van den Berg R J, Rijnsburger W H
J Membr Biol. 1980 Dec 30;57(3):213-21. doi: 10.1007/BF01869589.
In voltage-clamp configurations for nodes of Ranvier the axoplasm resistance functions as a voltage-current converter. In existing configurations this resistance cannot be measured directly. In the present arrangement the electrical resistances of the preparation (axoplasm, membrane and seals) can be measured only from two measurements. This allows us to: 1. calibrate the ionic current under voltage-clamp conditions, and 2. calculate the intensity of the current fluctuations, not arising from the membrane (background noise). The measured axoplasm resistances are considerably higher than the values calculated on the basis of fiber geometry and axoplasm resistivity. The difference is due to the presence of constrictions in the nerve fiber. Membrane current estimation based on geometrical parameters in the presence of wide seals may contain large errors. Variations in the axoplasm resistance for voltage-membrane current conversion were observed within 1.5 hr. In 68% of the fibers this resistance decreased with 30% of the original value. With our current calibration the values for the maximum sodium conductance gNa (at 0mV membrane potential), maximum potassium conductance gK and leakage conductance gL are 49.5 X 10(-8), 6.66 X 10(-8) and 1.71 X 10(-8) S. respectively. The contribution of the different noise sources to the total background noise was calculated at the holding potential. For frequencies below 10(3) Hz there is an excellent agreement between measured and calculated noise levels.
在郎飞结的电压钳配置中，轴浆电阻起到电压 - 电流转换器的作用。在现有的配置中，这种电阻无法直接测量。在当前的设置中，制剂（轴浆、膜和封接）的电阻只能通过两次测量来确定。这使我们能够：1. 在电压钳条件下校准离子电流；2. 计算并非由膜产生的电流波动强度（背景噪声）。测得的轴浆电阻显著高于基于纤维几何形状和轴浆电阻率计算得出的值。差异是由于神经纤维中存在缩窄。在存在宽封接的情况下，基于几何参数的膜电流估计可能包含较大误差。在1.5小时内观察到用于电压 - 膜电流转换的轴浆电阻存在变化。在68%的纤维中，这种电阻下降至原始值的30%。通过我们当前的校准，最大钠电导gNa（在膜电位为0mV时）、最大钾电导gK和漏电导gL的值分别为49.5×10⁻⁸、6.66×10⁻⁸和1.71×10⁻⁸S。在保持电位下计算了不同噪声源对总背景噪声的贡献。对于低于10³Hz的频率，测量的和计算的噪声水平之间有很好的一致性。
