Munson J B, Foehring R C, Mendell L M, Gordon T
Department of Neuroscience, University of Florida College of Medicine, Gainesville 32610-0244, USA.
J Neurophysiol. 1997 May;77(5):2605-15. doi: 10.1152/jn.1997.77.5.2605.
Chronic stimulation (for 2-3 mo) of the medial gastrocnemius (MG) muscle nerve by indwelling electrodes renders the normally heterogeneous MG muscle mechanically and histochemically slow (type SO). We tested the hypothesis that motoneurons of MG muscle thus made type SO by chronic stimulation would also convert to slow phenotype. Properties of all single muscle units became homogeneously type SO (slowly contracting, nonfatiguing, nonsagging contraction during tetanic activation). Motoneuron electrical properties were also modified in the direction of type S, fatigue-resistant motor units. Two separate populations were identified (on the basis of afterhyperpolarization, rheobase, and input resistance) that likely correspond to motoneurons that had been fast (type F) or type S before stimulation. Type F motoneurons, although modified by chronic stimulation, were not converted to the type S phenotype, despite apparent complete conversion of their muscle units to the slow oxidative type (type SO). Muscle units of the former type F motor units were faster and/or more powerful than those of the former type S motor units, indicating some intrinsic regulation of motor unit properties. Experiments in which chronic stimulation was applied to the MG nerve cross-regenerated into skin yielded changes in motoneuron properties similar to those above, suggesting that muscle was not essential for the effects observed. Modulation of group Ia excitatory postsynaptic potential (EPSP) amplitude during high-frequency trains, which in normal MG motoneurons can be either positive or negative, was negative in 48 of 49 chronically stimulated motoneurons. Negative modulation is characteristic of EPSPs in motoneurons of most fatigue-resistant motor units. The general hypothesis of a periphery-to-motoneuron retrograde mechanism was supported, although the degree of control exerted by the periphery may vary: natural type SO muscle appears especially competent to modify motoneuron properties. We speculate that activity-dependent regulation of the neurotrophin-(NT) 4/5 in muscle plays an important role in controlling muscle and motoneuron properties.
通过植入电极对腓肠肌内侧（MG）神经进行慢性刺激（持续2 - 3个月），可使原本异质性的MG肌肉在机械和组织化学上转变为慢肌（I型）。我们检验了这样一个假设：通过慢性刺激使MG肌肉转变为I型后，其运动神经元也会转变为慢表型。所有单肌单位的特性均均匀地变为I型（强直激活时收缩缓慢、抗疲劳、无松弛收缩）。运动神经元的电特性也朝着抗疲劳的S型运动单位方向改变。根据超极化后电位、基强度和输入电阻确定了两个不同的群体，它们可能分别对应刺激前的快（F型）运动神经元或S型运动神经元。F型运动神经元虽然受到慢性刺激的影响，但尽管其肌单位明显完全转变为慢氧化型（I型），却并未转变为S型表型。原来F型运动单位的肌单位比原来S型运动单位的肌单位更快和/或更强有力，这表明运动单位特性存在一些内在调节。对交叉再生到皮肤中的MG神经进行慢性刺激的实验产生了与上述类似的运动神经元特性变化，这表明肌肉对于所观察到的效应并非必不可少。在高频串刺激期间，正常MG运动神经元的Ia类兴奋性突触后电位（EPSP）幅度的调制可正可负，但在49个慢性刺激的运动神经元中，有48个为负。负调制是大多数抗疲劳运动单位运动神经元EPSP的特征。尽管外周施加的控制程度可能有所不同，但外周至运动神经元逆行机制的总体假设得到了支持：天然的I型肌肉似乎特别有能力改变运动神经元特性。我们推测，肌肉中神经营养因子（NT）- 4/5的活性依赖性调节在控制肌肉和运动神经元特性方面起着重要作用。
