Synaptic Integration in Crayfish Nonspiking Interneurons

为了研究小龙虾中间神经元的突触输入如何转化为运动神经元的突触输出，建立了小龙虾非刺激性中间神经元的多室模型。用细胞内电流注入实验研究了神经元间树突的电学性质与电极穿透树突的位置的关系。建立了一种现场识别侵彻部位的方法。该模型基于(1)填充荧光染料荧光黄的细胞的形态测量和(2)从阶跃电流注入的结果获得的膜电阻率。通过将电流注入试验的实验结果与在模型上模拟该试验的计算结果进行比较，对模型进行了评估。通过扩大模型的膜面积来补偿模型中由于细胞染色不完善而丢失的细小树突分支，直到最终获得最优的树突。对中间神经元模型的突触活动的模拟表明，在树突上的电紧张性扩散过程中，中间神经元的突触电位比运动神经元更有效地平滑了突触电位。这一结果与非棘突中间神经元的输入电阻和膜时间常数显著大于运动神经元树突的结果一致。结论：非刺激性中间神经元作为运动神经元的延伸树突，通过收集来自更大范围的突触前细胞的突触输入并对其进行处理，使运动神经元的总体兴奋性可以通过中间神经元所测量的连续的膜电位变化而以更精细的方式调节。

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.