Campbell D M, Rose P K
Department of Physiology, Queen's University, Kingston, Ontario, Canada.
J Neurophysiol. 1997 Mar;77(3):1470-86. doi: 10.1152/jn.1997.77.3.1470.
The specific membrane resistivity of motoneurons at or near the soma (Rms) is much lower than the specific membrane resistivity of the dendritic tree (Rmd). The goal of the present experiments was to investigate the contribution of tonically active voltage-dependent potassium channels at or near the soma to the low Rms. These channels were blocked with the use of intracellular injections of cesium. Input resistance (RN), Rms/Rmd, a conductance representing voltage-dependent potassium channels on the soma, GK, and a conductance attributed to damage caused by electrode impalement, GDa, were estimated from voltage responses to a step of current. The effect of intracellular injections of cesium on electrotonic structure was determined with the use of two strategies: 1) a population analysis that compared data from two groups of motoneurons, those recorded with potassium acetate electrodes (control group) and those recorded with cesium acetate electrodes after the motoneurons were loaded with cesium; and 2) an analysis of changes in electrotonic structure that occurred over the course of multiple injections of cesium or during similar periods of time in control cells. RN of control and cesium-loaded motoneurons was similar. Over the course of the recordings, RN of control cells usually increased and this increase was associated with a hyperpolarization. In contrast, increases in RN after successive injections of cesium were closely linked to a depolarization. At corresponding membrane potentials, Rms/Rmd of cesium-loaded motoneurons was greater than Rms/Rmd of control motoneurons. Over the course of cesium injections, Rms/Rmd increased and the membrane potential depolarized. In contrast, increases in Rms/Rmd observed during the course of recordings from control cells were associated with a hyperpolarization. Compared with control cells at corresponding membrane potentials, GK was less in cesium-loaded cells. Increases in GK that occurred over the course of recordings in control cells were closely coupled to a depolarization. In cesium-loaded cells, GK usually decreased as the cell depolarized during the injections. In control cells, increases in GDa that occurred during the recording period were closely coupled to a depolarization. In contrast, changes in GDa that occurred during cesium injections were not related to the change in membrane potential and did not explain the corresponding changes in Rms/Rmd and membrane potential. The results of this study indicate that voltage-dependent potassium channels contribute to the somatic shunt (low Rms) in neck motoneurons and provide a voltage-dependent mechanism for the dynamic regulation of motoneuron electrotonic properties.
运动神经元胞体处或其附近的比膜电阻（Rms）远低于树突树的比膜电阻（Rmd）。本实验的目的是研究胞体处或其附近的持续性激活的电压依赖性钾通道对低Rms的贡献。通过细胞内注射铯来阻断这些通道。根据电流阶跃的电压响应估计输入电阻（RN）、Rms/Rmd、代表胞体上电压依赖性钾通道的电导GK以及归因于电极刺入造成损伤的电导GDa。使用两种策略确定细胞内注射铯对电紧张结构的影响：1）群体分析，比较两组运动神经元的数据，一组是用醋酸钾电极记录的（对照组），另一组是运动神经元加载铯后用醋酸铯电极记录的；2）分析在多次注射铯过程中或在对照细胞的相似时间段内发生的电紧张结构变化。对照和加载铯的运动神经元的RN相似。在记录过程中，对照细胞的RN通常增加，且这种增加与超极化相关。相反，连续注射铯后RN的增加与去极化密切相关。在相应的膜电位下，加载铯的运动神经元的Rms/Rmd大于对照运动神经元的Rms/Rmd。在注射铯的过程中，Rms/Rmd增加且膜电位去极化。相反，在对照细胞记录过程中观察到的Rms/Rmd增加与超极化相关。与相应膜电位下的对照细胞相比，加载铯的细胞中GK较小。对照细胞记录过程中GK的增加与去极化密切相关。在加载铯的细胞中，注射过程中随着细胞去极化GK通常降低。在对照细胞中，记录期间GDa的增加与去极化密切相关。相反，注射铯期间GDa的变化与膜电位变化无关，也无法解释Rms/Rmd和膜电位的相应变化。本研究结果表明，电压依赖性钾通道对颈部运动神经元的胞体分流（低Rms）有贡献，并为运动神经元电紧张特性的动态调节提供了一种电压依赖性机制。
