Delayed currents in myelinated nerve fibres of Xenopus laevis investigated with voltage clamp technique

Cathodal step polarizations applied to a node of Ranvier in a myelinated fibre are associated with a complex pattern of membrane current. This current can be separated in the same way as the current in a squid fibre under voltage clamp (Hodgkin, Huxley & Katz, 1952; Hodgkin & Huxley, 1952a, b, c, d) into a number of different fractions: (a) the capacitive current, charging the membrane capacity, (b) the leak current through a more or less non-specific membrane leak, (c) the initial sodium current and (d) the delayed current. The currents a-c have been described and analysed for the myelinated fibre in previous papers (Dodge & Frankenhaeuser, 1958, 1959; Frankenhaeuser, 1959, 1960). The delayed currents have, however, not yet been analysed in detail. The aim of the present investigation was to find out by which ion or ions the delayed current is carried, and it will be shown that the current was carried essentially by potassium, as in the squid fibre (Hodgkin & Huxley, 1952 a). There were, however, some definite complications, since the instantaneous currents at repolarizations clearly deviated from the currents predicted. The potassium currents will be analysed in further detail in following papers (Frankenhaeuser, 1962 a, b). METHODS