealed that previously found differences in tetani profiles between FF and FR units resulted from faster development of described changes in the course of contractions summating into the tetanus in FF motor units.
研究了大鼠内侧腓肠肌快速运动单位中伴有松弛效应的非融合强直收缩过程。对这些强直收缩中连续收缩过程的分析表明，强直收缩开始时在松弛之前出现的高峰力是由于在此阶段FF（快速疲劳）和FR（快速抗疲劳）单位中收缩的暂时、非常有效的总和。尽管收缩和松弛平行缩短，但总和过程仍在发展。FF单位中强直收缩力的峰值平均出现在第2次收缩时，FR单位中出现在第5次收缩时。在强直收缩峰值之后，有效的总和过程完成，力下降，这表现为松弛。在强直收缩的后续阶段，主要在FF单位中，出现了增强作用，连续收缩的力增加。尽管收缩时间缩短，但力的增加仍然可见，这是由于强直收缩这一部分中松弛时间的延长。这些观察结果表明，在松弛之前和增强过程中连续收缩的总和过程经历了本文讨论的不同机制。有人提出，肌浆网中Ca2+离子的大量释放可能是强直收缩开始时非常有效的总和的一种机制，而肌肉纤维中肌球蛋白调节轻链（RLC）的磷酸化被认为是增强作用的原因。此外，目前的分析表明，先前发现的FF和FR单位之间强直收缩曲线的差异是由于FF运动单位中收缩总和形成强直收缩过程中所述变化的发展更快。
