Membrane current and noise measurements in voltage-clamped node ofRanvier

Summary

In voltage-clamp configurations for nodes ofRanvier 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\(\overline {g_{Na} } \) (at 0 mV membrane potential), maximum potassium conductance\(\overline {g_K } \) and leakage conductance\(\overline {g_L } \) are 49.5×10⁻⁸, 6.66×10⁻⁸ and 1.71×10⁻⁸ S, respectively. The contribution of the different noise sources to the total background noise was calculated at the holding potential. For frequencies below 10³ Hz there is an excellent agreement between measured and calculated noise levels.