Davis B M, Collins W F, Mendell L M
J Neurophysiol. 1985 Dec;54(6):1541-52. doi: 10.1152/jn.1985.54.6.1541.
Single medial gastrocnemius Ia-afferent fibers and motoneurons to which they projected were simultaneously impaled in anesthetized cats. Each Ia-afferent fiber was electrically stimulated once every 2 s with short high-frequency bursts (32 shocks at 167 Hz) followed by 1-11 test shocks. The resulting motoneuron excitatory postsynaptic potentials (EPSPs) were recorded and averaged in register. The interval between the end of one burst and the beginning of the next was 2 s; therefore, the amplitude of the first EPSP in the burst was considered to be a measure of efficacy of transmission 2 s after the burst. At most connections (23/29) the mean amplitude of the first EPSP in the burst was equal to or larger than the mean amplitude of control EPSPs produced by low-frequency (18-Hz) stimulation. Enhancement of transmission was maximum 50-100 ms after the burst, and the amplitude of the test EPSP delivered at this time was always greater than that of the control. The period of enhanced transmission appeared to decay more rapidly at connections with small EPSPs. The greatest amount of EPSP amplitude enhancement at 50 or 100 ms after the burst was observed at connections at which EPSP amplitude increased during the burst. The shape (rise time, half width) of potentiated EPSPs was the same as control EPSPs averaged during low-frequency (18-Hz) stimulation. Multiple shocks delivered at low frequency between bursts revealed that enhanced transmission following the high-frequency burst is very sensitive to the effects of low-frequency test stimulation. Furthermore, increasing the number of shocks during the interval between bursts reduced the enhancement of the first EPSP in the burst. We suggest that modulation of synaptic transmission after high-frequency bursts differs across Ia-motoneuron connections. These time-dependent changes associated with short bursts of firing (which are similar in frequency to those observed in Ia-fibers supplying hind-limb muscles during stepping) emphasize the necessity to consider the history of the discharge pattern of the group Ia fiber in assessing efficacy at individual Ia-motoneuron connections.
在麻醉猫中，同时刺入单根内侧腓肠肌Ia传入纤维及其投射的运动神经元。每隔2秒对每根Ia传入纤维进行一次电刺激，采用短高频脉冲串（167赫兹下32次电击），随后进行1 - 11次测试电击。记录并对齐平均所产生的运动神经元兴奋性突触后电位（EPSP）。一个脉冲串结束与下一个脉冲串开始之间的间隔为2秒；因此，脉冲串中第一个EPSP的幅度被视为脉冲串后2秒时传递效率的一种度量。在大多数连接（23/29）中，脉冲串中第一个EPSP的平均幅度等于或大于低频（18赫兹）刺激产生的对照EPSP的平均幅度。传递增强在脉冲串后50 - 100毫秒时最大，此时传递的测试EPSP幅度总是大于对照。在具有小EPSP的连接中，传递增强期似乎衰减得更快。在脉冲串期间EPSP幅度增加的连接中，观察到脉冲串后50或100毫秒时EPSP幅度增强量最大。增强后的EPSP的形状（上升时间、半宽度）与低频（18赫兹）刺激期间平均的对照EPSP相同。在脉冲串之间以低频施加多次电击表明，高频脉冲串后的增强传递对低频测试刺激的影响非常敏感。此外，增加脉冲串之间间隔期间的电击次数会降低脉冲串中第一个EPSP的增强。我们认为，高频脉冲串后突触传递的调制在Ia - 运动神经元连接中存在差异。这些与短脉冲发放相关的时间依赖性变化（其频率与在行走时供应后肢肌肉的Ia纤维中观察到的频率相似）强调了在评估单个Ia - 运动神经元连接的效率时考虑Ia纤维放电模式历史的必要性。
