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以其最大强直张力的固定百分比激活的慢肌运动单位的非均匀疲劳特性。

Nonuniform fatigue characteristics of slow-twitch motor units activated at a fixed percentage of their maximum tetanic tension.

作者信息

Cope T C, Webb C B, Yee A K, Botterman B R

机构信息

Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallas 75235.

出版信息

J Neurophysiol. 1991 Nov;66(5):1483-92. doi: 10.1152/jn.1991.66.5.1483.

Abstract
  1. The endurance of slow-twitch motor units from the soleus (SOL) and medial gastrocnemius (MG) muscles of the cat were tested by determining the length of time (endurance time, Et) that a unit could maintain its tension output at 85% of maximum. Motor-unit tension was clamped at the target level by altering the stimulation rate of a unit's motor axon through computer feedback control. Tested in this way, units of both muscles displayed a wide range of Ets, approximately 40- to 50-fold. 2. Electromyographic (EMG) waveforms of motor units subjected to force-clamp contractions were analyzed to access whether any activity-dependent changes in their waveform shape might predict Et. Three measurements of waveform shape were determined: baseline-to-baseline duration, peak-to-peak amplitude, and area. Typically, amplitude decreased and duration increased as a contraction proceeded, whereas area remained fairly constant. Because changes in each measure were very similar for units of widely different Ets, it was concluded that neuromuscular junction failure and changes in the excitability of the sarcolemma (excluding the t-tubule system) play a minor role in determining Et. 3. Et was highly correlated with the mean stimulation rate (Et/number of stimuli) used during the force-clamp contractions. Mean rate was seen to progressively decrease with increasing Et. This correlation could not be explained by measures of isometric contractile speed or relaxation (e.g., twitch contraction time or half-relaxation time) measured before the force-clamp contractions. Both contraction time and half-relaxation time were found to be unrelated to both Et and the rate used to stimulate the unit during the force-clamp contraction. 4. Among type S units of SOL and MG, maximum tetanic tension and Et were not related. A significant relation (r = -0.49) was found between axonal conduction velocity and Et for SOL units (n = 38). In addition, a significant correlation (r = 0.47) was found between conduction velocity and tetanic tension for SOL units. Perhaps because of the small sample of type S units from MG (n = 10), conduction velocity was found not be related to either Et or tetanic tension. 5. Others have shown that a motor unit's maximum tetanic tension and axonal conduction velocity are correlated with its order of recruitment among motoneurons innervating a muscle. Recent work has further shown that among type F units the order in which a motoneuron is recruited is highly correlated with the fatigue resistance of its muscle unit.(ABSTRACT TRUNCATED AT 400 WORDS)
摘要
  1. 通过测定猫比目鱼肌(SOL)和内侧腓肠肌(MG)慢肌运动单位在最大张力的85%时能够维持其张力输出的时间长度(耐力时间,Et),来测试这些运动单位的耐力。通过计算机反馈控制改变运动单位运动轴突的刺激频率,将运动单位的张力钳制在目标水平。以这种方式进行测试时,两块肌肉的运动单位都表现出广泛的耐力时间范围,约为40至50倍。2. 对经历强制钳制收缩的运动单位的肌电图(EMG)波形进行分析,以确定其波形形状的任何与活动相关的变化是否可以预测Et。确定了波形形状的三项测量指标:基线到基线持续时间、峰峰值幅度和面积。通常,随着收缩进行,幅度减小而持续时间增加,而面积保持相当恒定。由于对于耐力时间差异很大的运动单位,每项测量指标的变化非常相似,因此得出结论,神经肌肉接头功能衰竭和肌膜(不包括横管系统)兴奋性的变化在确定Et方面起次要作用。3. Et与强制钳制收缩期间使用的平均刺激频率(Et/刺激次数)高度相关。随着Et增加,平均频率逐渐降低。这种相关性无法用等长收缩速度或放松的测量指标(例如,单收缩时间或半放松时间)来解释,这些指标是在强制钳制收缩之前测量的。发现收缩时间和半放松时间与Et以及强制钳制收缩期间用于刺激运动单位的频率均无关。4. 在SOL和MG的S型运动单位中,最大强直张力和Et无关。对于SOL运动单位(n = 38),发现轴突传导速度与Et之间存在显著关系(r = -0.49)。此外,对于SOL运动单位,发现传导速度与强直张力之间存在显著相关性(r = 0.47)。也许由于来自MG的S型运动单位样本量较小(n = 10),发现传导速度与Et或强直张力均无关。5. 其他人已经表明,运动单位的最大强直张力和轴突传导速度与其在支配一块肌肉的运动神经元中的募集顺序相关。最近的研究进一步表明,在F型运动单位中,运动神经元的募集顺序与其肌肉单位的抗疲劳性高度相关。(摘要截断于400字)

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