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The membrane currents in the myelinated nerve fibre of the toad Xenopus laevis have been analysed in previous investigations (Dodge & Frankenhaeuser, 1959;Frankenhaeuser, 1959Frankenhaeuser, , 1960Frankenhaeuser, , 1962a Frankenhaeuser, , b, c, 1963a with the voltage clamp technique described by Frankenhaeuser & Persson (1957) and Dodge & Frankenhaeuser (1958). For this analysis, membrane potential was changed in steps and membrane current was recorded. Equations were then fitted to the results so as to express the component ionic currents as functions of membrane potential and time. In the present investigation these equations are used to compute the time course of the membrane action potential and of the ionic currents which accompany it, and the results are compared with action potentials recorded from real nerve fibres.It seems appropriate to sum up some of the findings, and especially some of the uncertainties, of the analysis, before the computations are dealt with.(1) The membrane currents associated with a potential step are treated as the sum of a capacitive current (IJ) and an ionic current (Ii).(2) The time resolution of the clamp technique is not sufficient for reliable measurements of I.. In addition, membrane capacitance (Cm) generally increases during the experiments, an effect which clearly has to do with the application of tight petroleum jelly seals (see Dodge & Frankenhaeuser, 1959).(3) The ionic currents are treated as the sum of a leak current (IL), an initial current and a complex delayed current. (4) The leak current has been measured at anodal steps and at the sodium equilibrium potential. Since the instantaneous IL-V relation seems to be nearly linear, IL is treated as IL = -L ( VL) where 9L is a leak conductance, which is independent of potential and time, and VL is the

With the voltage clamp technique it has been found that the membrane of the squid giant nerve fibre shows a sequence of specific changes in sodium and potassium conductances when the membrane is depolarized (Hodgkin & Huxley, 1952a-d; Hodgkin, Huxley & Katz, 1952). A step depolarization is associated with a rapid transient increase in sodium conductance followed by a slower but lasting increase in potassium conductance. The magnitudes and the rates of change of these permeability changes vary continuously with the membrane potential so that they are larger and more rapid at large cathodal polarizations than at small. The mechanism underlying these permeability changes is mainly unknown, but in further experiments with the voltage clamp technique it was found that the conductance-membrane potential (gNs-V and

The purpose of the present investigation was to determine the effects which changes in temperature have on the constants governing the permeability changes in the myelinated nerve fibre. Voltage clamp experiments were carried out with the node under investigation at various temperatures. The membrane current records so obtained were then analysed as in previous investigations (Frankenhaeuser, 1960(Frankenhaeuser, , 1963 and the rate constants am, Pm' Oh /Ah, oan and fn, and the permeability constants PNa and P'K were calculated. The temperature coefficients (Qlo) were then determined from these data.The significance of the constants is apparent from the following equations PNa = PNa m2h,PNa and P'K are constants with dimensions of permeability; m, h and n are dimensionless variables which can vary between 0 and 1; a and P are rate constants which vary with membrane potential but not with time, and have dimensions of (time)-'.Since a complete voltage clamp analysis is a rather tedious task it was decided to restrict the analysis to a few potential steps only. These potential steps were chosen such that a relatively reliable determination of one rate constant at each step was obtained in a simple manner. METHODSThe experiments were performed on single nodes of Ranvier in large isolated myelinated nerve fibres of the clawed toad (Xenopus laevi8). The membrane potential of node No was changed in rectangular steps by the aid of a feed-back amplifier system as described in previous reports (Dodge & Frankenhaeuser, 1958, 1959. The node No was kept in a con. 28-2