Amplification and linear summation of synaptic effects on motoneuron firing rate.
Amplification and linear summation of synaptic effects on motoneuron firing rate.
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突触对运动神经元放电率影响的放大和线性求和。
DOI：
10.1152/jn.2001.85.1.43
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发表时间：
2001
期刊：
影响因子：
--
通讯作者：
Cope,TC
中科院分区：
文献类型：
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作者：
Prather,JF;Powers,RK;Cope,TC
本研究的目的是测量突触输入对未麻醉猫制剂中运动神经元放电率的影响，其中电压敏感树突电导的激活可能会影响突触整合和重复放电。在麻醉猫中，稳定突触输入产生的放电率变化约等于静息电位 (IN) 下测量的有效突触电流与体细胞注入电流和运动神经元放电率 (f-Islope) 之间线性关系斜率的乘积。然而，之前对未麻醉的去大脑猫的研究表明，放电率调节可能受到电压依赖性树突电导的强烈影响。为了量化这些电导对运动神经元放电行为的影响，我们将阈上电流阶跃注入去大脑猫的内侧腓肠肌运动神经元中，并测量叠加的兴奋性突触输入产生的放电速率的变化。在相同的细胞中，我们测量了 IN 和 f-Islope 以确定放电率的预测变化 (ΔF=IN*f-Islope)。与之前在麻醉猫中的结果相比，突触诱导的运动神经元放电率的变化大于预测。这种增强的效果表明在重复点火过程中存在额外的内向电流。这种额外的内向电流放大了由两种不同的兴奋源（Ia组肌梭传入神经和尾部皮肤腓肠神经传入神经）产生的有效突触电流。 Ia 输入（14/16 细胞）比腓肠输入（11/16 细胞）有更普遍的扩增趋势。然而，在两个输入均被放大的细胞（10/16 细胞）中，每个源的放大幅度相似。当这两个突触输入同时激活时，它们的组合效应通常非常接近其放大的个体效应的线性和。在麻醉猫的内侧腓肠肌运动神经元中也观察到线性求和，其中不存在放大。这种相似性表明放大不会干扰突触整合的过程。对于此处研究的两个分段输入，放大输入的线性求和是明显的。如果这些现象也适用于其他突触源，那么放大背后的活跃树突电导的存在可能使运动神经元能够整合多个突触输入，并以相对简单的方式在整个生理范围内驱动运动神经元放电率。
The aim of this study was to measure the effects of synaptic input on motoneuron firing rate in an unanesthetized cat preparation, where activation of voltage-sensitive dendritic conductances may influence synaptic integration and repetitive firing. In anesthetized cats, the change in firing rate produced by a steady synaptic input is approximately equal to the product of the effective synaptic current measured at the resting potential (IN) and the slope of the linear relation between somatically injected current and motoneuron discharge rate (f-Islope). However, previous studies in the unanesthetized decerebrate cat indicate that firing rate modulation may be strongly influenced by voltage-dependent dendritic conductances. To quantify the effects of these conductances on motoneuron firing behavior, we injected suprathreshold current steps into medial gastrocnemius motoneurons of decerebrate cats and measured the changes in firing rate produced by superimposed excitatory synaptic input. In the same cells, we measuredINand thef-Islope to determine the predicted change in firing rate (ΔF=IN*f-Islope). In contrast to previous results in anesthetized cats, synaptically induced changes in motoneuron firing rate were greater-than-predicted. This enhanced effect indicates that additional inward current was present during repetitive firing. This additional inward current amplified the effective synaptic currents produced by two different excitatory sources, group Ia muscle spindle afferents and caudal cutaneous sural nerve afferents. There was a trend toward more prevalent amplification of the Ia input (14/16 cells) than the sural input (11/16 cells). However, in those cells where both inputs were amplified (10/16 cells), amplification was similar in magnitude for each source. When these two synaptic inputs were simultaneously activated, their combined effect was generally very close to the linear sum of their amplified individual effects. Linear summation is also observed in medial gastrocnemius motoneurons of anesthetized cats, where amplification is not present. This similarity suggests that amplification does not disturb the processes of synaptic integration. Linear summation of amplified input was evident for the two segmental inputs studied here. If these phenomena also hold for other synaptic sources, then the presence of active dendritic conductances underlying amplification might enable motoneurons to integrate multiple synaptic inputs and drive motoneuron firing rates throughout the entire physiological range in a relatively simple fashion.