Membrane currents at large positive internal potentials in single myelinated nerve fibres of Rana pipiens.

1. A voltage clamp for single myelinated nerve fibres was developed from commercially available operational amplifiers.2. Under voltage clamp conditions steady-state currents were measured. The currents reached a maximum value at about E = + 100 mV. Currents for greater potentials decreased.3. The preceding polarization markedly affected the magnitude of the delayed current at high cathodal potentials; however, the currents continued to reach maximum values.4. As predicted by the constant field theory, currents at high cathodal polarizations were essentially independent of the external potassium concentration. Also, these currents were independent of the external calcium concentration.5. Two pulse voltage clamp experiments showed that instantaneous outward currents did not reach a limiting value. The limiting steady-state current was reached with an exponential time constant of about 1 msec.