Ionic Currents at Resting Potential in Nerve Fibres from Xenopus laevis. Potential Clamp Experiments

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Abstract
The ionic currents in myelinated nerve fibres were measured while the membrane potential was held at its resting value and the external solution with one ionic composition was changed to a solution with a different composition. Equations were derived, based on the independence principle by which the net sodium, potassium and chloride currents can be calculated from the change in current associated with the concentration changes. The chloride currents were found to be negligibly small compared with INa and IK. The ratio INa/IK was found to be about – 5.8, which deviates significantly from — 1. This deviation either shows that the independence principle does not apply to INa and/or Ik at rest or that the membrane at rest has some other current carrier in addition to Na+, K+ and Cl. The slope conductance of the membrane was measured and the measured slope conductance was compared with the sum of the sodium and potassium slope conductances at resting potential calculated from the measured current changes caused by concentration changes. The measured slope conductance was was 2.7 times larger than the sum of the calculated ionic conductances. It is concluded that resting potential of the myelinated nerve is not satisfactorily described as a potassium electrode with a ‘sodium error’.