A Quantitative Description of Nodal Membrane Currents in Myelinated Nerve Fibres of the Lizard Anous Carolinensis

Experiments were performed on individual nodes of Ranvier from a lizard at 15–16°C, using the method of Nonner (1969). In addition to a leakage current (mean conductance of 19.7 nS), the membrane ionic currents consisted of a Na+ current, completely inhibited by lμmoll−1 external tetrodotoxin, and a K+ current, sensitive to external tetraethylammonium ions and to internal caesium ions. The inactivation time course of the Na+current at 0 mV was well fitted by the sum of two exponential phases, one fast and one slow, with mean time constants of 0.68 and 2.92 ms, respectively. In contrast to observations on amphibian and mammalian preparations, the peak Na+ current showed an almost linear potential dependence at large positive membrane potentials. A mean maximum Na+ conductance of 146.8 nS was calculated. Both the time and potential dependence of the K+ current indicated that it was similar to that found in amphibian nodes of Ranvier. The mean maximum K+ conductance was 51.9 nS. Furthermore, as described in amphibians, three different components of the K+ current (s, f1 and f2) could be separated. We conclude that, under similar experimental conditions, nodal ionic currents are qualitatively and quantitatively similar in lizard and amphibian myelinated nerve fibres, suggesting that the nodal membrane of these two preparations contains similar types of ionic channels. These results strengthen the view that the near absence of nodal K+ channels is a peculiarity of mammalian nerves.