Collins W F, Honig M G, Mendell L M
J Neurophysiol. 1984 Nov;52(5):980-93. doi: 10.1152/jn.1984.52.5.980.
Excitatory postsynaptic potentials (EPSPs) were recorded in medial gastrocnemius (MG) motoneurons following intraaxonal electrical stimulation of single spindle afferent fibers in anesthetized cats. High-frequency bursts of 32 shocks were delivered to the afferent axon and the EPSPs were averaged in the motoneuron. EPSP amplitude generally changed during the burst, in some cases increasing and in other cases decreasing, depending on the connection. Interpretation of these changes was complicated by potentiation of the initial EPSPs in the burst that occurred with the repeated bursts. The extent of the potentiation varied from connection to connection. The magnitude of facilitation or depression during a burst of standard frequency (167 Hz) was determined by comparison of EPSPs at the end of the burst with the mean EPSP obtained during low-frequency stimulation (18 Hz). Large amplitude EPSPs tended to depress, whereas the small amplitude EPSPs facilitated. Facilitation was more prevalent in motoneurons with large rheobases and depression was more often observed in small rheobase motoneurons. The use of partial correlations, which was necessary because of the inverse correlation between EPSP amplitude and motoneuron rheobase, revealed that facilitation-depression behavior during repetitive stimulation is correlated primarily with EPSP amplitude rather than with motoneuron rheobase. Acute transection of the spinal cord resulted in no change in motoneuron rheobase but considerable enlargement of mean EPSP amplitude at low frequencies of stimulation. A significant increase in the amount of depression during repetitive stimulation was noted under these conditions. These results indicate considerable heterogeneity in the response of individual connections to repetitive stimulation. We suggest that this heterogeneity results from differences in transmitter release at different connections. This heterogeneity must also have functional consequences related to susceptibility for firing of different motoneurons under various physiological conditions that can include afferent discharge frequencies equivalent to those used in this study.
在麻醉猫中，通过对单根肌梭传入纤维进行轴突内电刺激，记录腓肠肌内侧（MG）运动神经元的兴奋性突触后电位（EPSP）。向传入轴突施加32次电击的高频脉冲串，并对运动神经元中的EPSP进行平均。EPSP幅度在脉冲串期间通常会发生变化，在某些情况下增加，而在其他情况下减少，这取决于连接方式。这些变化的解释因脉冲串中初始EPSP的增强而变得复杂，这种增强随着重复脉冲串的出现而发生。增强的程度因连接方式而异。通过比较脉冲串结束时的EPSP与低频刺激（18Hz）期间获得的平均EPSP，确定标准频率（167Hz）脉冲串期间促进或抑制的幅度。大振幅EPSP倾向于抑制，而小振幅EPSP则促进。促进在具有大基强度的运动神经元中更普遍，而抑制在小基强度运动神经元中更常观察到。由于EPSP幅度与运动神经元基强度之间存在负相关，因此使用偏相关分析表明，重复刺激期间的促进-抑制行为主要与EPSP幅度相关，而不是与运动神经元基强度相关。脊髓急性横断导致运动神经元基强度无变化，但在低频刺激时平均EPSP幅度显著增大。在这些条件下，重复刺激期间抑制量显著增加。这些结果表明，各个连接对重复刺激的反应存在相当大的异质性。我们认为这种异质性是由不同连接处递质释放的差异导致的。这种异质性在功能上必然也与不同运动神经元在各种生理条件下的放电易感性有关，这些生理条件可能包括与本研究中使用的传入放电频率相当的频率。
