Synaptic Integration in Crayfish Nonspiking Interneurons

为了研究传递给中间神经元的突触输入如何转化为传递给运动神经元的突触输出，建立了小龙虾非尖峰中间神经元的多室模型。通过细胞内电流注射试验研究了神经元中间树突的电学特性与电极插入树突位置的关系。建立了一种原位识别侵彻部位的方法。该模型是基于(1)荧光染料路西法黄填充细胞的形态学测量和(2)阶跃电流注入结果获得的膜电阻率。将电流注入试验的实验结果与模型上电流注入试验的模拟计算结果进行对比，对模型进行了评价。在构建的模型中，由于细胞染色不完善而缺失的精细树突分支通过扩大模型的膜面积来补偿，直到最终获得最优的树突分支。在中间神经元模型上的突触活动模拟表明，在树突上的电紧张扩散过程中，中间神经元的突触电位比运动神经元的突触电位更有效地平滑。这与非尖峰中间神经元的输入电阻和膜时间常数明显大于运动神经元树突的结果一致。由此得出结论，非尖峰中间神经元作为运动神经元的延伸树突，通过收集来自空间范围更广的突触前细胞的突触输入，并以这样一种方式处理它们，从而通过中间神经元连续的膜电位变化以更精细的方式调节运动神经元的一般兴奋性。

Multicompartment models of crayfish nonspiking interneurons were developed in order to investigate how the synaptic input to interneurons could be converted into the synaptic output to motoneurons. Electrical properties of the interneuron dendrite were studied by intracellular current injection test with relation to the site of electrode penetration into the dendrite. A method for identifying the penetration site in situ has been established. The model was based on (1) morphological measurement of the cell which was filled with the fluorescent dye, Lucifer yellow and (2) the membrane resistivity obtained from the result of step current injection. The model was evaluated by comparing the experimental result of current injection test with the calculated one in the simulation of that test on the model. The fine dendritic branches missing in the constructed model due to imperfect staining of the cell was compensated by expanding the membrane area of the model until the optimal one was finally obtained.Simulation of synaptic activities on the interneuron model revealed that the synaptic potential in the interneuron was smoothed during the electrotonic spread over the dendrite more effectively than in the motoneurons. The result is consistent with the finding that the input resistance and the membrane time constant of nonspiking interneurons are significantly greater than those of motoneuron dendrites. It is concluded that the nonspiking interneurons function as an extended dendrites of motoneurons by gathering synaptic inputs from spatially wider range of presynaptic cells and processing them in such a way that the general excitability of motoneurons can be adjusted in a more delicate way through the continuous membrane potential change meicated by the interneuron.